[TowerTalk] Mast slippage

[Grant Saviers]

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 at contesting.com] On Behalf Of Les
> Brown
> Sent: Friday, July 31, 2020 9:50 PM
> To: john at kk9a.com; 'Jon Pearl - W4ABC' <jonpearl at tampabay.rr.com>;
> towertalk at 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 at contesting.com] On Behalf Of
> john at kk9a.com
> Sent: Friday, July 31, 2020 7:18 PM
> To: 'Jon Pearl - W4ABC' <jonpearl at tampabay.rr.com>; towertalk at 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 at tampabay.rr.com]
> Sent: Friday, July 31, 2020 5:21 PM
> To: john at kk9a.com; towertalk at 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 at 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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