Impeller Noise At Higher RPM 1800+

[valmeidan]

Ok so I have a water pump used in a computer liquid cooling solution. I am finding that the impeller whether it is in water or not makes a kind of whining noise when it hits around 1800rpm. Sometimes right around 1800 or above if you dont here the noise you can tap the fins on the impeller then the whine will start, I was wondering if anyone has had this experience, and whether or not it is a sleeve shaft tolerance issue, causing the impeller to wobble at higher rpm, or something else. The impeller has a magnet in it which is seated in a housing with a stator which spins the impeller. The fit is quite snug, but it was originally designed so a small amounts of fluid can pass down in between the sleeve and shaft to lubricate the two components. The sleeve attached to the impeller plastic is brass and the shaft is stainless steel. Thoughts?

 

[valmeidan]

Would be nice if someone knew of an ASME spec or similar that defined bearing/sleeve and shaft tolerance

 

[JJPellin]

You state that it was originally designed to use product to lubricate the bushing. Was this design changed? What you are describing sounds like a magnetically driven, centrifugal pump. The driven magnet assembly must be lubricated with the pumped product. You also stated that it makes the whine when spinning in water and without water. This implies that you have deliberately run the pump dry with no water. Is this correct?

What you describe sounds like an instability that occurs at a particular speed. It could be a resonance. That fact that you can initiate the noise by tapping on the assembly agrees with this possibility. The other possibility that occurs to me is a whirl instability in the bushing that supports the rotating assembly. This could be occurring as a result of excess clearance as a result of the dry running or inadequate lubrication suggested by my first set of questions.


Johnny Pellin

 

[valmeidan]

I was just dry running it to see if I could hear the sound, and I could. The sleeve is in molded to the plastic impeller, so would you say that I should increase the gap between the shaft and bearing or decrease it? I cannot increase it too much obviously or it will most definitely vibrate in the housing, but like I said it can spin nicely up until around 1800 rpm, but even at 1800 rpm if I tap the impeller blades as its spinning it will make a whining noise, and if I look closely it appears that the impeller is no longer spinning smoothly, almost like a slight wobble, but hard to distinguish. If you touch the impeller a few times it can go back to a smooth spin again. Once you ramp the rpm's up to around 2500, the noise starts whether you tap on the impeller or not. But even if you tap on it again, it will not go away at that point. I think this is a journal bearing. I would like to find a tolerance clearance spec if one exists

 

[valmeidan]

Also the tolerance I have on the shaft it 1.992mm +-.002mm, and the tolerance on the bearing sleeve is 2.0mm+-.05mm, which might not be tight enough?

 

[MikeHalloran]

Buy a bigger pump and run it slower.


Mike Halloran
Pembroke Pines, FL, USA

 

[1gibson]

Check motor amps whining vs not whining, to see if you are rubbing.

Record the whine and determine the frequency. If the frequency is approximately 1/2 the rotation speed, that usually indicates a whirl problem.

If you just want the noise to go away, try graphite dust??? If you have been rubbing enough to wear the material, it might help temporarily. Don't think it will make things worse.