Washer Effect on Bolt Torque 2

[Sparweb]

In the application I'm looking at, the specified bolt torque is being debated. The bolt is turned into a barrel nut, and under the head of the bolt it is specified that an aluminum washer must be used. I have access to tables and charts that would give the required torque if everything in the system were cadmium-plated steel, but the aluminum washer throws up a question mark.
Does anybody know if the washer material matters much when torquing a bolt? If so, do you have any data that backs this up?


STF

 

[CoryPad]

Yes, the washer material affects the results when torquing a bolt. During torquing, a reaction torque exists between the bolt and the washer. The magnitude of the reaction torque is a function of the friction coefficient. The friction coefficient is material dependent. Bare aluminium has a higher friction coefficient than cadmium-plated steel, so you will need more input torque to develop a target preload.

Data can be found in VDI 2230 Systematic Calculation of Bolted Joints available at

http://www.beuth.de

and Handbook of Bolts and Bolted Joints available at

http://www.dekker.com

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[drawoh]

CoryPad,

Yikes!

What happens if the aluminium washer is anodized? This massively reduces the coefficient of friction.

JHG

 

[arto]

Use Never Seez

http://www.neverseezproducts.com/bostik.htm

 

[Sparweb]

arto, I didn't ask for commercial advertising.

I realized (only after typing) that it would be in Mark's Standard Handbook of Mech. Engineering. Aluminum on Mild Steel has a static friction coefficient of 0.61, and sliding, it's 0.47. That applies to bare materials, so a cad-plated steel bolt on an anodized aluminum washer is really up in the air.

If we really need to nail down the difference in torques, we'll have to do tests. Perhaps sticking with steel washers is best!


STF

 

[MintJulep]

Spar,

Test it. Easy to do. Then you'll know.

 

[arto]

Not advertizing - that's the greatest stuff since duct tape & WD-40 ;-) Most torque spec's are for a "Well lubricated" bolt & nut - probably want to lube an aluminum washer to prevent galling.

 

[CoryPad]

SparWeb,

FYI, the data you state are for vacuum conditions. Also, a single value with two digit precision is nonsense unless you are recreating the same test (same material, temperature, humidity, etc.). For plated steel rotating against bare aluminium, I think the friction coefficient will be between 0.15 and 0.25.

drawoh,

I don't have any data for anodized aluminium. I think the friction coefficient would be between 0.10 and 0.20, which isn't that different considering that friction varies widely.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Sparweb]

Vacuum conditions? I guess 15,000 ft altitude doesn't qualify!

No offence taken, arto.

This discussion is opening up my eyes to a subtle kind of problem that can sneak into aircraft structures. In my brief five years of poking around airplane and helicopter innards, I've seen all sorts of fasteners used, and often, little consideration is given to the type of washers that get installed, and as long as the right bolt is used, the inspectors pass it.

Now, we've illuminated situations where using an aluminum washer instead of a steel one will increase the fastening torque. If the joint was designed with a steel washer, and an aluminum washer is used, then the pre-tensioning in the bolt is too low, and the joint experiences reduced fatigue life, perhaps can even "separate" under normal loads. The contrary situation, using a steel washer where an aluminum washer is specified, creates a joint where too much pre-tension is applied and working loads could exeed the tensile strength of the bolt!



STF

 

[bvi]

Another possible issue with aluminum washers is their relative softness. Through hardened washers are prefered, to avoid potential gauling which can consume much of your bolt torque.

 

[rmw]

Another troublesome characteristic of aluminum is cold flow, where the aluminum literally "creeps" out from under a pressure joint. Many an electrical fire has been caused by aluminum cable connections becoming loose after initial tightening, due to cold flow. Electrical people know the routine of repeatedly having to go back and re-torque electrical connections when Aluminum cable is involved. The washer should be expected to act the same.

RMW

 

[diamondjim]

Forget the Aluminum Washers unless
they have special large ODs to overcome
the bearing stresses applied by the
steel bolts. If you use steel washers
I would expect the same thing would
apply ie, use larger ODs as above.