Basic Hydraulic Principles
Contents
Objectives ................................................................................................................... 1
Introduction ................................................................................................................. 1
Fundamental & Scientific Principles ............................................................................ 1
Fundamental Principles ........................................................................................... 1
Force (F) ................................................................................................................. 2
Pressure (P) ............................................................................................................ 3
Pascal’s Law ........................................................................................................ 4
Joseph Bramah ....................................................................................................... 5
Application of pressure - modern example ............................................................... 7
Energy ................................................................................................................... 10
Energy types ...................................................................................................... 10
Flow (Q) ................................................................................................................ 11
Example of Velocity & Flow Rate ....................................................................... 12
A Practical Example of Velocity & Flow Rate: .................................................... 12
Fundamental principles of flow ........................................................................... 13
Bernoulli ................................................................................................................ 14
Bernoulli’s Principle ............................................................................................ 15
Flow & Pressure Drop ........................................................................................ 15
Pressure Drop Due To Friction ........................................................................... 15
Work ...................................................................................................................... 17
Power .................................................................................................................... 18
Hydraulic power ................................................................................................. 18
Power losses in hydraulic systems ..................................................................... 18
Torque (T) ............................................................................................................. 19
Advantages and Disadvantages ................................................................................ 20
Advantages of a hydraulic system ......................................................................... 20
Disadvantages of a hydraulic system..................................................................... 20
Revision Questions ................................................................................................... 21
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Basic Hydraulic Principles
List of figures
Figure 1 - Pascal's Law ............................................................................................... 4
Figure 2 - Bramah's Press ........................................................................................... 5
Figure 3 - Hydraulic lever (single shot system) ............................................................ 6
Figure 4 - Typical multi-shot system (hydraulic bottle jack) ......................................... 6
Figure 5 - Pressure in a closed circuit ......................................................................... 8
Figure 6 - Output force ................................................................................................ 8
Figure 7 - Pressure & pistons of different areas .......................................................... 9
Figure 8 - Velocity & flow rate ................................................................................... 12
Figure 9 – Flow rate example .................................................................................... 13
Figure 10 –Bernoulli's Principle ................................................................................. 15
Figure 11 - Pressure drop across a restriction ........................................................... 16
Figure 12 - Torque example ...................................................................................... 19
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Basic Hydraulic Principles
Objectives
Introduction
Hydraulics is the science of transmitting force and/or motion through the medium of a
confined liquid. In power hydraulics, power is transmitted by pushing on a confined
liquid which. The transfer of energy takes place because a quantity of liquid is
subject to pressure and that liquid is said to be incompressible. To maintain liquidpowered systems and aid fault diagnosis, the maintenance engineer requires a
sound understanding of the basic nature of liquids and how they react under certain
conditions.
In this lesson we will explore the fundamental principles governing hydraulic power,
the properties of liquids and how they act under different conditions; essentially, the
science behind the mechanics of how a relatively small power unit can move huge
loads. This lesson uses S.I. (Systeme International) units and these are explained
with each relevant topic.
Every system has its own distinct set of advantages and disadvantages so this
section will finish off by highlighting what mobile hydraulics is particularly useful for
and what it isn’t, including some of the inherent dangers of using this method of
power transmission.
Fundamental & Scientific Principles
Fundamental Principles
In order to fully understand hydraulic systems and how they operate the following
fundamental principles need to be considered and they will be examined as we
progress:
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•
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•
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•
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All liquids behave in a similar manner.
A liquid is infinitely flexible (it will assume the shape of its container).
It is as unyielding as steel (due to its virtual incompressibility).
It can readily change its shape.
It can be divided into parts to do work in different locations.
It can move rapidly in one place and slower in another.
It can transmit force in any or all directions.
Flow produces motion and governs speed.
Pressure provides the pushing force.
A liquid takes the path of least resistance.
For a liquid to flow there must be a pressure difference between two
points.
The greater the pressure difference, the greater the flow potential.
Heat is generated when a liquid flows from high pressure to low pressure
without doing work.
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Basic Hydraulic Principles
Force (F)
Force is that which produces, or tends to produce, a change in state or motion of a
body such as:
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•
•
from rest to motion or from motion to rest,
a change in rate of motion (acceleration or deceleration),
a change in direction or motion.
