Chapter 2.pptx
- 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
- 4. (a)
- 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.
- 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
- 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? 3