this report gives

1. TERM PAPER
HYDRAULICS ACTUATION SYSTEM
Caterpillar 797B mining truck. Source: Caterpillar
Submitted By:
RAJESH KUMAR P2009ME1100
LALIT AGGARWAL P2009ME 1088
1

2. Abstract
Table Of Contents
List of Figures
List of Tables
Objective
2

3. Fluid Power
Fluid power is the transmission of forces and motions using a confined pressurized fluid. In
hydraulic fluid power systems the fluid is oil or water. Fluid power is ideal for high speed,
high force, and high power applications. Compared to all other actuation technologies,
including electric motors, fluid power is unsurpassed for force and power density and is
capable of generating extremely high forces with relatively lightweight cylinder actuators.
Fluid power systems have a higher bandwidth than electric motors and can be used in
applications that require fast starts, stops and reversals, or that require high frequency
oscillations. Because oil has a high bulk modulus, hydraulic systems can be finely controlled
for precision motion applications.
Major advantage of fluid power lies in its compactness and flexibility. Fluid power cylinders
are relatively small and light for their weight and flexible hoses allows power to be snaked
around corners, over joints and through tubes leading to compact packaging without
sacrificing high force and high power. A good example of this compact packaging is Jaws of
Life rescue tools for ripping open automobile bodies to extract those trapped within.
But there are some disadvantages also.
1. Hydraulic systems can leak oil at connections and seals.
2. Hydraulic power is not as easy to generate as electric power and requires a heavy,
noisy pump.
3. Hydraulic fluids can cavitate and retain air resulting in spongy performance and loss
of precision. Hydraulic can become contaminated with particles and require careful
filtering.
4. The physics of fluid power is more complex than that of electric motors which makes
modelling and control more challenging.
Research is going on not only to overcome these challenges but also to open fluid power to
new applications, for example tiny robots and wearable power-assist tools.
3

4. Some main Applications Of Fluid Power (Hydraulics)
Fluid power is extensively used throughout industry and throughout the world because of its
major advantages and here are some examples.
 Earth moving machines such as excavators
 Winches on cranes and boats
 Rams in forging and extrusion processes
 Automated production lines
 Aeroplane controls
 Automated assembly units
 Machine tools
 Braking system
 Roller coaster
 Earthquake simulators
Figures showing application of fluid power
Caterpillar 797B mining truck. Source: Caterpillar
4

5. 40,000 ton forging press. Source: Shultz Steel
MAST Laboratory for earthquake simulation. Source: MAST Lab.
5

6. Caterpillar 345C L excavator. Source: Caterpillar.
Hypersonic XLC roller coaster with hydraulic lanuch assist. Source:
Wikipedia image
6

7. Introduction to Hydraulics
Hydraulics refers to the means and mechanisms of transmitting power through liquids.
Hydraulic Actuators, as used in industrial process control, employ hydraulic pressure to drive
an output member. These are used where high speed and large forces are required. The
fluid used in hydraulic actuator is highly incompressible so that pressure applied can be
transmitted instantaneously to the member attached to it.
In fluid power, hydraulics is used for the generation, control, and transmission of power by
the use of pressurized liquids. Hydraulic topics range through most science and engineering
disciplines, and cover concepts such as pipe flow, dam design, fluidics and fluid control
circuitry, pumps, turbines, hydropower, computational fluid dynamics, flow measurement,
river channel behaviour and erosion.
th
It was not, however, until the 17 century that the branch of hydraulics with which we are
to be concerned first came into use. Based upon a principle discovered by the French
scientist Pascal, it relates to the use of confined fluids in transmitting power, multiplying
force and modifying motions.
Then, in the early stages of the industrial revolution, a British mechanic named Joseph
Bramah utilized Pascal’s discovery in developing a hydraulic press.
Principle Behind
Pascal’s Law
“Pressure applied to a confined fluid at any point is transmitted undiminished and equally
throughout the fluid in all directions and acts upon every part of the confining vessel at right
angles to its interior surfaces.”
7

