Industrial Fluid Power

141 Pages·2011·3.39 MB·English

Dhanvantari College of Engineering, Nasik

Page | 1

Fluid Power System
B.E. Mechanical
SEM - II

• ADVANTAGES OF FLUID POWER

1. Fluid power provides highly accurate and precise movement of the actuator with relative
ease. This is particularly important in applications such as machine tool movement control
where tolerances are often specified in microns and must be repeatable during several million
cycles.
2. Fluid power is not hindered by the geometry of the machine and it can be used to actuate
devices that are located away from power source. This is a decided advantage over
mechanical systems that are dependent upon the machine geometry.
3. The power capacity of a fluid system is extremely large and is limited only by the strength
capacity of the material.
4. Fluid power provides flexible and easy control of variable force, distance and speed.
5. Fluid power provides an efficient method of multiplying forces.
6. Fluid power can be varied from a delicate touch of a few ounces to a gigantic force of several
hundred tons [@36000 tons or more].
7. In fluid power, small forces can be amplified to control large forces thereby providing
leverage.
8. Only fluid power systems are capable of providing constant force or torque regardless of
speed changes.
9. Fluid power systems provide instant and smooth reversible motion.
10. Fluid power systems also provide infinitely variable speed control.
11. Fluid power systems provide fast response to controls.
12. Fluid power systems provide automatic protection against overload.
13. No harm is done to a fluid power system should it stall.
14. Torque output continues even if hydraulic motor is stalled.
15. Fluid power systems have the highest power to weight ratio of any known power source.
16. Fluid power systems use fewer moving parts than comparable mechanical or electrical
systems. Hence, they are simpler to maintain and operate.
17. Fluid power systems are safe, economical, efficient and reliable.
18. Fluid power systems are compatible with either electrical, electronic or mechanical means of
control.
19. Fluid power is readily available.
20. Pneumatic power is free from fire hazards and hence preferred to electrical systems;
hydraulic power is self lubricating thereby reducing wear of moving parts and hence
preferred to mechanical systems.

• APPLICATION OF FLUID POWER

Fluid power systems are widely used in industry to perform hundreds of important
tasks. Some of these important applications are studied in this article.

• Fluid power application in machine tool

Over 90% of all machine tools are controlled or operated with fluid power. Due to this

it has been possible to summaries

Dhanvantari College of Engineering, Nasik

Fluid power application in machine tool

Over 90% of all machine tools are controlled or operated with fluid power. Due to this

summaries the application in tabular form (Table 1.3) for this industry.

Dhanvantari College of Engineering, Nasik

Over 90% of all machine tools are controlled or operated with fluid power. Due to this

n in tabular form (Table 1.3) for this industry.

• Fluid power applications in material handling

Fluid power controls the telescopic mast

and grabbing jaws of the forklift trucks used in
industry. Conveyor hois
tilting ramps and levelling
examples of fluid powered tools for modern
material handling.

A classic example showing fluid power

application in material handling is the hoist used
by automobile service stations t
servicing. This application of fluid power
generally uses both a liquid and a gas.

Tilting a container for the purpose of

emptying or discharging its contents is carried out as a pivoting motion performed by
cylinders acting on levers.
handling of bulk materials. (Ref. Fig. 1.1)

The feeder shown in Fig. 1.2, is used to pick up a part from an overhead conveyor and locate
it in a machining station. The verti
the conveyor carrier and locating it in the production station. Angular motion around the
vertical axis results from a second horizontally mounted cylinder whose piston rod is attached
to a rack engaging with pinion.

• Fluid power for automation

Parts supplied by vibratory cup conveyor normally need to be selected and advanced

at the end of the magazine chute. A set is to be up designed to take the parts singly from the
chute and load them on the work
advantageously used for automation.

Dhanvantari College of Engineering, Nasik

Fluid power applications in material handling

Fluid power controls the telescopic mast

grabbing jaws of the forklift trucks used in

industry. Conveyor hoists, cranes, dampers,

levelling docks are a few other

examples of fluid powered tools for modern

A classic example showing fluid power

application in material handling is the hoist used
by automobile service stations to raise a car for
servicing. This application of fluid power
generally uses both a liquid and a gas.

