On 7/23/2012 3:50 AM, Grant Saviers wrote:
> No doubt one can design and spend $$$ for boom clamps that will exceed
> the torsional strength of the mast,
No one has talked about exceeding the torsional strength of the mast.
It's been exceeding the internal strength of the rotator.
but why bother when off the shelf
> economical hardware will do the job? The major problem we all seem to
> agree about is adequate clamping to the mast when it is hardened (alloy)
> steel. In spite of galvanizing, it is still hard and slippery - just
> try to clamp hardened steel in a machinist hard jaw vise (which I
> regularly do).
Same here.
> So it takes a lot of contact area and a conforming
> material to generate the friction necessary to generate sufficient
> clamping force and hard steel on hard steel is a very poor choice. The
> cast aluminum surface in a DXE saddle clamp does that job well and two
> or three of them provide plenty of contact surface compared to anything
> I've seen in an out of the box antenna boom to mast clamp. A u-bolt
> holding a tube against a flat plate or against a muffler clamp style
> saddle are recipes for trouble.
Agreed.
In the first case there might be only
> two points of contact, one on the plate and one at the apex of the
> u-bolt unless it is perfectly sized to the tube. In the second case,
> the stamped saddle sides will spread out as force is applied.
>
> There is no benefit to having the saddle extend further than the DXE
> single saddle clamps as there is no force from the U-bolt to provide
> clamping force.
Here we disagree, but the clamping action does reduce with distance from
the bolt. OTOH a two inch wide saddle would give notably more grip than
a one inch wide. saddle.
If more clamp is needed, there are the DXE "super-duty"
> saddle clamps with two saddles and through straight bolts.
I prefer two saddles on one u-bolt.
However when it comes to anything larger than a 2" mast I prefer a
larger clamp with straight bolts.
>
> For mast to rotator, if a split sleeve is used such as in the K7NV
> rotator clamp there is force around much of the circumference clamping
> the mast and maximum contact between the surfaces if tolerances are
> correct. see http://www.k7nv.com/proppitch/id35.htm
Something similar to that is what I've been referring to. What I don't
like about that is the asymmetrical forces exerted at the base of it.
Of course if there is only a few thousandths clearance between loose and
tight it wouldn't be a problem. I'd rather see it as a split fitting as
the 4 bolts holding it to the rotator would be the weak point and for
that size (2 or 3") The clamp is far stronger than the attachment bolts
and both far exceed the needed strength.
Since most masts
> have multiple antennas, the strength needed at the rotator is a sum of
> all the torques and the upgraded rotator clamps (over stock) are
> probably worthwhile in many large antenna installations.
>
> The engineering tests show that double nuts, properly tightened are
> superior to lock washers and the nylock nut is somewhat better as the
> second nut. see http://www.boltscience.com/pages/twonuts.htm Even if
> the nylon degrades (there are non nylon inserts as well) the double nut
> friction still is better than a lockwasher. The most important factor is
> sufficient bolt torque to insure the nuts stay tight. I considered
> mentioning using a torque wrench to get the bolts adequately tight and
> that is definitely a good idea. There are other choices as well to
> insure the fastener stays tight. Perhaps the easiest and most reliable
> is Loctite.
Agreed
The medium strength grade (blue 242) can be disassembled
> with hand tools (no heat required). However, the bolt surface should be
> clean so use the Loctite to prevent SS galling.
I just use what are called double sided lock nuts. They appear to be a
normal nut, possibly a little thicker than normal. Looking closely you
can see two very small indentations 180 degrees apart on the flats.
They work very well and I've never had one come loose, but you do have
to remember to use either locktite, or neverseize to prevent galling.
The neverseize will not normally reduce the gripping force enough to
cause a problem.
Belleville washers are
> used when stress or temperature cycling can cause the tension in the
> bolt to reduce, potentially lowering the friction force holding it or
> the nut tight,
The last time I use Belleville washers was when working in industry, but
they are certainly superior to lock washers. That was over 35 years ago
and I'd even forgotten about them. I've never had conventional lock
washers last more than a few years. SS lock washers do last, but do not
appear to add much force. The only lock washers I've seen that worked
well, were *big*! Big as in one inch and up.
a problem in steam lines but not likely on a tower.
> Bellevilles are very high rate springs that are not flattened at the
The use of Belleville might be indicated with large Aluminum boom to
mast clamps with SS bolts of 3/8" and up where the expansion ratio
between the Al and SS is substantial. Probably not needed on tribander
and WARC 7 size antennas, but they certainly should not hurt.
> required bolt torque/tension. There is also a ratcheting 2 washer
> combination (Nord-Locks) that causes the tension to increase if the
> assembly starts to unthread but these are about $3 each in 3/8 SS from
> Fastenal (if you can find them ). Regular split ring lockwashers are
> now recognized as just about useless, they flatten at required torques
In over 50 years I've only had one boom to mast clamp come loose and
that was on a Hy-Gain TH-5, maybe 8 or 10 years ago.
One of my pet peeves is hardware on Force 12 antennas. So far I like the
antennas and would get them again, but I've had to replace all the
U-bolts with SS saddle clamps. I had to soak the nuts with PB-Blaster
for about 24 hours to get them off and then I still twisted off 3 of
them. That is a lot of saddle clamps and IIRC it cost over $100 to
replace them with the saddle clamps on the C19 XR. Now I'd like to
replace that with the C31 XR.
BTW although those saddle clamps are a bit pricey, you can't build them
yourself for that price unless you just happen to have some 1" thick Al
plate or rectangular rod handy or one whale of a good source.
73
Roger (K8RI)
<snip>
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