On 12/9/16 8:13 AM, Hardy Landskov wrote:
UHMW PE has a horrible coefficient of expansion. I don't know about
Nylatron.
But this isn't a micron clearance kind of application,is it?
Aluminum is 22 ppm/degree C
Steel is 12
UHMW PE is about 100
Nylons without glass seem to be in the 75-100 range
Acetal with glass filling is about 39 ppm, w/o the glass it's 106
So, let's assume grungy regular "plastic" is about 100 (although getting
a good glass filled, UV resistant wouldn't cost much more and would
probably be a better choice for other reasons)
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.
N7RT/4
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
charlie@thegallos.com
Sent: Friday, December 09, 2016 11:06 AM
To: George Dubovsky
Cc: towertalk; charlie@thegallos.com
Subject: Re: [TowerTalk] Which Thrust Bearing to Use
>I would expect either
tapered roller bearings, or today, do what most of industry does, and
use an engineering plastic bearing.
And, in fact, that's what I do. One mast has a brass shaft collar that
rides on a slab of Nylatron (GS grade I think), and the other mast
uses a scrap TB-3 that was machined to replace the ball assembly with
a Nylatron sleeve insert for axial and radial thrust. I expect both of
them to outlive the towers they're sitting on... ;-).
73,
geo - n4ua
Yeah, I suspect they will
I'm sort of surprised however that they aren't made that way, although by
FEEL, that may be how the Yaesu thrust bearing is made. Heck, a piece of
UMHW PE would probably work too. As I said, not a real high load situation.
You have room for LOTS of square inches of plastic, which will prevent any
creep so you don't need the high bucks stuff like rulon or vespil (shudder
when you price vespil), it'll never rust, it never needs lube, doesn't
matter if it gets wet
Looking at the design of the DXE bearing, and the TB3 - the DXE is a true
THRUST bearing - aka designed for a load along the axis of rotation (aka
carry weight off the rotor), where the TB3 is really just a regular bearing
for side to side loads. To be BOTH, they would really need to be angular
contact, or use a flanged plastic bearing, so it has bearing surfaces in
BOTH directions, and get the best of BOTH worlds
Heck, if I was designing a rotor today, or say updating the Ham V series
(including the TX2), I'd use plastic there too, with the added advantage of
it tending to act as at least a partial water seal. Never did the math for
what plastics you'd need keeping the contact size the same
I mean nylatron is practically begging for that use. No elevated temps
(folks reading - elevated temps here means greater than 50C on a continuous
basis aka 20 hours at a time - and even then, if we go reinforces with
either glass or Molly, you can go 75c - just in case you want to paint your
bearing black and put in in the sun in the Mideast), "moderate" loads per
square inch, low rotational speeds, no seriously significant shock loads.
Never have to worry about the balls indenting the races like the Ham IV,
never have to worry about rust etc. MFJ could get rid of the two steel
races, the balls, the retaining rings etc, and have a better product for
probably less money
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