Unit 2.

ACTUATORS and Power Unit

Introduction: An Actuator is a component of machines that is responsible for

moving or controlling a mechanism or system.

The supplied main energy source may be electric current,

hydraulic fluid pressure, or pneumatic pressure. When the

control signal is received, the actuator responds by converting

the energy into mechanical motion.

It is a device which converts fluid power into rotary power or

converts fluid pressure into torque or linear power.

Introduction- They extract energy from a fluid, and convert it to mechanical energy to

perform useful work.

Hydraulic cylinders, also called linear actuators provide a force that drives

an external load along a straight line.

Hydraulic motors, also called rotary actuators, provide a torque that drives

an external load along a circular path.

Hydraulic

Cylinder

Electric

Motor

T x ωV x IHydraulic

Pump

P x Q

Hydraulic

Motor

F x v

T x ω

Hydraulic

System

Difference between Hydraulic Motor and Hydraulic Pump

Hydraulic Motor Hydraulic Pump

It is a device for delivering torque at a

given pressure. The main emphasis is on

mechanical efficiency and torque that

can be transmitted.

It is a device for delivering flow at a

given pressure. The main emphasis is on

volumetric efficiency and flow.

Motors usually operate over a wide

range of speed, from a low RPM to high

RPM.

Pumps usually operate at high RPM.

Most motors are designed for

bidirectional applications such as

braking loads, rotary tables.

In most situations, pumps usually

operate in one direction.

Motors may be idle for long time Pumps usually operate continuously.

Motors are subjected to high side loads

(from gears, chains, belt-driven pulleys).

Majority of pumps are not subjected to

side loads.

Comparison between a Hydraulic motor and an Electric motor

Electric Motor Hydraulic Motor Electric motors cannot be stopped

instantly.

Their direction of rotation cannot be

reversed instantly.

This is because of air gap between the

rotor and stator and the weak magnetic

field.

Hydraulic motors can be stalled for any

length of time.

Their direction of rotation can be

instantly reversed and their rotational

speed can be infinitely varied without

affecting their torque.

They can be braked instantly and have

immense torque capacities.

Electric motors are heavy and bulky. Hydraulic motors are very compact

compared to electric motors. For the

same power, they occupy about 25% of

the space required by electric motors and

weigh about 10% of electric motors.

Applications:

Applied directly to the work.

They provide excellent control for acceleration, operating speed, deceleration,

smooth reversals and positioning.

They also provide flexibility in design and eliminate much of bulk and weight of

mechanical and electrical power transmission.

A hydrostatic transmission converts mechanical power into fluid power and

then reconverts fluid power into shaft power.

The advantages of hydrostatic transmissions include power transmission to

remote areas, infinitely variable speed control, self-overload protection, reverse

rotation capability, dynamic braking and a high power-to-weight ratio.

Eg: Material-handling equipment, farm tractors, railway locomotives, buses and

machine tools

Rotary actuators-

1.Gear Motors:

Working-

A gear motor develops torque due to hydraulic pressure acting

against the area of one tooth.

There are two teeth trying to move the rotor in the proper direction,

while one net tooth at the center mesh tries to move it in the opposite

direction.

In the design of a gear motor, one of the gears is keyed to an output

shaft, while the other is simply an idler gear.

Pressurized oil is sent to the inlet port of the motor. Pressure is

then applied to the gear teeth, causing the gears and output shaft to

rotate. The pressure builds until enough torque is generated to rotate the

output shaft against the load.

The side load on the motor bearing is quite high, because all the

hydraulic pressure is on one side. This limits the bearing life of the

motor.

2.Vane Motors:

3.Axial Piston Motors:

Axial Piston Motors:

4.Swash-plate piston motor

5.Bent-Axis Piston Motors:

6.Radial Piston Motors:

Performance of Hydraulic Motors:

1.Starting torque: The starting torque is the turning force the

motor exerts from a dead stop.

2. Running torque: Running torque is exerted when the motor is

running and changes whenever there is a change in fluid pressure.

3. Stalling torque: Stalling torque is the torque necessary to stop

the motor.

