Locking patch for 4-40 screw 1

[chopper75]

We are using a locking patch on 4-40 stainless screws. The patch is applied by the supplier and has a prevailing torque as high as 5 in-lbs. I find that rather high for a 4-40 screws. We have had some situations where we would engage a few clean threads, hit the patch and strip the threads on the end of the screw because the torque required to pierce the locking patch is too high.

Question is: what is a typical prevailing torque value for locking mechanism on 4-40 screws? I'm thinking of specifying MIN=1 in-lbs, MAX=2.5 in-lbs

 

[3DDave]

It may be that the threads are galling and that causes the stripping. I've seen stainless screws without locking features go in finger tight a few turns and require milling to remove because of galling.

A short search suggests the range you have, but I'd still look at an anti-galling material for such small threads.

 

[tbuelna]

This document provides some recommendations for prevailing torque of screws with a locking patch.

http://everyspec.com/MIL-SPECS/MIL-SPECS-MIL-DTL/download.php?spec=MIL-DTL-18240F.027710.pdf