Gerotor pressure and flow 1

[Pimech]

Hello,
I'm studying a transmission system.
A gerotor is joined to a shaft (that transmits about 100 KW) to pump lubricating oil.
Gerotor constuctor give informations only about theoretical flow rate.
Hydraulic power is a function of flow and pressure.
Flow should be proportional to rotating speed.
How can I control oil pressure and mechanical power absorbed?

Thanks
Pierluigi

 

[micalbrch]

You can control the pressure either by the flow (lower flow = lower pressure, higher flow = higher pressure) or at constant flow by a valve or an orifice.

 

[hydroman247]

To control pressure you need some sort of valve to create the pressure in the first place.

 

[Oldhydroman]

No, the pressure comes about because of the resistance to flow.

Your pump is a fixed displacement unit which pushes out a certain volume of fluid for each revolution (we'll conveniently ignore the volumetric efficiency for a minute). There will be a flow rate (litres/minute) from the pump which is basically proportional to the shaft speed (revs/minute). The flow from the pump (presumably lubricating oil) will be directed to the various places in the transmission that need the oil supplied to it. This might be the small clearance between a plain bearing and the shaft running in it, or maybe you have nozzles spraying oil onto some meshing gears. The pressure that you would measure at the outlet of the pump will be caused by the effort needed to squeeze that particular flow rate through all the galleries and into all the places that the oil has to go.

When the oil is very cold (and thick) it will be harder to squeeze it into all these places and the pressure will rise. Similarly the pressure falls when the oil is hot (and thin). If it's an engine and it's a bit worn out then the oil pressure can be a little low because the clearances have opened up. Years ago some people used to compensate for this by using a thicker grade of oil than the manufacturer recommended - adequate oil pressure was restored but fuel efficiency was reduced because of the extra work done pushing the "treacle" around the engine.

The oil pressure is also a function of flow rate (and so will be a function of shaft speed). All things being equal (and ignoring the fluid friction in the galleries) the pressure will increase by a factor of four for a doubling of the shaft speed. Except all things aren't equal: the clearances might open up as the oil pressure increases so the resistance to flow isn't quite what you expect and the increase in oil pressure is less than the theoretical rise for that particular increase in shaft speed. A similar effect occurs in the pump - as the shaft speed increases the pump flow rate increases, the outlet pressure increases (as described above) but this increases the internal leakage in the pump and the increase in flow rate isn't as much as you expect it to be. We would say here that the volumetric efficiency reduces as the pressure increases. If you only have the theoretical flow rate from the manufacturer then you need to make an allowance for the volumetric efficiency - it's not easy because it is a function of pressure, shaft speed, temperature, viscosity and wear. So let's just gloss over the unknowns and say "the pressure is going to rise when the shaft speed increases" (Hydroman247 - you're not right to say there has to be a valve there.)

The theoretical power absorbed by the pump will be the mathematical product of theoretical flow rate and the actual pressure (be careful with the units). The actual power will be greater than the theoretical power because of the mechanical losses and the fluid friction losses inside the pump. So - all things being equal again, and with a lossless pump pushing oil through simple sharp edged orifices, a doubling of the shaft speed would entail an eight fold increase in absorbed power [the pressure has gone up by a factor of four and the flow has gone up by a factor of two].

If you want to limit the increase in power absorbed by the pump as the shaft speed rises then you could install a pressure relief valve. Such a valve would limit the maximum pressure to a value high enough to ensure there was enough oil reaching all the places that needed it. Any excess flow from the pump would spill over the relief valve back to the sump. If we assume the relief valve had a flat pressure override characteristic (once the valve has opened the pressure remains the same no matter how much oil you push over it) then, as soon as the shaft speed reached a value high enough for the pressure to rise enough for the relief valve to open, any further increase in absorbed power would simply be proportional to the shaft speed.

If you wanted to save even more power you would have to control the speed of the pump independently of engine speed or you would need a [much more complicated] variable displacement pump.

DOL

 

[tbuelna]

PiE81-

Wow! A 100kW lube oil pump?

 

[hydroman247]

It is quite possible the shaft is powering other things with that 100kW.

 

[Pimech]

Thanks to all for replies.
Oldhydroman's explanation is very clear and useful and it's consistent with my researches.

tbuelna no, the shaft on which oil pump is mounted transmits a such high power; the pump should absorb only 0,5-1 KW.