Chapter 7 Fluid Power Systems Introduction Mechanical electromechanical

Chapter 7 Fluid Power Systems

Introduction • Mechanical, electromechanical, and fluid power systems are methods for transmitting power in industrial applications. • Fluid power: the use of a confined fluid flowing under pressure to transmit power from one location to another. • Fluid can be a gas or a liquid.

Introduction • Fluid power systems: pneumatics (gases); hydraulics (liquids). • Hydraulics: physical science and technologies associated with liquids that are at rest or flowing under pressure. • Pneumatics: physical science and technologies associated with mechanics of pressurized gases. • Power density: power per unit volume.

Introduction • Closed fluid power system: system in which fluid is confined to a container or series of containers that are linked together. • Pressure: type of load that occurs when a force is distributed perpendicular to the surface of an object.

Introduction • Hydraulic fluid power systems can generate tremendous pressures. • Hydraulics systems are for situations that require precise positioning of objects. • Almost all heavy construction machines use hydraulic systems.

Introduction • Pneumatic systems are for quick movement of relatively light objects across short distances. • Pneumatics systems operate at much lower pressure levels than hydraulic systems. • Hydraulic systems incur higher maintenance costs than pneumatic systems.

Milestones in the History of Fluid Power • Pascal’s law: pressure exerted on a confined fluid is transmitted equally and perpendicular to all of the interior surfaces of the fluid’s container. • This law describes the behavior of confined fluids under pressure. • See Figure 7 -6: Timeline of inventions and scientific discoveries related to fluid power.

© Cengage Learning 2012 Milestones in the History of Fluid Power Figure 7 -7 b: Pascal’s law.

Common Fluid Power System Components and Schematic Symbols • Standard: reference developed by an authority or through general consent; used as a basis for comparison and verification. • Schematic symbol: simplified graphic representation of an electrical, mechanical, or fluid power system component.

Common Fluid Power System Components and Schematic Symbols • Both pneumatic and hydraulic systems contain at least one of these components: –Device that serves to pressurize the fluid –Pathway through which the fluid can flow –Device to control pressure, flow rate, and the direction that a fluid will flow –Device that serves as the point of application

Common Fluid Power System Components and Schematic Symbols • Working line: fluid transport to and from an actuator or any other device that performs work in a fluid power system. • Pilot line: fluid pressure transmission for the purpose of controlling a valve. • Mechanical connections: indicated by a double line.

Common Fluid Power System Components and Schematic Symbols • Both systems use special filters to remove particulate matter that can damage inner workings. • Actuator: device that converts fluid pressure into mechanical motion for the purpose of moving a load.

© Cengage Learning 2012 Common Fluid Power System Components and Schematic Symbols Figure 7 -18 a: Hydraulic circuit powers a linear actuator on backhoe.

Common Fluid Power System Components and Schematic Symbols • Shutoff valve: turns all or part of a fluid power system on or off. • Check valve: one-way valve that allows fluid to flow in one direction only. • Shuttle valve: combines the functions of a T-connector and a check valve. • Flow-control valve: controls the volume of fluid as it flows in one direction only.

Common Fluid Power System Components and Schematic Symbols • Reservoir: used in a hydraulic system to store and protect hydraulic oil from outside contamination. • Hydraulic pump: generates hydrostatic pressure transmitted to the actuators in a hydraulic circuit. • Fixed displacement pump: provides constant pressure.

Common Fluid Power System Components and Schematic Symbols • Variable-displacement pump: allows the increase or decrease of pressure that a pump generates. • Pressure-relief valve: safety mechanism used to protect from damage caused by excess pressure. • Directional control valve (DCV): the control interface between a fluid power system and its operator.

Common Fluid Power System Components and Schematic Symbols • Pneumatic pressure: generated by an air compressor. • Compressors: most use an electric motor or internal-combustion engine to generate power needed to compress air. • Pneumatic pressure regulator: manually adjusts and controls the pressure of the compressed air source.

Common Fluid Power System Components and Schematic Symbols • Pneumatic components must be lubricated. • Receiver tank: holding device for compressed air before it is drawn into a pneumatic circuit. • Solenoid: electromechanical actuation device using principles of electromagnetism to control spool within a DCV.

© Cengage Learning 2012 Common Fluid Power System Components and Schematic Symbols Figure 7 -30 a: Schematic diagram of pneumatic circuit.

Basic Scientific Concepts of Fluid Power • Fluid mechanics: study of the properties of gases and liquids that are at rest or in motion. • Hydrostatics: study of the properties of fluids that are in a state of static equilibrium (at rest).

Basic Scientific Concepts of Fluid Power • Hydraulic systems achieve mechanical advantage through the hydraulic amplification of force. • Volume: amount of space occupied by a three-dimensional object or enclosed within a container.

Basic Scientific Concepts of Fluid Power • Hydrodynamics: study of fluids that are in a state of motion. • Viscosity: measure of a fluid’s resistance to flow. • Hydraulic oil purposes: transfers energy through the flow of pressurized fluid; lubricates moving components within the system.

Basic Scientific Concepts of Fluid Power • Laminar flow: under ideal conditions, fluid moving in a smooth, steady stream through a system. • Turbulent flow: fluid velocity that is too high. • Flow rate: volume of fluid that moves past a given point in a system per unit time.

Basic Scientific Concepts of Fluid Power • Flow meter: measures a liquid’s flow rate. • Flow velocity: speed of a fluid moving through a system. • Bernoulli’s principle: states the velocity of a fluid increases as the pressure exerted by that fluid decreases.

Basic Scientific Concepts of Fluid Power • Venturi tube: constriction that is placed in a pipe that causes a drop in pressure as fluid flows through it. • Vacuum generator: pneumatic device that incorporates a Venturi tube to generate suction by accelerating the flow of compressed air.

© Cengage Learning 2012 Basic Scientific Concepts of Fluid Power Figure 7 -43 b: Venturi tube.

Basic Scientific Concepts of Fluid Power • Gauge pressure: measured pressure in a pneumatic system. • Absolute pressure: the total pressure exerted on a system; includes atmospheric pressure. • Atmospheric pressure: air surrounding us that is constantly applying pressure against our body.

Basic Scientific Concepts of Fluid Power • Absolute temperature scale: Kelvin scale; predicts changes in a gas’s physical properties. • Boyle’s law: the absolute pressure of a confined gas is inversely proportional to its volume, provided its temperature remains constant.

Basic Scientific Concepts of Fluid Power • Charles’ law: the volume of a confined gas is proportional to its absolute temperature, provided its pressure remains constant. • Gay-Lussac’s law: the absolute pressure of a confined gas is proportional to its absolute temperature, provided its volume remains constant.

Summary • Fluid power systems use the energy contained in pressurized liquids and gases. • Hydraulic systems use pressurized liquid. • Pneumatic systems use compressed gases.

Summary • Hydraulic and pneumatic systems have common elements. • Hydrostatic systems involve fluids that are in a state of static equilibrium. • Pascal’s law is a fundamental principle of the science of hydrostatics.

Summary • Hydraulic amplification of force: a small input force generates a large output force. • Fluid’s flow rate: time it takes for a specific volume of fluid to move through a system. • Bernoulli’s principle: the velocity of a fluid will increase as the pressure exerted by that fluid decreases, and vice versa.

Summary • Kelvin temperature scale is based on the concept of absolute zero. • Boyle’s law, Charles’ law, and Gay. Lussac’s law are collectively referred to as the perfect gas laws.