Mike wrote:
> Thanks for the ideas and discussion guys. Food for thought.
>
> Can anyone address the 2 points I asked about?
>
> Rohn vs Glenn Martin bearings:
> http://www.texastowers.com/rohn_tb.htm
> http://www.glenmartin.com/catalog/page195.html
>
> Is a Rohn rotor shelf beefy enough for the bearings?
>
Remember it's a ROHN shelf in a ROHN tower with a ROHN bearing.
According the ROHN the bearing is for side loads and as it's a complete
ROHN system I'd stick with their recommendations. IOW the shelf is
plenty sturdy for side loads and will "probably" support quite a load.
However check the difference in the thickness of the tower top plate
compared to the shelf.
If it were me (and it's not) and I planned on letting a thrust bearing
support load down in the tower, then depending on the load I'd use the
ROHN shelf, or for heavier loads make a custom one out of 1/4" or even
1/2" steel plate. The shelf for the 25G is considerably lighter than the
one for the 45G, but the 25G isn't meant for supporting a lot of
weight. OTOH it depends on your definition of "a lot of weight". I
don't consider 200-300# a lot of weight and would expect the shelves of
either to support that much. BUT I'd not put that much weight on a 25G.
Check the catalog for ultimate ratings of each tower.
As I've said before, I use two bearings, but the system destroyed one
TB-3 and the second isn't in great shape so I'm going with sleeves
welded to 1/4" steel plate drilled to mount on the ROHN shelves. I'll
use collars to keep the mast from dropping or turning with the rotator
removed.
73
Roger (K8RI)
> New question:
> How about aluminum masts?
> http://www.heightstowers.com/accessories.htm
>
> They also have bearings which look like the same ones at Martin. Any
> experience here?
>
> 73, Mike NF4L
>
>> I like to get thoughts/experiences about Rohn TB3 bearings vs. Glenn
>> Martin TB25.
>> The antennae will be a 4 element SteppIR and an M2 6M7JHV on 20 feet of
>> 2" chrom-moly.
>>
What's the wind area of each antenna? Don't forget to multiply that by
the distance above the top of the tower and then add the two for your
real world wind load.The 6M7 has only 2.5 sq ft of wind load rated for
85 MPH wind survival. Mount that on a 10' arm above the top of the tower
and it'll be almost as much as the SteppIR.
>> I plan to use 2 bearings. Can the bearings be mounted on a rotor shelf
>> (25G tower)?
>>
>> 73, Mike NF4L
>>
>>
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