If a Force (F) acts on a mass (m) then the mass will be accelerated (a) in the
direction of the force. Therefore:
Force = mass x acceleration
F=ma
In the SI system, the unit of Force is the Newton (N).
Definition. One Newton (N) is the force required to accelerate a mass of one
kilogram (kg) a distance of one metre (m) in 1 second (s).
Therefore: 1 N = 1 kgm/s2
Any mass will be subject to the effect of gravity. Irrespective of the size of the mass,
acceleration due to gravity will remain constant at 9.81 m/s 2 at sea level. We all
have a mass and we are all subject to the forces of gravity. Therefore, to convert
mass (kilograms) to Newtons the mass must be multiplied by the acceleration due to
gravity.
Example: A mass of 10 kg is subject to gravity at 9.81 m/s2:
10 kg (mass) x 9.81m/s2 = 98.1 kgm/s2
Bearing in mind that 1 kgm/s2 = 1N, the above becomes 98.1N.
Hydraulic force
In hydraulics, Force is the result of Pressure acting on a Surface Area, therefore:
Force (N) = Pressure (Pa) x Area (m2)
Example: Calculate the force generated by a pressure of 750 Pa acting on an area of
0.5 m2.
Force = Pressure x Area
Force = 750 x 0.5
Force = 375N
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Basic Hydraulic Principles
Pressure (P)
Definition. The technical definition of pressure is force per unit area, therefore:
Pressure = Force / Area
In the SI system Pressure is measured in N/m2 or Pascal (Pa).
1 N/m2 = 1 Pa
There are two natural forms of pressure that will affect a hydraulic system:
1. Atmospheric Pressure. The earth has an atmosphere of air extending 80
km (50 miles) up, and this air has weight. This air creates a head of pressure
that is called atmospheric pressure. A column of air 25mm square (1 inch
square) in cross section and the height of the atmosphere would weigh 6.67kg
(14.7 lbs) at sea level. Thus, the earth's atmospheric pressure is 101325Pa
(101.3 kPa) at sea level (14.7 psi or 1.01 bar). The role of atmospheric
pressure in most hydraulic systems is significant and will be demonstrated in
later lessons.
2. Static head pressure. The weight of a fluid in a container exerts pressure
on the containing vessel's sides and bottom. This is called static head pressure
and it is caused by earth's gravitational pull. Before the concept of pressure
had been realised, head was the only way to express pressure measurement. It
was expressed as feet of water. Today, head is still used to define the vertical
distance between two levels in a fluid.
Pressure on a fluid can be created in two ways:
1. By the weight of the fluid itself (see also Static Head Pressure), as proven
by Torricelli, where fluid in a tank will flow out of a hole at the bottom with most
force when the bung is removed but will decrease in force as the fluid level
drops; this decrease in force is directly related to the fluid level therefore the
fluid weight.
2. As Joseph Bramah will show, by pushing on a confined fluid and only if
there is a load or resistance to flow will pressure be created.
Pressure in hydraulics is the tendency of the fluid to resist compression; therefore
pressure is responsible for pushing or opposing a force due to a fluid’s virtual
incompressibility. Pressure is the term used to define how much force is exerted on
a specific area.
Example: How much pressure will be generated by a force of 10kN acting on a
piston with a radius of 100mm?
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Basic Hydraulic Principles
First, we need to determine the area of the piston:
Area of a circle = π r2
Area of a circle = π x 1002
= π x 100 x 100
= 31416 mm2
31416 mm2 = 0.031416 m2
Now:
Pressure = Force / Area
Pressure = 10000 / 0.031416
Pressure = 318309 Pa (318.31 kPa)
Pascal’s Law
Blaise Pascal formulated the basic law of hydraulics in the mid-17th century. He
discovered that pressure exerted on a fluid acts equally in all directions. His law
states that:
Pressure in a confined fluid is transmitted undiminished in every
direction and acts with equal force on equal areas and at right angles
to a container's walls.
Figure 1 - Pascal's Law
Figure 1 shows the apparatus that Pascal used to develop his law. It consisted of
two connected cylinders of different diameters with a liquid trapped between them.
Pascal found that the weight of a small piston will balance the weight of a larger
piston as long as the piston areas are in proportion to the weights.
For example:
Piston A is 100mm2 and weighs 1kg.
Piston B is 500mm2 and weighs 5kg.
He also discovered that the movement of the each piston is proportional to their
sizes as well; so if Piston A can move 100mm, then Piston B will move 20mm.
