Hydraulic Fluid Power

ANDREA VACCA is the Maha Chair for Fluid Power Systems at Purdue University and he leads Purdue’s Maha Fluid Power Research Center. He obtained his MSc at the University of Parma and his doctorate in Energy Systems at the University of Florence. He has written over 150 technical papers on fluid power technology and was awarded the 2019 Joseph Bramah Medal by the Institution of Mechanical Engineers for contributions to fluid power research. GERMANO FRANZONI obtained his Ph.D. in mechanical engineering in 2006 from the University of Parma, Italy. Since then, he has worked in the Hydraulics Industry, both in Europe and USA. He is currently part of the Mobile Systems Engineering team for North America at Parker Hannifin. He specializes in systems design, business development and R&D. He works side by side with the major OEMs in the construction, mining, vocational truck and military markets. He holds two patents and has several patent pending applications. He is author of several scientific papers and has presented at various fluid power conferences and symposia.

PART I:Fundamental principles4

Objectives4

CHAPTER 1:Introduction to hydraulic control technology6

Historical perspective7

Fluid power symbology and its evolution12

Common ISO Symbols16

Problems25

CHAPTER 2:Hydraulic fluids28

Ideal vs. Actual hydraulic fluids28

Classification of hydraulic fluids31

Mineral oils (H)32

Fire resistant fluids (HF)33

Synthetic fluids (HS)34

Environmentally friendly fluids34

Water hydraulics34

Comparisons between hydraulic fluids35

Physical properties of hydraulic fluids36

Fluid compressibility: Bulk Modulus

Fluid density38

Fluid viscosity42

Viscosity as a function of temperature43

Viscosity as a function of pressure47

Entrained air, gas solubility and cavitation48

Entrained air48

Gas solubility48

Equivalent properties of liquid-air mixtures50

Contamination in hydraulic fluids57

Considerations on hydraulic filters59

Filter placement64

Considerations on hydraulic reservoirs68

Tank volume68

Basic design of a tank69

Problems71

CHAPTER 3:Fundamental Equations73

Pascal’s law73

Basic law of fluid statics74

Volumetric flow rate77

Conservation of mass80

Application to a hydraulic cylinder81

Bernoulli’s Equation84

Generalized Bernoulli’s equation85

Major losses calculation87

Minor losses89

Hydraulic resistance90

Stationary modeling of flow networks92

Momentum equation96

Flow forces100

Problems106

CHAPTER 4(*):Orifice Basics111

The orifice equation111

Fixed and variable orifices115

Power loss in orifices117

Parallel and series connection of orifices119

Functions of orifices in hydraulic systems123

Orifices in pressure and return lines123

Orifices in pilot lines126

Problems131

CHAPTER 5:Dynamic Analysis of Hydraulic Systems134

Pressure build-up Equation - hydraulic capacitance134

Fluid inertia Equation - hydraulic inductance140

Modeling flow network – dynamic considerations146

Validity of the lumped parameter approach151

Further considerations on the line impedance model152

Damping effect of hydraulic accumulators153

Problems156

References160

PART II:Main hydraulic components4

Objectives5

CHAPTER 6 (**):Hydrostatic pumps and motors6

Introduction6

The ideal case7

General operating principle9

ISO symbols13

Ideal equations14

The real case16

Losses in pumps and motors17

Fluid compressibility17

Internal and external leakage20

Friction21

Other types of losses23

Volumetric and hydro-mechanical efficiency24

Trends for volumetric and hydromechanical efficiencies28

Design types34

Swashplate type axial piston machines35

Bent axis type axial piston machines38

Radial piston machines39

Gear machines40

Vane type machines43

Problems46

CHAPTER 7(*):Hydraulic cylinders50

Classification50

Cylinder analysis52

Ideal vs. real cylinder55

Problems61

CHAPTER 8(*):Hydraulic control valves63

Spring basics64

Check and shuttle valves65

Check valve65

Pilot operated check valve66

Shuttle valve67

Pressure control valves68

Pressure relief valve68

Direct acting pressure relief valve68

Pilot operated pressure relief valve72

Pressure reducing valve75

Direct acting pressure reducing relieving valve75

Pilot operated pressure reducing valve77

Flow control valves80

Two-way flow control valve80

Fixed displacement pump circuit with a two-way flow control valve83

Three-way flow control valve87

Fixed displacement pump circuit with a three-way flow control valve89

Directional control valves95

Meter-in and meter-out configurations97

Neutral position100

Actuation methods103

Servovalves107

Characteristic of servovalves112

Servovalves vs. proportional valves123

Problems126

CHAPTER 9(*):Hydraulic Accumulators132

Accumulator Types132

Weight loaded accumulators132

Spring-loaded accumulators132

Gas-charged accumulators133

