[TowerTalk] Noob question re: Thrust Bearings

[Grant Saviers]

The bending stress on the mast is highest at the top of the tower 
support point.  So definitely don't drill a hole there!

The bolts or set screws (grub screws) in an aftermarket "thrust" bearing 
hold the mast axially and radially, a little tricky with a hardened 
alloy mast.  Grub screws at a point of significant bending stress of a 
shaft can walk, so that needs to be considered.  One alternative often 
used is a cam locking collar or split collar and some use pillow 
bearings made to do that.  At 2" id they have plenty of radial and 
thrust capacity.  A downside is a ball or roller bearing at the top of 
the tower that needs grease and may rust, but that is true for all 
commercial tower "thrust" bearings.  Worse, aftermarket tower "thrust" 
bearings have steel balls in aluminum races, which is a poor 
implementation of a bearing.

Whether the "thrust bearing" should handle thrust or radial loads or 
both is a matter of "controversy" as mentioned.  It is a rare mast load 
that would exceed the thrust load rating of the appropriately sized 
rotator.  Check the rotator specs vs your mast load.  My K7NV prop pitch 
has 500# plus thrust load and Kurt recommends all of it on the rotator. 
Yaesu and HyGain rotators I've disassembled all need a thrust load to 
center the balls in the races.  Some are angular contact designs that 
require thrust loading to have radial load capacity.

If two thrust bearings are on a shaft and the loading on each one 
becomes indeterminate.  So it is never done in machines.  So ideally the 
rotator should have to have no thrust load as mentioned.  Or if it is 
intended, you won't know what it is.  However, once the rotator clamps 
are tightened there will be something happening when the wind blows, the 
tower and mast bend, or the temperature changes, and the top "thrust 
bearing" wears.  What??

So there really can't be any clear engineering explanations of how 
aftermarket "thrust" bearings work.  However, the marketing department 
has them.

Then consider the well regarded US Tower crank-ups use a pipe sleeve as 
the top bearing.  Zero thrust capacity.  Works well.  Also, doesn't need 
separate mast tilt restraints when removing a rotator.  Mast axial 
support when servicing a rotator is the only reason IMO to have a thrust 
capable bearing on the mast.  A muffler clamp is cheaper.

My choice is a UV resistant UHMW radial top bearings for low cost, no 
maintenance, long life, and low friction.  A 6x6x4" thick block ($47) is 
what I made for the 3" mast radial bearing with 500# loading.  In 
perfect condition after 6 years.  Some use stacked HDPE cutting boards, 
but not UV resistant unless painted.

Grant KZ1W


On 5/9/2020 11:24, scott at nx7u.net wrote:
> I've never found any clear explanation of how thrust bearings are 
> practically implemented.
> If the bearing is to "hold up" the antenna (so that the rotator doesn't 
> have to), I assume that it's the force from the four radially-oriented 
> bolts that do so?  It seems to me that over time the mast would slip. Is 
> that really not an issue with sufficient bolt torque?
> It seems to me that the mast should be pinned through the bearing 
> instead--that is, a through-bolt that goes through holes drilled in the 
> mast.
> What obvious (but not to me) thing am I missing?
> Thanks,
> NX7U
> 45 years active & never had a rotator bigger than Radio Shack :-)
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