Compiled and Prepared in perspective of students dealing with H&P

1. HYDRAULIC ACTUATORS
AND MOTORS
PRESENTED BY,
SOMASHEKAR S M
ASST PROFESSOR

2. CONTENTS
• Learning Objectives
• Introduction
• Comparison b/w Pump and Motor
• Linear Actuators

3. LEARNING OBJECTIVES
• Differentiating hydraulic pump functions from hydraulic motors
• Describe the construction and working of various types of hydraulic cylinders/actuators
• Application based Mechanics of hydraulic cylinder loading.

4. INTRODUCTION
• Hydraulic systems are used to control and transmit power.
• In hydraulic systems, pumps perform the function of adding energy to the fluid for
transmission to some output location.
• They extract energy from the fluid and convert into mechanical energy to perform some
useful work by the hydraulic actuators and motors at application end.
• The power developed at the actuator depends on the flow rate, the pressure drop across the
actuator and the overall efficiency.
• There are three types of hydraulic actuators
 Linear actuators: Hydraulic cylinders which produce straight line motion. Ex:
Hydraulic rams and jacks.
 Rotary actuators(continuous): Commonly known as Hydraulic motors which
converts hydraulic energy into mechanical energy in the form of rotary motion.
 Rotary actuators (limited angle): Semi rotary actuators.

5. COMPARISON B/W PUMP & MOTOR

6. LINEAR ACTUATORS
• Single Acting Cylinder:

7. LINEAR ACTUATORS
Single Acting Cylinder:
 Hydraulic pressure is applied to only one side of the piston.
 Single acting cylinder may be either:
 Outward-Actuated: When an outward actuated cylinder has hydraulic pressure
applied to it, the piston and rod are forced outward to lift the load. When the oil
pressure is relieved, the weight of the load forces the piston and rod back into the
cylinder.
 Inward-Actuated: When an inward-actuated cylinder has hydraulic pressure applied
to it, the rod is pulled inward into the cylinder.
 One side of a single acting cylinder is dry. The dry side must be vented so that when oil
pressure on the pressure side is relieved, air is allowed to enter, preventing a vacuum.
 Single acting cylinders are further classified into three types. They are:
a) Gravity or self returned b) Spring return cylinder c) Telescopic cylinder

8. LINEAR ACTUATORS
• Single Acting Cylinder (Spring Return):

9. LINEAR ACTUATORS
Single Acting Cylinder (Spring Return):
• When a single acting cylinder is to be placed in a position other than vertical, then its
retraction can be affected by placing a compression spring on the rod end side of the
piston.
• Extension of the piston takes place due to the oil pressure at the cap end against the spring
force compressing the spring.
• When the pressure is removed the pistons retracts or return due to spring force.
• The cylinder can be placed in any position.

10. LINEAR ACTUATORS
• Another type of single acting cylinder.
• It consists of series of nested tubular
segments called Sleeves.
• These sleeves work together and provide a
long working stroke than possible with
standard single acting cylinder.
• Up to 4-5 sleeves can be used.
• The max force that can be exerted depends on
the diameter of the smallest sleeve.

11. LINEAR ACTUATORS
• Double Acting Cylinder

12. DOUBLE ACTING CYLINDER
• Cylinders whose extension and retraction are due to pressure of oil are called ‘double acting
cylinders’.
• Double acting cylinders are featured by two ports.
• Double acting cylinders are most common types of cylinders used in industrial hydraulics.
• In this type of cylinder, oil pressure is applied on either side of the piston for its extension
and reaction through ports.
• Since oil pressure is acting on either face of the piston, it is called double acting cylinder.

13. WORKING OF DOUBLE ACTING CYLINDER
•

14. WORKING OF DOUBLE ACTING CYLINDER
• The initial position of the piston is ready for expansion when the valve is actuated by lever
pressure oil enters the cap end side of the piston through port A.
• The cylinder extends and the oil present on other side of the piston flows out through port
B as the piston extends.
• The entry of oil through port A and exit of oil through port B continues till the piston is
completely extended.
• When the lever of the valve released, the valve is returned to its normal positon by the
spring .
• In this position pressurised oil ingress the rod end side of the piston through port B and
pushes the piston (retracts) and the oil present in the cap end side of the piston is expelled
out through port A.

