This document provides an overview of hydraulics and pneumatics. It discusses key fluid properties like density, pressure, viscosity, and compressibility. It also describes different types of pumps used in hydraulic systems, including dynamic pumps like centrifugal pumps and positive displacement pumps like gear, vane, piston, and screw pumps. Specifically, it explains how external gear pumps work by using meshing gears to carry fluid between their teeth and displace it from the suction to discharge side.

1. Hydraulics and Pneumatics
Prepared by
S.David Blessley AP/Mechanical Engg.,
Kamaraj College of Engineering and Technology,
Virudhunagar
Unit 1 Fluid Power Principles and Hydraulic Pumps
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

2. • Specific density
• Specific density or specific mass or mass density is the mass per unit volume of the
fluid.
• 𝝆 =
𝑴𝒂𝒔𝒔
𝑽𝒐𝒍𝒖𝒎𝒆
Unit : kg/m3
• With increase in temperature volume of fluid increases and hence mass density
decreases in case of fluids as the pressure increases volume decreases and hence
mass density increases.
• Specific weight
• Specific weight or weight density of a fluid is the weight per unit volume
• γ =
𝒘𝒆𝒊𝒈𝒉𝒕
𝒗𝒐𝒍𝒖𝒎𝒆
Unit : N/m3
• With increase in temperature volume increase and hence specific weight decreases.
With increase in pressure volume decreases and hence specific weight increases.
• Therefore specific weight = specific density x acceleration due to gravity
Properties of Fluids
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

3. • Specific volume:
• It is the volume per unit mass of the fluid.
• Specific volume ɏ =
𝑽𝒐𝒍𝒖𝒎𝒆
𝑴𝒂𝒔𝒔
Unit – m3/kg
• Specific gravity:
• It is the ratio of density of the fluid to the density of a standard fluid.
• S =
ρ 𝒇𝒍𝒖𝒊𝒅
ρ 𝑺𝒕𝒂𝒏𝒅𝒂𝒓𝒅 𝒇𝒍𝒖𝒊𝒅
• Dimensionless quantity having no unit
• Pressure:
• Pressure of a fluid is the force per unit area of the fluid. In other words, it is the ratio
of force on a fluid to the area of the fluid held perpendicular to the direction of the
force.
• Pressure =
𝒇𝒐𝒓𝒄𝒆
𝒂𝒓𝒆𝒂
Unit - N/m2.
• Pressure is denoted by the letter ‘P’.
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

4. • Viscosity
• Viscosity is a measure of a hydraulic fluid's resistance to flow. It is a hydraulic fluid's most
important characteristic and has a significant impact on the operation of the system.
• When a hydraulic oil is too thin (low viscosity), it does not seal sufficiently. This leads to
leakage and wear of parts. When a hydraulic oil is too thick (high viscosity), the fluid will be
more difficult to pump through the system and may reduce operating efficiency.
• Dynamic or absolute viscosity
• Absolute viscosity - coefficient of absolute viscosity - is a measure of internal resistance.
• Dynamic (absolute) viscosity is the tangential force per unit area required to move one
horizontal plane with respect to an other plane - at an unit velocity - when maintaining an
unit distance apart in the fluid.
• τ=μ
𝒅𝒚
𝒅𝒕
Unit – N-s/m2
• Kinematic viscosity
• Kinematic viscosity is the ratio of absolute (or dynamic) viscosity to density
• Kinematic viscosity can be obtained by dividing the absolute viscosity of a fluid with
the fluid mass density
• V=
𝝁
𝝆
Unit – m2/s
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

5. • Compressibility
• Compressibility is defined as the ratio of change in volume to the change in pressure.
It is denoted by the letter B. For fluids, compressibility depends on either the
adiabatic or isothermal process.
• Bulk Modulus
• Bulk modulus is defined as the ratio between increased pressure and decreased
volume of the material. It is denoted by the letter K.
• Oxidation Stability:
• Oxidation is caused by a chemical reaction between the oxygen of the dissolved air
and the oil.
• The oxidation of the oil creates impurities like sludge, insoluble gum and soluble
acidic products.
• The soluble acidic products cause corrosion and insoluble products make the
operation sluggish.
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

