HYDRAULICS AND PNEUMATICS
Hydraulics and Pneumatics is the technology that deals with the generation, control and transmission of power using pressurized fluid. This subject is designated as FLUID POWER, and is world wide accepted.
The word Hydraulics is derived from the Greek word “HYDOR” and means “Water”. This comprised all things in affiliation with ‘ Water’.
The word Pneumatics is derived from Latin Word “PNEUMO” means “ Breathing”.
It can be said that Fluid Power ( Hydraulics and Pneumatics ) is the muscle that moves Industry. This is because its application in industry is unlimited .
· It jacks up an automobile( Hydraulic jack)
· Drills out teeth
· Launches space ship
· Control submarines
· Mines coal and ores
· Moves earth ( earth moving equipment like excavators, bull dozers, borewell drills etc.,
· Harvests crops
· Presses
· Machine tools
· Material handling equipment
· Transportation
· Construction
And in general, makes our everyday living easier and more enjoyable.
Why Fluid Power?
Fluid power provides flexible and easy control of variable force, distance and speed. Fluid power can be varied from a delicate touch of a few grams to a gigantic force of 36000 tons or more. It provides constant torque at speed of nearly 100 km/hr within a few centimeter or give a creeping speed of a fraction of an cm/min.
Hydraulic system use liquids such as petroleum oils, water, synthetic oils etc. The first hydraulic fluid to be used was water because it is readily available but it has been replaced by oils.
Pneumatic system use air as the gas medium because air is very abundant and can be readily exhausted into the atmosphere
Fluid flow fundamentals:
The knowledge of the fundamental laws and equations which governs the flow of fluid is essential for the design of hydraulic control components and system.
The field of fluid mechanics is broken down as follows:
1. Hydrostatics : Mechanics of still fluid. It is the pressure which acts on the base of container filled with fluid and is dependent on the height of head of fluid inside the container.
Mathematically , p=ρgh
Where ρ = mass density, Kg/m3
h= height of fluid level ( head) in m
p = pressure intensity in bar, 1 bar = 105 Pascal = 105 N/ m2
g = acceleration due to gravity, m/ s2
Fig. 1.1 The Hydrostatic Paradox
The pressure intensity at all points along a horizontal plane remains the same
p1= p2 = p3
1. Hydrodynamics :
It is related to mechanics of moving fluid. It is concerned with the fluid flow laws and their effective forces.
a) Continuity equation : Matter such as hydraulic oil cannot be created nor destroyed and since it is incompressible the mass rate of flow of fluid into any fixed space is equal to the mass flow out of fluid. Thus the mass flow rate of fluid past all cross section of a tube is equal.
Fig. 1.2 Velocity of flow
In steady state, the mass of fluid passing through all cross section of a tube / unit time is the same.
i.e, ρ1A1v1 = ρ2A2v2
Since ρ1 = ρ2 ( same fluid is flowing in the pipe )
Where v1 , v2 = velocity of flow, m/s at cross section 1 and 2.
A1, A2 = cross sectional area at section 1 and 2.
Therefore, A1v1 = A2v2 -------1
Flow rate Q = V/ t , V = volume = A* S, where S= distance , t = time
i.e, Q = A * S / t
Distance S divided by time t is the quotient velocity v
i.e, v = S / t
therefore , Q = A* v
From 1 , Q= A1v1 = A2v2 = Constant which is the continuity equation.
Also Q is proportional to square root of pressure drop across the length
i.e Qα √ ∆ p , where ∆ p is the pressure drop
b) Bernoulli’s equation :
It states that the total energy of a flow of fluid does not change as long as energy is not supplied from the outside or transferred to the outside.
Total energy is made up of :
-- Potential energy
-- Pressure energy and dependent on the column of fluid and on static pressure.
-- Kinetic energy : It is dependent on the velocity of flow and on back pressure.
Hence, Ptotal = Z + p / ρg + v2/ 2g
Where Z =elevation
p= Pressure of fluid
v= Velocity of flow
ρ = Density of fluid
Fig. 1.3 Pressure in narrow points
Let us now consider both the continuity equation and Bernoulli’s equation. The following may be deduced.
