Jam Nut Torque Value

[toes112]

I have not been able to find much information regarding torque values applied to jam nuts, so hopefully this discussion will share useful knowledge.

Situation: I have two pieces of material that rotate about each other via a shoulder bolt. The shoulder bolt gets torqued to 55 ft-lbs into part "A", and part "B" rotates on the shaft of the shoulder bolt. The current design has a jam nut applied to the back end of part "A" to prevent backing out.

The orientation of components is:
Jam Nut--Part "A"--Bearing--Part "B"--Shoulder Bolt

Currently the jam nut is also torqued to 55-ft-lbs as well, but we are failing the threads inside the jam nut 40% of the time.

Should the jam nut be torqued to a percentage of full load? If so, how is this calculated?

Note: A jam nut is being used as opposed to a standard nut due to clearance issues.

 

[paddingtongreen]

If you are torquing the shoulder of the bolt against Part A, you shouldn't need a jam nut. The function of the jam nut is to have something for the regular nut to be torqued against when it is not tightened up on the substrates. Torquing the shoulder against Part A is the equal of tightening up on the substrates.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[rmw]

You don't say what size the fastener is so it is hard to tell if 55 ft-lbs is a lot of torque or too little torque.

60% of yield is what most of our torques are based on.

I personally wouldn't want to depend on shoulder jam torque to assure bolt tightness in a rotating moving part situation.

rmw

 

[toes112]

Shoulder bolt and jam nut are 1/2-13 thread.

 

[DLite30]

Jam nuts or double nuts are generally more complicated than what you would think, and I hate using them, or seeing them being used.

See this link for some helpful guidance, or maybe an alternate idea. Section 5.3.4

http://www.hnsa.org/doc/nstm/ch075.pdf

 

[dinjin]

What grade are the fasteners,nuts, and jam nuts?
That is a lot of failures. You also might want to
search on this site for the use of jam nuts.

 

[Tmoose]

Is the jam nut failure the only failure, and does it occur at assembly?

I think a 3/8 inch bolt torqued against a sleeve with 1/2" OD and a well controlled length might be more reliable.
How does the bolt interface with the support structure or housing?

 

[dinjin]

If these are indeed grade 5 quality, I would use the ratio of the heights times the torque value for the jam nuts as a maximum torque value.

 

[kellnerp]

You are using a "jam nut" as the main and only load carrying nut and then wonder why they are failing?

A jam nut should be torqued to 1/2 to 2/3 of that of a regular nut based on the thickness of the "jam nut". The 55 ft/lb torque is probably on the high side for a regular 1/2-13 nut, especially if lubricant is used.

But stand back and ask yourself why the nuts are failing. It is because the axial load on the threads is higher than the failure load for the nuts. Therefore, you either have to change the design.

Fastener strength should always be tested in the lab and not on assemblies that are shipped to customers IMHO.

TOP
CSWP, BSSE

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