It depends a bit on what you want to happen in an overload situation.
My PTO auger used grade 5 bolts which worked as shear pins if the auger
hit a rock, thus the auger shaft was not damaged.
An 8mm hole would insignificantly weaken the mast inside the clamp and
you have a thrust bearing above that takes most of the load. A quick
calc of 8mm grade 5 (65ksi steel) in shear at 2 places on 2" mast is
about 700 ft-lbs to shear it. I don't know how that compares to your
rotator max parked torque, so maybe a larger diameter or grade 8 bolt is
appropriate. Another consideration is how the rotator clamp bolt holes
might be deformed. Since the current clamps can't generate a
slip-critical fit condition (friction between the surfaces rather than
bolt shear strength as the limit) I don't think a single pin will have
sufficient clamping force for slip-critical fit. With the clamps
slipping and the shear bolt not a perfect fit the mast and bolt will
experience a shock load from the slop in the fit and over time the bolt
and hole wear will make that worse.
What will be difficult is to drill a straight accurately sized round
hole while on the tower in 120ksi steel. I'd use a USA cobalt split
point drill sized as close as possible to the bolt diameter and plenty
of coolant - about $15. Step drilling in hard steel is difficult unless
you have a core drill designed for that purpose.
You might consider an aftermarket mast clamp for the rotator that would
be an easier and better solution. It would be IMO.
Grant KZ1W
On 8/28/2016 9:47 AM, Rudy Bakalov via TowerTalk wrote:
I am getting really tired of my mast slipping during major wind gusts. The mast
is supporting a 4 el 40m M2 yagi and a 6 el M2 KT36XA. The clamp is the
standard clamp that comes with the ProSisTel PS61D rotator.
So the question is if I should pin the mast to the rotator clamp using a M8
bolt. The clamp has 8 bolts, but clearly the friction between the clamp and the
mast is not sufficient under extreme conditions. My one and only concern about
pinning is that the hole in the mast may weaken the mast. The PST61D is rated
for 36 sq.ft so, right or wrong, I am not concerned about the wind damaging the
rotator.
If you suggest I pin the mast, there are two follow up questions:
1) What grade bolt should I use for the pin? The mast is 120K psi. Should the
bolt be rated for less (e.g., Grade 5) or more (e.g., Grade 8 or 10.9) psi?
That is, do I want the bolt to break before the mast does under extreme
conditions?
2) Further, is it OK if the bolt is zinc plated? I could not find stainless
steel Grade 8/10.9 M8 bolts. Is the contact between zinc and the mast going to
lead to corrosion?
Rudy N2WQ
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