For various reasons (some inherent in bearing manufacturing tolerances)
three spaced bearings are almost never done in machinery - it's just too
hard to get them/keep them aligned. Two bearings can handle a degree or
two of face misalignment, and there is that much forgiveness in regular
ball bearings, but very little offset in the bearing centers of rotation
is tolerable. How much offset depends on the bearing quality and shaft
stiffness. Three bearings (two thrust plus rotator) spaced along a mast
is asking for trouble.
Application of bearings is too off topic, but briefly, angular contact
bearings are used for combined radial and thrust loads since neither
thrust or ball bearings can do both unless (only for ball bearings)
significantly oversized. For example a deep groove ball bearing should
have a axial load less than 15% of its rated radial load. Thrust ball
bearings have no meaningful radial load capacity. Tower "thrust
bearings" should be designed as angular contact bearings with a high
ball load angle, like 45 degrees. I've not measured any so any data
would be appreciated.
A tower should handle its spec'd wind load at the assumed 1' above the
top plate, with or without a rotator and/or mast inside. If a 1' stub
is welded to the top plate and the allowed beam boom is clamped to it in
a fixed direction, all should be ok. The leverage argument (Leeson?)
fails in extremis - a rotator 1" below the top plate doesn't multiply
the load by 12x and a zero length mast wouldn't make the leg load
infinity (wind load/zero).
Grant
KZ1W
DF3KV wrote:
> I don´t use any thrust bearing at all but a home made slew ring rotator with
> the mast flanged to the top of it.
> To align two thrust bearings will be quite a task with a heavy mast.
>
> 73
> Peter
>
> -----Original Message-----
> From: towertalk-bounces@contesting.com
> [mailto:towertalk-bounces@contesting.com] On Behalf Of Jim Thomson
>
>
> ## which is why you should always use TWO thrust bearings. Then you
> won't have a sideways load on the rotor at all ! Per Leeson [ dunno if he
> got this part right] you take the bending moment where the mast enter's the
> top bearing... and divide that number by the distance between the top
> bearing
> and what ever is below it... like a 2nd bearing.. or the rotor. If the
> distance between
> top bearing and next item below it is only 2'... VS 4'...... you can kiss
> the top 2'
> of the tower goodbye in a big wind storm.
>
>
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