If the rotator is rated to handle the weight load of your mast and
antennas, then you really don't need a "thrust" bearing i.e. one that
accepts load along its axis.
However, you still need a "radial" bearing, one that centers the mast
and resists side loads. US Tower larger crank ups have no provisions
for thrust bearings and the radial bearing is a galvanized steel sleeve
for a 2" mast.
I use black delrin (acetal) or dark color nylon or black UHMW (ultra
high molecular weight polyethylene) for radial bearings as all my
rotators can easily handle the thrust loads, including a 350# 80m yagi
with 170# 3" mast.
For a radial bearing you can take a plate of material that is large
enough to fit over the existing thrust bearing mounting holes (or drill
new holes in the top plate), bore it to the mast od (I go 0.040" over
for the galvanizing and some clearance), drill the mounting hole pattern
and then cut it in half on a bandsaw. You can leave the bad "thrust"
bearing above it held with a pipe clamp.
My bearings are 4" thick (see following). 2" thick or more are a good
idea. Black UHMW is my first choice material, cheap and slippery and
reasonably UV resistant, but it is sticky and gummy to machine so takes
sharp lathe tools and twist drills that are "daubed" so they don't
grab. Some folks propose using polypropylene or polyethylene kitchen
cutting boards stacked to enough height. That is an easy way to go if
the above materials are unobtanium, but paint either with a plastic
compatible paint for UV protection.
One reason I use 4" thick material is that it will take some bending
loads if I need to raise the mast to work on the rotator. I have a load
rated eyebolt in the top plate for attaching a hoist, but that still
puts a side load on the mast. After lifting to clear the rotator, I
have a fixture from 2x2x 1/4" angles that centers the mast and keeps it
from windmilling. I have a round flat plate above the radial bearing
that accepts a thrust load from the plastic radial bearing so the hoist
isn't required to hold the mast long term. Of course that had to be
installed on the mast when it went into the tower. If you really need a
thrust bearing, such as in a side mount arm assembly, then that plate
can do that job against the plastic, again why UHMW is a good choice.
I've found all materials I've needed on ebay at good prices.
Grant KZ1W
On 11/1/2015 7:46 AM, jcjacobsen@q.com wrote:
Ummmmmmmmmmmmm
<SNIP>
You might try loosening the Yagi mounting clamps and let them slide down
the mast to the top of the tower. Then slide the mast up using a gin pole
until you can insert the bearing. Then let the mast down and re-mount the
beams. I think you will be much happier with a proper bearing.
<Snip>
Let's think about that........ You slide the yagi down to the top of the tower.
Then raise the mast to insert the new bearing.
This would mean the mast AND yagi would be above the top of the tower. What
would support the mast and yagi while the mast is out of the tower??? Nope,
don't think so.
I like the idea of raising the mast out of the rotor just enough so you can
place the bearing on the bottom of top plate. It would be easier to keep the
mast braced vertical while doing it.... not to mention stable.
Just my $.02 worth.
K9WN Jake
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|