Rotary Actuators (Hydraulic Motors)
Definition : The hydraulic motor is a fluid power device which converts fluid energy into mechanical energy in the form of rotary motion of the shaft.
● The oil pressure is converted into torque and flow rate is converted into speed.
Application of Hydraulic motor
● The hydraulic motors are coupled to the resisting object for doing the work. It may be used for applications where rotary movement is required. E.g. winding and rewinding of cable.
● Hydraulic motors are similar in construction and size that of hydraulic pumps. The only difference is that instead of pushing the fluid as in case of pump, the rotating elements like gear, vane, piston, etc. are pushed by the oil to enable the motor shaft to rotate.
Types of Hydraulic Motors
1. Gear Motors
2. Vane Motors
3. Piston Motors
(a) Axial piston motor
(i) Swash plate type
(ii) Bent axis type
(b) Radial piston motor
4. Rack and Pinion Type Motor
5. Semi rotary (Limited Rotary) Actuators
Gear Motors
Definition: When a gear pair is used to obtain rotary motion by utilising fluid energy, it is known as gear motor
Principle :
It works on the principle of imbalance of oil pressure to act on the gear tooth and push the tooth for development of torque.
Construction:
Gear motors construction is similar to gear pump. mainly consists of:
(b) Casing : It is the outer body of the motor which encloses gear pair and passages for inlet and outlet ports.
(c) Inlet and outlet ports : The oil is allowed to enter through inlet port and drain out through the outlet port.
Working:
● When pressurised oil enters through the inlet port it exerts oil pressure on the gear teeths.
● As the casing inside surface and gear pair are closely fitted, the oil pressure acts on the one side of one tooth on the gear located near the inlet port.
● This will cause hydraulic imbalance and it push the gear tooth to develop torque. The larger the gear tooth or higher the pressure, more is the torque developed.
● This action continued to produce rotary motion of the shaft. The oil used has low pressure and of no use hence drained through the outlet port.
Vane Motors
Definition: When sliding vanes are used to obtain rotary motion with the help of pressurised oil, it is known as vane motor
Principle :
It works on the principle of hydraulic imbalance is caused by the difference in vane area subjected to hydraulic energy of oil due to eccentric mounting of rotor to the housing.
Construction:
Its construction is similar to vane pump. It mainly consists of following components:
(a) Cam ring (housing) : It is a circular ring which is stationary and it has inlet and out-ports provided.
(b) Rotor : The rotor is circular disc with adequate width and provided with number of slots to accommodate vanes. It is mounted eccentrically with respect to the cam ring axis.
(c) Sliding vanes : Vanes are rectangular, blades mounted with the help of spring.
(d) Inlet and outlet ports : They are provided for entry of pressurised oil and drain out used oil respectively.
Working :
● When pressurised oil enters in the vane motor, it will enter in the cam ring and rotor surface. It strikes on the vanes.
● Due to eccentric mounting of rotor, the unequal area of vanes occurs. The oil passing through unequal area pushes the vanes in the forward direction. It results in development of torque in the motor shaft.
● The oil strikes the vanes successively and develops rotary motion. Higher the oil pressure, more is the torque developed.
● This motor is unbalanced due to two different pressures the inlet pressure of oil is much greater than the outlet low pressure. Hence it may cause side loading in the motor shaft.
● To overcome above limitation, a balanced vane motor is developed which is modification of unbalanced vane motor.
Balanced Vane Motor
● Similar to balanced vane pump, here also the rotor containing vanes is concentrically placed with the cam ring.
● The two inlet and two outlet ports opposite to each other and thus maintains the balance of pressure on right side and left side of the motor.
Piston Motors
Definition : When reciprocating movement of piston is utilised for obtaining rotary motion by pressurised oil, it is known as piston motor.
Principle :
It works on the principle of conversion of reciprocating motion into rotary motion by using swash plate or connecting rod mechanism.
Types of piston motors :
(a) Axial piston motor
(b) Radial piston motor
(a) Axial piston motor:
Definition : When piston moves parallel to the axis of the shaft to produce rotary motion, it is called as axial piston motor.
It has two types:
(i) Swash plate type
(ii) Bent axis type
(i) Swash plate axial piston motor:
Principle :
It uses swash plate to obtain rotary motion from reciprocating movement of axial pistons.
