I agree with Jim.
There is a problem not mentioned with the "pin the mast" strategy. That
is the hole and pin need to be a "close" to "line" fit. Otherwise the
mast will move back and forth in the actual sizes of the 4 holes. That
leads to wear and high shock loads on the mast and antennas. Eventually
the holes (or bolt or both) wear enough to allow a large excursion and
more shock. When I fit a taper pin with a special reamer to pin a shaft
to a collar it is a tight fit that locks both parts in place with the
jam taper of the pin. Not used so much anymore, but it is the classic
way to prevent this problem.
Any hand drilling of alloy steel tube likely won't be round or anywhere
near desired diameter. Even done in a vise on a mill, if drilling thru,
it is likely the hole on the far side of the tube will wander from true
position.
IMO, if your mast clamp slips, install a better one.
Re clamps, I'll chime in also.
plain U-bolts only are the worse - if slightly off size, only two points
contact, one at the flat plate and one at the top of the bolt. Can be
made to work using lots of them of the right size. Prone to distorting
the tube so clamping force is limited for thin wall tubes.
muffler clamps are one better IF the right size - more contact area in
the saddle, but the stamped saddle is prone to splaying out when
tightened, see Cushcraft A3/4 designs
really good are the DX Engineering cast saddles of correct fit - lots of
contact area in the saddle. The double V ones are not as good with 4
points of contact but are better than muffler clamps when the exact size
isn't available, less distortion of the tube. My go to clamps for
everywhere on antennas and masts.
near perfection are full top an bottom saddles with thru bolts -
probably 300 degrees plus area contact, see JK Antenna designs, and the
SteppIR DB36 I am assembling right now. clamps tighten with very little
tube distortion. JK ones are CNC machined and SteppIR has invested in
extrusions they slice into saddles - nice. I make them when needed, eg
driven element insulators.
perfection are the sleeve clamps, fit to the exact diameter of the tube
- several folks make these as mentioned. Nearly impossible to distort
the tube.
Grant KZ1W
On 8/29/2018 4:07 AM, Jim Thomson wrote:
Date: Tue, 28 Aug 2018 15:29:03 -0400
From: Warren Munro <wmunro1@hawaii.rr.com>
To: towertalk@contesting.com
Subject: [TowerTalk] drilling chrome-moly mast
<All
<This may have been mentioned before, but my machinist brother-in-law advises drilling
starting with a quite small bit and then use new gradually larger bits as you step up in hole
size. This will cut down on the annealing, and will help eliminate the "bite" at the
<end of the final hole.
<73
<Warren KH6WM
## IMO, trying to drill a .375 hole into a 2.00 OD X .375 inch thick
heat treated 4130 chromolly mast..... while up at the top of a tower would be
a night mare. And trying to free hand it with a hand drill ?? Good luck
with that.
The original idea was to... pin the mast, so it wont slip. IMO, it would be
much easier and simpler to just use the correct rotor clamp in the 1st place,
like the K7LXC or K7NV units. They dont slip period...end of problem..at
least at the rotor
end of the mast. Where the boom meets the mast... that assy is easily done
with 2-3-4 of the DXE super mast clamps. They come in 2 inch ID, 2.5
inch ID..and also 3.0 inch ID. I used the F12 concept, which uses 2 al
plates, one is mounted to mast
using several of the DXE mast clamps. The horizontal al plate is mounted to
the boom with either DXE SS U bolts + solid al saddles, OR a pair of DXE
mast clamps. Each al plate is .375 inch thick. Plates are mated with 4 x
.375 inch bolts.
The 5th bolt is typ .5 inch. I drill and tap the 5th hole into the vert
plate. Vert plate is mounted to mast before hand. Horz plate is mounted to
boom. Boom brought up and mated to vert plate simply by sliding hole #5
over mating stud #5..
done deal. Then align the boom, such that the 4 x remaining holes line up.
Stuff 4 x remaining bolts in..and u are done. F12 called that assy the
..easy on mount. I just took the same concept, and beefed it up a bunch.
And no, the vert plate
will not spin on the mast, it cant. Those DXE super mast clamps are things
of wonder..with a massive a mount of clamping force on each clamp. 80m
rotary dipole gets a 8 x 14 vert al plate..and a 8 x 17 horz plate. 2 x DXE
super mast clamps used on vert plate...and 6 x dxe ss u bolts with solid
saddles used on Horz plate. The 40m assy gets a 8 x 17 vert plate..and a 8 x
14 horz plate. 40m vert plate gets 4 x DXE super mast clamp..and horz
plate gets 2 x DXE super mast clamps. Nothing can slip
on the mast, no need to pin the mast. K7NV small PP with its 2 inch mast
clamping assy. All TQ balanced, so minimal tq required to rotate..and hold it
in a wind.
Jim VE7RF
_______________________________________________
_______________________________________________
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
|