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Basic Hydraulic Principles
Joseph Bramah
Although Pascal’s experiments had very little practical use they laid the foundation to
modern hydraulics. Pascal’s Law wasn’t put into practical application until the
Industrial Revolution. In 1795 an Englishman called Joseph Bramah used Pascal’s
Principles to develop the first hydraulic press, using water as the hydraulic medium.
Bramah was able to achieve huge force multiplications never before imagined.
It can be seen from Bramah’s press (Figure 2) that an initial Force input of 4800N
acting on a surface area of 800mm2 will generate a pressure of 6 N/mm2 in the
cylinder. The pressure of 6 N/mm2 will now act on a surface area of 320000mm2
resulting in a force output of 1920000N.
This is equivalent to a force multiplication or a mechanical advantage of 400:1.
Figure 2 - Bramah's Press
The Single Shot Hydraulic Lever
So far the main emphasis has been on forces in balance, for example, a small effort
acting on a small plunger, which then pushes on the fluid and balances a large load
by the fluid acting on a large ram. As Pascal proved, if a smaller effort is exerted on
a confined fluid then it is possible to balance a large load. If the effort is increased
even slightly then it follows that the large load must move. If the small effort is
greater than the large load then an unbalanced situation arises, this results in
movement of the small plunger and therefore, because of the fluids virtual
incompressibility, the result must be a movement at the large ram.
This difference in movement between the plunger and ram is called the Velocity
Ratio.
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Basic Hydraulic Principles
Figure 3 - Hydraulic lever (single shot system)
The simple two piston (plunger and ram) hydraulic lever (Figure 3) is also known as
a Single Shot System which is really only suitable for short load travel or light loads.
The Multi-shot System
It can be seen from the single shot system (simple hydraulic lever) that for the ram to
move a useful amount then the plunger requires a large amount of travel (Velocity
Ratio). The large amount of plunger travel can be overcome by converting the
Single Shot System into a Multi-shot System which enables multiple short strokes
of the plunger to achieve a useful amount of travel at the ram.
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2
4
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Figure 4 - Typical multi-shot system (hydraulic bottle jack)
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Basic Hydraulic Principles
Figure 4 shows a typical example of a modern Multi-shot System – the hydraulic
bottle jack. The following components are required to convert the Single Shot to a
Multi-shot System:
1. A tank or reservoir, to supply additional fluid to the system.
2. A suction valve to allow fluid from the reservoir to act under the plunger as it is
drawn up.
3. A delivery valve to direct fluid to act under the ram as the plunger is forced down.
4. A release valve, when opened, allows the ram to lower by directing oil back to the
reservoir.
Keeping our hydraulic bottle jack in mind, oil from a pump flows into a cylinder that is
lifting a load. The resistance of the load causes pressure to build inside the cylinder
until the load starts moving. The pressurized oil is trying to get out of the pump,
pipe, and cylinder, but these mechanisms are strong enough to contain the fluid.
When pressure against the piston area becomes high enough to overcome the load
resistance, the oil forces the load to move upward. There will be an initial spike in
pressure until the pressure has built enough to overcome the inertia of the load.
While the load is in motion though, pressure in the entire circuit stays nearly
constant.
Notice that the pump did not produce pressure; it only produced flow. Pumps never
produce pressure; they only produce flow. Resistance to pump flow causes
pressure. It can therefore be stated that pressure is a measure of the resistance to
flow. This is one of the basic principles of fluid power that is of prime importance to
troubleshooting hydraulic circuits.
Suppose a machine with the pump running shows almost zero on its pressure
gauge. Does this mean the pump is bad? Without a flow meter at the pump outlet,
engineers may be tempted to change the pump, because many of them think pumps
make pressure. The problem could simply be an open valve that allows all pump
flow to go directly to tank. Because the pump outlet flow sees no resistance, a
pressure gauge shows little or no pressure. With a flow meter installed, it would be
obvious that the pump was all right and other causes such as an open path to tank
must be found and corrected.
Application of pressure - modern example
The pressure of a liquid in a closed system, for example a hydraulic brake system, is
the force exerted against the inner surface of its container, which is the surface of all
the lines, hoses, valves, and pistons in the system. As Pascal proved, pressure
applied to a liquid exerts force equally in all directions. If a hydraulic pump is capable
of producing 700 kPa (7 bar or 100 psi), there will be 700 kN of force on every
square metre of the system (Figure 5).