8. Amplification of Force or Hydraulic “Leverage”
As the pressure in the system is the same, the force that the fluid gives to the surroundings
is therefore equal to pressure multiplies by area. In such a way, a small piston feels a small
force and a large piston feels a large force.
The same principle applies for a hydraulic pump with a small swept volume that asks for a
small torque, combined with a hydraulic motor with a large swept volume that gives a large
torque. In such a way a transmission with a certain ratio can be built.
Note : Pressure remains same everywhere only force changes due to change in area and
work done in the process remains same i.e. input = output.
8

9. The illustration given below shows that 1 lb. of force exerted on a 1 sq. in. piston, moved 10
in. will lift 10 lbs. a distance of 1 in. with a 10 sq. in. piston. The larger piston will move a
shorter distance, but provides the mechanical advantage to lift a much heavier load. This
mechanical workforce advantage is hydraulic leverage.
Where hydraulic actuation should be used?
Hydraulic actuation should be used for any installations where there are a number of
systems that can be operated from a single system; installations where the gates, valves,
and actuators must be submerged; and installations where the system must operate in a
power failure or other emergency. Hydraulic actuators are particularly desirable where
equipment is to be operated frequently, where loads are high, where the speed of
operation is high or must be varied during operation, and where they are located in a
hazardous area requiring explosion-proof and intrinsically safe equipment.
9

10. Components of Hydraulic Actuation Systems
1. Hydraulic Fluid
Hydraulic fluid must be essentially incompressible to be able to transmit power
instantaneously from one part of the system to another. At the same time, it should
lubricate the moving parts to reduce friction loss and cool the components so that the heat
generated does not lead to fire hazards. It also helps in removing the contaminants to filter.
Figure below shows the role played by hydraulic fluid films in lubrication and sealing.
2. The Fluid Delivery Subsystem
It consists of the components that hold and carry the fluid from the pump to the actuator. It
is made up of the following components.
10

11. 3. Reservoir
It holds the hydraulic fluid to be circulated and allows air entrapped in the fluid to escape.
This is an important feature as the bulk modulus of the oil, which determines the stiffness of
hydraulic system, deteriorates considerably in the presence of entrapped air bubbles. It also
helps in dissipating heat.
4. Filter
The hydraulic fluid is kept clean in the system with the help of filters and strainers. Metal
particles are continually produced by mechanical components and need to be removed
along with other contaminants, which can cause blocking of the orifices of servo-valves or
cause jamming of spools.
The graphical symbol for Reservoirs and Filters
11

12. 5. Line
Pipe, tubes and hoses, along with the fittings or connectors, constitute the conducting lines
that carry hydraulic fluid between components. Lines convey the fluid and also dissipate
heat. There are various kinds of lines in a hydraulic system. The working lines carry the fluid
that delivers the main pump power to the load. The pilot lines carry fluid that transmits
controlling pressures to various directional and relief valves for remote operation or safety.
Lastly there are drain lines that carry the fluid that inevitably leaks out, to the tank.
The various kinds of lines in a hydraulic system
Connection Arrangement of Filter and Lines with a Reservoir
12

13. 6. Fittings and Seals
Various additional components are needed to join pipe or tube sections, create bends and
also to prevent internal and external leakage in hydraulic systems. Although some amount
of internal leakage is built-in, to provide lubrication, excessive internal leakage causes loss of
pump power since high pressure fluid returns to the tank, without doing useful work.
External leakage, on the other hand, causes loss of fluid and can create fire hazards, as well
as fluid contamination. Various kinds of sealing components are employed in hydraulic
systems to prevent leakage. A typical such component, known as the O-ring is shown below
in Figure.
Sealing by O-rings
7. Hydraulic Pumps
The pump converts the mechanical energy of its prime-mover to hydraulic energy by
delivering a given quantity of hydraulic fluid at high pressure into the system. Generally, all
pumps are divided into two categories, namely, hydrodynamic or non-positive displacement
and hydrostatic or positive displacement. Hydraulic systems generally employ positive
displacement pumps only.
The graphical symbol for Pumps
13

