Hydraulic Pumps

1. Hydraulic Pumps Positive Displacement Devices Displacement Formulae Characteristics

2. Gear Pumps(External Gear) • Pumping Mechanism

3. Gear Pumps(External Gear) • Displacement parameters and determination • Displacement = π/4(Do2 – Di2)L • Do = Outer diameter of the two gears • Di = Inner diameter of the two gears • (Actually it is the diameter of the circle defined by the center of one gear and the outer diameter of the other.)

5. Gear Pumps(External Gear) • Advantages: • Cheap (easy to manufacture) • Compact • Cheap • Did I say inexpensive?

6. Gear Pumps(External Gear) • Disadvantages • Limited pressure capability • Unbalanced (note where pressure is) Results in large bearing loads • Can be noisy (gear mesh noise) • Volumetric efficiency? • Fixed Displacement

7. Gear Pumps(Internal Gear) • Pumping Mechanism

8. Gear Pumps(Internal Gear) • Displacement is a function of the number of teeth on the internal and external gears and the size of the crescent divider. • ( I don’t have a formula for the displacement. Perhaps you can derive one.)

9. Gear Pumps(Internal Gear) • Advantages • Similar to external gear pumps in many respects • Quieter as gear slap is reduced • Disadvantages • Somewhat more difficult to manufacture • Same issues of volumetric efficiency • Same issues of unbalanced forces • Fixed displacement

10. Gear Pumps(Internal Gear - Gerotor) • Mechanism • External (inside) gear is shaft driven • Internal gear is driven by external • Single tooth space is displaced • Design keeps tolerance close throughout the cycle

12. Gear Pumps(Internal Gear - Gerotor) • Advantages • Cheap • Simple • Cheap

13. Gear Pumps(Internal Gear - Gerotor) • Disadvantages • Limited pressure capability • Unbalanced design • Fixed displacement • Frequently used as a charge pump

14. Vane Pumps • Pumping mechanism

15. Vane Pumps • Displacement • VD = π/2(Dc-DR)eL • C = Cam • R = Rotor • E = eccentricity • L= depth

16. Vane Pumps(Variations) • Vane tip pressure control options • Outlet pressure under the vanes • Surface pressure under the vanes • Intravanes: outlet pressure is applied always to a small area of the vane while surface pressure is applied to the rest of the area • These are probably Vickers innovations and hence are highlighted in the text

17. Vane Pumps(Variations) • Balanced designs

18. Vane PumpsAdvantages • Cartridges to quickly replace rotating group

19. Vane Pumps(Variations) • Variable Displacement Design

20. Vane Pumps • Advantages • Quieter than gear pumps • Higher pressure capability than gear pumps? • Better volumetric efficiency than gear pumps? • Can be balanced in design for longer life • Variable displacement an option • Disadvantages • More complex and expensive than gear pumps

21. Piston Pump Designs • Axial Piston

22. Piston Pump Designs • Displacement of an axial piston pump • VD = YAD tan(θ) • Y = Number of Pistons in the rotating group • A = the area of a single piston • D = is the diameter of the centerline circle of the piston bores • θis the angle of the swashplate or the bend angle

23. Piston Pump Designs • Radial piston design

24. Piston Pump Designs • Bent axis design

25. Piston Pump Designs • Bent axis – variable displacement design

26. Piston Pump Designs • Axial piston – variable displacement design

27. Piston Pump Advantages • Generally highest volumetric efficiency • Generally highest pressure capability • Variable displacement designs

28. Piston Pump Disadvantages • Higher cost (complexity)

29. 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 • Design of pumps is not taught here

30. General Issues • Our theoretical displacements can be used to determine theoretical pump flow • Actual flow is a linear function of pump displacement, speed, a units constant, and an efficiency term • Two kinds of inefficiencies • Volumetric losses • Friction losses

31. Actual Pump Output, Q • Qp = (Vp npηVp) /1000 where: • Q: L/min • Vp : cm3/rev • ηVp: Volumetric efficiency (decimal) • OR… Qp = (Vp npηVp) /231 where: • Q: GPM • Vp: in3/rev • ηVp: same as above (no units)