Tilting a container for the purpose of

emptying or discharging its contents is carried out as a pivoting motion performed by
cylinders acting on levers. Fluid power operated tilting units are used as accessory devices for

erials. (Ref. Fig. 1.1)

The feeder shown in Fig. 1.2, is used to pick up a part from an overhead conveyor and locate
it in a machining station. The vertical cylinder stroke controls the removal of the part from
the conveyor carrier and locating it in the production station. Angular motion around the
vertical axis results from a second horizontally mounted cylinder whose piston rod is attached

aging with pinion.

Fluid power for automation

Parts supplied by vibratory cup conveyor normally need to be selected and advanced

at the end of the magazine chute. A set is to be up designed to take the parts singly from the
chute and load them on the work holders of a rotary index table. Fluid power is more
advantageously used for automation.

Dhanvantari College of Engineering, Nasik

emptying or discharging its contents is carried out as a pivoting motion performed by

Fluid power operated tilting units are used as accessory devices for

The feeder shown in Fig. 1.2, is used to pick up a part from an overhead conveyor and locate

cylinder stroke controls the removal of the part from

the conveyor carrier and locating it in the production station. Angular motion around the
vertical axis results from a second horizontally mounted cylinder whose piston rod is attached

Parts supplied by vibratory cup conveyor normally need to be selected and advanced

at the end of the magazine chute. A set is to be up designed to take the parts singly from the

holders of a rotary index table. Fluid power is more

Dhanvantari College of Engineering, Nasik

Page | 4

• Fluid Power For Forming Operation
Force is a major criterion in motion taking place to form metals. Fluid powered actuators
(cylinders) in varied types and sizes are the dominating elements in the motions required for
metal forming.

• Fluid power in agriculture

The need for food and fibre production has caused unprecedented leadership in
agricultural equipment development. They are extensively used in forage harvesters,
backhoes, chemical sprayers and organic fertilizer spreaders. They are used for controlled
apportioning and supply of feed for animals, collection and removal of manure in mass
raising of animals, wool shearing and slaughtering. They are mainly used for tilting, lifting
and swivelling gear equipment for fieldwork, crop protection and weed control.

• Fluid Power in Construction

Another sector of our economy that has benefited from the brute power of hydraulics
and pneumatics is the construction industry. Crawler tractors, road graders, bucket loader,
trenchers, backhoes, hydraulic shovels, pan scrapers, bull dozers, vibrator screens, crushers,
rollers and asphalt mixers are just a few of the many applications. Even the smaller tools of
construction such as rock drills used for breaking of concrete are powered by fluid.

Bin gate controls used in large concrete mixing plant, weight batching mixer controls,
forming presses for concrete products, brick and block conveying and handling equipment,
spray painting equipment are some of the fluid powered equipment used in the building
industry.

• Fluid Power in aviation

Aerospace and aviation applications of fluid power include controlling of landing of
landing gear, elevens, rudders, elevators, payload bays and trim tabs.

• Fluid power in marine industry

Fluid power in marine shipping is basically used for automatic helmsman ship cargo
handling, actuation of hatch covers and a vast bulk of dock and shipyard machinery.

Under sea application of hydraulics include submersibles that are used for exploration
and development of the ocean resources.

• Fluid power in Transportation system
Transportation systems provide examples of the most varied used of fluid power.

Power brakes (both high pressur

Fig. 1.3 and Fig. 1.4) power windows, and powered seat adjustments are all typical fluid
power devices. Hydrostatic
systems are dampened with hydraulic shock
using the compressible nature of gases as the basis for air

Fig. 1.5 shows a hydraulic wheel motor which is a recent addition to the ransportation

line. They give almost unlimited flexibility to the design which can mount the power plant in
a convenient location to power individually driven wheels as they are required to support the
load and provide traction to propel it in almost any direction with a variety of step l
speeds.

