I have a 3/8 ball screw, .125" lead, with about .007" backlash.
It uses .0627" balls.

Would it be possible to reload it to reduce the .007" backlash?

yes, but you would be suprised how little of oversize will reduce backlash. I would advise that you check:(before re-load)
1) your fixed angular bearings ( search many , many discussions on this subject, especially by CNC Zones resident bearing engineer, NC Cams.Some are very technical and hard to digest the 1st reading, but essentially the gist of the white paper is cheap,improperly mounted, worn or improper preload equals backlash, not fixed by oversize ball nut bearings.
2) The ball nut mount : sloppy, loose improper mount

Both of the above can be checked with a good tenths dial indicator.

If, after careful inspection you do determine that it is in the ball nut, and the ball screw or shaft is not worn ( usually in the center ) then re-load with some increment of ten thousands ( as in .0003 to.0007).Some even re-load
with every other one an oversize. Better experiment with different oversize
1st, do not let the ball nut get in a bind, you could damage the ballscrew or nut.
Do not use a magnet to pick up the balls, they become magnetic and begin attracting metal slivers that really screw things up. as you can imagine.

Over size chrome steel balls,series 52100 can be viewed at www.salemball.com, click on chrome balls, click on inventory, go to "inches", should give you an idea of inventory in your needed size.

more than any thing good luck

ADOBE (OLD AS DIRT )

I ordered balls, from Salem, oversizes by .0003" and .0007", and .0014".

I'll report back with results

kool..just one more thing...seclude yourself ..have a real clean area with lots of light...no phones, wives, kids or dogs ( usless its a real neat dog )you will have it done and working in no time..

down load : www.rockfordballscrew.com/manual.htm....costs nothing..has the best pictures and instructions of " how to" re-load ball nuts I have found.
Good Luck

Adobe (old as dirt )

I developed my own technique:
I back off the nut on the screw so that when looking into end the ball nut the opening of the retun tube is visible. ( the nut has two holes in it for each return tube. the hole is obviously visible on the outside of thhe nut. It is also visible also from the inside of the nut too. With the tube inplace perform the stuffing operation)

one ball at a time stuff the balls into the return tube through the open end of the nut.

Every once and a while screw the nut fully onto the screw to test the action. Then back off the nut again from the scew, like before, so the opening of the return tubeis just visible from the open end of the nut, stuff the balls one by one into the tube. I do not use grease while stufffing.

I use tweezers with a small indentation in each jaw to hold the ball. I place the ball, then push it into place with thhe tweezer.... worked pretty good.

Well I reloaded two ballscrews
I used .0642" balls
I have about .003" backlash in both.

I was hoping for better, I don't know If i will continue or move on.

wow, you used the .0014 oversize and reduced backlash by .004...:
Just a couple of questions...was this a new or used shaft/ball nut combo ?
Have you checked the thrust on the ball shaft with a dial indicator ?

I have no experiance with this small of shaft, but have a friend that designs, repairs manufactures small equipment that uses the smaller ball screws. Will call him Monday and find what his input is on your problem.. Post as much info on the ball screws as you can so I can ask him intelligent questions.( he owes me one any way )

Adobe (old as dirt )

DON'T FORGET TO CHECK THE GIBB ADJUSMENTS. Lots of slop can occur here and no amount of nut preloading will eliminate it.

and

DON'T FORGET TO CHECK THE PRELOADED SUPPORT BEARINGS. Wear or lack of preload/rigidity in these bearings will cause LOTS of initial take-up lash, especially at direction changes. To eliminate slop here, you want HIGH preload with HIGH CONTACT ANGLE ball bearings mounted in DF configuration.

Once you get the ball nuts to the point of having a mechanical feel when they turn, you're probably going to have to look elsewheres for any further backlash elimination!!!

THERE IS NO SILVER BULLET FIX!!! It is a systematic and careful CUMULATIVE elimination of slop that you have to eliminate so as to get low/minimal backlash in an axis of travel.

NOTE: you didn't say if you had rolled or ground screws. Some rolled threads have a true "radiused" ball raceway. However, precision ground raceways are often ground in a "gothic arch". Thus, a rolled raceway looks like a "U" whereas a gothic arch is more like a "V" albeit with a slight radius on the sides of the V to better conform to the ball.

Since the U shaped groove has more horizontal travel potential at the ball contact point than a V shaped one, you might not be able to get the axial stiffness (lack of slop) in a rolled screw that you can in a ground screw due to the forming geometries used for the raceway.

Just a couple of questions...was this a new or used shaft/ball nut combo ?
Have you checked the thrust on the ball shaft with a dial indicator ?

