How does a 2-stage hydraulic pump work?

[davesisk]

This seems like as good a forum as any to post this question... /forums/images/graemlins/cool.gif

I'm referring to the type of hydraulic pump typically used on log splitters. It provides a high flow at lower pressure (to move the splitter wedge without resistance quickly) and a lower flow at high pressure (to move the wedge slower but with more force when resistance is encountered). How do these work internally to get the two different flows and pressures? Anybody know? It must be pretty simple, or it wouldn't be reliable (right?).

I have a small tractor/loader whose "transmission" is a hydrostatic drive pump and 4 hydraulic wheel motors. One of the disadvantages of this design is that you only have 1 "gear"...slow! The thought struck me that some sort of two stage pump might give you more speed when there's little resistance and more torque when it's needed...kind of like a automagic gear-selector... /forums/images/graemlins/shocked.gif Any thoughts?

Also, many hydraulic pumps can also be used as hydraulic motors. Does anyone know if that's true of any of these two-stage hydraulic pumps?

I'm not planning on modifying my current tractor, just curious. Who knows, I might decide to actually build something ludicrous one day, though!

Dave

 

[Youare]

There are several different designs when it comes to multi-stage hydraulic pumps.
The one you refer to on a wood splitter is usually a 2-stage pump somewhat like your heart,
two chambers with a vavle in between.

There is a high volume low pressure chamber and a low volume high pressure chamber. When
the resistance to flow is low in the hydraulic system (cylinder) the high volume low pressure side
supplies flow to the cylinder. As the resistance to flow increases (splitting wood) the relief valve
reacts to divert the flow to the low volume high pressure part of the pump until the pressure drops
below the pressure setting of the relief vavle then the high volume low pressure side takes over.

I"m not sure this type of pump would work as a motor also.

Randy

 

[Gary1944]

...but what exactly causes the pressure change. Large outlet orofice for volume and small outlet orofice for high pressure?
Gary1944

 

[oldnslo]

On two-stage pump The pressure change is caused by work being done externally and not in the pump itself. These typically will not work as a motor.

Using a two-stage pump on fixed displacement motor would provide you with two speeds depending on the pressure required to turn the motor.

 

[Egon]

http://www.multipower.us/pdf/hydraulic_pumps.pdf

This site may be of interest.

 

[Gary1944]

I'm afraid your quote applies to me. I don't know what you mean by "work being done externally". I am thinking of this as a garden hose that I squeeze off at the nozzle and water shoots farther...so, I picture a smaller orifice makes the greater pressure.

 

[J_J]

When the pump senses the pressure caused by the load, some components shift to change from LO to HI pressure, and is automatic with the LO stage pressure preset at 500 PSI.

http://www.northerntool.com/images/downloads/manuals/1053.pdf

 

[Gary1944]

WOW! What a fantastic publication. Thank you a ton!

 

[George G]

A 2-stage pump is just 2 pumps in one housing. Lets say we have a 11 gpm 2-stage pump. That means that the high volume side is pumping 8 gpm and the high pressure side is pumping 3 gpm. Now,when the splitters cylinder comes into contact with the block of wood. System pressure starts to build,When system pressure gets up to the pressure setting of the unloading valve. The unloading valve will open, the check valve will close. Pressure in the system will continue to build from the 3 gpm side of the pump. Meanwhile, the oil from the 8 gpm side of the pump flows though the unloading valve back to the supply"inlet" side of the pump

 

[Gary1944]

Thank you George G. I got a great PDF file from Haldex that pictures the two gear pump components...but doesn't indicate what is different about the high pressure pump component. I tried to copy a link (or anything!) to attach here, but couldn't. So, now I know there are separate pump components at least.
I have a gear pump that I hope to use on a logsplitter and hoped there was a way to get 2 pressures from the one set of gears. I suppose that is either not possible or too difficult/expensive. Thanks to all who replied.

 

[J_J]

Just as oldnslo and others have said, the load or resistance dictates the pressure.

Light load, low resistance = low pressure.

Heavy load, high resistance = high pressure.

You are actually getting pressure from 0 to the max pressure of the component, be it hyd cyl or hyd motor.