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Basic Hydraulic Principles
Figure 5 - Pressure in a closed circuit
When pressure is applied to a movable output piston, it creates output force. If the
system included a piston with an area of 0.02 m2, each square metre would receive
700 kN of force. This means there would be 35000 kPa of pressure applied to that
piston (Figure 6).
Pressure = Force / Area
Pressure = 700 / 0.02
Pressure = 35000 kPa (350 bar or 5000 psi)
Figure 6 - Output force
The use of the larger piston would give the system a mechanical advantage or
increase in the force available to do work. The multiplication of force through a
hydraulic system is directly proportional to the difference in the piston sizes
throughout the system. By changing the sizes of the pistons in a hydraulic system,
force is multiplied, and as a result, low amounts of force are needed to move heavy
objects.
Continuing our example of hydraulic braking systems, these systems use hydraulics
to increase the force applied by the driver for brake application.
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Basic Hydraulic Principles
3.5 kN
35 kN
2.5 kN
Figure 7 - Pressure & pistons of different areas
Figure 7 shows a hydraulic braking system with an input piston of 0.007 m 2. A force
of 3.5 kN is pushing on the piston. This produces a system pressure of 500 kPa (5
bar or 73 psi):
Pressure = Force / Area
Pressure = 3.5 / 0.007
Pressure = 500 kPa
A pressure gauge in the system shows the pressure to be 500 kPa.
There are two output pistons in the system:
One has 0.07 m2 of area. This means that the larger output piston has 500 kPa
applied to 0.07 m2 to deliver an output force of 35 kN:
Force = Pressure x Area
Force = 500 x 0.07
Force = 35 kN
The other output piston is smaller than the input piston with a 0.005 m 2 area. The
0.005 m2 area of this piston has 500 kPa acting on it to develop an output force of
2.5 kN:
Force = Pressure x Area
Force = 500 x 0.005
Force = 2.5 kN
In a hydraulic brake system, a small master cylinder piston is used to apply pressure
to larger pistons at the wheel brake units to increase braking force. A modern
vehicle system also uses larger operating pistons on the front brakes with smaller
ones on the rear. This principle of differing surface areas and pressures is widely
used in mobile hydraulics as we will see in later lessons.
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Basic Hydraulic Principles
Energy
Definition. Energy is the capacity to do work and is expressed in the same units as
work. For example, a lump of coal contains heat energy, a battery contains electrical
energy and a steam boiler would contain pressure energy.
Conservation Of Energy Law:
The Conservation of Energy Law states:
Energy can neither be created nor destroyed; it can only be
transformed from one form to another, or transferred from one point
to another.
In mechanics, conservation of energy is usually stated as:
E=T+V
where T is kinetic energy and V is potential energy.
In the SI system Energy is measured in joules (J).
Energy types
The types of energy we need to consider in a hydraulic system are:
•
Potential Energy. Potential energy is energy due to position. In
hydraulics, potential energy is a static factor. When force is applied to a
confined liquid potential energy is present because of the static pressure
of the liquid. The weight of the fluid in the reservoir, plus the pressure,
either atmospheric (vented reservoir) or positive pressure (non-vented
reservoir) acting on the free surface area of the fluid, are the
components of potential energy. Potential energy of a moving liquid can
be reduced by the heat energy released. Potential energy can also be
reduced in a moving liquid when it transforms into kinetic energy. A
moving liquid can, therefore, perform work as a result of its static
pressure and its momentum.
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Pressure Energy. Pressure energy is generated when the push on a
confined, virtually incompressible fluid is opposed by a resistance to
flow. Factors causing resistance to flow can take the form of:
o
o
o
o
o
o
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a physical load on the actuator,
pipe-work diameter,
pipe-work smoothness,
fluid velocity,
fluid flow rate,
fluid viscosity.
Kinetic Energy. Kinetic energy of an object is the extra energy which
it possesses due to its motion. In hydraulics this is generated by the
Basic Hydraulic Principles
mass density of the fluid and the fluid’s velocity. Pressure caused by
kinetic energy may be called velocity pressure.
•
Heat Energy. Heat energy is the energy a body possesses because of
its heat. Kinetic energy and heat energy are dynamic factors. Pascal's
Law dealt with static pressure and did not include the friction factor.