6. • Demulsibility:
• The ability of a hydraulic fluid to separate rapidly from moisture and successfully resist
emulsification is known as Demulsibility.
• If oil emulsifies with water the emulsion will promote the destruction of lubricating value
and sealant properties. Highly refined oils are basically water resistance by nature.
• Lubricity:
• Wear results increase in clearance which leads to all sorts of operational difficulties including
fall of efficiency.
• At the time of selecting a hydraulic oil care must be taken to select one which will be able to
lubricate the moving parts efficiently.
• Rust prevention
• The moisture entering into the hydraulic system with air causes the parts made from ferrous
materials to rust
• This rust if passed through the pumps and valves may scratch the nicely polished surfaces
• Thus additives named inhibitors are added to the oil to keep the moisture away from the
surface
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

7. • Pour point
• The temperature at which oil will clot is referred as pour plot
• Flash point and fire point
• The temperature at which a liquid gives off vapour in sufficient quantity to ignite
momentarily when flame is applied is called flash point
• The minimum temperature at which the hydraulic fluid will catch fire and
continue burning is called fire point
• Neutralization number
• The measure of acidity or alkalinity of a hydraulic fluid.
• Also referred as pH value of a fluid
• High acidity causes oxidation rate to increase rapidly
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

8. Pump:
A pump is a mechanical device, that is used to pick up water
from low-pressure level to high-pressure level. Basically, the pump is
a device which converts mechanical energy into hydraulic energy.
Pump operates on the principle whereby a partial vacuum is created
at the pump inlet due to the internal operation of the pump
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

9. As the piston is pulled to the left, a
partial vacuum is generated in pump
cavity and allows atmospheric
pressure to push fluid from the
reservoir into the pump via check
valve
when the piston is pushed to
the right, the fluid movement closes
inlet valve and opens outlet valve. The
quantity of fluid displaced by the
piston is ejected out the discharging
line leading to the hydraulic system.
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

10. Pump Classification
• There are two broad classifications of pumps
• Dynamic pumps (Non Positive displacement pumps)
• Positive displacement pumps
• Dynamic Pumps
• Generally used for low pressure, high volume flow applications
• Not capable of withstanding high pressures. i.e., limited to 250-300 psi
• Primarily used for transporting fluids from one location to another
• Positive displacement pumps
• As name implies, ejects a fixed amount of fluid per revolution of shaft
rotation
High pressure capability
Small, compact size
High volumetric efficiency
Great flexibility of performance
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

11. Pumps
Dynamic Pumps
Positive
Displacement Pumps
Axial flow
propeller pumps
Centrifugal
Pumps
PistonVaneGear
Fixed displacement
Vane
Variable
displacement Vane
Fixed displacement
Piston
Variable
displacement Piston
A fixed displacement pump is one in which the amount of fluid ejected per revolution cannot be varied
A variable displacement pump is one in which amount of fluid ejected can be varied by changing the physical
relationships of various elements
External Gear Internal Gear Lobe pump Screw pump
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

12. Dynamic Pumps
• These pumps are typically used for low pressure, high volume flow
applications
• In dynamic pumps, the clearance between the rotating impeller or
propeller and the stationary housing is high. Thus, as the resistance of
the external system starts to increase, some of the fluid slips back
into the clearance spaces causing a reduction in discharge flow rate
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

13. • Centrifugal Pump
• The most commonly used type of dynamic pump
• The fluid enters at the center of the impeller and is picked up by the rotating impeller
• As the fluid rotates with the impeller, the centrifugal force causes the fluid to move
radially outward
• This causes the fluid to flow through the outlet discharge port of the housing
• Although dynamic pumps provide smooth continuous flow, their output flow rate is
reduced as resistance to flow is increased.
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

