Hydraulic actuators are installed to drive
loads by converting the hydraulic power into
The hydraulic actuators are classified into 2
◦ Linear actuator - Hydraulic cylinders
◦ Rotary actuator - Hydraulic motors, rotary actuators
Converts hydraulic energy to mechanical energy.
Generates linear movements. Linear motors.
o Single acting cylinder
o Double acting cylinder
Only piston side is supplied with hydraulic fluid.
Only work in one direction.
Return stroke effected by spring or load.
◦ Moving workpiece
Single Acting Cylinder
A single acting cylinder is only powered in one direction
It needs another force to return it such as an external load
(e.g. in a car hoist or jack) or a spring.
No hydraulic fluid is present on the low pressure side.
To extend the cylinder
or to push the load,
pump flow and
pressure are sent to the
When pressure is
released, the spring
the cylinder to the fully
Gravity Return Single Acting Cylinder : In Circuit
Both piston surface is supplied with hydraulic fluids.
Fluid power works in both directions (extend & retract)
When piston extends, the fluid on rod are displaced in
When piston retract, fluid in piston area are displaced
Hydraulic Cylinder : Double Acting Cylinder
A double acting
cylinder is powered
in both direction.
In the case of
piston surfaces can
Pressure Port and
Vent port can be
o Working pressure determined by bore diameter
o Larger diameter produces larger forces
Piston rod diameter
o Normally 1/6 of bore diameter
o Length by which the piston rod moves from one
extreme position to another extreme position
o Pressure that can be handled by cylinder
o Limited by size of bore, rod and tube thickness
o Pressure used during testing (by manufacturer)
o Normally 1.5~2 times of normal working pressure
ϕ = AP/APR ; APR = AP – AST
AP : Area of Piston
AST : Area of Rod
Cylinder is selected to suit application load (F = P × A).
Can be used to calculate piston diameter. Even so,
hydraulic mechanical efficiency must be included,
F p A
A cylinder with area ratio 2:1 is to lift 40 kN load. The
max system pressure for pump is to be 160 bar.
Calculate the piston diameter, dp and piston rod
diameter dST for this system. The mechanical-hydraulic
efficiency of cylinder amounts is 0.95.
Piston diameter, dp
; 40,000 ; 160 1600 /
d F N p bar N cm
Piston rod diameter, dst
PR P ST
A A A
Hydraulic Cylinder : Extending
FE = p x Ap
Ap = Piston cross section area (m2) Q = Volume flow rate (m3/s)
AR= Rod cross section area (m2) vE= extend rod velocity (m/s)
FE= Extend force (N) p = pressure from pump (N/m2)
Hydraulic Cylinder : Retracting
FR = p x (AP-AR)
Ap = Piston cross section are (m2) Q = Volume flow rate (m3/s)
AR= Rod cross section area (m2) vR= Retract rod velocity (m/s)
FR= Retract force (N) p = pressure from pump (N/m2)
Convert hydraulic energy to rotary mechanical
( / min)
Q n V
p pressure Pa
M torque Nm
V Geometricdisplacement capacity cm
Q Flowrate L
n speed rpm
◦ A motor with capacity of V = 10 cm3 is to operate
at a speed of 600 rpm. What flow rate is required by
between P1 and
P2 (eg. P1 =