.......Post as much info on the ball screws as you can so I can ask him intelligent questions.( he owes me one any way )

The two sets are the same, about 10-12 years old.
They were not used often, but were improperly disassembled a few times and some balls were lost BUT I took a good look(10x magnifier) at the wear surfaces and no serious defects. In fact the black oxide finiish was still visible. Backlash is the same at centre ( where it is likely to have been used the most) as well as at the end of the screw ( where it is likely to have been used the least)

Black oxide to me means rolled not ground screws. I have no info or part numbers, I am taking this mmachine over from Dad. He completed it in the 50's and the ballscrews were added much later in a re-build.


I measured the backlash in the following 2 methods

method 1: lock down gib, verly lightly turn handlle and read off amount of turn before ANY restistance is encountered...about .004"

method 2: unlock gibs, dial indicate from saddle to table. Push and pull on tablle with the weight of my body. Take care to not twist table and therefore measure play in gib. Take care not to back drive ballscrew. Both axiis measured about .004"

I tried to measure the backlash of the screw/nut assemblly on thhe bench but couldn't get everything to stay still enough.

I have posted elsewhere about the axial play in my needle thrust bearings, I don't think this is the problem, but may just remove them and re-measure the assebmled machine ( minus bearings) for backlash

One night this week I'll post some pictures here of the relevant items, I just have to pull the machine apart...again.

cool,will go from there...now I guess that this machine was designed with needle thrust bearings in combination with possibly a simple radial bearing ?Possibly NC Cams could lend his knowledge here, but I do not think you could take up all axial thrust with needle bearings and thrust washers with out damaging the bearings or washers, this is why they have angular bearings mounted in DF ( to you and I thats face to face ) with fitted spacers for pre-load...could you draw a sketch of the support bearing arrangement for us to look at ?

Understand that measuring shaft movement is kinda difficult, but what I do is find the end of the shaft that was used for the live center on the lathe ( you should see a 60 deg. hole in the end of the shaft) find a ball bearing that will fit in the hole & I use marine trailler wheel bearing grease to hold the bearing in place...that will give the end of your dial guage test indicator a good surface to ride on.Roll the table manually to 3/4 of guage travel and reverse , if the guage moves then you have found the source of the rest of your back lash.

I do encourage you to post a simple drawing if the shaft support bearing arrangement..

Wow, Madagascar...really an intriguing place eh, only in the movies ?

Adobe ( old as dirt )

I suspect you have a needle bearing, followed by a bronze bushing to suppor the end of the screw followed by another needle (if you're lucky).

Needle bearings are not usually preloaded, per se, as they are typically designed to be drop in interchangeable with conventional thrust washers. Folks sometimes try to do it with belleville washers but you can overcome the deflection curve of these quite readily.

THis is why angular contact ball bearings are used - first they can be preloaded and still roll because the balls make "point contact" with the raceway.

EDIT: try replacing the thrust bearing with a solid washer that is essentially tight. Then push shove on the table to see if you have gibb/table slop. At that point, your slop should be confined to the ball nut and/or the giggs. Once you fix slop here, THEN go to the ball screw support bearings.

YOU CAN"T ELIMINATE BACKLASH/SLOP UNTIL YOU METHODICALLY QUANTIFY ALL THE PLACES WHERE IT IS COMING FROM. Thus, FIND THE ROOT CAUSE FIRST, then develop a sollution strategy, piece by piece, section by section, part by part.

END EDIT

You also have to consider the bearing geometry. Needle bearings make line contact and also tend to want to skid because the OD of the needle is trying to roll at a different surface speed than the ID of the rollor (think about the difference in circumference at the ID vs OD of a needle bearing and it will make perfect sense). EIther the ID or the OD wants to slip if you preload them - NOT GOOD.

In light of the fact that this machine has a needle thrust beariing or bearings, I'd be inclined to speculate that it has a fixed distance/clearance at the thrust end, or if you're luckly, there is a nut that can be adjusted to "preload" the washer and take up clearance. In such cases, you can't make them tight enough without experiencing binding in some way or another - that's what I've seen in the cases where I tried to fix needle thrust/bushed ball screw ends.

Either way, you probably won't/can't come up with a low friction/high stiffness/low clearance axial thrust absorber using needle thrust bearings. Besides, you shouldn't preload them due to the slip/slide problem associated with the skid potential discussed above.

Ball screw bearings are designed that way for a reason. Folks try to get around it by all sorts of gyrations but, eventually, the endemic limitations of the use of the less than optimum bearing rears its ugly head.

You either have to live with what you have or do a major redesign to try to package something more appropriate for/to the task.

