Hydraulic Pump / Motor question

[Hilbilly]

I have a pto pump that will produce up to 2500 psi and flows up to 37 uspgm. I will throttle it to produce 20 gpm max. This will match the hydraulic motor I have. I have performance curves for the pump and a performance table for the motor. The pump curves indicate it will produce much lower torque than the motor. My question is: does the pump torque have any impact on the motor torque or will the motor produce torque based on the flow it receives from the pump and the pressure produced by the load? The pump is 4.78 cu in and the motor is 6.2 cu in.

 

[oldnslo]

I have a pto pump that will produce up to 2500 psi and flows up to 37 uspgm. I will throttle it to produce 20 gpm max. This will match the hydraulic motor I have. I have performance curves for the pump and a performance table for the motor. The pump curves indicate it will produce much lower torque than the motor. My question is: does the pump torque have any impact on the motor torque or will the motor produce torque based on the flow it receives from the pump and the pressure produced by the load? The pump is 4.78 cu in and the motor is 6.2 cu in.

Torque is a function of displacement and pressure. Flow is speed.

To figure pump input torque: displacement in cubic inch per rev times pressure divided by (24 pi times efficiency)

To figure motor torque:displacement in cubic inch per rev times differential pressure times efficiency divided by 24 pi

 

[Hilbilly]

Thanks oldnslo. I'm getting this hydraulic stuff figured out slowly (maybe my name should be old n slow) but it's coming. I think I phrased the question wrong as I wasn't sure if the smaller pump displacement (ie torque) would impact the larger motor displacement torque but after some verbal discussion today with the tech at PA I believe I will be ok with the setup I'm planning on using.

I'll start another thread with the blower setup when I get a bit further into it.

 

[oldnslo]

Hillbilly,
In hydraulics flow is speed in motor RPM or cylinder velocity while pressure is force in either torque from a motor or thrust from a cylinder. A smaller pump driving a large motor the motor just turns slower but will develop the same torque for a given differential pressure.

 

[Hilbilly]

Perfect explanation and thanks for that.

I'm in the process of setting up a chain drive speed increaser that will drive the smaller PTO pump I have. I bought the pump years ago and wasn't able to find any info about it until recently. Now I have the pump specs and see it is capable of producing up to 2500psi at speeds from 200 rpm to 1800 rpm and flows up to 37 usgpm. According to my calcs the motor only requires about 15 usgpm to produce the 540 rpm needed for the blower. I'm planning to run the pump faster and adjust the tractor throttle to get the motor speed the blower needs. It will be trial and error initially and I will fine tune the set up after some testing.

 

[oldnslo]

Does your PTO pump already have a speed increaser? If yes be careful you don稚 overspeed your pump.

 

[dynasim]

One should realize that you can't "throttle" a positive displacement pump. Doing so would cause it to fail immediately. If you throttle a pd pump, and it has a relief valve/circuit, the excess flow will pass through the relief system. Typically, this is a very fuel/system inefficient process, and generates a ton of heat, eventually causing the relief system to fail.

To throttle a pd pump, you should lower the rpm of the power source.

Roughly, for a pump/motor system, the torque is directly proportional to the relative speed(or volume) of the system components. The higher the speed, the lower the torque. The larger the volume(lower the speed), the higher the torque.

 

[Hilbilly]

Sorry I took so long to respond. I've been away on vacation for 10 days.

In any case it appears I used the wrong term to describe my plans. By throttling the pump I meant control it's output by adjusting the motor speed of the tractor and therefore tractor PTO speed, to produce the desired flow and motor speed.