14. Different types of pumps
 Hydrostatic or Positive Displacement Pumps
These pumps deliver a given amount of fluid for each cycle of motion, that is, stroke or
revolution. Their output in terms of the volume flow rate is solely dependent on the speed
of the prime-mover and is independent of outlet pressure notwithstanding leakage. These
pumps are generally rated by their volume flow rate output at a given drive speed and by
their maximum operating pressure capability which is specified based on factors of safety
and operating life considerations. In theory, a pump delivers an amount of fluid equal to its
displacement each cycle or revolution. In reality, the actual output is reduced because of
internal leakage or slippage which increases with operating pressure. There are various
types of pumps used in hydraulic systems as described below.
Gear Pumps
The construction of a Gear Pump
A gear pump develops flow by carrying fluid between the teeth of two meshed gears. One
gear is driven by the drive shaft and turns the other, which is free. The pumping chambers
formed between the gear teeth are enclosed by the pump housing and the side plates. A
low pressure region is created at the inlet as the gear teeth separate. As a result, fluid flows
in and is carried around by the gears. As the teeth mesh again at the outlet, high pressure is
created and the fluid is forced out. Figure shows the construction of a typical internal gears
pump; Most gear type pumps are fixed displacement. They range in output from very low to
high volume. They usually operate at comparatively low pressure.
14

15. Vane Pumps
In a vane pump a rotor is coupled to the drive shaft and turns inside a cam ring. Vanes are fitted
to the rotor slots and follow the inner surface of the ring as the rotor turns. Centrifugal force
and pressure under the vanes keep them pressed against the ring. Pumping chambers are
formed between the vanes and are enclosed by the rotor, ring and two side plates. At the pump
inlet, a low pressure region is created as the space between the rotor and ring increases. Oil
entering here is trapped in the pumping chambers and then is pushed into the outlet as the
space decreases.
Principle of Operation of Vane Pumps
Piston Pumps
In a piston pumps, a piston reciprocating in a bore draws in fluid as it is retracted and expels it
on the forward stroke. Two basic types of piston pumps are radial and axial.
A radial pump has the pistons arranged radially in a cylinder block and in an axial pump the
pistons are parallel to the axis of the cylinder block. The latter may be further divided into in-line
(swash plate or wobble plate) and bent axis types.
15

16. 8. Motors
Motors work exactly on the reverse principle of pumps. In motors fluid is forced into the
motor from pump outlets at high pressure. This fluid pressure creates the motion of the
motor, shaft and finally goes out through the motor outlet port and return to tank. All three
variants of motors, already described for pumps, namely Gear Motors, Vane Motors and
Piston motors are in use.
The graphical symbol for Motors
9. Accumulators
Unlike gases the fluids used in hydraulic systems cannot be compressed and stored to cater
to sudden demands of high flow rates that cannot be supplied by the pump. An accumulator
in a hydraulic system provides a means of storing these incompressible fluids under
pressure created either by a spring or compressed gas. Any tendency for pressure to drop at
the inlet causes the spring or the gas to force the fluid back out, supplying the demand for
flow rate.
Accumulator
16

17. Types of Accumulators
 Spring-Loaded Accumulators
In a spring loaded accumulator, pressure is applied to the fluid by a coil spring behind the
accumulator piston. The pressure is equal to the instantaneous spring force divided by the
piston area. The pressure therefore is not constant since the spring force increases as fluid
enters the chamber and decreases as it is discharged.
Spring loaded accumulators can be mounted in any position. The spring force, i.e., the
pressure range is not easily adjusted, and where large quantities of fluid are required spring
size has to be very large.
A spring-loaded accumulator
17

18.  Gas Charged Accumulator
The most commonly used accumulator is one in which the chamber is pre-charged with an inert
gas, such as dry nitrogen. A gas charged accumulator should be pre-charged while empty of
hydraulic fluid. Accumulator pressure varies in proportion to the compression of the gas,
increasing as pumped in and decreasing as it is expelled.
A gas-charged accumulator
10. Cylinders
Cylinders are linear actuators, that is, they produce straight-line motion and/or force.
Cylinders are classified as single-or double-acting as illustrated in Figures with the graphical
symbol for each type.
Single Acting Cylinder: It has only one fluid chamber and exerts force in only one direction.
When mounted vertically, they often retract by the force of gravity on the load. Ram type
cylinders are used in elevators, hydraulic jacks and hoists.
18