15. MECHANICS OF HYDRAULIC CYLINDER LOADING
• Hydraulic cylinders are not just designed to move the load in the linear direction of the
piston rod.
• In many engineering applications, the load acts at an angle to the axis of the cylinder.
• Many mechanisms use the hydraulic cylinders to transmit motion and power.
• Among these, lever mechanisms such as the toggles, the rotary devices and the push-pull
devices use the hydraulic cylinder.
• In order to determine, the hydraulic cylinder force to drive non-axial loads we have first-
class, second class and third class lever systems.
• In these system designs, the cylinder rod and the load rod pin connected by a lever that
can rotate about a fixed hinge.

16. LEVER SYSTEM

17. FIRST CLASS LEVER SYSTEM

18. FIRST CLASS LEVER SYSTEM
• It contains first class lever system, which is characterized by the lever fixed-hinge pin
between the cylinder and load rod pins.
• The length of the lever portion from cylinder rod pin to fixed hinge is L1, whereas the
length of the lever portion from the load rod pin to the fixed hinge is L2.
• To determine the cylinder force, Fcyl required to drive the load force Fload we equate
moment about the fixed hinge, which is the pivot point of the lever.
• The cylinder force attempts to rotate the lever in CCW about the pivot end this creates
CCW moment.
• Similarly, the load force creates a CW moment about the pivot.
• At the equilibrium, these two moments are equal in magnitude.

19. FIRST CLASS LEVER SYSTEM
• Suppose the centreline of the hydraulic cylinder tilts by an offset angle from the vertical;
the relationship becomes.
• If L1> L2, the cylinder force is less than the load force and the cylinder stroke is greater than
the load stroke.

20. SECOND CLASS LEVER SYSTEM
•

21. SECOND CLASS LEVER SYSTEM
• In this lever system, the loading point is in between the cylinder an the hinge point.
• Using the same nomenclature discussed under first lever system, we can analyse by taking
moments about fixed hinge point.
• Compared to the first-class lever, the second-class lever requires smaller cylinder force to
drive the given load force for same L1 and L2 and load force. In other words, if we use a
second-class lever cylinder, a smaller size cylinder can be used.
• Compared to the first-class lever, the second-class lever also results in a smaller load
stroke for a given cylinder stroke.

22. THIRD CLASS LEVER SYSTEM

23. THIRD CLASS LEVER SYSTEM
• For a third-class lever system, the cylinder rod pin lies between the load road pin and the
fixed-hinge pin of the lever.
• Equating moments about the hinge point, we can write
• In a third-class lever system, cylinder force is greater than load force.
• In a third-class lever system, load stroke is greater than the cylinder stroke and therefore
requires a larger cylinder.

24. HYDRAULIC MOTORS
• INTRODUCTION:
 Hydraulic motors extract energy from a fluid and convert it to mechanical energy to
perform useful work.
 Hydraulic motors can be of the limited rotation or continuous rotation type.
 Hydraulic motors are of gear, vane or piston configurations.
 Hydraulic motors are repeatedly graphically by a circle with dark arrows towards the centre
as shown.

25. CLASSIFICATION
• Based on Speed and Torque
 High Speed and Low Torque Motor
 Low Speed and High Torque Motor
 Limited Rotation Motor
• Based on Displacement
 Fixed Displacement Motors (Gear, Vane & Piston Motors)
 Variable Displacement Motors (Vane & Piston Motors)
• Based on number directions of rotation
 Unidirectional Motors
 Bidirectional Motors

26. GEAR MOTORS
•

27. GEAR MOTORS
• A gear motor develops torque due to hydraulic pressure acting on the surfaces of the gear
teeth, as shown in fig.
• The direction of rotation of the motor can be reversed by reversing the direction of flow .
• The volumetric displacement of a gear motor is fixed.
• The gear motor shown in figure is not balanced with respect pressure loads.
• The high pressure at the inlet, coupled with low pressure at the outlet, produces a large
side load on the shaft and bearings.
• Gear motors are normally limited to 150 bar operating pressures and 2500 RPM operating
speed.
• They are available with a maximum flow capacity of 600 LPM.
• The main advantages of a gear motor are its simple design and subsequent low cost.

