Hydraulic Pump and Motor

Use a variable speed, constant flow pump, or something like a Parker load sense control.

what picture?

How is your pump controlled? If it is pressure-compensated or has load sensing, you may not need anything else. Do you have a schematic and more information - does this pump drive anything other than the fan?

Which would be a better means of control? I do know that pressure compensated is less complex but it keeps the output pressure of the hydraulic pump constant and varies the flow to the motor depending on the load. In this case, since the load is constant ( Fan), i suppose this will work here.

If the load is changing, i think it might be better to use a load sensing line. If the load is varying, in the case of pressure compensated system, it will try to keep the pressure output of the pump constant and thus the flow will decrease which in turn will cause the load to rotate at a lower speed. Am i right in saying the above statement.

I guess the next question comes too.. lets say the prime mover will vary speed from 1000-2000 rpm and the fan has to rotate at a constant speed of 2000 rpm. Lets say even if i were to size the pump and motor, am i right to say that i will have to size the variable displacement pump based on the prime-mover minimum speed of 1000 rpm such that the fan will spin at 2000 rpm. When the prime-mover speed changes to above 1000 RPM, if i were to use the load sense system, it will still keep the fan rotating at 2000 RPM. Am i correct in my concept?

I think you've got it. As sawmilleng says, load sensing would give the most precise speed control if the fan is the only thing the pump is driving. I suppose you could use remote load sensing if there are other loads, but that seems like overkill. You would probably need pressure relief in case the fan motor stalls.

Pressure compensated could be used if there are other driven devices, but then you might need a restriction to control the fan speed. But a pressure compensated pump would probably cost less than load sensing.

And yes, the pump must put out enough flow at full displacement at its slowest speed to run the fan at the desired speed. At higher speeds, it would reduce its displacement as required.

You'll need a load sensing control on your variable displacement pump. This is assuming that the pump is not driving anything else but the fan. The load sense maintains a given pressure drop across the load independent of the pump speed.

A simple Google search will get you tons of hits for this.

Jon.

Since you did not inform us of what the fan cools. If it cools the motor control components then the variable speed is just fine. As at a higher rate of speed the components would get hotter then at a slow speed.

Since the hydraulic motor is mechanically connected to some other device. Which is driving the speed of the prime mover. The fan speed will always be dependent upon the prime mover speed. A flow control may get you a constant speed but it will be slower. And you will always have a period from stop to when the flow control starts to restrict flow that the fan speed will vary.

Also if the prime mover is battery powered then restricting the flow of the hydraulic line will reduce the battery charge time. The restriction will be felt as a load on the speed motor requiring more current.

The latest generators we have are hydraulically powered. Gear, or belt driven hydraulic pump, connected to a hydraulic motor running the generator. Gives us precise generator output while the engine can operate anywhere from 600 to 2600 RPM. I believe it is a simple pump with a variable displacement hydraulic motor. Try looking into Harrison or Onan Hydraulic generators. Good luck.