Introduction to fluid power systems

INTRODUCTION TO FLUID POWER
SYSTEMS

Fluid Power System
• Fluid Power Systems are power transmitting assemblies
employing pressurized liquid or gas to transmit energy.
• Fluid power can be divided into two basic disciplines
> Hydraulics – Employing pressurised liquid
> Pneumatics – Employing Compressed gas

What is Hydraulics?
• Hydraulics is derived from the Greek word
- Hydor meaning Water
-Aulos meaning Pipe
Mechanical Power
Electrical Power
Fluid Power
Where T is Torque and N is speed
V is Voltage and I is current
P is liquid pressure and Q is flow rate
K is conversion constant

ADVATAGES OF HYDRAULIC SYSTEMS
Lubrication
Compactness
Components life
Safety
Variable speed
Reversible
Overload protection
Power to weight ratio
Multiple operations
Flexibility
Variable pressure
Maintenance

DISADVANTAGES OF HYDRAULIC SYSTEMS
Accuracy
Cost
Leakage
Prevention
Risk of fire

APPLICATIONS OF HYDRAULIC SYSTEMS
Hydraulic Jack
Hydraulic ram
Hydraulic press
Hydraulic elevators
Automobile industries
Aeronautical industry
Controller units
Machine tools

COMPONENTS OF HYDRAULIC SYSTEM

COMPONENTS OF HYDRAULIC SYSTEM
Reservoir system
Pump
Prime mover
Controlling device
Actuators
Pressure relief valve
Accessories

PASCAL LAW: Pascal Law – which is the basis for all
hydraulic systems, is named after the French Scientist – Blaise
Pascal, who established the law.

Do you remember Pascal’s Law?
It states that “ Pressure exerted anywhere in a confined fluid is
transmitted equally in all directions throughout the fluid.
• The basic idea behind all hydraulic system is based upon that
principle & can be simply stated as:
Force applied at one point is transmitted to another
point using an incompressible fluid.

FLUIDS FOR HYDRAULIC SYSTEM
Functions of Hydraulic Fluids
It transmits power.
Lubricates moving parts
It seals clearances
between moving parts
It dissipates heat

PROPERTIES OF HYDRAULIC FLUIDS
Lubrication
Viscosity
Viscosity index
Corrosion resistant
Demulsification
Chemical and Environmental stability
Fire resistance

PROPERTIES OF HYDRAULIC FLUIDS
Flash point
Pour point
Foam resistance
Good heat dissipation
Low co efficient of expansion
Non toxic
Easily available and easy to handle

CLASSIFICATION OF HYDRAULIC FLUIDS
1. Mineral oils
2. Fire resistant Fluids
a. Oil in water Emulsion
b. Water in oil Emulsion
c. Water glycol Fluids
3. Synthetic oils

Hydraulic Oils used in India
1. Servo system 311 6. Servo system 517

Requirements imposed on the Hydraulic fluids
1. Satisfactory flow properties
2. A high viscosity index
3. Good lubricating properties
4. Low vapour pressure to avoid cavitation
5. Compatibility with component materials
6. Chemically stable
7. Corrosion Protection
8. Rapid de-aeration and air separation
9. Good thermal conductivity
10. Fire resistance
11. Electrically insulating properties
12. Environmental acceptability

ADDITIVES
1. Oxidation inhibitors
2. Corrosion inhibitors
3. Antifoaming agents
4. Anti-wear additives
5. Viscosity index improvers
6. Pour point depressants
7. Friction modifiers
8. Detergents

EFFECT OF TEMPERATURE ON HYDRAULIC FLUID
1. Oxidation of the fuel
2. Formation of insoluble gums, varnishes, and acids
3. Deterioration of seals (they harden and leakage begins)
4. Loss of lubricity
5. Changes in viscosity

EFFECT OF PRESSURE ON HYDRAULIC FLUID
1. At elevated pressure the viscosity changes
2. Filtration and lubrication is difficult
3. Pressure surges

OIL SEALS
Function of seals
1. They prevent leakage.
2. They prevent dust and other particles from
entering into the system
3. They maintain pressure.
4. They enhance the service life and reliability of
the hydraulic system.