Dhanvantari College of Engineering, Nasik

Power brakes (both high pressure and vacuum assisted), power steering systems, (Ref.

Fig. 1.3 and Fig. 1.4) power windows, and powered seat adjustments are all typical fluid

Hydrostatic transmissions are very common in all types of vehicles. uspension

with hydraulic shock observers, and some combine pneumatics by

using the compressible nature of gases as the basis for air-oil suspension systems.

Fig. 1.5 shows a hydraulic wheel motor which is a recent addition to the ransportation

ost unlimited flexibility to the design which can mount the power plant in

a convenient location to power individually driven wheels as they are required to support the
load and provide traction to propel it in almost any direction with a variety of step l

Dhanvantari College of Engineering, Nasik

assisted), power steering systems, (Ref.

Fig. 1.3 and Fig. 1.4) power windows, and powered seat adjustments are all typical fluid

very common in all types of vehicles. uspension

observers, and some combine pneumatics by

oil suspension systems.

Fig. 1.5 shows a hydraulic wheel motor which is a recent addition to the ransportation

ost unlimited flexibility to the design which can mount the power plant in

a convenient location to power individually driven wheels as they are required to support the
load and provide traction to propel it in almost any direction with a variety of step less

Dhanvantari College of Engineering, Nasik

Page | 6

• Fluid power in Food – Processing Industry

The food processing industries include bakery product, dairy products, meat and fist
product and beverage industry. Extreme cleanliness and accurate weighing and vacuum seal
packing is of utmost importance. Hence, pneumatic power is used extensively in various food
processing equipment such as grinders, pulverizes, shifters, screeners, presses, cutters,
dehydrators, conveyors, oven, agitators, wrappers and cartooning machinery. But the most
important application is in canning – from the moment the can is formed, until it is finally
filled and labelled.

• Fluid power in Mining Industry

Fluid power has wide application in continuous coal and mineral mining and
quarrying. The coal-mining machine which can dig and load coal at the rate of 2 tons/min, is
equipped with several hydraulic cylinders, jacks and motors. Others mining application using
fluid power pneumatic include rock drills, hammers, chippers, hydraulic track laying
machines, shuttle cars, roof bottling machines and conventional hydraulic cranes, hoist and
jacks.

• Fluid power in Utilities and Communication

Utilities and communication are two industries where the use of flued power is vital.
Line utility vehicles are used to support persons working above the ground.

Other applications include hydraulic trenchers, cable boring machines, earth augers,
pipe laying machines and tempers.

• Fluid power of Numerical Control

The dimensions from drawing and the machining data are coded and put on control
tape. When this control tape is fed to the machine, the machining data is translated into
motion and force for hydraulic control operations hydraulic actuation provides a very suitable
link between the control tape signals and the actual machining motions because of ease of
control, rapid response, variable speed and amplification force.

• Some interesting fluid power applications

(a) The actuators that blink the eyes and move the fingers on the almost human
mannequins at Disney world are hydraulically operated.

(b) Hinged doors can be actuated pneumatically.

• Application with scope for technological advances

Three applications given recent attention in fluid power industry are miniature
pneumatic, moving part logic and fluidics. Miniature pneumatics makes use of small air
powered components such as cylinders and valves to carry out small assigned tasks as well as
to control large components. Moving part logic and fluidics make use of logic elements with
function similar to several electronic counterparts such as capacitors, resistors and amplifiers
to control other hydraulic and pneumatic systems.

• STATIC AND DYNAMIC POWER TRANSMISSIONS

According to general dictionary definition, hydraulics is that branch of physics, which
deals with the utilization of energy (either kinetic or pressure) of a liquid to do work.

Hence, a hydraulic device can perform work by

Dhanvantari College of Engineering, Nasik

Page | 7

(a) utilizing the momentum of a moving liquid, or
(b) utilizing the pressure energy of a confined fluid.
Hence, a hydraulic power transmission is classified into two general types as shown in Fig.
1.6

Hydraulic Power Transmission

Hydrodynamic / Hydrokinetic

Hydrostatic
It use the kinetic energy of a high

It uses high pressure and relatively

Velocity flow of fluid.

