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 | jeff r
Join Date: Jul 2003 Posts: 428 burton. michigan 
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2002-09-25 42911
HEY ART,
If a PTO hydraulic pump is rated at 7.2 gpm at 540 @ 2600 rpm does that mean it is pumping 3.6 gpm at 1300 rpm?
jeff
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 | Art White
Join Date: Jan 2000 Posts: 6898 Waterville New York 
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2002-09-26 42919
Jeff, that would be a logical assumption for many things but when you talk hydraulics the only consistant thing is fluid like many things takes the path of least resistance. I have not tested any pto pumps to see if that works, in working with the tractor pumps it's not the way it is so I would assume that it would not be that way on the PTO pump. ....
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 | TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2002-09-26 42925
Very likely, and Art is very likely to say one way of the other for certain. Depending on the reason for asking the question, it might be good to keep in mind that pressure also is part of a pump's specs. All pump volumes decrease as they pump into greater pressures. On a detailed specs sheet the rating is the pump's volumetric efficiency, which may be as low as 70%. I think that common gpm ratings are for no load at pto rpm. There also are other efficiency ratings that affect the volume actually pumped or the HP required to pump it.
Anyway, if the idea is to see if something will work at a desired speed at lower RPM's, the pressure the pump is working into and it's volumetric efficiency should be known to make an estimate of how fast a cylinder will move.
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| Duane
Join Date: Feb 2002 Posts: 53 Western PA
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2002-09-26 42949
Hey, while you're on the subject of pto pumps... do you think that a pto pump designed to run a backhoe attachment at 540 rpm would withstand the abuse of being ran at say 750 or 800 rpms? I built a logslitter last spring for my tractor and now I need a pump... buying a replacement pump for one of kubota's backhoes looks to be the cheapest way out but they only run 9 gpm or so... I would like to have about 12gpm. That would be easy to obtain by putting the pto selector switch on 1000 rpm and adjusting the throttle to obtain the required rpm. ....
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 | Peters
Join Date: Feb 2002 Posts: 3034 Northern AL
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2002-09-26 42955
I guess I am confused. The pump on my 955 is a gear pump. These pumps are nearly volumetric unless there is a lot of wear in the pump. There is some loss through the machined parts but you can use these pumps for metered pumping. Most of the PTO pumps I have seen are also gear pumps. Is there other types of pumps used? ....
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| jeff r
Join Date: Jul 2003 Posts: 428 burton. michigan
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2002-09-26 42965
All hydro pumps I have run into including the one my company puts on our tensile test machines are gear pumps. Gear pumps are made for high pressure pumping. The thing I'm trying to figure out is why hydro PTO pumps made for backhoes are normally being run at way less than PTO speed, but manufacturers have pump recommendation GPM's rated at 540? The last 3 backhoes I demo'ed the engine rpm were at about 1200-1400 and the hoe worked great. That would seem to me the hoes gpms were a lot less than the MFG's recommendations. In other words they recommended 6-8 gpms at 2000psi/ 540PTO but hoe runs peachy keen at half the speed?? ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2002-09-27 42984
My volumetric efficiency comment was taken from a 2000 Intertec publication that contained reprints from 1973 articles. The article gave a volumetric efficiency range of 70% - 90%. I guess that's for pumps actually found on tractors during the 60's - 70's. Within the range, vane pumps would have the lowest efficiency and piston pumps the highest with gear pumps in the middle.
I'm not certain, but the way I'd convert volumetric efficiency to pump flow is the take a rated no load gpm. Multiple the rating by the efficiency at the pump's rated pressure and then prorate the flow reduction to the operating pressure (the loss probably isn't straight-line but it should give a decent approximation). The same article gave typical intermittent pressure ratings of 4,000 lbs. for gear pumps (compared to 15,000 for good piston pumps). Perhaps pressure ratings too have changed in modern times.