Construction:
It consist of:
(a) Swash plate: It is positioned at an angle and acts as a surface on which the shoe side of the piston travel.
(b) Cylinder barrel : Cylinder barrel contains number of axially placed pistons. Minimum three pistons are necessary.
(c) Piston and piston shoes : The pistons are provided to move in the cylinder bore provided in the cylinder barrel. Piston shoes are provided at the end of piston rods and held in contact with the swash plate by the shoe plate.
(d) Shaft : The rotating shaft is connected to the cylinder barrel.
Working :
● When pressurised oil enters in the cylinder, it pushes the piston out of the cylinder barrel and slides over the swash plate.
● As the swash plate has angular surface, piston movement causes the twisting movement of the barrel and rotation of the shaft is created.
● The single piston develops only half rotation of the full circle of rotation of the cylinder barrel. The number of pistons should be 5 to 7.
● The amount of torque produced depends upon the angle of slide caused by the swash plate and the oil pressure in the system.
● For uniform rotation without much fluctuation the number of pistons should be above seven.
● Due to inclination of the swash plate, when one piston reaches its end, it starts with drawing and the next takes its position producing a movement, hence a torque is created.
● They are used for medium to high pressures and high power ranges.
(ii) Bent axis piston motor:
In this type of motor, the axis of the cylinder block is inclined at an angle to the axis of the shaft.
Construction and working :
● It consists of cylinder block in which number of pistons are mounted in number of bores, respectively.
● The cylinder block is making an angle with the shaft mounted with the help of drive shaft flange.
● When pressurised oil enters in the cylinder, it causes linear movement of the piston. This motion is transmitted to the drive shaft flange.
● As there is a angle between cylinder block and flange it develops twisting movement of the flange. Further it develops rotation of the flange and hence the drive shaft.
● These motors are used for machine tools like table movement of die sinking machine.
(b) Radial piston motor:
Definition: When pistons are arranged along the radius of the circular casing, it is called as radial piston motor.
Principle :
● It utilises the conversion of reciprocating motion of pistons produced due to oil pressure in to rotary motion by using common crank and connecting rod mechanism.
Construction: 
The radial piston motor consists of:
(a) Pistons arranged radially along the casing.
(b) Connecting rod is connected to each piston rod and other ends of connecting rod are connected to crank disc.
(c) Crank : Crank is a circular disc on which drive shaft is mounted.
Working :
● When oil enters in the cylinder O, it will push the piston out of the cylinder. This movement is given to the connecting rod.
● The connecting rod transmits the movement to the crank. As the end of connecting rod is mounted eccentrically with the centre of the crank, it creates rotary motion
● The motion is produced continuously with other pistons.
● This type of motors is suitable for low speed high torque applications. They are used in marine winches, rudders, construction equipment, high power industrial machines, etc.
Rack and Pinion Type Motor
● This motor consists of double acting cylinder, rack and pinion mechanism.
● The reciprocating movement is given to the rack and converted into rotary motion by using the pinion.
● When piston moves towards right, it will produce clockwise rotation while towards left will rotate in counterclockwise direction.
● It will give limited rotations of the shaft.
Semi rotary (Limited Rotary) Actuators
● This type of motor produces rotation of about 300° angle. The oil enters between fixed and moving vanes. The oil pressure makes the moving vane to rotate the shafts.
● When two sets of moving fixed vanes are used it can give rotation less than 180°.
Selection of Hydro-motors
The following factors are to be considered for selection of hydraulic motors.
1) Torque required :
The torque required to move the work and it is calculated as starting torque and running torque. Initial starting or breakaway torque is higher than the running torque.
2. System pressure:
As per guidlines by manufactures in the instruction manual system pressure is selected to produce the required torque. Radial Piston motor can work at very high pressures upto 450 bar.
3. Speed of rotation:
For low speed application, radial piston motors are suitable and for high speed, vane motors are suitable. The high speed motors are provided with cross over relief valve.
4. Type of application :
Torque needed with direction reversibility, space available for installation, precision machine tools, requires minimum speed fluctuations, etc.
5. Economical :
The motor is selected on the basis of cost and service life expected. It should be easily available of cheaper rate.
Advantages of Hydro Motors over Electrical Motor
1. Easy, smooth and instant reversing of motor shaft without any shock.
2. Easy controlling of torque throughout the operating speed.
3. Possibility of breaking action easily without any shock.
4. Better power-to-weight ratio.
5.Very low starting torque is required.
