George
Thanks for your thoughts and calculations. I do plan to get new gears
and
other parts tested and I'll keep everyone informed.
Dougfir
Thanks Doug - this is all good stuff, and is of particular interest to
me.
As soon as I get my other computer back on track ( again) I will send you
an
update listing of planetary ratios that I have put together.
I have had preliminary dealings with a local gear cutter and we come up
with
a design but I hadn't got to the stage of getting anything cut as yet,
perhaps with dimensions available from the Bell, it may be time to look
at
it again.
BTW There are a range of materials for gears of this type and you may be
surprised at what you find - I certainly was!
George (down under)
George
Yes I would like to hear about my options for gear materials because I
suspect I will have to make the planetary gears over.
I had the gear box taken apart and displayed on my workbench and ready
for
a picture but then Murphy struck. The 'new' batteries I bought in the
local
shop earlier turned out to dead:) I did count the sun and plant gear
teeth
though. They are 46 and 23 respectively. The gears are thinner than I
first thought 1 inch or 25.4mm. Tooth height from the valley to the
mountain
top is about 6mm or more lightly 1/4 inch.
DougFir
Doug,
I guess your saying their 1" long, not 1" in Diameter - the Ford Planet
gears are bigger than that, but 1" long is OK. If you were developing this
from scratch you could make it a bit longer if you wanted more contact area.
I will check with my contact on materials again, I can't find my notes on
materials for the gears.
You mentioned two planetary gear sets back to back in the Bell Planetary -
how does the other planetary differ in size etc?
6mm or 1/4" is OK for the tooth size, I believe I aimed for about that size,
but I was only targeting 300 hp and the gear sizes were smaller.
My initial sojourn into planetary development, resulted in a design of a
ring gear of approx 7", sun gear of 3.9" and planet gear of 1.325" with a
ratio of 1.8:1 ( 2.8:1) i.e. 81/ 45 = 1.8 and 81-45 = 36/2 =18 planet teeth,
with TBO of 3,000 hrs with MAX Take-off ( engine) speeds to 7,500 RPM and
6,000 RPM cruise. But that is as far as I got, it all got too hard working
from scratch. I hadn't got around to the needle bearings or the planetary
cage.
This wasn't a bad balance between planetary sun and ring gear sizes, a
bigger planet gear and smaller sun gear would, of course, give different
ratios. The Bell looks similar in design as what I had come up with, with
the Bell being bigger by about 2", I was trying to keep as compact as
possible. Interestingly the Mistral ring gear is 93 teeth to the Bell 92
teeth, so the Mistral may be of a very similar size, however the Mistral sun
gear is somewhat larger with 51 teeth compared to the Bell with 46 teeth(
1.82:1). It goes without saying the Mistral sun gear is therefore smaller
with 21 teeth compared to the Bell with 23 teeth.
I mention this as I feel ( how I rationalize the sizes) the smallest gear
(the planet gear) is doing the bigger job at the fastest speed, as it is
also the only gear with bearings and the bearing sizes are limited by
available space. So to my mind the bigger the planet gear the better,
therefore the 3:1 ratio better suites the application your seeking with high
HP use. I may be wrong but this may be part of the HP limiting factor in the
Mistral PSRU, as the rest of the planetary looks like it would handle in
excess of 400 hp IMHO.
Of course all this is true of all planetaries at this ratio and you can't
get much bigger planet gears unless you go much larger in size or increase
the ratio further - neither of which we want because of weight and prop
speed requirements.
For those not interested, excuse my ramblings - this is a pet interest of
mine.
George (down under)
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