Appendix B. Glossary of Terms. Absolute pressure: The pressure above absolute pressure. It is the sum of atmospheric pressure and gauge pressure.

Transcription

1 Appendix B Glossary of Terms Absolute pressure: The pressure above absolute pressure. It is the sum of atmospheric pressure and gauge pressure. Absolute zero pressure: The reference zero that is not the atmospheric pressure. It is the pressure of pure vacuum. Accumulator: A container in which fluid is stored under pressure as a source of fluid power. Accuracy: The ability of the system to achieve a desired output. Actuator: A device for converting hydraulic energy into mechanical energy. It can be linear or rotary type. Aeration: Air trapped in hydraulic fluid. Excessive aeration may cause system to operate erratically. Amplifier: A device used to amplify an electrical signal. The signal may be current, voltage or power and can be either direct current or alternating current. Annular area: Refers to effective area of the rod side of the single acting cylinder. Atmospheric pressure: Pressure exerted by the atmosphere at any specific location on the Earth s surface. Atmospheric pressure at sea level under normal barometric condition is bar. Baffle: Plate installed in hydraulic reservoir to separate flow of the return line from the flow to the pump inlet. Bar: An international standard pressure unit. 1 bar = 100 kpa. Bleed off: To divert specific controllable portion of pump delivery directly to tank. Brake valve: A valve used to in exhaust line of a hydraulic motor to prevent the overrunning load and excessive pressure while decelerating or stopping the load. Breather: A device that permits air to move in and out of the component or tank to maintain atmospheric pressure. Bypass: A secondary path for fluid flow. Cartridge: Replaceable part of filter.

2 Cavitation: A localized gaseous condition within liquid stream that occurs where pressure is reduced to the vaporpressure level. Cavitation can cause noise and pump wear. Check valve: A control valve that permits fluid in only one direction. Circuit: An arrangement of hydraulic, pneumatic or electrical components interconnected to form a specific function within a system. Closed circuit: A circuit arrangement in which pump output, after passing through various components returns directly to pump inlet, instead of tank. Closed loop: A system in which the feedback of one or more elements (hydraulic, pneumatic or electrical) is compared to a command signal, providing error signal to control the output of the loop. Closed ports: Ports of hydraulic valve which are blocked and do not permit flow. Compensator control:an automatic displacement control for variable pumps and motors. It alters the displacement in response to system pressure changes as related to the adjusted pressure setting. Component:A single hydraulic, electrical, pneumatic or mechanical unit. Compressibility:The change of volume of a unit volume of fluid when it is subjected to a unit change of pressure. The compressibility of liquid is very small compared to air or gas. Cooler:A heat exchanger that removes heat from the system. Counter balance valve:a pressure control valve that maintains back pressure to prevent load from falling. Cracking pressure:the pressure at which a pressure-operated valve begins to pass fluid. Cushion:A device that provides controlled resistance to motion. The cushion can be either built into either end of hydraulic cylinder, thus restricting the output flow and providing cushion to the load near the end of travel. Cylinder:A device that converts fluid power into linear mechanical power. Deadband:The region or band of no response where an error signal will not cause a corresponding actuation of the controlled variable. Delivery:The volume of fluid discharged by a pump in a given time. Usually expressed in liters per minute (LPM). Differential cylinder:any cylinder in which the two opposed piston areas are not equal.

3 Directional valve:a valve whose primary function is to direct or prevent flow through selected channels. Displacement:The volume of the fluid delivered by pump or required by motor during one revolution. Dither:A low-amplitude,high-frequency signal inserted into a servo loop to minimize the effect of column friction, hysteresis and deadband. Double-acting cylinder:a cylinder in which hydraulic force may be applied to movable element in either direction. Drain:A passage or line from which leakage flow returns. Drift:The percentage above or below the programmed operating level of a servo amplifier over a specified operating time Efficiency:The ratio of output to its input. Efficiency is usually expressed in percentage. Electrohydraulic servo valve:a type of servo valve, most of which are capable of providing an output flow directly proportional to the input electric current. Error:The amount by which a control system misses its target. Error signal:algebraic sum of input signal and feedback signal. Filter:A device used to separate and retain insoluble contaminates from a fluid. Flow control valve:a valve whose primary function is to control or regulate flow rates. Flow rate:the volume of fluid passing a given point in pipe or device in unit time Four way valve:a directional valve having four flow paths. Full flow filter:a hydraulic filter where all the flow must pass through the filter elements. Gain:The ratio of output signal of a device to its input. Gauge pressure:a pressure measured by a gauge or calculated using the atmosphere as zero pressure. Head pressure:pressure caused by the weight of the fluid. Heat exchanger:device which removes heat from hydraulic system using air or liquid. Hunting:The tendency of servo system to oscillate continuously. Hydraulic control:a control that is actuated by hydraulically induced forces