14. • Positive displacement pumps
• Ejects a fixed quantity of fluid per revolution of the pump shaft. This makes them
particularly well suited for fluid power systems
• However, positive resistance pumps must be protected against overpressure if the
resistance to flow becomes very large
• A pressure relief valve is used to protect the pump against overpressure by
diverting pump flow back to the hydraulic tank
Classification of Positive Displacement pumps
• Vane pumps
a. Unbalanced vane pumps
b. Balanced vane pumps
• Piston pumps
a. Axial design
b. Radial design
• Gear pump
a. External gear pumps
b. Internal gear pumps
c. Lobe pumps
d. Screw pumps
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

15. Gear Pumps
• External Gear Pump
• Develops flow by carrying fluid between the
teeth of two meshing gears
• One of the gears is connected to a drive shaft
which is connected to the prime mover
• Another one is the driven gear which meshes
with the driver gear
• Oil chambers are there between the gear teeth,
the pump housing and the side wear plates
• The suction side is where teeth come out of
mesh and here the volume expands bringing
about a reduction in pressure below
atmospheric pressure
• Fluid is pushed into this void by atmospheric pressure because the oil supply tank
is vented to the atmosphere
• The discharge side is where teeth go into mesh and the volume decreases between
mating teeth and ejects the oil into the outletS.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

16. • The volumetric displacement and theoretical flow rate of a gear pump
can be determined as follows
• Do = Outside diameter of gear teeth
• Di = Inside diameter of gear teeth
• L = Width of gear teeth
• VD = Displacement volume of pump
• N = rpm of pump
• QT = Theoretical pump flow rate
• QA = Actual flow rate
𝑉𝐷 =
π
4
𝐷 𝑂
2
− 𝐷𝐼
2
𝐿
𝑄 𝑇 =
𝑉𝐷 𝑋𝑁
60
𝜂 𝑣 =
𝑄 𝐴
𝑄 𝑇
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

17. • Internal Gear pump
• It consists of an internal gear, a regular spur
gear, a crescent shaped seal and an external
housing
• As power is applied to either gear, the motion of
the gears draws fluid from the reservoir and
forces it around both sides of the crescent seal
which acts as a seal between the suction and
discharge ports
• When the teeth mesh on the side opposite to
the crescent seal, the fluid is forced to enter the
discharge port of the pump.
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

18. • Lobe Pump
• Lobe pump operates in the same way as
external gear pump. But, unlike the
external gear pump both lobes are driven
externally so they do not actually contact
each other.
• Thus they are quieter than other gear
pumps
• As the lobes turn, fluid is held between
each lobe and the wall of the pump
chamber and carried around from suction
to discharge
• The volumetric displacement is generally
greater than other types of gear pumps.
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

19. • Gerotor Pump
• It operates much like a internal gear pump
• The inner gear rotor (gerotor element) is power driven and draws the outer
gear rotor around as they mesh together
• This forms inlet and discharge pumping chambers between rotor lobes
• The inner gear has one tooth less than the outer teeth and the volumetric
displacement is determined by the space formed by the extra tooth in the
outer rotor
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

20. • Screw pump
• The screw pump is an axial flow positive
displacement unit
• Three precision ground screws, meshing within a
close fitting housing deliver non-pulsating flow
quietly and efficiently
• The two symmetrically opposed idler rotors acts as
rotating seals confining the fluid in a succession of
closures
• The fluid enters from the inlet at the bottom of the
pump. The driver and the driven screw rotate in
opposite directions, which builds up a suction
pressure at the lower part of the pump.
• This pressure drives the fluid upwards. The fluid
crosses the small clearance between the two
screws and experiences a centrifugal force. The
combination of suction pressure and centrifugal
forces the fluid up and out of the pump.
• Axial hydraulic forces on the rotor set are balanced
and thus eliminating the need of thrust bearings
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

