>>> Rolf,
>>>
>>> Unlike the discussion above I will fix the ring gear and take power
>>> out of
>>> the planetary cluster. The cluster would be attached on the
prop shaft
>>> and
>>> be supported front and back on bearings. How would I calculate the
>>> load the
>>> planetary gears will be seeing? Total torque divided by the total
>>> tooth
>>> contact area? My engine will be putting out over 200 ft/lbs of
>>> torque at
>>> max rpm. With a supported prop shaft I believe the loads on the
>>> bearings
>>> will be from torque transmission and the centrifugal force of the
>>> planets
>>> themselves, right?
>>>
>>> Also if I understand it correctly the gear line velocity is
>>> circumference
>>> times rpm. Calculating it from the Bell sun gear in feet that is
>>> 377mm /
>>> 304.8mm = 1.24 ft. 1.24' X 8200 rpm = 10,168 ft per min?
>>>
>>> Mistral's unit at a 8000 designed redline with a slightly
larger sun
>>> gear
>>> would not be far off this figure. You mentioned maximum line
speeds of
>>> 5,000 ft/ min. Is that figure for light duty gearboxes?
>>>
>>> Thanks
>>>
>> Doug,
>>
>>
>>
>> You have a fixed ring gear and rotating planets. Give me the
dimensions
>> of all the gears, DP and pitch diameter and we can calculate it
>> together. As already mentioned, it is pitch diameter times pi
times rpm
>> /12. Pitch diameter in inches, pitch line velocity in fpm.
>>
>>
>> Since the pitch line velocity of all gears is the same, one need to
>> calculate just one. Best is the ring gear and the output speed or
>> propeller speed. The gear ratio is thereby irrelevant. The
torque load
>> is also the same on all gears. The load per tooth is divided by the
>> number of planets.
>>
>>
>> The bearing loads are essentially torque divided by radius. The
>> centrifugal force of the planets only need be considered on the
planet
>> bearings, for the output shaft they cancel each other out
through equal
>> spacing around the circumference.
>>
>> Rolf
>>
>> 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.
Doug,
Here is the way I would look at it: the 46 tooth sun gear rotating at
6000 RPM drives the 23 tooth planet gear at 12000 RPM. It does not
matter that the planet is rolling inside the ring gear. For the planet
bearing load estimate, I would say the 660 ft-lb prop torque is
transmitted to the planet carrier at the radius of the planet axles,
which appears to be about 4" or .25 ft in your photos. The torque
divided by this radius and by the six axles gives a radial bearing load
of about 440 lb per axle. The compound motion of the rotating planet
gears revolving in the carrier tends to make the bearing needles bunch
toward one side (Think of a carnival ride like the "Round-Up" or
"Twister" where you sit in something that rotates about its axis while
the whole ride is revolving.). So, you need a caged needle bearing.
Preferably one that has a large contact surface between the cage and the
needles. The easiest approach would be to find a drawn case, caged
needle bearing that would work with your existing planet axles and meet
the speed-load requirements. Then, grind that partition out of the
planet gear and bush the gear to fit the needle bearing. It looks like
your axles are around 5/8" diameter. A quick look at an SKF catalog
indicates it is probably feasible to get 1200 to 1500 hours life on a
caged needle bearing of this size with this speed/load.
The pitch diameter of the planet gears looks to be about 2.3". That
would give a pitch line speed of 7417 ft/min. With continuous oil
spray, that speed is probably OK. Here is a photo of how the Solar T-62
turbine planetary is oiled. The ratio is 10:1 and it runs with a pitch
line velocity around 12000 ft/min. The sun gear is not present, but
you can see the three brass tubes that squirt oil on the gears just
before they mesh.
Aubrey
Great picture Aubrey. Did you take it?
Good analysis. I am surprised T62 APU operates at that pitch
line velocity. I guess that is about right since it is rated at
60,000 RPM and the pinion gear is tiny. Around 3/4 inch
in diameter. Glorified turbo charger :)
I think it does matter if the planet is rolling inside
the ring gear and power is taken from the
rotating planet carrier.
I think that reduces the pitch line velocity.
In the case of the T62 the dwg of which I attached the
shafts of the planet gears are fixed to the case so the
pitch line velocity is indeed 12,000 FPM.
The output shaft of the T62 rotates in the opposite
direction to that of the turbine/compressor shaft.
A agree with you on the torque analysis but Doug is
shooting for 3,000 hours TBO. In a rotating planet
carrier the loads on the needles are more complicated
so that case is not covered in the bearing manuals.
I agree a cage will help. The fact of the matter
is the Bell box is not designed for 8200 RPM input.
My guess is something like 300 to 600 RPM in and
100 to to 200 RPM out.
Paul Lamar
-----------------------------------------------------
Aubrey
I haven't followed through with the calculations, but in your
comversion of the planet axis, 4 inches = 0.333...Ft.
Joel
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