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Jam nut first or last.

Jam nut first or last.

Jam nut first or last.

Does anybody know what should be installed first, jam nut over regular nut or the other way around?  I can not find any standards regarding this issue.  Any help will be appreciated.  Thank you.

RE: Jam nut first or last.

Thats a good question. Its been around for a long time and the instinct to tighten with the main nut and add a lock nut at the back of it, seems to rule everywhere, including as I have seen, slue rings on lifting cranes, etc. I understand this instinct is wrong, when you consider logic and Ajax Fastener Handbook. First, the lock nut, then the load carrying nut, tightened up the main nut to torque value and then back off the lock nut. The instinct way means the lock nut carries the load because it is pushing foward the main nut off the thread.

RE: Jam nut first or last.

Jam nut last.
It simply holds the load
nut in place and it only
stretches a short portion of
the thread maybe from the 1/3
from the center of the load nut
toward the jam nut and only sees
static load conditions.

RE: Jam nut first or last.

Having recently researched the issue of fastener integrity and was surprised how little logical information is available (compared to welding integrity)- I did come across this extract apertaining to use of Jam/Lock Nuts


There are two common usage’s of this term:

1.    A nut which provides extra resistance to vibration loosening by either providing some form of prevailing torque, or, in free spinning nuts, by deforming and/or biting into mating parts when fully tightened.

2. The term is sometimes used for thin (or jam) nuts used to lock a thicker nut. When used in this way the thin nut should be adjacent to the joint surface and tightened against the thick nut. If placed on top of the thick nut the thin nut would sustain loads it was not designed to sustain.

Jam nuts. These nuts are normally "jammed" together as shown in Figure 7, although the "experts" cannot agree on which nut should be on the bottom. However, this type of assembly is too unpredictable to be reliable. If the inner nut is torqued more tightly than the outer nut, the inner nut will yield before the outer nut can pick up its full load. On the other hand, if the outer nut is tightened more than the inner nut, the inner nut unloads. Then the outer nut will yield before the inner nut can pick up its full load. It would be rare to get the correct amount of torque on each nut. A locknut is a much more practical choice than a regular nut and a jam nut. However, a jam nut can be used on a turnbuckle, where it does not carry any of the tension load.

Hope this helps


RE: Jam nut first or last.

This is a question that has bothered me for a long time!  I attended a bolted joint seminar a few years ago and the instructor stated that the jam nut goes on first, then the heavy nut.  I have pondered this ever since and can't come to a satisfactory understanding as to why.

Looking forward to some more responses!


RE: Jam nut first or last.

This was the subject of a previous thread:


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