No doubt one can design and spend $$$ for boom clamps that will exceed
the torsional strength of the mast, 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). 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. 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. If more clamp is needed, there are the DXE "super-duty"
saddle clamps with two saddles and through 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 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. 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. 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, a problem in steam lines but not likely on a tower.
Bellevilles are very high rate springs that are not flattened at the
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
and have low spring rates. see
http://www.boltscience.com/pages/junkertestvideo.htm and various
engineering forums.
Grant KZ1W
no affiliation with any manufacturer yada yada
On 7/22/2012 10:32 PM, K8RI wrote:
> On 7/22/2012 11:43 PM, Grant Saviers wrote:
>> I'll second the motion that the DX Eng saddle clamps are the way to go.
> I use many of their saddle clamps, but I'd not use those for large
> antenna to mast clamps or rotator to mast clamps. There just isn't
> enough surface area.
>
>> Lots of surface area of the saddle in contact with the mast and
>> stainless u-bolts are provided. Use a SS nylock nut backing up the
>> (provided) regular heavy hex nut.
> The same for Nylock nuts. The nylon does not stand up well to the
> elements. I get 2 or 3 years out of them at most and I use a lot of
> them. Usually after a couple of years I can take them off with my
> fingers. More than that and they are just loose on the bolt.
>
> On the under side of antennas pointed down so the sun can't get at the
> nylon they do fairly well, but horizontally on a mast clamp or worse,
> vertically they do not do well at all. There are stainless Steel
> locking nuts that work very well, but be sure to use never seize as they
> will become part of the bolt in just a year other wise.
>
> I usually purchase this kind of hardware by the box of 100 at the local
> industrial supply "Prime Fastener" and they have bvery good prices, just
> be sure you don't get aircraft grade as they cost a fortune. I think it
> was near $70 or $80 USD for a box of 100 3/8" SS nuts. I said "I'll
> pass" give me the regular grade.
>
> The only downside is they are only
>> made in tube sizes, no pipe (IPS) sizes, when last I bought them. Your
>> mast to boom plate needs to be thick enough to handle the stress, I use
>> 1/2" thick 6061 T651 x 8" wide x 20" long and that has enough room for
>> four saddle clamps for a 2.5" boom to three saddle clamps for a 2" or 3"
>> mast.
> Go to a small machine shop and have them make up some mast clamps that
> are like double sided mast clamps, but wide enough to take 2, 3, 4, or
> even 5 U-bolts. The saddles should extend much farther around the mast.
>
> If you test the gripping force for a 2" saddle clamp which is only about
> an inch wide I think you will find it on par with normal mast clamps.
> Using 2, 3 or 4 mast clamps is a good start, but custom made clamps are
> far stronger and have considerably more gripping force.
>
> 73
>
> Roger (K8RI)
>
>
>
>> Grant KZ1W
>>
>> On 7/22/2012 7:00 PM, Gregg Seidl wrote:
>>> I just got down from my tower after turning my XM-240 in line to match the
>>> rotor and other antenna AGAIN.
>>> I have done this several times now and I just can't get a grip on why this
>>> continues to happen. I haven't had trouble with any other of my
>>> antennas,some have more windload than the XM-240.
>>> As I was up there enjoying the view and thinking why this one turns I had
>>> an idea and wanted to ask others ideas.
>>> First it is up 85 feet and has been up for 6 years and I think it works
>>> great. It is much more stout than the 40-2CD that I had up before but came
>>> down piece by piece in a bad windstorm.
>>> I am using the stock clamp which seems to be great,the only thing that I
>>> haven't used one other antennas but did on this one was a galvanized mast
>>> from Texas Towers. Could it be that the mast is too "smooth" to grip? The
>>> U-bolts are not loose when I go up to straighten the antenna. It only
>>> happens once a year or so nd always after a 70-80 MPH windstorm which we
>>> get here sometimes.
>>> Looking for ideas.
>>> Gregg K9KL
>>> _______________________________________________
>>>
>>>
>>>
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>>>
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>>
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