Thanks NC, knew there was some thing that prevented needle bearings from being good thrust. That was a good explination of slip /skid to illustrate."Tighten up" just never worked with needle thrust bearing, no matter how well they were lubricated.

Had a lot of "thrust bearings" )(needle) fail in High Performance out boards and out drives when lots of torque and horse power was applied ..argued endlessly with a domestic out board- inbord manufacture about their control of thrust..told their chief engineer he was still in the 1950's with his "new" design, he invited me out of his office, but they almost went broke fixing thousands of outdrives still in warrenty . The "other" marine manufacture solved their problem with precision matched angular thrust bearings..

Adobe (old as dirt )

Contacted my friend today. He is a M.E. involved in precision optical inspection equipment. Their tolerances are mind boggling ..000005 (+-).000002! Every thing is done in a controlled atmosphere room.
They use lots of small ball screw applications, all are ground and come to them at no more than .0002 back lash..He did say that in some instances the ball screws are reloaded to minimise back lash, but most accuracy is controlled by optic and other means for instant correction.
His impression to me that even a rolled 3/8 ballscrew that had .007 or more back lash was most likley not intended for any precision work.He also raised the possibility of mixing a metric ball screw with inch balls ?
In all instances they used precision angular bearings..

Adobe (old as dirt )

MOST ball bearings (even metric ones) still use inch sized balls. It seems that this was done eons ago so that load capacity equivalency could be maintained from maker to maker within each bearing size.

Yes, there are metric balls used but this is the exception rather than the rule.

If you think it is hard to find an oversized inch ball with micron size differentials, wait until you see how easy it is to find metric ones with comparable differentials.

Yes gang, ball screws are not merly ball screws. Although rolled threads are adequate for a lot of applictions, when you start looking for near zero backlash potential, you're probably going to have to spring for ground screws.

The Japanese have a phrase for this: shoganai (show- guh-nye). Translation: that's simply the way it has to be.

NC Cams: There were a lot of ball screws built in Europe that had metric ball screws, that in fact used metric balls, although most likley not a lot of them came to the US except in speciality equipment ( Telescopes, high end optic trackers)

Salem ball co and others keep a good inventory of metric balls.

A good rolled ball screw, specially where you can use a douple nut can be more than sufficient for the Hobbiest and small shop, tolerances less than
.0001, WITH THE CORRECT BEARING SET UP is obtainable.Just a lot of people can not afford high end ground ball screws, and most when delivered to a hobby consumer still have .002 to .003 backlash, unless you have paid "extra" for a "0" backlash ball nut re-load. My feeling is, invest in good bearings and rolled screws with double nuts..and then invest in a good set of duplex bearings to eliminate radial and axel play.

Helped a friend ( and former competitor) install a kit, including rolled ball screws with double nuts and Barden bearings mounted DB, in a BP clone that had already been CNC'd. He did have the table and ways ground and scraped , and cleaned /redid a good portion of the oiling system. After 7 weeks of 2 shift work, an accuracy check showed this machine was still at less than .0001., even in some hard corners.His total cost is $3100.00,
( not including refreshments) but he could not replace this nice of a macine for that amount.

In this thread started by CNCUSER1, there is still a possibility of a a mount bearing problem or possibly who ever retrofited the machine 10-12 yrs ago got a hold of some European ball screws that needed metric balls, or like most things, a combo of all of the above.

Adobe (old as dirt )

Adobe: your needle thrust inboard/outboard posting in #12 explains why thrust bearings in transmissions are/should be designed with spatial consideration for either needle or bushed thrust washers used interchangeably in critcal thrust postions.

This way, if they OOPS in figuring the use of one or the other, they have the option of substituting the other in case they boost up torque or power flow and don't want to redesign the case for the 0.020" thinner/thicker bearing that they need to install the RIGHT bearing.

Sadly, lots of people don't give TOTAL thought to the simple geometric limitations of bearings. Moreover, they don't even take the time to READ/UNDERSTAND what load capacity means for a bearing. Sadly, it is explained but few really take the time to learn.

It took racers quite a while to learn what happens when you put needle thrust bearings behind cam sprockets and they fail - surprisingly, some even learned that a bronze thrust washer works MUCH better there and elsewheres.

Oh well. That's why there are winners and losers. It is also why some are quite proficient at holding down their finishing positions in spite of MA$$IVE efforts to the contrary.

Do you mean those funny little bearings in my oil pan, that got into my High volumn pump and .....just another window in my K.B. I gotta tig up..me and that bearing engineer had a real discussion !
How are they doing with needle bearing cam bearings ? ?

Adobe ( old as dirt )

Needle cam bearings: Depends on the type.

