Accepting Roger's empirical results at face value, how about the
following hypothesis:
I don't know for sure, but I suspect that most of the friction that hold
a threaded bolt in a threaded hole is the friction that occurs at the
interface between the bolt head and the exterior surface of the material
that has the hole. I'm sure there is some friction between the threads
of the bolt and the threads of the hole, but it seem to me that when the
bolt is tightened down, you can't count on a high percentage of the
thread surfaces actually touching each other with appreciable force.
The holes are typically tapped for a loose enough fit to install the
bolt by hand, and the tolerances probably result in only a portion of
the thread surfaces actually tightly binding.*
Possibly even more relevant is that the bolt head has a larger diameter
than the bolt, so simple leverage accentuates the holding capability of
the head friction compared with the friction of the threads.
Now then, if I turn a bolt with a head into a threaded hole, the
rotational force I apply to tighten the head against the material
surface is lessened by the friction of the threads along the entire
length of the bolt. If instead I install a threaded rod and then
tighten down a nut against the surface of the material, virtually all of
the force I am applying goes toward creating a binding friction between
the nut and the material surface.
I can even apply a greater rotational force to the nut since I don't
have to worry as much about the threaded rod shearing as I do with the
bolt. Let's say the bolt and threaded rod are each capable of (purely
for the sake of example) 100 foot-pounds without shearing. If I exceed
that with my wrench on the bolt, the bolt head twists off. If I exceed
that with the nut, the nut just goes on tighter (unless I freeze or
strip the threads).
The jam nut, as many others here have stated, merely creates the
equivalent of a bolt head out of the first nut after it has been tightened.
Just some musings ...
73,
Dave AB7E
* Unless of course you use a stainless steel bolt, in which case every
single surface is virtually guaranteed to bind and freeze ;)
Steve Hunt wrote:
> Roger,
>
> I'm no mechanical engineer - just a lowly communications engineer - but
> I'm trying to understand the point you are making:
>
> Roger (K8RI) wrote:
>
>> When that nut is tightened it
>> pulls the bolt straight out against the threads in the rotator, or
>> objects base which adds force that adds to the torque required to remove
>> the bolt with out torquing the bolt down tighter.
>>
> Isn't this exactly what happens when you tighten the head on a
> conventional bolt? You turn the head but it can't move forward (because
> it's hard up against the washer or whatever) so it pulls the bolt
> against the threads. I would have thought that for the same thread
> pitch, the same torque on the nut or the head would result in the same
> "stretch forces" on the bolt. The jam nut simply locks the first nut to
> the bolt; so I still don't see that the situations are any different.
>
> Perhaps I'm "missing the obvious"?
>
> 73,
> Steve G3TXQ
>
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