Hydraulic Drive & Controls

Lesson 29 United States of American Standards institute ( USASI ) Graphical symbols

Reservoir

It is represented by a rectangle in which horizontal side is kept longer than the other side.  A reservoir if  vented to the atmosphere, the top of the symbol is open and  a reservoir if pressurized, the top is closed. Lines which connect to a reservoir usually drawn from the top. But  the line terminates below the fluid level, it is drawn to the bottom of the symbol.

Lines

Working line

It is made as  solid line which indicates a hydraulic pipe, tube, hose, or other conductor  carrying  the fluid between the components.

Pilot line

It is given by long dashes which indicates control lines.

Drain line

It is given by short dashes which indicates drain lines.

 Flexible line

It is made a solid, arced line which  is drawn between two dots which represents a flexible line in the system.

Pump

The hydraulic pump is a usually given by a symbol of circle with a black triangle in the circle pointing outward. The triangle indicates the flow direction The pressure line from the pump is drawn from the tip of the triangle, whereas, the suction line is drawn opposite it.. If a pump is reversible, it will have two triangles.

Motor

In the hydraulic system motor is also symbolized as circle like in pump but the black triangles pointing inward, which is a indication of  receiving of pressure energy. One triangle indicates a nonreversible motor; two triangles indicate a reversible motor.

Cylinder

A simple rectangle is the symbol for cylinder but it is different for different types of cylinders and is as below-

Single acting cylinder

 One of the cylinder is open in the cylinder symbol

Double acting cylinder

 Both ends of the cylinder  are closed in the cylinder symbol

Double end rod cylinder

 A rod line extends from each end of the basic cylinder symbol.

Pressure-Control Valves

The pressure control valve is given the  symbol as a square with external port connections. An arrow inside  shows the direction of flow of fluid.

Relief Valve

The relief valve's symbol goes between the pressure line and the tank. The flow-direction arrow points away from the pressure-line port and toward the tank port. When pressure in the system overcomes the valve spring, flow is from the pressure port to the tank port

Pressure Reducing Valve

It is a unit which  gives nearly constant output pressure provided that the inlet pressure remains higher than the required outlet pressure. The symbol indicates the outlet pressure opposite the spring

Flow Control Valve

This type of valve may be with fixed output in which variations in inlet pressure do not affect rate of flow will have the different symbols for the basic flow control valve, adjustable and nonadjustable valves.

Directional Control Valves

These are the valves which  provide full or restricted flow by opening or closing of one or more paths .A directional control valve symbol uses a multiple envelope system. It has separate rectangle for each position. All the port connections are made to the envelope, which shows the neutral condition of the valve. It is also having the arrows in each envelope which  indicates the flow paths when the valve is moved to that position.

Direction control valve

Sequence valve

In a sequence valve the inlet port is connected to a primary cylinder and  the outlet port is connected to the secondary cylinder. This valve is drained externally and therefore drain connection is added to the symbol.