George,
On stationary planets and rotating ring gear, the bearing for the ring
gear would not be so large as to be at the outside of the ring gear, but
rather much smaller, just to suit the shaft used to extend to the
spinner and prop flange. Lightest and most economical is a tube as a
shaft, especially if a certain length is required to be bridged as it
will be stiffer as well than a solid shaft.
The ring gear will have a flange going down to the bearings. Best of
course would be tapered roller bearings or an additional thrust
bearing. For that you check out a bearing catalogue or simply ask a
bearing supplier for help. The axial and radial loads and rpm you will
have to provide. Personally, I would prefer tapered roller bearings over
a thrust bearing.
Common bearings too work best with a low sliding speed and low area
pressure. The product of both tells you if it is within acceptable
limits. Speed and pressure individually must also be within practical
limits. The method of lubrication has a large impact on the bearing
capacity.
On the gears, a local gear shop can best advise you of the gear material
and the method of hardening. That is part of their job, they do it
frequently. There are basically two choices, through-hardened steels or
case hardened steels.
The first thing you have to specify is the service life, say 5,000 hrs
or perhaps 50,000 hrs.
The cheapest would be a through-hardened steel. You could even use
pre-hardened 4140 at 28 HRC, cut the gears without any further heat
treatment.
The next thing is 4140 or better 4340 due to larger section then
hardening and tempering it to the desired hardness of between 25 to 40
HRC. That should be good for 3000 to 5000 hrs if operated within an
acceptable load range.
For case hardened steels you can start with 1020, carburize, harden,
temper at 350ºF, the case is ~ 55 to 60 HRC but the core strength is low.
Next is 4130, 4140, hardened, tempered at 900ºF and nitrated.
Further steels along the same grade are 46xx and 86xx. Grinding before
nitrating also extends life.
Specifically for aerospace use for highly loaded gears with high pitch
line velocity and where reliability under extreme conditions is needed,
use AISI 9310. The case is 58 to 63 HRC and the core 250 to 350 Brinnel
hardness. The use should not be at high temperatures for this steel.
There are also 'Nitralloy N' alloys that do not need carburization and
hence are not distorted by this process. It is suitable for high
temperature operation. The steel is hardened and nitrated. Case is 90 -
94 HRC and the core 300 to 370 Bhn. These gears are usually ground after
cutting. Nitrating is the last operation.
As to practicality, for the relative low service life in private
aircraft use, a service life of 5000 hrs is likely adequate. On the
other hand it is a safety issue as well. For my taste, I would want
something 'reasonably safe' for flying. Even if the individual gears are
more expensive, the gear box itself is not affected by that, so that the
increase in overall cost is tolerable.
Perhaps you might argue that if it last for 50,000 hrs, it will take out
the fun of flying? Like in anything else, there is always a trade-off.
Best is to talk to your gear cutting shop and tell him what it is for. A
good trough-hardened steel should be fine. Grinding would be nice.
Don't forget that alignment, rigidity and lubrication are also crucial
of which the gear maker has no influence.
The gear pressure angle should be 20º, not 14.5º. A straight cut gear is
nosier, the helical cut is quieter but cost a bit more and thrust loads
have to be taken care of, best by tapered roller bearings.
A DP of 16 seems adequate for the 160 ftlbs of torque, a ~ 7 in dia
internal gear and ~ 2,700 rpm out with 1 in wide gears and 4 to 6 planets.
Of course, a high strength gear can be narrower, conversely a low
strength steel asks for a wider gear. The gear cutter should know if he
has an engineering department, which you would expect him to have.
Regards
Rolf Pfeiffer
Rolf,
Excellent information, some of which is in line with previous advice to me
from other quarters - however advice varies from person to person and
sometimes gets confusing.
I agree on the 5,000 hrs life, as a reasonable life expectancy.
I am a little confused about the benefits of Carburising verses Nitrating.
I'm wondering if Carburising at the higher heat is going to result in more
distortion requiring more grinding which takes off the surface hardness -
catch 22!
I'm aware of the core hardness needs ( and why) as well as the surface
hardness needs.
Thanks for confirming the pressure angle - '20 degrees'.
I take note of your choice of tapered roller bearings - my first choice also
but they can get quite heavy as they get bigger. Perhaps angular contact
bearings!?
You lost me with a DP of 16?, what's a DP?
Internal gear of 7" - do you mean internal ring gear?
Hearing you on alignment, rigidity and lubrication!
Other information provided isn't lost on me and it confirms previous info
provided.
George ( down under)
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
I don't think the Bell sun gear is 1.24 feet in diameter Doug :)
Heck the engine is only about 12 inches in diameter.
I think you mean ring gear diameter is 1.24 feet.
The pitch line velocity of the sun gear is not that easy
to calculate as the entire planet carrier is rotating in
the opposite direction to that of the sun gear.
I think the way to do it is subtract the tangential speed
of the planet carrier at the center of the shafts from
the tangential speed of the sun gear at the line contact point.
Imagine a pinion gear moving along a rack.
The loads on the planet gear needle bearings is very complex
as you have the loads due to centrifugal forces about the
sun gear axis and the centrifugal forces about the planet
gear axis combined with the tangential load on the shafts due
to output torque. Then you have the high rotational speed
of the needles themselves. This is the big unknown. Once
they lose contact they start to slide. This is a function
of how close the needles fit the bore of the planet gear
and the planet shaft itself. This is why Diamond hand
selects the needle bearing fit in the rotor of the
Wankel they make.
Paul Lamar ...No rotor no motor.
Paul,
No you are right the sun gear is not 1.24 feet in diameter. I was referring
to circumference. The sun gear is however 377mm in circumference, 120mm in
diameter. There are 304.8 millimeters in a foot so you can do the math:)
The pitch line velocity is relational to how the gears interface with each
other as if they ran together along a virtual straight line. Calculating
one of the gears pitch line velocity will result in knowing all of them.
I agree that the rpms of planetary gear and the stress they are seeing is a
big unknown for me.
Here is a informative website I found on different bearings and there use.
http://www.bearingstrade.com/needle-roller-bearings.html
The pictured setup is very similar to the original Bell insert.
On the left you can find links to different types of bearings.
It seems needle bearings and roller bearings are part of the same family.
Long and thin or short and fat respectively.
Doug Fir
OK I am wrong I missed the circumference bit.
10,000 feet per minute is TOO fast.
But that is not the case. The planet gear is not
stationary. If you use the along-a-virtual-straight-line
analogy the sun gear is moving and the planet
gear is also moving and going the same way but slower so
the relative speed is less than 10,000 feet per minute.
Yes relative to a fixed point the sun gear tooth is going
10,000 feet per minute. In other words the whole planet gear
is going say 6,000 feet per minute in the same direction so
the relative pitch line speed is 4000 feet per minute.
Think of the limit. Suppose the whole planet gear was going
as fast as the sun gear teeth. There would be no
rotation of the planet gear and no pitch line velocity.
In terms of angular velocity in degrees per minute the
sun gear is 360 x 6000 RPM or 2,160,000 degrees per minute.
The whole planet gear and planet carrier is only going
360 x 2000 rpm because there is a 3:1 reduction or 720,000
degrees per minute. Planet carrier revolves in the same
direction as the engine. Clock wise from the pilots point of view.
Difference is 1,440,000 degrees per minute.
What RPM is the planet gear revolving at? :)
Paul Lamar ...No rotor no motor.
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