If speed increases as the diameter decreases, the movement energy increases.
Since the total energy remains constant, potential energy or pressure energy, or both must change, i.e decreases when the diameter decreases. There is practically no measurable change in potential energy. However, the static pressure changes is depending upon the back pressure i.e dependent on the velocity of flow.
c) Friction and Pressure losses :-
If the fluid is still ( no fluid movement) , then pressure is same at all the point.
If fluid is flowing through the system, heat is created by friction. Thus part of energy is lost as heat energy, which means loss of pressure. The amount of friction loss is related to
· Length of pipe
· Roughness of the pressure
· Cross sectional area of pipe
· Number of pipe bends
· Velocity of fluid flow
· Viscosity of fluid flow.
Fig. 1.4 Pressure loss
d) Types of flow :
· Laminar flow : Up to a certain velocity fluid moves along pipes in layers (Laminar). The inner most fluid layer travels at the highest speed. The outer most fluid layer at the pipe wall does not move.
· Turbulent flow: It the velocity of flow is increased, the type of flow changes at the critical velocity and becomes turbulent. This results in an increase of flow resistance and thus the hydraulic losses increases. Therefore turbulent flow is not usually desirable. The critical velocity is not a fixed quantity, it is dependent on the viscosity of the fluid and on the cross sectional area through which flow passes.
e) Reynolds’s number Re :
The type of flow may be roughly determined Using Reynolds’s number
Re = v* dh /KV Where v = Velocity of flow m/s
dh = Hydraulic diameter in m, with circular cross- sections equal to
the pipe internal diameter, and otherwise calculated as
dh = 4 * A / U
A = Cross- sectional area,
U = Circumference
KV = Kinematic viscosity in m2 / s and
Re crit = 2300
At Re crit the type of flow changes from laminar to turbulent and vice versa.
Laminar flow occurs for Re < Recrit , and
Turbulent flow occurs for Re> Recrit
Pascal Law : Fluid power technology actually began in 1650 with the discovery of Pascal Law. Simply this law says that “ Pressure in a fluid at rest is transmitted equally in all directions “
This law states “ A pressure added to a confined fluid is transmitted undiminished throughout the fluid”.
It acts on all surfaces in a direction at right angle to those surfaces. The amount of pressure in the fluid is equal to the weight force with respect to the area being acted upon.
Fig 1.7 Pascal’s law
Mathematically, it is defined as Pressure = Force / Area
i.e, p = F / A
Hydraulic force transmission :
Fig. 1.8 Force transmission
As pressure distributes equally in all directions, the shape of the container is irrelevant. If we now pressurise surface A1 with force F1, we create pressure, p = F1 / A1
Pressure acts on all sides equally and simultaneously. It is therefore equal at all points. Therefore it acts also on surface A2 The force which can be achieved is
F2 = p A2
Thus F1 / A1 = F2 / A2
Or F2 / F1 = A2 / A1
The pressure in such a system always depend on the size of the load and effective surface. This means that the pressure rises until it can overcome the resistance, which builds up in opposition to the fluid movement.
If it is possible to achieve the pressure necessary to overcome the load F2 (Via surface A2 ) by means of force F1 and surface A1, Then load F2 can be raised . ( Friction losses need not be taken into consideration here).
The relationship of the paths S1 and S2 of the two pistons is then opposite to that of the surface. i.e., S1 / S2 = A2 / A1
Advantages of using Fluid Power :
1. Fluid power provides flexibility in the control of machines
2. Fluid power provides high forces (torque) with compact size i.e, high power density.
3. Stepless regulation of speed
4. Simple overload protection
5. Suitable for controlling fast movement and for extremely slow precision movement.
6. Hydraulic provides automatic lubrication for less wear
b) Bernoulli’s equation :
It states that the total energy of a flow of fluid does not change as long as energy is not supplied from the outside or transferred to the outside.
Total energy is made up of :
-- Potential energy
-- Pressure energy and dependent on the column of fluid and on static pressure.
