Planetary Gearbox Noise Reduction and Tolerancing

[TCookie]

Hi,
I'm a young engineer working out the details of a planetary gearbox for a power wheelchair. I have a 4.5K RPM motor, and need to get a total reduction of ~30:1 I have a few solutions worked out, with my favorite coming in at ~28:1 in two stages. I am concerned about the noise produced by such a gearbox, so I have been working on various ways to quiet the thing down, including:
1) Plastic gears on the first stage
2) Using Helical Gears
3) providing axial support to all parts to eliminate rubbing between stages
4) Lowering the speed of the first carrier
5) keeping contact ratios high
6) wisely choosing tooth numbers and spacing to keep the tooth of each planet from meshing in sinc with other planets
7) Choosing a motor with minimal cogging
8) fixing each carrier both axially and radially
9) adding a donut of rubber to each planet in such a way that the bushing is held inside the donut and a ring of teeth are pressed onto the outside
10) damping the gearbox to the fame and the wheel so as to not use either as a soundboard.

My design currently uses all of the above, except helical gears or rubber isolators. I've designed a higher ratio into the first stage so that I can use plastic gears, but also so that I can lower the speed of the first carrier at the expense of a lower contact ratio on the motor pinion (first stage sun) Since the design has the carriers on bearings, I saw no reason to stick with 3 planets in the second stage, so I went to as many as I could, which is 4.

I would like to discuss the relative importance of these design choices, especially regarding their impact on noise. I'm also fumbling on the tolerancing vs. noise issue. Also, tolerancing is a big issue for me if I choose to fix the carriers because without any other means for load balancing, I will be relying on tolerances for load balancing. Hmmm.

I don't have an experienced gear engineer to lean on here, so I'm hoping to find some advice on this forum. If anyone is willing, I would quite like to chat on the phone. Just let me know, and I will give you my number.
Thanks for your time!

 

[plasgears]

Using a pinion with extended addendum will result in quieter running. Also, use helical gears. Next, do not heat treat the motor pinion. That sounds suspect but it works. The first gear should be fiber, brass, or aluminum for sound absorption.

We tried all this on a breast pump, and the noise changed from 'zoom-zoom' gear noise to 'swish-swish,' the sound of the diaphragm pump. The customer was mystified about how we did this. We told him simply that we went to helical gears.

 

[TCookie]

plasgears,

First off, thanks for the response.

I'm not sure I can go to helicals, because it means a lot of little bearings all over the place. In my design, everything is a bit on the edge, including the pins and carriers, so adding an additional force might not be good in terms of cost and complexity. Maybe I could go with helicals on the first stage and squeeze by without putting in ball bearings, as the first stage isn't nearly as heavily loaded as the second. If I have helicals on one stage, it means I will have to toss out the idea of sharing a ring gear.

I'm certainly planning on having plastic planets on the first stage. Can you clarify if you think an unhardened sun would be quieter, even with plastic planet gears?

Lets say I have an 18t sun, meaning there is no undercutting, do you still think that profile shifting out will help me with noise? Or are you recommending profile shifting to get the contact ratio up, or maybe the pressure angle up?

On the first version of the breast pump, before you went to helical gears, did it have any of the quieting ideas that I proposed for my design? I would be especially interested to know if it had floating suns or carriers, and what was the method to get the planets to load balance?

Thanks for your time,

-TCookie

 

[dimjim]

I think plasgears is recommending the long
addendum sun for strength and also recess
action helping to reduce the noise. The
carrier will have the greatest strength
or Y factor so unhardened should be sufficient
and quieter. You might want to consider
cast iron for the carrier. Your planets
will be the weaker members. You might even
want to consider using quarter long addendums
on these and consider short addendums on the
carrier or stubbing the addendum.

 

[plasgears]

Tcookie,

The first cut design had helical gears and fiber first gear. The noise began to subside when we extended the addendum on all pinions, put in soft 12L14 non-HT pinions, and introduced gears with noise absorption materials (fiber, brass, alum.). My recollection is that we also went to viscous grease lube.

I hope you will be putting in thrust washers for helical gears. It's a small price to pay for quiet running.