Matching hydraulic motor to pump

Discussion in 'The Garage' started by DakarNick, Jan 2, 2013.

  1. DakarNick

    DakarNick Swabee

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    I have an idea for a project but need to know how to match a hydraulic motor to the engine-driven pump. Do I go off flow or displacement?

    Also, can a three position, center-off valve be used on hydraulic motors?

    Thanks!
    #1
  2. troidus

    troidus Long timer

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    Keep hooking up bigger and bigger motors until it quits blowing up, then go up one size.

    (Oh, wait. Not JM. :lol3)
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  3. DakarNick

    DakarNick Swabee

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    If it wasn't my money and time I would do that :D
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  4. troidus

    troidus Long timer

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  5. Crisis management

    Crisis management Latte riders FTW!

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    Depends on a few things...

    You need to know the displacement of the pump per rev, how may revs you will run it at, is it a variable output or fixed displacement?
    That will let you know how much flow of oil you will have available.

    How fast do you want the motor to turn? that will decide the size of your motor relative to the pump capacity.

    How much power do you have available? What pressure will the pump generate? This will give you the power / torque available to the motor and will assist in choosing the correct motor type for the application.

    Valving? Depends on the pump and relief valve arrangement and whether you want the motor stopped or able to freewheel.

    If all that makes sense carry on, otherwise look for a hydraulic engineer.
    #5
  6. Stan_R80/7

    Stan_R80/7 Beastly Gnarly

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    The hydraulic pump power (in HP or Watts) is Q*P, where Q is the flow rate at the pressure P: power=flow*pressure.

    The pressure is actually pressure differential. For a 3000 psig pump, just use 3000 psi. The engine (or motor) needs to output the required horsepower at the rated RPM of the pump. As such, a gearbox may be needed - dependent on the pump.

    As an example: a 1 gpm hydraulic pump operating at 3000 psi needs 1.75 horsepower (hp) or ~ 1.3 kW of power.

    power = (1 gal/min)*(231 in^3/gal)*(3000lb/in^2)*(1min)/(60sec)*(1 ft/12 in)*(1 hp/550 ft-lb/s)= 1.75 hp

    Hydraulic pumps that produce 3000 psi are positive displacement pumps - which means flow = displacement/revolution * RPM. You can have a more powerful engine than required. A normally closed directional valve can be used, but cooling for the full pump horsepower may be needed - such as a reservoir heat exchanger, dependent upon operation. Good luck!
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  7. DakarNick

    DakarNick Swabee

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    Thanks, guys. I want to run two hydraulic motors off a single pump. I'd like the vehicle to run 25mph, which means I would need a pump running 200 rpm if going direct drive.

    I found one that is 200 rpm @ 1000psi, 9 gpm, and 1230 lb/ft torque and 10 cubic inch/rev.

    So, I would need a pump that delivers 18 gpm. That is quite a bit.

    I found a calculator online and figured I could drive the motors with a 10.5 hp engine. Stan, your equation verifies that, also. Just need a pump to mate to the engine.

    How does cubic inch displacement work? The motors, combined, would be 20 ci/rev. I'd need a pump that displaces 20 ci/rev?
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  8. AUSROB

    AUSROB Adventurer

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  9. machinebuilder

    machinebuilder Long timer

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    If you want to borrow the engineering, look at Zero turn mowers. They are hydraulic drive.

    FWIW it takes a 3hp electric motor running a 6gpm pump for 1000psi (very Rough figures)

    We use these all the time at work mostly at about 600psi.
    #9
  10. fritzcoinc

    fritzcoinc Enjoying my last V8

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    Or you can connect the motors in series ( control valve port A to motor #1 port A, motor #1 port B to motor #2 port A, motor #2 port B to control valve port B). This way the exhaust from motor 1 drives motor 2 at the same speed as motor 1. This also makes a " posi track " set up. If motor 1 looses traction the flow goes to motor 2. With motors in parallel if one motor looses traction it will spin and rob pressure from the other motors.

    If you use a pump that displaces the same as the motor then the " reduction " is 1:1. For more torque use a smaller pump. Exp.: a 5 ci/rev pump and a 20 ci/rev motor gives you a reduction of 4:1. Of course speed is less. The hyd. motor will need some amount of reduction or it will index instead of spining up to speed. Sometimes called stepping.

    As to your question regarding the control valve, typically you would use an open center valve with a motor. This valve in the center position connects the pump and both motor ports to tank. The other type of valve blocks both motor ports and connects the pump to tank in the center position. This valve is usually called a tandum center valve. The open center valve prevents the motor from locking up when valve is moved from one direction to the center position. The down side of this valve is there will be no braking. The motor can free wheel with the valve in the center position.

    Another way to go is to use the tandum center valve and a crossover relief vavle in the motor lines. The crossover relief connects the lines of the motor together through relief valves. You set the relief valve pressure just high enough to launch the machine. When the control valve is centered and both motor ports are blocked, oil will flow from the exhaust side of the motor to the pressure side through the relief. When the pressure is low enough the relief closes and the motor is locked by the blocked valve ports.This sounds great but in actual pratice adjustment is tricky. Using the open center valve will be smoother but you will need some type of brake.
    #10
  11. Puchy

    Puchy Adventurer

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    I had a great Case garden tractor with hydraulic drive. 12 hp engine-hyd pump driven directly through lovejoy coupling to a hyd motor that drvies the trans axle. Very simple setup, just need to engineer pressure/flow rates to your application-interesting project you have.
    #11