150 Lecture 6 - University of Minnesota · •Lower pressure application: ... Pressure Compensated...

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Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2015)

M..E., University of Minnesota (updated 12.2013)

Systems review exercise

• To be posted this weekend

• Due next Friday (3/6)

Coming week:

• Lab 13: Hydraulic Power Steering

• Lab 14: Integrated Lab (Hydraulic test bench)

Topics today:

• Pumps and motors

• (Hydraulic Hybrids)

Lecture 6

150

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151

Pumps

• Source of hydraulic power

• Converts mechanical energy to hydraulic energy

• prime movers - engines, electrical motors, manual power

• Two main types:

• positive displacement pumps

• non-positive displacement pumps

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152

Pump - Introduction

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153

Positive displacement pumps

• Displacement is the volume of fluid displaced cycle of pump motion

• unit = cc or in3

• Positive displacement pumps displace (nearly) a fixed amount of fluid per cycle of pump motion, (more of less) independent of pressure

• leak can decrease the actual volume displaced as pressure increases

• Therefore, flow rate Q gpm = D (gallons) * frequency (rpm)

• E.g. pump displacement = 0.1 litre

• Q = 10 lpm if pump speed is 100 rpm

• Q = 20 lpm if pump speed is 200 rpm

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154

Non positive displacement pumps

Impeller PumpCentrifugal Pump

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Non-positive displacement pump

• Flow does not depend on kinematics only - pressure important

• Also called hydro-dynamic pump (pressure dependent)

• Smooth flow

• Examples: centrifugal (impeller) pump, axial (propeller) pump

• Does not have positive internal seal against leakage

• If outlet blocks, Q = 0 while shaft can still turn

• Volumetric efficiency = actual flow / flow estimated from shaft speed

= 0%

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156

Positive vs. non-positive displacement pumps

• Positive displacement pumps

• most hydraulic pumps are positive displacement

• high pressure (10,000psi+)

• high volumetric efficiency (leakage is small)

• large ranges of pressure and speed available

• can be stalled !

• Non-positive displacement pumps

• many pneumatic pumps are non-positive displacement

• used for transporting fluid rather than transmitting power

• low pressure (<300psi), high volume flow

• blood pump (less mechanical damage to cells)

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Types of positive displacement pumps

• Gear pump (fixed displacement)

• internal gear (gerotor)

• external gear

• Vane pump

• fixed or variable displacement

• pressure compensated

• Piston pump

• axial design

• radial design

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External gear pump

• Driving gear and driven gear

• Inlet fluid flow is trapped between the rotating gear teeth and the housing

• The fluid is carried around the outside of the gears to the outlet side of the pump

• As the fluid can not seep back along the path it came nor between the engaged gear teeth (they create a seal,) it must exit the outlet port.

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Gerotor pump

• Inner gerotor is slightly offset from external gear• Gerotor has 1 fewer teeth than outer gear

• Gerotor rotates slightly faster than outer gear• Displacement = (roughly) volume of missing tooth

• Pockets increase and decrease in volume corresponding to filling and pumping

• Lower pressure application: < 2000psi• Displacements (determined by length): 0.1 in3 to 11.5 in3

Inlet portOutlet port

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Vane Pump

• Vanes are in slots

• As rotor rotates, vanes are pushed out, touching cam ring

• Vane pushes fluid from one end to another

• Eccentricity of rotor from center of cam ring determines displacement

• Quiet

• Less than 4000psi

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Pressure Compensated Vane Pump

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PC Vane Pump (Cont’d)

• Eccentricity (hence displacement) is varied by shifting the cam ring

• Cam ring is spring loaded against pump outlet pressure

• As pressure increases, eccentricity decreases, reducing flow rate

• Spring constants determines how the P-Q curve drops:

• small stiffness (sharp decrease in Q as P increases)

• large stiffness (gentle decreases in Q as P increases)

• Preload on spring determines

• pressure at which flow starts cutting off

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Axial Piston Pump• Each piston has a pumping cycle

• Interlacing pumping cycles produce nearly uniform flow (with some ripples)

• Displacement is determined by the swash plate angle

• Generally can be altered manually or via (electro-) hydraulic actuator.

Displacement can be varied by varying swashplate angle

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Bent-Axis Piston Pump• Thrust-plate rotates with shaft

• Piston-rods connected to swash plate

• Piston barrel rotates and is connected to thrust plate via a U-joint

• More efficient than axial piston pump

(less friction)

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Radial Piston Pump

• Similar to axial piston pump, pistons move in and out as pump rotates.

