At 09:39 AM 6/8/2005, Gary Schafer wrote:
>Don't kid yourselves into thinking you are "distributing the load" over
>two or more thrust bearings. It is almost impossible to do unless the
>tower is being compressed as you add antennas.
>If you mount the mast in the rotator and add antennas and then add the
>top thrust bearing, how is it going to be pre loaded with some of the
>weight? Won't happen unless the tower compresses or the mast shrinks.
>
>A true thrust bearing has a tapered bearing like that of the front wheel
>bearings on a car. Or it has sizable top and bottom plates if it is ball
>bearing. A bearing that is designed for a horizontal shaft is not a
>thrust bearing just because you mount it in the vertical position.
On the other hand, the axial load (thrust) isn't all that huge, so a normal
non-tapered ball bearing can probably handle the load just fine. In a car,
the wheel bearings have to take thousands of pounds of axial load (i.e.
you're cornering that 6000 pound SUV and pulling .7g, with most of the load
on the outside front wheel, and you hit a pothole).
Not only that, the rotation rate is much higher in a automotive
application, but that goes more to bearing life than to load capability.
There's also a bearing known as a "angled contact" bearing, which still
uses balls (not tapered rollers like a auto wheel bearing) which is
designed to take axial loads.
I seem to recall that for ordinary plain bearings, they'll take an axial
load of 1/10th of their rated radial load without permanent damage, but I
might be misremembering
I ran across a table that shows a 3" diameter (ID) plain radial bearing
with a load rating of 5000 lbs (that would be the radial load).
Some angular contact bearings in similar sizes were rated at 17,000 lbs (40
degree contact angle.. so that probably means the axial rating is
sin(40)*17000=11,000 lbs and the radial rating is cos(40)*17000=13000.. but
I have no idea how bearings are rated in detail)
http://www.ntnamerica.com/ is a bearing company and has lots of useful info
on their web site.
As for what some random pillow block ball bearing you buy at Grainger or
Home Depot is going to be rated at (or what sort of bearing it is..
straight radial ball, angular contact, self aligning, etc.) who knows.
>The top bearing may be fine for lateral forces but it is going to do
>little for downward weight in addition to the rotator support unless the
>rotator is clamped after the weight is applied to the top bearing. Then
>it will support all the weight and very little will be supported by the
>rotator.
>
>73
>Gary k4FMX
>
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