LACR Helical gear noise vs. standard Helical?

[abt12]

I'm getting significant pushback from management and our vendor about the need for helical gears to have an axial/overlap contact ratio of greater than 1 for best performance. Our vendor says it won't help noise as long as the transverse ratio is over 1.5, and management doesn't want the longer packaging, but wants things as quiet as possible. We don't need the added width for strength or wear, but are already maxed on helical angle based on motor bearing life and as fine as we dare to get on pitch.

Unfortunately, I haven't found much supporting evidence for my assertion, except a lot of rules of thumb. Most of the research I've seen has been comparing the effect of higher contact ratios (1.25 vs 2+).

The only two pieces of supporting literature I've discovered so far are from Dudley's Handbook (5.1.13) - "... a certain amount of face width is required to get the benefit of helical-gear action. General experience indicates that it takes at least two axial pitches of face width to get full benefit from the overlapping action of helical teeth... if the face width is less than one axial pitch, the tooth action will be in between that of a spur gear and a true helical."

and Umezawa - "Clearly, choosing an overlap ratio over 1.0 lowers the vibration of a helical gear pair"

http://www.geartechnology.com/issues/0100x/umezawa.pdf

(also see Fig. 1)

Am I correct here, or totally off base? Is there another good source of supporting evidence either way?

 

[tbuelna]

Gear noise is basically due to transmission errors. So if you want quieter gears do whatever you can to reduce TE.

One thing you can do with your helical gears is increase the face contact ratio by increasing the helix angle. Which you already seem to have done to the maximum extent possible with your design.

You can also increase the contact ratio by using a lower pressure angle. This will reduce the bending strength of the teeth, but you noted tooth strength is not an issue with your gears.

You can reduce TE by applying profile and face modifications such as tip relief, crowning and lead correction. There are some pretty good gear analysis tools available that will give the optimum amount of each modification required for your gears if you want to avoid a lot of trial/error testing.

You can reduce TE by manufacturing your gears to a higher quality level. On one hand, higher gear quality often means higher cost. But you could do a thorough review of your current gear manufacturing process to see if there are any changes you can make that would improve quality of the finished gear without incurring a large cost penalty.

Lastly, you might consider doing an analysis of your gears to see if there are any structural modes coinciding with the meshing frequency of the gears over their operating range of speeds.

Good luck to you.

 

[abt12]

We've already pushed a lot of those around - 14.5 normal pressure angle, tip relief, etc. Crowning may be something we should look into. We're manufacturing to AGMA Q10 right now, I'm not sure my gear supplier can get any better (and we're locked in there).

Structural modes don't seem to be an issue from a quick analysis, but worth a second look.

Thanks!

 

[tbuelna]

You did not mention details of your gear mesh, such as tooth numbers, L/D ratio or PLV, which would help determine which changes would be most effective for addressing your problem.

In general, increasing total contact ratio (face contact ratio + profile contact ratio) of your helical gear mesh will help to reduce dynamic tooth loading, which in turn will reduce vibration of the gear. So using a lower pressure angle to increase profile contact ratio can definitely help.

If your helical mesh operates at high PLV (>10kft/min), then a gear quality better than AGMA Q10 would typically be used to minimize dynamic tooth loading caused by tooth-tooth index errors, PL runout, etc. With high PLV it is also essential to apply the correct amount of tip profile modification to minimize dynamic tooth loading.

You seemed to imply that your helical gear mesh has a face contact ratio <1.0, which would indicate either a shallow helix angle or a low L/D ratio (narrow gear face). With a narrow width gear, face profile modifications like crowning or lead correction are usually not required unless there is a stiffness/rigidity issue with the shafts/bearings/housings supporting the gears. Some design optimization of stiffness in your gear structures, bearings and housings might help reduce vibration/noise without adding cost. Reducing axial/radial play in any rolling element bearings by applying some preload (without creating an over-constrained condition) would be helpful in this regard.

Best of luck to you.
Terry

 

[abt12]

We have a few different sets, but one example set:

Teeth: 30/96
Normal Module: .45357 (56tpi)
Width: 5mm
Trans. Pitchline Velocity: 1.4 m/s Max
Center-to-center: 29.2 after addendum modification.
Normal PA: 14.5deg
Helix ang: 14 deg (and this is higher than we'd like already for bearing life).

Contact ratios:
Axial: .8093
Trans: 2.052

Motor bearings are preloaded from the motor supplier, and are the major driver of helix angle (we don't want to violate preload because we're already pushing life). Gear bearings are Abec 5, preloaded at ~1% of CoR (higher absolute value than the motor bearings).

There's additional reduction in another part of the gearbox but it's a separate system.

 

[gearcutter]

What are the addendum modifications for both pinion & gear?

Is there any form of heat treatment being done to the parts after machining?

What is the gear tooth finishing process being used?

