Torque specs on dissimilar fasteners

[finley31]

We are currently in a dilemma on deciding the correct torque for a 316ss 3/4"-10 unc bolt threaded into a block of 6061 aluminum approximately 2" deep. There is some confusion as to what the correct torque spec would be when two dissimilar fasteners are mated.

I have searched and cannot find any standards on this topic. Would you use the torque rating for a aluminum fastener or would it be higher? Also, if a threaded locking insert/helicoil was used would that change the situation any, other than just reducing the chance of galling/stripped threads? I should also note that this is an electrical connection.

Thanks

 

[rb1957]

i'd use the Al thread as critical ... i'd use a helicoil so that the thread dia is bigger in the Al, particularly if this is loosened and tightened in service (as opposed to a one time "fire and forget" torqueing)

 

[SnTMan]

"Machinery's Handbook" he said again.

 

[CoryPad]

You need a certain clamping force. That force is dependent on the application, including the stripping strength of the tapped hole material. You plan to generate the clamping force with an applied torque to the fastener. The relationship between force and torque includes factors for geometry and friction coefficient. You can use equations for both of these, and the Machinery's Handbook is one source for those equations.

 

[Gorpomon]

You don't mention what joint tightness you need. When that's not mentioned I assume user will want 75-85% bolt yield. Balance that with the bolt load and torque derived from the thread stripping strength and go with the lower installation value. Bickford's "Design and Behavior of Bolted Joints" will have thread stripping strength equations. It's also up to you to determine how to calculate torque from bolt load. I haven't looked at the method's in machine design manuals, but just be sure for both bolt yield and thread stripping strength you use the same calculation to get torque.

 

[Tmoose]

For charts of what UNBRAKO felt in 1996 were reasonable tapped hole thread strength to consider for a quality socket head capscrew into brass, aluminum, cast iron and mild steel, look at page 66/68 here -

http://www.dalecompany.com/pdf/UNBRAKO.pdf

Your stainless bolt will likely not tolerate SHCS torque, so could "get away" with less engagement.

Note their comment on page 64/66 about using figgerin' and cypherin' to come up with tapped hole thread strength being "not entirely satisfactory". Thus what they chose to present in the technical section of their "engieering guide" were tables created using empirical data from tests done with non-digital threaded test pieces, real bolts, and torque wrenches of some type.

As others said, depending on what you are securing, there is a minimum torque you need.

If loosening is much of a concern, I'd attack it with adequate general purpose torque, and making several diameters of bolt "grip length" available, and making the clamped component "thick enough" and nice mating surfaces .

 

[chicopee]

SnTman said it "Machinery Handbook" which has a section on dissimilar threaded materials and which gives formulae for lengths of engagement and torque.

 

[desertfox]

hi

Try this site:-

http://www.roymech.co.uk/Useful_Tables/Screws/Thread_Calcs.html

 

[finley31]

Thanks for the help everyone. The section in machinery's handbook on thread length engagement proved to be very helpful for us.