[TowerTalk] Thrust Bearings

Kurt Andress K7NV@contesting.com
Sat, 15 Jan 2000 06:17:40 -0800

Hi All,

The discussion about tower top bearings has been interesting.

I agree with others about there not being a great need for thrust
carrying capability, unless one is using an undersized rotator or a prop
pitch that does not have the top plate and thrust bearing installed (a
common practice).

Here's something else to consider when making a bearing selection.

Most masts with antennas stacked above the top are going to bend
significantly before they fail. Aluminum masts bend more than steel
This bending can produce fairly high axial rotation where the mast
passes through the bearing.

Any bearing like the Rohn TB is going to hate life because it is not
designed to rotate about the mast axis. So, all of the bending moment is
carried by the bearing. This puts very high loads on the ball/race
interface. It is not surprising that these bearings exhibit problems
after extended use with stacked antennas.

Most industrial grade pillow block bearings are made to absorb axial
deflections by allowing the entire bearing to rotate in the housing. So,
they don't have this problem.

As far as their corrosion problems, it's a matter of environment,
weatherproofing, and maintenance. 
I'm sure there are some places that are exceedingly difficult to deal
I know one that's been up for 15 years, 10 miles from the Pacific coast
that's still working. It's not real pretty, but still works fine. 
It has a hood fashioned from readily available galvanized steel ducting
parts, to prevent direct water flow into the bearing, and it gets
injected with waterproof marine grease at least once a year.
There also some fine corrosion preventative coatings that will help.

Plain bearings such as the hardwood block don't like the bending either,
but they will deform during use to eventually create clearance for this.
I wouldn't make the block with too tight a fit. It would be best to give
the top and bottom edges of the hole some extra clearance to prevent
them from fracturing before the thing wears in.

If you've just got one antenna mounted right above the bearing, most any
thing that can carry the lateral loads will do.

73, Kurt, K7NV 

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