Piston-type accumulators133

Diaphragm-type accumulators134

Bladder-type accumulators135

Operation of gas charged accumulators137

Typical applications138

Energy accumulation138

Emergency supply140

Energy recuperation140

Hydraulic suspensions140

Pulsation dampening – shock attenuation141

Equations and sizing142

Accumulator as energy storage device142

Accumulator as dampening device145

Problems151

References154

PART 3:Actuator control concepts3

Objectives3

CHAPTER 10 (*):Basics of actuator control5

Control methods: speed, force and position control5

Resistive and overrunning loads7

Power flow depending on the load conditions9

Problems11

CHAPTER 11:General concepts for controlling a single actuator13

Supply and control Concepts13

Flow supply – primary control18

Flow supply – metering control19

Flow supply – secondary control21

Pressure supply – primary control21

Pressure supply – metering control23

Pressure supply – secondary control25

Additional remarks26

CHAPTER 12:Regeneration with single rod actuators27

Basic Concept of regeneration27

Actual implementation32

Directional control valve with external regeneration valves32

Directional control valve with regenerative extension position33

Solution with automated selection of the regeneration mode34

Problems36

References38

PART 4:Metering controls for a single actuator3

Objectives3

CHAPTER 13:Fundamentals of metering control5

Basic meter-in and meter-out control principles5

Meter-in control

Extension with resistive loads

Retraction with overrunning loads

Meter-out control10

Extension with resistive loads 14

Retraction with overrunning loads18

Remarks on meter-in and meter-out controls19

Actual metering control components36

Single spool proportional DCVs41

Independent metering control elements38

Usage of anti-cavitation valve for unloaded meter-out51

Problems49

CHAPTER 14:Load holding and counterbalance valves53

Load holding valves53

Pilot operated check valve61

Counterbalance valves60

Basic operating principle67

CBV architecture69

CBV detailed operation72

Effect of the pilot ratio and of the pressure setting83

Counterbalance valve with vented spring chambers85

Problems78

CHAPTER 15:Bleed-off and open center circuits80

Bleed-off circuit operation91

Energy analysis94

Basic open center system97

Operation98

Open center valve design101

Energy analysis102

Advanced open center control architectures106

Negative flow control106

Basic Schematic106

Operation107

Pump displacement setting mechanism110

Positive flow control114

Basic Schematic114

Operation115

Pump displacement setting mechanism115

Energy analysis for advanced open center architectures116

Problems118

CHAPTER 16:Load sensing systems109

Basic load sensing control concept121

LS system with fixed displacement pump122

Basic Schematic122

Operation123

Energy analysis125

Saturation conditions126

Load sensing valve127

LS system with variable displacement pump137

Basic Schematic137

Operation138

Energy analysis139

Saturation conditions140

Load sensing pump148

LS solution with independent metering valves157

Electronic load sensing (E-LS)159

Problems162

CHAPTER 17:Constant pressure systems150

Constant pressure system based on a variable displacement pump163

Basic schematic and operation163

Energy analysis166

Constant pressure system with unloader (CPU)167

Constant pressure system based on a fixed displacement pump170

Basic schematic and operation170

Application to hydraulic braking circuits173

Problems175

References

PART 5:Metering control of multiple actuators3

Objectives3

CHAPTER 18:Basics of multiple Actuator Systems5

Actuators in series and in parallel5

Series configuration6

Parallel configuration8

Elimination of the load interference in parallel actuators12

Solving load interference using compensators12

Solving load interference with a volumetric coupling13

Syncronization of parallel actuators through flow dividers15

Spool type flow divider15

Spool type flow divider-combiner16

Volumetric flow divider-combiner19

Linear flow divider-combiner24

Rotary flow divider-combiner25

Problems23

CHAPTER 19:Constant pressure systems for multiple actuators27

Basic concepts for a Multi-user constant pressure system27

Basic schematic35

Flow saturation36

Energy analysis37

Complete schematic of a multi-user constant pressure system29

Problems33

CHAPTER 20:Open center systems for multiple actuators35

Parallel open center systems36

Operation46

Energy analysis48

Flow saturation49

Considerations on the open center spool design49

Opening areas39

Opening delay (valve timing)41

Load interference in open center systems41

Tandem and series open center systems47

Tandem configuration60

Series configuration63

Advanced open center circuit for multiple users: the case of excavators49

