61902 bearing from skf has sufficient max speed=> 500h L10 >5000h L10mh bearing life at 20k rpm https://www.bearingbasement.com/en/us/SKF-61902-Deep-Groove-Ball-Bearings--15x28x7mm/m-46605.aspx. This is good if we do regular spur gears. If we go with helical gears we’ll need some angular contact bearings
input torque: 22Nm assume first gear diameter is at most 30mm so Ft=22Nm/15mm=1500N contact angle around 20 deg => Radial 545N of force.
That was just an order of magnitude estimation, will be canceled out by the multiple gears. Therefore, nominally should be around 0. I am worried about minimum loading of the bearings at speeds >10k rpm. SKF seems to think minimum load is less important for speeds much less than the reference speed (30k rpm for these bearings)
| rpm | min rad. load (kn) | C. O. M. off center (mm*g) |
|---|---|---|
| 20k | 0.0378 | 34.4 |
| 15k | 0.0312 | 50.5 |
| 10k | 0.0238 | 86.8 |
100 mm g C.O.M. eccentricity would be 100N of radial load at 20k rpm which these bearings can easily take. For the planet gears, 1kN per planet is given at max. torque, so with four planets that would be 250N. However, if we add another 100N of eccentricity load (in the worst case where they’re lined up) 350N of load is still >1000 hours bearing life, and it allows the bearing to coast at max speed without skidding.
CRW1 or CRWA1 seals should be able to do the 20k RPM at 15mm = 15.7 m/s surface speed. <18m/s surface speed. This was confirmed to be fine by skf: https://www.motion.com/products/sku/00602697. This has no dust lip so the shield needs to protected from dust. I don’t why they don’t make a crwa1 in the 15mm size.
Note that the seals will be glued in with a pressure sensitive acrylic so they will not come out
"shaft plug" can just be a machined m4 screw with an oring slot cut in it ("piston seal"). It is a static seal so that should be easy (need to check lube compatibility for materials). Avoid providing axial stress to bearing since its not an axial type, force gear shaft against spline lip.
The housing for these parts (purple in the image) should probably be steel just to save us a lot of thermal expansion headaches.
The gear shaft around which the seal is going needs to be plunge ground smooth to Ra=8-17uin, Rz=64-115, Rpm=20-50. Lower value is actually a lower bound. No lathe marks should be left. Should be g7 tolerance. Needs to be case hardened to 45 HRC to a depth of at least 12 thou.
Bearing seat should be N7.
Seal seat should H8 with radius <31 thou and at least 16 thou deeper than nominal. Needs a 30 deg chamfer lead in
Prelim design for gears: Stage 1: Module 3. sun: 20, Planet 15: annulus 20. Pressure angle: 20 deg. 2 planet gears
According to Eassistant, if we use:
Standard gears:
According to eassistant the ideal profile shifts in order to limit specific sliding: 1.
[](<https://lh7-us.googleusercontent.com/K5g1bO2EnPzENf5OEmRgEW3osmd4QSkJZg7B1MqK-SFj09IZVv-nxxgdZPWkDG6eXLighekIYnTfJN_EnKTFmQSZ9f3X9UWovmo0FQ4TxmYC0TmIgys48e8ibvptku-qDfAGS9qXdLk1a2NvlFzuwKE>)