Doubt And Questions On Acme Screw Thread Efficiency

[Brenck]

Hi everyone,

Im gonna use ACME screw to build my second machine, i faced some doubts when choosing the leadscrews... Its not the pitch or diameter... I want to know how does the starts affect on the efficiency of everything...

Lets say I choose a ACME screw with 14mm diameter and 4mm pitch, with one start only... Lets supose that this will give me a efficency of 50% ( i dont know im just making the number for an example)

What about if I use the same 14mm diameter and 4mm pitch but with 2 starts... So the spacing of each thread it would be smaller... But it will still move 4mm with a turn.... Will it be more efficient or less efficient? I suspect it would be better with 2 starts, or maybe even more! If its possible to make...

Lets consider both situations using the same length, size, kind and material of nut

So... Any mechanicals enginners that knows the answer for this? Or other person who got the knowledge of that...

Thank you in advance to everyone

[evena4]

I was under the impression that the number of starts is responsible for the actual turns/inch(mm).

In fact I just remembered looking at Mcmaster.com under ACME PRECISION THREADED RODS a while back so I pulled it up here.


I am not sure if the "efficiency" you are seeking is related to the "speed-ratio" but hought I would throw 2 cents in anyhow.
Hope this helps,
-Manny



-----insert text-----
Start Type
Tne number of individual threads on a rod determine how fast a nut will travel on the rod. Standard-travel rods must be used with standard-travel nuts and fast-travel rods must be used with fast-travel nuts. Standard Travel Single

Start
The most common rods and have a single thread that runs the length of the rod.

Fast Travel Multiple Start
Have multiple threads that run the length of the rod.

Speed Ratio
Indicates how fast a nut will travel on the threaded rod. 1:1 | 2:1 | 3:1 | 4:1 | 5:1 | 8:1 | 10:1 | 12:1 | 16:1

Travel Distance Per Turn
The distance a nut will travel with one revolution of the threaded rod. 1/41.7" | 1/40" | 1/20" | 1/20.8" | 1/16" | 1/12" | 1/10" | 1/10.4" | 1/10.7" | 1/8" | 1/7" | 1/6" | 1/5" | 1/5.2" | 1/4" | 1/3" | 1/2" | 1/2.7" | 1" | 2" | 2 mm | 3 mm | 4 mm | 5 mm | 7 mm

[CWBAILEY]

The lead of any thread is the pitch multiplied by the number of starts.
So a 14mm Dia thread with pitch of 4mm and two starts will give a lead (distance travelled in one turn) of 8mm. The efficency of the thread or its mechanical advantage is determined by the wedge or thread angle. for a 14mm dia screw of 4mm pitch this is 14 x 3.142 divided by 4 which equals 10.997 or 11:1 if the lead is 8mm the mechanical advantage (the distance the effort moves/ the distance the load moves) is 5.5 only half the mechanical advantage. To get the same mechanical advantage as a 14mm dia x 4mm pitch you will need to increase the thread diameter to 27.15mm to get the same thread angle.

[Metalcutter]

First of all are we talking efficiency or precision? Multi start threads affect precision. The pitch of a screw is the total effective pitch so a 4mm pitch screw is 4mm regardless of the number of starts (leads). Each lead has a pitch of 4mm. The distance between the leads, from crest to crest is the pitch divided by the number of leads whereas the distance from crest to crest of the same lead is the screw pitch.
Efficiency would be how much energy is required to turn the screw. Since multi-lead screws have shallower thread depths it is possible it could be more efficient also.

[ger21]

Go to www.nookind.com and download there acme catalogs. They list the efficiency of each screw.

[Brenck]

What i wanted to say about the 2 starts thread its that after 1 turn it will still move 4mm like the one with 1 start, i tought that was called pitch, its actually called lead I forgot... But if we gonna consider pitch beeing the being the spacing of each peak of thread, lets say it would be 2mm then...

