Hi George, glad you asked, gives me an opportunity to answer it once for
everybody.
If you mean 'what problem would it be for me' the answer is "No
problem at all". Except for scrapping every part of the current
design, scrapping all the tooling developed for it, scrapping all the
production procedures (this is no small thing) and replacing all this with
new stuff.
Obviously, there has to be a good reason to do this to make it worth
while. It has to solve an existing problem (are there any?),
significantly improve performance (would it?) or reduce cost enough to
justify all the work and expense of changing (does it?).
I'm sure that Dodge gear set is a fine piece of hardware but what does it
give us? It has a smaller diameter ring gear (4.4" vs 5.0") narrower gears
(about 3/4" vs 7/8") and one less planet. On paper, this looks like a
minus, not a plus. The availability of straight cut gears is a possible
bonus but on the other hand, I have not had any problem dealing with the
helical gear thrust so far.
The splined sun gear might be nice but so far we are paying a pretty high
price for it. A lot of other factors need to be considered. For
instance, How do you isolate the oil in the gear box when using that mating
splined part? Is there a place to put an oil seal? There are
literally hundreds of questions like this that must be answered during the
course of designing a gear reduction drive.
As far as the ratio goes, 2.85 : 1 is actually a bit higher than optimum
for most applications. Going even higher is the wrong direction.
Note that I said "most applications". There will of course be some that
would favor a higher one. A detailed discussion of this would be
interesting but is way more involved than I can detail in an email
message. But, to summarize, I believe fuel economy, engine life, and
proper matching with a suitable prop would suffer with a higher ratio.
Anecdotal data from people in the auto transmission industry has been
contradictory at best. It's the Chevy vs Ford vs Mopar thing all over
again. I give the Ford guys as much credit as the Dodge
boys - Zero.
I know Paul L. pushes for more power at every turn (higher rpm (requiring
higher ratios), P porting, etc) but I think this focus is counterproductive
in too many other areas. I'm more in tune with Richard Vangrunsvan's
goal - Total Performance.
You also asked why I was previously opposed to changing from 2.17 to
2.85. Note that I did not change. The 2.85 is in addition to the
2.17 which is still in production. There are plenty of applications where
the 2.17 is clearly a better choice than the 2.85. There are still times
when I would prefer the 2.17 on my own plane. Some of the reasons are
subtle and not quantifiable. I like the BMW better than the Chevy I drove
but I couldn't give you any hard reasons for it.
Tracy (now stepping off soap box)
----- Original Message -----
Sent: Tuesday, January 03, 2006 5:56
PM
Subject: [FlyRotary] PSRU Ratios
A question for Tracy.
What problems would you envisage using a 3.21
ratio instead of the 2.85.
One of the Lads on the other site has found this
'U-beaut' Planetary out of a Cummings Diesel ( overdrive).
There are two 5 planet arrangements, one with
angle cut ( opposite to the Ford unit) whereby the thrust is between the
sun gear and planet ( internal thrust gear arrangement).
The other is a straight cut 5 planet
arrangement ( no thrust).
The beauty of this planetary is that the sun gear
has a spline at the aft end with a matching female splined flange ( for
bolting to the damper plate).
The ring gear is externally notched as in
the Ford 2.17.
The planet housing is similar in design and size
to the Ford 2.85.
Advice from people in the industry are that the
Dodge units are superior to the Ford units, however that would have to be
confirmed.
Originally you were opposed to going from the
2.17 to 2.85 for a number of reasons - can quite remember why? Was it because
of higher RPM ( engine workload)? overspeeding the water pump and alternator?
Anything else ?
What problems do you envisage with a 3.21:1
ratio?
George ( down
under)