Hydraulic pump Output question ....help

[jhous]

I know the output of the hydraulic pump increases with engine RPM ......the manual says that if the loader moves slow to increase engine RPM (Assuming a warm tractor). My question is: Does the hydraulic pump output continue to increase all the way up to max engine RPM or is there a spot (RPM) that you get maximum pump output and then no increase in output after that? I do not want to run the engine at max RPM if max pump output is achieved at some lower engine RPM.

 

[kennyd]

Good question. I think most people assume the output ratings are at PTO RPM, but without a flow meter or details in the technical manual we can only guess.

 

[buickanddeere]

Don't worry about things that don't require worrying about.

 

[ozley]

Don't worry about things that don't require worrying about.

He is right about not worrying. Just experiment a little with different RPM's and find where the loader moves fast enough to your liking.

 

[oldnslo]

My question is: Does the hydraulic pump output continue to increase all the way up to max engine RPM or is there a spot (RPM) that you get maximum pump output and then no increase in output after that? I do not want to run the engine at max RPM if max pump output is achieved at some lower engine RPM.

The pump output increases with engine RPM. Presuming you have a fixed displacement gear pump and the displacement is 1 cubic inch per revolution. At 1000 RPM you would be pumping 1000 cubic inches, at 2000 RPM would be 2000 cubic inches, etc. on up to full engine RPM.

Like others have said, operate at an engine RPM that gives you the speed and control you desire.

 

[jhous]

He is right about not worrying. Just experiment a little with different RPM's and find where the loader moves fast enough to your liking.

It's not really a matter of worrying. I've had a lot of experience with different engines (marine, aircraft, motorcycle, auto ......even rebuilt motorcycle and auto engines) and I have found that when operated within the design parameters they operate more smoothly and last longer.

I added a backhoe to my little LS and when I operate this device I will sit in one spot for maybe hours with the engine running at whatever rpm I set it at. The manual states that 'maximum rpm' for the engine is 2900 rpm. On a car that would imply 'red line' and not a good rpm to run the engine at for hours on end......you will be sorry if you do that. At full throttle my engine is turning 3000 rpm. Hopefully, this explains my concern a little. So if the hydraulic pump puts out it's maximum output at a lower rpm, then that's what I want to run my engine at.

While the advice to "...not worry..." is reassuring and if given in a spirit of helpfulness, is much appreciated. I'm looking for knowledge. I don't want to break my tractor either today or a year from now. Anyone with one of the larger tractors probably think this is foolish in that nothing they do puts their tractor in danger. That is not the case with the compacts and subcompacts. Look at the used ones and you will see bent loaders, loader arms that flex like crazy, and all sorts of weirdness at the PTO end. To me this 'tool' cost a lot of money and I don't want to spend more on repairing it.

Your advice to experiment is actually pretty good. The loader does not seem to speed up after a certain rpm (not sure what it is at the moment but will experiment to find out). That implies additional rpm does not increase pump output. The backhoe is somewhat a different story. Digging up stumps I constantly get into the relief valve. In that case I will have to experiment both with movement speed vs rpm and rpm vs relief valve. mmmmmm.

In any case, thank you for the thoughts.
j.

 

[jms396]

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.

 

[MMagis]

He is right about not worrying. Just experiment a little with different RPM's and find where the loader moves fast enough to your liking.

x3. You're overthinking it.

 

[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.

I think I proved you are right. As one of the other members suggested, I experiment. So I did. I timed the lift speed of the loader at various rpm's from 2500 rpm to 3000 rpm. The results were that the faster the engine speed the faster the loader went up. Starting at 2600 rpm the incremental speed increase for each 100 rpm was .16 seconds faster up to max of 3000 rpm. The interesting thing to me was that there was no difference in time between an empty loader and a full (total full bucket of top soil) loader.
The backhoe showed the same basic results (different numbers).

So. The pump does increase output with an increase in engine rpm...... but in the upper rpm range (2600 rpm to 3000 rpm) there is only a half second increase in speed of the lift. Does that mean anything in the real world? Probably to not worry about the pump and operate at an rpm that feels right for the situation................ Gosh! Just what several guys have said.

Thank you for your input.

 

[jhous]

x3. You're overthinking it.

Can't help myself. All my life I've had to know the how and why of every machine I've owned. Downside is that sometimes I worry about something that may have little impact on the whole machine. The upside is that My stuff tends to last longer and run better. My wife doesn't mind. She always knows where I am ......in the garage.

If you ever need anything 'overthought', just let me know.
j.

 

[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