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Thread: Hydaulic Circuit Question

  1. #1
    Associate Engineer
    Join Date
    Jul 2012

    Hydaulic Circuit Question

    I'm running a Hydraulic Pump off a Gas Motor. The pump then powers two Hydraulic Wheel Motors.

    My calculations show I need .485 gpm @ 2300 psi to move the motors at my desired speed (very slow). From this I calculated that I needed around 0.657 HP from my gas motor to power the pump using the equation HP = (GPM*PSI)/1714.

    I bought a 3.5 HP Honda Gas motor (b/c I got a great deal on it). I'm planning on running it at a comfortable spot in it's curve: 3000 RPM where it'll achieve 3.1 HP.

    I know I need to put a gear box between the motor and pump b/c my pump can't even safely be run at 3000 RPM. My question is how much do I gear it? Do I need to gear it so that the torque being put out by the gas motor would produce the needed 2300 psi. With it running at 3.1 HP @ 3000 RPM I calculate that's only ~54 in-lb of torque, which when connected to the pump running at 1 in3/rev will only produce a pressure of 331 psi according to my calculations unless I do around a 7:1 gear ratio, but then my pump will be running at around 428 RPM, which seems dangerously slow. Do I need to gear it this much or am I missing something here?

  2. #2
    Technical Fellow
    Join Date
    Feb 2011
    Um, you seem to be attacking this from the wrong end. Hate to be the bearer of bad tidings, but ---

    First off, the formula for HP is for the output of the hydraulic pump and not the horsepower to drive it. Even then that is based on 100% efficiency and that ain't gonna happen in the real world.

    Given that as a starter, there is too much to explain for this forum, well, for lazy old me anyway. You need to get a book or tutorial on hydraulic motors and start again. I would be VERY surprised if you 3.5HP engine is enough for what you are trying to do. No matter what the great deal on it was. I am guessing maybe 6HP or 10HP.

    I have been wrong before, but you need to hit the books for about a week. Then come back and ask f you still have questions.
    Last edited by PinkertonD; 07-30-2012 at 07:18 PM. Reason: speeling of coors

  3. #3
    Yes you miss hydraulic system education.

    I dont know how you came up with these numbers without even understanding the basics of an hydraulic system.

    Good luck!

  4. #4
    Principle Engineer
    Join Date
    Mar 2011
    "Do I need to gear it this much or am I missing something here?"

    You are missing almost everything.

    As Dave points out, you need a basic course in fluid flow, hydraulics, etc."

    But with all that lack of knowledge you might still be able to do the job, but first you must start with the pump curves and then add the system pressure curve (pvs gpm) to get the operating point on that curve.(485 gpm @ 2300 psi)

    Let's say that you have that point on the pump curve that delivers the .485 gpm and 2300 psi needed at say 900 RPM and the power is 0.657.

    Now you have a Honda engine that can deliver 6HP at 2000 RPM full throttle. If you use a governor to hold the engine speed at say 2000RPM, then the governor would move the throttle to accommodate the load much like an ordinary domestic generator will automatically adjust the throttle to accommodate the electric power required.

    Now all you need is a gearbox with a ratio of 2000/900 (well not exactly since that's an oddball number, just iterate to get new values for a realistic gear ratio)

    Just an example of how you should proceed.

    But please do some homework!
    Last edited by zeke; 08-01-2012 at 10:23 PM.

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