Have you had an acoustic/vibration analysis performed? You'd be surprised how much noise/vibration a bearing can generate. Even an electric motor can be a major source of noise/vibration, even more so if it's powered by a VFD. Too many times the assumption is made that the gears are the source of vibration.

With such a fine pitch; gear tooth geometry accuracy is difficult to maintain.
Have you had a third party verify the claims by your vendor that they are actually achieving AGMA Q10?

 

[tbuelna]

abt12-

Your total contact ratio is 2.86, which is pretty good. Your max PLV and pinion speed of ~2000rpm are also quite modest. So it looks like you did a pretty good job with the gears. But I think your bearing system might have room for improvement, so here's a couple suggestions to improve the stiffness/alignment of your gear mesh that could be implemented without much added cost.

First, it sounds like your pinion gear is overhung and only supported radially on one side by the adjacent motor bearing. And based on your comment that the motor bearings are axially preloaded I'll assume they are angular contact ball bearings. If your gear mesh only transfers torque in one direction of rotation, it will help the motor bearings to use a helix angle orientation (RH or LH) for the pinion such that its axial thrust force is reacted by the bearing at the far end of the motor. With an overhung pinion the adjacent motor bearing is already handling most of the radial force produced by the gear mesh, so relieving it of as much axial load as practical will help balance the L10 life of the bearing pair.

Second, you mentioned your motor bearings are preloaded to less than 1% of basic static load capacity. This amount of preload is suitable for a set of motor rotor bearings that are not subject any external loads, and it gives a good balance of stiffness, life and efficiency. But since you are using the motor bearings to support the pinion, I'd suggest doing an optimization study of the motor bearings to find a contact angle and preload (maybe 2-3% of COR) that gives better stiffness for the pinion while still providing acceptable L10 life. As long as the bearing cross section is not altered it should not be a problem for your motor supplier to implement the change.

Third, I'd suggest doing the same with the gear bearings. Applying as much axial preload as practical to the angular contact bearings supporting the gear, within constraints of L10 life, efficiency, etc, will ensure optimum alignment is maintained at the mesh. If the mesh only transmits torque in one direction of rotation, a good method of axially preloading angular contact bearings is to use something like a wave spring applying force to one of the outer races. This makes it easier to maintain a consistent amount of axial preload under variable operating conditions.

Hope that helps.
Terry

 

[abt12]

Sorry, should clarify - the gear bearings are 1% of CoR (and have a CoR 4x higher than the motor bearings). They're deep groove ball bearings, and are preloaded to ~3-5% of CoR. This is set by the motor manufacturer, and we've been informed we can't change out the bearings for an angular contact or play much with preload.

We're reciprocating on load, and fairly balanced in most cases, so handedness doesn't matter too much for us. There is one case where loading is unbalanced, so I'll adjust to make sure we push axial loading to the back bearing.

The gear is also supported on deep groove, but as they're much larger we're not as concerned about life. We may want to reconsider angular contacts here - the initial design had them in but we couldn't source accurate enough bearings at reasonable prices so we went back to deep groove. We're preloading with wave springs on the inner race due to assembly requirements (press fit on housing, slip on shaft) and order.

Sounds like we're on the right track, just limited by the under-specced motor bearings for the application. Thanks!

 

[Tmoose]

Is this a new design?

If I'm assuming correctly that gear shaft is rotates, and as you say is a slip fit in the bearings, what kind of "slp fit" was the bearing manufacturer on board with?

 

[abt12]

@Tmoose -

The gear is attached to the "hub", which rotates around a fixed shaft. the bearings are pressed into the hub (m6 fit) and g6 on the shaft - fairly standard for outer ring rotating load conditions. It does reduce life but we're not limited by these bearings so it's not a big consideration, and rotating shaft is not an option for this application.

 

[tbuelna]

Out of curiosity, how is power taken from the driven gear rotating about the fixed shaft?

As a general rule if you want to optimize your gear drive in terms of efficiency, reliability, cost and service life, you should adjust your bearings sizes so that they all have similar operating (L10) lives. It does no good to have some bearings in a system that have operating life greater than others.

With rolling element bearings where the radial force is fixed with respect to the direction of rotation, it's usually better to have a fixed outer race and rotating inner race. The reason for this is that the hertzian contact stress levels at the (convex) inner race surface are usually higher than at the (concave) outer race surface. And it is beneficial in terms of fatigue life to have the more highly stressed inner race surface rotate relative to the applied load so that the fatigue cycles are uniformly distributed.

 

[abt12]

Can't really describe it without divulging more than I'd be comfortable - suffice to say the downstream gearbox components have to be able to move around a shaft, fixed or otherwise, but we gain very little with a rotating shaft and have a large number of negatives (including increased weight, larger packaging, higher reflected inertia for the motor, and many others), with the only big downside being reduced bearing life. Since these bearings have a relatively fixed bore diameter and will outlast the motor bearings regardless, the sacrifices are minor.