CLASSIFICATION OF SEALS
According to method of sealing :
I. Positive sealing.
II. Non-positive sealing.
According to area of application:
I. Static seal.
II. Dynamic seal.

1.Positive seals-allows no leakage
2.Non-positive seals-allow some leakage for lubrication
3.Static seals-compressed between 2 rigid stationary
parts
4.Dynamic seals-installed between 2 moving parts
a)Wears quicker

According to shape configuration:
O-ring seal.
T-ring seal.
V-ring seal.
U-cup ring.
Wiper ring.
According to application:
External sealing.
Internal sealing.

DESIGN OF SEALS
a. Characteristics of the mating surface.
b. Characteristics of the medium.
c. Design and nature of the seal and seal material.

MATERIALOFSEALS
Leather.
Metal.
Polymers, elastomers and plastics
Asbestos, nylon.

FACTORS FOR SEAL SELECTION
Working pressure and pressure range.
Environmental condition.
Fluid medium.
Dynamic or static application.
Temperature of the system.
Functional reliability and expected life.

CAUSES FOR SEAL FAILURE
Damaged or worn out shafts.
Incorrectly installed seals.
Incompatibility of the seal material and oil.
Groove design and shape.
Working temperature.
Eccentricity of mating surfaces.
Poor installation.
Lack of cleanliness.

PIPING
Iron & steel pipe were the first conductors used
for hydraulic systems.
Still, widely used today because of their low
cost.
Pipe interior should be clean, no rust or scale.
Use seamless pipe.

SIZING PIPE
Pipe is classified by nominal size & wall thickness.
Wall thickness can vary and is classed by a schedule
number. Schedule 10 to schedule 160.
“Standard” pipe is schedule 40.
ANSI sets standards for piping. Pipe has tapered
outside (male) threads.
Joints are sealed by an interference fit as the pipe
joints are tightened.
Pipe joint compound or Teflon tape help make a
secure pipe joint.
Hydraulic systems should use “dry seal” type threads.

PIPE SEALING
Pipe has tapered outside (male) threads.
Joints are sealed by an interference fit as the pipe
joints are tightened.
Pipe joint compound or Teflon tape help make a
secure pipe joint.
Hydraulic systems should use “dry seal” type
threads.

PIPES/TUBES
Rigid pipes-Steel Seamless(Joint less) pipes
Semi-rigid pipes-Tubes
( copper/aluminium/steel)
Flexible pipes-Nylon/Plastic/PVC.

MATERIALS FOR PIPES
Steel.
Copper.
Aluminium.
Zinc coated Galvanized pipes.

TYPES OF PIPE CONNECTORS
Straight coupling.
Tee joint.
Bracketed elbow.
Bend.
Elbow.
Cross tee.
Split flange.
Reducer.
Blanking plug.
Nipple.

HOSE MATERIALS
1.Plastic:
Nylon.
Braided nylon hose.
Polyvinyl chloride.
Textile braided hoses.
Teflon.
2.Homogeneous synthetic rubber:
Buna N (nitrile rubber)
Neoprene.
Butyl.

HOSE SELECTION CRITERIA
Pressure .
Temperature.
Fluid compatibility.
Degree of vibration.
Abrasion resistance.
Flexibility.
Shock and mechanical load.

ADVANTAGES
Rubber hoses can be well equipped with quick
connect-disconnect end fittings.
It can be manufactured in long lengths.
It is capable of withstanding to very high
pressures.
They can absorb very heavy shocks than rigid
tubes.

DISADVANTAGES
Very poor in abrasion resistance.
Poor in resisting whether condition.
Initial cost is very high.
They can damage due to incompatible oil.

