X-Mozilla-Keys:
Matthew Cosman wrote:
I talked with goodyear engineer today and he did not have good news on the
topic.
Pulleys are limited by a 6500 ft/min. perimeter velocity. That puts the RPM
of a 14" pulley at about 1750. I dont think that is fast enough for prop
RPM. Due to the centifugal tension on the belt there is now a RPM limit
placed upon the smaller pulley. If you stacked three pulleys together you
will have enough to take the load but not the RPM. If the alighnment on the
three pulleys is not absolute it will mean failure. I talked with him about
maybe getting 50 hrs. he said " lucky to get 50 sec. at that RPM." I guess
I was wrong about how doable it was. He said the cat Iron pulleys will fly
apart at the RPM we are talking about. what about aluminum? Possible, but
goodyear will not touch the product. ( don't even think about producing one
ourselves. Goodyear will sue for even trying) what was funny is this guy
was a pilot who has built three of his own planes.
maybe gears is more cost effective? I have lots more interesting math as to
why any belt drives will fail at this Hp/RPM if anyone is interested. Sorry
for the optimizm :-)
Well that rules out the herring bone. I think he is wrong if you make
one
for your own use. If you make them and sell them to others GoodYear may
have a case.
There still exist the other kinds of belts with stright teeth which may
require wider pulleys to transmitt the same HP.
I am not convinced an aluminum four or five inch pulley made in
T6 will fly apart at 10,000 RPM. Heck 12 inch flywheels with steel
ring gears do that all the time.
I think a 4130 0.035 or 0.049 thick sheet metal 15 inch pulley with
stright teeth is the way to go. We are only talking 3300 RPM here
for the large pulley. Maybe less.
Several sheet steel diaphrams may be required to take
the centrifugal load of the rim however. It must be very
carefully formed in a press brake to be certain the teeth are
parrallel. It can be trued up somewhat after it is formed up into
a cylinder and welded to the diaphrams. It could also be heat treated.
I may have some pictures around here of such a pulley used in
car engine for belt driven overhead cams. These pulleys are running
up to 5000 RPM as the cam runs at half crank speed.
The belt tension due to centrifugal loads may be another matter entirely
but the cord being Kevelar gives one of the highest tensil strength
to weight ratios known to man. One of the tricks used to keep tires
together at high RPM is remove excess rubber. You see this
trick in certain high speed belts as there is just enough
material to keep the cords in place.
I am not convinced it is not doable. Maybe 15 inch is not optimum.
Perhaps smaller pulleys and finer teeth pitch all around are
the way to go.
On the other hand perhaps a huff and puff with PT6 gearboxes would
also work but the fuselage would be extra long to get everything inside.
I think that is the mistake Dave Garber made as he probably
underestimated
the cooling capacity needed and his diffuser ducts may have been
less than optimum.
Paul Lamar
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