Hydraulic Pumps

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Hydraulic Pumps

• Positive Displacement Devices
• Displacement Formulae
• Characteristics

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Gear Pumps(External Gear)

• Pumping Mechanism

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Gear Pumps(External Gear)

• Displacement parameters and determination
• Displacement p/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.)

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Gear Pumps(External Gear)

• Advantages
• Cheap (easy to manufacture)
• Compact
• Cheap
• Did I say inexpensive?

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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

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Gear Pumps(Internal Gear)

• Pumping Mechanism

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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 dont have a formula for the displacement.
  Perhaps you can derive one.)

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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

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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

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Gear Pumps(Internal Gear - Gerotor)

• Advantages
• Cheap
• Simple
• Cheap

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Gear Pumps(Internal Gear - Gerotor)

• Disadvantages
• Limited pressure capability
• Unbalanced design
• Fixed displacement
• Frequently used as a charge pump

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Vane Pumps

• Pumping mechanism

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Vane Pumps

• Displacement
• VD p/2(Dc-DR)eL
• C Cam
• R Rotor
• E eccentricity
• L depth

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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

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Vane Pumps(Variations)

• Balanced designs

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Vane PumpsAdvantages

• Cartridges to quickly replace rotating group

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Vane Pumps(Variations)

• Variable Displacement Design

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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

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Piston Pump Designs

• Axial Piston

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Piston Pump Designs

• Displacement of an axial piston pump
• VD YAD tan(?)
• 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
• ? is the angle of the swashplate or the bend angle

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Piston Pump Designs

• Radial piston design

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Piston Pump Designs

• Bent axis design

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Piston Pump Designs

• Bent axis variable displacement design

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Piston Pump Designs

• Axial piston variable displacement design

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Piston Pump Advantages

• Generally highest volumetric efficiency
• Generally highest pressure capability
• Variable displacement designs

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Piston Pump Disadvantages

• Higher cost (complexity)

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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

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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

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Actual Pump Output, Q

• Qp (Vp np ?Vp) /1000 where
• Q L/min
• Vp cm3/rev
• ?Vp Volumetric efficiency (decimal)
• OR Qp (Vp np ?Vp) /231 where
• Q GPM
• Vp in3/rev
• ?Vp same as above (no units)

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Torque to Drive a Pump

• Tp (?P Vp)/(2p ?tp) where
• Tp Newton meters torque required
• ?P pressure rise across the pump in MPa
• Vp Pump displacement in cm3/rev
• ?tp Pump torque efficiency a decimal
• OR

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Torque to Drive a PumpEnglish Units

• Tp (?P Vp)/(2p ?tp) where
• Tp inch lbs torque required
• ?P pressure rise across the pump in PSI
• Vp Pump displacement in inches3/rev
• ?tp Pump torque efficiency a decimal

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Power to Drive the Pump

• The hydraulic power is Qp?P/60 or Qp?P/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 / ?pp where
• ?pp ?vp ?tp which is total pump efficiency

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What Determines ?vp ?tp ?

• ?vp 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

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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

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Efficiencies
(µ n)/(?P x 1000)
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Sizing Pumps

• Component sizing begins with the LOAD
• Load and actuator will determine
• Flow requirement for this circuit
• Pressure range required by the circuit
• (Well 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)

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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

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Pump Sizing

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

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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
• pr2 83.33 in2 r 5.15 inches (lets use
  5)

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Car Crusher Pump contd

• What will the system pressure be?
• Cylinder area 52 p 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?

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Car Crusher Pump contd

• 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.

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Car Crusher Pump contd

• Pump speed
• Electric power available? - 1750 rpm
• Remote from grid? Diesel at 2200 rpm
• Determine approximate size
• Vp 1000Q/p (np ?Vp) or 231Q/p (np ?Vp)
• Vp 231196/(1750.95)
• Vp 27.2 inches3/revolution

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Car Crusher Pump contd

• 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

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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

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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)
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Pumps in series and parallel
Series
Equivalent pump
Parallel
Equivalent pump
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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.
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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.
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Pump-system operation
System resistance (losses) curves (typically H ?
Q2)
C operating point
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Positive Displacement Pumps

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

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Types of PD Pumps

• Rotary Pumps
• Gear Internal, External
• Lobe
• Vane
• Screw
• Reciprocating Pumps
• Piston
• Plunger
• Diaphragm

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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

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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
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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/inte
rnal.htm
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PD Pump Curve
Source http//www.driedger.ca/ce2_pdp/CE2_PDP.ht
ml