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[Towertalk] structural assumptions// bolts - Orion 2800

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Subject: [Towertalk] structural assumptions// bolts - Orion 2800
From: (Tom Rauch)
Date: Thu, 13 Jun 2002 11:23:40 -0400
> >  anyone with a race car knows it.) Virtually all automotive
> >  fasteners stay tight because of stretch, where the bolt or stud
> >  actually deforms and elongates. That's true even with lug nuts.
>     Interesting info. Which brings up a question I've been looking for
>     the 
> answer to for several years.
>     I've installed numerous M2 Orion 2800 rotators over the years and
> invariably the mast turns eventually due to loosening of the mast
> clamp. As you probably know, it's a couple of massive pieces of steel
> and with six bolts it doesn't seem likely that they'll loosen up but
> they do. 
>     I originally thought the loosening was due to temp cycling of the
>     big 
> clamps that was the culprit. Now I suspect that it's bolt stretch that
> does it. The nuts aren't getting loose - I've used double-nutting AND
> Lok-Tite and the danged masts still work loose.
Unless you are sealing leaks, Loctite is a band-aid for design 
problems. It almost never works anyway, as you found out.

Critical automotive bolts are actually lubricated rather than glued! 
I lubricate lug nuts and connecting rod bolts, and make sure they 
spin free before tightening them to the proper torque (stretch).

You probably are, if anything, tightening the bolts too tight. That 
may not be your doing, it might be a fault of poor materials.

Something is probably being pushed beyond the elasticity limits of 
the materials, either in the bolts, the clamping plates or the mast. 
Whatever material is being deformed does not "spring back" to its 
original physical size or shape.

"Real systems" are generally designed so the bolt stretches, but does 
not exceed the limits of elasticity. Sometimes hardware like conical 
or split-ring spring washers are used to control tension in low-
tension systems, but I would expect the required clamping force to a 
mast (or from a rotor to a rotor plate) to be so high it would need 
special hardened materials that actually stretched hardened bolts, 
and you use a **torque wrench** to set the tension properly.

If the clamp is soft you might try over-sized hardened washers, like 
those used on aluminum cylinder heads, under the bolts and nuts to 
spread the load. Or you might have to "girdle" the clamp with an 
additional plate if it is giving. 

Perhaps the bolts are too soft or short, but my bet is the used a 
relatively soft stamped plate that bends somewhat permanently under 
load, and doesn't allow enough clamping force to be applied to the 
mast. Bolts must be somewhat long in the area kept under load, so 
bolt stretch can suck up the distance that other things deflect. Also 
almost any lockwasher you would use a would make things worse, not 
better, because the lockwasher would push into the metal with time 
and allow the bolt to loosen. If you had to use a washer, it should 
be a very strong split-ring spring type against a very hard washer 
and definitely not a tooth washer, and you should only tighten it 
until it just compresses. You could also use a stack of washers to 
increase tension, or an opposing stack to increase 
distance tension is applied (I do this with very high power 
transistors on soft heatsinks).

Surface shape of the clamp is another problem, the design might 
concentrate pressure in a small smooth area. That would mean the 
installer would have to over-tighten the clamp to prevent slipping of 
the mast. If there was reasonable surface area, some sandy grit 
between the clamp and the mast would help. It would allow use of less 
clamping tension, so things wouldn't permanently deform.

Locktite or any form of locking device is not a cure when clamping 
loads are high. Something is almost certainly deforming or going past 
the limit of elasticity. You can bet it is a design problem, and not 
anything you are doing. Like the generally poor design of the rotor 
mounting bolt systems in many rotors, that require silly band-aids 
like wires or glue.73, Tom W8JI 

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