[TowerTalk] Thrust bearings

Thu Feb 28 12:54:07 EST 2013

Lot's of confusion because commercial "thrust" bearings are really some 
combination of thrust and radial load bearings.  What most hams need 
with tower installations is a bearing for radial loads, something that 
will take the side load of the mast above the top of the rotator when 
mounted in a tower.  If the rotator is mast mounted then the rotator 
bearings have to do all of the work, both thrust and radial and that is 
why the antenna height above the rotator and wind load is so limited.  
Many stock towers have sleeves at the top to handle the radial loads or 
a tapered top section can be used that ends in a sleeve.  Usually, these 
are all that is needed as a bearing.  If the tower has a flat top plate 
with a hole for a "thrust" bearing then a radial bearing needs to be 
installed to handle the side loads.

Let's apply logic to the situation with the commercial thrust bearing 
and rotator in a tower and compare them.  I don't have either apart in 
front of me so this is a bit from memory, please respond with the 
accurate numbers if you have them.

Products that I have used have similar load/stress/wear/corrosion 
limitations  - (not stainless) steel ball bearings in aluminum races 
(Rohn & Yaesu thrust bearings and HyGain, Yaesu  rotators).

The number of bearing balls (they are all about the same diameter) ratio 
thrust bearing to rotator in simply the ratio of pi * Diameter of the 
bearing.  That ratio varies between 1:2 and 1:4 and probably averages 
1:3.  Therefore the load is less per ball by that ratio in the rotator, 
and at first order approximation the rotator life will be greater by the 
inverse of same ratio.  Could you split the loads thrust bearing and 
rotator?  Highly unlikely given this requires a few thousands of an inch 
accuracy and thermal changes would exceed that amount even if attained once.

Then there is the method of clamping the thrust bearing to the mast.  If 
you are concerned about the stock Orion mast clamp (I have one rotating 
40m beam and 86' long 80m dipole, no problems). then surely you view the 
3 or 4 grub screws used in a thrust bearing to hold the mast thrust load 
as a joke.  Plus the castings have tapped threads in aluminum for those 
screws or the castings crack (my Rohn did) if the screws don't strip as 
tightened.  The Orion and Yaesu use grade 8 bolts and nuts to provide 
mast clamping force - quite a difference.  No doubt the K7LXC Orion 
clamp is better than the stock one, but compare either of these to a 
stock thrust bearing clamp system.

Could a commercial thrust bearing be used as a radial only bearing? 
Perhaps not tightening the grub screws onto the mast would work? Not 
considering the designs I've seen, they require some sort of thrust load 
to stay together and/or preload the bearing.  A plain radial ball 
bearing would work much better with a loose fit to the mast.  If water 
can be kept out this is an alternative, and be bought as a pillow block 
assembly, but be prepared to grease it periodically.  The plastic radial 
support bearing ideas presented on the forum are a better alternative 
IMO.  Stacking up 2 or 3 pieces of plastic cutting board with mast sized 
holes or bore a hole into a 1 to 2" thick chunk of nylon plate (black or 
MDS filled for UV resistance), will last a long time and be maintenance 
free.

If that logic isn't convincing, than plan out what it takes to replace a 
thrust bearing on a multi-antenna mast vs removing a rotator that will 
last 2 to 4 times longer.  It either case it makes good sense to replace 
the grease every 5 years or so to maximize the life of the bearings.  I 
replace the balls with stainless ones when rebuilding them.

Its clear to me, the standard commercial "thrust" bearing is to be avoided.

Grant KZ1W

On 2/28/2013 7:39 AM, Larry Loen wrote:
> To me, it's a no brainer.  150 dollars to almost certainly lengthen the
> life of a much more costly rotor?
>
> Let's suppose the rotor's specs will take that compressive weight.
> Wouldn't logic indicate that, regardless, it will last longer if we don't
> press all that weight on the rotor 24 by 7, forever?
>
> Moreover, the clamp that comes with even my M2 Orion rotor is not rated the
> best by many on this list.  There's a better replacement but (so far) out
> of stock.  Until I can get it replaced, it seems likely to last longer if
> _it_ doesn't have to deal with the compressive load, at least, especially
> as it seems plausible to me that ordinary physics will transfer some of
> that downward load from side to side (e.g. a strong wind) sometimes.
>
> I'm sure it's not a perfect solution, but it seems quite likely that it is
> at least worth the money.
>
>
> Larry Wo0Z
>
> On Tue, Feb 26, 2013 at 8:24 PM, Michael Goins <wmgoins at gmail.com> 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.
>>
>> 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. 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.
>>
>> 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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