CNCFusion mentioned in a thread that their "Deluxe" kit had "Preloaded" ballnuts. How can a SINGLE ballnut be preloaded? I was under the impression that you needed two ballnuts, with something between them to keep the tension (see pic below). Are they just referring to using oversized balls?

Oh, and NOOK said that they can add these oversized balls at the factory for an extra $35 per ballnut. They said this reduces backlash to from .01 to .003. I think I'm going to have to get the $40 SBN series ballnut, because all other options are $300+ :-O I'm planning on doing some sort of DIY preload using dual ballnuts (ala Cadmonkey) and something between them (at least on the X, and maybe the Y).

http://caleb-angie.com/wp-content/uploads/2009/02/preloaded-ballnut-illustration.JPG

If you have two sets of tracks in the nut, and you offset those tracks from each other by a set amount, you can pre-load things. This does not account for variances in the screw lead though, so in different places on the screw you have differing backlash.
I used bellville washers to pre-load my double nut setups, seems to have worked quite well. After all was said and done though, $90-$100 for a single pre-load might have been cheaper given my time in machining, but I'm happy with the result.

could the preloaded mean that they have the balls inside already, rather than just sending you an empty nut and some loose balls for you to put together?

my kit came with the ballnuts already on the screws, perhaps this is what was ment?

i know, just offering an alternative interpretation....

A single pre-loaded nut is one that has had oversize balls installed to minimize (not eliminate) backlash. Another option is double-nuts, with some kind of spring or elastic tensioning means, though adjustability is really not worth much, as there is a "correct" pre-load for each size screw, based on its load rating (usually 10-15% of the rated load). Some of these are adjustable, some are not. The final option is multi-circuit nuts, which have more than one (usually two, sometimes four) separate circuits, and are also pre-loaded to eliminate backlash. These also sometimes have integral elastic tensioning means.

Regards,
Ray L.

could the preloaded mean that they have the balls inside already, rather than just sending you an empty nut and some loose balls for you to put together?

my kit came with the ballnuts already on the screws, perhaps this is what was ment?

i know, just offering an alternative interpretation....

No, pre-load is a specific condition depending on the fit of the balls within the grooves of the screw. There are several ways it can be achieved. Pre-loaded A/C bearings don't mean that the balls are already in place in the raceways, it means that there is some initial force applied to the bearings before any loads are imposed upon them due to actual work done. Hence PRE load. The same thing applies with a ball nut(s).

Thanks for the info gentlemen.

Here's my conundrum:

Nook SBN standard no-frills ballnuts are $41. According to Andrew the Nook rep, they come with backlash of .01 (pretty bad).

I can either replace the balls with larger ones myself, or pay Nook $35 per nut to do it at the factory. I know that they will wear quicker, but according to Andrew, this would reduce the backlash from .01 down to .003 (better).

The next step up in Nook ballnuts (as far as precision) would be the pre-loaded line, which would cost me $320 each. This is ridiculous for a hobby mill.

Therefore, I am stuck with the SBN nuts, with .003 backlash at its best. I am considering doing a DIY pre-load with dual nuts similar to what CadMonkey did. However, he used BOTH larger balls AND pre-load bracket, and only got the backlash down to .0025. The trouble to make the pre-load bracket doesn't seem worth the .0005 reduction as compared to just using a single nut with larger balls.

What would you recommend as a "target" backlash to get down to? What amount of backlash can Mach3 or EMC2 successfully and accurately handle?

Thanks!

-Caleb105-

i know, i was trying to make a funny.. guess it didnt work.

Thanks for the info gentlemen.

Here's my conundrum:

Nook SBN standard no-frills ballnuts are $41. According to Andrew the Nook rep, they come with backlash of .01 (pretty bad).

I can either replace the balls with larger ones myself, or pay Nook $35 per nut to do it at the factory. I know that they will wear quicker, but according to Andrew, this would reduce the backlash from .01 down to .003 (better).

The next step up in Nook ballnuts (as far as precision) would be the pre-loaded line, which would cost me $320 each. This is ridiculous for a hobby mill.

Therefore, I am stuck with the SBN nuts, with .003 backlash at its best. I am considering doing a DIY pre-load with dual nuts similar to what CadMonkey did. However, he used BOTH larger balls AND pre-load bracket, and only got the backlash down to .0025. The trouble to make the pre-load bracket doesn't seem worth the .0005 reduction as compared to just using a single nut with larger balls.

What would you recommend as a "target" backlash to get down to? What amount of backlash can Mach3 or EMC2 successfully and accurately handle?

Thanks!

-Caleb105-

Getting down to low backlash by ball-loading alone is a crap shoot, as you're limited by the dimensions of the specific screw and nut, and the availability of different-sized balls - generally quite limited. The best way is a spring pre-load, in which case ball-loading is really not necessary. Properly done, they should be *zero* backlash in one direction under all conditions, and *zero* backlash in the other direction until the load exceeds the pre-load amount. For all practical purposes, this will always be zero backlash. If you build a spring-loaded double nut assembly and don't get zero backlash, you've done something wrong. The spring pre-load should be on the order of 10-15% of the rated load for the screw, which is typically around 180 pounds for a 5/8" screw. This is easily achieved with a stack of several Belleville washers. The tricky part is coming up with a reasonable way of keeping the nuts from rotating, and, if necessary, adjusting the pre-load. If you can do it without having to machine the nut, that can be a plus, as at least some ballnuts are made from unGodly hard stuff. I can't even make a dent in mine with carbide. It is important that provision be made for a slight amount (a few thou) of axial movement, to deal with imperfections in the screws. If you don't do this, you may have binding in some positions, and/or suffer premature failure of the screws or nuts.

Regards,
Ray L.

Ray,

Thanks for the great info. A few more questions. Is there enough room on an X3 to do double-nut pre-load on the Y travel? Would you need to worry/mess with this on the Z travel?

Also, I'm still not totally sure exactly what a double-nut preload setup looks like. Does anyone have close-up pictures of a setup like this? Cadmonkey's block just looked like a block with 4 holes for bolts, and a pass-through hole for the ballscrew. What do the 4 machine bolts screw into?

Also, I know nothing about these washers you speak of. Do you have any pictures of a setup like this?

ETA: I understand that the basic concept of a double-nut preload setup is to have something in the middle that pushes out against both nuts, but how do you push out in the middle, while still holding the nuts together? See pic:

http://caleb-angie.com/wp-content/uploads/2009/02/preloaded-ballnut-illustration-2.JPG

Thanks guys!

-Caleb105-

Thanks for the info gentlemen.

Here's my conundrum:

Nook SBN standard no-frills ballnuts are $41. According to Andrew the Nook rep, they come with backlash of .01 (pretty bad).

I can either replace the balls with larger ones myself, or pay Nook $35 per nut to do it at the factory. I know that they will wear quicker, but according to Andrew, this would reduce the backlash from .01 down to .003 (better).

The next step up in Nook ballnuts (as far as precision) would be the pre-loaded line, which would cost me $320 each. This is ridiculous for a hobby mill.

Therefore, I am stuck with the SBN nuts, with .003 backlash at its best. I am considering doing a DIY pre-load with dual nuts similar to what CadMonkey did. However, he used BOTH larger balls AND pre-load bracket, and only got the backlash down to .0025. The trouble to make the pre-load bracket doesn't seem worth the .0005 reduction as compared to just using a single nut with larger balls.

What would you recommend as a "target" backlash to get down to? What amount of backlash can Mach3 or EMC2 successfully and accurately handle?

Thanks!

-Caleb105-

I used a pair of $27 (from mcmaster) Thompson nuts per axis. A stack of Bellville washers (also mcmaster, search "disc spring"). I made a block, threaded one nut into it, and put dowel pins through the same block, just enough so that the second nut couldn't rotate on the screw. There are pictures in my thread on "New Homebuilt Mill project" that show what I'm talking about. Sucky part was the $60 tap to do the ball nut mount, you can find them for less, or you can single point the thread on a lathe if you have that capability.
2x$27 gives you $54, and then I'd add $20 per for the tap, since it's getting spread over 3 holes. Another $10 (generous) for the washers and you're looking at $84 per axis.
I've got next to no measurable backlash out of this setup, and although it cost just slightly less than I've seen some pre-loaded nuts go for (especially after counting my machine time). I am happier with this solution than the single fixed pre-load. YMMV.

Correction - I used each method on 2 different axes - I used oversized balls on Y to only use one nut and maintain max travel. I used double nuts on X. Both have some backlash but it is a small and consistent amount that is easily compensated for by my control software. (I think the .0025 was an early measurement, I thought I got it down to .001 or less, I'd have to check my .ini file for EMC to be sure).

And yes, my double nut setup didn't work like I intended as I don't have a strong enough stepper to preload the 2nd nut as much as it needs to take up the slop and still turn - I loose steps - I didn't do a spring separation setup, I compressed 2 nuts together and haven't been able to tune it 'just right'.

Correction - I used each method on 2 different axes - I used oversized balls on Y to only use one nut and maintain max travel. I used double nuts on X. Both have some backlash but it is a small and consistent amount that is easily compensated for by my control software. (I think the .0025 was an early measurement, I thought I got it down to .001 or less, I'd have to check my .ini file for EMC to be sure).

And yes, my double nut setup didn't work like I intended as I don't have a strong enough stepper to preload the 2nd nut as much as it needs to take up the slop and still turn - I loose steps - I didn't do a spring separation setup, I compressed 2 nuts together and haven't been able to tune it 'just right'.

CadMonkey,

I think perhaps I am misunderstanding your setup. On your X axis, didn't you have two ballnuts facing each other, both threaded into the "Preload Block"? Doesn't this put them at a fixed distance apart? Where does the preload come from? What pushes the two nuts apart?

Also, you didn't do either method (larger balls or doublenut) on the Z axis correct? How did that work for you?

And what kind of backlash were you getting on the Y axis after switching to larger balls?

AND...(last question for this post...I promise) is there a limit to the amount of backlash that Mach3 can correct for?

Thanks!

-Caleb105-

i would suggest that the backlash comp be used as a last resort. Correcting it mechanically will yeild better overall results.. then whatever you cant get out mechanically, then use the BL comp.

CadMonkey,

I think perhaps I am misunderstanding your setup. On your X axis, didn't you have two ballnuts facing each other, both threaded into the "Preload Block"? Doesn't this put them at a fixed distance apart? Where does the preload come from? What pushes the two nuts apart?
Doesn't matter is they are pushed apart or together. As long as the balls in each nut are touching opposite sides of the raceway there's preload. I have 2 blocks on my X - the second is not anchored to the saddle but is anchored to the primary nut's mounting block by 4 screws, so tighten them to remove backlash.

Also, you didn't do either method (larger balls or doublenut) on the Z axis correct? How did that work for you?
Well enough, I run the gib TIGHT so theres sticktion that makes more problem than anything. If I loosen the gib it runs nice and repeatable, but theres chatter...