Volumetric efficiency:

Mechanical efficiency:

Here,

Overall efficiency:

Example:

A hydraulic motor receives a flow rate of 72 LPM at a pressure of 12000

kPa. If the motor speed is 800 RPM, determine the actual torque delivered

by the motor assuming the efficiency 100%?

Cont.

Semi-Rotary Actuators-Devices used to convert fluid energy into a torque which turns through

an angle limited by the design of the actuator

1.Vane-Type Semi-Rotary Actuator

1.1(Single Vane)-A semi-rotary actuator allows only a

partial revolution.

A vane-type semi-rotary actuator

consists of a vane connected to an

output shaft.

When hydraulic pressure is applied to

one side of the vane, it rotates.

A stop prevents the vane from rotating

continuously. The rotation angle in the

case of a single-vane semi-rotary

actuator is 315°.

1.2.Two-Vane-Type Semi-Rotary

Actuator-

The advantage of this design is that

the torque output is increased because

the area subjected to pressure is

large.

However, two-vane models cannot

rotate as many degrees as can single-

vane models.

It is limited to 100°.

2.Chain and Sprocket Semi-Rotary Actuator-It is suitable for multi-revolution applications.

The larger cylinder is the power cylinder and the smaller cylinder is

the chain return or seal cylinder .

The larger piston moves away from the port due to differential areas of the

two pistons.

The movement of larger piston pulls the chain, causing the sprocket

and output shaft to rotate.

3.Rack and Pinion Rotary Actuator-

Used design for obtaining partial revolution actuation.

The cylinder drives a pinion gear and the rack is an integral part of the

piston rod.

The angle of rotation depends upon the stroke of the cylinder, rack

and the pitch circle diameter of the pinion.

Types of Hydraulic Cylinders/Linear Actuator:

Hydraulic Cylinders are of the following types:

1. Single-acting cylinders.

2. Double-acting cylinders.

3. Telescopic cylinders.

4. Tandem cylinders.

1.Single-Acting Cylinders

Graphic Symbol

Port

Extension

Retraction

PistonPiston Seal Rod

Barrel

According to the type of return, single-acting

cylinders are classified as follows:

a. Gravity-return single-acting cylinder.

b. Spring-return single-acting cylinder.

a. Gravity-return single-acting cylinder:

Figure :Gravity-return single-acting cylinder: (a) Push type; (b) pull

type

b. Spring-return single -

acting cylinder.

Working-Single Acting Hydraulic

Cylinders

Push Action

Oil to extend, Spring

for return

Pull Action

Oil to retract,

Spring to extend

2.Double-Acting Cylinder:

There are two types of double-acting cylinders:

a. Double-acting cylinder with a piston rod on

one side.

b. Double-acting cylinder with a piston rod on

both sides.

a. Double-Acting Cylinder with a Piston Rod on One Side

Graphic Symbol

b. Double-Acting Cylinder with a Piston Rod on Both

Sides

Graphic Symbol

3. Telescopic Cylinder : used when a long stroke length and a

short retracted length are required.

4. Tandem Cylinder

Used in applications where a large amount of force is

required from a small-diameter cylinder.

Cylinder Cushions

Prevention of shock due to stopping loads at the end of the piston

stroke, cushion devices are used.

Cushions may be applied at either end or both ends.

Cylinder Cushions (Cont.)

Cylinder Force, Velocity and Power

The output force (F) and piston velocity (v) of double-acting cylinders are not the

same for extension and retraction strokes.

Hydraulic Cylinder Calculation –

Power developed by a hydraulic cylinder

(both in extension and retraction) is,

Power =Force ×Velocity =F* V

Extending

Retracting

Cylinder Mountings-

Selecting a particular mounting is depends on whether the force applied

is tensile or compressive.

Alignment of the rod with the resistive load is another important

consideration while selecting cylinder mounts.

The ratio of rod length to diameter should not exceed 6:1 to prevent

bucking

Types-

1. Centre line Mountings- cylinder supported

along its centerline. Forces occurs only

along axis of cylinder

2.Foot Mountings- cylinder introduce torque

under loaded condition

3.Pivot Mounting-when cylinder allowed

to rotate while reciprocating