Filters and Strainers
For proper operation and long service life of a hydraulic system,
oil cleanliness is of prime importance. Hydraulic components are
very sensitive to contamination. The cause of majority of
hydraulic system failures can be traced back to contamination.
Hence, filtration of oil leads to proper operation and long service
life of a hydraulic system.
Strainers and filters are designed to remove foreign particles from
the hydraulic fluid. They can be differentiated by the following
definitions:

1. Filters: They are devices whose primary function is the
retention, by some fine porous medium, of insoluble
contaminants from fluid. Filters are used to pick up smaller
contaminant particles because they are able to accumulate them
better than a strainer. Generally, a filter consists of fabricated
steel housing with an inlet and an outlet. Because the filter
element is not capable of being cleaned, that is, when the filter
becomes dirty, it is discarded and replaced by a new one. Particle
sizes removed by filters are measured in microns. The smallest
sized particle that can be removed is as small as 1 μm.

2. Hydraulic strainers: A strainer is a coarse filter. Fluid flows
more or less straight through it. A strainer is constructed of a fine
wire mesh screen or of screening consisting of a specially
processed wire of varying thickness wrapped around metal frames.
It does not provide as fine a screening action as filters do, but
offers less resistance to flow and is used in pump suction lines
where pressure drop must be kept to a minimum. A strainer is a
device whose function is to remove large particles from a fluid
using a wire screen. The smallest sized particle that can be
removed by a strainer is as small as 0.15 mm or 150 μm.

Causes of Contamination
The causes of contamination are as follows:
 Contaminants left in the system during assembly or subsequent
maintenance work.
 Contaminants generated when running the system such as wear
particles, sludge and varnish due to fluid oxidation and rust and
water due to condensation.
 Contaminants introduced into the system from outside. These
include using the wrong fluid when topping up and dirt particles
introduced by contaminated tools or repaired components.

Filters may be classified as follows:
1. According to the filtering methods:
Mechanical filters: This type normally contains a metal or cloth
screen or a series of metal disks separated by thin spacers.
Mechanical filters are capable of removing only relatively coarse
particles from the fluid.
 Absorption filters: These filters are porous and permeable
materials such as paper, wood pulp, diatomaceous earth, cloth,
cellulose and asbestos. Paper filters are impregnated with a resin to
provide added strength. In this type of filters, the particles are
actually absorbed as the fluid permeates the material. Hence, these
filters are used for extremely small particle filtration.

 Adsorbent filters: Adsorption is a surface phenomenon and
refers to the tendency of particles to cling to the surface of the
filters. Thus, the capacity of such a filter depends on the
amount of surface area available. Adsorbent materials used
include activated clay and chemically treated paper.

2. According to the size of pores in the material:
 Surface filters: These are nothing but simple screens used to
clean oil passing through their pores. The screen thickness is very
thin and dirty unwanted particles are collected at the top surface of
the screen when the oil passes, for example, strainer.
 Depth filters: These contain a thick-walled filter medium
through which the oil is made to flow and the undesirable foreign
particles are retained. Much finer particles are arrested and the
capacity is much higher than surface filters.

 Intake or inline filters
(suction strainers): These are
provided first before the pump to protect
the pump against contaminations in the
oil as shown in Fig. 1.15. These filters
are designed to give a low pressure drop,
Otherwise the pump will not be able to
draw the fluid from the tank. To achieve
low pressure drop across the filters, a
coarse mesh is used. These filters cannot
filter out small particles.
Figure
3. According to the location of filters:

Pressure line filters (high-
pressure filters): These are
placed immediately after the
pump to protect valves and
actuators and can be a finer and
smaller mesh. They should be
able to withstand the full system
pressure. Most filters are
pressure line filters.

Return line filters (low-pressure filters): These filters filter
the oil returning from the pressure-relief valve or from the
system, that is, the actuator to the tank. They are generally
placed just before the tank. They may have a relatively high
pressure drop and hence can be a fine mesh. These filters have
to withstand low pressure only and also protect the tank and
pump from contamination.

