Washer Effect on Bolt Torque
Washer Effect on Bolt Torque
(OP)
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?
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





RE: Washer Effect on Bolt Torque
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.
RE: Washer Effect on Bolt Torque
Yikes!
What happens if the aluminium washer is anodized? This massively reduces the coefficient of friction.
JHG
RE: Washer Effect on Bolt Torque
http://www.neverseezproducts.com/bostik.htm
RE: Washer Effect on Bolt Torque
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
RE: Washer Effect on Bolt Torque
Test it. Easy to do. Then you'll know.
RE: Washer Effect on Bolt Torque
RE: Washer Effect on Bolt Torque
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.
RE: Washer Effect on Bolt Torque
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
RE: Washer Effect on Bolt Torque
RE: Washer Effect on Bolt Torque
RMW
RE: Washer Effect on Bolt Torque
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.