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post #1 of (permalink) Old 10-23-2002, 09:24 PM Thread Starter
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What determines a hydraulic pump\'s PSI? RPM\'s?

I'm trying to figure what pump to use with my pto to power my winch, but don't know how to determine the psi I'll be getting at different rpm's, or if that's even what it is that I should be thinking about. What I want it to do is to put out the 3.5gpm @ 1500 psi that I need, without having to mess with raising the rpm's every time I use it. The pumps i've been looking at say min. rpm of 600, max of 3600 rpm, and max psi of 3000. So does this mean i'll get 1500 psi at 1800 rpm? Is the psi output of a hydraulic pump directly effected by rpm's? Or is it just the flow rate that changes with rpm?
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Adam
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post #2 of (permalink) Old 10-24-2002, 05:50 AM
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Re: What determines a hydraulic pump\'s PSI? RPM\'s?

It's good to see that you're finally getting on with that project. How long have you been planning it now?

The pumps you're looking at are positive displacement. For all practical purposes, they'll discharge a fixed amount of oil with every revolution, regardless of the pressure they're working against. I say 'for practical purposes' because at very low speeds and high pressures, the unavoidable leakage will decrease the output, and of course, at some point the pressure will get so high that something will break.

First you need to decide what input RPM you want to use. Something around 1000 to 2000 is probably what you will want. Then get a pump with a displacement that will give you 3.5 GPM at that speed.

Don't worry about pressure - it will take care of itself as long as all components are rated above 1500 PSI. I'd recommend at least 2000 PSI rating on everything, and then put in a relief valve set at 1500.

The pump output volume will be directly proportional to speed, again, for practical purposes, and in the speed ranges you'll be using. The pressure in the system will be determined by how hard the winch is working. It might be 300 PSI when you're just reeling in the cable, and 1800 PSI just before the cable pops.

I'd also recommend putting a pressure gauge in the system where you can see it while running the winch. It will tell you how hard the winch is working.

I'd also recommend that you find an industrial hydraulics shop in your area. An interested, sympathetic salesman will help you pick the right valves, fittings, pump, hoses, reservoir . . .
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post #3 of (permalink) Old 10-24-2002, 06:48 AM Thread Starter
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Re: What determines a hydraulic pump\'s PSI? RPM\'s?

Thanks Jim, I was hoping I would catch your attention. [img]images/graemlins/smile.gif[/img] It has been a while I've been wanting to do this, I've just been waiting for a good deal on the right parts, which finally came along. I remembered most of what you had told me about figuring out the gpm's, plus muncie's website has some nice charts for calculating stuff, but I couldn't remember anything about the psi. I have all of those emails archived you sent me, but buried them somewhere on one of my servers and couldn't find them. [img]images/graemlins/crazy.gif[/img]

I was hoping I'd be able to use the pto at idle, so I wouldn't have to mess with raising the rpm's. Is that a bad idea? With the ratio's available for the pto I have, it would be easier to get the right flow rate at a higher rpm. But, there is a pump which will give me just over the 3.5 gpm(3.8gpm) @ 600 rpm (pto shaft rpm). This is the minimum rpm of the pump. I thought I could use this with a .84 efficiency, as long as idle rpm is around 725, and be okay. The highest ratio available for it is 99% efficiency, so as long as the engine idle isn't below about 610 rpm, i'd still be at 600 rpm on the pto. Would this work for me? Or would I get enough leakage at that low of rpm to lose power from the pump?
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post #4 of (permalink) Old 10-24-2002, 10:34 AM
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Re: What determines a hydraulic pump\'s PSI? RPM\'s?

That's really a question for the pump manufacturer. Second choice would be a salesman at the hydraulic supplier. However, my first reaction would be "If the pump's rated for 600 RPM, it should work at 600 RPM."

On the other hand, hydraulic wreckers have a throttle pickup to raise engine speed while working the system. On the third hand, inefficiency will show up as heat in the oil. The more slippage in the pump and motor, the more the oil temp will rise. (You won't lose any significant power at the winch - it will just slow down a little) That can be countered with a larger reservior, a cooler, or shortened duty cycles. In a Jeep, I don't think you'll have any problem unless you use the winch hard all day (as I have on quite a few occasions.) Using it to get yourself or a buddy unstuck won't cause a problem.

Remember, the ratio between pump speed and winch speed is going to be almost purely linear. And there will be lots of times when you want the winch to be really slow. If I were doing it, I think that I'd size the pump and motor based on a high-idle speed - maybe 1200 RPM - for everything to work at spec. It should still work fine at idle - just half as fast.

Is your PTO on the transfer case or the transmission? If it's on the transfer case, you have the transmission speeds to play with. In that case, set it up to normally use second gear. Then you have first for real slow pulling, and high gear to reel in your cable.

I'll send you those old e-mails if I can find them.
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post #5 of (permalink) Old 10-24-2002, 07:23 PM
 
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Re: What determines a hydraulic pump\'s PSI? RPM\'

well, as for speeds, and displacement, you will find that displacement is proportional to $ You may find it more economical to run a smaller pump at a higher PTO speed than a gigantic pump at idle. That $20.00 locking hand throttle may really be worth some money compared to a $1,000 pump that puts out gigantic volume at idle.

Remember also that your tranny is geared down. If you have a 4:1 1st gear, 600 RPM idle is only 150 rpm at the pump in 1st..... you will have to be in 4th gear 1:1 to even get your 600 RPM idle speed at the pump. The other option is to get a belt driven pump with a little teeny pulley on the end to take advantage of your engine's drive belts. I bet you could easily double the pump speed with the right sized pulley.

As for figuring out how much fluid you get at a certain speed and pressure, they have pump curves. It is a graph where you match the speed with pressure and it shows flow rate.

The deal with pressure is that the MAXIMUM pressure is set by your relief valve. The operating pressure is just whatever resistance it needs to do the work at the flow rate. You will only get maximum pressure when you are under full load.... but it gets the maximum flow at lower pressure.

Best regards

John
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post #6 of (permalink) Old 10-24-2002, 09:16 PM Thread Starter
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Re: What determines a hydraulic pump\'s PSI? RPM\'

Thanks for the help guys. Jonh, the pto is on the tranny, so my only means of chaning pump speed is by engine rpm.

Right now, the two pumps I'm deciding between are the two smallest ones available from muncie for my tranny/pto combination, which appears to be a good thing if price is reflected by pump size. The smaller has a .98 ci and the larger 1.47. The dealer told me the jeep's idle rpm is 800, so I would be just below the 3.5 gpm with a .99 pto gear ratio and the .98 ci pump, and not be bottomed out on the pump rpm range as I had planned with the 1.47 ci pump. So I think I've figured out what I need.

One question for you John. I've seen the hand throttle, which would be nice to have anyway, but haven't seen a locking one. Can you tell me where I might find one?

Thanks
Adam
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