9. Unit 1: Introduction to fluid power
Objective of subject: To understand design, analysis, operation, maintenance and application of fluid
power.
What is fluid power
Technology that deals with the generation, control and transmission of power using pressurized fluids.
Used to push, pull, regulate or drive virtually all machines of modern industry eg.
1) Steers and brakes of automobile
15. Fluid power is called hydraulics when fluid is liquid and is called
pneumatics when the fluid is gas.
Hydraulic systems use liquids such as petroleum oil, synthetic oil
and water. The first hydraulic fluid used was water, it has many
deficiencies such as freezes readily, poor lubricant and tends to rust
metal components.
There are two different types of fluid systems i.e. fluid transport
and fluid power
Fluid transport: used to transport fluid from one location to another.
E.g. pumping stations, cross country gas lines, chemical processing
etc.
18. Pressure applied to piston “A” is transferred
equally to a piston of the same size “B”
because pressure is the same throughout the
system.
Basic
principle
19. If the surface area of piston “B” is 10 times
the surface area of piston “A”, then the force
applied to piston “A” is multiplied 10 times as
the force exerted on piston “B”. pressure is
equal throughout the system.
hydraulic
advantage
21. Liquids provides very rigid medium for transmitting of power,
operate under high pressure and provide huge forces and torques with
utmost accuracy and precision.
On the other hand pneumatic system exhibit spongy characteristics
due to compressibility of air. It can be used in applications where low
pressures can be used.
Hydraulic chain saw Pneumatically
controlled dextrous hand
22. Advantages of fluid power
Power can be transmitted by three basic methods i.e. electrical,
mechanical and fluid power. Most applications use combination of
three methods to obtain efficienct overall system.
Fluid power can transmit power economically over greater distances
than mechanical types but restricted to shorter distances than electrical
systems.
Fluid power is versatile and manageable, it is not hindered by the
geometry of machine as in case of mechanical system.
Advantages:
1) Ease and accuracy of control: By the use of simple levers and push
buttons, operator can start, stop, speed up and slow down and
position forces with very close tolerances.
23. Drawbacks of fluid power
Hydraulic oils are messy, leakage is impossible to eliminate
completely
Hydraulic lines can burst resulting in injuries to people due to high
speed oil jets and flying pieces of metal if proper design is not
implemented.
Prolong exposure to loud noise, such as that produced by pumps can
result in loss of hearing.
Hydraulic oil can cause fire if leaks.
24. Components of a fluid power system
Six basic components are required in hydraulic system
1. A Tank (Reservoir) to hold the hydraulic oil
2. A pump to force the oil through the system
3. An electric motor or other source to drive the pump
4. Valves to control oil direction, pressure and flow rate
5. An actuator to convert the pressure of the oil into mechanical
force or torque to do useful work
6. Piping which carry oil from one location to other
25. Input / Output
Force input on a hydraulic
system or component results
in transfer of power to output
of force by the system or
component.
26. Pump
Functions in a hydraulic
system by pressurizing
and moving fluid from one
part of the system to
another.
27. Hydraulic pumps are usually one
of four types:
PISTON
GEAR
VANE
INTERNAL ROTOR
28. Piston Pumps
• Work well at PSI’s of 2000 or more
• Single piston pump used in bottle jacks
• Require several pistons working together to
generate enough volume for tractor
applications
• Necessarily involve many moving parts
29. Gear Pumps
• Work well at 1500 PSI and below
• Work with a minimum of moving
parts
• Less expensive to manufacture
than piston type pumps
41. Check Valve
Restricts flow of fluid to only one direction;
allows pressure to build up or be maintained.
42. Spool Valve
Controls direction of flow of fluid in a hydraulic system
to cause the different parts of the system to function.
43.
44.