21. S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

22. • Vane pumps
• The rotor which contains radial slots is splined to the
drive shaft and rotates inside a cam ring
• Each slot contains a vane designed to mate with the
surface of the ring as the rotor turns
• Centrifugal force keeps the vanes out against the
surface of the cam ring
• During one half revolution of rotor rotation, the
volume increases between the rotor and cam ring
• This volume expansion causes a reduction of
pressure and thus suction occurs which causes the
fluid to flow through inlet port and fill the void
• As the rotor rotates through the second half
revolution, the surface of the cam ring pushes the
vanes back into their slots and the volume is
reduced. This positively ejects the trapped fluid
through the discharge port
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

23. S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

24. There is an eccentricity between the centerline of the rotor and the centerline of the cam ring. If the
eccentricity is zero, there will be no flow.
𝑒 𝑚𝑎𝑥 =
𝐷 𝐶 − 𝐷 𝑅
2
This maximum value of eccentricity produces a maximum volumetric displacement:
𝑉𝐷𝑚𝑎𝑥 =
𝜋
4
𝐷 𝐶
2
− 𝐷 𝑅
2
𝐿
Theoretical flow rate can be given by,
Some vane pumps have provisions for mechanically varying the eccentricity. A handwheel or a
pressure compensator can be used to move the cam ring to change the eccentricity.
Fixed Displacement Unbalanced Vane pump
In the fixed displacement pump, the rotor housing eccentricity is constant. Hence the
displacement volume is fixed. A constant volume of fluid is discharged during each revolution of rotor
Variable Displacement Unbalanced Vane Pump
Variable displacement can be provided if the housing can be moved with respect to the rotor.
This movement changes the eccentricity and hence the displacement
𝑄 𝑇 =
𝑉𝐷 𝑋𝑁
60
DC = Outside diameter of gear teeth
DR = Inside diameter of gear teeth
L = Width of gear teeth
VD = Displacement volume of pump
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

25. • Piston Pumps
• A piston pump works on the principle that a reciprocating piston can draw in fluid
when it retracts in a cylinder bore and discharge when it extends
• There are two basic types of piston pumps. One is the axial design having pistons
parallel to the axis of the cylinder block
• The another type of piston pump is the radial design which has pistons arranged
radially in a cylinder block
• Axial piston pumps can be either of bent axis type or swash plate type.
• Axial Piston Pump
• In the axial piston pump, rotary shaft motion
is converted to axial reciprocating motion
which drives the piston
• Most axial piston pumps are multi piston
designs and utilize check valves or port plates
to direct liquid flow form inlet to discharge.
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

26. • In line axial piston pump
• An axial piston pump in which the pistons are in line with the axis of the drive
shaft and is shown in fig.
• The drive motion is converted to reciprocating by means of swash plate mounted
on the drive shaft
• Thus the rotation of the swash plate produces in and out motion of the pistons in
their cylinders and hence the fluid is discharged.
• Fixed displacement In line axial pump
• the axial pump which discharges a constant volume of oil is the fixed
displacement pump
• Variable displacement In line pump
• Axial displacement pump can be designed to have variable displacement
capability. This can be achieved by altering the angle of the swash plate
• When the piston carrying body turns, the exit passages in the cylinder bores
move along the control slots of a firmly positioned control plate and thus
connected alternatively to the suction or discharge pipelinesS.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

27. • Fixed displacement bent axis piston pump
• In fixed displacement pump, the pumps are
mounted in a fixed casing so that swing angle
cannot be adjusted. So displacement of the
piston and hence the constant discharge of
fluid are achieved
• Variable displacement bent axis piston
pump
• In variable displacement pumps, the swing
angle can be varied. Because the volumetric
displacement of the pump varies with the
offset angle.
• The increase in offset angle will increase the
piston stroke and hence the fluid
displacement. When the offset angle is zero,
then the displacement will be zero. However,
the practical reasons, angle is to be varied
from 0 to a maximum of 30
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