And what kind of backlash were you getting on the Y axis after switching to larger balls?
I don't honestly remember, it's set in the .ini file for EMC2 so I'd have to go back and look.

AND...(last question for this post...I promise) is there a limit to the amount of backlash that Mach3 can correct for?
Don't know - I use EMC2 :) I'm open-source. But I think it was project 5k that said do as much mechanically then use backlash comp for the remainder. For hobby it certainly wasn't worth spending $1200 in ballnuts to get the ground nut, though I may get a preloaded double nut for the Z axis and take care of that one with that.

As for preloading yourself - there is a wide variety of ball sizes available. Get the nut and the screw first, see what your backlash is and then measure all the balls and visit the seller toolsupply (I think...) on eBay. He has steps of .0001 up from the nominal ball diameter if I remember. I should have gone one size larger for the ones I ordered for Y, but I had them and wanted it together so I just said the hell with it and was done.

stupid idea here-

but what about say on a hobby mill. you have a ballscrew thats fixed to both ends on the table and in the middle goes through the ballnut. What about using 2 cheap ballnuts, 1 of the ballnuts if fixed to the table the other is loose on the screw, but you screw and thread 4 corners of both ballscrew mounting blocks and attach (screws+nuts?) and get it very tight? this way you have a ballnut pushing in each direction of slop, would this not extremely reduce the backlash?

Is anyone following me or should i post a MS paint picture?

cheers

caleb105,
There are many types of ball nuts and I'm sure some are much better than others. I have a Syil SX3 machine which I have had disassembled several times so I know what kind of ball nuts it has. They use a single ball nut, however, they are in effect a double ball nut. The nut is sctually two separate nuts that are spaced apart with a two piece spacer. The nuts are screwed towards each other against the spacer and locked in place with double locks, thus eliminating nearly all of the slack. Take a look at my post of 12-1-07 on this site.

OM

stupid idea here-

but what about say on a hobby mill. you have a ballscrew thats fixed to both ends on the table and in the middle goes through the ballnut. What about using 2 cheap ballnuts, 1 of the ballnuts if fixed to the table the other is loose on the screw, but you screw and thread 4 corners of both ballscrew mounting blocks and attach (screws+nuts?) and get it very tight? this way you have a ballnut pushing in each direction of slop, would this not extremely reduce the backlash?

Is anyone following me or should i post a MS paint picture?

cheers

That's exactly what I did - there is a section showing the X axis in the file of mine you got :) Sheet 3 page 7 of 31 in the PDF, the top section which is colored. I know it will work, but it's very persnickety (is that a word?) to tune and requires a stronger X axis motor than I have.

the PDF is HERE (http://www.distinctperspectives.com/X3/X3_CNC_CONVERSION.PDF) for those that are curious.

picture are always fun

Greg (CadMonkey) could you possibly be binding the second nut by tightening one of the bolts more than another thus skewing the nut so it rides the screw at an angle? I was looking at this before I started adding the screws to my machine and since you didn't mention any problems with it I didn't think it worth mentioning but now that I hear you're troubles I'm wondering if this could be part of the problem. I haven't added a second nut yet because I would like to see what larger balls would do but I was looking for an alternative to let the nut self-align on the screw but as of yet I haven't figured out a way to keep them tight and aligned properly to prevent binding. I got something in mind but right now its just a thought. I have to draw it up and see if it has potential.

I'm getting .0035' backlash from my $25 Roton ballnuts and when I saw that Nook has .01" for theirs I thought there has to be a mistake somewhere. It just doesn't sound right.

Rick

Greg (CadMonkey) could you possibly be binding the second nut by tightening one of the bolts more than another thus skewing the nut so it rides the screw at an angle? I was looking at this before I started adding the screws to my machine and since you didn't mention any problems with it I didn't think it worth mentioning but now that I hear you're troubles I'm wondering if this could be part of the problem. I haven't added a second nut yet because I would like to see what larger balls would do but I was looking for an alternative to let the nut self-align on the screw but as of yet I haven't figured out a way to keep them tight and aligned properly to prevent binding. I got something in mind but right now its just a thought. I have to draw it up and see if it has potential.

I'm getting .0035' backlash from my $25 Roton ballnuts and when I saw that Nook has .01" for theirs I thought there has to be a mistake somewhere. It just doesn't sound right.

Rick

It's not a mistake. Single nuts are simply not designed to have zero backlash. Ballnuts are for low-friction drives. If you want zero backlash (there are many applications where backlash doesn't matter), you use a double nut. It is *critical* that the second nut be able to "float" a bit, or you WILL get binding in some positions and/or backlash in others, as the screw pitch is not absolutely constant. Rigidly locking the two nuts together is NOT a good idea, and will generally lead to rapid wear.

Regards,
Ray L.

It's not a mistake. Single nuts are simply not designed to have zero backlash. Ballnuts are for low-friction drives. If you want zero backlash (there are many applications where backlash doesn't matter), you use a double nut. It is *critical* that the second nut be able to "float" a bit, or you WILL get binding in some positions and/or backlash in others, as the screw pitch is not absolutely constant. Rigidly locking the two nuts together is NOT a good idea, and will generally lead to rapid wear.

Regards,
Ray L.

Yeah Ray I understand fully what your saying I just thought the specs would be smaller than .010 inch. I guess I shouldn't worry about .0035 inch but I'd still be happier with .001 or less. :) Even the spring washers would let some backlash appear with a heavy enough cut but thats another reason to take a small final cut.

Rick

So,

Do any of you have PICTURES of a double-nut setup using these spring/bellville washers?

I'd like to see what other people have done.

Thanks!

-Caleb105-

Here's how I did mine:

http://cid-f31d82e92e218e66.skydrive.live.com/self.aspx/Big%20Mill%20CNC%20Conversion/P1040521.JPG

The Belleville washers are not shown in the photos, as I didn't have them yet when I took these. They simpy go between the two nuts, held in place by the aluminum "sleeve" between the nuts. Pre-load is easily set by adding/removing/shimming the Belleville washers. These are 1" screws, set to about 250# preload.

BTW - MSN SUCKS!!!! It seems they've "enhanced the experience", and I can no longer order the photos as I want. You upload them, and they go in whatever order the Gods of Redmond deem appropriate, and there is no way to change it! What a bunch of morons....

Regards,
Ray L.

Here's how I did mine:

http://cid-f31d82e92e218e66.skydrive.live.com/self.aspx/Big%20Mill%20CNC%20Conversion/P1040521.JPG

The Belleville washers are not shown in the photos, as I didn't have them yet when I took these. They simpy go between the two nuts, held in place by the aluminum "sleeve" between the nuts. Pre-load is easily set by adding/removing/shimming the Belleville washers. These are 1" screws, set to about 250# preload.

BTW - MSN SUCKS!!!! It seems they've "enhanced the experience", and I can no longer order the photos as I want. You upload them, and they go in whatever order the Gods of Redmond deem appropriate, and there is no way to change it! What a bunch of morons....

Regards,
Ray L.

Ray,

Thanks for the picture. A couple more questions:

The two ballscrews are being held together b/c the 2nd ballnut can't rotate (b/c of the pin), and therefore can't spin away from the 1st ballnut?

Also, you have 250# of preload? Per the previous posts, I was assuming 18-27lbs (10-15% of 180lbs) of preload was good for a .621 ballscrew!

Thanks!

-Caleb105-

Ray,

Thanks for the picture. A couple more questions:

The two ballscrews are being held together b/c the 2nd ballnut can't rotate (b/c of the pin), and therefore can't spin away from the 1st ballnut?

Also, you have 250# of preload? Per the previous posts, I was assuming 18-27lbs (10-15% of 180lbs) of preload was good for a .621 ballscrew!

Thanks!

-Caleb105-

Yes, the roll pin prevents the nuts from rotating w.r.t. each other. In commercial nuts, this is usually done with a dowel pin pressed into one nut, and a hole in the other, or something similar. These nuts were so hard, I could not make a dent in them, much less drill a hole.

Preload is 10-15 of the rated load of the screw, which is typically on the order of 1600# for a 5/8" screw. Mine are 1" screws, so the preload is higher. You ideally want the preload to be higher than the maximum machining load, as once the load on the nut exceeds the preload , you WILL get backlash due to compression of the Bellevilles. This will happen only in one direction, when the load is being carried by the "floating" nut. In the other direction, the load is carried entirely by the fixed nut, so the Bellevilles never see the load. That's why I sized the sleeve to allow 0.005" of movement, to limit the maximum deflection under this condition.

With square nuts, or nuts which are soft enough to be machined, you have many more options for how to constrain them.

Regards,
Ray L.

Greg (CadMonkey) could you possibly be binding the second nut by tightening one of the bolts more than another thus skewing the nut so it rides the screw at an angle?
I think that is a major part of it, even though I have the bores for the countersunk SHCS a tight fit and tightened equally. I never mentioned a lot about it (I know I did mention something at some point but didn't make a big deal about it) because I left that as an "optional" item and couldn't completely blame it on the nut assembly as the stepper could be responsible too - underpowered. My thinking at this point is to go back and make a very similar floating block, but use threaded studs the block can ride on to stay oriented and then use heavy springs to push them apart or use 2 studs and 2 screws and compress (as I initially intended) instead of expand. Not sure yet. My initial desire to go with compression instead of expansion was to eliminate the possibility of the cutting forces overcoming the preload and as the screws/nuts wear, it would be possible to tighten things and continue to take up the lash as the systems wears to the point of replacement (maybe by the time I hand the archaic machine down to my grandkids at the rate I've been using it...)

I'm getting .0035' backlash from my $25 Roton ballnuts and when I saw that Nook has .01" for theirs I thought there has to be a mistake somewhere. It just doesn't sound right.

I think Nook quotes a worst case scenario so that you get what is printed or typically better - there is variation in all rolled screws just because of how they are made, they're going to elongate and contract during hardening and straightening. Also, the SBN nuts aren't intended for the XPR screws I used anyways, they are intended for the SRT screws are the general transport screws not intended for precision positioning and have an even worse lead variance. The nuts intended for the XPR screws are the same as those for a ground ballscrew - they are ground instead of cut, thus their higher cost, but preloading the cheaper nuts is less expensive and can get the same or better results than using the higher priced nuts if you do it right (which I haven't...yet...).

The spring pre-load should be on the order of 10-15% of the rated load for the screw, which is typically around 180 pounds for a 5/8" screw.
Regards,
Ray L.


Preload is 10-15 of the rated load of the screw, which is typically on the order of 1600# for a 5/8" screw. Mine are 1" screws, so the preload is higher.

Regards,
Ray L.



So is it 180 lbs or 1600 lbs?

So is it 180 lbs or 1600 lbs?

Rated load is typically around 1600 pounds. Preload is typically around 180 pounds. Most of the ballscrew manufacturers have app notes or specs defining how to determine preload, and recommended values.

Regards,
Ray L.

