What is the most likely reason for the wear on the bolt's thread? 3

[rollingcloud]

The supplier informed us that the test bolt threads are experiencing wear during a reusability test for a self-locking nut. They observed a change in thread dimensions after just 2-3 cycles, along with increased heat generation. The nut is silver-plated and waxed, and it is crimped to create the self-locking feature. Both the nut and the test bolt are made of Waspaloy N07001, with hardness measurements of ~32–34 HRC.

https://imgur.com/a/mwK6fKf

This is a standard nut that is part of the legacy parts, so deviation from the specification is not allowed. I am trying to understand the issue and help the new supplier address it. The supplier passed the test using a test bolt that is reduced by 0.003’’ from class 3A tolerance and is proposing that we accept this reduced tolerance deviation. Obviously, we can’t agree to this, as class 3A tolerance is very common in the industry. I have only seen reduced test bolt tolerance used when the actual mating bolt in the assembly has a reduced tolerance.

I suspect the wear might be caused by the silver plating being too thin, a lack of lubrication (oil) between cycles, or—less likely—torquing too fast or over-torquing. The test bolt thread is also unnecessarily long. Since the prevailing torque of the first cycle was acceptable, the locking feature seems to be functioning properly, so I don't believe over-crimping is the cause. What do you guys think
Test data:

https://imgur.com/a/jbNyNSC

 

[TugboatEng]

Does the assembly need to be reusable? Was that part of the original specification? Perhaps you should consider replating the nuts between uses.

 

[3DDave]

A similar situation I was informed of was resolved by taking the bolts over to the buffing wheel and polishing the threads. This removes the highest points that tend to gouge the mating coating and removes the points of extremely high contact pressure that will remove any oxide on the bolt, allowing galling/cold welding to take place.

Silver is very soft so making it thicker just increases the interference fit and increases the contact pressure to wear through anyway.

Give it a try - the cost is minimal to check it out.

On a similar situation involving a Teflon seal a visit to the polishing operation turned a 10 hour to failure into several hundred hours of operation. My company didn't believe in follow-up with engineering - they just stop calling when the problem is resolved and, they stopped calling.

 

[dvd]

Somebody put thought into developing your specification. When I looked at my copy of IFI 100 (not sure if it is still current) it lists the following, obviously not steel, but it does raise some questions of how this testing is being conducted -

 

[EdStainless]

1. Your bolt must be harder than your nut.
The HT should be adjusted to get you at least 3-4 points HRC between them.

2. I agree with Dave, polish those threads.
Also make sure that they are well cleaned between test cycles.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[rollingcloud]

Tugboat,
The nut needs to be reusable; the test assembly is only used for a single test run (15 cycles). I don't believe it's meant to be re-plated; the test procedure only allows oil lubrication in-between cycles.

 

[EdStainless]

Wipe clean and re-lube between cycles.
But a difference in hardness will help a lot.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[rollingcloud]

Thanks Dave, definitely going to try that first.

 

[rollingcloud]

dvd,
I am not seeing a direct correlation between the proof load test and the wear on the thread.

 

[rollingcloud]

Thanks Ed, the actual mating bolt is made of Inconel 718 with a min hardness of 36 HRC.
I wonder why they allow the nut hardness to be 32 - 44 HRC and the test bolt to be 32 - 42 HRC in the first place.

 

[mfgenggear]

What this again.
What no the threads appears to have galling, and damage. Profiles has been damaged. I don't belive buffing will correct this issue.
The contact stress has been compromised.
Surface finish has been damaged.
Question I have if these are most likely
Rolled threads. It work hardens at the surface.
My question why are Penny pinching bean counters trying to save low cost parts.