Torque Settings for differing materials

[KeninCA]

I'm torquing a stainless steel 4-40 socket head machine screw into a 6061-T6 aluminum threaded hole.

I've located some torque tables that give differing values for different screw materials, but I'm wondering how this applies since I'm going into a threaded hole of a softer material.

We've chosen to use torque values as a manufacturing standard, and I don't want to over-torque what the aluminum threads can handle.

We are using a small amount of blue thread-locker, which may act as a lubricant during torquing.

Does anyone out there have experience in this application?

 

[drawoh]

KeninCA,

I use the standard torque equation of T=CDF, where D is the screw major diameter, F is the clamping force, and C is a friction factor. C=0.15 is a popular value for lubricated joints. I have had no problems using this.

Stainless steel and aluminium are high friction materials, and you should be cautious about standard assumptions. This is an excellent case for the lubricating thread-locker.

If you have torque tables, it is worthwhile to work out how they did the calculations. Did they assume a lubricated joint? What yield/proof stresses did they assume?

Are your torque tables for stainless steel screws into aluminium? 18-8 stainless steel work hardens and it is often annealed, so stress values can be all over the place. One of our vendors claims that their stainless steel cap screws have a yield stress of 55,000psi. That is the lowest value I have seen, so I use it.

I hope you are aware that most threadlockers attack the plastics used on most electronics components.

JHG

 

[israelkk]

Before using any torque value you need to calculate the pull strength of the thread in the 6061 aluminum. It depends on how deep is the active thread and on the length of threaded part of the screw which mates with the threaded aluminum.