The real early 'drop in style' like Crower tried to pioneer (rollers contained in groove cut into journal with split busing outer rings) have long since fallen by the wayside.

The true press in place caged needles that the blocks are recut for are quite common in Pro Stock drag racing and Nextel/Bush/Crafstman truck racing. I dunno about the sportsman classes as this ends up being big buck due to the trick cams you have to use in concert with them.

Only trick with needles is to properly harden and grind the cam journals (not all are done properly!!! some are real lame) and also to grind the journals to the proper size and tolerance (some are grossly improperly sized).

One night this week I'll post some pictures here of the relevant items, I just have to pull the machine apart...again.

I pulled my Y axis apart and have attached some pics.
The close up of the nut is self explanitory

The other two pictures show the screw&nut. Moving to the left of the nut:
(midway , to the left of the nut, is a piece of hardware, it is not relevant)
- Is a shoulder machined onto the screw, a hardened washer (.0625" thick)is press fit up to the shoulder.
- next to the left is another hardened washer ( .032" thick), then the bearing pictured in a previous post, then another hardened washer ( .032" thick).
- Next is a 1/2" steel plate
- next to the left is another hardened washer ( .032" thick), then the bearing pictured in a previous post, then another hardened washer ( .032" thick).
- Next is a pulley keyed to the screw shaft
then a hand keyed too the shaft
then finally a 1/4 nut clampimg everything up

So the nut clamps everything between it and the machined shoulder.

In my humble opinion, when two fingers lightly ride on the nandle OD and can turn the handle .003", the problem is not in the clamping.

I tested the assembly by holding it all down with toolmakers clamps to a 1/2" brass plate, to try to simulate the backlash, it did not appear on the bench set-up!

The only guess i can make is that the backlash comes from a springyness within the nut, nut an actual gap.

I just ordered slightly larger balls from salem

Recap: stock .0627" balls gave .007" backlash
oversize .0642" give .003" backlash
I just ordered .0660" ( but they didn't have enough for both of my lead screws, I'll cross that bridge when i come to it)

I'll report back with results

M

Thanks..one other point you might look at is the flange attaching the ball nut to the table pinned or set screwed ?I had one drive me to tears, just could not find the backlash, and had the nut stuffed so hard that it was getting hard to turn, finally I had one of my better machinests look at it ..took about 45 seconds and he turned to me and said "Boss, can you not see the ball nut moving on the flange ?" Not only had I missed it, but the allen set screw was lost or long gone..Drilled for a hard pin and installed the set screw with blue lock-tight..backlash gone, had to restuff the ball nut with smaller balls.

Is there a way to set up a dial indicator on the end of the shaft when assembled ?..

Keep us informed.

N.C. Cams: Just courious, will he be able to preload the arrangment above to eliminate all back lash under cutting conditions ? He is getting closer, but with out having to redo the whole system, would you have any economical suggestions ?

Adobe ( Old as dirt )

What does this mean?
[QUOTE=Adobe Machine;199632]Drilled for a hard pin and installed the set screw with blue lock-tight..backlash gone,....[ QUOTE]

The assembly in the pictures, I clamped it during bench tests. I looked for axial float of the lead screw...none.
I tried to look for backlash in the nut, in my bench set up. Under no-load conditions, there was no backlash.

I clamped the ball nut to my bench set-up and tried to turn the handle but I ouldn't tell if there was backlash because the clamps induced strain on the setup.

I looked for play in the ball nut holder in the machine( lock gib, dial indicator on ball screw holder, push,pull by hand), no play.


I noticed that the ball nut has some slop when it is on the screw, as you alued to. I tried really hard to constran the nut in it's holder.

So as oof right now my working theory is a springy type of backlash that is onlly visible under load.

I'll wait for the O/S balls and see what happens, it that doesn't fix it then I'll really have to tear down the machine and go at it, since I have all this spare time on my hands it won't be a problem, not.

Balls are shipped by postal service so it may be a week or more, stay tuned and thanks again.

OK, the problem I had in the above post ( 10 + years ago ) was that the flange for the ball nut ( in your picture, that would be what the ball nut threaded into ,was loose..it could not be tightned against the ball nut fully due to the bolt pattern on the flange not alinging , so it was about 1/2 thread off being tight, and was allowing some motion between the flange mount and the ball nut. I just drilled and pinned and put in a new flange set screw, no more float..but spent several days..hours looking for the problem,on my part it was kinda a dah uh !

Hope the larger balls reduce the backlash some more, and keep us informed.
What kind of industry do they have in Madagascar ?


