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[TowerTalk] K7LXC replacement Orion mast clamp

To: "Tower" <>
Subject: [TowerTalk] K7LXC replacement Orion mast clamp
From: "Dick Green WC1M" <>
Reply-to:, "Tower and HF antenna construction topics." <>
Date: Sun, 21 Mar 2010 14:00:53 -0400
List-post: <">>
Last week I installed the K7LXC replacement mast clamp for the M2 Orion
rotor and am happy to report that the job went very well. It'll probably be
months before I know how well the new clamp performs (we don't get major
windstorms here very often), but it sure looks like it's going to hold a lot
better than the stock clamp, which allowed the mast to slip several times,
including a whopping 60-degrees during a recent 50-60 MPH windstorm.


The most tedious part of the job was removing the old mast clamp. Had to
remove 10 bolts threaded with aircraft nuts, which have to be unscrewed with
a wrench nearly to the end of the bolt before they can be unscrewed the rest
of the way by hand. Took a long time. It's amazing that with so many bolts
the clamp does such a poor job gripping the mast!


I only had to jack up the mast a couple of inches to slip the K7LXC clamp in
place over the center bolt of the rotor's top platform. I had been worried
that the round (not oval) holes in the clamp might not line up perfectly
with the holes in the platform. Steve said the holes are precision drilled,
so I gambled and didn't bring a drill up the tower. I did bring a reamer in
case there was some extra galvanizing around the holes, as one person
reported, but turned out to be unecessary. The holes were indeed precisely
drilled and lined up perfectly with the holes in the platform.


I found that the metal wedge provided by M2 for centering the mast does fit
between the walls of Steve's clamp, if oriented lengthwise across the
opening. Although the wedge really isn't necessary, I used it with Steve's
clamp for two reasons. First, my 40m beam sits very close to the top thrust
bearing and the extra height of the wedge is needed to keep the beam's
mounting bracket off the thrust bearing. Without the wedge, I'd have had to
raise the beam an inch or two up the mast (the beam weighs 165 lbs.) Second,
the wedge raises the bottom edge of the mast off the weld at the bottom of
the clamp walls. Probably no big deal, but I prefer keeping the mast off the
weld. However, after I had lowered the mast onto the wedge and tightened the
bolts, I found the wedge was loose -- i.e., the weight of the mast wasn't on
the wedge. I loosened the clamp bolts and the thrust bearing bolts again to
let the weight of the mast rest on the wedge, but when I tightened the clamp
bolts the wedge was again free to move. It appears that the u-bolts either
pull the mast off the wedge or deform the (aluminum) mast such that it
doesn't make full contact. In the end, it might have been better to skip
using the wedge and raise the beam on the mast. I'm reasonably certain the
weight of the mast and beam are on the bolts of the clamp, and are thus
being transferred to the rotor. Probably better to have the weight supported
by the rotor platform, but given the number and heft of the bolts I don't
think it's a big deal.  


Anyhow, I'm a happy camper now.


73, Dick WC1M



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