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# Hydraulic pumps performance and Characteristics

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Volumetric efficiency,Actual Pump Output, Q
Mechanical efficiency
overall efficiency
Power to Drive the Pump,
What Determines ηv & ηm ?,
Comparative analysis of pumps
Cavitation
Sizing Pumps
Pumps Selection,
Pump Symbols

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### Hydraulic pumps performance and Characteristics

1. 1. Hydraulic Pumps >>Performance and Characteristics
2. 2. General Issues  Pumps are not strictly continuous flow devices. Discrete chambers are involved.  Flow is collected for discharge through valve plates  Design of the valve plate and the pump mechanism affects pressure pulses and variation (ripple) of torque and pressure
3. 3. General Issues  Our theoretical displacements can be used to determine theoretical pump flow  Qth =Displacement (cc/rev) * Speed (rpm)  Actual flow is a linear function of pump displacement, speed, a units constant, and an efficiency term  Two kinds of inefficiencies to account for losses:  Volumetric efficiency (slip)  Mechanical efficiency (Friction losses)
4. 4. Volumetric efficiency This indicates the amount of leakage, which takes place within the pump and involves considerations such as manufacturing tolerances and flexing of the pump casing.
5. 5. Actual Pump Output, Q  QA = (VD np ηV) /1000 where: Q: L/min VD : cm3 /rev ηV: Volumetric efficiency (decimal)  OR… QA = (VD np ηV) /231 where: Q: GPM VD: in3 /rev ηV: same as above (no units)
6. 6. Mechanical efficiency  This indicates the amount of energy lost by friction in bearing and other moving parts and Energy losses due to fluid turbulence.  mech eff =
7. 7. Mechanical efficiency  Mechanical efficiency can also be computed in terms of torque, and called torque efficency:
8. 8. overall efficiency The ratio of power output to power input to the pump Or the Product of both volumetric and mechanical efficiencies is known as the overall efficiency
9. 9. Torque to Drive a Pump  TA = (ΔP VD)/(2π ηm) where: TA : Newton meters torque required ΔP : pressure rise across the pump in MPa VD : Pump displacement in cm3 /rev ηm: Pump mechanical (torque) efficiency – a decimal  OR…
10. 10. Torque to Drive a Pump English Units  TA = (ΔP VD)/(2π ηm) where: TA : is torque required ΔP : pressure rise across the pump in PSI VD : Pump displacement in inches3 /rev ηm: Pump torque efficiency – a decimal
11. 11. Power to Drive the Pump  The hydraulic (theoretical) power delivered by the pump is QActualΔP/600 or QactualΔP/1714 for SI English units (note this is actual pump flow, not theoretical)  Shaft power to drive the pump is given by Psp = Phydr / ηo where:  η o = ηv ηm which is total pump efficiency
12. 12. What Determines ηv & ηm ?  ηv is a function of clearance spaces, system pressure, viscosity and pump speed  Leakage flow at a given pressure is relatively fixed regardless of pump speed  It is also affected by fluid viscosity as lower viscosity fluid will result in higher leakage and lower volumetric efficiency
13. 13. What about Torque (mechanical) Efficiency?  Torque efficiency is a function of speed and fluid viscosity  Higher pump speeds will result in lower efficiency as viscous friction is speed dependent  Lower viscosity fluid can reduce viscous losses but acts negatively on volumetric efficiency
14. 14. Typical Performance curves for pumps
15. 15. Other Factors affecting pump performance • Presence of foreign particles cause damage to the internal surfaces of a pump. • Foams and bubbles Generate noise and causes cavitation • Overheating of oil poor lubricant and increases the internal leakage, reducing pump capacity • Wrong selection of oil. select the oil in accordance with the ambient temperature and follow the instructions of pump manufacturer
16. 16. Comparative analysis of pumps
17. 17. Cavitation  Pump cavitation can occur due to entrained air bubbles in the hydraulic fluid or vaporization of the hydraulic fluid  To control cavitation keep the suction pressure above saturation pressure of fluid by:  Keeping suction line velocities below 4 ft/sec (~1m/s) and pump inlet lines as short as possible  Minimize inlet line fittings; mount pump close to reservoir; use low-pressure drop filters on inlet, and use proper oil
18. 18. Catalogue example
19. 19. Double pumps
20. 20. Sizing Pumps  Component sizing begins with the LOAD  Load and actuator will determine  Flow requirement for this circuit  Pressure range required by the circuit (We’ll do this with cylinders and motors… soon)  Total and simultaneous flow requirements  Select for the maximum load pressure  Add pressure drops that will occur in valves, lines and fittings
21. 21. Pump Sizing  With pump outlet pressure and flow known we will consider speed.  Industrial apps will use synchonous speed of electric motors. Generally 1750 rpm, or possibly 1100. (\$ decides)  Small diesel apps such as skid loaders can operate directly from engine crankshaft and will have engine speed. (2000-3000 rpm).  Larger diesel apps – pump splitter with gear reductions possible to optimize speed
22. 22. Pump Sizing  Determine appropriate speed for your app  Use the equation for pump flow, solved for displacement  VD = 1000Q/p (np ηV)  What shall we use for ηV ??  This is a function of speed, pressure, and fluid viscosity  Look for vendor data or curves and adjust…
23. 23. Pumps Selection • Flow rate requirement • Operating speed • Pressure rating • Performance • Reliability • Maintenance • Cost and • Noise.
24. 24. Pump Symbols  Any fixed displacement pump  Variable displacement pump  Variable displacement – Pressure Compensated