I'd like to add a caution to using multiple thrust bearings and using a
thrust bearing to hold the weight of the antenna system. Under some
conditions this works just fine, but if the coefficient of expansion of
the mast is different than the tower you can at times end up with a
substantial *pull* to the top of the rotator, or between thrust bearings.
Normally I doubt it'd be much of a problem, but it is possible to
literally pull the rotator apart if there is enough distance between the
top of the rotator and the bottom thrust bearing.
Also thrust bearings normally do not support the weight of the system
but keep it aligned. By doing so they keep the bending moment off the
top of the rotator and reduce the bending moment at the top of the tower
where they can serve more as a pivot point. Some rotators are designed
to operate with a load and I would think they will last longer with the
load than without.
I'm replacing the top thrust bearing in my system with a sleeve.
followed by another about 10' below it.
The sleeve makes a great thrust bearing, BUT it does not allow the
pivoting action of a thrust bearing so the length of the arm from the
sleeve to the top of the array can end up as a bending moment added to
the top of the tower which is the reason for the second sleeve about 10'
below . This reduced the bending moment to more of a linear, or
perpendicular side force applied to the top section or sections of the
tower. I'd not want to put the load of the system I have "up there" on
a single sleeve at the top of the tower. The weight and bending moment
have literally destroyed two TB-3 thrust bearings. I have also run into
the problem of the mast coefficient of expansion pulling on the top of
the rotator which also increases the load on the bearings. Fortunately
the PST-61 has two bearings to handle the load whether it pulls up or
pushes down.
73
Roger (K8RI)
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