Rated load is typically around 1600 pounds. Preload is typically around 180 pounds. Most of the ballscrew manufacturers have app notes or specs defining how to determine preload, and recommended values.

Regards,
Ray L.

Aaaaaaaaah. Thanks Ray!

I'm looking at the XPR line by NOOK. They don't list a "Pre-load" range:

http://www.nookindustries.com/ball/BallChartXPRSize.cfm?id=0631%20-%200200

Do I use that static load? I'm looking at the 2nd one down on this list. For some reason that one has a much higher load (3360lb) than the rest (2110lb).

Or do I use the dynamic load? Seems like the preloaded ones on this list are limited to 450lbs of dynamic load. Does that mean that they have 450lb of Pre-load? That's about 13.4% of the 3360 static load. Thoughts?

Thanks!

-Caleb105-

Aaaaaaaaah. Thanks Ray!

I'm looking at the XPR line by NOOK. They don't list a "Pre-load" range:

http://www.nookindustries.com/ball/BallChartXPRSize.cfm?id=0631%20-%200200

Do I use that static load? I'm looking at the 2nd one down on this list. For some reason that one has a much higher load (3360lb) than the rest (2110lb).

Or do I use the dynamic load? Seems like the preloaded ones on this list are limited to 450lbs of dynamic load. Does that mean that they have 450lb of Pre-load? That's about 13.4% of the 3360 static load. Thoughts?

Thanks!

-Caleb105-

Use the static load. The most important thing is to make sure the preload exceeds your maximum machining load, which is not a problem on a small machine. Too loose, and you'll have backlash. Too tight, and it won't turn smoothly. On a small machine, that does not need to run 40 hours a week for 20 years, you really don't need to be too fussy about it. I just set mine to the point that I could feel going further made the motion less smooth.

Regards,
Ray L.

Quote from the Hiwin ballscrew catalog, page 19:

http://www.hiwin.com/pdf/bs/ballscrews.pdf

Single nut preloading
There are two ways of preloading a
single nut. One is called “the oversizedball
preloading method”. The method
is to insert balls slightly larger than the
ball groove space (oversized balls) to
allow balls to contact at four points (Fig.
4.18).
The other way is called “The offset
pitch preloading method” as shown in
Fig. 4.19. The nut is ground to have a
δ value offset on the center pitch. This
method is used to replace the traditional
double nut preloading method and has
the benefit of a compact single nut with high stiffness via small preload force. However, it should not be used in heavy
duty preloading. The best preload force is below 5% of dynamic load (C).

The CNCFusion deluxe kits use the second method to achieve preload. The ballnuts do not contain oversize balls. The balls are .125", and the nuts are preloaded by ABBA.

Hope that answers any questions you may have had on the deluxe kits.

cheers!
Michael

I have an RF45 retrofit from Chris Rosequist, who has a great deal of CNC building experience. It came with Thomson ball screws with fixed preloaded double nuts. It does not bind with the small amount of preload I have set and I have .002" backlash total in each axis (and I'm sure some of this is from the bearing). Using a floating second nut with a spring backing it up sounds great, until cutting force exceeds the spring rate, then it's anybody's guess where your table really is. I am far more confident in the machines positioning accuracy with fixed nuts.

Joe

I have an RF45 retrofit from Chris Rosequist, who has a great deal of CNC building experience. It came with Thomson ball screws with fixed preloaded double nuts. It does not bind with the small amount of preload I have set and I have .002" backlash total in each axis (and I'm sure some of this is from the bearing). Using a floating second nut with a spring backing it up sounds great, until cutting force exceeds the spring rate, then it's anybody's guess where your table really is. I am far more confident in the machines positioning accuracy with fixed nuts.

Joe

So you'd rather have 0.002" uncertainty at ALL times, rather than just when you've exceeded the design load of the scrdouble nut assembly? Doesn't make sense. With double nuts, you can pre-load the nuts with over-size balls to achieve a very small amount of backlash, then use the spring pre-load to remove that. Only when you over-drive the machine will you see any backlash at all.

Regards,
Ray L.

Surely if you have a double nut with preload then you should not have any measurable backlash in the nut assembly (when not under load). Are you 110% sure the backlash is not somewhere else.

If it is fixed preload then I don't understand when you say "with the small amount of preload I have set".

I guess spring loaded double nuts are more forgiving that fixed preload nuts when it comes to low cost screw manufacturing tolerances.

Phil

I have an RF45 retrofit from Chris Rosequist, who has a great deal of CNC building experience. It came with Thomson ball screws with fixed preloaded double nuts. It does not bind with the small amount of preload I have set and I have .002" backlash total in each axis (and I'm sure some of this is from the bearing). Using a floating second nut with a spring backing it up sounds great, until cutting force exceeds the spring rate, then it's anybody's guess where your table really is. I am far more confident in the machines positioning accuracy with fixed nuts.

Joe

As I said, I believe that the lash I measured is introduced elsewhere. And yes, I'd rather have a guaranteed upper limit of error than uncertainty at random times. Look at the specs of a real VMC; There's no variable in there for "when forces exceed some predetermined limit".

It's fixed but adjustable preload. There's a block with a hole tapped straight through, one nut tight (and loctited in) in one end and the other nut fixed axially (that is prevented from rotating), once the desired preload is achieved, with 2 setscrews in the other side of the block. The preload still needs to be set. I tightened it up myself when I tore down the entire machine, down from .005 per axis to .002. Not sure how you measure backlash but here's what I do.

Drill a 3/4" hole in a piece of alu clamped in the vise (or clamp a piece with a hole approx that size in the vise).

Jogging only in the x+ and y+ directions, indicate the spindle in to the center of the hole, zero x and y @ the center of the hole.

z+ out of the hole, move x-1 y-1, then x0 y0 and drop into the hole again to verify/compensate. Usually takes 2 tries before I am dead on, then it will repeat within a few tenths all day.

z+ out of the hole, move several inches x+ y+, then x0 y0 (negative direction moves in both axis)

drop into the hole again. The backlash is the amount that the spindle is off center.


Joe

As I said, I believe that the lash I measured is introduced elsewhere. And yes, I'd rather have a guaranteed upper limit of error than uncertainty at random times. Look at the specs of a real VMC; There's no variable in there for "when forces exceed some predetermined limit".

It's fixed but adjustable preload. There's a block with a hole tapped straight through, one nut tight (and loctited in) in one end and the other nut fixed axially (that is prevented from rotating), once the desired preload is achieved, with 2 setscrews in the other side of the block. The preload still needs to be set. I tightened it up myself when I tore down the entire machine, down from .005 per axis to .002. Not sure how you measure backlash but here's what I do.

Drill a 3/4" hole in a piece of alu clamped in the vise (or clamp a piece with a hole approx that size in the vise).

Jogging only in the x+ and y+ directions, indicate the spindle in to the center of the hole, zero x and y @ the center of the hole.

z+ out of the hole, move x-1 y-1, then x0 y0 and drop into the hole again to verify/compensate. Usually takes 2 tries before I am dead on, then it will repeat within a few tenths all day.

z+ out of the hole, move several inches x+ y+, then x0 y0 (negative direction moves in both axis)

drop into the hole again. The backlash is the amount that the spindle is off center.


Joe

"I'd rather have a guaranteed upper limit of error than uncertainty at random times." - Even though that "uncertainty at random times" is of *exactly* the same magnitude as the uncertainty you have ALL the time.... VMCs use ground screws which don't require spring pre-load. But, then those screws cost 5-10X what rolled screws do....

You're saying that the maximum distance the "floating" nut backup up by disc springs can deflect is .002"? You can't be serious. What is the total accumulated backlash per axis with your setup? I bet we're talking about a little x3 table, right?

And what does the quality of the screw have to do with backlash-induced positioning error?

Joe

You're saying that the maximum distance the "floating" nut backup up by disc springs can deflect is .002"? You can't be serious. What is the total accumulated backlash per axis with your setup? I bet we're talking about a little x3 table, right?

And what does the quality of the screw have to do with backlash-induced positioning error?

Joe

My machine is a very fast, very strong, very accurate servo-driven 9x49 Bridgeport Clone with a 3HP spindle, not a "little x3". I can do things on this machine no X3 could ever even dream of doing. I typically run a 4 cubic inch per minute removal rate in aluminum, so I'm applying FAR greater machining forces through my screws than any benchtop machine could possibly apply.

Your statement indicates you understand how double-nuts actaully work, and the forces acting on them. At, and above, the load where you overcome the pre-load, the maximum backlash you will ever see is the same backlash you would see if the springs were not there. If you load over-sized balls, this can easily be no more than 0.001" if you're using good quality screws and nuts. In any case, it is *EXACTLY*, no better, no worse, than the backlash you can achieve by rigidly mounting both nuts and pre-loading balls alone using whatever screws and nuts you're using.

The quality of the screw has everything to do with how much compliance you must provide to accomodate lead error in the screw. If you rigidly mount the nuts, you will have backlash in some screw locations, and binding in others, as the pitch of rolled screws is *not* perfect - hence the "lead error" spec on the screws - typically 0.004"/foot for rolled screws (other than Nook XPRs). This is precisely why spring-loaded double nuts are necessary with rolled screws. Ground screws are FAR more precise, typically having a maximum lead error spec of less than 0.0005"/foot. This requires less compliance in the nuts.

Regards,
Ray L.

What is the spring force exerted by the preload springs on the floating nut? If the tool pressure exceeds that spring pressure, the floating nut moves relative to the table (introducing positioning error) until coil bind occurs. I have an exact understanding of how preloaded double nuts work, thanks. If you can't understand that, then believe what you want.

If my Thomson rolled screws have an accuracy of .003" per foot, that's less than .001" of deviation in the less than 4" that my 2 ballnuts occupy.

Joe

What is the spring force exerted by the preload springs on the floating nut? If the tool pressure exceeds that spring pressure, the floating nut moves relative to the table (introducing positioning error) until coil bind occurs. I have an exact understanding of how preloaded double nuts work, thanks. If you can't understand that, then believe what you want.

If my Thomson rolled screws have an accuracy of .003" per foot, that's less than .001" of deviation in the less than 4" that my 2 ballnuts occupy.

Joe

"If the tool pressure exceeds that spring pressure, the floating nut moves relative to the table (introducing positioning error) until coil bind occurs." - Nope, not true. The screw can move off-position *only* as far as it takes for the load to be picked up by the balls on the fixed nut, and that distance will be *precisely* equal to the amount of backlash in the fixed nut. It is 100% impossible to achieve "coil bind" in a double-nut assembly, unless you have a HUGE amount of backlash in the fixed nut. Or perhaps you can explain how further motion would be possible?

