acme thread wear issue

[GlennD]

Hi all

On a design that been used for 15 years or so, both in stainless and mild steel, witout any problems, has developed severe wear on the stainless version shortly after assembly. we have what is basically amd acme thread jack. And these are on machines built and sent to market. The stainless ones seem to stick and dig wearing the threads and leaving small chips and burs. This never happened before. The unit consistes of inch 8 thread rod, and a weldment that contains a tube with an acme nut welded to the end and turned down to blend with the OD of the tube. the tube sort of guides on the flat OD of the inch 8 t-rod. Everyone seems to be at a loss to explain It Including the supplier since this was fine before

*The plain steel version still works fine. I had noticed some small diferences on manufacturing which were eliminated and now the parts are apparently the same but still the stainless version has problems. I am suspecting some thing changed in the material used. Which is 303 for the rods amd we have *samples of 304 and 303 *for the weldment both acting the same way. After a few cycles after assembly the threads on the ID have sharp edges and burs. anyone have any idea on what else we can check for?

[jboggs]

Whenever I have run into this kind of problem in the past it usually gets back to a difference in the actual chemistry or treatment of the steel. You might submit a sample to an independent lab for analysis. If you have any of the of older more reliable screws around, you might submit one of those at the same time for comparison. You could also try samples from a different supplier. Can you set up a test sequence to simulate the conditions to try to duplicate the failure internally?

I have some very painful memories of watching specially built samples of trailer jacks fail miserably one after the other at a potential customer's site after all our internal trials had been very successful. Turned out the customer test units were made from a different heat of the material at the steel mill.

[Kelly_Bramble]

I agree with jboggs, this a base material problem...

[RWOLFEJR]

Your first screws that worked well were most likely made from one of the 400 series stainless steels... A martensitic grade that is heat treatable.

[GlennD]

Thanks guys. I will have them look closer at this. Its very apparent that there is a problem just putting the parts together by hand you can feel the diference between the steel and stainless

[GlennD]

Client made parts out of 303 304 and some 400 series materials and in all cases the shafts show galling in a short time. I'm not there anymore so I can't describe it any better than that. They made another unit up where the threaded guide tube was hollow but put Poly Acetyl (POM) bushings in, one end with about an inch of thread and the other with a POM plain bushing. It's obviously not eating the shaft now but they asked my opinion and I don't know how to apply the shear strength spec for POM reported to be 150-500PSI (that's a huge variable) to the unit to gauge if it's even strong enough. Another concern is that this POM material dissolves in bleach which is common to use in food places as a cleaner and while these units never come into contact with food or are used in those kinds of areas I can see where a user might use the same cleaning chemicals inside and outside of a clean area just because it's available. How much of a problem could it be?