HYDROSTATIC PUMPS AND MOTORS

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Ashlee Stevenson
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1 HYOSAIC PUMPS AN MOOS High-pressure hydraulic pups have a fixed displaceent that is they generate a fixed volue of fluid per revolution so that operating at a fixed speed produces a constant flow. In soe units the displaceent can be varied. ifferent types of fixed displaceent pups and otors are described in Chapter 2, soe of which are shown here: Gear type Inlet flow Inlet Outlet Inlet Outlet Vane type

2 Axial piston type Low speed otor Flow and speed relationship For the ideal achine with no leakage, the displaceent of the achine and its speed of rotation deterine the flow rate Q. hus: Q ω (1) where is voluetric displaceent [ 3 rad -1 ] ω is the rotational speed [rad sec -1 ]

3 For pups that are driven by electric otors the speed is often constant. However for otors, the speed depends on the level of the supplied flow: hus: Q ω (2) Voluetric efficiency he internal flow leakage in pups and otors affects the relationship between flow and speed and is taken into account by the use of the voluetric efficiency ( v ). hus for pups equation 1 becoes Q vω (3) And for otors equation 2 becoes Q ω v (4) he voluetric efficiency varies with the fluid viscosity, pressure and rotating speed as discussed in ore detail in chapter 8. Manufacturers will usually give values for the voluetric efficiency for operation at specified conditions. orque and pressure relationship For the ideal achine, the echanical power is entirely converted to fluid power, Power ω P Q (5) Where is the torque [N] P is the differential pressure [N -2 ] Fro equation 5 we get: QP which fro equation 2 gives P (6) ω hus the ideal torque is proional to the pressure for a given displaceent. In a pup this is the input torque required fro the prie over and for a otor, it is the output torque available fro the otor shaft. Mechanical efficiency he presence of friction between the oving parts creates echanical losses that are represented by the echanical efficiency ( ). hus: For pups the required input torque is given by And for otors the output torque is given by: P (7) P (8)

4 he echanical efficiency, as for the voluetric efficiency, will vary with the fluid viscosity, pressure and rotating speed as discussed in ore detail in chapter 8. he power input, H, to a pup is: PQ H (9) v he power output fro a otor is: H v PQ (10) he total efficiency of both units is therefore: v Figure 7 Pup Perforance Characteristics Figure 8 Motor Perforance Characteristics Figures 7 and 8 show how the easured perforance of pups and otors are presented for use with a particular fluid at a particular viscosity. For the pup it can be seen that the flow output reduces with the output pressure at constant speed because of the effect of the increasing leakage flow loss. For the otor, the output torque varies with increasing speed at constant pressure as a result of the variation in the echanical efficiency. he theoretical analysis given in chapter 8 shows how the efficiencies are related to the syste paraeters which enables the perforance for operation at other conditions to be predicted. Worked exaple fro Chapter 11 4 Winch Application

5 P ω ω r r M Figure 1 - Winch driven by otor and reduction gearbox. 4.1 Lifting the load Winch dru torque Motor torque Motor pressure P M g r d d M g r M g r d d otor echanical efficiency reduction gearbox echanical efficiency winch dru echanical efficiency 4.2 Lowering the load M g r P P P M g r up 1 1 down ( ) 2 d 2 M g r 4.3 Nuerical Values ata r 0.25 M 2.5 onne 574 c 3 rev x rad -1 5:1 (reduction) 0.78 (starting), 0.92 (running) d 0.94 Ideal pressure

6 M g r 2500 x 9.81 x0.25 P 133 bar -5 5 x 9.1x10 herefore, during start-up fro rest: 0.69 P up 193 bar & P down 92 bar And when operating at speed: P up 164bar &P OWN 108bar ypical application of hydraulic otors for winch and wheel drive (worked exaple in Chapter 11)

Unit 24: Applications of Pneumatics and Hydraulics

Unit 24: Applications of Pneumatics and Hydraulics Unit code: J/601/1496 QCF level: 4 Credit value: 15 OUTCOME 2 TUTORIAL 3 HYDRAULIC AND PNEUMATIC MOTORS The material needed for outcome 2 is very extensive