Shake proof washer - earth bolt - torque settings?

[Mattylad]

Hi,
It's my understanding that for a protective earth/bonding point that uses a bolt, it is best done (as opposed to one with no washers at all or spring washers) using a shakeproof/star washer and that it should be tightened to a specific torque rather than tightened as if your life depended upon it (see what I did there?).

Tightening up on a shakeproof washer can flatten it completely (removing the tangs from digging into the metals) or deform the washer.

Although I must admit, I have never come across it, can anyone confirm what the torque settings should be for M4,M5,M6, M8 bolts and shakeproof washers?

Or have I got the wrong end of the stick?
If you have links to technical resources with this info that would be most welcome.
Thank you.

[ogden]

Although I must admit, I have never come across it, can anyone confirm what the torque settings should be for M4,M5,M6, M8 bolts and shakeproof washers?

It depends. There are washers with datasheets and torque specified, then there are just washers. In case of the latter use torque specs of the screws you use.

[timelessbeing]

" star washers are not being used under any ground connection. It has been proven by GM engineering that with a star washer installed, it will increase the chance for corrosion and prevent the terminal from achieving proper torque"

https://archive.is/gtsPx

[Monkeh]

" star washers are not being used under any ground connection. It has been proven by GM engineering that with a star washer installed, it will increase the chance for corrosion and prevent the terminal from achieving proper torque"

https://archive.is/gtsPx

GM are not exactly known for quality anything. Also, that's a specifically adverse environment.

One way or another you need to get through paint and oxides.

[WattsThat]

Ah, yes, GM. The same group that gave us a torque specification for self-tapping fasteners:

“Fully driven and seated but not stripped”.

Yep, that’s how I always do it 

[timelessbeing]

One way or another you need to get through paint and oxides.

remove the paint and oxides, and apply anti-corrosive compound.

[bdunham7]

Tighten it until it just snaps off and then back off 1/8 turn.

[timelessbeing]

GM aren't the only ones who think so.

This is NASA reference publication 1228 (March 1990) Fastener Design Manual

[timelessbeing]

A study and paper concluding that "serrated  washer  shows  only  marginal improvement  of  anti-loosening  nature"

http://www.inacomm2013.ammindia.org/Papers/123-inacomm2013_submission_34.pdf

[Mattylad]

Marginal improvement over what though?
While it may only have a marginal improvement in anti loosening (and an improvement is better than none) surely given that a serrated washer digs into the metal either side, it's an improvement electrically?
As for GM's corrosion issues, an electrical control panel etc. does not see the environment that a car does.

I cannot remember ever buying an electrical item that has earth fastenings that do not use a serrated washer so there must be something in it?

[Kjelt]

Since the cable lug is usually pure galvanised copper the washer might penetrate through the layer making it a triple metal contact. In humid conditions this could result in electrolytic corrosion depending on the metals.

In the Rittal cases I have the earth bond nuts have ribs so kind of a washer integrated.
For motion shaking purposes a drop (not too much) of loctite should suffice.

[timelessbeing]

Marginal improvement over what though?

Flat washer or no washer.

an improvement is better than none

that depends on production environment, tolerances, specifications, cost, statistical significance etc.

surely given that a serrated washer digs into the metal either side, it's an improvement electrically?

I'm sure they know how serrated washers work.

I cannot remember ever buying an electrical item that has earth fastenings that do not use a serrated washer so there must be something in it?

Convention isn't proof of efficacy. If it was, nothing would ever change.

an electrical control panel etc. does not see the environment that a car does.

They said  "star washers are not being used under ANY ground connection." Cars have many interior ground connections in a similar environment to your control panel.

Of course there are many variables that affect threaded fasteners. I think the point is that you need to be aware of limitations.

[CatalinaWOW]

I spent an incredible amount of time over my career determining correct torque settings for fasteners.  What I learned is that what appears to be a simple subject has an incredible number of variables and no universal answers.

This situation appears to be one of the more complex due to the susceptibility of various materials commonly involved to cold creep, discontinuous load curves and significant property variability over time, supplier and operating environment.  Things like solder, gold, pcb materials and layer adhesives.

Unless you are willing to commit a lot of time and effort to understanding the details of your application and it's environment, and then another huge batch of effort on process controls ranging from no re-used of hardware, control of details of lubrication, stringent controls on thread forms and surface finish and others you are probably just as well off to use a generic formula and recognize that it won't always work.  This is one of the reasons that many applications have many more ground connections than required, hoping they will not all fail.  And it isn't a vain hope, several of the failure sources are partially or completely independent.

Which approach you take will depend on your reliability requirements, warranty provisions and ability to screen for those loser cases.