Low velocities of fluid.

Fig. 1.6: Classification of Hydraulic Power Transmission.

• Hydrodynamic / Hydrokinetic Power Transmission: The branch of hydraulic, which uses
the impact or momentum, or kinetic energy of a moving liquid to transmit power is called
hydrodynamics.

e.g.: Centrifugal pumps, and Water turbines etc.
• Hydrostatic Power Transmission: The branch of hydraulics which uses the pressure force
obtained by pushing a confined fluid to transmit power is called as hydrostatics.

e.g. Hydraulic jacks, hydraulic rams, hydraulic presses, hydraulic elevators etc.

The term hydrostatic transmission refers to the use of hydraulic pumps and motors for
converting fluid power into mechanical rotary motion.

A brief comparison of hydrodynamic and hydrostatic transmission is given in the
Table 1.4.

Table 1.4
Hydrostatic Transmission
Hydrodynamic
Transmission

1. A change in fluid pressure
yields output energy.
2. Positive displacement units.
3. Output torques available at all
speeds.

4. A speed ratio of 60 : 1 is
practical.

5. No creeping tendency.
6. Fully reversible.
7. Input and output can be remote
or directly coupled.
8. Torque
remains
practically
constant over full speed range
of hydraulic motor.
9. Direct braking available.
10. Substantially constant speed
with load variations.
11. Positive control available.
12. Both open circuit and closed

1. A change in fluid velocity
yields output energy.
2. Non-Positive
displacement
units.
3. Output torque is not available
at low input speeds.
4. Input speed ratio 6 : 1 output
speed maximum.
5. Tends to creep.
6. Non-reversible.
7. Input and output directly
coupled.

8. Torque varies over a wide
range.

9. Practically
no
internal
braking.
10. Speed drops off with load.

11. Only limited control.

Dhanvantari College of Engineering, Nasik

Page | 8

circuit systems are available.
12. Only open circuit systems are
available.

As seen from the comparison above, since hydrostatic transmissions utilize positive
displacement pumps and motors and because they are flexible, this system offers a number of
advantages over hydrodynamic transmission system. During the last few decades,
improvements in design and operational performance have opened new fields of usage which
hold unlimited possibilities for machines and equipment of the future.

Int the field of fluid power, almost all the systems in the industry are hydrostatic
devices. Hence, the various equipments and systems covered in this text book will be
hydrostatic in nature. It should be noted that although the term hydraulic is used frequently in
the text book, it mainly refers to hydrostatic. Hence, the term hydraulics as treated in this text
book pertains to power transmitted and controlled thorough the use of pressurized liquids.

Questions

1. Define fluid. Define fluid power.
2. What do you mean by hydraulic and pneumatics?
3. How did fluid power develop in the 19-20the century?
4. What are the various methods by which power can be transmitted?
5. Compare the various methods of power transmission.
6. What are the advantages of using fluid power method of transmission?
7. List 10 fields of applications where hydraulics and pneumatics can be used more effectively
than the other power sources.
8. What effect has fluid power had on automation?
9. Why is hydraulic power especially useful with heavy work?
10. Explain in brief the areas of application of fluid power.
11. What is hydrostatic and hydrodynamic power transmission? Explain in brief.
12. Compare hydrostatic and hydrodynamic power transmission.

Dhanvantari College of Engineering, Nasik

Page | 9

Chapter – 3

Hydraulic Fluids -

• INTRODUCTION

A fluid has been defined in Chapter 1 as any liquid or gas. However, in hydraulics the
term fluid is generally used to refer to hydraulic liquids used for power transmission. Hence,
in this section on hydraulics, the term ‘fluid’ will mean ‘hydraulic fluid’.

