[TowerTalk] Which Thrust Bearing to Use

jimlux jimlux at earthlink.net
Fri Dec 9 12:07:47 EST 2016


On 12/9/16 8:53 AM, charlie at thegallos.com wrote:
> <snip>
>>   Aluminum is  22 ppm/degree C
>>   Steel is 12
>>
>> UHMW PE is about 100
> <snip>
>> SO let's take an example of a 2" diameter mast.  going from -40C to 60C
>> is 100 degrees, and probably wider than the range most ham towers will
>> see.  So that's a change in diameter of, worst case 100*100 ppm or 1%
>>
>> Your 2" hole that just clears at -40 will be 2.02" at +60C.  I don't
>> think you're going to have a big problem with that 10 mil extra clearance.
>>
>> If you have a 20 foot mast sticking up above the top, and 5 feet in the
>> tower, that 0.02" slop will turn into 0.1" at the very top of the mast.
>
> Don't forget that your MAST is going to expand 12*100 ppm too, so it'll be
> a bit less than that

Yeah, I was worst-casing it, assuming that the mast was made of Invar<grin>



>
> Nylatron 66 is at 80ppm, go with MD grade it is 50ppm
>

I think the filled materials would be better choices in general, but I 
think from a practical standpoint, you could probably use almost ANY 
hunk of plastic you find sitting around.
I'd choose on the basis of pleasantness of fabrication - which mitigates 
against glass filled, because the glass will cut up your tooling, 
although if you're just using a hole saw in the garage, it's probably 
not a big deal.



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