PTO hydraulic pump questions

[jdcoolidge]

I am new to hydraulics so as usual have questions.

My tractor is an 18 hp diesel Yanmar 187D with an integral hydraulic pump.
I have been unable to find out what the actual GPM of the pump is, but installed an hydraulic log splitter using a diverting valve to insure full output to the splitter. Using the out stroke time I back calculated and the pump is putting out about 4.4 GPM. So for a one person operation, and with a locking retract cycle it works "OK". I suspect that for the amount of wood needed and for a multiperson operation a faster cycle time will be needed. There are several sources for PTO hydraulic pumps all with about the same range of specs. How do I determine the pump best suited for this particular application? The splitter has 1/2" ID hoses as are all the hoses and fittings I added to hook it to the tractor. $$$$ ouch. Is there any benefit to using a larger capacity pump at a lower rpm to get a faster cycle time, or am I better off to run a pump at the full PTO rpm (540) with higher engine rpm. My thought was that less fuel would be used running the tractor (and PTO) at a lower rpm. Splitter specs: Ram 3.5" Rod 1.5" Stroke 18.5" Thanks for any thoughts and help.

Duncan

 

[J_J]

Duncan,

It sounds like what you have will work, but you want more. The PTO hydraulic pump below will put out 7.1 GPM at 2000 psi, and take 11.9 HP to run it. The next size hydraulic pump up will take 18 HP but will pump 11.4 GPM.

Another way is to use a fan belt driven hydraulic pump.

https://www.surpluscenter.com/item.asp?UID=2009072717392326&item=9-1047-7&catname=hydraulic

 

[jdcoolidge]

JJ I thought you might respond.

Any thoughts on the use of a larger capacity pump at lower rpms.
Can you still get 2000 psi, but at 1/2 or so of the rated pump rpm. I have
no idea what the HP vs RPM curve for the tractor looks like.

As I understand pumps are generally constant flow so they require some way of allowing that flow even when equipment is static. An oversized pump
I presume would require some bypass if the attached equipment did not utilize the excess flow when operating? Or for that matter if the open center
of the control valve was small. How is that dealt with? How do you match hose/fitting ID with pressure and GPM to minimize resistance losses and not get too large or costly?

Thanks

Duncan

 

[kennyd]

These pumps have a fixed pressure, but the flow (GPM) is RPM dependent. So you could use a larger pump and slow the RPM's down a bit.

You use a open center valve that has a built in PRV (pressure relief valve) that is set to the max system pressure you want/need-most valves already have that built in so you would need nothing extra.

The valve on you splitter is probly rated for 20 or 25 GPM...And you can find specs for flow rates HERE.

 

[J_J]

JJ I thought you might respond.

Any thoughts on the use of a larger capacity pump at lower rpms.
Can you still get 2000 psi, but at 1/2 or so of the rated pump rpm. I have
no idea what the HP vs RPM curve for the tractor looks like.

As I understand pumps are generally constant flow so they require some way of allowing that flow even when equipment is static. An oversized pump
I presume would require some bypass if the attached equipment did not utilize the excess flow when operating? Or for that matter if the open center
of the control valve was small. How is that dealt with? How do you match hose/fitting ID with pressure and GPM to minimize resistance losses and not get too large or costly?

Thanks

Duncan

You will have flow as long as the pump is running. It only develops pressure when a restricting force force is applied. If you have open center, the fluid is just making a loop from pump through the valve, to reservoir. At a certain rpm you will have a certain volume in GPM. Yes, you can run a larger pump at say 270 rpm, and that will pump about half of the pumps capacity. The next size PTO pump will pump at 7.1 GPM at max tractor rpm. If you run that same pump at 270, then you will get about 3.55 GPM.

On the valve GPM capability, you try and match the output of the pump, so as to not restrict fluid. You can use a larger valve, but don't go smaller. Hoses should be 3/8 or 1/2 in.

 

[J_J]

You do know that a log splitter pump has two speeds, and that pump only needs a min of 5 HP to run it. It runs in the fast mode most of the time. All the specs are in the link.. An electric clutch hooked to one of these, and the engines fan belt, you have a separate hyd system. You can get 5 HP out of the tractor at very low rpm's.

 

[jdcoolidge]

I want to thank you all for your help. It has brought my level of understanding up a notch more. Considering I started at virtually zero a few days ago the education I've recieved has been substantial and all due to the thoughtfulness of helpful people like yourselves. One other question: Why the rather large hydraulic fluid reservoirs? I thought the volume was fairly constant with push-pull hydraulic pistons. Is it a high duty cycle heat dispersion requirement? If so why isn't some sort of finned radiator used on the low pressure or suction side? It would seem cheaper.
Thanks again,

Duncan

 

[J_J]

I want to thank you all for your help. It has brought my level of understanding up a notch more. Considering I started at virtually zero a few days ago the education I've recieved has been substantial and all due to the thoughtfulness of helpful people like yourselves. One other question: Why the rather large hydraulic fluid reservoirs? I thought the volume was fairly constant with push-pull hydraulic pistons. Is it a high duty cycle heat dispersion requirement? If so why isn't some sort of finned radiator used on the low pressure or suction side? It would seem cheaper.
Thanks again,

Duncan

Anything you can so to help reduce the heat developed, is good, I have a 20 gal tank on my machine which is a Power-Trac, all hydraulic. It has a radiator and a cooling fan that comes on automatically.

Some of the radiators are only big enough to supply the pump GPM volume. The cooler , the better.