Does washer diameter impact friction in a bolted joint? 3

[CB77]

Consider a clevis with a bolt going through it, with an elastomeric washer on both sides to prevent rotation.

In terms of maximizing the friction, I don't think the washer's diameter matters because friction doesn't care about surface area (assuming a constant pre-load from the bolt and that the washer is always completely covered by the bolt/nut heads for even clamping).

The counter-argument is that with a larger diameter washer, the resistance to rotation is greater since it's a greater moment (the friction is acting further away from the axis of rotation). But surely that's countered by the frictional force being lower throughout making this a moot point?

Who's right here?

 

[3DDave]

Various simplifications make the calculus trivial, but without exact dimensions and material properties, those simplifications are meaningless.

It's a surprise if the integrating dTorque = P*2*pi*r^2*dr from inner radius to outer radius is difficult; noting that P = Clamp force/Area

However, you have a piece of weak plastic in an airspaced joint, so P will not be uniform and the simplification won't give the answer. For that you'll need some FEA modeling.

It makes little sense to worry about the mathematics on a poorly defined problem.

Maybe you can say what rotation you are trying to prevent, what you think causes that rotation, why it's important to prevent it, why the elastomeric washer was chosen as the only solution, why it matters what the diameter is, ...

Edit: Forgot to include the lever length; aka radius.

 

[3DDave]

drawoh,

That's mostly welding and welding problems are not bolt problems. I see sometimes people ask about threaded fasteners, but it hasn't got much visibility for them.

 

[dvd]

There is some reasonable discussion, based on Shigley textbook, in this set of slides -

http://me.utep.edu/cmstewart/documents/ME3334/Lecture 17 - Screws and Bolts.pdf,

starts around p. 50

There is an integral shown, but the simplifying assumptions are provided afterward.

 

[MintJulep]

@3DDave

I'm inspired to suggest adding a new forum - bolts, screws, nuts, and washers. Collect these questions in one spot

There is forum725 which over the years has been the place for "nut and bolt" questions.

 

[electricpete]

> In terms of maximizing the friction, I don't think the washer's diameter matters because friction doesn't care about surface area (assuming a constant pre-load from the bolt and that the washer is always completely covered by the bolt/nut heads for even clamping).

I'd like to examine that assumption bolded above.

I assume it's the case that a torque is specified or at least there is some qualitative judgement of torque by the assembler. We rely on that to establish the preload.

I wonder if the size of the washer can affect what fraction of applied torque transfers into preload. I don't know the answer, just asking the question.

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(2B)+(2B)' ?