The brake spec is 2000 ft-lbs.
The problem with a pin is a zero tolerance fit to the 4 holes, as others
have advised. Very hard to do on the tower and 120ksi mast. The
"limited slip" idea is a bad one IMO, since the shock from a stop will
break bolts or rotator.
IMO, the factory mast clamp design from bent sheet metal is worthless
for the antennas you have. It seems you have proven that.
Grant KZ1W
On 8/28/2016 13:01 PM, Rudy Bakalov via TowerTalk wrote:
Thank you everybody for the thoughtful responses. Lots of advise to consider.
A couple of follow up questions:
1) The responses lead me to believe that the answer slightly tips in favor of
pinning assuming the pinning is done right. Doing it right at 90' will be a
challenge. So, how do I know if the PST61D will take the load if the mast is
pinned? What specs do I need to review and how do I calculate the load for my
particular antennas? I know the wind load surface is less than max specs, but
how do I consider the torque that comes from the 40m yagi boom.
2) What about an alternative solution that allows for some, but manageable
slips? Take a look at the mast clamp at
http://arraysolutions.com/image/data/products/Rotators/PST-61%20with%20controller.jpg
What if I drill two holes in the mast that are perpendicular to the current
bolts and insert two long bolts that are long enough to touch the clamp.
Basically, will insert two pins between the clamp halves. So the slipping will
still happen, but will be limited to the angle created by the gap between the
clamp halves.
Rudy N2WQ
Sent using a tiny keyboard. Please excuse brevity, typos, or inappropriate
autocorrect.
On Aug 28, 2016, at 3:26 PM, Grant Saviers <grants2@pacbell.net> wrote:
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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