On 10/13/2011 11:20 PM, Jim Thomson wrote:
> Do what the manufacturer says ... that is a familiar statement repeated here
> many times. But no one has been able to find out what the manufacturer,
> Rohn, says until now. This should answer the question. It is left to the
> discretion of the user.
Taking this from a mechanical perspective, you have "steel ball
bearings" running in an "Aluminum race".
To begin this is not a good combination for several reasons and not just
due to the difference in hardness of the two materials, but that extreme
difference is one major reason.
Another is the way aluminum wears. In the case of a high pressure
contact point (steel ball) the Aluminum "flakes" off in tiny flat pieces
leaving an irregular surface exposed. Aluminum Oxide forms on that new
surface almost immediately to protect it. Unfortunately you now have an
uneven surface with many contact points that will wear or erode even
faster. You also have those little flakes floating around in there and
every time one gets between a ball and race you have an increase in
contact pressure that is likely to result in another piece flaking off.
Sand grains or other impurities blowing in would have the same effect.
There is a LOT of space for wind to blow through a TB-3 or 4 to remove
these flakes. Whether grease is good or not, with grease any flakes and
impurities that get in will stay.
One other point is these bearings *appear* to be cast Aluminum which is
very soft and granular meaning that in the best of conditions I'd not
expect it to wear well.
You have "roughly" 16 contact points around half the bearing surface
with the pressure varying with the angle to the applied pressure. The
first 3 or 4 contact points on each side are nearly perpendicular to the
force and can almost be ignored with the remaining 8 or 9 tiny contact
points taking progressively more of the force with the 3 opposite the
applied force taking the majority of that force. This results in the
load being supported by a very tiny area and resultant high pressure per
square inch. This results in what is called "Brinnelling". In plain
English: "Dents" caused by pressure.
Just think what the pressure is on that surface with a big tribander or
monobander in a strong wind, or worse yet with the antennas mounted 10
or 15 feet above the bearing.
One other point. All bearings that are designed to be lubed are
supposed to have the *viscosity* of the lube specified as every bearing
has a specified minimum viscosity for the lube. Often this figure can
be very difficult to find, but it exists if the bearing is designed to
Think of how putting lube on a bolt changes the required torque or how
much more force the same torque exerts when the threads have been lubed
compared to dry.
> It is up to the user whether they want to use lubricant or not. We do not
> feel that it is necessary. We have some that have been installed at our
> location for a number of years without lubricant, with no issues.
> Tim Rohn
> ##IMO, Rohn TB-3 and TB-4 thrust bearings are at the bottom of the heap,
> more junk.
> Both my new Rohn TB-4 bearings ran rough, after being outside for 1 yr.
> Mine came lubricated too.
Mine were really rough when new.
> Trbl is, the lube drys out, and no zerk fitting to pump new grease in.
> Steel ball bearings in an Aluminum
> bearing is about as dumb as it gets.
> ## On both my TB-4 bearings, the 3 x 3/8" set bolts, every 120 deg, also
> rusted out real bad, seized up good.
Yup. Never seize before ever installing.
> After loads of pb blaster, etc, I managed to get 4 of the 6 x bolts out,
> and 2 x sheared off. Rohn hits the hole
Heat the bolt up with a torch (BUT don't melt the aluminum)
This *usually* results in the bolt coming out easily due to the Aluminum
oxide at the junction of the threads.
> where the ball bearings gets inserted, [via set screw] with a drift punch.
> A zerk fitting would have been a better
> deal instead.
I think a Sleeve bearing would have been better still. <:-))
> ## so far, on the new HDX-689 tower, the PEER brand thrust bearings, with
> their locking collars works good.
> The set screw for the locking collar has to be slopped with never seize
> goop though....and also more goop to be
> applied to the female thread.
> ## Thrust bearings are not easy to replace..after the fact.
That's what makes the wood and other split blocks so nice.
> The Rohn bearing's are not only expensive, they are poor value
> for the money, and poorly designed. If u do use the TB3/4's... add a zerk
> fitting, so at least u can pump it with grease
> once or twice per year.
I'm not sure if that will make them last any longer, but it will make
them run smoother.
> ## You can also get Peer and similar brand thrust bearings with or without
> the self aligning feature.
> Later.... Jim VE7RF
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