[TowerTalk] Thrust Bearings
K8RI-on-TowerTalk at tm.net
Wed Dec 5 22:11:40 EST 2007
Sorry for the length, but when I started I didn't mean to type quite this
much. It just kept growing. I hope I at least got the high points
accurately and didn't miss too many.
The life of a thrust bearing:
In the ham world we have a relatively limited source for thrust bearings and
then the use of a thrust bearing is not well understood and often they are
not used properly. In a typical ham installation the thrust bearing's
primary duty is (should be) to handle thrust in the horizontal plane (side
to side). From what I've seen and that is admittedly limited even over 50
some years, hams use the thrust bearing to support loads, but often the
bearings end up doing double duty within both the vertical and horizontal
planes. Some times they are doubled up with a relatively long, heavy duty
mast to get more antennas higher on one tower. Each of these installations
comes with it's own good and bad points, or strengths and weaknesses.
The problem with thrust bearings is service. They could be constructed of
much better and stronger materials. They could be precision bearings such as
wheel bearings. Any one with a lathe could produce retainers for readily
obtainable, good rugged thrust bearings with many times the load capacity of
the typical thrust bearing, but with just two major drawbacks. They need
protection and service.
Even then they are at the mercy of the installer and who ever takes care of
the service. As several of us on here can attest, wheel bearings do not
survive well or long when water is present. In automotive use they survive
for hundreds of thousands of miles, or many years in one of the worst
environments possible. The average wheel bearing probably never sees service
such as repacking
Commenting on the TB-3 and 4 bearings the races are actually designed to
handle both vertical and side thrust. The bearings must be removed before
you can get the races apart and they drain (for the most part) more or less,
naturally. The split(s) in the casing are at the inner top and outside
bottom. The design is such that the inner race is well above the bottom of
the casting with the outside dropping well below the inner race. So both
races are covered and fairly well protected from the elements. You almost
have to try to get dirt in the things, but Nature is quite ingenious.
These particular bearings are designed to run dry. Any dirt that does blow
in has to be small and light. It follows that they should come out just
about as easily. They don't but aren't usually a problem. Greasing the race
and bearings actually increases the point load between bearing and race. It
also causes the bearing to move up and down a bit (due to space from
tolerances) more so than dry. As the both races are an Aluminum casting,
greasing can increase the rolling off of partials due to the pressure on the
roller more so than when dry.
The only direction the bearings do not provide support is UP. You can put
one whale of a load on them from above or the side, but _don't pull_up_ as
the bearings are marginal in this direction at best ( This can create major
problems and I'll come back to this). I think many of the problems come from
the marriage of the hard steel balls and the soft, cast casing which has a
granular structure. Under some conditions the hard steel balls will peel off
(or roll off) the surface of the race in flakes. When overloaded this can be
pronounced. Sometimes this overload is unwittingly built into the
installation. One other problem is getting the three clamping/centering
bolts out of the upper housing. These bolts are "cad plated" 3/8" NC and
used with a locking nut in the units with which I am familiar. The "cad
plating" is a sacrificial metal which is supposed to save the Aluminum from
corrosion. The problem being the cad plating is usually gone within a year
or two at most and then those bolts almost become a permanent part of the
upper housing. Cad plated bolts just do not hold up well out in the
elements. And those bearings are a bear to change when on a long mast inside
I've had TB-3's up for quite a few years with SS bolts on the old tower. In
the case of the new tower I just did a regular installation and had the
usual problems getting them out. (PB-Blaster to the rescue.) In the case of
the 3 clamping/positioning bolts Pentrox and even the Moly Disulfide grease
has kept corrosion to a minimum where it's never been a problem for me. The
ones with the cad plated (sacrificial plating) rust and for all essential
purposes can turn a $125 bearing into a single use item. If you figure the
time required to save the ones where the bolts need to be drilled out and re
tapped it takes little to exceed the cost of a new bearing.
Now going back to the weakness in the upper direction: Normally a thrust
bearing never sees thrust in the UP direction, but there is an installation
in which they may, or are likely to do so. This happens when multiple
thrust bearings are used. If the mast has a different thermal expansion than
the tower, one of them is going to get longer than the other when the temps
go up and the other will get longer when the temps go down. Under some
conditions this can produce more force in the vertical axis than the
antennas and mast we can put up there. Getting warmer will reduce or even
reverse the load on one of the bearings while increasing it on the other.
Getting cooler will reverse the process and put the stresses on the opposite
bearings. In many of the cases I've seen the installation would have been
better using that sloppy, crude sleeve bearing at the top of the tapered
tower and letting the rotator support the weight instead of a thrust
bearing. Most rotators are designed to be loaded from the top when in use.
Some even need that weight. Eliminating, or even using a negative load
(lifting) can adversely affect a rotator's life. Many of todays rotators are
designed to support as much or more weight than we'd have even with a large
array above it, but they are not designed for a negative load. OTOH many
rotators are not designed to handle the large antennas they are struggling
to hold or turn.
A mast or shaft is required to get from the rotator out through the top of
the tower and to support the antenna. This greatly reduces the forces on the
rotator and greatly enhances its ability to handle larger antennas, but only
within reason. Now all of us can end up putting the logic together and
follow the train of thought that if we gain strength by placing the rotator
in the tower compared to on top and we have to run a mast up to the antenna
we should be able to go with a longer shaft and put up another antenna
without exceeding the previous rating which might have been adequate. We all
know these towers are built with a safety factor so we should be able to
operate in that safety factor zone without much of a risk. Right? Ask the
manufacturer why they put that safety factor there.
Well, we've gone from the logic of an extended mast and another antenna and
we've all seen large arrays supported like this. It then seems to follow
that if we do away with that pipe at the top of the tapered tower top
section and replace it with a thrust bearing and another on the mast down to
the rotator we should be able to handle more and/or larger antennas and
reduce the stress on the tower and rotator.
Without going too deep and already having covered some of the pitfalls of
multiple thrust bearings there are several problems that may show up when
mounting the rotator down in the tower, or near the bottom and using a
longer mast between the antenna(s) and rotator along with the required
That mast between the antennas and rotator may be of a very strong steel
alloy, but it is still flexible from side to side and radially as a torsion
bar. That makes it a spring and springs have resonances in two directions
and multiple modes.
That means thrust bearings need to be properly placed within the tower in
relation to these resonances AND guy placement on the tower which also has
resonances of its own. Where and how? It differers with every installation.
The longer the mast the more rigid it must be and that usually means a
larger diameter and that usually means more mass which means more inertia.
In the end that changes the resonant points higher except one due to inertia
where period goes up with mass. It's all a compromise and the farther we get
from standard the farther we are into experimenting and uncharted territory.
BTW, a rotator mounted in a tower at or near the bottom exchanges torsional
movement of the tower into shear at the tower legs
It pays to follow good engineering practice, but most hams aren't engineers,
or at least of the right type. No mater what designs we look at we are
likely to find some singing the praises while others tell us that may be a
bad way to go. Some installations survive for years in spite of them selves
while well engineered systems fail in a relatively short time. However I'll
stick with the odds that favor the engineered approach.
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