Hydraulic pump - positive displacement?

[turnkey4099]

In a discussion of a log splitter problem a guy is complaining that the new motor is the problem with lack of pressure. I say that the motor has zip to do with pressure as long as the motor is not stalling and that his problem is in the pump/hydro system.

Guy 2 insists that the proper rpm is required as the rated rpm is needed to develope pressure, i.e., run at idle, there won't be enough pressure to operate the ram. He is basign that on an 80% efficient pump.

That seems totally at odds with my, admittedly limited, experience. While there may be _some_ degradation of pressure at less than rated rpm, I doubt one would even notice it.

Harry K

 

[J_J]

If you have a good pump. as soon as the pump is turning, you have flow, and potential pressure, however very low, if any. Pressure is developed by some resistance to the flow of fluid such as a valve, cyl or hyd motor.

Volume/GPM is based on the rpm of the pump, the faster it turns, the more GPM.

If you can put that fluid under some pressure, it can be by motor and pump, or a hand pump. Some of those hand pumps can put out 10,000 psi.

A lot of people think that if a hyd pump is turning, it is putting out fluid under pressure. Not true.

On a running tractor, the fluid from the pump is flowing through all the valves back to the tank/reservoir, with little or no pressure, however the flow in GPM's is there all the time. When ever a valve is activated, the flow is interrupted and sent to a cyl or motor which uses the fluid to do work, and then expels the used fluid to tank.

 

[turnkey4099]

All true, in any hydro system there is no real pressure other than due to fittings/hose/pipe, etc restrictions until a force is applied.

I see I forgot to specify that in my view the only thing the engine (if it is not sstalling) is the cycle time of the splitter, e.g. it will develop the same working pressure at idle, or nearlyso, as at rated rpm. Thus the volume pumped is all it affects.

Harry K

 

[AKKAMAAN]

A positive displacement pump is designed to move fluid forward. There are both fixed and variable displacement pumps. A fixed displacement pump need a pressure relief valve, PRV, to protect pump and down stream valves from over pressure. A positive displacement pump always create a pressure that's required to move the fluid forward. If we ignore the internal leakage in a positive displacement pump (3-20% depending on design and pressure), pressure is not RPM dependent, but if we consider the internal leakage we need a RPM enough to move (displace) more fluid than the volume that leaks backwards (down stream internal leaks in spool valves etc., will also affect), to make enough pressure to move a load on a cylinder. Basically we can say that all positive displacement pumps can create enough pressure, even at any reasonable low RPM. The internal leakage is necessary for lubrication. The higher pressure a pump is designed for, the the tighter must the design between moving parts be.

All pumps are less efficient at lower RPM's, how much less, depends on design and pressure.

Pumps designed for the highest pressure are the small volume (displacement) radial piston pumps used in two stage pumps for injection molding 15000psi+ is not uncommon

Guy2 is not wrong in his statement. He consider the efficiency, 80%. Also comparing idle, with rated RPM for the pump. Theoretically the rated RPM have to be lower than 3600RPM, if the idle RPM he consider is lower than 720RPM. At 80% efficiency and the number above his statement is ok. Most new log splitter pumps are more efficient than 80%, and should for that create enough pressure even at idling.

 

[Egon]

the motor has zip to do with pressure as long as the motor is not stalling a/QUOTE]

Sure, as long as the PD pump is pumping, irregardless of volume, the point will eventually be reached where the pressure is greater than the capability of the prime mover and it stalls out or part of the system relieves the pressure by design or bang.

Small motor/pump just means it takes longer to build up pressure.

 

[turnkey4099]

Thanks guys. That discussion finally died out but hardly anyone paid any attention to the base problem and blamed it all on the new motor. At most they considered whether the motor wasn't at the right rpm (but still far from idle).

What happened was the guy loaned the splitter, user ran it out of oil, he replaced themotor with a new Chinese Honda knock-off and instantly blamed it on the "crap" motor. Operation was so bad it would barely move the cylinder.

For a bunch of people running log splitters I was some surprised that only I and, I think, 3 others pointed out that the motor was _not_ the problem. Discussion finally ended with the majority still talking about the "motor problem".

Harry K

 

[J_J]

Was there a question in the original post?. Whatever it was, I think it was answered, I thought the answers were precise and to the point.

 

[SPYDERLK]

Thanks guys. That discussion finally died out but hardly anyone paid any attention to the base problem and blamed it all on the new motor. At most they considered whether the motor wasn't at the right rpm (but still far from idle).

What happened was the guy loaned the splitter, user ran it out of oil, he replaced themotor with a new Chinese Honda knock-off and instantly blamed it on the "crap" motor. Operation was so bad it would barely move the cylinder.

For a bunch of people running log splitters I was some surprised that only I and, I think, 3 others pointed out that the motor was _not_ the problem. Discussion finally ended with the majority still talking about the "motor problem".

Harry K

OK. Theres something wrong and it isnt the engine unless it is spinning the wrong way. Was hyd oil run dry? Is there enuf in it? Does the engine labor as the ram advances? Is there a suction side filter on the pump - perhaps clogged?
larry

 

[turnkey4099]

OK. Theres something wrong and it isnt the engine unless it is spinning the wrong way. Was hyd oil run dry? Is there enuf in it? Does the engine labor as the ram advances? Is there a suction side filter on the pump - perhaps clogged?
larry

Per the OP the new motor ran fine but he was certain that the motor, being 'chinese crap', was the sole problem. Guy who borrowed it blew the motor by running it out of oil. No mention of hydraulic oil. Most replies bit right into it and suggested things such as motor not up to rpm (although it apparently was running at whatever the factory setting called for), slipping coupler, etc. Seems noone except a few of us bothered to anyalyze the problem. Even after we pointed out that the motor was the last thing to be looking at the discussion continued merrily on about the motor.

Thread died before the OP told us what the real problem was.

I wasn't sure of the "needs rpm to build pressure' bit but I had my doubts. That is why I posted it here.

Harry K

 

[J_J]

Guy2 is totally wrong, in the fact that if the pump is turning, the fluid is being displaced and flowing. Doesn't matter about the rpm, or rated rpm, fluid is flowing, and potential pressure is there. In a log slitter situation, the resistance caused by the cyl trying to split the log, or holding the valve lever to full extend or retract, will cause the pressure to build , depending on the force required and if the pressure becomes as much as the relief setting, the relief valve will relieve.

In a direct engine to pump coupling, I don't believe there is any efficiency lost. Belt driven pumps might lose some efficiency, but not direct drive or chain.