Adobe (old as dirt )

The geometry of a needle thrust washer is NOT conducive to properly affecting a preload. About the time you get solid, you then run into the slip/slide problem with the radii conflict described earlier. Maybe if you add multiple parallel belleville washers to provide some "spring", you can conjure up something BUT you won't get around the slip/slide problem.

It is nye on impossible to explain the internal geometry of bearings on a message board. If you want to learn and/or understand what's going on, Tedrick Harris wrote a book on ball bearings. In it are most if not ALL the formulae that are used for:

calculating axial stiffness of bearings

calculating load vs deflection curves

measuring load/deflection curves

how to calculate the affect of raceway curvature on axial clearance in deep groove ball bearings

plus tons of other information.

When you REALLY want to have axially stiff bearings (what you need in ball screw supports to avoid axial deflection) with low/no friction, you're pretty much relegated to packaging angular contact balls.

Many mill suppliers have tried to cheap out and use preloaded ball bearings to absorb thrust but when push comes to shove (no pun intended), they can't be made stiff enough.

Bridgeport even went to the extreme of custom making special 6204's for their ball screws and they essentially suck if you want dead nuts accuracy - I"ve documented my escapades with this n a previous thread. I discussed what and how I over-preloaded some 6204's to support the axial thrust in my Bridgeport - even with all my knowledge of bearings and bearing trickse, it flat didn't work.

So, if needles won't work, preloaded deep groove ball bearings don't work adequately and A/C's are too expensive, what about tapered rollers?

Check the design manuals and you'll find that for minimum friction, you need to run tapers with 0.000 to 0.005" axial clearance.

YES, you can preload them - they do it all the time with pinions to make them stiff so they maintain position gear toot postion.

Result: huge friction penalty, you can't hardly turn a pinion by hand if properly preloaded.

If you can find an explanation of "bearing spring rate", you'll see that ball bearings are NOT solid as load is initially applied. Between the radial clearance and raceway curvatures, there is a "springyness" that occurs until you load the thing so hard axially that it does become SOLID.

When you finally see and understand the transition that takes place between initial deflection and final "infinite" load versus deflection rate curve that develops, you'll see how and why high contact A/C bearings make the ultimate ball screw support bearings. PERIOD.

You can mess with all kinds of bearing tricks. But eventually you'l find that some things simply can't be improved upon. I tried with needles and tapers myself until I became a bearing engineer and then I learned how the darn things REALL work. Didn't work before, won't work now, won't work tomorrow.

Get the Harris book and check it out for yourself... The math behind the you "can't get around it rocket science" aspect of bearing functions are all in there. Try as you wish or feel you must, you won't change the laws of physics and mechanics when it comes to bearing reactions under radial and axial loading....

Well, we need to wait untill he reloads with the incomming oversize balls,then can comment better...understand this guy is 1/2 the world away and trying to solve the problem with out rebuilding the entire machine he got from his dad...see if the really oversize helps, then go from there..most likly a combination problem.

Adobe (old as dirt )

Already been thru this aspect of machine improvement on similar starter machines that I don't use anymore. So I already know what he's trying to go thru as I did it on my own and you can only get SO good.

Having screwed with needle thrusts, tapers, kluged up deep groove bearings, bellevilles and other scenarios including o'ring preload "washers", it is a case of BTDT - been there done that and realizing what can and can't be done - and what won't ultimately work due to the "awe shoot factor".

At this point, I elect to merely try to explain how to REALLY fix it.

Rather than deprive folks of the learning experiences I went thru, you do have to realize that you can only teach/explain so much - folks have to eventually learn to reinvent the wheel on their own.

You can't wipe your nose with your elbow but who am I to say that you shouldn't even try to refo - even if you're halfway around the other side of the world...

First of all I am not in Madagascaar, I am in Canada, i just trust this internet stuff is all.

So can you point me in the right direction as far as A/C bearings go.

Ball park...my machine is about 75% of a Sieg X2, I have 3/8" dia ball screws, turned down to 1/4" ( or 5/16 don't remember ) for the input for CNC.

I'lll look into it while I wait for the balls

Having read all this, I totally agree on the action = reaction, so a load can't be taken without deformation. In the end you can imagine everything is made out rubber, and find what you're looking for. The very last fight involves solidity, concentricy and accuracy, and still there will be a gray area.
So would the use of ceramic hybrids not be a possible next step?

As for solidity:
http://www.ina.com/content.ina.de/en/branches/production_machinery/productronic/productronic_product_range/screw_drive/zklf/zklf.jsp
And INA about ceramic:
http://www.ina.com/content.ina.de/en/branches/production_machinery/machine_tools/maschine_tool_-_product_range/main_spindle_1/x_life_1/x_life.jsp

high contact A/C bearings make the ultimate ball screw support bearings. PERIOD.....