32. Torque to Drive a Pump • Tp = (ΔP Vp)/(2πηtp) where: • Tp : Newton meters torque required • ΔP : pressure rise across the pump in MPa • Vp : Pump displacement in cm3/rev • ηtp : Pump torque efficiency – a decimal • OR…

33. Torque to Drive a PumpEnglish Units • Tp = (ΔP Vp)/(2πηtp) where: • Tp : inch lbs torque required • ΔP : pressure rise across the pump in PSI • Vp : Pump displacement in inches3/rev • ηtp : Pump torque efficiency – a decimal

34. Power to Drive the Pump • The hydraulic power is QpΔP/60 or QpΔP/1714 for SI and English units • (note this is actual pump flow, not theoretical) • Shaft power to drive the pump is given by Psp = Phydr / ηpp where: • ηpp = ηvpηtp which is total pump efficiency

35. What Determines ηvp & ηtp ? • ηvp is a function of clearance spaces, system pressure, 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 flow and lower volumetric efficiency

36. What about Torque 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

37. Efficiencies (μ n)/(ΔP x 1000)

38. 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 the simultaneous flow requirements • Select for the maximum load pressure • Add pressure drops that will occur in valves, lines and fittings ( another topic to come…)

39. 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

40. Pump Sizing • Determine appropriate speed for your app • Use the equation for pump flow, solved for displacement • Vp = 1000Q/p (npηVp) • What shall we use for ηVp?? • This is a function of speed, pressure, and fluid viscosity • Look for vendor data or curves and adjust…

41. Example Pump ProblemCar Crusher • Need 125,000 lbs of force • 8 foot stroke • 10 seconds to extend? • Target system max pressure of 1500 psi • What is the cylinder size needed? • 125,000 lbs/ A (area) = 1500 psi • Area = 83.33 in2 • πr2 = 83.33 in2 r = 5.15 inches (let’s use 5”)

42. Car Crusher Pump cont’d • What will the system pressure be? • Cylinder area = 52π = 78.53 in2 • 125,000 lbs / 78.53 in2 = 1592 psi • We study our plumbing and valves and allow for 300 psi drops in our system • Set PRV to 1900?

43. Car Crusher Pump cont’d • What is flow is required of the pump? • Q = cyl stroke x area /time • Q = 96 in x 78.53 in2/ 10 sec = 754 in3/sec • 754 in3/sec x 1 gal/231 in3 x 60 sec/min • Q = 195.8 GPM • Note that we have sized for one cylinder. We might have others (a cylinder to kick your crushed Hummer bale out of the machine). Size for those that will be used simultaneously.

44. Car Crusher Pump cont’d • Pump speed: • Electric power available? - 1750 rpm • Remote from grid? Diesel at 2200 rpm • Determine approximate size • Vp = 1000Q/p (npηVp) or 231Q/p (npηVp) • Vp = 231*196/(1750*.95) • Vp = 27.2 inches3/revolution

45. Car Crusher Pump cont’d • Large pump (27.2 in3/rev) • Now we would look at vendors • For this large, a piston design is likely • Could also select two or more smaller pumps operating in tandem with outlets coupled • Selection will be based upon costs of installation, costs of operation, and required life • Continuous use favors efficiency • Intermittent use may favor low initial cost

46. Pumps Selection • Fixed or variable displacement? • So far our circuit is simple and we would likely use a fixed displacement pump • Later we will look at more efficient circuits and may wish to select a variable displacement pump with appropriate controls

47. Positive displacement pumps: External gear pump Reciprocating piston Double screw pump Sliding vane Three-lobe pump (left) Double circumferential piston (centre) Flexible tube squeegee (peristaltic)

48. Pumps in series and parallel Series Equivalent pump Parallel Equivalent pump

49. Pumps in Series Add the heads (H) at each flow rate (Q) For example, for two identical pumps the head will be double that of a single pump.

50. Pumps in Parallel Add the flow rates (Q) at each head (H) For example, for two identical pumps the flow rate will be double that of a single pump.