References:

• Anthony Esposito, Fluid Power Systems and control, Prentice-Hall,

1988
• R. Srinivasan Hydraulic and Pneumatic Control, published by Vijay
Nicole Imprints Private Ltd. 1990
• Herbert R. Merritt, Hydraulic control systems, John Wiley & Sons,
Newyork, 1967
• Dudbey.A.Peace, Basic Fluid Power, Prentice Hall Inc, 1967
• Peter Rohner, Fluid Power logic circuit design, The Macmillan Press
Ltd., London, 1979
HYDRAULIC ACTUATORS

Depending on the type of actuation, hydraulic actuators are classified as follows:

1. Linear actuator: For linear actuation (hydraulic cylinders).
2. Rotary actuator: For rotary actuation (hydraulic motor).
3. Semi-rotary actuator: For limited angle of actuation (semi-rotary actuator).
Linear actuator:

Hydraulic cylinders are of the following types:

• Single-acting cylinders.
• Double-acting cylinders.
• Telescopic cylinders.
• Tandem cylinders.
Rotary Motors:
• Gear type
• Vane type
• Piston type
• Axial type
• Radial type
Single acting cylinder
A single-acting cylinder is simplest in design and is shown schematically in figure. It consists of a piston Inside a cylindrical
housing called barrel. On one end of the piston there is a rod, which can reciprocate. At the opposite end, there is a port for
the entrance and exit of oil. Single-acting cylinders produce force in one direction by hydraulic pressure acting on the
piston. (Single-acting cylinders can exert a force in the extending direction only.) The return of the piston is not done
hydraulically. In single-acting cylinders, retraction is done either by gravity or by a spring.

According to the type of return, single-acting cylinders are classified as follows:

Gravity-return single-acting cylinder.
Spring-return single-acting cylinder.
 Gravity cylinder
cylinder lifts the
cylinder extends to weight by retracting
lift a weight against
the force of gravity by
applying oil pressure
at the piston end

Push type Pull type

Spring return single acting cylinder

Due to inherent mechanical problems associated with spring, single acting cylinders with spring return are not used in
application where larger stroke length is required
Double-Acting Cylinder
There are two types of double-acting cylinders:
Double-acting cylinder with a piston rod on one side.
Double-acting cylinder with a piston rod on both sides.

• Double acting cylinders

are operated by oil
pressure applied to the
cylinder in both sides.
• For a given pressure
single rod end cylinders
exert greater force when
extending than when
retracting.

• When it is required equal

force in both directions,
the double rod end
cylinder is used.
Double-acting cylinder with a piston rod on both sides.
Telescopic cylinders.

• It consists of nested tubular segments called sleeves.

• These sleeves work together and provide a longer working stroke or
extension than possible with a standard single acting cylinder.
• The maximum force that can be exerted depends on the diameter of
the smallest sleeve of the cylinder

Truck dump bodies

Fork lift trucks
Hydraulic cranes
Tandem cylinders.
• Two cylinders are connected in series
• Higher force for a given pressure and cylinder bore

Through-Rod Cylinders
• similar in construction to the standard double-acting cylinders
• cylinder rod extending through both cylinder end caps
• piston rods with different diameters at each end of the cylinder
Application:
• same speed is required in both the directions
• both ends of the rod can be utilized to do work
• non-working end is used to indicate or signal the position of the load
• rod is fixed at both the ends and the cylinder body carrying the load moves on the rod
Disadvantages: correct alignment and concentricity of cylinder bore, piston, end caps and rods.
Displacement Cylinders
Rod is displaced from inside a tube by pumping hydraulic fluid into the tube
Volume of the rod leaving the tube is equal to the volume of fluid entering the tube

The rod of the displacement cylinder is guided by bearings in

the nose or neck of the cylinder body. A collar on the end of the
rod prevents it from being ejected and limits the stroke of the
cylinder. Elastomer seals in the neck prevent any leakage of
fluid along the outside of the rod. This design is a single-acting
“push” or extension cylinder, which has to be retracted by
gravity, a spring or some external force. The bore of the cylinder
body does not require machining other than that for the neck
bearing and the inlet port; the manufacturing cost is, therefore,
low when compared with other types or hydraulic cylinders.

