Hi Jim.
I don't like your idea, Jim, but that assumes you are planning to use a
standard Rohn TB3 thrust bearing. I have not consulted Rohn on this, but I
believe just by looking at it, that the Rohn TB3 is made to be used in
compression and not in tension, that is, with the two halves that surround the
bearings geing pressed together. This is the mode it operates in when mounted
on the top of a shelf. If you mount it underneath, the bearing would be
in tension with the halves wanting to separate. It think this would put
unusual stress on the bearing race as it attempts to support the weight of the
mast and antennas.
If you were planning on using a different type of bearing, then all bets are
off. It would depend on the specific construction of the bearing itself to
determine if it will work in either direction.
Bearings seldom fail. I would not bother to design the system for easy bearing
replacement. Easy antenna removal: YES. Easy rotator removal: YES.
On the other hand, if I really wanted easy bearing removal, I would look at
these two possible solutions:
1. Use a bearing other than a Rohn TB3 that is designed to work in either
direction and mount it underneath the shelf, as you suggest.
2. Consider using a bearing shelf design that comes apart so that it can be
removed allowing the bearing to slide past that point for removal at the
bottom of the mast. I would consider building a bearing shelf that is made of
several overlapping parts, like a sandwich, that bolts together. For
example, you could use two Rohn rotator shelves, back to back, and each cut
into two pieces, right through the center, with cut lines that cross at
roughly 90 degrees. That shelf would come completely apart and allow bearing
removal at the bottom of the mast.
I would not use a bearing at the top. Rather, I would use a 25AG3 two inch
clearance ID pipe top. With both the rotator and bearing removed, this top
will hold a pretty good sized mast and antennas upright as long as no real
strong winds come up. You could secure the bottom of the mast inside the
center of the tower during this repair operation, just for safety, in case a
stiff wind DID come up.
Jim Idelson wrote:
> Hi all,
>
> Let me begin by informing you that I am fully aware of the K7LXC
> Prime Directive.
>
> I am making plans for the top section, rotor, mast and thrust bearings
> in a new Rohn 25 installation. The mast will be about 18-20 feet long. The
> rotor will be mounted at the bottom of the top section. One thrust
> bearing will be mounted on the flat top. A second thrust bearing will be
> mounted on a plate 24 inches below the flat top. With this setup 10-12
> feet of the mast will extend over the top of the tower for mounting antennas.
>
> Future serviceability of the installation is on my mind as I complete this
> design. Here are some thoughts:
>
> 1. If an antenna breaks, it can be removed directly for repair,
> 2. If the rotor breaks, it can be removed directly for repair,
> 3. If a thrust bearing breaks what do I do?
>
> Let's talk about the upper thrust bearing. Removal process would for a
> bearing mounted on top of the flat top would be to remove all antennas and
> slide the bearing up and off the top of the mast. Ugly process.
>
> The lower bearing is mounted on the top of a plate inside the tower. Removal
> would require sliding both the plate and the bearing down to the rotor and
> removing them there. Somewhat ugly.
>
> After all that discussion, here's the idea:
>
> Why not mount the thrust bearings on the UNDERSIDE of each of the
> mounting plates? The Rohn TB-3 appears to operate just fine upside-down.
> I'd use big, wide washers on the top to help distribute the downward
> forces across the mounting plate. Wouldn't this substantially ease the
> removal of either thrust bearing? Taking the upper bearing off by bringing it
> down to the rotor sounds much more appealing than taking it off over the top!
>
> Okay . . . let me know everything that's wrong with this idea . . .
>
> 73,
>
> Jim K1IR
>
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> James S. Idelson
> President
> DesigNET International, Inc.
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>
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