On nooks products catalog show me something that i never known,

Look at thoose 2 leadscrews:

8 x 10, Lead 10.0 Efficiency with bronze nut 72.7
10 x 10, Lead 10.0 Efficiency with bronze nut 69.8

What its ok because the higher the diameter the less efficient you get ( same leads of course )

But what got me surprised is that the loads that each one hold is different, the 8 x 10 holds 1040 N static and dinamic, and the 10x10 holds only 780 N...

Its strange that with plastic nut makes the situation goes the opossite way, the 10x10 wins on efficiency but it loose even more load... While the 8x10 looses efficiency but keeps its load

So it looks like the best thing to do its to get the smallest diameter you can get for your set up with the lead you want...

It didnt answer my questions about the starts but it helped a lot...

Does anyone knows how to calculate the diameter of leadscrew you need according to the lengh between bearings and RPM it will rotate?

Thanks for the answers

[VWSatOz]

I agree with Bailey also from OZ.
Efficiency from a friction type thread screw (ie not a ball nut) is to do with the helix angle of the thread to the axis. In a normal type screw thread the efficiency is less than 50% and when you press on the nut the screw locks up and will not rotate. ie as in a nut &bolt you tighten it and it says locked up.
So if you have a single start thread on say a 1" diam shaft with a LEAD of 1/8inch ie 8TPI then you "unwind" one thread on paper you get pi times 1" about 3" long circumference triangle diagram with only 1/8" rise of the wedge/slope, a very shallow edge angle, less than 50% effiency so it is self locking.
But if you were to cut the same LEAD of 1/8 on a 1/4" diam shaft(4times smaller)... (ie you would need it to be a multistart thread as the tip of of the tool in a single start would be near the centre of the shaft!).... then the above "diagram" would have a triangle "wedge shape" with much steeper angle,
pi times 1/4" =prox 3/4" long with the same 1/8" rise, the angle is steeper, if the surfaces are slippery, ie low coefficient of friction, then the screw may no longer be self locking, and if not self locking the efficiency will be above 50%.
So it is related to LEAD vs diameter, not just the number of starts.
If you have 14mm "mean" diam (pitch diam) with 4mm LEAD you will get the same efficiency with single start or 4 starts or 8starts or 16starts.
The multi start "bolt" will be stronger as the core diam is fatter.
If you have 14 mm diam with 8mm pitch you will achieve greater efficiency, ie less loss of work input energy compared to work output energy, but you get twice the speed of motion from the same rpm and you will need twice the torque input from your CNC axis drives to get the same feed force, or you will need 2:1 gear or belt reduction to halve the feed speed (adjust all this with our increase in efficiency, but you get the idea).
It's complicating everything to just gain a little bit more cutting feed force from the same axis drive motor, and much harder to make a multistart screw & nut with no back lash.
I suggest to get a high accuracy acme screw, design for two opposing Acme nuts that can be adjusted around to get absolutely minimal backlash along the whole length of the screw (tight spots might need lapping), and solidily lock/bolt the 2nuts in place against the FACES of their mounting block. Keep the CNC screws shielded so they are always clean and oiled, (no grit etc to wear it out in the middle working area), and good end bearings preferably at BOTH ends with the screw in slight preloaded tension, you will have a very good rigid result, with ability to climb mill, and if the nuts are nice and long the oil will sop up vibrations form the cutting.

I hope this clears up the thoughts about all this!!

[Brenck]

Thanks!!!

I didnt know that it was harder to take out the backlash from a screw with lots of starts...

It probably have the same efficiency.

I think with 2 starts the only thing it will change its that it will hold more load...

[BillTodd]

If you're looking to reduce the friction associated with backlash free lead-screws, have a read though the A different (better) kind of screw thread.

If you've room to build a roller nut instead of a fixed bronze or delrin nut, you will be amazed how the much lower the friction is under heavy loads (i.e.my single start 23mm x 2.5mm pitch Acme threads will back-drive rather than lock! )

[phil c]

You might also take a look at Kerk Motion. They have several series of anti backlash nuts that automatically take up any wear in a very rigid fashion.

http://www.kerkmotion.com/products/l...s-overview.asp

I haven't priced any of these, so take your chances.