45. Hydraulic power transmission
• Hydraulic power transmission:
Hydro = water, aulos = pipe
The means of power transmission is a liquid (pneumatic gas)
Hydrodynamic power transmission:
• Turbo pump and turbine
• Power transmission by kinetic
energy of the fluid
• Still the relative spatial position is
fixed
• Compact units
Hydrostatic power transmission:
• Positive displacement pump
• Creates high pressure and through
a transmission line and control
elements this pressure drives an
actuator (linear or rotational)
• The relative spatial position is
arbitrary but should not be very
large because of losses (< 50 m)
A continuously variable transmission is possible
Most of this lecture will be about hydrostatic systems (in common language it is
also called simply hydraulics)
46. Structure of a hydrostatic drive
Aggregate Control
elements
Actuator
These components and their interaction is the subject of
this semester
Valves,
determining the
path, pressure, flow
rate of the working
fluid
Elements doing
work
• Linear
• Rotational
• Swinging
Pump, motor
Fluid reservoir
Pressure relief
valve
Filter
Piping
47. 2005/2006 I. Hydraulic and Pneumatic Systems 47
A typical hydraulic system
1 – pump
2 – oil tank
3 – flow control valve
4 – pressure relief valve
5 – hydraulic cylinder
6 – directional control valve
7 – throttle valve
48. Advantages of hydrostatic drives
Simple method to create linear movements
Creation of large forces and torques, high energy density
Continuously variable movement of the actuator
Simple turnaround of the direction of the movement, starting
possible under full load from rest
Low delay, small time constant because of low inertia
Simple overload protection (no damage in case of overload)
Simple monitoring of load by measuring pressure
Arbitrary positioning of prime mover and actuator
Large power density (relatively small mass for a given power
compared to electrical and mechanical drives)
Robust (insensitive against environmental influences)
49. Disadvantages of hydrostatic drives
Working fluid is necessary (leakage problems, filtering, etc.)
It is not economic for large distances
50. Hydraulic fluid
Hydraulic fluid characteristic have crucial effect on equipment
performance and life. Clean, high quality fluid achieve efficient
hydraulic system operation.
Modern hydraulic system contain fluid with special additives to
provide desired characteristics
Hydraulic fluid has the following four primary functions
1. Transmit power
2. Lubricate moving parts
3. Seal clearances between mating parts
4. Dissipate heat
51. Hydraulic fluid
Hydraulic fluid characteristic have crucial effect on equipment
performance and life. Clean, high quality fluid achieve efficient
hydraulic system operation.
Modern hydraulic system contain fluid with special additives to
provide desired characteristics
Hydraulic fluid has the following four primary functions
1. Transmit power
2. Lubricate moving parts
3. Seal clearances between mating parts
52. Hydraulic fluid should have following properties
1. Good lubricity
2. Ideal viscosity
3. Chemical stability
4. Compatibility with system materials
5. High degree of incompressibility
6. Fire resistance
7. Good heat transfer capability
8. Low density
9. Foam resistance
10.Nontoxicity
11.Low volatility
53. Fluids
It refers to both Gases and Liquids
Liquid is a fluid that for a given mass, will have a definite volume
independent of the shape of its container.
Liquids are considered to be
incompressible i.e. their volume does not
change with pressure change. This is not
exactly true, but the change in volume
due to pressure changes is so small that
it is ignored for most engineering
applications.
Gases are readily compressible, their
volume will vary to fill vessel containing
54. Properties of fluids are density, pressure, compressibility, viscosity
and viscosity index.
1. Specific weight, density and specific gravity
Weight: All objects solids or fluids are pulled towards the center of
the earth by the force of attraction. This force is called as weight
and is proportional to the object mass, and defined by
F = W = mg
Specific weight: It is the ratio of weight of fluid to its volume,
denoted by symbol ϒ.
55. 2. Force, Pressure and Head
Pressure is defined as force per unit area, i.e. amount of force acting
over unit area
P = F/A
Head: 1 meter height of water is commonly called a pressure head
57. Viscosity
It is single most important property of hydraulic fluid. It is measure
of fluid resistance to flow, when fluid viscosity is low, the fluid flows
easily and is thin in appearance.
