[TowerTalk] Advice for Thrust Bearings for 3 inch Mast and HeavyAntennas

[Dubovsky, George]

Jack,

I am just now putting up a slightly smaller 40m 3-el, with a 10m beam
above it, and I faced the same decision. My mast weighs just over 100
pounds, the 40m weighs about 130 pounds (it started life as a Telrex
40M335), and I didn't want to contemplate replacement at ANY time in the
future, and maintenance-free would be a plus. So I designed and fabbed a
bushing out of Nylatron GS, which is a moly-disulfide loaded Nylon, upon
which rides a flanged steel collar which clamps to the mast. The lower
mast support is a prop-pitch, which is intentionally set up to take none
of the vertical load, but is capable of accepting large radial loads.

I ran a steel on brass bushing setup for 20 years, but it required
periodic lubrication to keep wear to an acceptable limit - it wouldn't
have gone another 5 years. This time, I have a larger bearing area
(lower PSI) and, I hope, permanent lubrication. Yeah, it's an
Engineering experiment, but it's based on sound information.

Whatever you decide to do, you need to select EACH bearing to take the
full axial load, as it is just about impossible to get them to share the
vertical load. Good luck from central VA...

geo - n4ua

> -----Original Message-----
> From: towertalk-bounces at contesting.com [mailto:towertalk-
> bounces at contesting.com] On Behalf Of K4VV at aol.com
> Sent: Tuesday, September 06, 2005 1:48 PM
> To: towertalk at contesting.com
> Subject: [TowerTalk] Advice for Thrust Bearings for 3 inch Mast and
> HeavyAntennas
> 
> This message is to seek advice on the choice of thrust bearings.  I
have
> checked the archives, but still need advice.  I am just returning to
> TowerTalk
> after a two year absence.
> 
> A current tower project for my new station includes a 120' PiRod tower
> that
> will support two large antennas at 121' and 132' on a Chrome Molly
mast.
> The
> tower and mast are suitable to handle these antennas, with a safety
> margin.
> 
> The mast extends 7' into the top section of the tower.  Three 1/2"
steel
> plates are welded into the structure of the top tower section at the
112',
> 116'
> and 120' levels for the large ProSistel rotator and two 3" thrust
> bearings.  The
> bearings are intended to handle both horizontal and vertical thrust,
and
> provide a stable mast support to permit removal of the rotator for
> maintenance.
> 
> The antennas are a full size 4-element K3LR 40m yagi (on a 48' boom
and
> ~30
> square foot wind load) and Force 12 Ultra 80m dipole (102' long and
11.5
> square
> foot wind load).  The 80m dipole will be mounted parallel to the 40m
boom
> to
> spread the wind load more evenly.
> 
> The antennas and mast together weigh about 550 pounds.
> 
> Advice on the choice of thrust bearings would be appreciated.  Low
> maintenance is desired.  Periodic service is not an issue, whereas
> replacement would be
> a challenging and costly project.
> 
> Jack Hammett, K4VV
> k4vv at aol.com
> _______________________________________________
> 
> See: http://www.mscomputer.com  for "Self Supporting Towers",
"Wireless
> Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041
with
> any questions and ask for Sherman, W2FLA.
> 
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> 


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