[TowerTalk] Re: Long mast, intermediate bearing

[Chuck Lewis]

Stan Griffiths wrote:

<<much snippage>>
> 
> So I regard this whole issue as something we can't really control and,
> besides, does not seem to matter much in the overall picture.
> 
> Of course, there ARE mechanical schemes that you COULD use to actually
> control the weight distribution between rotator and thrust bearing but I
> have not seen any used in practice.  In fact, every large rotating tower I
> have ever seen has the rotator mounted outside the tower and it only rotates
> and does not support weight.  They seem to work just fine without any "preload".
> 
> My recommendation:  don't sweat it.
> 
> Stan  w7ni@teleport.com
> 

Before we can dismiss axial preload as a factor, we need to look at the
method whereby the rotator acquires its radial (sideways) strength.
This  is important when a rotator sits below a single upper bearing in a
tower, and provides the lateral (radial) strength to keep the mast
vertical. Depending on the design of the rotator bearings, axial loading
may or may not be required to allow the rotator to generate sufficient
radial strength. For example, if the bearing races are conical or flat
(I'm not a bearing designer, so my terminology is suspect) a certain
amount of axial load is required to keep the ball bearings from riding
up the walls of the race when placed under a lateral load. On the other
hand, if the rotator has other provisions for resisting radial loads,
then it's another story. And of course, as Stan points out, when
rotators  are outside the tower and only provide torque, then radial
loads are much less of a factor. 
My bottom line, FWIW, is that without any more info than a maximum axial
load spec. from the manufacturer, we should try to provide some preload
as long as it's within spec. That doesn't mean 'no' load on a thrust
bearing, nor does it mean we have to worry too much about differential
expansion.

It sure would be helpful to hear from the rotator designers; They're the
only ones who know for sure...the rest of us, me included, are just
guessing. Sometimes K7LXC's prime directive is hard to follow! :) 

73,
Chuck N4NM


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