Hydraulic Actuators 1

Actuators

An actuator is used to convert the energy of the fluid back into mechanical power. Basic Types of Hydraulic Actuator

1- Linear: Cylinder 2- Semi rotary: Limited angle of movement 3- Continuous rotary: Motor

Hydraulic Cylinders Types:

1- Displacement (Simple, Telescopic) 2- Single acting 3- Double acting

♦ In displacement cylinder, the volume of the rod leaving is equal to the volume of fluid

entering the tube, hence the name “displacement cylinder”. It is used as a jack. ♦ The bore of the displacement cylinder does not require machining other than the neck

bearing and the inlet port, the manufacturing cost is therefore low. ♦ Telescopic cylinders are used when a long stroke is required and the length available for

installation is limited. A typical application is the tipping gear of a lorry. ♦ For double acting cylinder:

Full bore area A, Annulus area (A-a), a is the piston rod area, P1 and P2 are the pressure at inlet and outlet ports respectively.

Piston in extending: Velocity vE=QE/A=qE/(A-a) Where qE=QE(A-a)/A Net Thrust FE=P1A-P2(A-a)

Piston in retracting: Velocity vR= qR/(A-a)= QR/A Where QR=qRA/(A-a) FR= P2(A-a)- P1A

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Regenerative Circuits

Extension: Qin=Q+q Velocity vE=Q/a Thrust FE=P*a Retraction: Qin=Q Velocity vR=Q/(A-a) Thrust FR=P*(A-a)

Standard Metric Cylinders

Acceleration and deceleration

u initial velocity v after time t s distance moved during time t a acceleration during time t

v= u+ at v2= u2+ 2as s= ut+ 1/2* at2 s= 1/2*(u+v)/t Force=Mass * acceleration= (W/g)* a

Load Deceleration Cylinder Cushioning

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Cylinder maximum speeds Uncushioned cylinder → 8 m/min Cushioned cylinder → 12 m/min Exteral cushioned cylinder → 30 m/min

Buckling of Cylinders Buckling load K=π2 Ε J/L2

E Young’s modulus, for steel E=20*1010 N/m2

J Second moment of inertia (m4) L free equivalent length (m) The maximum safe working thrust or load F on the piston rod is given by: F=K/S where S is the factor of safety which is usually taken as 3.5.

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Semi rotary actuators: Limited angle of movement

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Hydraulic Motors ♦ Gear motors ♦ Vane motors ♦ Cam rotor motor ♦ Piston type motor: Axial piston motors, radial piston motors

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External Gear type motor

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Hydraulic motor circuits

♦ Open loop: All the fluid discharged by the motor returns to the oil reservoir ♦ Closed loop: Most of the fluid from the motor is returned to the pump inlet

Open loop circuits

Definition n speed (rev/min), D swept volume per revolution, Subscript m motor, Subscript p pump Motor speed: nm= np*Dp/Dm

Work done W=Torque T*2π= Dm*pm

Motor Torque: Tm=Dm*pm/2π Motor power: Pm=Tm*nm=Dm*pm*nm/2π Hydraulic motor efficiencies Actual Qm=Dm*nm/mήv

Actual Tm= mήt * Dm*pm/2π Actual Pm= Qm *pm *mήo

mήo= mήv* mήt pm : motor pressure, Pm motor powe, nm: motor speed (rpm), Tm: motor torque

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