Friction is the resistance to relative motion between two bodies. When
liquid flows in a hydraulic circuit, friction produces heat. This causes
some of the kinetic energy to be lost in the form of heat energy.
Although friction cannot be eliminated entirely, it can be controlled to
some extent. The main causes of excessive friction in hydraulic
systems are:
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o
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o
Extremely long lines.
Numerous bends and fittings or improper bends.
Excessive velocity from using undersized lines.
Sudden changes in pressure with no work produced.
Flow (Q)
Flow is the movement of fluid caused by a difference in pressure at two points. In a
hydraulic system, flow is usually produced by the action of a hydraulic pump - a
device used to continuously push on a hydraulic fluid. It can be measured by its:
•
Velocity. Velocity is the average speed at which a fluid's particles move
past a given point, measured in metres per second (m/s). Velocity is an
important consideration in sizing the hydraulic lines that carry a fluid
between the components.
Velocity = Distance / Time
In SI units this will result in metres per second (m/s).
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Flow Rate. Flow rate is the measure of how much volume of a liquid
passes a point in a given time, measured in litres per second (L/s). Flow
rate determines the speed at which a load moves and, therefore, is
important when considering power.
Flow Rate = Volume / Time
In SI units this will result in litres per second (L/s).
Note: The capitalised L is normally used to denote litres as a lower case
l can be confused with the number 1 or capital I.
Example: A mass of oil is required to move 25 metres in 5 seconds. At what velocity
must it travel to achieve this?
Velocity = Distance / Time
Velocity = 25 / 5
Velocity = 5 m/s
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Basic Hydraulic Principles
Example: A pump produces 15 litres per second. How much oil will it displace in 6
seconds?
Flow Rate = Volume / Time
So: Volume = Flow Rate x Time
Volume = 15 x 6
Volume = 90 litres
Example of Velocity & Flow Rate
Consider Figure 8, where a pump provides a constant rate of 5 litres per second
through two pipes of differing diameter, where each pipe section holds 5 litres.
Therefore, as each pipe section will allow 5 litres each second to pass through it, this
means that:
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Oil flows through pipe A at a Velocity of 4 m/s.
Oil flows through pipe B at a Velocity of 2 m/s.
Figure 8 - Velocity & flow rate
It can therefore be seen that a constant flow rate will result in:
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Lower velocity as diameter increases.
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Higher velocity as diameter decreases.
A Practical Example of Velocity & Flow Rate:
Consider Figure 9, which shows two cylinders:
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•
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Cylinder A is 400 mm long and holds l litre.
Cylinder B is 200 mm long and also holds 1 litre.
Basic Hydraulic Principles
Figure 9 – Flow rate example
If 1 L/m is pumped into each cylinder, then both cylinder pistons will travel their full
length in 1 minute. However, the Piston in Cylinder A must move twice as fast as
the Piston in Cylinder B, due to the fact that it is twice as long.
It therefore follows that if the Flow Rate is doubled to each cylinder, then each
cylinder would fill in half the time and therefore both cylinder piston speeds will
double.
There are now two ways of increasing the speed at which the load moves:
•
By decreasing the size of the cylinder.
•
By increasing the flow rate to the cylinder.
Conversely if the cylinder size is increased, or the flow rate is decreased, then the
cylinder will slow down.
The speed of the cylinder must therefore be proportional to flow, and
inversely proportional to the piston area.
Fundamental principles of flow
It can be seen that there are three fundamentally important principles regarding flow:
1. Flow makes it go. The actuator must be supplied with flow for anything in a
hydraulic system to move.
2. The rate of flow determines speed. Flow in hydraulic systems is normally
provided by the pump. The speed of the actuator will vary with changes in
pump outlet flow.
3. Actuator volume affects actuator speed. Changes in actuator volume
displacement will change actuator speed for a given flow rate.
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Basic Hydraulic Principles
The maintenance engineer must also bear in mind that hydraulic oil will always take
the path of least resistance. This fact can have great significance when
troubleshooting hydraulic systems as a faulty valve or blocked restrictor may make
the flow act differently to that expected from studying a circuit diagram.
As previously stated the hydraulic system will be subject to three forms of energy,
although for practical purposes only Pressure Energy and Kinetic Energy are
obvious in horizontal fluid flow. Therefore with a constant flow rate, energy at the
input and output will remain constant; this is known as the Total Energy of the
Hydraulic System which is the result of Pressure Energy & Kinetic Energy.