"If my Thomson rolled screws have an accuracy of .003" per foot, that's less than .001" of deviation in the less than 4" that my 2 ballnuts occupy." - Also not true. You are assuming the spec implies a constant, monotonic lead error, which it absolutely does not. In fact, quite the contrary. The spec simply defines the maximum error you will ever see between any two points on a one foot length of screw. It says absolutely nothing about how the error varies over that one foot length. If they reliably could produce a screw with a constant, monotonic error, they would have the ability to fix the error and sell perfect screws. But manufacturing processes simply don't work that way, hence the need for that spec. And hence the reason even high-end VMC use large screw mapping tables to correct the lead errors in their screws. A screw which had an error that increased to +0.0015" over nominal during the first two inches, then decreased to -0.0015" under nominal during the second two inches, and so on, would MEET the spec to the letter, but your 4" long double ballnut would see a worst case error of +0.003" in some positions, and -0.003" in other positions. This extreme error profile would be unusual, but the spec does NOT preclude this possibility, so designers must design around it.

Regards,
Ray L.

The 2 nuts are opposed, which means that the load is handled by 1 nut when table moves in 1 direction, and by the other nut when the table reverses. You have to admit that the floating nut is free to move relative to the fixed nut, or there would be no advantage at all to using spring preload to compensate for relatively small deviations in the screw pitch. If the floating nut can move relative to the fixed nut, then obviously it can move relative to the table. Since the preload is created by a spring force that creates the opposition, then when the table moves in the direction that loads the floating nut, if the resistance of the table (bind, tool pressure, table lock) is greater than the amount of force the spring exerts holding said nut in its preloaded position, the spring will compress and the nut moves. Once it moves beyond the amount necessary to remove lash, it is only limited by the spring coil binding (or in this scenario disk binding). It is then anybody's guess what the floating nuts position is relative to the table that it is supposed to be precisely positioning. It's very simple.


As for real VMCs having their screw deviations mapped, yes I'm aware of this. I have a fair amount of manufacturing experience.

Joe

edit; Ah I see what you are saying, that in such a scenario the spring would not coil bind because the lash in the fixed nut would limit the error. Which in the case of my machine would be .01" per axis, which would be unacceptable, but an unlikely deviation while taking a finishing pass on a contour and would never happen on a rapid positioning move. Again, what kind of preload spring rate are we talking about, say, on a 5/8" 5 tpi ballscrew?

The 2 nuts are opposed, which means that the load is handled by 1 nut when table moves in 1 direction, and by the other nut when the table reverses. You have to admit that the floating nut is free to move relative to the fixed nut, or there would be no advantage at all to using spring preload to compensate for relatively small deviations in the screw pitch. If the floating nut can move relative to the fixed nut, then obviously it can move relative to the table. Since the preload is created by a spring force that creates the opposition, then when the table moves in the direction that loads the floating nut, if the resistance of the table (bind, tool pressure, table lock) is greater than the amount of force the spring exerts holding said nut in its preloaded position, the spring will compress and the nut moves. Once it moves beyond the amount necessary to remove lash, it is only limited by the spring coil binding (or in this scenario disk binding). It is then anybody's guess what the floating nuts position is relative to the table that it is supposed to be precisely positioning. It's very simple.


As for real VMCs having their screw deviations mapped, yes I'm aware of this. I have a fair amount of manufacturing experience.

Joe

edit; Ah I see what you are saying, that in such a scenario the spring would not coil bind because the lash in the fixed nut would limit the error. Which in the case of my machine would be .01" per axis, which would be unacceptable, but an unlikely deviation while taking a finishing pass on a contour and would never happen on a rapid positioning move. Again, what kind of preload spring rate are we talking about, say, on a 5/8" 5 tpi ballscrew?

Joe,

Your edit got it. Pre-load is typically spec'ed as a percentage of maximum static load rating for the screw, which is usually around 1600 pounds for a 5/8" screw. Recommendation is up to 30% for maximum stiffness, but less than that for maximum life. "Typical" runs between 10 and 20%, or around 150-200 pounds. There is also some mechanism (it's mentioned in several manufacturers design guides, but never explained) that makes the actual stiffness greater than the pre-load would imply, supposedly by 2-3X. Even 180# is more than you'll ever see on a small machine, short of a crash.

Regards,
Ray L.

so, let me ask a question, how much spring are we talking about, and we'll have to use some kind of units that i can understand.. are we talking something like a screen door spring, or something like a valve spring out of a car motor?

I have no basis to compare spring stiffness, how thier really measured or any of that.. i just know that a valve spring is pretty darn stiff, and a screen door spring isnt(ok yea i know that one is a pull and one is a push, but you get my point...

how could someone like me know how much force a spring is putting out? is it as simple as just pushing on it on a scale, or pulling on it with a fish scale, soemthing to that effect, or is there a bunch mroe to it?

so, let me ask a question, how much spring are we talking about, and we'll have to use some kind of units that i can understand.. are we talking something like a screen door spring, or something like a valve spring out of a car motor?

I have no basis to compare spring stiffness, how thier really measured or any of that.. i just know that a valve spring is pretty darn stiff, and a screen door spring isnt(ok yea i know that one is a pull and one is a push, but you get my point...

how could someone like me know how much force a spring is putting out? is it as simple as just pushing on it on a scale, or pulling on it with a fish scale, soemthing to that effect, or is there a bunch mroe to it?

The pre-load is typically provided by Belleville washers (search at www.mcmaster.com to see examples), which can provide a very high force in a very small space. It generally takes a stack of several Bellevilles (I have 7 on my 1" screws) to get enough force. The spec for the Bellevilles tells you what the force is at a given deflection, so you simply design for that deflection, and you know what the force for one Belleville is. Stack as many as you need, and the forces add. So, if one provides 30#, and you want 180#, use 6 of them.

Regards,
Ray L.

well thats easy enough... i think i've seen these before.. these are the ones that you stack like this ()()()() to get more travel, and then like this (()) to add thier forces.. right? so in theory you could end up with something like (())(())(()) for more force and more travel?

well thats easy enough... i think i've seen these before.. these are the ones that you stack like this ()()()() to get more travel, and then like this (()) to add thier forces.. right? so in theory you could end up with something like (())(())(()) for more force and more travel?

Stack them like this: )))) to increase force per unit displacement. (()) would have 2X the force of a single spring for a given displacement, and twice the total travel. (())(())(()) is equivalent to (((((()))))) or ))))))(((((( and would have 6X the force of a single spring for a given displacement, with twice the travel.

Regards,
Ray L.

Wouldn't it be 3x the travel?

I thought I understood Belleville washers, but maybe not. Wouldn't (()) give you the same force as a single for a given displacement and twice the travel.

Doesn't (( give half the displacement of a single for a given force then if you add another pair reversed that will give you another half displacement for the given force.

Just trying to check my understanding
Phil

.... (()) would have 2X the force of a single spring for a given displacement, and twice the total travel.

Regards,
Ray L.

Phil,

We aren't talking about travel for a given force, we are talking about total allowable travel.

Therefore, () would be 2x the distance, b/c you have 2 washers to compress.

)) gives 2 times the force, but still only has the compression distance of 1 washer b/c they compress at the same time.

I thought I understood Belleville washers, but maybe not. Wouldn't (()) give you the same force as a single for a given displacement and twice the travel.

Doesn't (( give half the displacement of a single for a given force then if you add another pair reversed that will give you another half displacement for the given force.

Just trying to check my understanding
Phil

This is making my head hurt! :-)

If you stack Bellevilles so they "nest" (i.e. -concave sides all pointing the same direction), you will increase the load at a given displacement by the number of springs you've stacked. This is typically referred to as "parallel" stacking. So, if one spring gives 10# when displaced by 0.050", then two springs pointing the same way will give 20# when displaced by 0.050". No matter how many you stack, the maximum displacement of the stack will remain the same as the maximum displacement of a single spring. You can visualize this as being exactly equivalent to putting coil springs between two metal plates. If you put two springs between the plates, placed next to each other, the force required to achieve a given displacement doubles. With three springs, it triples, etc.

If you now reverse one of them them, you increase the available displacement, with no increase in load for a given displacement. This is typically referred to as "series" stacking, in which each spring is facing opposite the direction of its neighbors. This is exactly equivalent to stacking coil springs one on top of another in series between two metal plates. Each spring you add, allows you to increase the maximum displacement by the maximum displacement of that spring. So, if the above spring has a maximum displacement of 0.100" (at which point it's completely flat), then two springs facing opposite directions will still give you 10# when displaced by 0.050", but you will be able to compress the pair by 0.200" before they become flat.

When you combine parallel and series stacking, things get a little more complicated. The coil spring analogy only works well here if you have a symmetrical stack. Suppose you have a stack of two springs pointing up, then two pointing down, then two more pointing up, then two more pointing down. Think of this as two stacks of four springs between the metal plates. You'll have twice the force at a given displacement, and four times the maximum displacement.

When springs are stacked in parallel, you can think of each parallel sub-stack as a single spring having a spring constant equal the n times the spring constant of a single spring, where n is the number of springs in the sub-stack. So, in the above example, you can think instead of a stack of four springs, each with a spring constant equal to twice the constant of a single real spring. Again, this will double the force for a given displacement, and quadruple the maximum displacement.

If you have a parallel/series stack with unequal numbers of springs facing the two directions it gets confusing, so I'm not going to go there.

Regards,
Ray L.

That seems to be a yes!

Phil

This is making my head hurt! :-)

If you stack Bellevilles so they "nest" (i.e. -concave sides all pointing the same direction), you will increase the load at a given displacement by the number of springs you've stacked. This is typically referred to as "parallel" stacking. So, if one spring gives 10# when displaced by 0.050", then two springs pointing the same way will give 20# when displaced by 0.050". No matter how many you stack, the maximum displacement of the stack will remain the same as the maximum displacement of a single spring. You can visualize this as being exactly equivalent to putting coil springs between two metal plates. If you put two springs between the plates, placed next to each other, the force required to achieve a given displacement doubles. With three springs, it triples, etc.

If you now reverse one of them them, you increase the available displacement, with no increase in load for a given displacement. This is typically referred to as "series" stacking, in which each spring is facing opposite the direction of its neighbors. This is exactly equivalent to stacking coil springs one on top of another in series between two metal plates. Each spring you add, allows you to increase the maximum displacement by the maximum displacement of that spring. So, if the above spring has a maximum displacement of 0.100" (at which point it's completely flat), then two springs facing opposite directions will still give you 10# when displaced by 0.050", but you will be able to compress the pair by 0.200" before they become flat.

When you combine parallel and series stacking, things get a little more complicated. The coil spring analogy only works well here if you have a symmetrical stack. Suppose you have a stack of two springs pointing up, then two pointing down, then two more pointing up, then two more pointing down. Think of this as two stacks of four springs between the metal plates. You'll have twice the force at a given displacement, and four times the maximum displacement.

When springs are stacked in parallel, you can think of each parallel sub-stack as a single spring having a spring constant equal the n times the spring constant of a single spring, where n is the number of springs in the sub-stack. So, in the above example, you can think instead of a stack of four springs, each with a spring constant equal to twice the constant of a single real spring. Again, this will double the force for a given displacement, and quadruple the maximum displacement.