28. • Radial Piston Pump
• The radial piston pump has a number of radial
pistons in a cylinder block which revolves
around a stationary eccentric cam
• In these pumps, the pistons move
perpendicularly to the shaft centerline. As the
cylinder block rotates, the eccentricity of the
causes an in and out/pumping motion of the
pistons
• As seen in fig., the fluid inflow and outflow at
each piston is controlled through revolving
parts.
• During the downstroke, each piston is
connected to the fluid inlet and hence the fluid
is drawn inside the cylinders. During the upward
stroke, each piston is connected to the fluid
outlet and hence fluid is discharged outside the
pump.
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

29. Pump Performance.
1. Volumetric efficiency (ᾐvol ):
volumetric efficiency indicates the amount of leakage that takes
place within the pump. This involves considerations such as
manufacturing tolerances and flexing of the pump casing under
design pressure operating conditions:
Volumetric efficiencies typically run from 80% to 90% for gear
pumps, 82% to 92% for vane pumps, and 90% to 98% for piston
pumps.
100A
T
Q
Q

𝜂 𝑉 =
𝐴𝑐𝑡𝑢𝑎𝑙𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑 𝑏𝑦 𝑝𝑢𝑚𝑝
𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝑝𝑢𝑚 𝑠ℎ𝑜𝑢𝑙𝑑 𝑝𝑟𝑜𝑑𝑢𝑐𝑒
=
𝑄 𝐴
𝑄 𝑇
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

30. 2. Mechanical efficiency (ᾐmech):
Mechanical efficiency indicates the amount of energy losses that
occur due to reason other than leakage. This includes friction in
bearings and between other mating parts.
It also includes energy losses due to fluid turbulence.
Mechanical efficiencies typically run from 90% to 95%.
theoretical powe required to operate pump
power delivered to pump
pump output power assuming no leakage
m
m
actual
or
input power delivered to pump




𝜂 𝑚𝑒𝑐ℎ =
𝑃 𝑥 𝑄 𝑇
𝑇𝐴 𝑥 𝜔
𝑥100
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

31. 3. Overall efficiency (ᾐoverall):
The overall efficiency considers all energy losses and is defined
mathematically as follows:
volumetric effeciency mechanical efficieancy
efficiency=
100
overall

1
1
/1714100
100
100 100 / 63,000
o m A
o
Q PQ
Q TN
 
   
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

32. PUMP
TYPE
PRESSURE
RATING
(PSI)
SPEED
RATING
(RPM)
OVERALL
EFFICIEN
CY
(PER
CENT)
HP
PER
LB
RATIO
FLOW
CAPACITY
(GPM)
COST (DOL
LARS PER
HP)
EXTERNAL
GEAR
INTERNAL
GEAR
VANE
AXIAL PISTON
RADIAL
PISTON
2000-3000
500-2000
1000-2000
2000-12000
3000-12000
1200-2500
1200-2500
1200-1800
1200-3000
1200-1800
80-90
70-85
80-95
90-98
85-95
2
2
2
4
3
1-150
1-200
1-80
1-200
1-200
4-8
4-8
6-30
6-50
5-35
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.

33. Pump Selection.
• Select the actuator (hydraulic cylinder or motor) that is appropriate based on the
loads encountered.
• Determine the flow-rate requirements. This involves the calculation of the flow
rate necessary to drive the actuator to move the load through a specified distance
within a given time limit.
• Select the system pressure. This ties in with the actuator size and the magnitude of
the resistive force produced by the external load on the system.
• Determine the pump speed and select the prime mover.
• Select the pump type based on application.
• Pump type based on the Application & Cost.
• Select the reservoir and associated plumbing, including piping, valving, hydraulic
cylinders, and motors and other miscellaneous components.
• Consider factors such as noise level, hp loss, pump wear etc.,
• Calculate overall cost
S.David Blessley AP/Mechanical Engg., Kamaraj College of
Engineering and Technology, Virudhunagar.