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Hydraulic design

  1. Oct 14, 2014 #1
    am currently designing a servo motors for a spillway gate for a dam , I have some doubts on how to determine the pushing force( the force needed to drop the spillway gate at a safe rate) for a piston carrying a heavy load of 275tons at a speed of 0.4447 m/min(this speed was pre-determined by me because I wanted a for the spillway gate to close in 40 min having a total stroke of 8m) can anyone chat suggest books or articles that I should read or who to speak to in this community that could get me closer to the solution
    Best regards
    Sigamana
     
  2. jcsd
  3. Oct 14, 2014 #2

    Baluncore

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    To help with this question we need to know the geometry of the spillway gate and actuators.
    Is the gate a radial gate that is hinged on pins?
    Is gate moved by a hydraulic cylinder using hydraulic oil from a pump?
    How many and what size cylinders are you considering. Stroke, rod and cylinder diameter?
    What is the maximum operating pressure of your cylinders?
     
  4. Oct 14, 2014 #3
    Thank you Mr Baluncore

    The Gate is radial with a skin plate bent to a radius of 13.5m height 7.8 length 6m the gate is hinged on pins (specifically trunnions) , its moved by 1 double acting hydraulic cylinder using oil from 1 pump .

    Gate is attached to a suspension bar that consequently is atteched to the hydraulic cylnder

    the total stroke is 8730mm
    Cylnder diameter - 590mm
    Rod diameter - 133mm
    WEIGHT of gate - 275 tones taking in to account the suspension bar that weighs 58 tons
    Sum of friction forces - 291.245KN
    Time estimated for opening gate - 20 min v=0.44 m/min
    time estimated for closing gate 40min v=0.220 m/min

    I will attach a file with a picture of gate please tell me if i missed any details that could help understanding this problem .
     

    Attached Files:

  5. Oct 14, 2014 #4
    Sorry forgot
    Maximum operating pressure = 75 bars


     
  6. Oct 14, 2014 #5
    I'm not a hydraulic engineer or mechanical engineer, but don't you just need to balance forces? You are not trying to accelerate the gate, you are just trying to overcome frictional forces. So maybe the pushing force is = sum of friction forces?
    I am assuming that frictional forces includes the effect of water pressure on your 7.8x6 m^2 area gate. http://en.wikipedia.org/wiki/Floodgate#Physics
    Is 75 bar the pressure in the hydraulic system or on the spillway gate?
     
  7. Oct 15, 2014 #6

    Baluncore

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    In the OP you are concerned by “some doubts on how to determine the pushing force (the force needed to drop the spillway gate at a safe rate)”.

    Firstly the gate will close under it's own weight. You will need to regulate the flow of hydraulic oil to limit the closing rate of the gate. Potential energy will be released as the gate closes so it will tend to over-run a servo motor and cause the motor to operate as a generator.

    In that situation a DC motor would always operate with current flowing in the same direction. That current would generate the torque needed to oppose the suspended load through the hydraulic system. By increasing the current slightly the gate would rise, by reducing the current slightly the gate would fall. The mechanical system will have low friction, so the difference in motor current either side of that needed to support the suspended mass will be small.

    If the flow of oil is controlled without the use of the servo motor, then the oil flow rate regulator will reduce the oil pressure generated by the suspended mass and geometry. The product of pressure drop and flow rate would heat the oil in the pressure reducing flow regulator, that heat is usually radiated from an oil cooler or the bulk oil reservoir tank.
     
  8. Oct 16, 2014 #7
    Thank you ...........
    May i also ask does a vertical gate same size under the same frictional forces and a radial gate same size same fricional forces have the same PULLING and PUSHING FORCE? ........i ask this because i was comparing the results for pulling and pushing forces of my system and the one that they already built and they didnt take to account the fact that the gate is radial
     
  9. Oct 16, 2014 #8

    Baluncore

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    A gate of height 7.8m and length 6m will have an area of 46.8 m2. When closed, with a 7.8m difference in water levels between the faces, there will be a hydrostatic force of about 6 * 7.8 * 1.5 = 70.2 tonnes force against the gate. The hydrostatic pressure against the gate will be zero along the upper edge but maximum along the lower edge.

    A radial gate transfers all those hydrostatic forces to the trunnions. There will be very little friction from the small area of the lip seals at the sides of the gate when it is moving. A radial gate should therefore close under it's own weight.

    A vertical sliding gate has friction on the surfaces that bear the differential hydrostatic force. Because of the friction, if there is a difference in water levels across the gate a vertical gate will require an additional force to it's weight alone when raising or when lowering the gate.

    I notice in the provided diagram, that the hydraulic cylinder is being used to retract against the weight of the gate. That places the rod in tension which is good. It requires greater pressure and less flow though because of the piston face area and cylinder volume differences. There will never need to be oil pressure in the top of the cylinder against the piston. The speed of gate movement will therefore be proportional to hydraulic oil flow rate, independent of whether the gate is rising or falling. It is normal to expect a cylinder to have a factor of two difference in velocity between extending and retracting, but that is not the case with this design since only the rod side of the piston is working under pressure.
     
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