Machinery's Handbook, 31st Edition
Standard Fluid Circuit Components
2753
Fluid Circuits Fluid circuits contain compressed and flowing fluid, either liquid or gas, and convey it to perform work. They usually employ a valve to stop and/or control the direction of flow. Standard Fluid Circuit Components.— Fluid circuits typically include fluid prepara tion components, valves, actuators, and connecting lines. Pneumatic circuits also nor- mally include provisions for gas to be exhausted to the atmosphere, through a silencer, to a reclassifying filter, or to an exhaust manifold where it can be ducted elsewhere. Quick exhausting of gas is essential for fast pneumatic circuit response. Hydraulic circuits are generally closed, that is, recirculating fluid rather than discharging it. Fluid Circuit Types.— There are two main fluid circuit types, which are classified in terms of how the valve functions. Open and Closed Center Circuits: Open center circuits permit nearly unrestricted flow through the valve and back to the source when the valve is de-energized. Closed center circuits block flow through the valve when the valve is de-energized. This type of circuit is recommended when you have multiple circuits on a single supply. Flow Control Circuits: To control the speed of an actuator in a pneumatic circuit, use flow controls. Flow controls always generate some heat in hydraulic circuits, so with hy - draulics they should be avoided where possible and other methods used. There are three types of flow control circuits: meter-in, meter-out, and bleed-off (or bypass). Pneumatic and some hydraulic systems use meter-in and meter-out circuits, while many hydraulic circuits use bleed-off types. Meter-in circuits have a flow control valve before an actuator. Meter-in control can be used for actuator speed control. For hydraulic systems, a regulated bypass circuit is recommended rather than a restrictive flow control to reduce generated heat. Meter-out circuits are best for pneumatic actuator speed control, and are required for applications with large negative loads on actuators. For example when lowering a heavy weight, a meter-out circuit is best. Flow control is placed after the actuator to restrict flow coming out. Because the positive side of the actuator sees full system pressure and flow, the mechanical system is very stable under loading. Care must be taken to avoid overload ing actuators when using meter-out circuits. Fluid Component Symbols and Circuit Diagrams.— Fluid power circuits are docu mented through the use of circuit diagrams or schematics. The components in a circuit are represented symbolically and connected with lines representing pipe or tubing. The symbols are used to identify and represent the function of the components. Symbols for fluid power components have been standardized by ANSI Y32.10 and ISO 1219. Some common symbols can be found in Table 5. Directional control valve symbols show the valve’s positions, methods of actuation, flow paths, and ports. The valve positions are represented by boxes. The methods of left and right valve actuation are represented by symbols on each end of the valve symbol. The flow path for each position is shown within its respective position box. Ports are marked on the position box that corresponds to the de-energized state of the valve. Table 5. Selected ANSI Fluid Component Symbols
Solid Line Main Line Conductor, Physical Outline, or Shaft
Dashed Line Pilot Line for Control
Dotted Line Exhaust or Drain Line
Center Line Enclosure Outline
Circle Device, Connector, or Component
Triangle Indicates Flow Direction a : (See footnote a )
Square Reservoir (Closed, Open)
Oval Accumulator or Receiver
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