Planetary gear whine under load
Planetary gear whine under load
(OP)
thread406-295989: Planetary gear noise problem
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!
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!





RE: Planetary gear whine under load
Is the output load radial or pure torque?
The bearing manufacturers FAG and SKF go into some detail about the importance of establishing ElastoHyrdoDynamic lubrication in rolling bearings, and the requirements of lube viscosity in achieving it.
http://mountingmanager.schaeffler.com/library/libr...
http://www.skf.com/group/products/bearings-units-h...
Kappa or viscosity ratio is a measure of whether EHD can exist
Gear lubrication has a strong EHD component, so I'd be looking at my Kappa.
RE: Planetary gear whine under load
If you're asking for specific reasons as to why your gears are generating noise; please provide more info. Supplying drawings that also shows the gear data along with any load conditions data is always a good start.
RE: Planetary gear whine under load
gearcutter - I guess the difference to the other thread is that this gearbox is very, very quiet when running under no load. The noise only appears once load is applied, even light load. I have attached the relevant gear data and a 3D view of the gearset. The original data was not in English and I've tried to translate it as best as I could. Hope it's OK if some of the nomenclature isn't AGMA.
Due to the low noise requirement, all gears are made of plastic. The sun and ring gears are PA6/6, planet gears are acetal.
RE: Planetary gear whine under load
RE: Planetary gear whine under load
The geometry, however, needs optimising.
I'm getting undercut & non-involute contact warnings from the software that I'm using.
This may well explain the noise issue that you're having. The interference will only get worse under load.
Are you using lube, if so what type?
What is the operating environment's temperature?
What are the operating environment conditions.....clean, dusty, etc?
What are the tip diameters of all the components?
What are the base tangent length spans for the sun & planets?
What is the measurement between pins for the annulus?
RE: Planetary gear whine under load
Adding some face crown to the planet gears would help a bit, but it would also reduce the load capacity of the gears. And since they are made of plastic this might not be practical. The only other thing I might suggest would be to redesign the carrier structure so that it is supported at both ends, and also so that the planet pins are supported at both ends. This would reduce the deflections in the carrier structure due to torsional wind-up, and would maintain much more accurate alignment of the planet gears.
Hope that helps.
Terry
RE: Planetary gear whine under load
If that's true, you might be able to reduce the pin deflection that tbuelna conjectured, by increasing the axial thickness of the planet carrier.
E.g., doubling the thickness of the planet carrier in the 'back' direction and using correspondingly longer pins would certainly help.
If you are painted into a corner already, you might be able to help the situation a little by adding a boss around each pin, extending in the 'forward' direction nearly to the centerplane of the planets, and a corresponding counterbore in the planets. ... balanced of course against changes in the various bearing pressures.
A tapered boss would be even better for supporting the pins, and a corresponding tapered counterbore would retain most of the planets' strength.
Mike Halloran
Pembroke Pines, FL, USA
RE: Planetary gear whine under load
I'm not familiar with some of the terms - what is tip diameter and base tangent length span?
The pin positioning is highly accurate with a measured tolerance of only 0.01mm. Since we are dealing with only a very small amount of torque, I can't imagine that deflection of the carrier structure can be a significant factor. The carrier is made using powder metallurgy so should easily withstand the rated load and hold the pins in place. The pins are 4mm in diameter by 16mm length, 6mm of which is planted into the carrier plate.
I may have miscommunicated the noise issue. The noise is only apparent when the gearset comes into contact with a resonating surface such as a wooden desk. Just holding it in the hand, noise is still very quiet when load is applied.
RE: Planetary gear whine under load
I would recommend that you do a study of the combined clearances/misalignments in the bearing supports of the carrier, planet pins and sun gear. If there is more than just .002" or so of misalignment along the mesh line, then you likely have a problem.
As I noted above, the cantilevered design of your planet carrier is not a good choice. With the small size of the gears you are using it only takes a tiny amount of misalignment at the mesh line to cause problems.
RE: Planetary gear whine under load
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.
Is it unrealistic to think that this problem might be solved with some simple tweak?
RE: Planetary gear whine under load
Or you can just go ahead and measure it, with a dial indicator and some deadweights. Since P/M properties are subject to some variation, direct measurement may be preferred.
Mike Halloran
Pembroke Pines, FL, USA
RE: Planetary gear whine under load
There are three noise control methods that can be very effective without redesigning the gear set:
1) Constrained layer damping for the ring gear (the usual suspect for your type of problem)
2) Vibration isolator material/device between ring gear and rigid stationary support (eliminates the "hard desk" effect you mentioned)
3) Torsional vibration damper on high speed shaft
These NC methods can be relatively inexpensive, and only one or two methods may be necessary. Typically the sound and vibrations are measured for a baseline and to identify the sound/vibration properties for selecting the appropriate noise control method and materials. Spur gears are usually the noisiest type of gear, so now you have to control it! A reduction of 10-dB (1/2 as loud) or perhaps more should be possible.
Walt
Sound-Vibration Control Engineer
RE: Planetary gear whine under load
This is the first time I've heard of constrained layer damping. Looks very interesting! We may not be able to implement this concept in the current product due to lack of space but will definitely keep it in mind for the future.
The other two methods are in fact already a part of the design, although that is not apparent in the sketch. They are part of the overall system designed to minimize transmitted noise. These measures have certainly been very effective for us in the past. For the present product, it seemed the base vibration is too great to be effectively dampened.
Fortunately, the noise problem has improved significantly after we reassembled all the components. I'm not even sure what we did wrong the first time around. However, it has been noted that turning the planet gears upside down has a noticeable impact. I'm guessing this has something to do with draft angles on the mold.
RE: Planetary gear whine under load
Can you fit constrained layer damping about 3 mm thick, or perhaps less, on outside of ring gear?
Walt
RE: Planetary gear whine under load
RE: Planetary gear whine under load
i don't see how the required tolerance could be held and especially after meshing.
RE: Planetary gear whine under load
Why would a plastic helical gear cost more than a plastic spur gear? The helical mold might cost a little more, but after producing a few hundred of each kind, why would there be a price difference?
Understand that I am asking questions, not stating facts. I don't have any facts here.
RE: Planetary gear whine under load
Brian - You are right that, with shrinkage, accuracy is not great on our molded gears. I was no aware there were any machining methods for plastic gears post-molding. Can you tell me a bit more on what can be done?
Windward - I agree that once tooling has been completed, part costs for helical molded gears won't be a lot different from spur gears. However, my understanding is we would have to upgrade our current molding equipment due to the more complex demolding process. Whereas with spur gears can be simply ejected via ejector pins, I imagine helical gears would entail some sort of twisting motion. I might be wrong on this matter and would gladly be enlightened.
RE: Planetary gear whine under load
RE: Planetary gear whine under load
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
RE: Planetary gear whine under load
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
RE: Planetary gear whine under load
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.
RE: Planetary gear whine under load
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.
RE: Planetary gear whine under load
RE: Planetary gear whine under load
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.
RE: Planetary gear whine under load
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
RE: Planetary gear whine under load
je suis charlie