Hydraulic Actuators and MotorsTeaching hours:05At the end of this chapter student should be able to:Explain working principle of linear as well as rotary actuators. Demonstrate Mechanics of Hydraulic Cylinder Loading. Discuss Cylinder cushioning with sketch. Solve problems on motors to determine discharge, power, torque and efficiency. Hydrostatic transmission. Evaluate volumetric, mechanical and overall efficiency of motors.

Lesson scheduleLinear Hydraulic Actuators (Cylinders)Mechanics of Hydraulic Cylinder LoadingCylinder cushioning, Hydraulic Rotary ActuatorsGear Motors, Vane Motors, Piston MotorsHydrostatic Transmission open and close circuitPerformance of motors

Linear Hydraulic Actuators (Cylinders)Fluid energy Cylinder or Rotor Mechanical energy Hydraulic cylinderorex: Piston & cylinder arrangementLinear actuators

Hydraulic motororex: Gear motorRotary actuator

Single acting cylinder-

Exerts force in only extending directionRetraction not hydraulic

Double acting cylinder

Extension & retraction hydraulically

Cylinder mountings

Force, velocity and power analysisOutput force and piston velocity not equal for extension and retraction

Cross sectional area 1/ velocity for same flow rate

Cross sectional area output forcefor same flow rate

Need to consider weight of piston, inertia, friction

Extension stroke F = p * Apistonv = Qin / Apiston

Retraction stroke F = p * (Apiston Arod)v = Qin / (Apiston Arod)

Power during extension = power during retractionPower = F * v = p * Qin

Double rod cylinder Same force and velocity at both end for both extension and retraction steps

Telescopic cylinder Multiple cylinders slide inside each otherLong work strokeConsumes less space

Cylinder loading through mechanical linkagesUsually load does not act along axis of cylinder i.e non-axial loading

First class lever system If cylinder side lever is greater than load side leverFcyl < Fload , butStroke cyl > Stroke load

Second class lever system Smaller Fcyl is required for same load compared to First class, so smaller cylinder-piston areaBut smaller load stroke

Third class lever system Fcyl > Fload, but load stroke is more

Hydraulic cylinder cushionsTo slow the piston speed near the end of stroke and to prevent excessive impactDeceleration starts when taperedplunger enters opening in capNow oil must exhaust through an adjustable opening Check valve to allow free flow to piston during direction reversalMaximum pressure developed by cushions at ends of cylinder

Hydraulic shock absorbersBrings a moving load or shock to a gentle rest using metered hydraulic fluidFilled completely with oil and can be spring returnAccumulator accommodates oildisplaced by piston rod, as the rodmoves inwardMultiple orificeMoving piston pushes oil through series of holesPiston progressively shuts off theseorifices as it moves inwardSo total orifice area continually decreases and load decelerates uniformly, hence no bounce back (like springs)

Hydraulic motorsFluid energy into mechanical energy to perform useful work

Rotary actuatoror less than one complete revolutionOscillating motor(limited rotation)Produce high instantaneous torque in either directionRequire small spaceSimple mounting

Hydraulic motor - continuous rotation

Applications of rotary actuators

Analysis of torque capacityRr outer radius of rotorRv outer radius of vaneL width P - hydraulic pressureF hydraulic force acting on vane faceA surface area of vane in contact with oilVD volumetric displacementT torqueF = P*A = P*(Rv-Rr)*LT = F* radius = F*(Rv+Rr)/2 = PL(Rv2-Rr2)/2ORVD = *(Rv2-Rr2)*LT = PVD/2 = PL(Rv2-Rr2)/2

Gear motorDevelops torque due to hydraulicpressure acting on the surfaces ofgear teethDirection of rotation of motor canbe reversed by reversing directionof flowDisplacement volume is fixedPressure is not balanced, so Unbalance motorSimple design and low costSymbolic representation

Vane motorHydraulic pressure acting onthe exposed surface of vaneRotor connected to drive shaftSprings placed between vanesand rotorCan be Balance or Unbalance motor

Piston motor (swash plate)Fixed or variable displacementGenerates torque by pressure acting on the ends of pistons reciprocating inside a cylinder blockIt generates a force against an angledswash plateThis causes cylinder block to rotate with a torqueSwash plate mounted in a swinging yokeTorque capacity is directly proportional to Swash plate anglei.e if the swash plate angle is increased, the torque capacity is increased, but the drive shaft speed is reduced

Bent axis designSpeed and torque depends onangle between cylinder block and drive shaftLarger the angle, greater displacement and torque butsmaller speedPiston motors are most efficient

Motor torque, power & flow rateP pressureVD - displacementN speed

Theoretical torque TT = VD *p/2

Power = TT * N = VD *p* N/2

Flow rate, Q = VD * N

Hydrostatic transmissionConsists of variable displacement pump and a fixed or variable displacement motor, operating togetherIn CLOSED circuit, fluid from motor outlet flows directly to pump inlet without returning to tankIn OPEN circuit, fluid from motor outlet flows to tank.

Control of the variable displacement pump is the key to controlling the vehiclePrime mover power is transmitted to pump, so when operator moves the control lever, the swash plate in the pump is tiltedFluid flows from pump to motor. Volume of fluid flow is depends upon the tilt of swash plate ( controlled by operator )If discharge from pump to motor is increased, the motor rotation speed increases

Hydraulic motor performancePerformance depends upon precision of its manufacture and operating conditionsGear motor-70-75%Vane motor-75-85%Piston motor-85-95%

o = v * m

v = Theoretical flow rate motor should consume = QT Actual flow rate consumed by motor QAIts inverse of that for a pump, because pump does not produce as much as it should theoretical due to leakagesWhereas motor uses more flow than it should theoretically, due to leakages

QT = VD * Nm = Actual torque delivered by motor = TA Torque motor should theoretically deliver TTItsinverse of that for pump, because pump requires a greater torque than it should theoretically , due to frictionWhereas motor produces less torque than it should theoretically, due to frictionTT = VD * P 2 TA = actual power delivered by motor / speed in rpmo = Actual power delivered by motor Actual power delivered to motoro = TA * N P*QA