Often we get by in spite of what we do, not because of what we do and
this is probably more true concerning antennas, towers, and rotators.
Thrust bearings that I've seen were designed to run in the manner they
would probably be used, not the way they should be used. They were also,
according to ROHN in the older catalogs, designed to run *DRY*. That
they run and last lubricated by many hams does not mean it's the way
they are supposed to be used.
Most rotators are designed to run supporting a positive load of up to
some specific value. They are not designed to serve as the pivot point
for an arm. (IE mounted on top of a tower with a mast sticking up and an
antenna on top of that). Nor are they designed to work against a side
thrust. OTOH THRUST bearings ARE designed to work against a side THRUST.
IF the bearings in the rotator or the thrust bearing are causing erosion
in the race then the ratings of said rotator or bearing are/were
exceeded. The rotator has a completely different design for the bearing
races and is designed to *support* weight. My rotators use "car wheel
bearings" at the top and bottom. So even if in an installation where the
mast can pull against the rotator there is a bearing to protect it.
Thrust bearings operate under a wide variety of conditions and are often
prime examples (the epitome) of misuse, or surviving in spite of the way
they are used.
One problem with the way thrust bearings are often used is with rotators
designed to support weight. I think the Hy-gain series will handle 500#.
According to "Norm's" most of them will support upwards of 1000#, or
half a ton. When you put a shaft through two anchored points that are
held together by a tower, it's likely the shaft and tower will have
different thermal coefficients of expansion. This means that under some
conditions you will be adding more than the total weight of the antenna
system to the rotator and in some conditions you can actually be pulling
up on the rotator housing.
The first defeats the reason the person put the bearing there in the
first place even though that's not what it was designed for. The second
can ruin a rotator in a hurry as much of the torque translates into side
forces that the bearings are no longer resisting to the extent designed,
thus transferring the side load to a rather fragile motor using thin,
stamped, spur gears.
Another problem greasing thrust bearings designed to run dry is it
dramatically increases the point load where the steel balls contact the
soft Aluminum races. I'm not sure how to explain this other than it's
pretty much the same phenomena as the difference in linear pull on a
bolt torqued to the same value with greased threads, Vs dry threads.
This is particularly bad when you have such a wide difference in
hardness between the bearings and the races.
This leads to two points. The first as Steve has already said, "Follow
the prime directive". IOW do it the way the manufacture tells you to.
The other is when using large antennas or reasonable sized antennas in a
windy area, I do not trust any rotator that has to rely on a wedge type
brake particularly when there are comparably priced rotators that are
much more powerful such as the Alpha Spid that do not require such a
brake and will not free wheel. I've had too many bad experiences with
rotators using wedge brakes and some of them were both expensive and
very inconvenient. Of course I don't care if the wind is blowing 70
MPH, if the band is open, I'll be on.
73
Roger (K8RI)
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