non-positive displacement pumps - me.umn. · PDF fileTypes of positive displacement pumps • Gear pump (fixed displacement) • internal gear (gerotor) • external gear • Vane

1

Fluid Power Controls Laboratory (Copyright – Perry Li, 2004-2010)

M..E., University of Minnesota (updated 12.2010)

181

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

2



182

Pump - Introduction

3



183

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

4



184

Non positive displacement pumps

Impeller PumpCentrifugal Pump

5



185

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%

6



186

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)

7



187

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

8



188

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.

9



189

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

10



190

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

11



191

Pressure Compensated Vane Pump

12



192

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

13



193

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

14



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)

194

15



195

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

16



197

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 piston Total flow

• Displacement = # Cylinders x Stroke x Bore Area

17



# of Pistons Effect on Flow Ripples

198

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

18



199

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

19



200

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

• dissolved air in fluid evaporates in partial vacuum to form bubbles

• bubbles expands then collapse

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

20



201

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)

21



202

Aeration and Cavitation

22



203

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

23



•

Models for Pumps and Motors

204

24



205

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

Qloss

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

25



206

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

26



Hydrostatic Transmission

207

27



208

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 !

28



209

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

29



210

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

30



211

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!

31



Hydraulic Hybrid Vehicles

32



How Hybrid Vehicles Save Energy?

With a secondary power source/storage, it is possible to:

Required for maximum

performance

Normal drivingoperating

range10-15%

38%

Example vehicle on EPA-UDDS cycle: • Baseline (10% engine efficiency): 24 mpg• Engine management (38% efficiency): 95 mpg• Above with regeneration: 140 mpg

• Manage engine operation

• Store/reuse braking energy

• Turn off engine

• Downsize engine for continuous power

Engine

Energy storage

Drive-train wheel

33



Why Hydraulic Hybrids?

Why not stick with electric hybrids?

• Electric batteries / ultracaps (cost, reliability, recycling, power density)

• Electric motors & inverters (cost, power density)

• Affect overall cost, weight, and power

Toyota PriusMetrics

• Fuel economy

• Cost

• Performance

34



Hybrid Hydraulic versus Hybrid Electric Vehicle• Hydraulic pump/motor have significantly higher power density than electric

motor/generator (16:1 by weight , 8:1 by volume)

• Hydraulic drives have much lower torque density than electric drives

• Accumulators are 10x more power dense than batteries

• Efficient power electronics are expensive

• Batteries have 2 order magnitude higher energy density than accumulators

• Current hydraulic systems tend to be noisy and leaky

• Overall tradeoff: Hydraulic hybrids can be significantly lighter and cheaper than electric hybrids if energy density limitation can be solved.

Accumulator

Accumulator

EngineHydraulic

pump/motor

Battery &ultracapacitor

EngineElectricmotor/

generator

limits acc/braking and hence regenerative

braking

Parallel Hybrid Electric Parallel Hybrid Hydraulic

35



Hydraulic Hybrids Versus Electric HybridsElectric Fluid

Power

Performance

• acceleration -- ++

• regenerative braking -- ++

Component efficiency + -

Regenerative efficiency

-- ++

Weight -- ++

Cost -- ++

Reliability -- ++

Environmental impact - +

Energy storage ++ --

NVH ++ --

Realize opportunities for • Both performance & efficiency• Cost and reliability

Overcome threats in• Inefficient components • Low density energy storage • Noise, vibration, harshness

+

+

36



Parallel ArchitectureExample: HLA system for F150 & garbage trucks

• Regenerates braking energy

• Utilizes efficient mechanical transmission

• Does not allow full engine management

Achievable engine op. points

37



Series Architecture• Example: Eaton/UPS (truck), Ford/EPA (Escape), Artemis (BMW-5), …

– Regenerates braking energy

– Allows for full engine management

– Independent wheel torque control possible

– All power must be transmitted through fluid power components

Normal driving

operating range

10-15%

38% Required for maximum

performance

Normal drivingoperating

range10-15%

38%

38



Power-Split: Hydromechanical Transmission (HMT)

• Power split between mechanical and hydraulic paths

• Hybridized HMT – i.e. w/ regeneration

Efficient Mechanical

Transmission

RegenerativeBraking

Full enginemanagement

39



Hypothesized hydraulic & overall efficiencies

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Mean hydraulic efficiency

Overa

ll eff

icie

ncy

Urban

series

parallel

hmt

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Mean hydraulic efficiency

Overa

ll eff

icie

ncy

Highway

series

parallel

hmt

• Series / HMT at peak engine efficiency (38.5%)• Parallel at lower engine efficiency (33%)

Documents

non-positive displacement pumps - me.umn. · PDF fileTypes of positive displacement pumps • Gear pump (fixed displacement) • internal gear (gerotor) • external gear • Vane