Gerotor Pump Harmonics

[TrannyNoise]

I'm looking for insight into causes of higher order harmonics in the noise (and vibration) of a gerotor oil pump.

The inner gear has 9 teeth, so the fundamental pump frequency is too low to be an issue at the operating speed; but the 3rd, 4th, and 5th harmonics create an issue. The 3rd harmonic is the worst and varies greatly (>3 dB) among several units tested.

Any clues as to the mechanical or hydraulic reasons for such behavior would be appreciated.

Regards,
Tranny

 

[electricpete]

If I understand correctly you see low 9x running speed and higher 2x9x, 3x9x etc.

Normally what they will suggest is that at 9x per revolution there is a sharp pulse, either mechanical impact or hydraulic pulse.

The pump is similar to a gear as you know. I would think improper backlash may create mechanical impacting as the tooth goes into the valley (sorry I don't know the correct terms).

We have one experience with these style pumps installed as aux oil pump for a large chiller. In that application they "frequently" (perhaps 5x over 10 years among 6 units) fracture they key or keyway such that the pump stops rotating. We suspect possibly some misalignment induced by pipe strain but we really don't know the root cause.

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[GregLocock]

Third (and fifth) order is strongly associated with 'square wave' pressure waveforms, and also with knocking, ie clearances.

So, look at your hydraulic pressure patterns and see if they vary between pumps, failing that look at your clearances.

3 dB variation is damn good, actually.


Cheers

Greg Locock

 

[TrannyNoise]

Thanks

We'll look at those things

 

[ronfrend]

On ANY hydraulic delivery pump (including Gerotor) you WILL get many harmonics of run speed. It is in the nature of the beast and (as Greg) indicates is due to the asymetric pressure pulsations.

So don't worry about it but it makes analysis more "interesting".

Ron Frend

http://www.predicon.net

 

[TrannyNoise]

Ron,

Yes I realize any pump will generate lots of harmonics...any advice on how to reduce their amplitudes (other than reducing the pump pressure pulsation amplitude)?

Thanks!

 

[ThomasKing]

I have a simillar effect on a race engine right now. I have pump fundamental and 2 times pump fundamental that create pressure fluctuations of roundabout 100%, what is quite a bit.

I am looking for a way to reduce it. Any ideas about the porting geometry? I used Nichols Portland standard geometry and I was told that there is no better design possible.

Any comments? Apreciate you help. Thanks!

 

[MikeHalloran]

I'm guessing that unit to unit variations will correlate with end clearances, i.e., axial clearance between rotors and end plates, with the looser units being quieter because of the extra internal leakage.

All the Gerotor teeth I have ever seen are 'straight', i.e., not twisted like a helical gear. Straight cut gears also resemble the rotor of a mechanical siren. Which leads me to this; I haven't tried it, but it should be possible to reduce the noise from a gear pump with a little porting work, skewing the edge of the inlet port so it's not parallel to the gear teeth or the shaft. No such opportunity presents for a Gerotor.

Somebody with a CNC id/od grinder could probably produce one-off Gerotor sets with twisted teeth, or it could be done in quantity with powder metal, but is there a market for really quiet gear pumps?

Okay, there is, but the pumps are called screw pumps.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA