Hydraulic motor as pump

[JoshNZ]

I wondered if anyone here knew if you can use a gerotor/geroller hydraulic motor as a pump, i.e. drive the motors output shaft and put a load on the oil pressure?

I think I've seen both pumps and motors of the geroller type, I dont know why they wouldn't specify both. Might be a question for @fluidpowerpro . 

I'm looking for a ~200cc/rev pump to drive a splitter from a pto shaft. Which seems near impossible to find but,plenty of motors with this rating.

[fluidpowerpro]

Although I have seen it done once as an r&d project, no, it shouldn't be done.

 A gerotor motor, inherent to it's design has about an 18:1 reduction. For every revolution of the shaft, the gerotor goes around 18 x. This was the perceived benefit of the project in that you could get lots of output flow at low input rpm. The only issue was you had to charge the inlet of the motor because it's porting is not designed to draw fluid in, and without charge pressure it would surely cavitate and self destruct in short order.

[fluidpowerpro]

From an efficiency standpoint this is not ideal but for your PTO application, you could use a PTO type pump, drive a hydraulic motor, that then drives a pump. Doing this you could increase the speed of the last pump so it wouldn't have to be so big. Obviously, in the end, your final flow and pressure cannot exceed your input HP.

[PoginyHill]

Although I have seen it done once as an r&d project, no, it shouldn't be done.

A gerotor motor, inherent to it's design has about an 18:1 reduction. For every revolution of the shaft, the gerotor goes around 18 x. This was the perceived benefit of the project in that you could get lots of output flow at low input rpm. The only issue was you had to charge the inlet of the motor because it's porting is not designed to draw fluid in, and without charge pressure it would surely cavitate and self destruct in short order.

Could you use a high-speed hydraulic motor as a pump? The internals look similar to a gear pump. Or is it the same issue with cavitation if there isn't any net positive suction head?

[fluidpowerpro]

Maybe but probably no. Yes, it's a porting/suction issue. 
Also most gear pumps and motors have pressure balanced wear plates on each side of the gear. These might be designed slightly different depending on if it's a pump or a motor. Some gear units, typically small displacement, are fixed clearance design with no wear plates so in those that part is not an issue.
Next is the shaft seal. If the motor is one that is bi-directional, it will probably have small check valves in the front cover to insure the area behind the shaft seal is always drained to the low pressure side. If it's a single direction motor, you would want to make sure your turning it the right way so the seal is drained to the low pressure side. Some motors have an external shaft seal drain in which case it's not an issue however in either case , if you charge the suction port, you can't exceed the pressure rating of the shaft seal.
In summary, yes, it could be done, but not without looking into the specifics of the motor as well as application details such as rpm and pressure.

[fluidpowerpro]

Josh. Thinking about your PTO application more. Assuming that because your PTO probably runs at a low RPM, that is why you need such a large displacement pump. You could use a smaller pump if you addressed the rpm issue by speeding it up, either with a gearbox, or an intermediate shaft, belt and bearing arrangement. If you need 200cc and you increased the speed 4:1, now your down to 50cc.

[fluidpowerpro]

Take a look at this type of pump.

11.4 GPM Prince HC-PTO-2A Tractor PTO Pump 540 RPM | PTO Pumps | Hydraulic Pumps | Hydraulics | www.surpluscenter.com

With this design you could stack multiple pumps on a common PTO shaft to achieve the flow you need.

[JoshNZ]

Thanks for the response.

I have looked around for gearboxes there's no obvious solution that isn't going to cost the earth. Plenty of Chinese gearbox/pump combos available but they're usually multi stage pumps and they're rated for 540rpm, I don't want to have a tractor sitting there near wide open throttle for splitting wood either.

We have a new Holland t4 which we pull from the aux on now and it splits wood at a decent speed with just 1200rpm so whatever pump is inside them is what we're after hah. That tractor is just busy all year. We have a fleet of MF tractors for harvest that sit around the rest of the year, I'd love to get it going on one of them but for now the project has been sidelined again. 

[bitternut]

You need a tractor with an economy pto feature. My JD has such that runs the pto at 540 rpm at 1725 engine rpm max. My JD 5210 runs my 6' brush hog just fine using the economy mode.

Its just possible that you could power the hydraulic needed at less than 1725 rpm in economy mode since the tractor would be stationary.

[JoshNZ]

Yeahhh, but a tractor is worth a bit more than a gearbox again haha 

[Hilltop366]

I've seen some DIY tractor driven wood splitters that used a chain and sprocket to step it up, the pump sprocket (the small one) was directly attached to the pump. no doubt not recommended by the manufacture but they seen to hold up as these were 40 year old splitters and still going. No doubt the chain/sprocket has a bit of side thrust but only a fraction that a V belt would have.

I see some pretty big two stage pumps that aren't too spendy (one would have to make sure the rotation is correct) but maybe by the time you get a couple of sprockets, chain, jackshaft and bearings you might be up to the price of a PTO pump , 

[Al_Smith]

I've seen home builds where they used a pump from like a dump truck driven from  a tractor PTO .They did okay .One used a low speed pump from an electric high lift .Problem with that deal was it could stall out an 8 HP Kohler engine on a tough round .