Lubrication system. Please explain.

Rich_j said:

Are you saying positive displacement pumps are not affected by cavitation?

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I'm saying cavitation is not a significant consideration in their typical applications. Sure, if you used one to pump a fluid with a high vapor pressure, such as, say, alcohol, you could get cavitation if the inlet pressure is insufficient and you would get less pump throughput (mass per unit time) than you might have wanted. If you did use a positive displacement pump for this application, you would have to design the system to provide a high enough inlet pressure to avoid cavitation (as you would with any other type of pump used here). But typically you choose this type of pump where you can't or don't want to control the inlet conditions very well, and you don't have to because the fluid's vapor pressure is low - such as oil in a Cdo. One other thing that bears remembering: bubbles visible in a flow are not necessarily evidence of cavitation. If they are air bubbles mixed with, say, oil, then they are definitely not. Cavitation, due to a drop in pressure, creates voids in a fluid where the fluid, usually just instantaneously and momentarily, goes from a liquid state to a gaseous state. These voids are pockets of vapor of the same substance as the liquid. When the pressure subsequently recovers at the pump outlet, they collapse and that's the end of their existence - they do not travel downstream. Sucking air is not cavitation.

"Carry on, and dread nought" - Winston Churchill

kartiste said:

lynxnsu said:

...the speed of the feedline matters of course as well or you get cavitation ,but that is nothing to do here

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Just a point about cavitation and the Cdo oil pump: being a gear pump, it's generically a "positive displacement" pump, meaning it simply moves whatever appears at its entrance - air or oil - by increments of volume, not by increasing the energy of the flow, as in a centrifugal pump. Cavitation (defined as a reduction in fluid pressure below the vapor pressure of the fluid) is not relevant to the operation of a gear pump, and bubbles in the flow - supply or return - are not a result of cavitation, since the fluid pressure is above the vapor pressure of the oil at all times except possibly within the pump somewhere, but probably also even there - oil has a very low vapor pressure, so it takes a pretty good pressure drop to get under it.

Very intressting
But when fitting a high capacity pump to a round crankcase Duke we found that mains would not last long and one should see the pumpgears after afew races (very pitted to say the least)
any thoughts what happened there?
when fitting std pump no more problems....
ps a duke uses gear oil pump

lynxnsu said:

But when fitting a high capacity pump to a round crankcase Duke we found that mains would not last long and one should see the pumpgears after afew races (very pitted to say the least)

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Well perhaps the pump did cavitate! The pitting could be a result. The collapse of vapor cavities can be microscopically violent, and commonly leads to erosion of pump impellers where cavitation occurs.

A gear pump can draw a pretty high vacuum and cavitate but normally only if the inlet is severely restricted. I have seen some hydraulic pumps that were destroyed because the inlet was restricted.
Pumping air is not a problem except for the fact that you have to get the air back out of the oil. Jim