19. Cross Sectional View of Single-acting Cylinder
19

20. Double-Acting Cylinder:
The double-acting cylinder is operated by hydraulic fluid in both directions and is capable of
a power stroke either way. In single rod double-acting cylinder there are unequal areas
exposed to pressure during the forward and return movements due to the cross-sectional
area of the rod. The extending stroke is slower, but capable of exerting a greater force than
when the piston and rod are being retracted.
Double-rod double-acting cylinders are used where it is advantageous to couple a load to
each end, or where equal displacement is needed on each end. With identical areas on
either side of the piston, they can provide equal speeds and/or equal forces in either
direction. Any double-acting cylinder may be used as a single-acting unit by draining the
inactive end to tank.
Cross Sectional View of Double-acting Cylinder
20

21. 11. Control Valve
Control valves are essential and appear in all fluid power systems. Valves are sometimes
categorized by function, which includes directional control valves for directing fluid flow to
one or the other side of a cylinder or motor, pressure control valves for controling the fluid
pressure at a point and flow control valves for limiting the fluid flow rate in a line, which in
turn limits the extension or retraction velocities of a piston.
On/off valves can only be in the states defined by their positions while proportional valves
are continuously variable and can take on any position in their working range. A servo valve
is a proportional valve with an internal closed-loop feedback mechanism to maintin precise
control over the valve behaviour.
Types of control valves Left to right: hand-operated directional valve for a log splitter, On-off
miniature, solenoid actuated pneumatic valve, Precision proportional pneumatic valves, High
precision, flapper-nozzle hydraulic servo valve.
Valve actuation symbols Left to right:
push-button, lever, springreturn, solenoid, pilot-line.
21

22. 12. Hydraulic Circuit Drawings
Accurate diagrams of hydraulic circuits are essential to the technician who must diagnose and repair
possible problems. The diagram shows how the components will interact. It shows the technician
how it works, what each component should be doing and where the oil should be going, so that he
can diagnose and repair the system.
There are two types of circuit diagrams.
1. Cutaway Circuit Diagrams show the internal construction of the components as well as the
oil flow paths. By using colors, shades or various patterns in the lines and passages, they are
able to show many different conditions of pressure and flow.
2. Schematic Circuit Diagrams are usually preferred for troubleshooting because of their ability
to show current and potential system functions. A schematic diagram is made up of
consistent geometric symbols for the components and their controls and connections.
Schematic symbol systems:
I.S.O. = International Standards Organization.
A.N.S.I. = American National Standards Institute
A.S.A = American Standards Association
J.I.C. = Joint Industry Conference
22

23. A Complete Hydraulic Schematic
23

24. Different Kinds of Hydraulic Actuators
A hydraulic actuation system is a drive or transmission system that uses pressurized
hydraulic fluid to drive hydraulic machinery. The term hydrostatic refers to the transfer of
energy from flow and pressure, not from the kinetic energy of the flow.
All hydraulic systems are essentially the same regardless of the application. There are four
basic components required; a reservoir to hold the fluid; a pump to force the fluid through
the system; valves to control the flow; and an actuator (motor) to convert the fluid energy
into mechanical force to do the work.
1. Hydraulic Jack
The principle behind most hydraulic systems is similar to that of the basic hydraulic jack. Oil
from the reservoir is drawn past a check ball into the piston type pump during the piston's
up-stroke.
When the piston in the pump is pushed downward, oil will be directed past a second check
ball into the cylinder. As the pump is actuated up and down, the incoming oil will cause the
cylinder ram to extend. The lift cylinder will hold its extended position because the check
ball is being seated by the pressure against it from the load side of the cylinder. More liquid
24