1. Depending on the amount of oil
filtered by a filter:
 Full flow filters: In this type,
complete oil is filtered. Full flow of oil
must enter the filter element at its inlet
and must be expelled through the outlet
after crossing the filter element fully.
This is an efficient filter. However, it
incurs large pressure drops. This
pressure drop increases as the filter gets
blocked by contamination. FULL FLOW FILTER

Proportional filters
(bypass filters): In some
hydraulic system applications,
only a portion of oil is passed
through the filter instead of
entire volume and the main
flow is directly passed
without filtration through a
restricted passage.
X ZY
VENTURI
FILTER ELEMENT
PROPORTIONAL FLOW FILTER

GASES IN HYDRAULIC FLUIDS
1. Free air
2. Entrained air
3. Dissolved gases

Eliminating of pump cavitation
1. Keep suction line velocities below 1.2m/s
2. Keep pump inlet lines as short as possible.
3. Minimize the number of fittings in the pump inlet line.
4. Mount the pump as close as possible to the reservoir.
5. Use low pressure drop-pump inlet filters or strainers.
6. Use a properly designed reservoir that will remove the entrained
air from the fluid before it enters the pump inlet line.
7. Use proper oil, as recommended by the pump manufacturer.
8. Keep the oil temperature from exceeding the recommended
maximum temperature level (usually 650C).

Beta Ratio of Filters
Filters are rated according to the smallest size of particles they can trap. Filter
ratings are identified by nominal and absolute values in micrometers. A filter
with a nominal rating of 10 μm is supposed to trap up to 95% of the entering
particles greater than 10 μm in size. The absolute rating represents the size of
the largest pore or opening in the filter and thus indicates the largest size
particle that could go through. Hence, absolute rating of a 10 μm nominal
size filter would be greater than 10 μm.
A better parameter for establishing how well a filter traps particles is called
the beta ratio or beta rating. The beta ratio is determined during laboratory
testing of a filter receiving a steady-state flow containing a fine dust of
selected particle size. The test begins with a clean filter and ends when
pressure drop across the filter reaches a specified value indicating that the
filter has reached the saturation point. This occurs when contaminant
capacity has been reached.

By mathematical definition, the beta ratio equals the number of
upstream particles of size greater than Nμm divided by the
number of downstream particles having size greater than Nμm
where N is the selected particle size for the given filter. The
ratio is represented by the following equation:
Beta ratio =
No. of upstream particles of size > Nμm
No. of downstream particles of size > N μm
A beta ratio of 1 would mean that no particle above specified N are trapped
by the filter. A beta ratio of 50 means that 50 particles are trapped for every
one that gets through. Most filters have a beta ratio greater than 75:
Beta efficiency = No. of upstream particles -No. of downstream particles
No. of upstream particles
Thus,
Beta efficiency = 1 − 1
Beta ratio

FILTER SPECIFICATIONS
Mesh number
Standard sieve number
Beta ratio
Weight of particles to be held
Flow capacity
Pressure rating
Pressure drop

FILTERING MATERIALS
Absorbent filters are divided into 2 types; depth &
surface filters.
Surface filters are for coarse filtration
Depth type filters are better for fine filtration.
Cellulose, synthetics, glass fibers or a combination of
these are used to make filters.

Heat exchangers
Heat is generated in hydraulic systems because no component
can operate at 100% efficiency. Significant forces of heat
include the pump, pressure relief valves, and flow control
valves. Heat can cause the hydraulic fluid temperature to
exceed its normal operating range of 400C to 700C. Excessive
temperature hastens the oxidation of the hydraulic oil and
causes it to become too thin. This deterioration of seals and
packing and accelerates wear between closely fitting parts of
hydraulic components of valves, pumps and actuators.

The steady-state temperature of a fluid of a hydraulic system
depends on the heat-generation rate and the heat-dissipation
rate of the system. If the fluid operating temperature in a
hydraulic system becomes excessive, it means that the heat-
generation rate is too large relative to the heat-dissipation rate.
Assuming that the system is reasonably efficient, the solution is
to increase the heat-dissipation rate. This is accomplished by
the use of coolers, which are commonly called “heat
exchangers”.

