Lomakin Effect 2

[mjpetrag]

I am planning on replacing the metal wear rings in the circulating water pumps here with Vespel wear rings. I was reading the manual on Dupont's website, and they mentioned the Lomakin Effect. I was wondering if I am correct in my understanding of it...

Basically, what I got from it was that as the wear ring clearance decreases to the stationary part of the pump, the fluid pressure caused from the force of the rings flinging it outward makes a stronger cushion for wear rings.

Also, does anyone have experiences with Vespel wear rings?

 

[electricpete]

The Lomakin effect is described in Rotating Machinery Vibration by Maurice Adams section 4.2.1.

He has two pages, but here's my understanding of what he's saying (please remember I'm an electrical guy): When there is axial flow in an annular passage between stationary and rotating components, it provides a centering force (increased stiffness). Consider an offcenter rotor which has narrow gap at 12:00 and large gap at 6:00. The pressure drop across the seal from end to end is the same at the narrow gap as at the large gap. That means higher flow velocity in the higher gap than the lower gap. Bernoulli's equation tells us this gives higher pressure applied at the small gap than at the large gap. This net force from these two pressures is dominated by the pressure in the small gap and therefore acts in the centering direction.


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[JJPellin]

I am not the best expert to describe the Lomakin affect. But I will explain my understanding. In any wear ring or bushing with pressure differential, there will be axial flow passing through the clearance. If the rotating part moves away from the center of the stationary part, there will be different clearances on each side. The side with the greater clearance will pass more flow than the side with the tighter clearances. More flow passing through the clearance will result in locally higher velocity. Since the velocity is higher, the effective local pressure will be lower. This affect tends to be stabilizing. The force of lower pressure on the side with larger clearance and higher pressure on the side with tighter clearance makes the rotating part want to move back toward the center. In effect, the wear ring or bushing becomes a stable bearing. This reduces the chance of a rub, even with a flexible rotor. When you change to Vespel, you will run tighter clearances. This increases the Lomakin affect and the rotor is even more stable and likely to stay close to the center of the clearances. Overall shaft deflection and vibration should be reduced.

We have used Vespel in a large number of applications. It has saved some wrecks when pumps were run dry or dead-headed. But be very, very careful about the design. If the cross section of the Vespel is too thin, it may collapse into the rotating parts. And the axial thermal growth is much, much greater than steel and this must be accounted for. We have had a couple of very dramatic failures when these properties were not properly taken into account.


Johnny Pellin