Problems52

CHAPTER 21:Load sensing systems for controlling multiple actuators53

Load sensing system without pressure compensation (LS)53

Basic circuit69

Energy analysis72

Valve implementation and extension to more actuators74

Load sensing pressure compensated systems (LSPC)61

LSPC with pre-compensated valve technology61

Basic circuit79

Energy analysis82

Valve implementation and architecture84

LSPC with post-compensated valve technology70

Basic circuit90

Energy analysis92

Valve implementation and architecture94

Flow saturation and flow sharing in LS systems79

Flow saturation with pre-compensated LSPC80

Flow saturation with post-compensated LSPC82

Pre vs. post compensated comparison84

Independent metering systems with load sensing88

Problems91

CHAPTER 22:Power steering and hydraulic systems with priority function102

Hydraulic power steering103

Classification of hydraulic power steering systems103

Hydrostatic power steering111

Hydrostatic steering unit description114

Types of hydrostatic steering units119

Priority valves121

Priority valve for a fixed displacement flow supply121

Priority valve for load sensing circuits128

Problems131

References

PART 6:Hydrostatic transmissions and hydrostatic actuators3

Objectives5

CHAPTER 23:Basics and classifications6

Hydrostatic transmissions and hydrostatic actuators6

Basic definitions6

Supply concepts used in HTs and HAs9

Primary units for hydrostatic transmissions and actuators10

Constant speed prime mover and variable displacement pump10

Variable speed prime mover and fixed displacement pump10

Variable speed prime mover and variable displacement pump11

Over-center variable displacement pump11

Typical applications12

CHAPTER 24:Hydrostatic transmissions15

Main parameters of a hydrostatic transmission15

Theoretical layouts19

Pump and motor with fixed displacement (PFMF)19

Variable displacement pump and fixed displacement motor (PVMF)20

Fixed displacement pump and variable displacement motor (PFMV)21

Variable displacement pump and variable displacement motor (PVMV)23

Variable displacement pump and dual displacement motor (PVM2)25

Open circuit hydrostatic transmissions29

Open-circuit HT with flow supply: basic circuit29

Open circuit HT with flow supply: common implementation31

Open circuit displacement regulator33

Open circuit HTs with pressure supply35

Closed circuit hydrostatic transmissions40

Charge circuit and filtration41

Cross-port relief valves45

Flushing circuit47

Closed circuit displacement regulators54

Electro-hydraulic displacement regulator for closed circuit pumps54

Automotive control for closed circuit pumps56

Conceptual schematic58

Actual implementation60

Electro-hydraulic displacement regulator for motors59

Automatic pressure regulator for motors60

Problems61

CHAPTER 25:Hydrostatic transmissions applied to vehicle propulsion67

Basic of vehicle transmission67

Classification for variable ratio transmission systems71

Power-split transmissions74

Planetary gear train76

Hydromechanical power split transmission78

Analysis of an output coupled hydromechanical power split transmission

Analysis of an input coupled hydromechanical power split transmission

Hybrid transmissions92

Series hybrids93

Parallel hybrids95

Series-parallel hybrids (or power split hybrids)97

Sizing hydrostatic transmissions for propel applications100

Step 1: Maximum tractive effort calculation101

Step 2: Fixed or variable displacement motor selection102

Step 3: Sizing of the motor (secondary unit)104

Step 4: Sizing of the pump (primary unit)105

Step 5: Check results106

Problems112

CHAPTER 26:Hydrostatic actuators113

Open circuit hydrostatic actuators113

Closed circuit hydrostatic actuators116

Cylinder extension117

Extension in pumping mode117

Extension in motoring mode118

Cylinder retraction120

Retraction in motoring mode121

Retraction in pumping mode122

Further considerations on the charge pump and the accumulator124

Final remarks on hydrostatic actuators127

CHAPTER 27:Secondary controlled hydrostatic transmissions129

Secondary control circuit with tachometric pump132

Secondary control circuit with tachometric pump and internal force feedback135

Secondary control circuit with electronic control137

Multiple actuators139

References

APPENDIX 1 – Prime movers and their interaction with the hydraulic circuit

Objectives

Corner power method and its limitations

Diesel engine and its interaction with a hydraulic pump

Diesel engine regulation

Engine stall

Overrunning loads

Fuel consumption

Electric prime movers

Brushed DC electric motors

DC hydraulic power units

Induction motors (or asynchronous motor)

Synchronous motor

Power limitation in hydraulic pumps

Torque limiting using fixed displacement pumps

Torque limiting using variable displacement pumps

References