28. WORKING
• Oil under pressure enters on one side of the casing where the gears un-mesh forces the gears
to rotate.
• The oil under high pressure follows the path of least resistance which is around the
periphery of the inner side of the housing and comes out at low pressure through the outlet
provided on the opposite side of the inlet.
• The torque developed is a function of hydraulic imbalance of only one tooth of one gear at a
time.
• Close tolerances between gears and housing helps in controlling of oil leakage and increase
volumetric efficiency.
• The gear motors are simple in construction and have good dirt tolerance, but their
efficiencies are lower than those of vane or piston pumps and they leak more than the piston
units.

29. INTERNAL GEAR MOTORS
• DIRECT DRIVE GEROTOR MOTOR:

30. CONSTRUCTION
• It consist of a set of inner and outer gear and an output shaft connected to the inner gear .
• The inner gear has one teeth less than the outer gear.
• The shape of the both inner and outer gears are same and both the gears are in contact all
the times.
• Stationary kidney shaped inlet ad outlet ports are built into the motor housing.
• The centre of rotation of the two gears is separated by a small distance called eccentricity.
• The inner gear coincides with the axis of the output shaft.

31. WORKING
• The pressure oil enters the motor through the inlet port as shown in fig.,
• Since the inner gear has one tooth less than the outer gear, a pocket is formed between the
two inner teeth and an outer socket A.
• The kidney shaped inlet port is so designed such that once the pocket is full, the fluid flow is
shut off with the tip of the inner gear providing a seal as shown in fig.
• As the inner and outer gear rotate in pair to another pocket is formed and the previous
pocket has moved around and the opposite kidney shaped outlet port steadily draining the
oil in the pocket.
• This continues and continuous rotation is generated.
• Reversing the flow reverses the direction of rotation of the motor.

32. ORBITAL GEROTOR MOTOR
•

33. CONSTRUCTION
• An orbiting gerotor consists of a set of matched gears, a coupling, an output shaft and a
commutator (cylindrical drum mounted on the output shaft which helps to produce
unidirectional torque) or a valve plate (acts as seal).
• In this types also, the inner gear also called rotor contains one tooth less than the outer gear.
• The outer gear is stationary.
• The coupling has splines, which match with the same speed that of the rotor.
• The commutator rotates with the same speed as that of the rotor and provides pressure oil
and passage to the tank and to the proper areas of spaces

34. WORKING
• In operation, tooth 1 of the inner gear is aligned exactly in socket D of the outer gear (Fig).
• Point y is the centre of the stationary gear, and point x is the centre of the rotor.
• If there were no fluid, the rotor would be free to pivot about socket D in either direction.
• It could move toward seating tooth 2 in socket E or, conversely, toward seating tooth 6 in
socket J.
• When pressure fluid flows into the lower half of the volume between the inner and outer
gears, if a passageway to tank is provided for the upper-half volume between the inner and
outer gears, a moment is induced that rotates the inner gear counter clockwise and starts to
seat tooth 2 in socket E.
• Tooth 4, at the instant shown in Figure, provides a seal between pressure and return fluid.
• However, as rotation continues, the locus of point x is clockwise.
• As each succeeding tooth of the rotor seats in its socket, the tooth directly opposite on the
rotor from the seated tooth becomes the seal between pressure and return fluid

35. WORKING
• The pressurized fluid continues to force the rotor to mesh in a clockwise direction while it turns
counter clockwise.
• Because of the one extra socket in the fixed gear, the next time tooth 1 seats, it will be in socket J.
• At that point, the shaft has turned one-seventh of a revolution, and point x has moved six-
sevenths of its full circle.
• In Figure c, tooth 2 has mated with socket D, and point x has again become aligned between
socket D and point y, indicating that the rotor has made one full revolution inside of the outer
gear.
• The commutator or valve plate contains pressure and tank passages for each tooth of the rotor.
• The passages are spaced so they do not provide for pressure or return flow to the appropriate port
as a tooth seats in its socket.
• At all other times, the passages are blocked or are providing pressure fluid or a tank passage in the
appropriate half of the motor between gears.