The primary purpose of the hydraulic fluid is to transmit power. Fluid is picked up by
the pump from the reservoir, fed through the control valves to cylinders and motors where the
power is expended, and then returned to the reservoir where it is cooled and settled before
starting the cycle again. How well a fluid transmits power is determined how easily it is
pumped, how stiff it is and a number of other service related properties that determine how
suitable a certain fluid is to a particular application system and environment. Proper selection
and care of hydraulic fluid is to a particular application system and environment. Proper
selection and care of hydraulic fluid for a system will have an important effect on the
efficiency of the hydraulic system, on the cost of maintenance and on the service life of
hydraulic components.

• FUNCTIONS OF HYDRAULIC FLUID

The hydraulic fluid performs the following functions:

• Primary Function: Power Transmission

The primary function of the hydraulic fluid is to transmit power to perform useful
work. The hydraulic fluid must transmit an applied force from one part of the system to
another and must respond quickly to reproduce any change in magnitude and direction of the
applied force. Hence, it should have good flow ability and it should be incompressible to
make the pump start instantaneously.

• Secondary Functions

In addition to transmission of power, the hydraulic fluid should perform the following
secondary functions.
1. Lubrication: In an hydraulic system, the internal lubrication is provided by the fluid.
The hydraulic fluid minimizes the wear due to friction by providing adequate lubrication for
bearing, sliding surfaces in pumps, valves, cylinders motors and other components of the
system. Hence, for long service life the fluid must contain anti wear and antitrust additives.
2. Sealing: In an hydraulic system, there are many instances where due to the close
mechanical tolerances between the moving parts, providing a mechanical seal is impractical.
Here, the hydraulic fluid itself provides the film strength to seal the close clearances against
leakage.
3. Cooling: In an hydraulic system, while circulating the hydraulic fluid carries away
the heat generated and dissipates it in the reservoir. Thus it cools the system.

• SERVICE RELATED FLUID PROPERTIES

An hydraulic fluid has some inherent properties, while some service required
properties can be added by the addition of suitable additives. These properties which affect
the performance of an hydraulic system include specific gravity, viscosity, bulk module, pour

Dhanvantari College of Engineering, Nasik

Page | 10

point, neutralization number, flash point, fire point, auto ignition temperature, anti wear
properties, anti-rust properties, defoaming and detergent dispersing properties. The object is
to select a fluid that has the properties for a specific application and then have them remain
stable during continuous use for the recommended time between changes. This could be as
long as 1500 hours (or 3 years) in some cases.

• Specific Gravity (s)
Specific weight of a liquid indicates weight per unit volume

…(3.1)

Specific weight of water is 9810 N/m3
Specific gravity of a liquid is the ratio of specific weight of that liquid to the specific weight
of water.

S.I Unit: Specific gravity is a dimensionless quantity.
Specific gravity is also known as ‘relative density’.
Specific gravity of water is 1.
For commercially available hydraulic fluids, the specific gravity may range from 0.80 to
1.45.

• Viscosity

The ability of a fluid to be pumped and transmitted through the system is most
important. This ability to flow is determined by the fluid viscosity.

Viscosity is a measure of the internal resistance of a fluid to shear and is related to the
internal friction of the fluid itself. Thick fluids flow more slowly than thin fluids because they
have more internal friction. The term fluidity is the reciprocal of viscosity. Thus a fluid
having a high fluidity has low viscosity and a fluid having a low fluidity has a high viscosity.

Viscosity can be defined in the following manner:
1) Absolute (dynamic) viscosity
2) Kinematic viscosity
3) Relative viscosity in Say bolt Second Universal (SSU) and
4) SAE numbers (for automotive oils).
1.
Absolute or Dynamic Viscosity (H): Absolute viscosity or dynamic viscosity is
defined as the force required to move a flat surface with an area of one unit at a velocity of
one unit, when it is separated from a parallel stationary flat surface by an oil film one unit
thick.
S.I. unit; Pascal second (Pa - s)
However, the commonly used unit is Poise and centipoise

1 Poise =
0.1 Pa – s

1 Centi Poise (cP) = 0.01 Poise = 0.001 Pa – s
= 1 MPa - s

Speci�ic weight w �

������ �� ���

������ �
= ρg

Specific Gravity (s) =

��
� �

ρ�
ρ

Dhanvantari College of Engineering, Nasik

Page | 11

2. Kinematic Viscosity (v): Kinematic viscosity is defined as the ratio of absolute viscosity and
mass density of fluid.

… (3.3)

S.I. unit:
����
��
��

�

�
����

��
��

�
�����

��
�
����
������

��
�
��

�

However, the commonly used unit for kinematic viscosity is stoke and centistokes.