If you have a parallel/series stack with unequal numbers of springs facing the two directions it gets confusing, so I'm not going to go there.

Regards,
Ray L.

Something to ponder...

Your ballnuts already have a maximum amount of backlash before the ballscrew starts riding on the opposite side of the threads. No point in having washers that provide 180lbs of force when fully compressed, if they are only going to be compressed part-way and therefore only provide partial pre-load force.

What you want is a washer that BEGINS to compress at 180lbs. As long as you don't exceed 180lbs of tool force, then you will have NO backlash. This also means that in order to get FULL backlash, you would have to put MORE than 180lbs of tool force against it.

Something to ponder...

Your ballnuts already have a maximum amount of backlash before the ballscrew starts riding on the opposite side of the threads. No point in having washers that provide 180lbs of force when fully compressed, if they are only going to be compressed part-way and therefore only provide partial pre-load force.

What you want is a washer that BEGINS to compress at 180lbs. As long as you don't exceed 180lbs of tool force, then you will have NO backlash. This also means that in order to get FULL backlash, you would have to put MORE than 180lbs of tool force against it.

What you seem to be describing is exactly how it works. The preload is just that - preload. The springs are compressed sufficiently to provide 180# of static force pushing the nuts away from each other, and into the screw threads when there is zero load on the machine. Unless and until the longitudinal force from machining exceeds 180#, there will be *no* loss of contact between any of the balls in either nut and the screw threads, and hence *no* backlash. The magnitude of the preload will diminish as the machining load increases, but until the preload reaches zero, no backlash can occur.

You *need* partial compression of the springs to allow some movement to accomodate the lead error, variations in ball diameter, and other imperfections in the screws and nuts.

Regards,
Ray L.

You *need* partial compression of the springs to allow some movement to accomodate the lead error, variations in ball diameter, and other imperfections in the screws and nuts.

Regards,
Ray L.

THIS is what I had not considered. But how do you know what force the spring is exerting at PARTIAL compression? The only values you are given are:

- Force required to START compression

- Force required for COMPLETE compression

Obviously this is not a linear relationship (you can't split the difference and say that happens at 1/2 compression).

THIS is what I had not considered. But how do you know what force the spring is exerting at PARTIAL compression? The only values you are given are:

- Force required to START compression

- Force required for COMPLETE compression

Obviously this is not a linear relationship (you can't split the difference and say that happens at 1/2 compression).

Not true. It's a spring, and behaves exactly like any coil spring - i.e. - linearly according to: F = k * X. They are spec'ed for a specific force at a specific displacement - generally NOT maximum displacement - and there is a spec for maximum deflection as well (usually just the thickness of the material). You can assume, with reasonable accuracy, that the force *will* vary linearly from zero to maximum.

Regards,
Ray L.

So it sounds like I need a washer that has an initial compression force below 180lbs, and a full compression force above 180lbs. My problem is that the only washer with an I.D. larger than my ballscrew (.621") takes 410lbs of force just to START compression. (at least at McMaster Carr).

Anybody got any other sources?

Thanks!

-Caleb105-

So it sounds like I need a washer that has an initial compression force below 180lbs, and a full compression force above 180lbs. My problem is that the only washer with an I.D. larger than my ballscrew (.621") takes 410lbs of force just to START compression. (at least at McMaster Carr).

Anybody got any other sources?

Thanks!

-Caleb105-

This is the exact reason I used "wave washers" also from McMaster. It has lower ratings than "Belleville" as you have found. Yes, I had to stack them to achieve the desired rating.

So it sounds like I need a washer that has an initial compression force below 180lbs, and a full compression force above 180lbs. My problem is that the only washer with an I.D. larger than my ballscrew (.621") takes 410lbs of force just to START compression. (at least at McMaster Carr).

Anybody got any other sources?

Thanks!

-Caleb105-

Nah! You're not looking in the right place. There are many different kinds of disc springs. Here's one that's 23# at 0.020 deflection:

http://www.mcmaster.com/#94065k54/=qfjtl

Search on "disc spring", and look at ALL the different kinds.

Regards,
Ray L.

Sweeeeeeeet Himy.

Looks like 8 of those would give me 184 lbs.

(((())))

-Caleb105-

Nah! You're not looking in the right place. There are many different kinds of disc springs. Here's one that's 23# at 0.020 deflection:

http://www.mcmaster.com/#94065k54/=qfjtl

Search on "disc spring", and look at ALL the different kinds.

Regards,
Ray L.

Exactly the ones I'm using for my 5/8" screws. Work perfectly.

Excellent!

Just placed an order for a 10 pack

-Caleb105-

This is a great thread! BUT... I have another question regarding preloaded ballnuts. It seems that the best way to get the least amount of backlash for sure is to use two ballnuts and preload them with disc springs.

Obviously you would do this in the X axis, but what are people doing for the Y? It seems as if you would lose a lot of travel in preloading your Y axis.

Also, for the Z axis, will the mill head provide enough downward force to effectively "preload" a single ballnut or do you still need to use two ballnuts preloaded in the normal way?

It seems a good way to do this might be to handle the backlash differently in each axis, preload the X, maybe use larger balls in the Y and maybe nothing in the Z? Thoughts?

This is a great thread! BUT... I have another question regarding preloaded ballnuts. It seems that the best way to get the least amount of backlash for sure is to use two ballnuts and preload them with disc springs.

Obviously you would do this in the X axis, but what are people doing for the Y? It seems as if you would lose a lot of travel in preloading your Y axis.

Also, for the Z axis, will the mill head provide enough downward force to effectively "preload" a single ballnut or do you still need to use two ballnuts preloaded in the normal way?

It seems a good way to do this might be to handle the backlash differently in each axis, preload the X, maybe use larger balls in the Y and maybe nothing in the Z? Thoughts?
I did it for all three axis. I might not absolutely need it on the Z, but at least now I know that it's addressed instead of having to refit later. Of course this is my own personal design, not a retrofit, so it's a little easier to make sure the travel is there.

This is a great thread! BUT... I have another question regarding preloaded ballnuts. It seems that the best way to get the least amount of backlash for sure is to use two ballnuts and preload them with disc springs.

Obviously you would do this in the X axis, but what are people doing for the Y? It seems as if you would lose a lot of travel in preloading your Y axis.

Also, for the Z axis, will the mill head provide enough downward force to effectively "preload" a single ballnut or do you still need to use two ballnuts preloaded in the normal way?

It seems a good way to do this might be to handle the backlash differently in each axis, preload the X, maybe use larger balls in the Y and maybe nothing in the Z? Thoughts?

On a small machine, getting a double nut on the Y axis can be difficult. Many people end up hogging out part of the base, or even extending it, to recover the lost travel.

A single nut will work fine on Z, if you're doing only 2.5D milling, since you'll almost always be approaching from the top down. Of course, if your gibs are tight enough the head won't move down under its own weight, you'll still have problems. But, if you're doing 3D profiling, the backlash will still kill you when you're making upward moves while cutting.

Regasds,
Ray L.

I'm still trying to figure out how I want to compress my two ballnuts, but here's what I drew up last night. These are 2 NOOK SBN10325 ballnuts with those McMaster belleville washers in between. I have 8 of them, which will give 184lbs of force when properly compressed. You can see that the diameters of the washers fit nicely between the ballnuts.

Would anyone who has used these washers be willing to post some pics of their setup?

http://caleb-angie.com/wp-content/uploads/2009/02/preloaded-ballnut-autodesk-illustration.JPG

Just to correct a misunderstanding. No simple spring requires 410lbs to start compression. The load to compress is proportional to the deflection, starting from zero. So minuscule deflection requires minuscule load.

I’m interested why 2 sets of 4 (((()))) would be needed when all that is need is to have a couple of thou deflection available. Surely you can get this from just 1 set of four ((((.

Phil

....My problem is that the only washer with an I.D. larger than my ballscrew (.621") takes 410lbs of force just to START compression. (at least at McMaster Carr).

-Caleb105-

well one thought that i had was to use the 2 stacks on either side of the mounting.. my thinking was to have it spring loaded equally in both directions of travel, rather than one way being hard mounted, and the other totally relying on the springs.

so something like @(((([]))))@ where the [] is the mount for the nuts... and the @ is the nuts


just a thought...

Philbur,

I guess I'm confused then. Why does McMaster Carr give a load value for when deflection STARTS?

Also, I went with 8 of the smaller washers (23lbs each). And I realized after I typed that, that I would need them as:

))))))))

not:

(((())))

-Caleb105-

You will need a means to lock the angular position of one nut relative to the other nut.

Phil

I'm still trying to figure out how I want to compress my two ballnuts, but here's what I drew up last night. These are 2 NOOK SBN10325 ballnuts with those McMaster belleville washers in between. I have 8 of them, which will give 184lbs of force when properly compressed. You can see that the diameters of the washers fit nicely between the ballnuts.

Would anyone who has used these washers be willing to post some pics of their setup?

http://caleb-angie.com/wp-content/uploads/2009/02/preloaded-ballnut-autodesk-illustration.JPG

Or let one nut freely rotate, but be able to adjust the distance between the two. right?

-Caleb105-

Or let one nut freely rotate, but be able to adjust the distance between the two. right?

-Caleb105-

Absolutely not! The floating nut MUST be locked to prevent rotation, but allow axial motion. This is typically done using pins or tangs rigidly attached to the fixed nut, and sitting in slots in the floating nut. And, unless you use shims along with the spring washers, you must have a means of rotating the floating nut to the position that gives the desired pre-load, then locking it in that position against rotation, but still allowing the axial movement. They will almost certainly NOT come out with the flats aligned as your drawing shows.

Regards,
Ray L.

You need to give a link for the first point. The spring you have selected from McMaster shows that 23.2lbs is achieved at 0.0207" deflection. It doesn't give a "start deflection" load as far as I can see. Machinery’s Handbook shows that disc springs have a linear relationship between load and deflection, the graph of which which passes through the origin (0,0).

With regard to the second point: The spring you have selected )))))))) will give your 184lbs provided you compress them by 0.0207".

If you can find a stiffer spring, all be it with less total deflection, you could use less springs and minimise lose of axis travel. However it does look like your selection is a reasonable one.

I have not looked at disc springs in any detail before but it would appear to be a good solution for removing backlash from a low cost screw/nut set-up.

Phil


Philbur,

I guess I'm confused then. Why does McMaster Carr give a load value for when deflection STARTS?

Also, I went with 8 of the smaller washers (23lbs each). And I realized after I typed that, that I would need them as:

))))))))

not:

(((())))

-Caleb105-

They will almost certainly NOT come out with the flats aligned as your drawing shows.

Regards,
Ray L.

This I am aware of. I did not feel like calculating the exact angular relationship between the two.

You need to give a link for the first point.

Look 1/2 way down the page at the definition of "LOAD":

http://www.mcmaster.com/#disc-springs/=qvnug

The spring you have selected from McMaster shows that 23.2lbs is achieved at 0.0207" deflection. It doesn't give a "start deflection" load as far as I can see.