I'd sure be interested in knowing there are design improvements that have improved pump performance since the '70's. An alternative is that hydraulics on tractors has become much more important since the '70's and maybe manufacturers are simply using more expensive pumps.
I don't know about the rpm thing either. I run my hoe from tractor hydraulics (7.8 gpm minus the PS priority valve). I usually run below pto rpm at around 2,000. Feathering and precise control is a little easier there than at pto rpm. I ran at quite a bit lower rpm when I first got the hoe until I got my hoe skills up a bit.
If I had to guess why PTO pump manufacturers recommend speeds lower than pto rpm, I'd guess something to do with efficiency. Volumetric efficiency is only one component of losses. Mechanical efficiency gives other losses--mostly due to heat that is generated by the pump. It's possible that the type of pumps used on pto's are more subject to heat loss and have greater overall efficiencies at lower speeds.
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| Art White
Join Date: Jan 2000 Posts: 6898 Waterville New York
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2002-09-27 43005
Jeff, I just went to a company that I get some pumps from for different applications. They offer several different senarios that tell me that there is no set standards to say 1/2 rpm gives you one half output. They use both vane and gear pumps and have several comparisons on the different style pumps to work from all useing the same psi. You made me have to put my thinking cap on! I love it! ....
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| Art White
Join Date: Jan 2000 Posts: 6898 Waterville New York
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2002-09-27 43006
Jeff, I just went to a company that I get some pumps from for different applications. They offer several different senarios that tell me that there is no set standards to say 1/2 rpm gives you one half output. They use both vane and gear pumps and have several comparisons on the different style pumps to work from all useing the same psi. There are still piston pumps that they don't sell so I won't commit on that but I also know that you don't have that style. You made me have to put my thinking cap on! I love it! ....
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| Peters
Join Date: Feb 2002 Posts: 3034 Northern AL
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2002-09-27 43015
Tom;
The question was are there differences since the 70's in gear pumps? Yes! The main components in the gear pumps are the gears and the 8 shaped cavity. CNC machining has tighened these tolerances greatly increasing volumentic efficienies. The pumps I was using (not hydraulic) where near 100% effiecient in the operating pressure ranges.
On the down side a small particle could jam the pump.
I doubt PTO pumps are near this efficient though, but the tractor pumps could be. ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2002-09-27 43018
Thanks for the info Peters. The article describes two types of gear pumps 'gear-on' and 'gear-in'. The gear on is diagrammed to have a slightly 8-shaped cavity, but I doubt it's the same technology you mentioned. If the new technology produces performance at a cost of reliability, then maybe tractor pumps are still similar to those of the '70's. I'm not sure anybody is using vane pumps nowadays through.
I guess the thing to do is get some detailed specs for modern tractor and pto pumps and compare them to my '70's article. I'll see what I can come up. Not sure when I'll get around to it.
This thread is sort of ironic because I can't recall if it was you or Murf who originally convinced me that a pump's volume decreases as the pressure increases.
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2002-09-29 43095
I did a quick web search and came up with Plessey who makes gear hydraulic pumps for ag and other applications. They spec at: max rpm = 3000; max continuous press = 210 bar; volumetric efficiency (VE) exceeding 95%. If I've got my conversion right, 210 bar is 3,000 lbs.--funny thing is that I couldn't seem to find a direct conversion table.
I don't know how representative their pumps are of modern tractor pumps. However, their continuous pressure rating seems to be similar to '70's pumps, but the VE is higher. Too bad they didn't spec their max intermittent pressure rating. If modern pumps have much higher intermittent pressure ratings, then the volume reduction at typical operating pressures may not be much even if the pump has a VE significantly less than 100%.
I recall an earlier discussion that might shed some light. Some JD models have a system where the power steering and hydraulic pumps are chained together when high pressures in the hydraulic system occur. It's my understanding that the design speeds operations at high operating pressures. If true, the design implies that the hydraulic pumps have reduced flow at high operating pressures.
Anyway, I'd appreciate any information that turns up on this subject
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