1 Stoke = 1
cm2/s

1 Centistoke (cSt) = 1
mm2/s

3.
Relative Viscosity in SSU units: During the selection of fluids, it is very convenient
to known the relative viscosity of the fluid. It is measured by using the Say bolt Viscometer.
Hence, it is measured in Say bolt Second Universal, abbreviated as SSU.

Here, the resistance of the fluid to flow is measured as the number of seconds it
takes for a fixed quantity of 60 ml sample of oil to drain through a small orifice of standard
length and diameter at a constant temperature of 100o F (37.7o) or 210oF (98.9 oC). The
elapsed time is the SSU viscosity for the fluid at the given temperature. For thicker fluids, the
same test is carried out using a larger orifice to derive the say bolt Seconds Furol (SSF)
viscosity.

For most applications, the viscosity is in the range of 100 SSU to 200 SSU.
However, it is a general rule that viscosity should never go below 45 SSU and above 4000
SSU, regardless of temperature.

4. SAE number: The Society of Automotive Engineers has established standard SAE
numbers to specify the range of viscosities of engine oils at specific test temperatures.

Winter number (0W, 5W, 10W, 15W, etc…) are determined by tests at cold temperatures.
Summer number (20, 30, 40, etc…) designate the SSU range at 100o C.

Table 3.1: SAE Viscosity Grades for oils.

SAE
Absolute
Viscosity (cP) at
Temperature oC
(oF)

Kinematics
Viscosity (cSt)
at 100o C (212o
F)

Viscosity
Grade
Max
Min
Max

0W
3250 at – 30 (-
22)
3.8

5W
3500 at – 25 (-
13)
3.8

Kinematic viscosity (v) =

�������� ��������� �µ�

���� ������� �ρ�

Dhanvantari College of Engineering, Nasik

Page | 12

10W
3500 at – 20 (-4)
4.1

15W
3500 at – 15 (5)
5.6

20W
4500 at – 10 (14)
5.6

25W
6000 at – 5 (23)
9.3

20

5.6
Less than 9.3

30

9.3
Less than 12.5

40

12.5
Less than 16.3

50

16.3
Less than 21.9

Viscosity is generally considered to be the most important physical property of an
hydraulic fluid an is the starting point in the selection of a fluid. If the fluid does not have the
proper viscosity, it cannot perform regardless of other superior characteristics.

If the viscosity is too low (lightweight oil), although the transmission efficiency will
be high, the following drawbacks may be encountered.

1. Less film strength causes more wear and tear of moving parts. The oil film may also break
down causing seizure.
2. Increase in internal leakage causes more pressure loss.
3. Leakage losses may result in increased temperatures.
4. Lower volumetric efficiencies in pumps and motors.
5. Slower response of actuator and hence less precision control.
If the viscosity is too high (heavy weight oils), although the self sealing obtained between
the mating surfaces is excellent, the following drawbacks may be encountered:

1. High resistance to flow results in sluggish operation.
2. High pressure drop due to friction.
3. Excessive heat generation.
4. Increased power consumption due to frictional losses.
5. Low mechanical efficiency.
6. Starvation of the pump inlet, causing cavitation.
7. Difficulty in separating air from oil in reservoir.

5. Viscosity Index (VI): The viscosity index (VI) is a measure of the relative change in
viscosity for a given change in temperature. An oil with a high viscosity index shows less
change in viscosity for a given change in temperature than does an oil with a low viscosity
index.