Can you suggest a part number.
That would give me a start

Ceramic balls are used for ultra high speed applications. They are overpriced, unnecessary alternatives that are not particularly well suited for a low speed application like a ball screw.

Sadly, there are NOT a lot of angular contact ball bearing options for ball screws that small. The smallest bore one that you could probably consider would be a 7005A5TYDUH from NSK.

Take what you can get in the way of ABEC level but be prepared for high cost as most of these are "machine tool grade".

Be sure to pack them with 30% fill of a #2 viscosity extreme pressure lubricant when you install them if they are not prelubed when you get them.

Ceramic balls are used for ultra high speed applications. They are overpriced, unnecessary alternatives that are not particularly well suited for a low speed application like a ball screw.

Sadly, there are NOT a lot of angular contact ball bearing options for ball screws that small. The smallest bore one that you could probably consider would be a 7005A5TYDUH from NSK.

Take what you can get in the way of ABEC level but be prepared for high cost as most of these are "machine tool grade".

Be sure to pack them with 30% fill of a #2 viscosity extreme pressure lubricant when you install them if they are not prelubed when you get them.

so this is not overkill for a 3/8" ball screw on tabletop mill using a 3/16 endmill cutting at 10 inches per minute?

I'll look into it, thanks

NC Cams: Would you explain for all of us why you only fill the bearings with
30 % lube..?.Truthfully, I have been carefull to get all air pockets out of the lube ( I hand pack, rolling the bearing in the palm of my hand to expel grease from the other side untill I can see there are no more air pockets). I musta missed something at the Timken School ( yeah, it was kinda a wild week )?
Or have I misinterpeted your note ?Learn something new every day !

Adobe ( old as dirt )

Ceramic balls are used for ultra high speed applications. They are overpriced, unnecessary alternatives that are not particularly well suited for a low speed application like a ball screw.

Sadly, there are NOT a lot of angular contact ball bearing options for ball screws that small. The smallest bore one that you could probably consider would be a 7005A5TYDUH from NSK.

Take what you can get in the way of ABEC level but be prepared for high cost as most of these are "machine tool grade".

Be sure to pack them with 30% fill of a #2 viscosity extreme pressure lubricant when you install them if they are not prelubed when you get them.


The ID's of these bearrings are too big(10mm), they do not seem to come small enough for me.

Do you have a second best recomendation?
( I have A 3/8" ball screw on tabletop mill using 3/16 endmill cutting at 10 inches per minute)

I reiterate:

"Sadly, there are NOT a lot of angular contact ball bearing options for ball screws that small. The smallest bore one that you could probably consider would be a 7005A5TYDUH...."

Why this one?

Because that size is the smallest bore A/C bearing with the highest, stiffest cross section, heaviest preload that I know of that might serve as a ball screw support bearing.

Sadly, they simply don't make A/C's small enough to fit your application. For that matter, they don't even make ANY small bore ball, radius'd raceway thrust bearings with bores smaller than 10mm. That is why you have what you have in the way of thrust bearings in your machine.

IF you're bound and determined to upgrade the performance of your screw, see what you can find in the way of a deep groove ball bearing that will fit. Then try to find some belleville washers that could be packaged to act as preload springs.

By stacking them in series and parallel, you should be able to achieve the spring rate curve that gives you the stiffness you need in concert with any remaining ability to turn the screw....

I reiterate:

".

I saw that 10mm is the smallest A/C bearing available.

So I was wondering about a second best alternative.

spherical roller thrust type ball bearings possibly?

This will fit but it essentially reaches an infinite "spring rate" very, very quicky as you apply load. The charm in an A/C is that there is some initial "spring" in the assembly that you can take advantage of during the preloading process. Once you use up the initial springiness, then you have infinite stiffness.

Once you reach infinite stiffness, friction can start to skyrocket and bearing life plummets.

The roller thrust bearing that you illustrate will offer better/lower friction than a needle bearing but it still may/will be capacity limited. It DEFINIITELY does NOT have much if ANY radial load carrying capacity as an A/C does.

If you use that type of roller thrust bearing and can find one in an appropriate size, you'll have to watch preload amount versus rated capacity of the bearing REAL CLOSELY.

For the bearing to live, you have to run preload at about 10% ~20% of the rated static capacity. Don't think for a minute that you can run 100% of rated capacity - this factor is sort of a misnomer.

Rated capacity is a actually used as a life calculating factor - it is NOT the "absolute load capacity" that the bearing can withstand and live at/with.

Hi guys!

