Positive Displacement Devices Hydraulic Pumps Positive Displacement Devices

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)

Gear Pumps (External Gear) Pumping Mechanism

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

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

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 Source: http://www.pumpschool.com/principles/internal.htm

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.

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

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

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

Vane Pumps Pumping mechanism

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

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

Vane Pumps (Variations) Balanced designs

Vane Pumps Advantages Cartridges to quickly replace rotating group

Vane Pumps (Variations) Variable Displacement Design

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

Piston Pump Designs Axial Piston

Piston Pump Designs Radial piston design

Piston Pump Designs Bent axis design

Piston Pump Designs Axial piston – variable displacement design

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

Piston Pump Disadvantages Higher cost (complexity)

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

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

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

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

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

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 Source: http://www.watson-marlow.com/wna-se/p-fmi.htm