The hyd motor or cyl will develop the pressure.

You want two different pressures, then run the pump output through a splitter with check valves and relief valves, and put different loads on the legs of the splitter.

Pressure also dictates the torque..


By the way, the original post was from 2003. by davesisk

 

[Gary1944]

Thanks, J.J. It seems that you have answered my early question of orifice sizes. Assume a large orifice for volume then a small orifice for pressure. These are simply holes in a steel plate with controls. The orifices equal load. Right???

 

[triptester]

Small orifices will increase back pressure but will not increase working pressure. Small orifices or restrictions increase friction which causes heat.

 

[Gary1944]

Please ignore this reply if you're tired of my thick head...but, I'm making hydraulic components as a hobby, and gathering pumps to learn from. I have a 12V Mercury 140 trim pump I adapted to a small tractor. It operated in a 2-stage manner but has one set of gears (gear pump). Pump outlet seemed to go first to a larger orifice in a steel plate, then via various control components, to a VERY small orifice in the same plate. I operated it @ 1,000 psi but seem to remember spikes to 3,000 a couple different times. My latest home-made cylinder is 18" X 7", so a 2-stage would be desirable. 'Nough said, I'm getting embarrassed but still learning a lot from your replies.

 

[oldnslo]

T
I have a gear pump that I hope to use on a logsplitter and hoped there was a way to get 2 pressures from the one set of gears. I suppose that is either not possible or too difficult/expensive. Thanks to all who replied.

You can not get two separate pressures at the pump outlet with one set of gears unless you do as JJ said and use a flow divider to provide two separate flow paths from the one set of gears. You will NOT have the same effect as using a Hi-Lo pump though.

 

[oldnslo]

Please ignore this reply if you're tired of my thick head...but, I'm making hydraulic components as a hobby, and gathering pumps to learn from. I have a 12V Mercury 140 trim pump I adapted to a small tractor. It operated in a 2-stage manner but has one set of gears (gear pump). Pump outlet seemed to go first to a larger orifice in a steel plate, then via various control components, to a VERY small orifice in the same plate. I operated it @ 1,000 psi but seem to remember spikes to 3,000 a couple different times. My latest home-made cylinder is 18" X 7", so a 2-stage would be desirable. 'Nough said, I'm getting embarrassed but still learning a lot from your replies.

Oil is like people, it will take the path of least resistance.

A larger orifice will pass a greater volume of oil than a smaller orifice at the same pressure loss. Formula to figure pressure loss across an orifice is:

Flow in GPM = (24.12 times orifice area in square inches) times (square root of the pressure drop (PSI)).

 

[sfd]

hello guys im also new on tractor net. I have a 11gpm pump on a 18hp briggs. will that pump work to motors. im wanting to put on a homemade dozer.

 

[J_J]

Did you look at the data on this page?

Surplus Center - 11 GPM 2 STAGE HYD PUMP S21202-5292

At high pressure , the pump flow is 1.8 GPM's at 3000 psi

Not designed for hyd motors.

 

[sfd]

good morning guys. mr j.j. I have not but I will . I don't know that much about hydraulics. the dozer I have just stop pulling and when I made it I had in mind to convert to hydraulics. but finding theres more to it thanks for your info. I tell you what I want you to do. HAVE A GREAT DAY.

 

[J_J]

sfd ,

Your 18 HP engine can support a 9 GPM hyd pump with a .5 cu in displacement turning at 3600 rpm.

If you can use a hyd motor of.5 cu in, that 8 GPM will produce a shaft speed of about 370 rpm.

Will you drive the track sprocket directly or gear it down some more.

You need to start at the source of your HP and work to the speed you want the dozer to run.

Or determine the speed and work the figures backward to determine the HP you will need to get the job done.

I have one of the older Magnatrac mini-dozers M-1800 produced by Struck corp, with a 16 HP engine and they use a 4 speed trans axle to drive the track.

In low gear, you can get off the machine and eat lunch and the dozer has only traveled a short distance. 4th gear might give about 5 MPH.

Magnatrac

http://youtu.be/Zq8ESOgg5aQ

http://youtu.be/3Gqt_TfRlgo