1. Positive Displacement Devices
Hydraulic Pumps
Positive Displacement Devices

2. 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)

3. Gear Pumps (External Gear)
Pumping Mechanism

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. Internal Gear Pumps Smaller gear rotating within a bigger gear
Partial vacuum created by meshing and unmeshing of internal teeth with external teeth
Crescent divides liquid flow between rotor and idler gears
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8. Gear Pumps (Internal Gear)
Pumping Mechanism
Displacement is a function of the number of teeth on the internal and external gears and the size of the crescent divider.

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

11. Gear Pumps (Internal Gear - Gerotor)
Advantages
Cheap
Simple
Disadvantages
Limited pressure capability
Unbalanced design
Fixed displacement
Frequently used as a charge pump

12. Vane Pumps
Pumping mechanism

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

14. 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

15. Vane Pumps (Variations)
Balanced designs

16. Vane Pumps Advantages
Cartridges to quickly replace rotating group

17. Vane Pumps (Variations)
Variable Displacement Design

18. Vane Pumps Advantages Disadvantages 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

19. Piston Pump Designs
Axial Piston

20. Piston Pump Designs
Radial piston design

21. Piston Pump Designs
Bent axis design

22. Piston Pump Designs
Axial piston – variable displacement design

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

24. Piston Pump Disadvantages
Higher cost (complexity)

25. 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

26. 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

27. Positive Displacement Pumps
Typical Characteristics
Constant Flow at Various Pressures
Pulse Flow is possible
Most can pump solids suspended in liquids
Self-priming

28. Types of PD Pumps Rotary Pumps Reciprocating Pumps
Gear – Internal, External
Lobe
Vane
Screw
Reciprocating Pumps
Piston
Plunger
Diaphragm

29. Rotary vs. Reciprocating Pumps
Rotary pumps transfer liquid through the action of a rotating mechanism (gear, lobe or vane) operating inside a rigid container
Pumping rates varied by changing speed of rotor

30. Rotary vs. Reciprocating Pumps
Reciprocating pumps move liquids by changing the internal volume of the pump
Require valves on the suction and discharge sides
Pumping rates varied by changing the frequency or the stroke length
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