Hi everyone
I am now measuring and preparing to draw the Bell 47 housing parts in Rhino
so I can share with other individuals. I think before I put it back together
I may have some molds of the front magnesium housing and the intermediate
aluminum ring made incorporating a few enhancements. I will also want to
draw up and spec out the other parts in case some of you may want a similar
unit in the future. These parts are pricey though. The planetary holder
appears to be forged and the flange that transfers power from the planetary
to the prop shaft was cut from a huge chunk of chrome alloy steel. At almost
three inches ID this internal spline flange is way over designed for our
use.
I also need to purchase a manual so I have an idea on the tolerances and
different nomenclature use in identifying the parts. Next I'll need to seek
help in identifying the different alloys and heat treatments used.
One person that sells Bell parts recommended I should call the following
number. Could one of you state side guys call for me? I can't dial a toll
free number from Japan obviously. Here is the message I received from him.
Mr. Fir
You may contact Bell Helicopter Textron (PSE Light) at
1-800-363-8023.
Ask the individual for the phone # of publications in Texas; you may
purchase support manuals direct from publications."
Good luck with your project.
............
My preliminary research has shown that the Bell unit had two back to back
planetary gears sets. Both had 3:1 ratios which together translates to a
9:1 total reduction.
First the specs.
*All measurements taken at the gear tooth mid line. Inches are
approximations.
Ring gear 234mm - 9.25"
Sun gear 120mm - 4.72"
Planet gear 57mm - 2.25"
Proof 2 x 57 + 120 = 234
Gears are approx. 35mm thick.
Planet gears have an internal 5mm boss and their roller bearings are
approx. 8mm in diameter and 15mm long. They are individually inserted from
each side of the gear sort of like loading a revolver with 38 caliber
blanks. The outer diameter of the inner race OD is about 38mm.
Circumferences:
Ring 736 mm
Sun 377 mm
Planet 179 mm
Inner race 119 mm
Gear shaft 57 mm
Roller bearing 25 mm
The PSRU was designed to deliver low speed and transmit the high torque of a
large helicopter rotor. It probably was over designed and could handle
1000Hp but I have not been able to confirm that claim. Here is what we do
know: For each revolution of the final sun gear, each planet gears rolls 2.1
times as much. (377/179)
On the Bell 47 unit dual planetary, the maximum rpm output would be about
300. This means the INPUT to the final planetary sun gear was about 900
rpms. This translates to the planetary gears rolling around their bearings
at 2.1 times that speed or 1890 rpms. Not so fast. Now from here on I am
unclear about what happens in the actual dance within bearings captivity so
anyone please feel free to jump in and correct me.
They appear to react like little planetary sets. Each roller bearing
contacts the outer part of the race (within the gear) and the inner 18mm
shaft at the same time. Well almost. Obviously some clearance is needed.
Reason would dictate that they are thrown outwards at speed, but then again
the whole gear is thrown outwards within the ring gear. Anyway, as the
outer bearing race is 119mm in circumference, each individual bearing would
roll around the outer track about 4.76 times times the gear rotation if
calculated from the OD. (119/25)
If calculated from the ID (the inner gear shaft pin) about 2.25 times (119/
57mm)
The high number (1890 X 4.76) is about 9,000 rpm. and the low number is
about 4,250 rpm. Probably neither number is correct.
The elliptical paths of bearings is academic as we usually we pick a bearing
based on the shaft speed, and different loads applied to it and leave it up
to the bearing engineers to figure out what is actually going on inside the
races. My curiosity has been piqued because I noticed that some of the used
bearings had large flat spots on them. Hmmm???? I was told the original
Bell 47 gear box had trouble with the fiber bearing spacer/holders because
they would often deteriorate after 300 hours of use. Dave Garber eliminated
them and stuck another bearing in the slot. His clearance between bearings
looked minimal if any at all and I wonder if his bearings could spread out
enough at higher speed. Dave, has flown his plane a few times to 1000
feet AGL but how reliable his bearings are is my biggest concern. Dave is
determined to seek help in getting his plane prepped for the RENO air races
and of course my plane has to be reliable for much longer flights.
When we pump the Bell unit up to the speeds Dave and I will be using,
things become interesting. With shaft inputs of 8200 rpms, and using only
the larger of the planetary clusters, the prop rpm will max at 2733 rpms for
take off. The input to the sun gear is still 8200 rpm however and as such
and the planetary gears will be spinning on their bearings 2.1 times as
fast or 17,200 rpm. Ouch!
I can just imagine what those large roller bearings logs are doing at those
speeds. The phrases 'oil heater' and 'spin weld' come to mind.
Obviously I need another bearing and possibly another design profile for the
planet gears.
Notice I haven't discussed gear line speeds, planetary bearing loads due to
centrifugal forces, or oiling and windage yet. That is for another day. I
have noticed though that the Bell unit as well as Mistral's PSRU support the
prop shaft and planetary cluster via an internal sleeve or bearing that is
integral to the e-shaft. That is wise IMHO. Those planetary gears have
enough work to do without handling prop flywheel permutations as well.
About now is the time I am starting to feel all warm and fuzzy seeing the
fine work Mistral has done:) But hey I am not bailing just thinking.
DougFir
I think it is 2.4:1. As I recall we measured it.
You have to put .jpg extensions on those file names
or browsers will not display them. In fact they are
not even jpg's. What gives? What kind of camera did you use?
Where did you get those files? What kind of computer are
you using? Perhaps you need an American computer instead
of a Japanese computer :)
OK I have some pictures of the Bell box so here they are.
Can you add a hyd constant speed prop feature?
Paul Lamar ...No rotor no motor.
Paul
Ok thanks. At 2.4:1 those gears are spinning at 19,680 rpms. Time to
investigate caged roller bearings you mentioned last year although with the
lighter loading a ball bearing might be possible.
My picts were Adobe files created on a MADE IN THE USA (actually Singapore)
Mac. The system soft is Japanese. Can you read this. どですか？ I just
chose the wrong format to save them in. Sorry now falling on rusty sword.
Yes a hydraulic constant speed prop would be possible because I have to
design and construct the prop shaft anyway. For initial engine tests I will
try to make something fairly simple but in the end I think a new front
housing including an additional bearing mount and perhaps an accessory
mount as well would be wise. Tom Parks experience with the Ross gearbox
encouraged me to also plan for a small built-in oil pump, sump, heat
exchanger, temperature sensor, oil filter and metal particle sensor. Any
other things on your wish list? Please don't say 'split case':)
BTW picture # 2 needs some explaining. In our use we take this upper bronze
slip ring off. It has a function with the interface of the primary
planetary. That picture caused me hours of grief last year trying to
figure out how the unit was driven:) The third picture shows it properly
installed only on the bottom.
The slotted drive plate shown in picture #1 is actually one of the plates
off of the primary planetary set sans gears. It now is used as a very
robust sun gear drive mechanism by slipping it over the partially shaved sun
gear of the secondary planetary system. The shaved sun gear design is
stock. Picture #3 shows it laying on the workbench to the right of the
gearbox. I plan to install six rubber isolators behind this plate ala
Tracy's idea. Dave does not have any isolators in his system and to date he
has not experienced any torsional resonance problems. This doesn't mean
they are not there or won't show up with a lighter carbon fiber prop. His
plane is designed only for short fast flights around racing pylons.
Doug Fir
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