6. Can be used for an indefinite period without damage.
Pneumatic/Air motors (Rotary actuators)
Definition : It is the pneumatic actuator which converts energy of compressed air into mechanical energy in the form of rotary motion.
● Air motors are used for pneumatic hand tools like grinders, drilling machines, mixers, etc.
Actual air motor
Types of Air Motors
The pneumatic motors are classified as per the mechanism used in construction :
(a) Vane motor
(b) Turbine motor
(c) Piston motor
(d) Gerotor motor
(a) Vane motor:
Definition. When sliding vanes with rotor used to obtain rotary motion by using compressed air, it is known as vane motor.
Construction :
It is similar to vane compressor in it construction. It mainly consists of
(1) Rotor with slots : It is the circular disc with slots accommodate the sliding vanes.
(2) Cam ring : It is the outer casing in which rotor is mounted eccentrically.
(3) Inlet/exhaust ports : They are provided for inlet o high pressure air and exhaust low pressure air to the atmosphere.
Working :
● The compressed air enters through inlet of the motor and strikes on the vanes. It will push the vane in the forward direction.
● As vanes are mounted with rotor, it will produce angular movement and produces rotary motion.
● The air at low pressure at the outlet is exhausted to the atmosphere.
● The motor has possibility of air leakage due to wear of vane tips. It is suitable for speeds 100 to 200 m.
(b) Turbine motor :
Definition : When rotating wheel has similar feature of turbine runner is used to obtain rotary motion from compressed air, it is known as turbine motor.
Construction :
It consists of
(i) Nozzle : It is used to increase the velocity of compressed air and produced jet of air.
(ii) Runner wheel : It is the circular disc with number of curved vanes on it peripheries.
(iii) Casing : It encloses the nozzle and runner wheel with inlet and outlet ports.
Working:
● When compressed air enters in the nozzle, it increases its veto city by dropping the pressure, and jet of air strikes on the curved vanes of the runner wheel.
● The jet will push the curved vanes and produces rotary motion by striking on vanes one by one in series.
● They are low torque high speed motors, suitable for low temperature application.
(c) Piston motor:
Definition: When reciprocating movement of piston is converted into rotary motion with the help of compressed air it is known as piston motor.
● It consists of radially mounted cylinders along with pistons. The piston rod is attached to the connecting rod for each cylinder.
● The connecting rods are mounted on the crank disc along with output shaft.
Working :
● When compressed air enters in the each cylinder, it will push the piston in the downward dissection. This movement is given to the connecting rod.
● The connecting rod end is attached to the crank disc away from its centre, hence it will push the crank in the forward dissection to produce angular motion.
● This cycle will continue to produce rotary motion by the sequence of cylinder 1 to 4.
● It has 4,5,6, cylinders mounted radially to obtain rotary motion.
(d) Gerotor motor:
Definition : When gerotor shaped elements are used to obtain rotary motion using compressed air, it is called as greater motor.
Construction :
It consists of:
(1) Gerotor pair : The gerotor shaped elements are mounted in a pair similar to gear pair,
(2) Casing : The gerotor pair is enclosed in a casing with proper clearance at the casing surface.
(3) Inlet and exhaust ports are provided for compressed air entry and exit.
Working :
● When compressed air enters in the casing through inlet port, it will strike on the gerotor and pushes it forward, the next gerotor shape will come in front of inlet and moves in forward dissection.
● This action gives rotary motion at the output shaft.
● They are used for low rpm up to 200 – 300 rpm.
Specification of Air Motors
1. Kilowatt rating :
They are available in market at low to high kW rating e.g. 0.05 kW to 20 kW.
2. RPM of the motor :
Hand tools like grinders, drills has rpm requirement is high, motors may be available in the rpm range up to 20000 rpm.
3. Tool holder size :
The tools like grinding wheel, drills are hold in the motor opening, the size of tool to be held is also specified on label showing specifications.
Applications of Air Motors
1.Hand tools
Grinders, borers, drills, hammers, etc.
2. Material handling devices:
To operate trolleys, conveyors, etc.
3. Dentist :
Tooth cleaning is done by pneumatic motor.
4. Mixers :
Small mixers in chemical industries, etc.