The Total Energy concept is the result of the studies carried out by Daniel Bernoulli.
Bernoulli
Previously it has been seen that with a change in pipe diameter there will be a
change in flow velocity, therefore to maintain the Total Energy Equation (E = T + V),
it is apparent that there must be a change in pressure each time that there is a
change in velocity.
Daniel Bernoulli, a Swiss scientist of the 18th Century, studied the relationship of
fluid speed and pressure. When a fluid flows through a narrow constriction its
speed increases. This is easily noticed by the increased speed of a stream when it
flows through the narrow parts. The fluid must speed up in the constricted region if
the rate of flow is to be continuous. Bernoulli wondered how the fluid got the energy
for this extra speed. He reasoned that the energy is acquired at the expense of a
lowered internal pressure. His discovery, now called Bernoulli's Principle, states:
The static pressure of a moving liquid varies inversely with its velocity.
Simply stated, as velocity increases, pressure decreases.
Therefore:
Pressure + Velocity = Constant (ignoring friction)
Bernoulli's Principle is a consequence of the Conservation of Energy Law. When a
fluid flows, it has kinetic energy because of its motion. It also has gravitational
potential energy or stored energy due to the Earth's gravitational field. If the fluid
picks up speed, or accelerates, it has more kinetic energy than before. If the fluid
does not move up or down as it travels through the constricted region, then its
gravitational potential energy does not change.
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Basic Hydraulic Principles
Bernoulli’s Principle
Figure 10 shows a simple experiment used to prove Bernoulli’s Principle. Chamber
A has a piston which will be used to force the liquid through the system. Chambers
A and B are of equal initial capacity.
Figure 10 –Bernoulli's Principle
The force on the piston is sufficient to create a pressure of 100 kPa on chamber A.
As the piston moves down, the liquid that is forced out of chamber A must pass
through passage C to reach chamber B. The velocity increases as it passes through
C because the same quantity of liquid must pass through a narrower area in the
same time. Some of the 100 kPa pressure in chamber A is converted into velocity
energy in passage C so that a pressure gauge at this point registers 90 Pa. As the
liquid passes through C and reaches chamber B, velocity decreases to its former
rate, as indicated by the static pressure reading of 100 kPa.
It is important to understand that a change in Velocity will produce a change in
Pressure, thus a change in Force. If flow should cease, pressure will balance
throughout the system.
Flow & Pressure Drop
A basic rule of hydraulics is that:
•
where there is flow there must be a pressure drop between the two
points;
•
where there is a pressure difference there must be flow or a difference in
fluid level between two points.
The pressure difference when a fluid is flowing is used to overcome friction and to lift
the fluid if required. When a fluid is flowing, the upstream pressure is always higher
than the downstream pressure, the difference between the pressures being known
as Pressure Drop.
Pressure Drop Due To Friction
Pressure Drop becomes more apparent when the flow is restricted.
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Basic Hydraulic Principles
An orifice is a restriction often placed in a line to deliberately create a pressure
difference. There is always a pressure drop across an orifice so long as there is
flow; this is also true of fluid flowing through any hydraulic line or hydraulic valve, and
the smaller the restriction or valve passage then the greater the pressure drop.
Figure 11 - Pressure drop across a restriction
The energy “lost” due to pressure drop is converted to heat.
Bernoulli's Principle holds for steady flow. In steady flow, the paths taken by each
little region of fluid do not change as time passes, and can be represented in a
diagram with streamlines which are the smooth paths of the neighbouring regions of
fluid. The lines are closer together in the narrower regions, where the flow speed is
greater and the pressure within the fluid is less.
If the flow speed is too great, the flow may become turbulent and follow changing,
curling paths known as eddies. Under these conditions, Bernoulli's Principle will not
apply.
Laminar Flow is a streamlined flow free from eddies or turbulence, it is this type of
flow that is desirable to keep friction to a minimum. Laminar Flow is usually only
possible at flow rates less than 2 m/s, and with gradual changes in both flow
direction and flow velocity.
Where Laminar Flow cannot be maintained the flow is said to be Turbulent Flow.
This causes friction, thus causing heat with a consequential pressure drop. The
causes of turbulent flow are high fluid velocities, with sudden changes in direction
such as too small pipe bend radius.
