[TowerTalk] How do I stop 2"-OD Chromoly mast from slipping in PST-61C rotator clamp?

[David Gilbert]

I have a PST-61D with a large tribander and a shorty 40 above it on a 2 
inch chrome-moly mast.  Not long after I put it up this spring we 
experienced some of the strongest clear weather winds I've ever seen 
here in Arizona, with 90 mph gusts on several occasions and one that 
topped out my wind gauge at over 100 mph.  These are typically 
mini-twisters caused by the wind swirling around the ridgelines on the 
side of the mountain range I live on.  They roar down the hillside like 
a freight train and I've watched them rip branches off trees 200 feet 
away from me while I stood in completely calm air.  When one of these 
little monsters hits the tower dead on, it is entirely possible for the 
wind to be pushing on one end of the antenna boom while simultaneously 
pulling on the other end.  The torque on the system would be incredible 
in such situations.

I had tightened every bolt on the rotator and boom-to-mast clamps as 
tightly as I could but still suffered some slippage during a couple of 
those twisters.  My solution was to realign everything on a calm day and 
then pin everything to the mast with 3/8 inch grade 8 bolts.  There is, 
I suppose, some potential for the bolts enlarging the holes in the mast 
over time, but I again tightened all the clamp bolts as tightly as 
possible and I don't expect the pins to come into play very often.  I 
realize some folks try to avoid pinning the mast in a rotator, figuring 
that slippage offers a bit of safety factor against overstressing the 
rotator, but it seems to me that it would be pretty silly then to try to 
totally avoid slippage in the first place.  It doesn't matter whether 
the mast is securely clamped in place in the rotator or if it is pinned 
... the net result is the same.

My attitude is that I didn't buy a strong enough rotator if I have to 
worry about it getting damaged without slipping.  And if your PST-61C 
clamp is the same as the one on my PST-61D, it already has holes in it 
that are just the right size for a 3/8 bolt.

By the way, chrome-moly is exceptionally tough but not overly hard, and 
drilling through the 1/4 inch wall mast was not difficult.  I used a 
cordless drill and the gold-colored "titanium pilot point" bits made by 
DeWalt that are sold at Home Depot.

73,
Dave   AB7E



David J Windisch wrote:
> Hi, all concerned:
>
> The rotator clamp held the mast just fine for more than a year since 01/03/2007; now, even though all 6 of the 1/2" clamp-bolts have been retightened, the mast is slipping again.
>
> We've had several "microbursts", as defined by neighbors witnessing trees toppling down and/or being heavily damaged along straigh-line paths in the area.  One incident was within 30M of the tower; thank Goodness for the grace of near-misses.
>
> Fwiw, 2 homebrew antenna-to-mast clamps, in a TH11 and 402CD shortstack, made with DXE U-bolts and half-inch aluminum angle stock, have not budged.
>
> Been reading the searchable archives.  Is a SlippNott, purchased or homebrew, the way to go?
>
> Help, please.
>
> Tia & 73,
>
> Dave, W8FGX
> Cincinnati
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
>