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@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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