36. BALANCED VANE MOTOR
•

37. VANE MOTOR
• Basically, a vane motor consists of a rotor which develops torque at its output shaft by allowing oil
pressure to act on the rectangular vanes which side in and out of slots provided in a rotor
splined to drive the shaft.
• Construction:
 Basically, a vane motor consists of a rotor, vanes, cam ring, port plate with kidney shaped inlet
and outlet ports casing.
 The rotor has radial slot in which the vanes will slide in and out.
 The rotor is connected to the driven shaft; both the shaft and the inner hole of the rotor are being
splined.
The assembly of the rotor, vanes and shaft are placed inside the cam ring eccentrically.

38. UNBALANCED VANE MOTOR
• Vane motor exists with two different pressures such as system and outlet pressure is known as
unbalanced vane motor.
• System pressure will be greater than the outlet pressure resulting in side loading on the motor
shaft.
• This side loading is due to only one pair of inlet and outlet ports.
• This side loading causes unbalanced forces on the shaft and its bearing resulting in
discontinuous and non uniform speed and torque.
• This reduces the efficiency of the motor in particular and that of the hydraulic system in general.

39. BALANCED VANE MOTOR
• Side loading in unbalanced cane motor and thereby its effect on torque can be prevented by replacing
circular cam ring by elliptical cam ring.
• By this arrangement two pressure quadrants oppose each other and the forces acting on the shaft are
balanced.
• Since the centre position of the rotor cannot be varied with the centre of the cam ring, balance vane
motors are fixed displacement motors.
• In vane motors, the extending of the vanes cannot takes place by centrifugal force but due to the
hydraulic fluid exerts force on the vane so that it rotates the shaft.
• The tip of the extended vanes provide a positive seal between vane and cam ring..
• Vanes are loaded either by coil spring in the slot of the rotor or by using small wire is attached to the
vane is fixed to the post.
• The springs housed inside the rotor slot is preset to the required tension so that the vane is always
pushed out of the slot continuously and creates positive seal between the cam ring and the vane tip for
all positions of the rotor.

40. BALANCED VANE MOTOR
• . In spring loading vane, pressure oil is directed to the underside of the vane as soon as the torque
is developed.
• In hydraulic pressure extending method, oil is not allowed to enter the vane chamber area until
the vane is fully extended and positive seal exists at the vane tip.

41. PISTON MOTORS
• Piston motors are hydraulic linear motors where in the pressure energy contained in the fluid is
converted into to and fro motion of the piston and in turn into rotary motion.
• Piston motors are probably the most efficient motor of the three types of hydraulic motors i.e.,
Gear, Vane and Piston Motors.
• These are positive displacement motors and develop an output torque at the drive shaft by
allowing oil pressure into act on pistons.
• Types: Piston motors are classified, based on the following factors.
 The position of the axis of the cylinder block and the drive shaft.
 Axial Piston Motors
 Radial Piston Motors
 Displacement of the motor
 Fixed displacement piston motors
 Variable displacement piston motors

42. AXIAL PISTON MOTORS
• Axial piston motors the pistons reciprocate parallel to the axis of the cylinder block.
• The main parts of an axial piston motors are,
 Cylinder block
Piston with shoe
Shoe plate
 Spring
 Shaft
 Port plate

43. AXIAL PISTON MOTORS
Principal of generation of torque in piston motors:
• Consider a fluid of pressure p acting on a piston of area A. The force exerted on the piston will be
F= P x A
• The force F act on the plate through the shoe and the shoe plate.
• If the force acting perpendicular to the plate known as radial force Fn, if the force acts parallel to
the plate known as tangential force Ft.
• The normal reaction of the plate will be equal to Fn, but there is no force to resist Ft.
• If the plate on which Ft is acting causes a moment of magnitude Ft x r, where r is the
perpendicular distance of the centre of rotation the plate from the force Ft, is free to rotate about
its centre, then the moment so developed rotates the plate.
• This is the principle of torque generation in the case of piston motors.
• Axial piston motors are further classified into:

44. AXIAL PISTON MOTORS
a. Inline-axial piston motors (swash plate design)
i. Fixed displacement
ii. Variable displacement
b. Bent axis piston motor
i. Fixed displacement
ii. Variable displacement

45. IN-LINE AXIAL PISTON MOTORS

46. IN-LINE AXIAL PISTON MOTORS
• In the inline design the motor, drive shaft and cylinder block are centered on the same axis.
• The principle of converting reciprocatory motion into rotary motion is by use of surface
inclined to the axis of reciprocating pistons.
• The inclined surface may be a part of the component or the component itself is tilted to
obtain an inclined surface and such components is called “swash plate”.
• If the inclined surface is an integral of the plate then such plate is called “ Swash Plate” and
the inclined surface obtained by inclining the surface then it is called “Wobble or tilting
plate”.
• If the displacement of a motor fixed i.e., stroke length is fixed (in the case of piston motor)
then such motors are called fixed displacement motors.
• If the displacement of a motor can be varied i.e., stroke length can be varied then such
motors are called variable displacement motors.

47. INLINE AXIAL PISTON MOTOR

48. SWASH PLATE

49. PISTON

50. RETAINER PLATE

51. CYLINDRICAL BLOCK

52. CYLINDRICAL BLOCK

53. DISTRIBUTOR PLATE

54. IN-LINE AXIAL PISTON MOTORS
• Fixed displacement type

55. IN-LINE AXIAL PISTON MOTORS
Fixed displacement type:
• This type of motors functions similar with piston pumps, but in reverse process.
• Here the pressurized oil is fed to the inlet side of the unit.
• The pistons are located inside the cylindrical barrel in a circular array.
• At other end of the pistons are connected to shoes which slide on inclined swash plate.
• In this type the swash plate is fixed and hence its stroke length.
• The high pressure hydraulic oil exerts force on the piston, that tend to rotate the cylindrical
barrel.
• This inturn generates useful torque.

56. IN-LINE AXIAL PISTON MOTORS

57. IN-LINE AXIAL PISTON MOTORS
Variable displacement type

58. IN-LINE AXIAL PISTON MOTORS

59. IN-LINE AXIAL PISTON MOTORS
Variable displacement type:
• In variable displacement motor the swash plate (called the tilting plate) is mounted on s
swinging yoke.
• The angle can be varied by various means such as a lever, hand wheel or a servo control.
• If the offset angle is increased (swash plate angle) the displacement and hence the torque
capacity is increased but the speed of the drive shaft decreases.
• Conversely reducing the angle reduces the torque capability and increases drive shaft speed.
• Minimum angle stops are usually provided so that the torque and speed stay within the
operating limits.
Working:
• In a swash-plate inline piston motor, the swash plate is held at some angle initially and the
cylinder block containing the axially place pistons in the moveable block is made to rotate by
admitting the pressure oil.

60. IN-LINE AXIAL PISTON MOTORS
Variable displacement type (working):
• The force F exerted by the oil pressure p on piston of area Ap, is p x Ap and this acts in the
inclined surface i.e., swash plate.
• The force F has two components Fn and Ft
• Ft acting at a radial distance ‘r’ from the centre of the swash plate develops torque T = Ft x r
• Due to this torque, the cylinder block on carrying the piston glides over the swash plate on a
circular path and rotation of the shaft is generated.
• The torques continues to be developed as long as the piston is pushed out of the cylinder block
by the oil pressure.
• A single piston develops half rotation of the full circle of the cylinder block.
• Number of pistons, higher the torque is and it is to have add number of pistons.
• The direction of rotation of the cylindrical block and the shaft may be reversed by changing
the direction of flow.

61. IN-LINE AXIAL PISTON MOTORS
Variable displacement type (working):
• Torque developed in the piston is a function of the oil pressure P and the volumetric
displacement VD.
• Volumetric displacement is a function of the area of the piston Ap number of pistons n, pitch
circle diameter of the bores D and the swash plate angle ϴ and is given by
Tth = (VD x p)
VD = n Ap D tan ϴ
Power developed Pth = Tth x N
Where N is the rpm of the motor

62. BENT AXIS PISTON MOTORS

63. BENT AXIS PISTON MOTORS

64. BENT AXIS PISTON MOTORS
• Bent axis design is similar to axial piston motor.
• This type of motor develops torque due to pressure acting on the reciprocating piston.
• In this motor, the cylinder block and drive shaft mount at an angel to each other so that the
force is exerted on the drive shaft flange.

