Hydraulic Systems_2 - Hydraulic Systems Chapter 11 Material...

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1 Hydraulic Systems Chapter 11 Material taken from Goering, 2003, Off-Road Vehicle Engineering Principles Hydraulic Actuators A hydraulic actuator is a device for converting hydraulic power into mechanical power. There are two types of actuators: rotary and linear. Rotary actuators are called hydraulic motors, while linear actuators are called hydraulic cylinders. Hydraulic Motors Hydraulic motors are similar in appearance to hydraulic pumps. If care is taken to avoid damaging the seal, pumps and motors can often be used interchangeably.
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2 Hydraulic Motors con’t The theoretical speed of a hydraulic motor is calculated using: N mt = (1000Q ma ) / D m Where N mt = theoretical motor speed, rpm Q ma = liquid flow rate into the motor, L/min D m = motor displacement, cm3/rev Hydraulic Motors con’t Internal leakage from the inlet port to the outlet port causes the actual motor speed to be less than the theoretical speed. The volumetric efficiency of a motor is defined as: e mv = N ma / N mt = Q ma / (Q ma + Q ml ) Where e mv = motor volumetric efficiency, decimal N ma = actual speed of motor, rev/min Q ml = internal leakage in motor, L/min Hydraulic Motors con’t The equation for calculating the theoretical torque produced by a motor is similar to T pt = pD p / 2 π , except that the p-subscripts are replaced by m: T mt = pD m / 2 π Where T mt = theoretical torque from motor, N*m p = pressure drop across motor, Mpa D m = motor displacement
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3 Hydraulic Motors con’t Internal friction causes the actual torque production to be less than the theoretical torque. Motor torque efficiency is: e mt = T ma / T mt = (T mt – T mf ) / T mt Where e mt = motor torque efficiency, decimal T ma = actual motor torque, N*m T mf = motor friction torque, N*m Hydraulic Motors con’t P h = Q p / 60 calculates the hydraulic power into a motor, given the actual flow into the motor and the pressure drop across the motor. The power efficiency of a hydraulic motor is: e mp = P ms / P mh = e mv e mt Where e mp = power efficiency of motor, decimal P ms = shaft power out of motor, kW P mh = hydraulic power into motor, kW The volumetric, torque, and power efficiencies of a hydraulic motor vary in a manner similar to the next figure for hydraulic pumps. Goering, 2003, Off-road Vehicle Engineering Principles Motor Efficiencies •e pp = pump power efficiency •e pt = pump torque efficiency •e pv = pump volumetric efficiency
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