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[TowerTalk] Thrust bearings

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Subject: [TowerTalk] Thrust bearings
From: (Chris Pedder)
Date: Sun, 12 Apr 1998 18:16:53 +0100

In the 'Mother Country' the definition of THRUST may not be the same as that 
used in the 'Colonies'. The Oxford English Dictionary regards thrust, in a 
mechanical sense, as being 'A pushing force exerted by one part of a structure, 
etc. upon another contiguous part'. In a mathematical sense, thrust in a rod 
refers to compression as opposed to tension.

I use a a Creative Design RC5B-3 rotator from whose manual I quote:


A bearing or similar item is usually installed at the top of an antenna tower 
to prevent the antenna from swaying. Such a bearing must be used only for this 
purpose. Using it to support any of the weight of the antenna or antenna mast 
would have an adverse effect on the rotor. This is not only because the rotor 
is more thrust effective than the bearing but also because eccentricity arising 
from structural imprecision cannot be absorbed at the top of the tower and so 
the resultant, waste force would work on the bearing and rotator. Accordingly 
the fastening bolts should not be tightened to the point of holding the antenna 
mast when a standard bearing for "ham" radio antennas is used. With bearing 
having both top and bottom bolts, the top ones should be removed.

With an antenna having a wind surface area greater than 2 sq m padding should 
be inserted between the bolts and the antenna mast. Do not tighten the bolts 
down directly onto the mast, as doing so will increase the danger of buckling 
in strong winds.


A diagram shows a recommended gap of 0.5mm between mast and bearing bolts. I 
use a Creative Design CK-46 bearing at the top of my tower. The specification 
for the RC5B-3 allows a vertical load of 700kg, sufficient for most 

A further quote:

... The most important thing to keep in mind here is that the central axes of 
the antenna mast and rotor must be within 0.5 degrees of each other.




Chris Pedder     G3VBL/8P9EM
DX-Cluster       g3vbl > gb7dxd

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