Types of Heat exchangers
1. Shell and Tube type
The shell and tube type has a series of tubes inside a closed
cylinder. The oil flows through the small tubes, and the fluid
receiving the heat (typically water) flows around the small
tubes. Routing of the oil can be done to produce a single pass
(oil enters one end and exists the other end) or a double pass (oil
enters one end, makes a u-turn at the other end, and travels back
to exit the same end it entered).

2. Finned tube
The finned tube exchanger is used for oil-to-air exchange. The
air may be forced through the exchanger with a fan or may flow
naturally. If an oil cooler is used on a mobile machine, it is the
finned tube type.

Hydrostatic Systems
A hydrostatic system uses fluid pressure to transmit power.
Hydrostatics deals with the mechanics of still fluids and uses the
theory of equilibrium conditions in fluid. The system creates high
pressure, and through a transmission line and a control element,
this pressure drives an actuator (linear or rotational). The pump
used in hydrostatic systems is a positive displacement pump. The
relative spatial position of this pump is arbitrary but should not be
very large due to losses (must be less than 50 m). An example of
pure hydrostatics is the transfer of force in hydraulics.

Hydrodynamic Systems
Hydrodynamic systems use fluid motion to transmit power. Power
is transmitted by the kinetic energy of the fluid. Hydrodynamics
deals with the mechanics of moving fluid and uses flow theory.
The pump used in hydrodynamic systems is a non-positive
displacement pump. The relative spatial position of the prime
mover (e.g., turbine) is fixed. An example of pure hydrodynamics
is the conversion of flow energy in turbines in hydroelectric
power plants.
In oil hydraulics, we deal mostly with the fluid working in a
confined system, that is, a hydrostatic system.

Objective Type Questions
Fill in the Blanks
1. Fluid power is the technology that deals with the generation,
_______and transmission of forces and movement of mechanical
elements or systems.
2. The main objective of fluid transport systems is to deliver a
fluid from one location to another, whereas fluid power systems
are designed to perform _______.
3. There are three basic methods of transmitting power:
Electrical, mechanical and _______.
4. Only ________are capable of providing constant force or
torque regardless of speed changes.
5. The weight-to-power ratio of a hydraulic system is
comparatively _______than that of an electromechanical system.

State True or False
1. Hydraulic lines can burst and pose serious problems.
2. Power losses and leakages are less in pneumatic
systems.
3. Pneumatic system is not free from fire hazards.
4. Hydraulic power is especially useful when performing
heavy work.
5. Water is a good functional hydraulic fluid.

Review Questions
1. Define the term fluid power.
2. Differentiate between fluid transport and fluid power systems.
3. State Pascal's law. Explain with a neat sketch, the basic
hydraulic system with respect to force and pressure in an enclosed
tank.
4. Sketch and explain the structure of a hydraulic control system.
5. Explain the benefits and drawbacks of using water as a
hydraulic fluid.
6. Name 10 hydraulic applications.
7. List five advantages and five disadvantages of hydraulics.
8. What is a fluid? What are the functions and characteristics of
hydraulic fluids
9. List the classification of hydraulic fluids.
10.Explain the effect of temperature and pressure on hydraulic
fluid.

11. What is the main difference between an open-loop and a closed-loop fluid
power system?
12. What is the importance of seals in hydraulic fluid system? List the functions
of seals.
13. What are the types of hydraulic conductors? Explain.
14. What are filters ? Classify and explain them briefly.
15. List any five applications of fluid power systems.
16. Define heat exchangers. What are the types of heat exchangers ?
16. Discuss in detail the future of fluid power industry in India.

Answers
Fill in the Blanks
1. Control
2. Work
3. Fluid power
4. Fluid power systems
5. Less
State True or False
1. True
2. True
3. False
4. True
5. False

Introduction to fluid power systems

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