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Re: [TowerTalk] Thrust bearing thread

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Thrust bearing thread
From: Grant Saviers <grants2@pacbell.net>
Date: Fri, 14 Oct 2011 16:04:20 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
I agree that the steel balls on cast aluminum race "thrust bearings" are 
NFG.  Unless the application is a super stack on a big diameter heavy 
mast, then the thrust load belongs on the rotator and the side load 
(wind reaction torque) at the mast top should be handled by a sleeve 
bearing.

Having removed some "real ball bearings" (flange bearings) from masts, 
the spherical od  was rusted tight (now NOT self aligning) and the race 
had water in it.  Even with seals, they don't hold up to standing water 
on the seal.  Maybe a "hat" can keep enough water out so they don't fail 
quickly.  I would also note that the mechanics of two ball bearings on 
the mast (rotator + "thrust beraing") separated by 5' is not ideal.  As 
noted a standard ball bearing can only handle a thrust load of about 20% 
of its radial load rating.  And then there is only a few thou of axial 
slop before the bearing is too highly preloaded.  That is one reason why 
machine tool spindles use back to back angular contact bearings with 
super tight tolerances to set a known and constant (temperature 
independent) preload. (about $600 a set in 2" bore).

IMHO, as others have mentioned, the ideal "not thrust" bearing for the 
top of the tower is a sleeve bearing.  I say "not thrust" because a 
sleeve "radial" bearing is usually what is needed to resist the mast 
side forces and the rotator handles the thrust/weight of mast and 
antennas. .  A corrosion resistant polished sleeve running in a plastic 
bearing is not so hard to make either, for a standard 2" mast.

Nylon is also a good sleeve bearing choice, more UV resistant and 
cheaper than delrin.  Check ebay Business and Industrial, search "nylon 
plate"  - there are plenty of inexpensive remnants in 1 to 2" 
thickness.  There is also a lot of delrin/acetal plate if that is your 
preference.  Nylon is also a good choice to make custom diameter 
insulators for driven elements.  However, delrin/actal is a lot more fun 
to machine.

A 2" sch 40 stainless 12" long pipe (likely enough for two bearings) 
costs $27 from On Line Metals in 316, a very corrosion resistant alloy.  
It is about 2.067" id.  And 2.375" od.   It would not be beyond common 
ham skills to drill and tap it for 3 top and 3 bottom holding/centering 
bolts (1/4-28 with locking nuts in 18-8 SS would be a good choice for 
the 2" pipe wall thickness) and to sand the od smooth by hand.  No lathe 
required.

Then use the plastic bearing material of choice - McMaster has some 
useful guidance http://www.mcmaster.com/#about-plastics/=ehckkx
or stack up a few chunks of polyethylene cutting boards - I think the 
bearing should be 1 to 2" thick.  Plastics used as bearings have PV 
ratings (psi load times velocity) but since V is essentially zero for 
rotators, only the deformation and creep limit of P needs to be 
considered. (computing the approximate side wind load of the antennas + 
mast on the bearing is not so hard but beyond this TT post).

As noted in this thread, a 2 3/8 standard hole saw can cut the hole in 
the plastic.  Go slow and keep a hole saw flooded with water when 
drilling plastic to reduce the melting.  The plastic can then be split 
for easy install and removal.

Glancing at a pipe size chart, sch 40 pipe will also fit over 3, 4, 5, 
and 6" od tubing sized masts.  3" ips id=3.068"; 4" ips id=4.026"; 5" 
ips id=5.047"; 6" ips id=6.065".  Welded ips pipe might need some clean 
up id machining though of the weld for small clearances. (probably 
affordable for those with 4", 5", and 6" masts ).  On Line Metals sells 
316 sch 40 pipe up to 8" in as little as 1 ft" lengths.

Grant  KZ1W

b.t.w. my visit to the OLM web site found "A513 type 5" steel DOM tubing 
in several diameters & wall thicknesses that would make good masts.  The 
yield stress is higher that normalized 4130 and the cost is about 40% 
less.   Any Tower Talkians using it?




On 10/14/2011 4:34 AM, w8zn@comcast.net wrote:
> It has been interesting seeing all the posts about this. The real question is 
> why would you ever use a TB3 or TB4 at $161 and $185 (from 3Star) when you 
> can have a REAL bearing for only $30.98???? Made with REAL steel, seals and a 
> zerk fitting?
>
> Check 
> outhttp://www.thebigbearingstore.com/servlet/the-88/2%22-Four-Bolt-Flange/Detail
>
> A nice  1" thick plate of Delrin with a two inch hole cut in it using a hole 
> saw from your local hardware store works really well also - Delrin however is 
> way more expensive than these bearings.
>
> Terry Price - W8ZN  ex K8ISK
> FM18dv - 1.8MHz thru 47GHz
> K8GP - The Grid Pirates - FM19bb
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