1. Theoretical discharge is:
If the gear is specified by its module and number of teeth, then the theoretical discharge can
be found by:
2. Gear pumps as the name suggests make use of the principle of two gears in mesh in order to generate
pumping action. They are compact, relatively inexpensive and have few moving parts. Gear pumps are
further classified as:
3. Figure 2.4Illustration of pumping theory
2.2.3.1 External gear pumps
External gear pumps are the most popular hydraulic pumps in low-pressure ranges due
to their long operating life, high efficiency and low cost. They are generally used in a simple
machine. The most common form of external gear pump is shown in Figs. 2.5a and b It consist
of a pump housing in which a pair of precisely machined meshing gears runs with minimal
radial and axial clearance. One of the gears, called a driver, is driven by a prime mover. The
5. Figure 2.6: Terminology of spur gears
Where
Do =the outside diameter of gear teeth
Di= the inside diameter of gear teeth
L = the width of gear teeth
N = the speed of pump in RPM
VD = the displacement of pump in m/rev
M = module of gear
z =number of gear teeth
α = pressure angle
2.2.3.2 Internal Gear Pumps
Internal gear pump is another variation of the basic gear pump. Figure 2.7 illustrates clearly, the internal
construction and operation of an internal gear pump. The design consists of an internal gear, a regular
6. 2.2.3.3 Lobe pump
The lobe pump is yet another variation of the basic gear pump. This pump operates in
a fashion quite similar to that of an external gear pump, but unlike external gear pumps, the
gears in these pumps are replaced with lobes which usually consist of three teeth. Figure 2.8
shows the operation of a lobe pump. Unlike the external gear pumps, both the lobes are driven
externally so that they do not actually make contact with each other. They are quieter than the
other gear pumps. Due to the smaller number of mating elements, the lobe pump will show a
greater amount of pulsation. However, its volumetric displacement is generally greater than other
types of gear pumps. Although these pumps have a low-pressure rating, they are well-suited for
applications involving shear-sensitive fluids.
7. Figure 2.8 Schematic diagram of Gerotor pump operation
Example 2.1: The inlet to a hydraulic pump is 0.6 m below the top surface of an oil reservoir.
If the specific gravity of the oil used is 0.86, determine the static pressure at the pump inlet.
8. Example 2.3:
A gear pump has an outside diameter of 80mm, inside diameter of 55mm and a width of 25mm.
If the actual pump flow is 1600 RPM and the rated pressure is 95 LPM what is the volumetric
displacement and theoretical discharge.
9. of the gear is 20. Pump speed is 1600 RPM. Outer diameter of gear is 108 mm and Dedendum
circle diameter is 81 mm. Volumetric efficiency is 88% at 7 MPa.
2.2.3.4 Screw pump
A schematic diagram of a screw pump is shown in Fig 2.9. A two-screw pump consists
10. The following is an expression for the geometric volume of a twin-gear screw pump:
2.2.3.5 Advantages and disadvantages of screw pump
1. The advantages are as follows:
2. They are self-priming and more reliable.
3. They are quite due to rolling action of screw spindles.
4. They can handle liquids containing gases and vapor.
5. They have long service life.
The disadvantages are as follows:
1. They are bulky and heavy.
2. They are sensitive to viscosity changes of the fluid.
3. They have low volumetric and mechanical efficiencies.
4. Manufacturing cost of precision screw is high.
11. Figure 2.10 Schematic diagram of Unbalanced vane pump
Schematic diagram of variable displacement vane pump is shown in Fig.2.11. Variable
displacement feature can be brought into vane pumps by varying eccentricity between the rotor
and the cam ring. Here in this pump, the stator ring is held against a spring loaded piston. The
system pressure acts directly through a hydraulic piston on the right side. This forces the cam
ring against a spring-loaded piston on the left side. If the discharge pressure is large enough, it
overcomes the compensated spring force and shifts the cam ring to the left.
12.
13. 2.2.3.7 Axial piston pump
Axial piston pumps convert rotary motion of an input shaft to an axial reciprocating motion of the
pistons. They in turn are categorized as:
(a) Bent-axis-type piston pumps and
(b) Swash plate-type inline piston pumps.
These two types are discussed separately below.
(a) Bent-axis-type piston pumps
In these pumps, the reciprocating action of the pistons is obtained by bending the axis of the
cylinder block so that it rotates at an angle different than that of the drive shaft. The cylinder block is
turned by the drive shaft through a universal link. The centreline of the cylinder block is set at an offset
angle, relative to the centerline of the drive shaft. The cylinder block contains a number of pistons along
its periphery. These piston rods are connected to the drive shaft flange by ball-and-socket joints. These
14. Figure 2.12: Bent axis piston pump
(b) Swash-Plate-Type Piston Pump
Schematic diagram of swash plate type piston pump is shown in Fig. 2.13a and b. In
this type, the cylinder block and drive shaft are located on the same centreline. The pistons are
connected to a shoe plate that bears against an angled swash plate. As the cylinder rotates, the
pistons reciprocate because the piston shoes follow the angled surface of the swash plate. The outlet
and inlet ports are located in the valve plate so that the pistons pass the inlet as they are being pulled
out and pass the outlet as they are being forced back in. This type of pump can also be designed to
have a variable displacement capability. The maximum swash plate angle is limited to 17.5° by
construction. Figure 2.14 describes the motion of one piston during a single rotation of the
cylinder block.
15. (b)
Figure 2.13 (a), (b) Schematic diagram of Swash-Plate-Type Piston Pump
Figure 2.14: Schematic illustrating the motion of one piston during a single rotation of the
cylinder block
16.
17.
18.
19. Example 2.9:
A positive displacement pump has an overall efficiency of 88% and a volumetric efficiency
of 92%. What is the mechanical efficiency?
Example 2.10:
Determine the overall efficiency of a pump driven by a 10 HP prime mover if the pump
delivers fluid at 40 LPM at a pressure of 10 MPa.
20. Example 2.12:
A pump has a displacement volume of 98.4 cm3. It delivers 0.0152 m /s of oil at 1000 RPM
and 70 bar. If the prime mover input torque is 124.3 Nm. What is the overall efficiency of
pump? What is the theoretical torque required to operate the pump?
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