[TowerTalk] Lock nuts revisited

[David Gilbert]

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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