If you stay with metric bolts the grade 10.9 is the same as gr8 SAE.
150ksi tensile strength, 2x 18-8 SS. Use the correct washers and nuts.
Grade 9 is only 20% higher and more expensive. Plenty of torque tables
on line, including lubricated and dry.
Grant KZ1W
On 7/30/2020 21:55, Les Brown wrote:
Thanks for all the advice guys. I think I'll start with replacing the SS M8
bolts with 3/8 inch grade 8 bolts (or grade 9 or 10?) to see if that solves the
problem. If it doesn't, I'll have to try the K7NV clamp, but that will require
me to remove the rotor to slip the clamp on to the bottom of the mast, which
won't be so easy. Given that half the mast is sticking out of the top of the
tower, I'll have to rig up some pipes to hold the bottom of the mast in place
for the operation.
It sounds like drilling and pinning isn't a good solution, and not so easy
to do.
73, Les VE3NNT
-----Original Message-----
From: Grant Saviers [mailto:grants2@pacbell.net]
Sent: Wednesday, July 29, 2020 12:42 AM
To: Les Brown; 'towertalk@contesting.com'
Subject: Re: [TowerTalk] Mast slippage
I looked at the 71D picture at Array Solutions. Not impressed with the
bent up clamps. Grade 8 bolts can apply 2x the force of the stock SS if
torqued to spec, the easiest thing to try. Torque the bolts with a
torque wrench, wait a day and re-torque. I've had SS bolts take an
initial stretch, a known property. One project I worked on required
torquing the grade 9 bolts 3 times and temperature cycling the joint in
between. Quite amazing the change each cycle.
Or the 71D ears may bend around more. Measure the separation top and
bottom to check for that with what you have done so far.
A way to improve the existing bracket is bracing welded at least across
the top, say 5/16 thick x 3" with a V cutout. Even better if a second
one was cut to fit mid height and welded in. Or buy better one if it
exists.
The problem with pins is it is very difficult to get a round straight
thru hole of correct diameter in both parts by hand, especially on alloy
steel tube. So the mast will still work back and forth, elongating the
hole and wearing the bolt. The back and forth is hard on the rotator.
Your mast will still be "slipping slightly".
If the pin is your choice, I'd try a pilot point drill (Dewalt) of the
diameter that just fits the bracket hole. It has a center drill built
in and they are excellent quality steel. You might go thru with a couple
of these. If they don't hack it a Co/M42 drill will have a good start
hole. You will need lots of thrust force and endurance and coolant.
Water in a spray bottle is better than oil, WD40 is easy and good too, a
helper is necessary to have continuous application. The mast material
cert can tell you what you have and the possible hardness range is
greater than 2:1 Hrc. (there are 6 spec 4140 standards). Hand drilling
at the hardest level is doubtful.
A punch might provide a drill start on round alloy steel tube for maybe
a 1/16" drill but how would you get it centered? It is possible to step
up in a few steps to reduce the thrust needed but by hand it's unlikely
to preserve the position.
Drill from both sides, don't attempt to drill thru from one side.
There is no way you want to mess with a MagDrill (I own 2) on a tower
and there is nothing for the magnet to clamp to.
Good luck, be safe,
Grant KZ1W
On 7/28/2020 16:00, Les Brown wrote:
I have 2 large Yagis attached to a 3 inch OD ?? inch wall thickness Chromoly
mast running down the tower to a ProSisTel PST-71D. On a windy day, I hear
creaking sounds from the tower. The mast is slipping slightly in the rotator
mast bracket. I???ve already tried to tighten the 6 bolts, but it still slips.
I don???t want to tighten them to the point where they might snap. I???m
thinking that what I need to do is to go up the tower, drill a hole through
the mast (the rotator mast bracket already has holes), and pin the mast and
bracket. I understand that it???s not easy drilling through Chromoly pipe, so
before I try this, I thought I would ask if any of you have any other
suggestions.
73, Les VE3NNT
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|