[TowerTalk] Best mast location?

[Jim Thomson]

Date: Fri, 14 May 2010 12:52:16 -0700
From: Grant Saviers <grants2 at pacbell.net>
Subject: Re: [TowerTalk] Best mast location?


For various reasons (some inherent in bearing manufacturing tolerances) 
three spaced bearings are almost never done in machinery - it's just too 
hard to get them/keep them aligned.  Two bearings can handle a degree or 
two of face misalignment, and there is that much forgiveness in regular 
ball bearings, but very little offset in the bearing centers of rotation 
is tolerable.  How much offset depends on the bearing quality and shaft 
stiffness.

  Three bearings (two thrust plus rotator) spaced along a mast 
is asking for trouble.

## Actually it's not that bad.  Just align the top bearing ....and the
rotor.  With the middle bearing,  I don't use the set screws, or
bolts that are usually spaced every 120 degs.  I align the middle
bearing by feel.. and tweak the bearing mounting bolts, till the mast
rotates a full 360 deg.. and middle bearing doesn't bind at any  point
of the mast rotation. 

# at that point, even if the mast just barely touches the middle bearing,
no big deal.  Now if you actually tightened the set screws or bolts
in the middle bearing [every 120 deg], yes, you can have a problem
with binding, etc.   Since I don't use the side bolts, the bearing supports
no vertical load at all [and neither does the top bearing].  The function
of the middle bearing is to just provide some lateral support in high winds
IF the mast decides to flex a bit... [ between top bearing and rotor]. 

## It's real purpose... is in the case of rotor repair.  In that case, the
mast must be elevated high enough to clear the rotor.. and allow for
rotor removal.  In that case... I'll use something like a spare OR-2800
mast clamp, and install it ..loosely just above the middle bearing.
A cum-along is used to elevate the mast say 1-2' max.   Then OR-2800
mast clamp is cranked down tight.  Rotor can now be safely removed. 

## In the meantime, the middle bearing takes any side load. Without it,
and mast elevated out of rotor... yikes, there is nothing to support the 
lower end of a mast at all... which is a disaster, esp  with 16' of mast
above the tower..and only 4' inside the tower.         
,




Application of bearings is too off topic, but briefly, angular contact 
bearings are used for combined radial and thrust loads since neither 
thrust or ball bearings can do both unless (only for ball bearings) 
significantly oversized.  For example a deep groove ball bearing should 
have a axial load less than 15% of its rated radial load.  Thrust ball 
bearings have no meaningful radial load capacity.  Tower "thrust 
bearings" should be designed as angular contact bearings with a high 
ball load angle, like 45 degrees.  I've not measured any so any data 
would be appreciated.

## agreed.  And I have always suspected what you just stated.
Pillow block bearings are designed to be mounted on end.. with the
rotating shaft parallel to the grnd.  We used em at work for HVAC
stuff, like high HP motor's  turning huge squirrel cage blowers.
Pillow block bearings were never designed to be mounted with
the shaft vertical... taking a vertical load.    They WILL work just
fine on  a tower.. since they will take a lateral load nicely,
just let ALL the weight sit on the rotor bearing.  They still
end up being a dead loss.. since they will all eventually
rust badly, regardless of what you do to protect em. 

later... Jim   VE7RF





Grant
KZ1W

DF3KV wrote:
> I don?t use any thrust bearing at all but a home made slew ring rotator with
> the mast flanged to the top of it.
> To align two thrust bearings will be quite a task with a heavy mast.
>
> 73
> Peter
>