Planetary gear whine under load 6

[whyeye]

thread406-295989
Hello all, this is my first post but I have loved reading all the technical discussions on this forum for quite a while. We have run into into a potentially show-stopper situation involving the planetary reducer in a new product.

We are dealing with very low power levels by transmission standards, but low noise is an absolute requirement. The input pinion rotates up to 2000RPM with a load of no greater than 0.2Nm. The gears are metric spur gears with 0.75 modulus. Due to cost and manufacturing considerations, helical gears are out of the question.

Here's the problem: under no load, the reducer is virtually silent. This leads me to believe there are no fundamental issues with backlash or center distances etc. However, as soon as any load is applied to the output shaft of the planet carrier, a very noticeable whine becomes present. The noise is not of a whirring nature but is constantly tonal.

Other than profile error, what else could cause an otherwise very quiet gearbox to exhibit so much whine as soon as load is added? I am not a gear designer by profession so am having a very hard time trying to pinpoint the issue. Any help would be greatly appreciated!

 

[mfgenggear]

Plastic can be machined but it does not lend it self to it.
far as plastic. I don't see how this going to be resolved if it's injected molded gears.
what agma class are the gears rated at. very precise gear geometry and gear attributes have to be held.

Mfgenggear

 

[Strong]

whyeye,

A gap of 0.6 mm may be enough for viscoelastic material between the ring gear and housing (assumed to be rigid for CLD). Ring gear rotation could be prevented with metal/plastic pins or tabs. I think you are pushing the limits of using this Forum to assist you for redesigning your product.

Walt

 

[tbuelna]

I'm not too sure how the number of planet gears can have an impact on TE? The number of planets is governed by the teeth numbers on the sun and ring gear. In our case, 2 or 4 gears is needed for even spacing.

Think of it like this. In kinematic terms, the optimum number of contact points to constrain a circle is three equally spaced about the circumference. With four contact points you can have an over-constrained condition if the contacts are not precisely located relative to each other. This is one reason why many simple epicyclics use three planets. Besides using three planets, either the sun or carrier is usually floated so that it will self-align under load. While it was not entirely clear from your sketch how your carrier and sun gear were constrained, I would suggest that you take another look at the design to make sure your arrangement is not over-constrained.

Noise from TE in gears is mostly the result of interference between multiple tooth contacts. And with very small gear tooth sizes it doesn't take much tolerance error to create a problem.

 

[whyeye]

A gap of 0.6 mm may be enough for viscoelastic material between the ring gear and housing (assumed to be rigid for CLD). Ring gear rotation could be prevented with metal/plastic pins or tabs. I think you are pushing the limits of using this Forum to assist you for redesigning your product.

I was thinking that any pins to hold the ring gear (base layer) in place would end up transmitting some of the vibrations to the housing. Anyhow, my apologies if I was asking too much/too in-depth. Certainly my intention was to better understand what we might be doing wrong.

I will try various configurations with the components that we have, including adding float etc. and report back if we find anything of note.

 

[mfgenggear]

X2 what terry said. It takes very close control of the mow of the sun and the planets. Their is a previous post about the correct teeth numbers to prevent or better said to have correct tooth contacts and ratios.

 

[Tmoose]

OP said the "pinion rotates up to 2000RPM" .

Have you tested a noisy-by-hand unit in opoeration?

I'd be inclined to dry test an assembly bluing the teeth, looking to understand the tooth contact of every gear installed in both orientations. My first hunch would be wider contact would be better than line contact.

 

[gruntguru]

The four cantilevered planet shafts look worrying to me too. Perhaps you could hand make a one-off cut-down version with all gear widths and pin lengths halved, to investigate the effect on noise level.

Actually an easy way to do this would be to modify a set of planets in a lathe. Shorten the teeth by 50%, starting from the outer end (furthest from the carrier.)

Easier still - shorten the sun only again removing from the end furthest from the carrier. This may not be as quiet as modifying the planets but may still give an indication.

je suis charlie

 

[gruntguru]

Just to clarify the term "shorten" - I mean remove the teeth completely for half the length of the gear.

je suis charlie