[TowerTalk] [Bulk] FW: Rotor Position in tower

[Grant Saviers]

Matt summarizes the issues well.  As a crusader for correct terminology 
a comment: A "radial" bearing at the tower top plate will reduce the 
bending moment on the rotator, such as the tube welded into some crank 
up tops in lieu of a flat top plate.  That is a pure radial bearing as 
all thrust is handled by the rotator bearings.

The question not answered is "how much mast between rotator and top 
plate bearing?"  Any 2" or 3" mast will have a bending moment of inertia 
a small fraction of its likely sized welded lattice tower so will add 
very little strength to a top section.  So long masts inside towers are 
a waste of mast and for any fixed length above the top increase wind 
loading and weight on the rotator and tower. Other factors are the type 
of top bearing as a ball bearing will tilt several degrees in the race 
before it is damaged, but the 6" (or so)  pipe sleeve at the top of my 
HDX589 won't allow much mast tilt at all.  Some commercial "thrust" 
bearings let the mast tilt at surprising angles when free from the 
rotator - not a good idea!

Another constraint is the amount of headroom needed to use a come-a-long 
or lever hoist to raise a mast to remove a rotator.  I think that makes 
the practical lower limit of inside to tower mast of about 4 to 5'.  So 
IMO, more mast inside is for mounting convenience or required placement 
of the the rotator mounting plate (or where the tower maker welded it in!)

Another constraint:   if the mast is partially sleeved for strength in 
the lower half, that sleeve should extend a couple of feet above the top 
bearing.

Grant KZ1W

On 11/1/2015 15:45 PM, Matt wrote:
> Assuming you are using a thrust bearing at the top of the tower, mounting
> the rotor lower in the tower reduces side loading on the rotor - usually not
> a limiting factor.   The thrust bearing normally supports the full weight of
> the mast - assuming it is locked down to the mast.   Mounting lower in tower
> does not reduce the maximum bending stress in the mast other than any net
> shortening of the mast's projection above the thrust bearing (which would be
> the same as using an equivalently shorter mast section).   Mounting lower in
> the tower does distribute the lateral loads from the mast to the tower over
> a wider span, but does not reduce the net bending moment transferred from
> the mast to the tower.   Neither of these are typically limiting stress
> factors within the top tower section.  Mounting the rotor lower in the tower
> does also reduce the rotor's sensitivity to any radial misalignment if
> present.
>
> Hope this helps.
>
> Matt
> KM5VI
>
>
>
>
>
> -----Original Message-----
> From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of
> h.duck
> Sent: Sunday, November 01, 2015 5:14 PM
> To: towertalk at contesting.com
> Subject: [TowerTalk] Rotor Position in tower
>
> Could I get some input on placement of a rotor in a tower.  Is there an
> advantage to mount the rotor, say ten feet down from the top verses the
> usual three or four feet?
>
> My rotor plate is not welded in place so I could place it at any level.
>
> Thanks for any input.
>
> Howard WA9YBW
>
>
>
>
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