[TowerTalk] Mast slipping

K8RI K8RI-on-TowerTalk at tm.net
Sun Jul 22 22:19:38 PDT 2012


On 7/22/2012 11:19 PM, Frank wrote:
> 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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> I used to have problems with mast slippage too.  It was aggrevating to
> have to crank the tower down and re-calibrate.  So I pinned the mast to
> the rotator housing.  The next time we had a strong wind I lost the
> rotator.  That slipping mast was a blessing in disguise.

That's a very good point. When moving to stronger mast clamps than come 
with the rotator the rotator needs to be strong enough to stand up to 
the extra stress.  They normally leaves out all of the ham type rotators 
as well as the lesser other models on the market.
Any time you pin a mast you are moving into uncharted territory and 
putting "at least" the rotator at risk if not more.

OTOH the mast clamps that come with the antennas may not be up to the 
task, or no where near the capacity of the rotator, but I do not trust 
spur gear rotators when it comes to torque. If you've ever rebuilt a ham 
series rotator those are some pretty weak, stamped steel gears.  The 
rotator gets its torque by running through many gear reductions, but 
that last gear looks pretty flimsy for handling much torque, hence the 
wedge brake.

I've torn up a number of ham series as well as two HDR 300s with the big 
array. I tried to move the array on windy days and although the HDRs 
have a lot of torque, they have no holding power except that wedge 
break.  I released the brake and the array rotated almost 180 degrees in 
2 or 3 seconds The motor must have been close to 10,000 RPM or even more.

So for big antennas and arrays from now on it's only double worm gear 
drives or home built rotators with heavy duty gear reductions  for me.

73

Roger (K8RI)


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