torque calculations for screws into base material

[windwardtack]

I'm a newbie here, so be gentle.

I've read through the Machinery's Handbook and other texts I can find. My eyes have rolled into the back of my head and my head now hurts.

I would like to find a document that takes into consideration screw material,"base" material screw is going into, screw size and a few other attributes. For example;
18-8 stainless steel 6-32 screw into 6061 extruded alumin heat sink with a thichness of 0.125", no lubrication and roll formed threads in the aluminum. I'd like to be able to calculate the reasonable amount of torque at which the screw will not yield and the threads will not pull out of the aluminum. Or in other words... never fall apart.

And when I run out of things to do I can maybe build an estimator using the necessary formulas in Excel or other spread sheet programs for future assembly applications.

I want to avoid differential equasions, calculus and other complex math.

Can anyone point me in the right direction?

 

[CoryPad]

faq725-536

thread725-35222



Regards,

Cory

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

 

[EHudson]

You’ve no doubt seen stripped out threaded holes. The internal threads are usually still wound around the external threaded member. The area sheared off in the stripping is roughly equal to the circumference of the stripped out hole times the length of the thread engagement. You can improve upon the approximation by reducing this area about 20% for the root radius portion of the internal thread that wasn’t bearing load.

So you can approximate the amount of force required to strip the threads as follows:

Pi x thread OD x screw engagement x SHEAR strength of the material x .8.

Note: The shear strength is not the tensile strength. If you lack a published number you might use 60% of the ultimate tensile as a crude guess.

Screw force is often approximated by dividing the screw torque by both the screw OD and a friction factor, (around .2; published values vary greatly. Your friction factor will vary.) Approximate Force = torque/(screw dia. x 0.2)

Remember these are approximations. Good luck.