Greetings! At N3RS this summer we are installing a 3 element full
sized 40 meter yagi to replace the 2 element full sized yagi we have
been using for many years. This necessitated an upgrade to the mast
we use from large diameter water pipe to a 4130 alloy pipe, as well
as a new rotator upgrade from the marginal Tailtwister to an Emoto
1300MSAX. To offload the rotator from vertical load as well as make
rotator repairs easier, we use a home made thrust bearing inside the
tower about 10 feet down from the top. The bearing is a permanently
lubricated truck wheel bearing. A piece of solid steel rod was
turned down to just pass through the bearing, with a machined lip to
sit on the inner chase. The top of the rod was turned down to fit
inside the upper mast. The bottom of the rod was bored to accept the
bottom "drive shaft" to the rotator. Here are a couple of questions
we are wrestling with.
How should be connect the mast and drive shaft to the bearing? In
the past, we have used #8 tapered pins. Tapered pins are widely used
in heavy machinery to connect shafts and prevent any "slop", which is
common with bolted connections. Our experience is however, that the
pins usually refuse to come out. We recently had to cut the pipes
above and below the bearing for system disassembly. This was
unpleasant at 145 feet with mild steel water pipes. Probably
impossible with hand tools on the new 4130 mast. Would several
machine bolts be adequate, in lieu of the tapered pins? Anybody else
using tapered pins? Is there a way to treat or coat the tapered pins
before driving them home which will allow future disassembly without
the risk of them vibrating out?
The lower "drive shaft" is a 20 foot length of 1-1/4 inch, Schedule
40 galvanized water pipe. The idea of this long length between the
bearing and the rotator was to effect a "torque tube", hopefully
saving the rotator some torsional stress. Are we at risk of
torsional failure of this pipe with a 3 element full sized 40? The
antenna is an old HyGain LongJohn, and is moderately imbalanced from
a wind loading standpoint. We have no current plans to balance it
with a dummy PVC element. Any Mechanical Engineers out there know
how to convert foot-pounds of torque imposed by the antenna into
torsional stress on the drive shaft and then make a determination of
safety factor?
Any help with these two questions would be much appreciated.
Dave, N3RD <34703@vlf03v31.ueci.com>
|