week 3 2_hydraulic_actuator

Hydraulic actuators are installed to drive loads by converting the hydraulic power into mechanical power.

The hydraulic actuators are classified into 2 category: ◦ Linear actuator - Hydraulic cylinders

◦ Rotary actuator - Hydraulic motors, rotary actuators

Converts hydraulic energy to mechanical energy.

Generates linear movements. Linear motors.

Basic types:

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.

Applications: ◦ Lifting

◦ Clamping

◦ 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

pressure port.

When pressure is

released, the spring

automatically returns

the cylinder to the fully

retracted position.

Gravity Return Single Acting Cylinder : In Circuit

Hydraulic Cylinder : In Application

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 reservoir.

When piston retract, fluid in piston area are displaced in reservoir.

Hydraulic Cylinder : Double Acting Cylinder

A double acting

cylinder is powered

in both direction.

In the case of

double-acting

cylinders, both

piston surfaces can

be pressurized.

Pressure Port and

Vent port can be

change during

extending or

retracting.

Double Acting Cylinder : In Circuit

Bore diameter o Working pressure determined by bore diameter

o Larger diameter produces larger forces

Piston rod diameter o Normally 1/6 of bore diameter

Stroke length o Length by which the piston rod moves from one

extreme position to another extreme position

Working pressure o Pressure that can be handled by cylinder

o Limited by size of bore, rod and tube thickness

Test pressure o Pressure used during testing (by manufacturer)

o Normally 1.5~2 times of normal working pressure

Area ratio

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,

2

. .

. 4

4

. .

hm

hm

hm

F p A

F dA

p

Fd

p

Standard bore diameter, dp

Standard rod diameter, dst

25 32 40 50 60 63 80 100 125

φ dst (mm)

1.25 12 14 18 22 25 28 36 45 56

1.4 14 18 22 28 32 36 45 56 70

1.6 16 20 25 32 36 40 50 63 80

2 18 22 28 36 40 45 56 70 90

2.5 20 25 32 40 45 50 63 80 100

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

24

; 40,000 ; 160 1600 /. .

33.5

5.79

6

p

hm

Fd F N p bar N cm

p

cm

cm

Piston rod diameter, dst

2

28.272

28.274

4.24

4.5

P P

PR P ST

ST

ST

A A

A A A

d

d

cm

Hydraulic Cylinder : Extending

FE = p x Ap

FE

AP

vE AR

Where :

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)

p

Hydraulic Cylinder : Retracting

FR = p x (AP-AR)

AP AR

vR

FR

Where :

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)

p

Convert hydraulic energy to rotary mechanical energy

Motor capacity:

3

( )

( )

( )

( / min)

( )

Mp

V

Q n V

p pressure Pa

M torque Nm

V Geometric displacement capacity cm

Q Flowrate L

n speed rpm

Example ◦ A motor with capacity of V = 10 cm3 is to operate

at a speed of 600 rpm. What flow rate is required by the motor?

Establish relationship between P1 and P2 (eg. P1 = 9P2 +3F1-5).

Show the calculation steps.

A1 A2

F1 F2

P1 P2

5cm 1.5cm

A1

A2

Cylinder Force : Examples

Cylinder Velocity : Examples

Cylinder Delivery: Examples

A

Qv

21 vv and