Reading your articles about replacing balls in a ballscrew,i would like to replace my balls to my ballscrew.Its a THK BNF 2005 ground screw.Of What material the balls should be(chrome steel,carbon,etc)?I have checked salemball.com but i dont know what material!

"Many questions, One Aim"

Thanks...

52100 carbon alloy bearing steel should be more than adequate.

NC Cams,
What do think of my bearing setup? I recently converted a X3 benchtop mill( Full thread in bench top forum) and I reused the factory grooved thrusts but added 2 deep grooves sandwiched between them.
These are pics of the Y axis block and bearings. I use the cogged pulley which has a threaded insert to take up end play at the screw and add a little bearing preload. A setscrew locks the adjustment in place.
With careful adjustment and my dual ballnut setup I measure .0005" backlash on the y axis and near .0003" lash on the x- axis which is direct drive with a large oldham coupling.
It seems to work well so far.

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Steve

As most of the readers know by now, I tend to be a "high end" (costly) solution provider. Why? Because I worked in the machine tool industry and small time, DIY user business simply was not in the business plan.

Thus, my solutions can often be beyond the realm of adaptability or affordability of many DIY'ers, especially those with small, hobby machines. Besides, these machines are small and bolt-in bearings simply don't come that small. In this case, I"m honestly impressed with your solution.

You found a way to package radial bearings to provide the needed vertical support and repeatablity of the screw support housing.

You also found a way to package roller thrust bearings so as to absorb pure axial thrust with what is probably an adjustable preload means.

Irrespecitive of the fact that I"m impressed with your solution, is the fact that it works - an apparently to your satisfaction. I'll bet it felt REAL satisfying to see those backlash numbers, didn't it???

To achieve such low/reasonable backlash with essentially off-the-shelf parts is quite impressive. You are to be commended.

The bottom line with any "fix" is this: are you satisfied with the results???

If so, great,

If not, try harder and look deeper into the workings of the device -

There is a solution although it may not be staring you in the face nor may it be a bolt in.

PS: keep an eye on the coupling(s) - slop can develop in any set screw/non-shaft claming type coupling over time.

NC Cams, thank you for the positive feedback. I have read your posts and value your opinion.
I'll admit on the very first test my numbers were less than spectacular. I spent an entire 10 hour day tearing it all back down and finding a few issues and fixed them, then by trial and error I found just how this setup needs to be adjusted to work properly.
I do use another deep groove with a slight fit on the opposite end of the x axis. The Y axis is floating and that was a compromise to get 2.25" of additional Y axis travel from factory specs.
When I converted the mill I envisioned good backlash numbers and it had to be done with my limited budget. It didn't happen on the very first try. I had to add more spring force in my ball nuts and the bearing preload has to be just right I found out, it was not just a simple matter of taking up endplay. When I did finally get it right, yes I was one happy camper to see those backlash numbers!
Plus the table gibs I put some time into as well.
I'll keep an eye on the locking set screws. I did use large 5/16" setscrews so a good amount of torque could be used to lock them.

Steve

oh just to be clear, the setscrew sticking out of the pulley side is just there for extra insurance. The threaded insert in the pulley was a tight pressfit and bonded with loctite.I just kept there as it came with the pulley.
The center 5/16 setscrew locks the preload against the tip of the ballscrew. Both have a mating 60' seat.
Steve

Your post is/should be required reading for ANYBODY who's having problems with motion slop/backlash in their CNC!!!!!

A systematic, detailed "step by step/check/change/verify/repeat" process is the ONLY way to solve this or any other lost motion problem in a machine tool.

Thank YOU for your belief and following of my postings and those of others who helped you solve your problems.

Thanks more for proving that there are folks out there who read and follow what some of us have to say and turn it into successful machine modifications.

It is nice to know that we can help make a difference in some member's machines.

I'm glad you had success - with your detailed and systematic approach, you surely will have many more experiences of the kind.

grooved thrusts.....
deep grooves .....
threaded insert..


Can you provide part numbers and manufacturers (if you have them).
I am intrigued by the thhreaded insert, what is this?