65. RADIAL PISTON MOTOR

66. RADIAL PISTON MOTORS
• In radial piston-type motors, the piston reciprocates radially or perpendicular to the axis of
the output shaft.
• Radial piston motors are low-speed high-torque motors which can address a multifarious
problem in diverse power transfer applications.
Working:
• When the oil pressure is introduced into the cylinder, the piston will be pushed outward.
• For this action to take place the cylinder block containing the pistons will rotate and hence
the output shaft of the motor connected to the cylinder block.
• As the cylinder block rotates along with one of the pistons, pressure oil is introduced other
into the bore of piston thus, creating torque in the cylinder block and hence continuous
rotation is generated in the motor.
• The rate at which the pressure fluid is introduced or pumped into the cylinder determines
the speed of the motor.

67. RADIAL PISTON MOTORS
• Radial piston motors may be of either fixed or variable displacement type.
• Variable displacement can be achieved by changing the length of the piston stroke.
• The length of the stroke is controlled by eccentricity or off-centre of the cylinder block
with respect to the rotor.

68. THEORETICAL TORQUE, POWER AND DISCHARGE
Wkt, Actual torque developed by hydraulic motors are less than the theoretical torque that
the motor should develop. This is due to frictional losses. Theoretical torque is the torque
developed by a frictionless hydraulic motor. Theoretical torque and hence theoretical power
may be computed as follows:
Theoretical Torque 𝑇𝑡ℎ = 𝑉𝐷 × 𝑝 𝑁 − 𝑚/𝑟𝑎𝑑𝑖𝑎𝑛
Where, 𝑉𝐷 = Theoretical discharge 𝑚3
/rev
p = Pressure of the oil entering the motor N/𝑚3
The theoretical power developed in a motor is given by,
𝑃𝑡ℎ = 𝑇𝑡ℎ × 𝑁 𝑟𝑎𝑑/𝑠𝑒𝑐
𝑃𝑡ℎ =
2𝜋 𝑇 𝑡ℎ ×𝑁
60000
kW N-Speed in rpm

69. THEORETICAL TORQUE, POWER AND DISCHARGE
The theoretical flow rate 𝑄𝑡ℎ = 𝑉𝐷 × 𝑁 𝑚3/𝑠𝑒𝑐
Where, N- speed in rps

70. PERFORMANCE OF HYDRAULIC MOTORS
• The performance of any mechanical system depends on the precision or manufacture of its
elements and maintenance of the close tolerances under the design operating conditions.
So also it is the case with hydraulic motors.
• Hydraulic motor efficiency is also evaluated on the basis of the three efficiencies namely
Volumetric Efficiency η 𝑣𝑜𝑙
Mechanical Efficiency η 𝑚𝑒𝑐ℎ
Overall Efficiency η 𝑂

71. • Volumetric efficiency (η 𝑣𝑜𝑙): Due to leakages, the hydraulic motor uses more flow than the
theoretical flow. Thus the volumetric efficiency of a hydraulic motor is the inverse of the
pump volumetric efficiency.
η 𝑣𝑜𝑙 =
𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒
𝑎𝑐𝑡𝑢𝑎𝑙 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒
× 100 =
𝑄 𝑡ℎ
𝑄 𝑎𝑐𝑡
• Mechanical efficiency (η 𝑚𝑒𝑐ℎ): The mechanical efficiency of a hydraulic motor is again the
inverse of an equivalent pump. Since frictional losses in the actual torque produced by the
motor, the mechanical efficiency is given by,
η 𝑚𝑒𝑐ℎ=
𝐴𝑐𝑡𝑢𝑎𝑙 𝑡𝑜𝑟𝑞𝑢𝑒 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑
𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑡𝑟𝑜𝑞𝑢𝑒
× 100
Where, 𝑇𝑎𝑐𝑡 =
𝑃
2𝜋𝑁
𝑇𝑡ℎ =
𝑉 𝐷 ×𝑝
2𝜋
PERFORMANCE OF HYDRAULIC MOTORS

