[TowerTalk] Fwd: [Bulk] Re: Top Bearing Plate for 55G vs Rotor Plate

[Roger (K8RI) on TT]

On 8/15/2014 12:44 AM, Hans Hammarquist via TowerTalk wrote:

when it comes to top bearings and bearing plates, the basically have a 
lateral load, not vertical. Most rotators , even the Ham series, will 
handle far more weight than a 4 or 5L 40 meter Yagi and are designed to 
work with a vertical load.  OTOH I'd not want to use any rotator with a 
wedge brake and spur gears that can be turned by hand with anything more 
than a tribander.  Yes, I know there are many out there doing just that, 
but will they still let you work when the wind is gusting 40 mph plus?  
Most rotators with a dual worm-gear drive will and you don't need a 
brake either.

Rotators designed to run with weight on them should not have a thrust 
bearing supporting the load! With different expansion coefficients, it's 
possible to have the hast lifting the top of the rotator instead of 
supplying the needed weight.

Better a top bushing serving as a lateral thrust bearing rather than one 
that rigidly holds the mast.  If the bushings/bearings are 10 feet 
apart, binding will rarely be a problem and alignment ts relatively easy.

If the weight is a bit much for the rotator, it'd be better to let the 
weight be supported by a bearing near the base of the mast and drive the 
mast with a heavy gear, worm gear, or industrial chain designed for that 
load.

The proper design of a mast and rotator for a large weight and wind load 
is not a trivial matter although we often through something together 
"that oughtta do it!".  There is give, or spring, plus slack in any 
drive train.  The longer the mast, the more it acts like a torsion 
bar/spring.  With long masts and many rotators this can easily match the 
natural resonance of the mechanics.  When that happens, the strain is, 
or can be multiplied many times, leading to the destruction of rather 
substantial components.  That we don't see this happening often is we 
don't have that many "large systems" and unless they are well known, we 
probably don't, or won't hear of the failures.

73 and be careful,

Roger  (K8RI)


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