Hydraulic pump Output question ....help

[4570Man]

It does continue to increase GPM with increasing RPM, but it's not linear.

 

[jhous]

I'm not an expert but my thoughts would be that you would use the max HP rating. Most of the specs I see for the LS show that the HP rating is achieved at say 2600 rpm's so even if you go above 2600, you won't achieve any more than HP than at that rpm. Kind of like a HP curve from a dyno.

Realistically you are right. Horsepower rating for my LS is at 2700 rpm. My experiment showed that increasing engine rpm past that will indeed increase pump output but the amount of output increase is very small and probably not significant in the real world. My guess is exactly what you say in that the output curve of the hydraulic pump is very similar to the HP curve of an engine on a dyno.

Thanks for your input.
j.

 

[jhous]

It does continue to increase GPM with increasing RPM, but it's not linear.

The experiment I did (detailed elsewhere) agree with you (about it not being linear) up to about 2600 rpm. From 2600 to 3000 rpm the amount of increase is very close to the same for each 100 rpm increase.

Thanks for speaking up.
j.

 

[4570Man]

The experiment I did (detailed elsewhere) agree with you (about it not being linear) up to about 2600 rpm. From 2600 to 3000 rpm the amount of increase is very close to the same for each 100 rpm increase. Thanks for speaking up. j.

How did you conduct your experiment. I was thinking the simplest method would be to time how long it takes to raise the loader to full height. Then I would repeat the test with the backhoe arm.

 

[LD1]

Running a tractor at WOT is not like running a car at red-line.

The tractor is made to run that speed all day long with no issues. Your max RPM is not a red line, rather its the governed speed of the engine. For backhoe work, I run WOT because I want every bit of speed I can get.

 

[SPYDERLK]

It should be the same increase with rpm ideally, but -- All pumps have leakage as they pump. Leakage is a function of time and pressure. At the same load pressure but at a lower speed the leakage is greater in proportion to the volume pumped because each revolution takes longer.
... The output increase you observe to a load should become closer to linear at hi rpm. Graphing from a lo rpm would show an upward turning curve that became more close to a straight line when you approach the pump rated rpm - and even a bit beyond if the pumps suction line is free flowing.

 

[jhous]

How did you conduct your experiment. I was thinking the simplest method would be to time how long it takes to raise the loader to full height. Then I would repeat the test with the backhoe arm.

Yep. That's exactly what I did. I used a stopwatch (actually a stopwatch app on my phone) and I determined a starting point and end point (for both the loader and backhoe) that could be replicated repeatedly. I also did the loader tests with an empty bucket and a fully loaded bucket. I had the backhoe arm fully extended for the tests.
Loader results were:
2500 rpm = 4.1 seconds to full height
2600 rpm = 3.83 seconds to full height
2700 rpm = 3.66 to full height
2800 = 3.5 to full height
2900 rpm = 3.46 to full height
3000 rpm = 3.3 to full height
A fully loaded bucket did not change the times ......which surprised me.

The backhoe ranged from 2.6 seconds to full height at 2500 rpm to 2.0 seconds to full height at 3000 rpm.

It was an interesting exercise. Proved to me (at least) that the top couple of hundred rpm does not really buy you that much. However as one of the other members pointed out these engines are designed to run at WOT continuously (and I believe he is right) so ......"Pedal to the metal" may not buy you much extra but it shouldn't hurt anything either.

J.

 

[jhous]

It should be the same increase with rpm ideally, but -- All pumps have leakage as they pump. Leakage is a function of time and pressure. At the same load pressure but at a lower speed the leakage is greater in proportion to the volume pumped because each revolution takes longer.
... The output increase you observe to a load should become closer to linear at hi rpm. Graphing from a lo rpm would show an upward turning curve that became more close to a straight line when you approach the pump rated rpm - and even a bit beyond if the pumps suction line is free flowing.

Thanks for the insight. I think my little experiment showed exactly what you predicted.
J.

 

[jhous]

Running a tractor at WOT is not like running a car at red-line.

The tractor is made to run that speed all day long with no issues. Your max RPM is not a red line, rather its the governed speed of the engine. For backhoe work, I run WOT because I want every bit of speed I can get.

Agree totally. When using the backhoe, I want all the ripping force available.

 

[npalen]

I suspect that at very high rpm the internal friction of the oil flowing thru the plumbing starts to diminish the slope of the curve. The friction generates heat which will tend to reduce the fluid viscosity in turn reducing pump output.

This is an interesting nomograph although is doesn't directly apply to this discussion:
Hose Size Selection Nomograph - RYCO