25. is pumped under a large piston to raise it. To lower a load, a third valve (needle valve)
opens, which opens an area under a large piston to the reservoir. The load then pushes the
piston down and forces the liquid into the reservoir.
Because the pump displacement is usually much smaller than the cylinder, each stroke of
the pump will move the cylinder a very small amount. If the cylinder is required to move at a
faster rate, the surface area of the pump piston must be increased and/or the rate which
the pump is actuated must be increased. Oil FLOW gives the cylinder ram its SPEED of
movement and oil PRESSURE is the work force that lifts the load.
2. Hydraulic brake
The hydraulic brake is an arrangement of braking mechanism which uses brake fluid,
typically containing ethylene glycol, to transfer pressure from the controlling unit, which is
usually near the operator of the vehicle, to the actual brake mechanism, which is usually at
or near the wheel of the vehicle.
Construction
The most common arrangement of hydraulic brakes consists of the following:
 Brake pedal or lever
 A pushrod (also called an actuating rod)
 A master cylinder assembly containing a piston assembly (made up of either one or
two pistons, a return spring, a series of gaskets/ O-rings and a fluid reservoir)
 Reinforced hydraulic lines
 Brake calliper assembly usually consisting of one or two hollow aluminum or
chrome-plated steel pistons (called caliper pistons), a set of thermally conductive
brake pads and a rotor (also called a brake disc) or drum attached to an axle.
The system is usually filled with a glycol-ether based brake fluid (other fluids may also be
used).
System Operation
Within a hydraulic brake system, as the brake pedal is pressed, a pushrod exerts force on
the piston(s) in the master cylinder causing fluid from the brake fluid reservoir to flow into a
pressure chamber through a compensating port which results in an increase in the pressure
of the entire hydraulic system. This forces fluid through the hydraulic lines toward one or
more callipers where it acts upon one or two calliper pistons sealed by one or more seated
O-rings which prevent the escape of any fluid from around the piston. The brake calliper
25

26. pistons then apply force to the brake pads. Subsequent release of the brake pedal/ lever
allows spring(s) to return the master piston(s) back into position. This relieves the hydraulic
pressure on the calliper allowing the brake piston in the calliper assembly to slide back into
its housing and the brake pads to release the rotor. The hydraulic braking system is designed
as a closed system: unless there is a leak within the system, none of the brake fluid enters or
leaves it, nor does it get consumed through use.
Advantages and disadvantages of Hydraulic Actuators
Hydraulic actuators are widely used in many systems such as drives of machine tools, rolling
mills, pressing, road and building machines, transport and agricultural machines. There are
great advantages of hydraulic actuator which differentiate them from mechanical and
electric transfers which explain such their widespread application.
Advantages:
1. Power-to-weight ratio: Hydraulic components, because of their high speed and
pressure capabilities, can provide high power output with vary small weight and size,
say, in comparison to electric system components. It is one of the reasons that
hydraulic equipment finds wide usage in aircrafts, where dead-weight must be
reduced to a minimum.
2. Infinitely variable control of gear-ratio in a wide range and an opportunity to create
the big reduction ratio which can be used in high power appliances for example to
open of the gate of a canal or a dam.
26

27. 3. Tall Condition and Overload Protection: A hydraulic actuator can be stalled without
damage when overloaded, and will start up immediately when the load is reduced.
The pressure relief valve in a hydraulic system protects it from overload damage. During
stall, or when the load pressure exceeds the valve setting, pump delivery is directed to tank
with definite limits to torque or force output. The only loss encountered is in terms of pump
energy. On the contrary, stalling an electric motor is likely to cause damage. Likewise,
engines cannot be stalled without the necessity for restarting.
4. Variable Speed and Direction: Most large electric motors run at adjustable, but constant
speeds. It is also the case for engines. The actuator of a hydraulic system, however, can be
driven at speeds that vary by large amounts and fast, by varying the pump delivery or using a
flow control valve. In addition, a hydraulic actuator can be reversed instantly while in full
motion without damage. This is not possible for most other prime movers
Disadvantages:
It is also necessary to highlight the disadvantages of hydraulic actuators:
1. Efficiency: Efficiency of a volumetric hydraulic actuator is a little bit lower, than efficiency of
mechanical and electric transfers, and during regulation it is reduced.
2. Conditions of operation: It’s operational condition influence its characteristics.
3. Hydraulic system is susceptible to contaminations & foreign object damage (FOD).
4. Mishandling and constant exposure to hydraulic fluid and its gas fumes without proper
equipment and precautions is a health risk.
Conclusion
Modern robotic systems are difficult. drives are a mechanical part of this systems. Three
types of drives are basically used now: electric, pneumatic and hydraulic. Each type has its
own advantages and disadvantages.
References
Artemieva T.V., Lisenko T.M. Hydraulic, hydromachines and hydropneumoactuator,
Moscow, 2005
4. Majumdar S.R. Oil Hydraulic Systems : Principles and Maintenance, McGraw-Hill, 2001
Ian C. Turner, Engineering Applications of Pneumatics and Hydraulics, Butterworth-
Heinemann, 1995
Appendices
.
27

28. .
28