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[TowerTalk] Re: Long mast, intermediate bearing

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Subject: [TowerTalk] Re: Long mast, intermediate bearing
From: (Robert Hummel)
Date: Mon, 06 Apr 1998 15:21:20 -0400
>The weight of the antenna/mast is perpendicular to the axis of
>revolution, not parallel.  The parallel forces are those left to right
>or "lateral" in nature.  Thus I believe that Webster has it right in
>this case in indicating that the thrust bearing's purpose is to limit
>the sideways thrust

Matt -- Are you confusing the plane of revolution with the axis of
revolution? The axis of revolution is coincident with and (hopefully)
concentric with the mast. Same for the thrust bearing. The axis that the
ball bearings revolve around runs through the center of the bearing,
perpendicular to the plane of its revolution.

Although some bearings (such as the simple sleeve bearing) are designed to
counter lateral forces (side-to-side slop), the Rohn thrust bearing
specifically does not do that. The slippery stuff (ball bearings) reduces
the effort required to turn the mast due to downward thrust only.

Picture this: a heavy pole terminates at the bottom end with a single large
caster. The caster absorbs the downward thrust of the pole and makes it
easy to move the pole around. But, it does nothing to keep the pole
vertical. That's what the thrust bearing in your tower is doing.

Of course, because the bearing is mounted on a plate, the plate does keep
the mast from slipping sideways, but only up to the point where the
sideways thrust will crush the bearing and plate. The bearing has no design
ability to resist lateral or torsional forces.

Robert L. Hummel (WS1A)

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