With speed of operation of the actuators dependant on flow rate, it is important for
system designers to minimise any disruption to flow at the design stage. It is equally
important that maintenance engineers do not affect that flow when completing any
repairs. From what we have already seen it is important to minimise any disruption
to flow and therefore pressure losses by:
a. Good system design which reduces the number of unnecessary restrictions in
the system.
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Basic Hydraulic Principles
b. Smooth pipe and hose layout with suitable internal diameters.
c. Allowing the hydraulic oil to quickly reach a suitable working temperature and
then maintaining that temperature. We will see later that cold oil does not flow
as well as oil at the correct working temperature.
Cavitation is a condition where the available fluid does not fill the existing space at
pressure less than atmospheric. The fluid tries to expand to fill the space, thus
forming cavities of vapour. External pressure greater than the cavity vapour causes
the cavities to implode, fluid now rushes in causing a violent action of high intensity
blows, which cause severe vibration and in some cases will force metal to
breakaway. Cavitation is not normally caused by air in the system: a hydraulic circuit
is normally sealed and it is very difficult to get air into the system.
Cavitation is a serious condition and will very rapidly cause severe damage,
especially in pumps where a restriction at the suction side of the pump is the main
cause. Therefore servicing of the suction strainer/filter and the reservoir breather
vent (atmospheric reservoirs) must be carried out in accordance with the
manufacturer's instructions to ensure a good flow of oil to the pump.
It is also possible for actuators to cavitate, although this should be less of a problem
due to service line control valves.
Work
Work is a measurement of accomplishment, where it requires motion to move a force
(load).
Therefore in a hydraulic system, due to the fluid’s virtual
incompressibility, work is the result of fluid flow (motion),
moving a force (load). This action could happen in three
seconds, three minutes, or three hours without changing the
amount of work.
Work is the result of a force being moved through a
distance, therefore:
Work = Force x Distance
The SI unit of work is the Joule (J), which is defined as the
work done by a force of one Newton acting over a distance
of one meter. The dimensionally equivalent Newton-meter (N-m) is sometimes used
instead; however, it is best reserved for torque to distinguish its units from work or
energy.
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Basic Hydraulic Principles
Power
When work is done in a certain time, it is called Power. Power is the rate of doing
work, or the rate of energy transfer, such as moving a force (load) through a given
distance in a given time. Therefore:
Power = Force x Distance
Time
In the SI system Power is measured in Watts (W).
1 Watt = 1 J/s
Hydraulic Power is more likely to be in the kilo Watt
region.
Since the expression for velocity is displacement/time,
the expression for power can be rewritten as force x
velocity:
Power = Force x Velocity
For historical reasons, the horsepower is occasionally used to describe the power
delivered by a machine. One horsepower is equivalent to approximately 750 Watts.
Hydraulic power
Although the above is an accepted method of finding hydraulic power, it is more
commonly used in Mechanical Power calculations.
The more common way of expressing Hydraulic Power is the result of Flow Rate (Q)
and Pressure (P). Hydraulic power is defined as Flow x Pressure.
Example: Find the hydraulic power supplied by a pump that delivers (Q) 130 L/min
and has a pressure (P) rating of 350 bar:
Pump power output = (130 x 350) = 74.3 kW
612
Where 612 is a constant used to convert the result into kW.
Power losses in hydraulic systems
Power losses generally occur due to leakage and heat.
1.
Leakage losses such as:
•
•
•
2.
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pumps and motors.
sliding spool valves.
control valve drain lines.
Heat losses in valves that naturally run warm, such as:
Basic Hydraulic Principles
•
•
3.
any valve used to control flow.
pressure reducing valves.
Heat losses in valves that only run warm during operation, such as:
•
•
•
pressure relief valves.
counter balance valves.
sequence valves.
Note: For every 1000 kPa (10 bar or 145 psi) pressure drop the fluid temperature
rises by approximately 0.7°C
Torque (T)
Definition. Torque is the tendency of a force to cause or change rotational motion
of a body. Torque is calculated by multiplying Force and distance.
In the SI system Torque is measured in Newton-metres or N-m.
In more basic terms, torque measures how hard something is rotated. Figure 12
shows a spanner being applied to a nut on a screw. To loosen or tighten the nut, the
mechanic must apply a force to the spanner. The amount of "twist" (torque) depends
on how long the spanner is, how hard the mechanic pushes down on the spanner
and how well you are pushing it in the correct direction. To achieve maximum torque
the force should be applied to the spanner at the furthest point from contact and at
90o to the required movement.