For some reason, McMaster gives a "Load" rating on Belleville washers, but not on the ones that are specifically for ball bearings. So yes, .0207 deflection would be required. More deflection would give more force.



With regard to the second point: The spring you have selected )))))))) will give your 184lbs provided you compress them by 0.0207". If you can find a stiffer spring, all be it with less total deflection, you could use less springs and minimise lose of axis travel. However it does look like your selection is a reasonable one.

Yes, per my previous posts, the only other washer I could find with a large enough I.D. required 410lbs of force for "Load" (the still undetermined value). I would prefer to use fewer washers.



I have not looked at disc springs in any detail before but it would appear to be a good solution for removing backlash from a low cost screw/nut set-up.

Phil

Do you have a different setup for removing backlash?

-Caleb105-

Nevermind. For some reason I failed to miss the "DEFLECTION AT GIVEN LOAD" value. :withstupi

-Caleb105-

Look 1/2 way down the page at the definition of "LOAD":

http://www.mcmaster.com/#disc-springs/=qvnug



For some reason, McMaster gives a "Load" rating on Belleville washers, but not on the ones that are specifically for ball bearings. So yes, .0207 deflection would be required. More deflection would give more force.



Yes, per my previous posts, the only other washer I could find with a large enough I.D. required 410lbs of force for "Load" (the still undetermined value). I would prefer to use fewer washers.



Do you have a different setup for removing backlash?

-Caleb105-

Spring washers require *two* specifications to define their behavior - load, and deflection at that load. So, if the "load" spec is 25#, and the deflection is 0.020", that says when the spring is deflected 0.020", the load will be 25#. If you deflect it 0.040" (assuming it can deflect that far), the load will be 40#. If you deflect it only 0.010", the load will be 12.5#. They could just as well express the spring rate in pounds/inch as is done for normal coil springs, but for some reason (no doubt buried in the dust of history) they don't.

Regards,
Ray L.

The only definition I can find on the link you give is:

"Load—The force that is applied to a spring that causes deflection. Metric loads are listed in terms of Newtons (N).
Deflection at Load—Distance the spring compresses when the given load is applied."

No reference to "start deflection" as far as I can see.

My machine has ground ball screws with fixed preload by the manufacturer, using a precision ground spacer. It seems to me that transport grade screws (rolled) will be far less forgiving with a fixed preload due to manufacturing tolerances and intended use. Rolled screws are not intended for precision applications so spring preloading is a very forgiving means of improving their performance with a lower risk of failure.

Phil



Look 1/2 way down the page at the definition of "LOAD":

http://www.mcmaster.com/#disc-springs/=qvnug



For some reason, McMaster gives a "Load" rating on Belleville washers, but not on the ones that are specifically for ball bearings. So yes, .0207 deflection would be required. More deflection would give more force.



Yes, per my previous posts, the only other washer I could find with a large enough I.D. required 410lbs of force for "Load" (the still undetermined value). I would prefer to use fewer washers.



Do you have a different setup for removing backlash?

-Caleb105-

So do you guys think this one would work:

http://www.mcmaster.com/#9713k98/=qwh8b

It looks like it gives 270lbs of force with deflection of .023"

Therefore, I SHOULD be able to get 180lbs of force (66% of 270) with deflection of .0153" (66% of .023").

Right?

And if my ballnuts have total backlash of .01 (NOOK ballnuts without larger balls), then it would have to deflect a total of .0253 in order to get total backlash. However, it would require 297lbs of force to achieve max backlash.

ETA:
Or if I got the ballnut backlash down to .003, it would take total deflection of .0183" to get max backlash, which would require 215lbs of force.

-Caleb105-

I can't find the 0.01" backlash number for NOOK SBN10325 on the Nook website. Assuming this is maximum backlash then you need to consider what the minimum backlash is to ensure you have enough stroke in the spring(s) to ensure you don't get "bottom out" as the two nuts fight one another for position and over load the screw/balls at some point along the length.

What I think you are not tuned into yet is that if 0.01" is the maximum backlash then at some other point on the screw the backlash could be zero or even maximum preload (for a specific life). Adding oversize balls would potentially reduce the life, by how much you have no way of knowing.

The 0.01" backlash implies that the tolerance on the grove profile on the screw is not particularly good, otherwise the manufacturer would have fitted large balls as standard.

If in doubt go for extra stroke in the springs to avoid disappointment.

Phil


So do you guys think this one would work:

http://www.mcmaster.com/#9713k98/=qwh8b

It looks like it gives 270lbs of force with deflection of .023"

Therefore, I SHOULD be able to get 180lbs of force (66% of 270) with deflection of .0153" (66% of .023").

Right?

And if my ballnuts have total backlash of .01 (NOOK ballnuts without larger balls), then it would have to deflect a total of .0253 in order to get total backlash. However, it would require 297lbs of force to achieve max backlash.

ETA:
Or if I got the ballnut backlash down to .003, it would take total deflection of .0183" to get max backlash, which would require 215lbs of force.

-Caleb105-

I can't find the 0.01" backlash number for NOOK SBN10325 on the Nook website.

According to an e-mail I received from NOOK, these ballnuts have max backlash of .01". If fitted with larger balls at the factory then this is reduced to .003".



The 0.01" backlash implies that the tolerance on the grove profile on the screw is not particularly good, otherwise the manufacturer would have fitted large balls as standard.

If in doubt go for extra stroke in the springs to avoid disappointment.

Phil

Again, NOOK offers to put in larger balls at the factory, which would reduce the backlash to .003". The issue is not with their screws, as their XPR line is rated to an accuracy of .001"/ft as compared to most rolled screws that have an accuracy of .004"/ft.

Also, those washers have a maximum deflection of .039"

If I preload them using .0153" of that displacement, that still leaves .0237" of possible deflection. That should be more than enough to absorb the MAX nut backlash of .01", leaving an additional .0137" of room for variance in the screw.

Since they're rated for .001"/ft, I doubt there will be a variance of .0137" anywhere along the screw.

Correct?

-Caleb105-

In that case I would suggest that you take out the backlash to 0.003" with larger balls first.

You should not plan on using all the spring stroke to take out the backlash the load decrease or increases depending on the "stroked" distance. Using 10 or 20% of the total stroke would sound like a good principle. Also if you have 2 nuts then the stroke length required potentially doubles (see next paragraph). If you get the backlash down to 0.003" with large balls you can reduce the required spring stroke length/number of springs.

I think you are still missing the point when you mention the positional accuracy per foot. This has nothing to do with the precision of the grove profile, which is what will impact on changes in preload along it's length. I also note that Nook have nuts with spring preload for backlash removal, actually in the range you are looking for.

I'm no expert, just trying to give you somebody to bounce your thoughts off.

Phil

According to an e-mail I received from NOOK, these ballnuts have max backlash of .01". If fitted with larger balls at the factory then this is reduced to .003".



Again, NOOK offers to put in larger balls at the factory, which would reduce the backlash to .003". The issue is not with their screws, as their XPR line is rated to an accuracy of .001"/ft as compared to most rolled screws that have an accuracy of .004"/ft.

Also, those washers have a maximum deflection of .039"

If I preload them using .0153" of that displacement, that still leaves .0237" of possible deflection. That should be more than enough to absorb the MAX nut backlash of .01", leaving an additional .0137" of room for variance in the screw.

Since they're rated for .001"/ft, I doubt there will be a variance of .0137" anywhere along the screw.

Correct?

-Caleb105-

Phil,

I appreciate your thoughts.

Believe me, I would LOVE to just go out and buy 3 pre-loaded NOOK ballnuts. However, my options are either a plain-jane $41 ballnut (actually two of these in a double-nut configuration), or a $320 preloaded ballnut, or a $500 ground, pre-loaded ballnut.

Also, I'm not sure that the ballnuts ACTUALLY have .01" backlash. I think he was just telling me what they COULD be.

Thanks for pointing out that I don't want to use all of the deflection to take care of backlash.

However, even though I have 2 ballnuts, I don't end up with DOUBLE the possible backlash do I?

Also, would putting in larger balls to reduce backlash, AND pre-loading using washers cause too much wear?

Thanks!

-Caleb105-

First point, you are correct. I haven't thought this through fully but there is possibly an issue with two nuts when you are at a position on the screw where there is naturally zero backlash or even positive preload, Your springs will potentially need to relieve preload rather than apply it.

The second point. If Nook are happy to reduce the backlash to 0.003" for a single nut then as long as you correctly select your springs so that the second nut doesn't fight for position with the first nut you should not increase the wear significantly. It is most probable that a single nut with 0.003" max backlash will have a higher wear than a single nut with max backlash of 0.01" because at some points on the screw the 0.003" will actiually have positive preload. If this were not the case why not use balls for 0.003" backlash as standard. However for hobby use it will still probably last longer than you.

The large cost difference between a single nut/screw arrangement and a spring loaded double nut/screw arrangement gives you a clue that there is possibly more to it than just adding a couple of 10 cent washers.

All engineering is a compromise, it's all about identifying compromises you can live with versus those you can't.

Phil

Phil,

However, even though I have 2 ballnuts, I don't end up with DOUBLE the possible backlash do I?

Also, would putting in larger balls to reduce backlash, AND pre-loading using washers cause too much wear?

I found this interesting point on a belleville washer's deflection:

"For static, or dynamic applications, select a disc spring that at 75% of its total available deflection offers the maximum force and/or deflection required. Between 75% deflected and the 'flattened' position, the actual force/stress characteristics become considerably greater than those calculated."

So make sure that you try not to exceed that I guess.

-Caleb105-

There are several things that have been said here that are not correct. First:

1) Pre-loading balls has *zero* impact on life, as long as you don't get carried away, and load balls so large that binding occurs in some positions. As long as you have *some* backlash at all times, wear will be the same, whether the minimum backlash is 0.001" or 0.010". Since the Nook XPR screws in question here are spec'ed at a maximum lead error of 0.001"/foot, with 0.003" backlash, binding will NOT occur, so the wear impact of using the larger balls is zero.

2) A double ballnut *does* not double the backlash that will occur when the machining forces overcome the pre-load. The amount of backlash in an over-loaded double ballnut will be precisely equal to the amount of backlash in the fixed nut only. The amounf of backlash in the floating nut is totally irrelevent.

With a spring-loaded double nut, backlash *only* occurs when the load is in one direction. In the other direction, backlash NEVER occurs, no matter how great the load. Consider the forces we're talking about here. To make the descriptions easier, lets' assume the ballscrew is oriented vertically. You have one nut which is rigidly mounted to the table of the machine. Call this the "fixed" nut. Call the second nut the "floating" nut, and let's assume it is mounted above the fixed nut. The floating nut is pushed away from the fixed nut by the pre-load force applied by the spring washers. Assume each nut has 0.001" of backlash, which means (approximately) that the ball grooves are 0.001" larger diameter than the balls themselves.