The viscosity index measures the stability between two temperature extremes. Viscosity
index is computed by using SSU designation for the reference oils (Pennsylvania crude
paraffin base fraction with a VI of 100 and Coastal crude naptha base fraction with a VI of 0)
and for the oil for which the VI is to be determined. The viscosity index is calculated as
follows

Dhanvantari College of Engineering, Nasik

Page | 13

L = SSU viscosity of a reference oil at 100oF with a VI of 0 that has the same viscosity at
210o F as the oil to be calculated.

H = SSU viscosity of a reference oil at 100oF with a VI of 100 that has the same viscosity at
210o F as the oil to be calculated.

U = SSU viscosity at 100oF of the oil whose VI is to be calculated.

The proper viscosity index of a fluid for a specific application is determined from the fluid
temperature change requirements of the system. For example.

(a) In production machinery, the operating temperature range of the oil is small and hence a
low VI is suitable.
(b) In mobile hydraulic equipments which may be required to operate in extreme temperature
conditions like below freezing to near 160o F, a fluid with a high VI above 100 is required.

• Bulk Modulus of Elasticity (K)

The bulk modulus of elasticity is a measure of the rigidity of the fluid. It gives an
indication of how much the fluid compresses under pressure.

Mathematically, the bulk modulus is the reciprocal of compressibility i.e. it is the ratio of
change in pressure to the change in volume.

…(3.5)

• S.I. Units: Pascal (Pa): The negative sign accounts for the fact that as the pressure increases,
the volume decreases from the eqn (3.5). Hence, we can say that higher the bulk modulus, the
less compressible or stiffer the liquid. Air entrained in hydraulic oils reduces its bulk modulus
making it spongy. This particularly affects the positioning circuits where the fluid is required
to be incompressible to maintain accuracy even with changes in load.

Hydraulic fluids have a bulk modulus in the range of 2068 MPa to 2758 MPa at room
temperatures in the pressure range of 6.8 to 41.5 MPa.

• Pour Point

Pour point is the lowest temperature at which an oil will flow. Low temperature
hydraulic applications, particularly those involving mobile equipment, use pour point as an
indication of the ability of the oil to be pumped as the temperature drops. As a general rule,
the pour point should be 15o to 20o F below the lowest temperature of the system during start
up to be sure that the pump will not cavitate and become damaged.

Chemical additives may be used to lower the pour point. The addition of these pour
point depressant does not vary the viscosity over the selected temperature range.

VI � L � U

L � H � 100

Bulk Modulus (K) �

������� �� �������� �∆��

������ �� ������ �∆�/��

Dhanvantari College of Engineering, Nasik

Page | 14

• Neutralization Number

Neutralization number is a measure of the acidity or alkalinity of a hydraulic fluid.
This is referred to as the pH factor of a fluid.

Petroleum base fluids have a tendency to become acidic with time high acidity causes
oxidation rate in an oil to increase rapidly. Hydraulic fluids are fortified with additives to
reduce the tendency to become acidic and keep the neutralisation number below 0.1 during
normal service. Fluids with a low neutralization number are recommended to prevent harmful
chemical reaction.

• Flash Point, Fire Point and Auto- Ignition Point

The flammability of a hydraulic fluid is described from the flash point, fire point and
auto ignition temperature.
1.
Flash Point: The flash point is a temperature at which a liquid gives off
sufficient quantity so as to ignite momentarily or flash when a test flame is passed over the
surface. A high flash point is desirable because it indicates good resistance to combustion and
a low degree of evaporation at normal or working condition.
2.
Fire Point: The fire point is a temperature at which the fluid will ignite and
remains ignited for five seconds when a test flame is passed over the surface.
3.
Auto Ignition Temperature: The auto ignition temperature is reached when
the sample will self ignite and combustion continues.

All the three temperatures indicate how hazardous the fluid will be in the presence
of metal open flames or elevated temperatures. Typical application include coal mines, ships,
aircraft and space craft.

