There are many opinions posted about the stock mast clamp from great to
junk. Presumably, based on experiences either way. This post is about a
modification I made to improve the clamp reliability at high load.
I'm wondering if any other towertalkians have modified the stock clamps
and the results and advice.
My prop pitch failed atop my tower turning a JK 2L 80m beam (390# x 100'
elements) on a 3" x 0.25" wall alloy mast. Having a spare 2800, a call
to JK advised me that the 2800 rotator has been up to the task. Long
lead times for a new/repaired PP is causing me to try and understand the
variety of results with stock 2800 clamps. Then, since I have a big
machine shop, how I might modify the stock clamp to reliably clamp the
80m beam mast. Aftermarket 3" clamps seem to be currently unobtainium.
I have a stock 2800 & clamp turning a DB36 and 86' 80m rotatable dipole.
It works great, 9 years no slip at 91' and 102' respectively on a 2"
alloy mast. I did use grade 8 clamping bolts and torque 3 times to the
max spec. So consider me open minded re what a detailed clamp
investigation might find.
Taking a machinists look at the design, it is basically a 120 degree
included angle v-block with serrations. That enables a variety of mast
diameters to be clamped. One or two serrations will contact the mast
(over the permitted diameter range) in four spots at four levels.
Cast iron may have surface hardness, it depends on the casting process
and heat treating. They weren't hard in my clamps. So some yielding of
the clamp protrusions happens on high strength mast steel. Another
reason they might work well on some masts is because the mast is
yielding in roundness. Another possibility with the 4 sets of 3/8 clamp
bolts is the 3/8" thick oval clamp mounting plate bends as bolts are
tightened to compensate for the un-squareness base to bore. This seems
unlikely as a design intent since cast ribs and footprint of the welded
base greatly stiffen this plate.
One knowledgeable opinion offered to me was the stock clamps are too
stiff. They don't conform to the mast with increasing tension as do
U-bolts, which might distort the mast as well.
An issue with any cast iron part is nothing will be flat or square out
of the mold, and will move around with heat treating. Every use of the
pattern or mold will cause wear. This makes fixturing and reference
surface selection for machining tricky. I have an half clamp as M2 sent
that by mistake without drilled base bolt holes on a 2800 I bought new.
That part gives me a bit more insight and is a nice doorstop. To their
credit, they immediately air shipped a machined replacement.
Cutting to the issues, I found one side out of square ~ 0.030"
referenced on the machined bottom. Checking serration squareness
yielded significant protrusion variations in both clamps. Wear from the
prior 3" mast this rotator held showed maybe a half dozen total
serrations in contact with the mast. For the current 80m beam
application, I didn't think this was going to work.
The modification process for me was to make a flat fixture plate to hold
the clamps in a four jaw lathe chuck and bore them to 3.000". I move
the clamps together on the fixture to obtain at least 10 contact
surfaces per level at 3.000" diameter (~ 40 total). The contact areas
vary from 0.050 to .15" in width by about an average 0.5" long. The
boring shows significantly more material removed at the bottom than at
the top of each clamp. Thus the "bore" was more out of square to the
base than I had initially measured.
btw, the clamps have enough material that the boring to a mast diameter
can be done for diameters from 2" to slightly larger than 3". They also
appear to be very strong so unlikely to break with max grade 8 bolt
tensions. It would take an finite element analysis to know for sure and
that would also show how the mast is distorting.
The M2 supplied bolts are grade 5 3/8-18 and at spec torque (~40ft-lbs)
have 4940# tension. The grade 8 3/8-24 I will use have 7900# tension at
the same torque.
The 2800 with modified clamps are installed and so far so good, but
there has not been a severe wind event to challenge them.
Grant KZ1W
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