On 2/26/2013 10:24 PM, Michael Goins wrote:
Thrust bearings (like baluns) seem to be one of those things that create a
lot of confusion in the ham community. They are used to keep the antenna
centered on the rotor to eliminate side strain, or to take compressive
strain off the rotor, or both, depending on who one talks to about them.
Look at the bearings and races usually tells you whether it's designed
for a lateral (side) load, or end load. Beyond that, read the manual or
call the manufacturer. Hy-Gain has always been very helpful.
But as to bearings. Look at the Ham series of rotators. The bottom race
is a simple half circle, or "U" looking up with the mating race an
identical "U" facing down. It's a typical, low RPM, load supporting
bearing. I believe most og them are good fo 500#, give or take.
If you put a side load on this bearing, where is the force applied?
Directly on the gap between the two races which is where you don't want it.
As the bearing is a large diameter, "leverage will cause one side to
lift and the other to have a corresponding increase in pressure, so
these rotators can handle "some" leverage, but not a lot.
A thrust bearing used with these does not need to support weight, but it
does need to eliminate lateral force. A good example is the "pointy
tube top on some towers. In typical cases, it es more than enough
These rotators are not good at handling negative loads
BUT, "call HY-Gain" to get their take on what to use. It takes but a few
minutes, they are very helpful, and you have information, right from
"the horses mouth"
BTW if you are putting enough weight on a Ham series rotator to need a
weight supporting thrust bearing, you are probably turning more mass
than the rotator was designed to handle.
Look at the ROHN TB series of thrust bearings. They do use that "U"
shaped race BUT the inner and outer edges are offset meaning a side
force does not have the bearings pushing directly on the gap.
Typically, ball and roller bearings are designed for lateral loads.
Taper bearings (like wheel bearings are designed to take considerable
loads in both axes. Some rotators us them, but lateral force can put
loads on rotator housings they were not designed to handle
There is one ball bearing designed to take linear lloads and that is the
throwout bearing in a clutch.
I'm putting a T-8 on a HG-52-SS that does not have a thrust bearing on it
at the moment. Everything I read about CDE rotors indicate that there is no
need because the rotor can handle the weight if a short mast is used
minimizing the leverage effect. Other readings say to use one because it's
important to hold the antenna weight off the rotor.
The only rotators I know of that are not designed to support vertical
loads art inexpensive TV antenna rotators.
As the old CD rotators were beefed up to handle larger ham antennas, a
wedge brake was added with better direction indication and they evolved
into thr rotators Hy-Gain now sells..
I think you will find this design is created to support considerable
weight and should not be operated without a load, but again, ask Hy-gain.
Many thrust bearings do
not have any way to hold the antenna off the rotor seemingly to be only for
keeping the mast centered in the rotor. That appears to me the purpose - to
minimize any lever-like lateral pressure on the rotor generated by the
antenna and mast induced by wind.
Isnt there a pipe or sleeve at the top? This is usually more than suficient.
73
Roger (K8RI)
There are lots of HG-52-SS towers in ham service and apparently no factory
thrust bearing available, and while the Rohn TB-3 and Array Solutions
version (and others) are adaptable, they require a few holes to be drilled,
not necessarily an easy project 20+ feet off the ground.
Is a thrust bearing really needed or is it something that most of us do
just because?
Mike, k5wmg
Fast cars, slow boats, good dogs, and summers off to write
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