[TowerTalk] [Fwd: Re: Lock nuts revisited]

[Roger (K8RI)]

-------- Original Message --------
Subject: 	Re: [TowerTalk] Lock nuts revisited
Date: 	Tue, 24 Jun 2008 00:08:53 -0400
From: 	Roger (K8RI) <K8RI-on-TowerTalk at tm.net>
To: 	Doug Renwick <ve5ra at sasktel.net>
References: 	<050c01c8d58b$457a6a00$ed68a58e at doug4>



Doug Renwick wrote:
> Roger,
>
> Can you quote a source which would confirm your application of a jam
> nut?
>   
Just years of experience working in industry and I make no claim to a 
proper interpretation as it seems to be one of the most widely used and 
misused terms out there.  In my application I'm talking about normally 
using one nut to keep another from turning which I think is accepted. I 
think the confusion comes particularly when talking about something like 
rotator bolts that we want to keep from loosening where many of us, me 
included, have learned specific or sometimes broad definitions of the 
term, jam nut.  Until researching the term and on here I had never heard 
of the "half thick" jam nut, but it does make sense.
> I Goggled a number of sites that describe the application of a jam nut
> and here are what I found.  The only purpose of a jam nut is to prevent
> nuts from loosening.
>   
That is basically the way I see it, but maybe a bit more broadly.
> Doug
>
> >From Wikipedia:
> A jam nut is a type of locking nut that avoids compression of the part
> being fastened. It is commonly used to fasten parts that must be able to
>   
This is the part where in industry we had diverged and I think the nut 
on the bolt in the base of a rotator is a bit of a cross between 
several.  The nut on the bolt stuck through the rotator plate should 
serve as a locking nut as tightening it only exerts a linear force on 
the threads in the base of the rotator which adds a good deal of 
resistance to turning the bolt without actually turning or "torquing" 
the bolt in. It can lock the bolt to the point where it can not be 
turned out without loosening the lock nut.  I'm hesitant to use the term 
"lock nut" as that usually has a specific meaning. In this case 
tightening the nut snugly will "usually" add quite a bit to the amount 
of torque required to turn the bolt.  Sometimes enough that the head of 
the bolt can be twisted off if the nut is not loosened before trying to 
remove the bolt.

I think you will find that most any one can do this as an experiment, 
but the results do depend greatly on technique (and materials). Tighten 
a bolt through a rotator mounting plate into the rotator to the proper 
torque. Note the torque required to remove the bolt.  Then replace the 
bolt with one that is longer with a nut placed well up on the threads. 
Turn the bolt in by hand until it stops and then back it up at least one 
turn. (This is to protect the threads in the rotator from differential 
expansion between the bolt and rotator material - probably not important 
for the experiment). Now tighten the nut down onto the rotator mounting 
plate, again to the proper torque.  Now measure the torque required to 
back the bolt out without loosening the nut.  With possibly the 
exception of lubricated, cad plated bolts the torque required to remove 
the bolt without loosening the nut should be substantially more than the 
torque applied to the nut.  The reasoning I was given is (in this case) 
the material in the rotator base is slightly malleable and will stretch 
with the linear pull of the bolt caused by the nut. This supposedly 
causes full or nearly full engagement of the edges of the bolt threads 
against the mating threads in the rotator base and that greatly 
increases the torque required to turn the bolt which should prevent the 
bolt from loosening.

One thing pointed out farther down is the sequencing of the lock nut and 
jam nut. If done improperly it can render the whole thing ineffective.
> spin freely, such as bearings. It looks like a thin hex nut, but half as
> tall. It is threaded inside of and tightened against a full-thickness
> nut. This puts opposing tension on a localized segment of the screw
> thread, preventing either loosening or tightening of the pair.
>   
In industry we just used full thickness nuts.
> <snip lots of good material for space>
> The idea of a jam nut is to cause the two nuts to press on the threads
> in opposite directions, so that the thread is stretched between them,
> and this stretch is not relieved if the nuts should turn slightly on the
> bolt, so that unscrewing is discouraged.
This is the use to which I was referring. Farther down is a good 
explanation as to why technique is so important. Improper sequencing can 
completely eliminate the desired effect.
>  If this condition does not
> exist, and it usually will not be if the thinner nut is on top and
> tightened second, then the top nut and after it the lower nut can loosen
> without restraint, and there will be no locking action. Two nuts in this
> case are not much better than one. The proper connection is made by
> tightening the jam nut snugly first, then tightening the upper nut so
> tightly that the stress on the jam nut is reversed as the bolt strains.
> The two nuts could be the same thickness, but it saves space if the
> lower one is thinner. The thinner nut must counter only part of the
> compressive force of the regular nut, which does double duty. It is
> surprisingly difficult to find illustrations of a jam nut used properly,
> or torque specifications. This is also a matter that is of no interest
> to the academic engineer, who only talks about locknuts and does not
> actually use them.
>
>
> References
> T. E. French, A Manual of Engineering Drawing for Students and
> Draftsmen, 5th ed. (New York: McGraw-Hill, 1935).
>
> T. E. French and C. J. Vierck, Engineering Drawing and Graphic
> Technology, 11th ed. (New York: McGraw-Hill, 1972).
>
>
> We have seen both variations of jam nut installations but only one can
> be correct. 
> When a nut is torqued a deformation of the bolt threads take place. You
> can visualize that one nut, fully torqued will tend to deform the
> threads upward.
> Now a half of a nut (the jam nut) will have less overall force than a
> fully torqued full nut. So if the full nut is placed on first, the
> threads are deformed upward. The jam nut, placed on top will bear
> against the full nut, tending to reverse the deformation. However since
> the full nut applies more force, the thread deformation under both the
> full nut and the jam nut is in the same relative direction.
> If the jam nut is torqued first, the threads are still deformed upward.
> But when a full nut is fully torqued, the bearing against the jam nut
> will tend to reverse the thread deformation. The threads under the full
> nut are deformed upward, while the threads under the jam nut are
> deformed downward. This thread deformation reversal provides the locking
> mechanism. 
> We have seen two full nuts used in an attempt to provide locking. If
> both nuts are torqued to the same value, this is the worst of all
> worlds. Theoretically the bottom nut will impose no deformation on the
> thread leaving it useless.
>
>   
The point is, the nut does work and prevents the bolts from coming loose 
in the rotator housing.
I've never had one come loose yet, but even with all the antenna work 
I've done over the years, that number is not a statically significant 
sample and can serve only as an individual reference point.  Quite 
possibly I've used the terms jam nut and lock nut improperly, thus 
confusing the issue.

Roger (K8RI - ARRL Life Member)
www.rogerhalstead.com
N833R (World's oldest Debonair)




-- 
Roger (K8RI - ARRL Life Member)
www.rogerhalstead.com
N833R (World's oldest Debonair)