Durability of plastic ACME threads in lead screw application?

[jcrags]

I have a lead screw application where a mechanism needs to be sealed up and run for long periods of time. Currently, we are using a brass nut on a stainless steel lead screw. Because of space constraints, the length of the nut is very short: 0.2 inches (4 threads on our 1/4-20 lead screw). We have had some field problems that might be related to lubrication issues, so I'm considering the possibility of using a plastic nut so that friction is reduced regardless of lubrication. The concern, of course, is the wear life of the plastic thread. I am unable to find much info on this issue, can someone point me in the right direction? Any way to calculate the expected cycle life time of 1/2-20 ACME lead screw with stainless screw and your choice of nut material with 25 lb axial load, no moment (linear rail supported nut), max speed 1000 RPM, ~5% duty cycle, 3 in stroke?

Thanks much

 

[Pud]

The critical factors with plastics as bearings is the PV ratio.
Here is a paper with some data: Link

Personally, I would suggest trying POM first (aka Acetal) as it has better creep properties than the best wear materials such as UHMWPE.

H


www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[Tmoose]

How much wear can you tolerate?

 

[MintJulep]

How long is "a long period of time"?

What material is "plastic"?

Is the load applied only while moving, or constantly?

As a point of reference, the door operators on most subway cars use stainless lead screws with "plastic" nuts. I forget the exact plastic, polyamide I think. The nuts last in excess of a million cycles.

 

[mikekilroy]

ask app engineers at BSA (ballscrews and actuators co) in Ca - believe they still use website wallscrews.com. they sell special plastic material (it is listed on their sit) lead screws and clearly state that if used within spec it will last the same 1,000,000 inches of travel as their ballscrews So your answer is, YES, you can get same life from good leadscrew with right 'plastic' nut.

Killers btw are high speed (above a few hundred rpm) and of course loads higher than rating. the engineers at link above will be happy to give you reply on what life to expect with their product for your application. I am not associated with this company, simply know they make good stuff and will give u ur engineering application info.

 

[mikekilroy]

can we have an edit button please?

ballscrews.com of course, not wallscrews

 

[jcrags]

Hello everyone and thank you for your interest. I had fires to put out today and am just now getting back to this.
I'll address issues in the order they were raised:

Pud: The paper you linked has some great general info, especially related to material selection, but I don't have the knowledge necessary to take specific wear rates and calculated estimated lifetimes in my application. I appreciate your material selections, some PTFE filled acetal looks like a good choice once I get that far.

Tmoose: In this application, the position of the nut is monitored by external sensors and so I can tolerate any amount of wear up until the nut either binds on the screw or gets completely stripped out.

MintJulep: A long period of time is something on the order of 50,000 cycles over a period of ten years. The load is applied constantly with very little inertial load.

mikekilroy: I like your suggestion and your numbers: 1,000,000 inches would definitely get me where I need to go. I'll give the folks at (w -> b)allscrews.com a call as soon as possible. The only problem I see is with the length of my nut: most commercially available lead screw nuts for 1/4-20 are on the order of .750in long, and mine is limited to just over .200in long. I'll see what the pros say, and if they aren't too discouraging I'll get something put together and run some cycles on it in the old test fixture.

Thanks everyone, I'll provide some feedback as I learn more.

 

[MikeHalloran]

You're basically screwed with that short nut space.

If you can paint yourself out of that corner, there are a couple manufacturers of stainless/plastic lead screw assemblies (Acme and multiple start) that use similar variations; the nut is split like a collet, externally tapered, and fitted with a lightly spring loaded ring that closes the jaws of the nut as it wears. The bad news is that they require more than an inch of axial space for the nut.

You might buy a few and mess with them anyway; there may be some topological transformations available to use space you haven't mentioned.



Mike Halloran
Pembroke Pines, FL, USA

 

[jcrags]

Topological transformations, huh? Do you know where I can get a nut with a fractal thread profile, so that there will be infinite surface area to wear away? Really, no such luck though. The actuator assembly is already in a J configuration, the pulley on the lead screw has been substantially modified, etc. The only space savings I can imagine would be to support the lead screw at only one end, but I don't really want to go down that road. I'll see what I can do with the short length... maybe I can squeeze another 50 thou in there.

 

[MikeHalloran]

Break a Popsicle stick in half, put half-threads on the broken faces, and spring load the halves toward each other, with e.g. brush springs. Of course do the half-nuts in plastic, but the stick idea should convey the geometry. That's all I can think of that might fit in the space.



Mike Halloran
Pembroke Pines, FL, USA