In my opinion, people are needlessly getting hung up on terminology.
A thrust bearing is designed to do exactly what it says ... support forces that
are in line with the axis of rotation. If you use it to support the weight of
your mast by tightening the set screws, you are using it for it's designed
purpose. That may not be a good idea, though, if you have a reasonably long
mast and the mast is also secured to the rotator ... differences in
expansion/contraction between the tower and the mast can put undesirable (i.e.,
lifting) forces on the rotator that don't allow the bearing to seat properly.
Most rotators are designed to handle the vertical forces put on them by the
mast and antennas and should be used as such.
It often happens, however, that it is also useful to have a bearing around the
mast that handles lateral forces (perpendicular to the axis of rotation), such
as at the top of the tower or just above the rotator to keep the mast
positioned even when you remove the rotator. A simple sleeve can often perform
that function, but usually so can a thrust bearing if the set screws aren't
tightened (I do it on my tower). You just aren't using it for it's primary
purpose, that's all.
You can use a crowbar to pound in a nail, but it's still called a crowbar.
73,
Dave AB7E
------Original Mail------
From: "Al Williams" <alwilliams@olywa.net>
To: <towertalk@contesting.com>
Sent: Tue, 2 Jun 2009 17:17:12 -0700
Subject: Re: [TowerTalk] Purpose of Thrust Bearing
I hope that some there will come a posting in simple terms about the purpose
of a thrust bearing?
It seems to me that the "axis of rotation" for a mast mounted thrust bearing
is vertical, thus the design is to support
loads around the mast that are not equalized?
However an jet engine is rated in thrust which is in line with the motion of
the air?
These two are in conflict, whats wrong?
k7puc
--- Original Message -----
From: "Mike" <nf4l_NO_SPAM@nf4l.com>
To: "Brian Machesney" <nekvtster@gmail.com>
Cc: <towertalk@contesting.com>
Sent: Monday, June 01, 2009 3:41 PM
Subject: Re: [TowerTalk] Purpose of Thrust Bearing
> Brian Machesney wrote:
>> I'm trying to clear up a point of my own confusion and/or ignorance
>> w.r.t.
>> "thrust bearings."
>>
>> I used to think that the "thrust" taken up by the bearing, e.g. when
>> supporting an antenna mast, was a vertical force. I have read a lot on
>> this
>> reflector recently that contradicted that belief. In searching
>> McMaster-Carr
>> for a bearing to take a vertical load, I find that they describe a
>> "thrust
>> bearing" as being designed to, "Support loads parallel to their axis of
>> rotation."
>>
>> This sure sounds to me like it's meant to support the weight of a mast
>> and
>> antennas - a vertical force when we mount a thrust bearing in the hole at
>> the top of a tower or on an "accessory shelf."
>>
>> Which is it?
>>
>> 73 -- Brian -- K1LI
> It's the McMaster-Carr definition.
> Some people describe using two bearings, one above the other, to
> prevent lateral forces from acting on a rotor (like a pry bar on a
> fulcrum), but that's not the designed purpose.
>
> 73, Mike NF4L
> _______________________________________________
>
>
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