Example: The spanner is 180mm long and the mechanic is able to exert a maximum
force of 100N. What will be the maximum torque applied to the nut?
Torque = Force(N) x Distance(m)
= 100 x 0.18
= 18N-m
Force
Distance
Figure 12 - Torque example
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Basic Hydraulic Principles
Advantages and Disadvantages
Advantages of a hydraulic system
•
•
•
•
•
•
•
•
•
•
•
Hydraulic power is easy to produce, transmit, store and regulate.
It is possible to generate a high gain in force and power amplification.
Hydraulic systems can provide constant force and torque.
Hydraulic systems are smooth, generate stepless motion and variable speed
and force to a greater accuracy.
Frictional resistance is much less compared to a comparable mechanical
system.
Hydraulic elements can be placed anywhere and still be easily controlled.
Noise and vibration created by hydraulic pumps is minimal.
Hydraulic systems are easy to maintain.
Hydraulic output can be linear, rotational or angular.
Hydraulic system can provide absolutely accurate feedback of position, load,
etc.
Hydraulic systems are able to provide braking and deceleration controls.
Disadvantages of a hydraulic system
•
•
•
•
•
•
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Hydraulic components need to be manufactured to a high degree of precision,
thus increasing manufacturing costs.
Hydraulic systems are highly susceptible to contamination.
Hydraulic oil leaks can be dangerous for users and maintainers.
Petroleum based oil may pose a fire risk, thus limiting its applicability if used
at high temperatures.
Hydraulic components need to be specially treated for protection against
corrosion, etc.
Manual systems can be laborious and slow to operate, and jerky in operation.
Basic Hydraulic Principles
Revision Questions
1. A hydraulic system with a 3 cm2 input piston and a 9 cm2 output piston is being
discussed. Engineer A says that the output piston will have three times as much
force as the input piston. Engineer B says that the output piston will move one-third
as far as the input piston. Who is correct?
A.
B.
C.
D.
Engineer A only.
Engineer B only.
Both Engineer A and Engineer B.
Neither Engineer A nor Engineer B.
2. Atmospheric pressure at sea level is equal to:
A.
B.
C.
D.
14.7 Hg
14 PSI
1.01 bar
30 PSI
3. A liquid will always take the path of least resistance. True or False?
4. Pressure in a fluid power system comes from:
A.
B.
C.
D.
pump flow.
resistance to flow.
motor horsepower.
pump volume.
5. Modern hydraulics is defined as the use of a confined liquid to transmit power.
True or False?
6. Pascal's Law states that pressure in a confined body of liquid will act
_______________ in all directions.
A.
B.
C.
D.
erratically
equally
forward
sequentially
7. A pump produces 120 litres per minute at 1500 rpm. How much oil will it displace
in 1 minute at 2000 rpm?
8. Force = __________ x ____________
A.
B.
C.
D.
flow x distance
work x time
pressure x flow
pressure x area
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Basic Hydraulic Principles
9. The basic idea behind any hydraulic system is very simple: Force that is applied
at one point is transmitted to another point using ______.
A.
B.
C.
D.
a compressible fluid.
an incompressible fluid.
a cold compress.
pump pressure.
10. Which of the following is not an advantage of hydraulic systems?
A.
B.
C.
D.
Hydraulic output can be linear, rotational or angular.
Hydraulic systems can provide constant force and torque.
Manual systems can be laborious and slow to operate, and jerky in operation.
Hydraulic power is easy to produce, transmit, store and regulate.
11. Hydraulic Power = __________ x ____________
A.
B.
C.
D.
flow x distance
work x time
pressure x flow
pressure x area
12. Laminar flow is:
A.
B.
C.
D.
caused by too high a flow velocity.
the cause of cavitation in a hydraulic circuit.
usually only possible at flow rates less than 7 m/s.
a streamlined flow free from eddies or turbulence.
13. Cavitation is caused by air in the system. True or False?
14. Pressure across a restriction will:
A.
B.
C.
D.
decrease.
stay the same.
increase.
fluctuate.
15. There are two ways of increasing the speed at which a hydraulic actuator
moves:
A.
B.
C.
D.
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By decreasing the size of the cylinder.
By increasing the size of the cylinder.
By increasing the flow rate to the cylinder.
By decreasing the flow rate to the cylinder.