72. PERFORMANCE OF HYDRAULIC MOTORS
• The overall efficiency is the product of the volumetric and mechanical efficiencies, and is
given by,
η 𝑂=
𝐴𝑐𝑡𝑢𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝑑𝑒𝑙𝑖𝑣𝑒𝑟𝑒𝑑 𝑏𝑦 𝑡ℎ𝑒 𝑚𝑜𝑡𝑜𝑟
𝐴𝑐𝑡𝑢𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝑑𝑒𝑙𝑖𝑣𝑒𝑟𝑒𝑑 𝑡𝑜 𝑡ℎ𝑒 𝑝𝑢𝑚𝑝
× 100
η 𝑂=
2𝜋𝑁𝑇𝑎𝑐𝑡/60
𝑃×𝑄 𝑡ℎ
× 100% =
2𝜋𝑁𝑇𝑎𝑐𝑡/60
𝑃×𝑉 𝐷×𝑁
× 100%
η 𝑂=
η 𝑣𝑜𝑙 × η 𝑚𝑒𝑐ℎ
100

73. CUSHIONED CYLINDERS

74. CUSHIONED CYLINDERS
• In certain applications, it is essential to decelerate the moving piston gradually at
the end of stroke.
• Such cylinders are provided with cushioning mechanism and are termed
cushioned cylinders.
• Cushioned cylinders are used when delicate loads are being moved, or when the
cylinder loading is to be gradual or in some heavy cylinders to avoid damage to the
cylinder at the end of stroke.
• The cylinder has built-in flow control mechanism, which acts only at the end of
the stroke.
• It has a cushion spear on the piston end, a matching recess in the end cap, a flow
check valve and a restricted path with a adjustable needle screw valve.
• Cushioning is achieved by throttling the rate of exhaust or return of oil, from
cylinder.

75. OPERATION OF CUSHIONED CYLINDERS

76. OPERATION OF CUSHIONED CYLINDERS

77. OPERATION OF CUSHIONED CYLINDERS
• In operation, until the piston-spear retracts and reaches the recess, the free flow of oil
takes place through the recess.
• But as soon as the spear enters the recess, it blocks the free flow.
• Then the oil starts flowing through the restricted path, thus decelerating the piston
movement.
• The check valve allows the free flow to the piston only during the extension stroke and
stops the flow in the retraction stroke.
• Since the oil has to flow through the restricted path, large amount of energy is absorbed,
giving a cushioning effect to the cylinder.
• The needle valve can be adjusted to vary the size of the restricted flow path, hence
controlling the deceleration speed.

78. CUSHIONING PRESSURE
• During deceleration, extremely high pressure may develop within a cylinder cushion.
• The action of the cushioning device is to set up a back-pressure to decelerate the load.
• Ideally, the back-pressure is constant over the entire cushioning length to give a
progressive load deceleration.
• In practice, cushion pressure is the highest at the moment when the piston rod enters the
cushion.
• Some manufacturers have improved the performance of their cushioning devices by using a
tapered or a stepped cushion spear.
• Wherever high inertia loads are encountered, the cylinder internal cushions may be
inadequate but it is possible for the load to be retarded by switching in external flow
controls.

79. CUSHIONING PRESSURE
•

80. MAXIMUM SPEED IN THE CUSHIONED CYLINDERS
• The maximum speed of a piston 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 occur when the
piston movement is arrested by the cylinder end plate.
• In an uncushioned cylinder, it is normal to limit the maximum piston velocity to 8m/min.
• This value is increased to 12 m/min for a cushioned cylinder, and 30 m/min is permissible
with high-speed or externally cushioned cylinders.
• Oversize ports are necessary in cylinders used in high-speed applications.
• In all cases, the maximum speed depends upon the size and type of load.
• It is prudent to consult the manufacturer if speeds above 12 m/min are contemplated.
• When only a part of the cylinder stroke is utilized, cushions cannot be used to decelerate
the load.
• In such cases, it may be necessary to introduce some form of external cushioning
especially where high loads or precise positioning is involved.