• Antiwar Properties

A good hydraulic fluid must be able to provide full lubrication for all integral moving
parts of the system. However under extreme speed and pressure condition, the fluid film
thickness depletes and a condition called as boundary lubrication occurs. Here, due to metal
to metal contact, wear of the moving parts occurs. Hence, antiwar additives are added to the
hydraulic fluid to reduce this wear caused by friction between moving parts.

Petroleum based fluids provide excellent lubricating qualities.

• Oxidation

Oxidation is a chemical reaction in which the oxygen combines with the fluid to result
in the formation of acid and sludge. Air provides the oxygen necessary to promote oxidation.
Petroleum base hydraulic oils are particularly susceptible to oxidation, since oxygen readily
are soluble in the oil and additional reactions take place in the products to from gum, sludge
and varnish. The first stage products which stay in the oil are acidic in nature and can cause
corrosion throughout the system, in addition to increasing the viscosity of oil. The insoluble
gum, sludge and varnish plug orifices, increase wear and cause valves to stick.

The two main accelerators of oxidation are:

1. High Temperature and High Pressure: Oxidation rate increases at elevated
temperatures. The rate approximately doubles for every 18o F (10o C), and it is estimated that
the life of the oil is halved for each 15o F rise in temperature.

When the pressure is increased, the viscosity increases. This will result in more
frictional heat, thus raising the operating temperature of the system which in turn increases

Dhanvantari College of Engineering, Nasik

Page | 15

the rate of oxidation. Also as pressure increases, the amount of air that can be held in solution
by an oil increases rapidly. More air results in more oxidation.

2.
Effect of contamination on oxidation: Contaminats like cutting oil, grease, dirt,
moisture paint, and insoluble oxidation products themselves act as catalysts and accelerate
oxidation.

Also metals such as copper, iron, aluminium which are used aid oxidation, especially
in the presence of water.

Additives are added to hydraulic fluids to resist oxidation. They may be of the type
that breaks the chain of reactions thus preventing oxidation, or they may reduce the effect of
oxidation catalyst (metal deactivator type).

Advantages:
1.
They have excellent lubricating and antiwear qualities, and for practical purposes are
equal to petroleum base fluids.
2.
Since they are available in the low range of viscosity (VI from 80 to - 40), they are
used in plants and outside, from high speed precision machine tools which require low
viscosity fluids, to sub-zero aircraft and mobile equipment application.
3.
Since they do not contain any water or other volatile material, they operate well at
higher temperature than water containing fluids.
4.
They are suitable for high pressure system than water containing fluids.
5.
Replenishing fluid can be added directly without regard for changing the viscosity or
chemical composition of the fluid.
6.
Here there is neither the separation of the continuous phase from the emulsion, nor
periodic replenishment of additives which evaporate with water.

Disadvantages:
1. It is the costliest hydraulic fluid being used. It is about 7 times more costly than petroleum
base fluid.
2. They can be used only where the operating temperature is relatively constant.
3. They do not operate well in low temperature systems. Auxiliary heating may be required in
cold environment.
4. These fluids have a high specific gravity and may cause pump cavitation.
5. Seals which are normally used for petroleum base fluids are not suitable for use with
synthetic fluids. Seals should be changed when the system is being converted to this fluid.
Suitable sealing materials are butyl, rubber, Teflon, viton.
6. Avoidance of continued skin contact is advised. When this fluid comes in contact with hot
surface, irritating fumes are developed.

• SELECTION OF FLUIDS
Fluid selected for a particular application is governed by following factors
1. Operating pressure of fluid in the system
2. Operating temperature and variation in the system
3. Environmental conditions
4. Component material for compatibility with the selected fluid

• EFFECT OF TEMPERATURE AND PRESSURE ON HYDRAULIC FLUID

For all oils, viscosity decreases as the temperature increases. Viscosity index is a
measure of change in viscosity with the change in temperature

Viscosity index can be improved by some additives. Fig. 3.1 shows variation in
Kinematic viscosity with temperature for oils with different viscosity index.

With the increase in the pressure of hydraulic oils, the viscosity is found to increase. Modern
hydraulic systems employ very high operating pressures, often exceeding 1000 bar. At