The centrifugal load of the rotating planet assemble is
contained by the ring gear and the separating tooth forces.
Paul Lamar ...No rotor no motor.
Hi Rolf,
Here is the info again on the Bell 47 gear sets.
*Diameter is measured at gear tooth contact point. All measurements in mm.
Name teeth number diameter gear width
Sun 46 120 27
Planet 23 57 22
Ring mm 92 234 25.4
Proposed input max rpm = 8200
Proposed max hp 343Hp
Proposed max input torque 220 ft/lbs
The planet gears tooth dimensions from the valley to the top of the 2mm
wide flat top is 5.9mm. The teeth are 4.6mm at their widest root.
In application Bell used the inner part of the hardened gears as the
bearing
race. A hardened collar (sleeve?) was placed over the gear shaft to
become
the inner race. This was 1950's technology and designed for low rolling
speeds but great torque. Keeping them center in the race where thick
washers on the outside and a fiber cage on the inside. TBO on the whole
craft was 1200hrs. The fiber cages lasted about 300 hours. Perhaps on
later
models Bell improved the bearings cages.
The inner hardened roller race that fits on the gear shaft is 23 mm in
diameter. The outer part of the bearings (actually the inner part of the
gear itself) is 39mm. The recess axially dimension to the central inner
rib
is 9 mm. Therefore the central rib must be 4mm wide. 22 - (2X9).
Calculations of bearing loadings:
Please check I am on the right track here.
The sun gear has to transfer a maximum of 220 ft/lbs torque at full chat.
Most likely less. The torque output to the prop shaft with the 3:1 ratio
would increase to about 660 ft/lbs. Each of the six planetary gears would
see 110 ft lbs. on what is basically a 2.5mm x 22mm (or 55 square mm) one
tooth contact area. The bearing is seeing 110 ft/lbs divided by the gear
28.5mm radius which works out to 3.85 lbs distributed over a line 18mm
long
contact line.
In conclusion. I believe I will have to have new planetary gears made and
ground with different internal bore specs to accept a standard high speed
sleeved roller bearing. Perhaps to increase bearing contact surface area
22%
the internal gear rib will have to be eliminated. If gear deflection
strength becomes an issue the bore can be reduced in diameter slightly.
Depending on the design of the gear case the gear also can be widened a
bit
more thus giving even more bearing surface area. Presently the hand
inserted
roller bearings are held in place by two 3.2mm (one on each side) thick
spacer washers.
The housing has several ports to direct a stream of oil at the planetary
set
and at the front bearing. Since this design unit was meant to fly on a
helicopter I would like to find a way to introduce pressurized oil through
the sun gear.
For pressurized oil into the prop shaft the Bell unit offers several
possible locations. The easiest manner would be to feed it into a
slightly
thicker outer collar that contained the proper shaft seals. This would be
a
'two birds killed with one stone' possibility. The present collar is about
25mm thick.
All in the all the unit is rugged, compact and the case designed to take
the
large lateral transient loads placed on it which I believe my plane will
require.
Doug Fir
And too big to run at 8200 RPM.
--
Paul Lamar ...No rotor no motor.
Rolf,
Load per planet is the load on one tooth - however I don't think all the
load is on the one tooth, as discussions with other engineers who have done
computer stress analysis suggest that a load on such dimensions would cause
the tooth to fail on every occasion. So I assume from this information that
the tooth with the most load is being supported by the following tooth to a
degree. To what degree is unknown to me, but my gut feelings is that it
could be as high as the third of the load?
George (down under)
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