Well this forum has been a big help to me.
NC Cams, it is I who have to thank you and others with greater knowledge who take the time to explain.
I was once into another hobby where I have a decent level of skill and helped others. Now I am enjoying being a beginner and student learning cnc machining and I especially enjoy all things mechanical.
cncuser1,
I had a dozen various abec 5 bearings given to me. Some were nsk and NBK I think. I paid no real attention to the part numbers and I took the bearings that by hand seemed in best condition. They were 8mm and I looked up the numbers at the time and found references to them on this forum and of course google.
I took the shields off to clean them and re lube. Bad idea? I don't know but they seemed much better after I did that and a lot of grime came off of them.
The thrust bearings pictured were the factory bearings that came with the x3 mill. I am sure just cheap Chinese made but they work in this mill and since the mill was brand new it seemed a shame to not try and reuse some of the existing hardware.
Inserts?
Ok, I machined my ball screws with 5/16-18 male ends. I purchased a 1/2" bore Oldham coupling for the x axis and a 1/2" bore GT2 3mm pitch pulley for the y axis . These parts were fitted with inserts I made on the lathe for a press fit 1/2 ID with internal female 5/16" thread.
These were pressed and bonded into the pulley and oldham coupling half to be clear.
The oldham coupling and pulley now thread onto the ball screw end with excellent runout.
So the oldham coupling and y axis pulley now also serve double duty as bearing preload adjusting nuts for the ball screws. Once preload is set then the centered 5/16" threaded set screws lock my adjustments in place.
I did this to keep things compact and simple.


Steve

Re: removing shields to clean and relube =

The most common reason why bearings fail is due to grease contamination and/or failure.

Particulate contamination of grease is pretty easy to understand and prevent/deal with (clean and relube) so I won't bore you with details.

"Grease failure" is something a bit more insidious.

Should grease become contaminated with water, solvents, or vaporous substances, it can start to deteriorate chemically.

Or, should it be run at elevated temperatures, it WILL start to oxydise (sic) an lose its lubricative properties. The higher the operting temp, the faster the deteriouraton.

The fix: clean and relube.

At this point, those with a keen eye for the obvious would realize that although bearings are usualy listed as being 'lubed for life', the life (of the bearing and the grease) can be extended via the cleaning and regreasing of bearings.

A ball bearing run at low hand cranked speeds can be packed fairly full of grease and suffer no appreciable effects one way or another asside from grease purge.

Bearings run at speed (IE: motor speeds for example) should only be packed 25% ~30% full of grease. Too much grease causes excessive temp rise (see above comment about temp rise) due to viscious drag and even more pronounced grease purging.

Yes, and contrary to what my father told me, you can 'overlube' a bearing.....

Thanks for that info NC Cams.
After cleaning them I used a light oil however instead of grease with the thinking it would be a little less friction or a "free'er" turning bearing setup.
I have lube ports in the bearing blocks to keep them oiled.

Bad or good idea?
The bearings will only see 375 max rpm with my 75 IPM rapids if that makes any difference.

Steve

Oil tends to be drippy/messy and can actually attract dirt and dust like a magnet. Light grease is a bit less messy and easier to contain within the bearing - especially if you use the fills noted previously.

A needle style syringe is the best way to insert and concentrate the grease into the ball path. This is preferred as opposed to jambing it it with the palm of your hand as one might pack wheel bearings. It is also easier to monitor fill amount with a syringe.

A good low viscosity grease that channels readily for low friction could be found in a product called Beacon 325. Pretty sure it was made by Exxon but don't quote me.

An option would be a light "sewing machine" grease or perhaps model train grease (if they still sell such an item - they did in my formative Lionel train years).

I'll wait for the O/S balls and see what happens, Balls are shipped by postal service so it may be a week or more, stay tuned and thanks again.

So I recieved the .0660" balls.
long story short... even using one .0660" for every five .0642"" balls the nut would jam up.


So for the time being I will have to bbe satisfied with what I have.

i amm still curious about some of the other quuestions that came up in this thread.

Your oversized balls are off by a factor of about 8 to 10!!!!

To preload a screw, you need to go O/S in steps of 0.0002" or so, not 0.0020". Granted, these will not be easy to find as increments that size are only stocked by the screw makers or outfits that specialize in screw reworking.

When clearance in the screw is measured in 0.001 to 0.010 MILLIMETERS, it should not be at all surprising if a single 0.002" O/S ball would jamb things up.

Your oversized balls are off by a factor of about 8 to 10!!!!

To preload a screw, you need to go O/S in steps of 0.0002" or so, not 0.0020". Granted, these will not be easy to find as increments that size are only stocked by the screw makers or outfits that specialize in screw reworking.

When clearance in the screw is measured in 0.001 to 0.010 MILLIMETERS, it should not be at all surprising if a single 0.002" O/S ball would jamb things up.


Yup I figured as much.I would have taken something smaller but it wasn't available.

know any sources of O/S balls aside from Salem?

Google "precision steel balls" for potential sources.

Might try Winstead Precision Ball as they make them. This is now a division of SKF if memory serves correct.

First go for grade 25,

Then 10, then 5 then 3.

Grade 3's are unbelieveably round and smooth and as "perfect" as you can get.