• Displacement is determined by cam profile (i.e. eccentricity)

• Displacement variation can be achieved by moving the cam (possible, but not common though)

• High pressure capable, and efficient

• Pancake profile

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Piston Pump - flow ripples

• Each cylinder has a pumping cycle

• Total flow = flow of each cylinder

• More cylinders, less ripple

• Frequency:

Even # cylinders n*rpm

Odd # cylinders (2n)*rpm

• Can be problematic for manual operator (ergonomic issue)

• Noise

1 piston

Filling

Pumping

2 pistonTotal flow

• Displacement = # Cylinders x Stroke x Bore Area

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# of Pistons Effect on Flow Ripples

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0 1 2 3 4 5 60

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Angle - rad

Flo

w -

n=2

n=3

n=4

n=5

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Pumping theory

• Create a partial vacuum (i.e. reduced pressure)

• Atmospheric / tank pressure forces fluid into pump

• usually tank check valve opens

• outlet check valve closes

• Power stroke expels fluid to outlet

• outlet check valve opens

• tank check valve closes

• Power demand for prime mover (ideal calculation)

• (piston pump) Power = Force*velocity = Pressure*area*piston speed

= Pressure * Flow rate

• If power required > power available => Pumps stall or decrease speed

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Aeration and Cavitation

• Disastrous events - cause rapid erosion

• Aeration

• air bubbles enter pump at low pressure side

• bubbles expand in partial vacuum

• when fluid+air travel to high pressure side, bubbles collapse

• micro-jets are formed which cause rapid erosion

• Cavitation

• fluid evaporates (boils) in partial vacuum to form bubbles

• bubbles expands then collapse

• as bubbles collapse, micro-jets formed, causing rapid erosion

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Causes of cavitation and aeration

• For positive displacement pumps, the filling rate is determined by pump speed; (Q-demand) = D * freq)

• Filling pressure = tank pressure - inlet pressure

• Q-actual = f(filling pressure, viscosity, orifice size, dirt)

• If Q-actual < Q-demand, inlet pressure decreases significantly

• This causes air to enter (via leakage) or to evaporation (cavitates)

• To prevent cavitation/aeration

• increase tank pressure

• low viscosity, large orifice

• lower speed (hence lower Q-demand)

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Aeration and Cavitation

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Hydraulic Motor / Actuator

• Hydraulic motors / actuators are basically pumps run in reverse

• Input = hydraulic power

• Output = mechanical power

• For motor:

• Frequency (rpm) = Q (gallons per min) / D (gallons) * efficiency

• Torque (lb-in) = Pressure (psi) * D (inch^3) * efficiency

• efficiency about 90%

• Note: units

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•

Models for Pumps and Motors

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Non-ideal Pump/Motor Efficiencies• Ideal torque = torque required/generated for the ideal pump/motor• Ideal flow = flow generated/required for the ideal pump/motor• Torque loss (friction) • Flow loss (leakage)• Signs different for pumping and motoring mode

Qi deal

Qact ual

Ql oss

Ideal pump

leakage

Friction

Functions of speed, pressure and displacements

(Reverse if motor case !! )

Pump volumetric eff:

Pump mechanical eff:

Total efficiency:

Tin/out

Tideal vol

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Hydro-static Transmission

• A combination of a pump and a motor

• Either pump or motor can have variable displacement

• Replaces mechanical transmission

• By varying displacements of pump/motor, transmission ratio is changed

• Various topologies:

• single pump / multi-motors

• multi (pump-motor)

• Open / closed circuit

• Open / closed loop control

• Integrated package / split implementation

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Hydrostatic Transmission

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General Consideration - Hydrostats

• Advantages:

• Wide range of operating speeds/torque

• Infinite gear ratios - continuous variable transmission (CVT)

• High power, low inertia (relative to mechanical transmission)

• Dynamic braking via relief valve

• Engine does not stall

• No interruption to power when shifting gear

• Disadvantage:

• Lower energy efficiency (85% versus 92%+ for mechanical transmission)

• Leaks !

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Closed Circuit Hydrostat Circuit

Notes:

• Charge pump circuit (pump + shuttle valve)

• Bi-directional relief

• Circuit above closed circuit because fluid re-circulates.

• Open circuit systems draw and return flow to a reservoir

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Hydrostatic Transmission

• Let pump and motor displacements be D1 and D2, with one or both being variable.

• Let the torque (Nm) and speeds (rad/s) of the pump and motor be (T1, S1) and (T2,S2)

• Assuming ideal pumps and motors:

Transmission ratioVariable by varying

D1 or D2

Infinite and negativeratios possibleif pump can

go over-center

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Hydraulic Transformer

• Used to change pressure in a power conservative way

• Pressure boost or buck is accompanied by proportionate flow decrease and increase

• Note: Hydrostatic transmission can be thought of as a mechanical transformer (torque boost/buck)

Q1 Q

2

D1D2 Research opportunity!

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Hydraulic Hybrid Vehicles

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150 Lecture 6 - University of Minnesota · •Lower pressure application: ... Pressure Compensated...

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