The pre-load force is pushing the two nuts away from each other. This results in all the balls in the fixed nut being pushed against lower side of the grooves in the screw, and the upper side of the grooves in the nut. This means there is *no* contact between the balls and the upper side of grooves in the screw, or the lower side of the grooves in the nut.

The situation in the floating nut is exactly opposite, with the balls being pushed against upper side of the grooves in the screw, and the lower side of the grooves in the nut. This means there is *no* contact between the balls and the lower side of grooves in the screw, or the upper side of the grooves in the nut.

Now apply an upward force to the screw. What happens to the forces applied on the nuts and their balls? The balls in the floating nut see no change in force whatsoever. They still see the pre-load force only. The force pushing up on the screw is transmitted directly from the screw, through the balls in the fixed nut, into the fixed nut, and into the table of the machine. Because the floating nut is supported by the spring washers, it can only see the force applied by the spring washers. In order for the force in the spring washers to increase, there must be an increase in their deflection, which can only occur if the floating nut moves closer to the fixed nut. That force can only come from the screw. But an upward force on the screw cannot move the floating nut downward, so no increased deflection of the spring washers can occur.

No matter how much force you apply in a upward direction, there will *never* be any backlash, and the entire load will be carried by the fixed nut.

Now apply a downward force to the screw. What happens to the forces applied to the nuts and their balls? This is essentially the reverse of the above situation. This time, the load is carried by the floating nut only. Notice, however, that in this case, the downward force applied to the screw is acting on the floating nut in a direction opposite the upward pre-load force, so it is trying to move the floating nut closer to the fixed nut. The only thing preventing the floating nut from moving downward is the pre-load force. The spring washers cannot deflect further until the downward force on the screw exceeds the pre-load force. So, as long as the downward force is less than the pre-load force, the two nuts remain the same distance apart, and hence there is no backlash. The load applied to the screw is carried from the screw, through the upper side of the grooves in the screw, to the balls, from there to the lower side of the grooves in the floating nut, through the spring washers, into the fixed nut, and to the machine table.

While all this is happening, the pre-load force seen by the balls in the fixed nut is decreased by the downward force applied to the screw. So, if there is a 180# pre-load, and 100# downward force is applied to the screw, the balls in the fixed nut will see their pre-load force reduced to 80#.

Now, what happens when the downward force reaches the pre-load force? At this pount, the balls in the fixed nut no longer see a pre-load force. Since the pre-load force is what was keeping the balls in contact with the grooves in the screw and the nut, the balls are now suddenly no longer constrained. Also at this point, the downward force on the screw exactly equals the force on the spring washers.

If the downward force is now increased further, the spring washers begin to compress further, moving the nuts closer together. Since there is now no pre-load force on the balls in the fixed nut, the screw is free to move downward, until the balls in the fixed nut again come into contact with the grooves in the screw and nut. But, since the grooves are 0.001" larger in diameter than the balls, the screw must move down 0.001" before this contact occurs. So, the machine will now move 0.001" that was not commanded, to take up the backlash. At this point, the load is now transferred away from the floating nut, and is once again being rigidly carried by the fixed nut, in exactly the way it was when the load on the screw was in an upward direction. The machine position will be off by 0.001" for as long as the load remains above the pre-load force.

When the load is removed, at the point where the load on the screw again equals the pre-load, the load will again be transferred back to the floating nut, as the spring washers again relax, and there will then be a 0.001" move back to "correct" position.

3) Imperfections in the screw and nut (lead error, etc.) will do nothing more than cause minor position-dependant variations in the pre-load force, but are otherwise irrelevent, *provided* the imperfections are small, relative to the spring rate of the spring washers. If they are large enough to cause the spring washers to either become fully compressed, or fully relaxed, obviously that's a very different matter.

Regards,
Ray L.

Hi Ray,

I think you miss understood what was being said. I was talking about the single nut arrangement as it comes from the factory, not after you have fitted a second nut with springs. The whole point of course is that the spring loading arrangement goes a long way to overcoming the difficulties of low tolerance ball screws.

I don't think your first point is correct, for a single nut or double nut with fix preload. My particular double nut/screw has three preload possibilities low, medium and high. The life for each setting is different. I can agree that if you have backlash (negative preload) over the whole length of the screw then 0.003 or 0.01 has no effect on life.

The second point is that the quoted value of 0.01" backlash appears to be maximum backlash not minimum backlash for a single nut arrangement from the factory. If it where minimum why would the manufacturer not just use larger balls and reduce the minimum to 0.003 for no down side. So if the max is 0.01 you have to ask what is the minimum over the entire screw length. For example the groove in the screw does not have "EXACTLY" the same profile and depth over its whole length, so a nut/screw that has max backlash at one point on the screw could have positive preload on another part of the screw due to groove tolerances. The more expensive the screw the better the tolerances and the less the variation. A 40 dollar screw is going to be as bad as it gets in this area, hence the need for a high maximum backlash with a single nut. There is no such thing as a fixed backlash. None of the above is based on facts, it's just my interpretation of what is going on between expensive screws and cheap screws.

We have to be careful that we are not passing one another in the doorway. The above post may make things worse.

Phil

There are several things that have been said here that are not correct. First:

1) Pre-loading balls has *zero* impact on life, as long as you don't get carried away, and load balls so large that binding occurs in some positions. As long as you have *some* backlash at all times, wear will be the same, whether the minimum backlash is 0.001" or 0.010". Since the Nook XPR screws in question here are spec'ed at a maximum lead error of 0.001"/foot, with 0.003" backlash, binding will NOT occur, so the wear impact of using the larger balls is zero.

Hi Ray,

I think you miss understood what was being said. I was talking about the single nut arrangement as it comes from the factory, not after you have fitted a second nut with springs. The whole point of course is that the spring loading arrangement goes a long way to overcoming the difficulties of low tolerance ball screws.

I don't think your first point is correct, for a single nut or double nut with fix preload. My particular double nut/screw has three preload possibilities low, medium and high. The life for each setting is different. I can agree that if you have backlash (negative preload) over the whole length of the screw then 0.003 or 0.01 has no effect on life.

The second point is that the quoted value of 0.01" backlash appears to be maximum backlash not minimum backlash for a single nut arrangement from the factory. If it where minimum why would the manufacturer not just use larger balls and reduce the minimum to 0.003 for no down side. So if the max is 0.01 you have to ask what is the minimum over the entire screw length. For example the groove in the screw does not have "EXACTLY" the same profile and depth over its whole length, so a nut/screw that has max backlash at one point on the screw could have positive preload on another part of the screw due to groove tolerances. The more expensive the screw the better the tolerances and the less the variation. A 40 dollar screw is going to be as bad as it gets in this area, hence the need for a high maximum backlash with a single nut. There is no such thing as a fixed backlash. None of the above is based on facts, it's just my interpretation of what is going on between expensive screws and cheap screws.

We have to be careful that we are not passing one another in the doorway. The above post may make things worse.

Phil

Phil,

I can see that a big part of the problem here is one of very unfortunate terminology. "Two people separated by a common language". :-) A "pre-loaded single nut" does not, in fact, have any pre-load force on the balls. It will *always* have some backlash, unless it was pre-loaded incorrectly. "Pre-loading" is simply a means of reducing, not eliminating, backlash. If you load balls large enough to remove the backlash entirely, then you are correct, you will almost surely have binding in at least some positions, and wear will be very rapid indeed. So, my first statement is correct, and so is yours. It's just that we're using "pre-load" to mean different things. Sadly, the way I used it is the way the term is used in the industry when talking about single nuts. You''d think they would have come up with a more accurate, and less misleading, term.

You are correct that the magnitude of the pre-load force in a double ballnut does indeed affect life. Were it not so, we'd just use the highest pre-load we could get away with, and life would be better. Standard recommendation is up to 30% of the static weight rating of the screw if maximum stiffness is required, knowing this will reduce lifetime. 10-15% of the static weight rating seems to be a common trade-off point for both reasonable stiffness and reasonable life.

As for the 0.010" maximum backlash spec, I think that's mostly the manufacturer exercising some CYA. I've never seen one anywhere near that bad out of the box. And the consistency of the groove dimensions is also very stable across a single screw, so I doubt you'll ever see a brand-new screw and nut where it's possible to have several thou of backlash in one position, and binding in another. I suspect they spec it that way more the cover the change in groove dimension that occurs between one made on a fresh die, and one made on a die nearing end of life. Mixing and matching screws made on new and old dies, and nuts made on new and old dies you'll get very different fits. But, I believe, in every case you would still be able to pre-load (there's that lousy word again...) with over-size balls, to get the backlash down to no more than a few thou, and not have binding anywhere on the screw. But with standard sized balls, there will be a wide variation in the total amount of backlash present as you mix and match screws and nuts. In other words, I think the absolute magnitude of the backlash will vary considerably, but the variation in backlash over the length of the screw with any given set of parts will remain fairly small. A screw/nut combination which had 0.010" backlash in one position, and actual ball pre-load or binding in another would almost certainly not meet the lead-error spec, particularly not the 0.001"/foot spec for the XPR screws.

The manufacturer does not use larger balls to minimize backlash on bulk produced nuts for the simple reason that it would cost them money. Also, truly minimizing backlash by means of oversize balls requires sizing the balls for both the nut and screw to be used as a pair, which you can't do on bulk-packaged parts. This is also why they do charge more for doing this, and do then fit the nut to the screw.

In addition, the mass-produced nuts are ball-loaded automatically by a machine, with the balls coming en masse from a hopper. Fitting over-sized balls would require them to have some means of determining the proper ball size *before* putting the nuts in the machine. AFAIK, no such means exists. Instead, I'm pretty sure loading of over-size balls is a manual process - Assemble the screw and nut with standard-sized balls, measure the backlash and determine the required ball size to achieve minimum backlash without binding, then dis-assemble, and re-assemble with the correct over-sized balls. Sometimes, this is even done using two different sized balls, alternating between large "loaded" balls, and small "spacer" balls. This would be difficult (though certainly not impossible) to do in an automated machine, as it requires the balls to be fed into the grooves serially to maintain the order of the balls, where the normal manufacturing process in effect loads all the balls into the grooves at once, with no control whatsoever over which ball goes where.

Regards,
Ray L.

Hi Ray,
The more expensive the screw the better the tolerances and the less the variation. A 40 dollar screw is going to be as bad as it gets in this area, hence the need for a high maximum backlash with a single nut.
Phil

Phil,

I just wanted to clarify. The BallNUT is $41....NOT the ballSCREW. The Nook XPR ballSCREW is about $25 per foot.

Nook is NOT intentionally putting .01" backlash in their ballnuts in order to leave room for the errors of the screw.

Also, Himy...thank you for your thoughts.

-Caleb105-

Do you know what the purpose of the 0.01" backlash is.

Phil

Phil,

Nook is NOT intentionally putting .01" backlash in their ballnuts in order to leave room for the errors of the screw.

