[TowerTalk] Purpose of Thrust Bearing

[xdavid at cis-broadband.com]

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 at olywa.net>
To: <towertalk at 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 at nf4l.com>
To: "Brian Machesney" <nekvtster at gmail.com>
Cc: <towertalk at 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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