4 Hydraulics:The science and application of fluid power, pertaining specifically to fluid pressure, flow and restriction. Hydrodynamics:Engineering science pertaining to the energy of liquid flow and pressure. Hydrokinetics:Engineering science pertaining to energy of liquid in motion. Hydropneumatics: Combination of hydraulic and pneumatic fluid power. Hydrostatics:Engineering science pertaining to the energy of liquid at rest. Hysteresis:The difference between the response to an increasing signal and that to a decreasing signal having the same slope. Kinetic energy:energy that a body has by virtue of its mass and velocity. Lag:Delay in response, usually given in degrees or in milliseconds. Laminar flow:a flow condition where flow particles move in continuous parallel paths without momentum or mass transfer. Lands:Portion of a valve spool that seals flow from undesired passages while permitting flow to desired ports. Line:A hose, pipe or tube used to conduct hydraulic flow. Linear actuator:adevice for converting hydraulic energy into linear movement. Linearity:The degree of straightness of the hysteresis plot of servo valve. How closely output follows input. Liter:A unit of volume. 1L= 1000 cm 3. Manifold:A fluid conductor with many connection ports. Manual control:a control actualled by the human operator. Manual override:a means of manually actuating an automatically controlled device. Micron rating:the smallest size particle, measured in microns, that a filter will remove. Motor (hydraulic):adevice that converts hydraulic energy into rotary mechanical energy. Null:The condition where servo valve provides zero output. Open-centered valve:valve in which all ports are interconnected in the center or neutral position.

5 Pilot pressure:auxiliary pressure used to actuate or control other hydraulic components. Pilot valve:the controlling valve of two- or three-stage valve or any auxiliary valve used to operate another valve. Piston:Movable part of cylinder. A rod is smaller than piston. Plunger:Movable part of cylinder. A rod is same size as piston. Poppet:The part of a valve that blocks flow when it closes against a seat. Port:The beginning or end of the passage in a component. Positive displacement:the characteristics of a pump or motor in which the inlet is positively sealed from the outlet. Since the fluid cannot readily circulate within the unit, the rate of fluid delivery is virtually constant with rotation. Precharge pressure:the pressure of compressed gas in an accumulator prior to the admission of liquid. Pressure:Force per unit area. Expressed in Pascals or heads of liquids such as meter or mm of mercury. Pressure differential:the difference in pressure between anytwo points in a system. Pressure drop:the pressure differential across component or system. Pressure override:the difference between the cracking pressure of a valve and the pressure reached when the valve is passing its full flow. Pressure-reducing valve:avalve that lowers and regulates the lower output pressure. Pressure switch: An electrical switch turned on and off by fluid pressure. Proportional flow filter:a filter designed to pass part of flow through the filter element proportional to the pressure drop. Proportional valve:a valve that allows remote proportional control of pressure,flow, acceleration and direction. It has most of the characteristics of a servo valve but it not as responsive as servo valve. Pump (hydraulic):a device that converts mechanical force and motion into hydraulic fluid power. Ram:A single-acting cylinder with a single diameter plunger rather than a piston and rod.

6 Regenerative circuit: A circuit arrangement for a differential cylinder in which discharge fluid from the rod end is combined with pump delivery to be directed into the cap end. Repeatability:Exactness with which servo motion or position can be duplicated. Reservoir:A container for storage of liquid in a fluid power system. Restrictor: Component designed to offer restriction to a hydraulic flow. Return line:a line used to carry exhaust fluid from working device to reservoir. Reversing valve:afour-way directional valve used to reverse a double-actingcylinder or reversible motor. Rotary actuator:the name reserved for a hydraulic motor that has less than 360-degree rotation. Sensitivity:The minimum input signal to a servo device required to produce detectable output signal. Sequence:The order of a series of operations or movement of a device. Sequence valve: A pressure-operated valve, which at a predetermined pressure setting, diverts flow to a secondary circuit while holding the predeterminedminimum pressure on the first circuit. Servo:Using feedback to obtain automatic control. Servo mechanism:a servo loop where the output load tracks as if it were driven by input signal, even when the output power is many times that of the input. The output power is derived from the main or auxiliary power source and not from the servo input. Servo system: A servo mechanism or any other closed loop system where the output position, speed, pressure, torque or power is automatically controlled by the feedback signal as a function of input signal. Servo valve:a flow valve or a valve that modulates output flow and pressure as a function of the input signal. Signal:Acommand for a desired position, speed, torque and acceleration in servo. Single-acting cylinder: A cylinder in which hydraulic force may act in only one direction. The cylinder must be returned by gravity or spring or other mechanical force. Solenoid: Electromechanical device that converts electrical energy into linear mechanical motion. Spool:moving part of a valve that directs the flow through the system

7 Strainer:Acoarse filter. Stroke:Length of travel of a piston or plunger. Subplate: An auxiliary ported plated for mounting hydraulic components. Suction line: The hydraulic line connecting the inlet of pump to a reservoir. Sump: Reservoir. Surge pressure: Increased pressure due to dynamic condition in the system. Swash plate: A stationary canted plate in an axial type piston pump that causes the pistons to reciprocate as the cylinder block and pistons rotate Synchro: Arotary electromechanical device used as AC position feedback or reference signal in servo. Tank: Reservoir or sump. Throttle: Used to restrict flow. May be used as noncompensated flow control or to produce a pressure drop as a part of compensated flow valve. Turbulent flow: A flow condition where flow particles move in a random manner rather than in parallel paths as in laminar flow. The usual cause of turbulence is use of sharp-edged orifice in the control valves. Unload: To unload pressure, by dumping the flow directly to the tank. Unloading valve: Allows the pressure of one circuit to be unloaded when the pressure of another circuit reaches the predetermined setting. Valve: Apart that controls fluid flow, pressure and direction. Vent: To connect to atmosphere. Viscosity: Ameasure of internal resistance of fluid or resistance of a fluid to flow.