There are a number of issues I would not agree with here but it's getting to much like hard work to continue. However for example:

Quote
.......................................
A screw/nut combination which had 0.010" backlash in one position, and actual ball pre-load or binding in another would almost certainly not meet the lead-error spec, particularly not the 0.001"/foot spec for the XPR screws.
.......................................
End quote

It is quite easy to imagine variations in the groove profile, even on a screw that has a lead-error of only 0.00001" per foot, that would still result in widely varing preload along its length for a single ball-nut. I thought this was self apparent. If you take it to an extreme you could have a perfectly constant pitch but with a groove depth that varies by say 10% of groove width.

Phil

Phil,

I can see that a big part of the problem here is one of very unfortunate terminology. "Two people separated by a common language". :-) A "pre-loaded single nut" does not, in fact, have any pre-load force on the balls. It will *always* have some backlash, unless it was pre-loaded incorrectly. "Pre-loading" is simply a means of reducing, not eliminating, backlash. If you load balls large enough to remove the backlash entirely, then you are correct, you will almost surely have binding in at least some positions, and wear will be very rapid indeed. So, my first statement is correct, and so is yours. It's just that we're using "pre-load" to mean different things. Sadly, the way I used it is the way the term is used in the industry when talking about single nuts. You''d think they would have come up with a more accurate, and less misleading, term.

You are correct that the magnitude of the pre-load force in a double ballnut does indeed affect life. Were it not so, we'd just use the highest pre-load we could get away with, and life would be better. Standard recommendation is up to 30% of the static weight rating of the screw if maximum stiffness is required, knowing this will reduce lifetime. 10-15% of the static weight rating seems to be a common trade-off point for both reasonable stiffness and reasonable life.

As for the 0.010" maximum backlash spec, I think that's mostly the manufacturer exercising some CYA. I've never seen one anywhere near that bad out of the box. And the consistency of the groove dimensions is also very stable across a single screw, so I doubt you'll ever see a brand-new screw and nut where it's possible to have several thou of backlash in one position, and binding in another. I suspect they spec it that way more the cover the change in groove dimension that occurs between one made on a fresh die, and one made on a die nearing end of life. Mixing and matching screws made on new and old dies, and nuts made on new and old dies you'll get very different fits. But, I believe, in every case you would still be able to pre-load (there's that lousy word again...) with over-size balls, to get the backlash down to no more than a few thou, and not have binding anywhere on the screw. But with standard sized balls, there will be a wide variation in the total amount of backlash present as you mix and match screws and nuts. In other words, I think the absolute magnitude of the backlash will vary considerably, but the variation in backlash over the length of the screw with any given set of parts will remain fairly small. A screw/nut combination which had 0.010" backlash in one position, and actual ball pre-load or binding in another would almost certainly not meet the lead-error spec, particularly not the 0.001"/foot spec for the XPR screws.

The manufacturer does not use larger balls to minimize backlash on bulk produced nuts for the simple reason that it would cost them money. Also, truly minimizing backlash by means of oversize balls requires sizing the balls for both the nut and screw to be used as a pair, which you can't do on bulk-packaged parts. This is also why they do charge more for doing this, and do then fit the nut to the screw.

In addition, the mass-produced nuts are ball-loaded automatically by a machine, with the balls coming en masse from a hopper. Fitting over-sized balls would require them to have some means of determining the proper ball size *before* putting the nuts in the machine. AFAIK, no such means exists. Instead, I'm pretty sure loading of over-size balls is a manual process - Assemble the screw and nut with standard-sized balls, measure the backlash and determine the required ball size to achieve minimum backlash without binding, then dis-assemble, and re-assemble with the correct over-sized balls. Sometimes, this is even done using two different sized balls, alternating between large "loaded" balls, and small "spacer" balls. This would be difficult (though certainly not impossible) to do in an automated machine, as it requires the balls to be fed into the grooves serially to maintain the order of the balls, where the normal manufacturing process in effect loads all the balls into the grooves at once, with no control whatsoever over which ball goes where.

Regards,
Ray L.

Do you know what the purpose of the 0.01" backlash is.

Phil

Phil,

Like I said, I think it's just an a$$-covering spec for the tolerance stack-up over a large number of units. There will be dimensional differences between screws and nuts made on different dies, and ones made on brand new dies vs ones made on nearly worn-out dies. I think you would rarely, if ever, see one as bad a 0.010", but if you did, I'd bet you'd find the backlash varied from 0.010" to perhaps 0.008" or 0.009" (for XPR stock - perhaps down to 0.005" or 0.006" for more typicall rolled stock) over the entire length of the screw, not from 0.010" to 0.001".

Keep in mind, in many ballscrew applications, backlash is totally irrelevent. They are often used for positioning in applications where the load is always in one direction, so even 0.100" of backlash would not impact the performance of the machine one bit. So, minimizing backlash in a single nut is likely not a key design goal. But, high manufacturing yield is ALWAYS a key desing goal, so specs are set to help ensure this, even if that sometimes means relaxing a secondary specification, like backlash. When you need low or no backlash, you *always* use dual nuts, or a dual-circuit single nut, and/or oversize balls.

Regards,
Ray L.

That was kinda my point. The spec for the ball/screw arrangement is - max acceptable backlash equals 0.01, minimum acceptable backlash equals "must run freely over entire length". The screw/nut you actually receive may have a maximum and minimum backlesh anywhere between those 2 points. You can't assume that the manufacturer has maintained a narrower range than that. Although I agree that they almost probably do, if only to ensure an unacceptable number do not fall outside the acceptance criteria and become rejects.

Phil

Phil,

Like I said, I think it's just an a$$-covering spec for the tolerance stack-up over a large number of units. There will be dimensional differences between screws and nuts made on different dies, and ones made on brand new dies vs ones made on nearly worn-out dies. I think you would rarely, if ever, see one as bad a 0.010", but if you did, I'd bet you'd find the backlash varied from 0.010" to perhaps 0.008" or 0.009" (for XPR stock - perhaps down to 0.005" or 0.006" for more typicall rolled stock) over the entire length of the screw, not from 0.010" to 0.001".

Keep in mind, in many ballscrew applications, backlash is totally irrelevent. They are often used for positioning in applications where the load is always in one direction, so even 0.100" of backlash would not impact the performance of the machine one bit. So, minimizing backlash in a single nut is likely not a key design goal. But, high manufacturing yield is ALWAYS a key desing goal, so specs are set to help ensure this, even if that sometimes means relaxing a secondary specification, like backlash. When you need low or no backlash, you *always* use dual nuts, or a dual-circuit single nut, and/or oversize balls.

Regards,
Ray L.

That was kinda my point. The spec for the ball/screw arrangement is - max acceptable backlash equals 0.01, minimum acceptable backlash equals "must run freely over entire length". The screw/nut you actually receive may have a maximum and minimum backlesh anywhere between those 2 points. You can't assume that the manufacturer has maintained a narrower range than that. Although I agree that they almost probably do, if only to ensure an unacceptable number do not fall outside the acceptance criteria and become rejects.

Phil

Phil,

I think you missed my point earlier about the impact of the lead error spec on this. If the lead error spec is +/-0.001"/foot, you *can't* have 0.01" backlash in one position, and 0.001" backlash in another position, without violating the lead error spec, as the dimensional variance that creates the backlash variance would also create a lead error of roughly the same magnitude. Hence my conclusion that the actual variance backlash on *any* given screw and nut will be far less than 0.010" - actually, more like 2X the lead error spec, or 0.002" peak-to-peak in this case. Now, with the looser spec screws, at +/-0.004"/foot, a peak-to-peak backlash variance of 0.008" would be possible, but that would indeed be a limit sample part I think you'd very rarely, if ever, actually see from a quality manufacturer.

As a point of anecdotal evidence, I initially assembled my 1" XPR screws with no springs, just shims to reduce backlash to essentially zero. The variance in torque and backlash across the entire 60" length of the screws was essentially zilch. I had no good way to actually measure, but it was definitely well below 0.001". The XPR screws are really a remarkable value for the money.

Regards,
Ray L.

I have a quick question regarding the relationship between ballscrews and ballnuts. I just got off the phone with a guy from Nook Industries. I explained to him that I wanted the XPR ballscrew, lead error of .001"/foot and was going to go with the SBN10325 ballnuts (the 41 dollar ones), to which he asked me why I was then going with the XPR ballscrew. I told him I was going to preload by using two ballnuts on each axis. He said that even so, I was not going to gain any accuracy by going with the XPR screw, and I should just use the SRT screw, which has an accuracy of .004, the same as the ballnut.

DOES this make any sense? I figure if I preload the ballnuts, I should be getting ALL of the accuracy of the XPR screw, regardless of the ORIGINAL accuracy of the individual ballnuts by themselves.

I have a quick question regarding the relationship between ballscrews and ballnuts. I just got off the phone with a guy from Nook Industries. I explained to him that I wanted the XPR ballscrew, lead error of .001"/foot and was going to go with the SBN10325 ballnuts (the 41 dollar ones), to which he asked me why I was then going with the XPR ballscrew. I told him I was going to preload by using two ballnuts on each axis. He said that even so, I was not going to gain any accuracy by going with the XPR screw, and I should just use the SRT screw, which has an accuracy of .004, the same as the ballnut.

DOES this make any sense? I figure if I preload the ballnuts, I should be getting ALL of the accuracy of the XPR screw, regardless of the ORIGINAL accuracy of the individual ballnuts by themselves.

Sounds to me like you talked to an idiot. Try asking the same question of Catherine Kastelic (ckastelic@nookind.com). She seemed quite knowledgable when I bought mine.

Regards,
Ray L.

i agree with ray, i'm not sure that he fully understands...

No backlash on my new toy
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=180331696897
Now I just have to finish my shop and get a rotary phase converter. Anybody interested in a nice RF45 conversion in SouthEastern PA?
Joe

You are doing an RF 45 conversion? Got any pictures as I am doing the same thing right now....peace

BTW just out of curiosity how much do you want for it and is it stepper or servo based?

No backlash on my new toy
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=180331696897
Now I just have to finish my shop and get a rotary phase converter. Anybody interested in a nice RF45 conversion in SouthEastern PA?
Joe

ADD TO CART :cheers: nice find

The Cincinatti is 90 miles from my house no less... the owner's brother in law will deliver it for $350.
The machine I will be selling was built my Chris Rosequist and features thomson preloaded rolled ballscrews, servos with gecko 320 drives, a tormach tooling system, touch probe, and 4th axis. It's actually pretty impressive as benchtop machines go. I would let it go for what I paid for it ($5500) keeping in mind that I have added a 17" monitor on a monitor/keyboard arm and a PC, as well as basically rebuilding the whole thing to properly align the ballscrews and preload the ballnuts.

PICS PLEASE!!!! Good luck with that new machine, god you suck having a VMC in your garage, Serious envy here man..... haha peace...