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

 

[Jay4200]

Pressure is a function of restricting the flow. You will get maximum pressure at idle to full throttle if you restrict the flow.

Do these pumps actually create full rated pressure (or internal bypass pressures) at idle? The pump is just a spinning impeller, isn't it? I'd imagine that pressure capacity would have some sort of relationship to speed, at least from and efficiency standpoint - well, it must, but I guess it would be moot provided low-RPM pressures can exceed the internal blow-offs.

I don't have one of those flow/pressure meters - wish I did.

Thanks - JayC

 

[SPYDERLK]

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

I know what youre saying and, on face, with positive displacement it has to be true. However, under pressure, fluid bypasses thru case/rotor clearances and must be pumped again. This amount should be mostly a function of pressure rather than rpm. So bypass at 2500PSI at 1krpm would be [~]nearly the same as at 2krpm at that pressure.That would lead to a conclusion that the bypass results in a higher proportion of low rpm flow being lost than is lost at the greater flow that accompanies hi rpm. This would say that, at pressure, net flow at 2krpm is more than twice that at 1Krpm. So one would think that pump efficiency must rise with speed until suction side issues develop.
larry

 

[mmurphy]

Jay, are you thinking centrifugal pump?

Not really a spinning impellar, unless you might be considering a vane pump in that regard. Most impellar or centrifugal type pumps are used in low pressure applications. (water)

I do not recall, but I think you have a open system. I think your question is then what is it doing at idle? If no valves are being used, it is under no pressure and returning to sump under no pressure, except for friction restrictions. Yes full pressure can be met at idle.
When using valves then you are under pressure, lot lower volume of course at idle.
Increased RPM of course will help build up pressure quicker cause of volume.

 

[wdchyd]

Do these pumps actually create full rated pressure (or internal bypass pressures) at idle? The pump is just a spinning impeller, isn't it? I'd imagine that pressure capacity would have some sort of relationship to speed, at least from and efficiency standpoint - well, it must, but I guess it would be moot provided low-RPM pressures can exceed the internal blow-offs.

I don't have one of those flow/pressure meters - wish I did.

Thanks - JayC

Kubota Flow/Load Meter Test - Tractor Videos - Kubota, John Deere, New Holland and More

You mean like this one...??

 

[Jay4200]

Kubota Flow/Load Meter Test - Tractor Videos - Kubota, John Deere, New Holland and More

You mean like this one...??

Exactly. Please drop one off the next time you're in the area. Thankssomuch...

JayC

 

[IceStationZebra]

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.....

and pump speed.

I have a pressure vs. speed efficiency chart for a specific gear pump and the efficiency goes up with the speed. Basically the gear clearance is a leak path that is fairly constant at a constant pressure, so it will leak Xgpm. As the pump speed increases the total flow is going up but the leak path is staying roughly constant - so the ratio of total flow to leakage gets better. At low speed the leakage can get pretty serious.

System pressure can affect you two ways. The obvious route is the leak path around the gears, and as pressure increases so does the leakage. But good gear pumps also have pressurized wear plates that work to close that gap and keep leakage to a minimum. From the data I had this wasn't a linear relationship. I suspect this is due to changing flow forces inside the pump.

So for high speed, low pressure applications your efficiency could be up to the mid-90's. Then for low speed, high pressure applications it could drop the the low 70's.

ISZ

 

[SPYDERLK]

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

I know what youre saying and, on face, with positive displacement it has to be true. However, under pressure, fluid bypasses thru case/rotor clearances and must be pumped again. This amount should be mostly a function of pressure rather than rpm. So bypass at 2500PSI at 1krpm would be [~]nearly the same as at 2krpm at that pressure.That would lead to a conclusion that the bypass results in a higher proportion of low rpm flow being lost than is lost at the greater flow that accompanies hi rpm. This would say that, at pressure, net flow at 2krpm is more than twice that at 1Krpm. So one would think that pump efficiency must rise with speed until suction side issues develop.
larry

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.....

and pump speed.

I have a pressure vs. speed efficiency chart for a specific gear pump and the efficiency goes up with the speed. Basically the gear clearance is a leak path that is fairly constant at a constant pressure, so it will leak Xgpm. As the pump speed increases the total flow is going up but the leak path is staying roughly constant - so the ratio of total flow to leakage gets better. At low speed the leakage can get pretty serious.

System pressure can affect you two ways. The obvious route is the leak path around the gears, and as pressure increases so does the leakage. But good gear pumps also have pressurized wear plates that work to close that gap and keep leakage to a minimum. From the data I had this wasn't a linear relationship. I suspect this is due to changing flow forces inside the pump.

So for high speed, low pressure applications your efficiency could be up to the mid-90's. Then for low speed, high pressure applications it could drop the the low 70's.

ISZ

Yup. Makes perfect sense. :thumbsup: ... Actually, efficiency can drop to zero at low speed hi pressure. At lo idle when hot the 7520 pump wont pop the relief valve.
larry