Hydraulic power and RPM

[VirginiaAgent]

I have been told conflicting things about this subject.

Does higher RPM provide more power to the hydraulic pump translating into greater digging / lifting power;

Or, does higher RPM simply make the hydraulics operate and move faster, but not increased power?

Any thoughts? By the way, my machine is a NH 555E backhoe loader.

VA

 

[SPYDERLK]

Reving it up increases force only a little-you can notice the extra in some situations where your right on the edge of max. However 2X revs comes quite close to doubling the speed of the implement. Since power is force X speed the implement has double the power altho essentially no more force that at lower rpm. If that force cant move the load all your power goes thru the relief valve, just wasting twice as much.
larry

 

[Kendrick]

another way to put it is if you have a straw that can handle 120 psi and a drain pipe that can handle 120 psi which will fill the bucket first. low rpm's = straw high = 2" drain. a cilinder takes for an example 1 galon of hydrolic fluid to fully extend. filling that gallon at low rpm will take some time because it is pushing the straws worth of oil to the cilinder. you have it run at operating speed and your filling it with a much larger amount of fluid in the same amount of time. since both of those pipes are still only rated for 120 psi you go beyond that pressure some how like by dropping a big rock on the loader either a line breaks or the saftey valves kick in. your tractor/hydro system only has x ammount of hp getting it to operating speed only does 1 thing bring all the pressures and pto speeds up to what they were designed to run at. the main reason you have more variance in a geared tractor is that you drive impliments and the gears for movement through the same throttle. as i under stand it. hst you have a throttle lever and then the shuttle on the floor to move the tractor forwards and backwards. the shuttle controls speed so you can leave the tractor in operating speed all the time. with gears when you shift from h1 to h2 you probably will go from 2k on the tack to 1k just so you dont do a nose plant on the stearing wheel as 2nd engages. if it werent for that i would leave it at operating speed when running the tractor except for when im just driving from 1 location to another. then i would throttle down to what is needed to keep the speed so idont waste fuel.

I hope i havent explained it poorly or forgot somthing.

 

[john_bud]

I have been told conflicting things about this subject.

Does higher RPM provide more power to the hydraulic pump translating into greater digging / lifting power;

Or, does higher RPM simply make the hydraulics operate and move faster, but not increased power?

Any thoughts? By the way, my machine is a NH 555E backhoe loader.

VA

The reason you have conflicting answers is that the situation is both simple and complex.

The simple answer is that hydraulic pumps will pump more volume at higher rpms, not higher pressure. So, you won't get any more digging / lifting power by jacking up the rpms.

The complex answer is that if the power needed to move the cylinder exceeds the power being produced by the engine at that rpm (the engine will decrease rpms when you activate the control), they you will be getting more digging / lifting power by increasing engine rpms.


If you are at 700 rpm and you try to lift 2000# the engine will bog down and maybe even die. Increasing rpms to 2000 will allow you to lift the weight because the engine now has the power to transfer to the pump.

If you are at 2000 rpm, increasing the rpms to 2100 or even 2400 rpm won't do diddly for power, but will increase speed slightly.

 

[MasseyWV]

My experience has been that:

2000 rpm's is the "sweet spot" for most machinery I have operated and that faster rpm's may increase the speed slightly but little if any actual power gain is noticable.

Anything below 2000 rpm's will slow things down a bit and can even cause the hydraulic pump to stall the engine if the rpm's are too low.

 

[VirginiaAgent]

Very interesting. More of a multi-dimensional issue than I previously thought.

Thanks for the insight!

 

[Wayne County Hose]

I have been told conflicting things about this subject.

Does higher RPM provide more power to the hydraulic pump translating into greater digging / lifting power;

Short answer, yes.

Or, does higher RPM simply make the hydraulics operate and move faster, but not increased power?
VA

Short answer, no.

Pumps do not create pressure, they create flow. Your hyd system supplies the resistance, which makes pressure. Pushing more fluid thru the same resistance makes higher pressure which makes more digging force.

On most properly designed machines, the hydraulic system is most efficient at the engine's torque peak, at which the engine runs most efficient. You should know where your engine's torque peak is at and run the machine there. That, is your "sweet spot."

 

[VirginiaAgent]

On most properly designed machines, the hydraulic system is most efficient at the engine's torque peak, at which the engine runs most efficient. You should know where your engine's torque peak is at and run the machine there. That, is your "sweet spot."

Okay, does it do any damage to the pump, or the engine, to operate at low RPM? I am usually not in a hurry, so I run the machine at it's lowest RPM. (Also to save fuel).

 

[Wayne County Hose]

Okay, does it do any damage to the pump, or the engine, to operate at low RPM? I am usually not in a hurry, so I run the machine at it's lowest RPM. (Also to save fuel).

In this case, use only the power you need. If you bog the engine down, increase the rpm's. You won't damage anything by using at lower rpm's unless you do bog the engine down. Bog it down far enough, and a diesel may even sputter, clank, and then start running backwards. Not all of them do it, and I don't know which ones will and won't, but i have seen it happen.

 

[SPYDERLK]

Okay, does it do any damage to the pump, or the engine, to operate at low RPM? I am usually not in a hurry, so I run the machine at it's lowest RPM. (Also to save fuel).

No damage. You will have 5 or 10% less pressure available than you would if the engine were singing, so you cant apply quite as much force. If your system is tight youll still pop your check valve when the implement stalls, but pressure wont go quite as high as it would if you were really pumping a lot and making it open wide to relieve. Of course power [as distinguished from force] would be down in proportion to your lower flow - power being force X speed. But as you said, youve got the time to work at lower power level. Lowest rpm may be a little low for keeping your engine happy tho.
larry