-- Kinetic energy : It is dependent on the velocity of flow and on back pressure.
Hence, Ptotal = Z + p / ρg + v2/ 2g
Where Z =elevation
p= Pressure of fluid
v= Velocity of flow
ρ = Density of fluid
Fig. 1.3 Pressure in narrow points
Let us now consider both the continuity equation and Bernoulli’s equation. The following may be deduced.
If speed increases as the diameter decreases, the movement energy increases.
Since the total energy remains constant, potential energy or pressure energy, or both must change, i.e decreases when the diameter decreases. There is practically no measurable change in potential energy. However, the static pressure changes is depending upon the back pressure i.e dependent on the velocity of flow.
c) Friction and Pressure losses :-
If the fluid is still ( no fluid movement) , then pressure is same at all the point.
If fluid is flowing through the system, heat is created by friction. Thus part of energy is lost as heat energy, which means loss of pressure. The amount of friction loss is related to
· Length of pipe
· Roughness of the pressure
· Cross sectional area of pipe
· Number of pipe bends
· Velocity of fluid flow
· Viscosity of fluid flow.
Fig. 1.4 Pressure loss
d) Types of flow :
· Laminar flow : Up to a certain velocity fluid moves along pipes in layers (Laminar). The inner most fluid layer travels at the highest speed. The outer most fluid layer at the pipe wall does not move.
· Turbulent flow: It the velocity of flow is increased, the type of flow changes at the critical velocity and becomes turbulent. This results in an increase of flow resistance and thus the hydraulic losses increases. Therefore turbulent flow is not usually desirable. The critical velocity is not a fixed quantity, it is dependent on the viscosity of the fluid and on the cross sectional area through which flow passes.
e) Reynolds’s number Re :
The type of flow may be roughly determined Using Reynolds’s number
Re = v* dh /KV Where v = Velocity of flow m/s
dh = Hydraulic diameter in m, with circular cross- sections equal to
the pipe internal diameter, and otherwise calculated as
dh = 4 * A / U
A = Cross- sectional area,
U = Circumference
KV = Kinematic viscosity in m2 / s and
Re crit = 2300
At Re crit the type of flow changes from laminar to turbulent and vice versa.
Laminar flow occurs for Re < Recrit , and
Turbulent flow occurs for Re> Recrit
Pascal Law : Fluid power technology actually began in 1650 with the discovery of Pascal Law. Simply this law says that “ Pressure in a fluid at rest is transmitted equally in all directions “
This law states “ A pressure added to a confined fluid is transmitted undiminished throughout the fluid”.
It acts on all surfaces in a direction at right angle to those surfaces. The amount of pressure in the fluid is equal to the weight force with respect to the area being acted upon.
Fig 1.7 Pascal’s law
Mathematically, it is defined as Pressure = Force / Area
i.e, p = F / A
Hydraulic force transmission :
Fig. 1.8 Force transmission
As pressure distributes equally in all directions, the shape of the container is irrelevant. If we now pressurise surface A1 with force F1, we create pressure, p = F1 / A1
Pressure acts on all sides equally and simultaneously. It is therefore equal at all points. Therefore it acts also on surface A2 The force which can be achieved is
F2 = p A2
Thus F1 / A1 = F2 / A2
Or F2 / F1 = A2 / A1
The pressure in such a system always depend on the size of the load and effective surface. This means that the pressure rises until it can overcome the resistance, which builds up in opposition to the fluid movement.
If it is possible to achieve the pressure necessary to overcome the load F2 (Via surface A2 ) by means of force F1 and surface A1, Then load F2 can be raised . ( Friction losses need not be taken into consideration here).
The relationship of the paths S1 and S2 of the two pistons is then opposite to that of the surface. i.e., S1 / S2 = A2 / A1
Advantages of using Fluid Power :
1. Fluid power provides flexibility in the control of machines
2. Fluid power provides high forces (torque) with compact size i.e, high power density.
3. Stepless regulation of speed
4. Simple overload protection
5. Suitable for controlling fast movement and for extremely slow precision movement.
6. Hydraulic provides automatic lubrication for less wear
