ACTUATORS

ACTUATORSHydraulic systems are used to control and transmit power.A pump driven by prime mover such as electric motor creates a flow of fluid in which the pressure, direction and flow rate are controlled by valvesAn actuator is used to convert the energy of fluid back into mechanical power.The amount of output power developed depends upon the flow rate, pressure drop across the actuator and its overall efficiencyThus they are devices which used to convert pressure energy of the fluid into mechanical energy.

TYPES

Depending on the type of actuation, hydraulic actuators are

classified as,

1. Linear actuator: for linear actuation (hydraulic cylinder)

2. Rotary actuator: for rotary actuation(hydraulic motor)

3. Semi-rotary actuator: for limited angle of actuation (semi-

rotary actuators)

HYDRAULIC LINEAR ACTUATORSAs their name implies, it provides motion in a straight line.

The total movement is a finite amount determined by the

construction of the unit

They are referred to as cylinders, rams and jacks

The function of a hydraulic cylinder is to convert the hydraulic

power into linear mechanical force or motion

Hydraulic cylinder extend and retract a piston rod to provide a

pull or push force to drive the external load along a straight path

TYPES OF HYDRAULIC CYLINDERS

Hydraulic cylinder are of following types:

• Single-acting cylinders

• Double-acting cylinders

• Telescopic cylinders

• Tandem cylinders

SINGLE ACTING CYLINDER

SINGLE ACTING CYLINDER• It consist of piston inside a cylindrical housing called as

barrel• On one end is attached a rod, which can reciprocate• At the opposite end, there is a port for the entrance and

exit of the oil • They produce force only in one direction by hydraulic

pressure acting on the piston• The return of the spring is not done hydraulically, it is

either done by gravity or spring.

DOUBLE ACTING CYLINDER

There are two types of double acting cylinder:

• Double acting cylinders with piston rod on one side

• Double acting cylinders with piston rod on both side

DOUBLE ACTING CYLINDER

DOUBLE ACTING CYLINDERDOUBLE ACTING CYLINDER WITH PISTON ROD ON ONE

SIDE

• To extend the cylinder, the pump flow is sent to the blank-

end port as shown in fig.

• The fluid from the rod end port returns to the reservoir

• Now to retract the cylinder, the pump flow is sent to the

rod end port and the fluid from the blank end port returns

to the tank as shown in another fig.

DOUBLE ACTING CYLINDERDOUBLE ACTING CYLINDER WITH A PISTON ROD ON BOTH SIDES

DOUBLE ACTING CYLINDER• A double acting cylinder with piston on both sides is a

cylinder with a rod extending from both the ends

• The application involves in a process where work can be

done by both the ends of the cylinder, thereby making the

cylinder more productive

• Double rod cylinder can withstand higher side loads

because they have an extra bearing on each rod to

withstand the loading.

TELESCOPIC CYLINDERA telescopic cylinder is needed when a long stroke length

and a short retracted length are needed.

The telescopic cylinder extends in stages, each stage

consisting of a sleeve that fits inside the previous stage

One application for this type of cylinder is raising a dumping

truck bed.

Telescopic cylinders are available both in single acting and

double acting models.

TELESCOPIC CYLINDER

TELESCOPIC CYLINDERConstruction:• They generally consist a nest of tubes and operate on the

displacement principle• The tubes are supported by bearing rings, the innermost set of

which have grooves or channels to allow the fluid flow• The front bearing assembly on each section includes seals and

wiper rings• Stop rings limit the movement of each section, thus preventing

the separation• For a given input flow rate, the speed of operation increases in

steps as each successive section reaches the end of its stroke. Similarly, for a specific pressure, the load shifting capacity decreases for each of the successive section

TANDEM CYLINDER

TANDEM CYLINDER• A tandem cylinder is used in application where a large

amount of force is required to be applied from a small

diameter cylinder

• Pressure is applied to both the pistons, resulting in a

increased force because of a large area.

• The only drawback is that this cylinders must be longer

than a standard cylinder to achieve an equal speed because

the flow must go to both the pistons simultaneously.

GRAPHICAL SYMBOLS

GRAPHICAL SYMBOLS

CUSHIONING OF CYLINDERS

CUSHIONING OF CYLINDERS• For prevention of shock due to stopping loads at the end of

piston stroke, cushion devices are used.• Cushion may be applied at either end or both the ends• They operate on the principle that as the cylinder piston

approaches the end of the stroke, an exhaust fluid is forced to go through an adjustable needle valve that is set to control the escaping fluid at the given rate.

• This allows the deceleration characteristic to be adjusted for different loads

• When the cylinder piston is actuated, the fluid enters the cylinder port and flows through a little check valve so that the entire piston area can be utilized to produce forces and motion.

OPERATION OF CYLINDER CUSHIONING

OPERATION OF CYLINDER CUSHIONING

CUSHIONING PRESSURE• During deceleration, extremely high pressure may be

developed within a cylinder cushion.• The action of the cushioning device is to set up a back

pressure to decelerate the load.

MAXIMUM SPEEDS IN CUSHIONED CYLINDERSThe maximum speed of the cushion rod is limited by the rate of fluid flow into and out of the cylinder and the ability of the cylinder to withstand the impact forces that occurs when the piston motion is arrested by the cylinder end plate. • For uncushioned cylinder: 8 m/min• For cushioned cylinder : 12 m/min• For high speed or externally cushioned cylinders: 30

m/min

ACCELERATION & DECELERATION OF CYLINDER LOADSCylinders are subject to acceleration and deceleration during their operation. Cylinders are decelerated to provide cushioning and cylinders are accelerated to reduce cycle time of the operation.Acceleration Equation:

2

22

212

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QUESTIONS

• Explain the classification of hydraulic actuators.

• Explain various types of hydraulic cylinders.

• Describe the construction and working of double-acting cylinders.

• Derive an expression for force, velocity and power for hydraulic cylinders.

• Explain the importance of cylinder cushioning.

• Explain various types of cylinder mountings used in fluid power.

• Evaluate the performance of hydraulic systems using cylinders.