[TowerTalk] Thrust Bearings
tod at k0to.us
Thu Dec 6 01:05:48 EST 2007
A nice discourse on this subject.
Roger's statements support the Second Law of Life;
"There are NO solutions in Life, only different sets of problems. You have
to pick the problem you want to cope with"
> -----Original Message-----
> From: Roger (K8RI) [mailto:K8RI-on-TowerTalk at tm.net]
> Sent: Wednesday, December 05, 2007 8:12 PM
> To: TOWERTALK at contesting.com
> Subject: [TowerTalk] Thrust Bearings
> 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 a tower
> 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 thrust bearings.
> 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.
> Roger (K8RI)
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