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Bryan,
I understand your confusion, as I had to get my
head around this stuff a long time ago.
That's 85 minus 45 = 36 divided by 2 =18 which is
the number of planet teeth. I see what you have done, you have divided 85
by 45 - that's the overall ratio 1.8 then divided by 2 to get .9.
There are 4 major components of the
planetary the ring gear, the planet gear
housing, the planet gear itself and the sun
gear.I threw the calculations for planet
teeth as a bonus for those unfamiliar with the process, thinking it may be
confusing, but for the overall picture it's a necessary part of a planetary
system, as is the difference between holding the planet housing as compared to
holding the ring gear, the difference here is + 1. The + 1 is when you run
the planetary opposite to the crank. In our case we hold the ring gear to attain
the +1.
Showing the reason for the +1 was necessary to show
the comparison between a 3 gear system (like the planetary 2.85) and the 2 gear
system ( 1.8) like the belt and pulleys/ sprockets.- which are very similar in
ratio but turn the opposite direction.
Hope that helps!?
George ( down under)
George you lost me with
(81-45)/2=1.8
I get
.9
Bryan
From:
Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of George Lendich
Sent: Friday, July 10, 2009 3:21
AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Interested in
cog-belt PRSUs?
The calculations for ratio of this
kind is (ring gear divided by sun gear). Using teeth numbers on the ring
gear divided by the teeth numbers on the sun gear; If you had 81 teeth on
the ring gear and 45 teeth on the sun gear i.e.
(81/45=1.8).
Now if there were an intermediate
gear as on the planetary i.e. planetary gear - then the calculation for the
planetary gear teeth. is; ring gear minus sun gear divided by two i.e.(
81- 45)/2 = 18. As a planetary runs with 3 gears, the prop runs the same
way if the planetary is held (in place) by the ring gear, the
calculations are 1.85 +1 = 2.85:1 Ratio.
As the rotary runs anti-clockwise
in out orientation ( from the rear of the motor forward ) we have a clockwise
prop direction when using a planetary PSRU, with ring gear held. With the
planet housing held it would run anti-clockwise and would be 1.8:1
However we only have two gears
(sprockets) in this belt PSRU so ir must run in the direction
the motor runs i.e. anti-clockwise. The ratio is similar to the Ford planetary
which is 1.85:1.
I don't really know the numbers of
the teeth on the belt PSRU sprockets but used the numbers abovementioned as
demonstration only.
"With the rotary, that would
mean an anti-clockwise prop rotation, view from
the cockpit, 1.8:1 is
right on the mark as compared to the Ford planetary
reduction which is
2.85:1 with clockwise rotation i.e. 1.85+1.
George (down under)."
I am totally confused about having two different ratios going from
clockwise prop rotation to anti-clockwise for the same engine speed and prop
speed. I am way out of my depth here. HELP!!!
Bernie
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