Maximum thrust = Pressure × Rod area

Cylinder Force, Velocity and Power
SOLVE
Common problems with cylinders:

1. Sticky and slow start

2. Scoring of tube and pistons

3. Leakage of oil

4. Wear out seal

5. Mechanical problems with links and other machine members

A displacement-type cylinder has a rod of 65 mm diameter and is powered by a hand pump with a displacement of 5 mL per
double stroke. The maximum operating pressure of the system is to be limited to 350 bar. (a) Draw a suitable circuit diagram
showing the cylinder, pump and any additional valves required. (b) Calculate the number of double pumping strokes needed
to extend the cylinder rod by 50 mm. (c) Calculate the maximum load that could be raised using this system.

Rod volume displaced = Fluid volume entering

S=33.17 double strokes

Maximum thrust = Pressure × Rod area

=116.080 kN
First-Class Lever System • fixed-hinge point is located in between the cylinder and the
loading point.
Second-Class Lever System

loading point is in between the cylinder and the hinge point

Third-Class Lever System

cylinder rod pin lies between the load

road pin and the fixed-hinge pin of the
lever.
Cylinder Cushions

For the 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. They operate on the principle that as the cylinder piston approaches the end of stroke, an
exhaust fluid is forced to go through an adjustable needle valve that is set to control the escaping fluid at a given rate. This allows
the deceleration characteristics to be adjusted for different loads. When the cylinder piston is actuated, the fluid enters the
cylinder port and flows through the little check valve so that the entire piston area can be utilized to produce force and motion.
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. Deceleration can then take place over a greater
part of the actuator stroke.
Cylinder Mountings

• Type of force- tensile or compressive

• bucking load must be avoided
• The ratio of rod length to diameter should not exceed 6:1 to prevent bucking
• Alignment of the rod with the resistive load

The various kinds of mountings normally used in industries are as follows

1. Foot mounting: It should be designed to give a limited amount of movement on one foot only to allow for thermal or
load expansion. That is, the cylinder should be positively located or dowelled at one end only.

2. Rod-end flange or front flange mounting: During the extend stroke, pressure in the hydraulic fluid acts on the cylinder-
end cap, the force set up being transmitted to the front mounting flange through the cylinder body.
3. Rear flange, back flange or head-end flange mounting: No stress is present in the cylinder owing to load on the
extend stroke; only hoop stress is present. The load acts through the fluid onto the rear flange.

4. Trunnion mounting: It allows angular movement. It is designed to take shear load only. Bearing should be as close to
the cylinder body as possible.

5. Eye or clevis mounting: There is a tendency for the cylinder to jack knife under load. Side loading of bearing must be
carefully considered.
Pumping theory
Gear pumps are rotary pumps

Spur gear- Noisy at relatively high speed

Helical gear- excessive end thrust
Herringbone gear- greater flow rate for less pulsating
Advantages of gear pumps The disadvantages are as follows:
1.They are self-priming. 1. The liquid to be pumped must be clean, otherwise it
2.They give constant delivery for a given speed. will damage pump.
3. They are compact and light in weight. 2. Variable speed drives are required to change the
4. Volumetric efficiency is high. delivery.
3. If they run dry, parts can be damaged because the
fluid to be pumped is used as lubricant.
• Ideal discharge speed characteristic
Leakage loss (Flow back)

Pressure (P)
Discharge (Q)

Actual Ideal Discharge Curve

Discharge
Curve

Discharge (Q)
Speed (N)