Ideal viscosity for a given hydraulic system is a compromise.
Too high viscosity results in
1. High resistance to flow, sluggish operation
2. Increased power consumption due to frictional losses
3. Increased pressure drop through valves and lines
4. High temperatures caused by friction
Too low viscosity results in
59. Kinematic viscosity
Calculations in hydraulic system often involve the use of kinematic
viscosity rather than absolute viscosity. It is the ratio of absolute
viscosity to the density of fluid. Unit m2/s
60. Hydraulic fluids - tasks
They have the following primary tasks:
o Power transmission (pressure and motion
transmission)
o Signal transmission for control
Secondary tasks:
o Lubrication of rotating and translating
components to avoid friction and wear
o Heat transport, away from the location of heat
generation, usually into the reservoir
o Transport of particles to the filter
o Protection of surfaces from chemical attack,
especially corrosion
61. Hydraulic fluids - requirements
Functional
o Good lubrication characteristics
o Viscosity should not depend strongly on
temperature and pressure
o Good heat conductivity
o Low heat expansion coefficient
o Large elasticity modulus
Economic
o Low price
o Slow aging and thermal and chemical stability
long life cycle
62. Hydraulic fluids - requirements (contd.)
Safety
o High flash point or in certain cases not
inflammable at all
o Chemically neutral (not aggressive at all
against all materials it touches)
o Low air dissolving capability, not inclined to
foam formation
Environmental friendliness
o No environmental harm
o No toxic effect
63. Hydraulic fluid types
1. Water (3%)
2. Mineral oils (75%)
3. Not inflammable fluids (9%)
4. Biologically degradable fluids
(13%)
5. Electrorheological fluids (in
development)
64. Hydraulic fluid types (contd.)
- Clear water
- Water with additives
o Oldest fluid but nowadays there is a renaissance
o Used where there is an explosion or fire danger or hygienic problem:
Food and pharmaceutical industry, textile industry, mining
1. Water:
Advantages:
No environmental pollution
No disposal effort
Cheap
No fire or explosion danger
Available everywhere
4 times larger heat conduction coefficient than mineral oils
2 times higher compression module than mineral oils
Viscosity does not depend strongly on temperature
65. Hydraulic fluid types (contd.)
1. Water:
Disadvantages:
Bad lubrication characteristics
Low viscosity (problem of sealing, but
has good sides: low energy losses)
Corrosion danger
Cavitation danger (relatively high
vapour pressure)
Limited temperature interval of
applicability (freezing, evaporating)
Consequences: needs low tolerances and very good materials (plastics, ceramics,
stainless steel) components are expensive
66. Hydraulic fluid types (contd.)
- Without additives
- With additives
o „Conventional” use, stationary hydraulics
o Always mixtures of different oils, often with additives
Additives:
- decrease corrosion
- increase life duration
- improve temperature dependence of viscosity
- improve particle transport
2. Mineral oil:
Advantages:
Good lubrication
High viscosity (good for sealing,
bad for losses)
Cheap
Disadvantages:
Inflammable
Environmental pollution
67. Hydraulic fluid types (contd.)
- Contains water
- Does not contain water
o mines, airplane production, casting, rolling, where there is explosion
and fire danger
o Water-oil emulsions (oil synthetic) or water-free synthetic liquids
3. Non inflammable fluids:
Disadvantages:
Higher density, higher losses, more inclination to cavitation
Limited operational temperature < 55 °C
Worse lubrication characteristics, reduction of maximum load
Worse de-aeration characteristics
Sometimes chemically aggressive against sealing
materials
68. Hydraulic fluid types (contd.)
- Natural
- Synthetic
o Environmental protection, water protection
o Agricultural machines
o Mobile hydraulics
Characteristics similar to mineral oils but much more
expensive.
If the trend continues its usage expands, price will drop.
4. Biologically degradable fluids:
