Durability of plastic ACME threads in lead screw application?
Durability of plastic ACME threads in lead screw application?
(OP)
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
Thanks much





RE: Durability of plastic ACME threads in lead screw application?
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.
RE: Durability of plastic ACME threads in lead screw application?
RE: Durability of plastic ACME threads in lead screw application?
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.
RE: Durability of plastic ACME threads in lead screw application?
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.
RE: Durability of plastic ACME threads in lead screw application?
ballscrews.com of course, not wallscrews :)
RE: Durability of plastic ACME threads in lead screw application?
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.
RE: Durability of plastic ACME threads in lead screw application?
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
RE: Durability of plastic ACME threads in lead screw application?
RE: Durability of plastic ACME threads in lead screw application?
Mike Halloran
Pembroke Pines, FL, USA