Volumetric efficiency= (Qact/Qth)*100

Pump slippage (S) = (Qth–Qact)
S= ((Qth–Qact)/Qth)*100
 VANE PUMPS
• Vane pumps are positive-displacement,
fixed or variable delivery, rotary units.
• Design is commonly used in industrial
applications
• Delivery can range up to 283.9 lpm (75
gpm)
• Maximum pressure of about 2000 psi
(13.79 N/mm2)

33
 VANE PUMP
• Vane pump consists of a slotted rotor,
fitted with moveable vanes, that
rotates within a cam ring in the pump
housing
• Rotor is off center in the ring, which
creates pumping chambers that vary in
volume as the pump rotates
• As chamber volume increases, pressure
decreases, bringing fluid into the pump
• As volume decreases, fluid is forced out
into the system

34
Parts of a typical vane pump

35
VANE PUMPS
• Vane pump may be pressure unbalanced (single acting) or pressure
balanced (double acting)
• Unbalanced has only one inlet and one discharge, which places a side load on
the shaft
• Balanced has two inlets and two discharges opposite each other, creating a
pressure balance and, therefore, no load on the shaft

36
UNBALANCED VANE PUMPS
• Components: cam surface and the rotor, into which vanes are slip fit.

• The rotor is keyed to the drive shaft and therefore rotates as the pump
is driven.

• As the rotor spins, the vanes are kept in contact with the cam surface
by centrifugal force, which may be supplemented by a spring or by fluid
pressure.

• The cam surface and rotor are mounted eccentrically to one another,
which causes the vanes to stroke as the rotor spins.
Unbalanced vane pump…
• As the rotor turns, pumping chambers between the vanes are opened
near the inlet, creating a vacuum that allows atmospheric pressure to
push the fluid in.
• The fluid is then carried between the vanes to the outlet where the
vanes are pushed back in and the pumping chamber volume is
reduced.
• The reduction in volume near the outlet causes the fluid to be forced
out.
Unbalanced vane pump…
• The pressurized pumping chambers are located on only one side of the
drive shaft.

• The outlet port at the top is under pressure, while the inlet Port at the
bottom is at vacuum.

• Results in a net force on the pump shaft bearing that can cause excessive
vibration and wear at high speeds or pressures.

• For this reason, these pumps are best suited to low-pressure applications.
BALANCED VANE PUMP
• Has an elliptical cam surface, which causes the vanes to stroke twice per
revolution of the pump shaft.

• The pumping chambers therefore undergo an increase and decrease in volume

twice per cycle, which necessitates two inlet and two outlet ports.

• The inlets and outlets are combined into a common inlet and outlet within the
pump housing.

• This configuration results to equal pressure on opposite sides of the pump shaft.
• Vane pumps are more efficient than gear pumps, but
less efficient than piston pumps.
• They are moderately tolerant of contamination.
• Unbalanced vane pumps are low-pressure pumps,
typically operating between 500 psi and 2000 psi.
• Balanced vane pumps can handle higher pressures.
They can be rated for pressures as high as 4000 psi.
Additional Design Features of Pumps

Unbalanced-vane pumps can be designed to produce

variable flow outputs
• A cam ring moves in relation to the center of the rotor
• No flow output is produced when the centers of the ring and
rotor are concentric
• Flow increases as the centers move apart

44
Additional Design Features
of Pumps
• A variable-flow, unbalanced-vane pump

45
Expression for theoretical discharge of Vane Pump

e
e
Both the gear wheels are driven
and one of the gear wheels is
extended shaft to provide output
torque
Piston Pumps
PISTON PUMPS
✓The piston is drawn back quickly, creating a vacuum at its inlet, allows
atmospheric pressure to push fluid from the reservoir into the pump.
✓The piston is then driven forward to expel the fluid towards the
system
✓Check valves are used on the inlet and outlet lines.
✓Two main types of power piston pumps that are commonly used in
hydraulics: radial and axial piston pumps.
PISTON PUMPS
1. Axial piston pump
Bent-axis-type piston pump.
Inline or Swash-plate-type piston pump.

