The minor diameters of both 5/16-24 and M8x1 are within 1% of each
other, so the area resisting tension is the same.
Grade 8 tensile strength 150,000 psi. 18-8 SS 70,000 psi so a simple
comparison is the Grade 8 is over 2x the strength of 18-8 for same minor
diameters.
There is some data about 18-8 stretch at less than yield which from
experience I think is true. My mast clamp choice is always Gr8 in NF
threads as the NF has a slight gain in minor diameter and is usually the
choice for high strength bolts.
Higher strength materials are available if you are willing to pay $$.
My belief is most slip problems are caused by not using a torque wrench
and sequential tightening plateaus up to the specified value. A dry Gr8
3/8-24 tension is 7900# when torqued to the spec 47 ft-lbs.
Then check in a few days and retighten whatever has yielded/moved a bit
(everything does).
Grant KZ1W
On 5/26/2021 18:06, Les Brown wrote:
Hi guys,
Last year, I reported mast slippage with my PST-71D clamshell mast clamp.
Well, after replacing the clamshell clamp with the nice K7NV 3 inch mast
clamp, no more mast slippage. However, the weak link now seems to be the
rotator itself. In a strong wind, I hear a creaking sound as the rotator
slips a couple of degrees one way then the other. Is this actually turning
the gears all the way back to the worm gear or is there a clutch pad in the
PST-71D that could be slipping?
I have 2 Optibeams up top for a total of 44 square feet of wind load and
the PST-71D is rated at 80 square feet.
73, Les VE3NNT
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Les
Brown
Sent: Friday, July 31, 2020 9:50 PM
To: john@kk9a.com; 'Jon Pearl - W4ABC' <jonpearl@tampabay.rr.com>;
towertalk@contesting.com
Subject: Re: [TowerTalk] Mast slippage
I was wondering about the size of the holes in the rotor mast clamp. I
didn't measure them to see if there was any slop. I know that 3/8 inch is a
little larger than M8. I was worried though that a 5/16 grade 8 might not be
much stronger than the M8 SS bolt. I may be wrong. Maybe you guys know the
tensile strength differences. I was prepared to ream out the holes to fit
3/8 inch if necessary. Do you think 5/16 inch would still be a good choice?
73, Les VE3NNT
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
john@kk9a.com
Sent: Friday, July 31, 2020 7:18 PM
To: 'Jon Pearl - W4ABC' <jonpearl@tampabay.rr.com>; towertalk@contesting.com
Subject: Re: [TowerTalk] Mast slippage
The bracket's hole size was never mentioned in the VE3NNT's post. I thought
it was odd that he was going up a size so I questioned if 3/8 will fit.
John KK9A
-----Original Message-----
From: Jon Pearl - W4ABC [mailto:jonpearl@tampabay.rr.com]
Sent: Friday, July 31, 2020 5:21 PM
To: john@kk9a.com; towertalk@contesting.com
Subject: Re: [TowerTalk] Mast slippage
Hi John,
3/8" is a tad fat for the M8 hole.
5/16" = .3125" and will drop in a M8/.3149" hole
3/8" = .3750" and will drop in a M10/.3937" hole
73,
Jon Pearl - W4ABC
On 7/31/2020 12:36 PM, john@kk9a.com wrote:
Will 3/8 bolts fit into a plate designed for M8? I believe that
stainless steel bolts are typically not much stronger than grade 2.
Grade 8 bolts are available with Armor Coat which has great corrosion
protection.
John KK9A
Les Brown VE3NNT 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
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