[TowerTalk] Thrust bearings

[Chris Pedder]

Hi,

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:

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1.6 MAST BEARING INSTALLATION

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.

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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 installations.

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.


73,

Chris

________________________________________

Chris Pedder     G3VBL/8P9EM
E-mail           g3vbl@netcomuk.co.uk
AX25-mail        g3vbl@gb7bnm.#45.gbr.eu
DX-Cluster       g3vbl > gb7dxd
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