2. Radial piston pump.

• A basic piston pump consists of a housing that supports a pumping

mechanism and a motion-converting mechanism
• Pumping mechanism is a block containing cylinders fitted with pistons and
valves
• Motion converter changes rotary to reciprocating motion via cams, eccentric
ring, swash plate, or bent-axis designs
• Rotating the pump shaft causes piston movement that pumps the fluid
53
Pump Design & Operation: Inline
• Inline has the cylinder block and pistons located on the
same axis as the pump input shaft
• Pistons reciprocate against a swash plate
• Very popular design used in many applications
• The pistons are aligned or parallel with the pump shaft axis
• Pistons are arranged in a circular pattern around the pump
shaft.
• Turning the pump shaft causes the pistons & piston block
to rotate.
• The housing, end cap, and piston shoe remain stationary.

54
• The pistons ride the piston shoe, which is mounted at an
angle to the shaft axis, causing the pistons to reciprocate
in the piston block as the pump shaft is turned.
• The pistons pull back and draw in oil during 180° of their
rotation and push out and expel oil during the other 180°.
• The oil is fed to and from the system by semi-circular feed
grooves that are connected to inlet and outlet ports
located in the end cap
Axial Piston Pump
Pump Design and Operation: Bent Axis
• Bent axis has the cylinder block and pistons set at an
angle to the input shaft
• Geometry of the axis angle creates piston movement
• Considered a more rugged pump than inline
• high flow rates and maximum operating pressures

58
 Fixed displacement pump
angle is not adjustable

Bent axis pump

Characteristics of piston pump

• Piston pumps are the most expensive and the most efficient

• Have the highest pressure ratings as high as 10,000 psi.

• Least tolerant of contamination, making them best suited to clean

environments.
Radial Piston Pumps
• Radial piston pumps have the highest continuous operating pressure
capability of any of the pumps regularly used in hydraulic systems
• Models are available with operating pressure ratings in the 10,000 psi
range

61
Types of Radial Piston Pumps
• Two variations of radial piston pumps:
• Stationary-cylinder design uses springs to hold pistons
against a cam that rotates with the main shaft of the
pump. Also called as rotating cam radial piston pump.
• Rotating-cylinder design uses centrifugal force to hold
pistons against a reaction ring
• When the main shaft is rotated, each piston
reciprocates, causing fluid to move through the
pump

62
 Rotating Cylinder Radial Piston Pump

• Pistons radiate out from the shaft axis.

• The pistons and piston block are keyed to the drive shaft

• They rotate, while the housing and cam surface remain stationary.

• The piston block is mounted off-center, or eccentric, to the cam

surface.
 Rotating Cylinder Radial Piston Pump

• As the piston block is rotated, the pistons are kept in contact with the
cam ring by centrifugal force.
• The eccentricity between the piston block and the cam ring causes
the pistons to stroke, drawing in fluid and expelling it with each
rotation.
• The inlet and outlet ports are separated by the nonrotating pintle.
Rotating Cylinder Radial Piston
Pump
Rotating cam
Radial-Piston Pump.
• The rotating cam type has a cam that rotates with the
shaft while the piston block is stationary.
• The pistons ride the eccentrically shaped cam, which
causes them to stroke.
• The pistons are held in contact with the cam with
springs,
A stationary-cylinder radial-piston pump
•

68
Radial piston pump
􀀀 The operation and construction of a radial piston pump
􀀀 This design consists of a pintle to direct fluid in and out of the
cylinders, a cylinder barrel with pistons, and a rotor containing a
reaction ring
􀀀 The pistons remain in constant contact with the reaction ring due
to centrifugal force and back pressure on the pistons.
􀀀 Types:
1) Radial piston pump with a stationary cylinder block
2) Radial piston pump with a rotating cylinder block