Hydro pump characteristics vs. RPM

[Jay4200]

I'd assume that the output of a hydro pump vary with RPM, but exactly how? Linearly or non-linearly? Volume and/or pressure?

For example, my tractor hydraulics are rated at 8.2GPM - I don't know what the max pressure rating is...probably around 2500PSI, I'd guess. I assume these ratings are with the engine running at PTO speed - so what would the max pressure/volume be at 1/2 PTO speed?

thanks - JayC

 

[kennyd]

Pressure is constant, it's the flow (GPM) that varies with the engine RPM. If I had to guess, I'd say it's close to a linear relationship. I would also assume the rating of 8.2 GPM is at PTO speed.

 

[brain55]

The flow is linear. Say the flow is 8.2 gpm at 2600 RPM the flow will be 4.1 gpm at 1300 RPM and 2.05 gpm at 650 RPM. The pump efficiency will determine how much of that flow you will retain at a given speed and pressure. If the pump is rated for 8.2 gpm at 2600 RPM and 2500 psi that would be at 100% efficiency. At 90% efficiency you will see 7.4 gpm at 2600 RPM and 3.7 gpm at 1300 RPM at 2500 psi.

Brian

 

[wdchyd]

Jay'

Is this question regarding your Snowblower problem possibly..?.....? Control Valve rating not large enough....maybe...?.....

 

[rdoiron]

I'd assume that the output of a hydro pump vary with RPM, but exactly how? Linearly or non-linearly? Volume and/or pressure?

For example, my tractor hydraulics are rated at 8.2GPM - I don't know what the max pressure rating is...probably around 2500PSI, I'd guess. I assume these ratings are with the engine running at PTO speed - so what would the max pressure/volume be at 1/2 PTO speed?

thanks - JayC

The volume would be roughly 1/2 of 8.2 GPH if the pump is a gear pump (not a variable displacement pump). Pressure is a function of restricting the flow. You will get maximum pressure at idle to full throttle if you restrict the flow.

 

[J_J]

You also have to take into account the efficiently of the hyd pump which is about 85%.

Just do a flow meter test if possible. That should give you real time volume and pressure, whether the pump is new, or worn.

 

[rswyan]

You also have to take into account the efficiently of the hyd pump which is about 85%.

Isn't that 85% efficiency a total covering both ends - 7% loss at the pump, and another 7% at the motor ?

From Surplus Center's website:

"Efficiency - typically assume hydraulic pump/motor efficiency of 85%"

I was looking at the spec sheets on some cheap Dynamic pumps recently and they were claiming an efficiency of around 93% ....

 

[J_J]

Both the pump and motor have approximately an 85% efficiency. If you input 20 HP to a hyd pump. you can expect to get 85% from the pump. 17 HP. The fluid then goes to the hyd motor, and if the pump flow and pressure is matched up to the hyd motor, there is another 85% efficiency through the hyd motor, which will develop 14.45 HP. This is with gear pumps. Other pumps may have a different efficiency.

 

[AKKAMAAN]

Volumetric efficiancy is depending on pump design, viscosity and pressure. A gear pump and a gear motor have similar efficiancy properties. See manufacturer ratings for detailed info. Power equation need info about the mechanical losses too

example.
gear pump volumetric efficiancy 85%
gear motor volumetric efficiancy 85%
(Internal leakage in spool valve X%)
Total volumetric efficiancy 0.85 x 0.85 x (X) = 0.7225 x (X)=72.25%
If you want 10hp output power on motor, you need to add mechanical friction losses to equation to...so you might end up with almost twice input power than output power.....

 

[Jay4200]

Jay'

Is this question regarding your Snowblower problem possibly..?.....? Control Valve rating not large enough....maybe...?.....

Yes, sort of and No - also just for general knowledge. My control valve is plenty big at 10GPM.

On the topic, I may have a flow restrictor in place at the cylinder (as suggested by KWentling), which could cause the overpressure condition I described earlier, provided (a) the pump can create full pressure at low RPM - which is what has been suggested here, and (b) that the restrictor allows flow less than what the pump can do at idle.

I think I remember seeing a fitting in place at one of the cylinder ports that wasn't at the other 3 ports - might be a functional piece. It makes sense, because the cylinders are really teeny and move fast. The flapper function, which appears unrestricted, cycles in no more than a second (and that is at idle). Cycling the chute rotator that fast would probably rip the assembly apart.

JayC