Torque Calculations

This might help....

http://www.plasticsintl.com/sortable_materials.php?display=mechanical&sort=dynamic_coefficient

http://www.sdplastics.com/phenolic.html

http://k-mac-plastics.com/data%20sheets/mds_filled_nylon_typical_properties.htm

You're right about the info you need being tough to find.

I tried for "Bakelite" and lucked out.

You might just have to experiment with different torques and fastener/tap sizes yourself and come up with your own table.

The torque calculation I'm familiar with, for steel nuts and bolts, uses a design bolt stress/load, and assumes 10% of applied torque goes to producing bolt stress, the rest being lost to friction.

Maybe for bolts in plastic the coefficient of friction is different, meaning the 10% changes. But if you base the load on the bolt characteristics for metal bolts, you're likely to strip the plastic thread before you reach calculated torque, unless the thread is very long.

It might be better to do it by trial and error and a bit of judgement. After you've stripped a few you'll begin to get a feel for it .

If you experiment as Wal suggests, the only way I can think of is to torque to stripping point, and use a safety factor of say 0.5 to get working torque.

i try to find it on the web, but there is not the values.

google it ,maybe you can find the values.

Geez, I had thought by now that most people understood that using coefficients of friction with the ultimate goal of achieving "properly tightened" fasteners is nothing more than an engineered crap shoot

Whenever one uses any friction coefficient in a torque calculation, one "guesses" that this will be applicable in all cases. Just ain't so...

"Torque", the Misunderstood Process

That was a good link.

The case for finding alternative methods for determining fastener loading was nicely explored.

Like it said: "Torque does not and cannot indicate how "tight" a bolt is! Torque is only a measurement of the degree of resistance encountered when trying to turn a nut (or a bolt)...."

Like it goes on to say, elongation is the only real way to tell if a bolt is properly loaded.

I wouldn't expect much elongation for a metallic bolt being screwed into a plastic though....hmmm.

I wouldn't expect much elongation for a metallic bolt being screwed into a plastic though....hmmm. Exactly, that's what I was getting at in #3

Right you are mate.

He'll just have to suck it and see.

In this case, he'll be relying on the spring characteristics of the flange rather than the bolt.

What case? What flange?