FUNDAMENTALS
Josh Cosford • Contributing Editor
Hydraulic Symbology 201: industrial directional valves This article is the third in my series on hydraulic symbology, this time going beyond the basics to discuss symbols in higher detail. If you haven’t read Hydraulic Symbology 101 and Hydraulic Symbology 102, please find them at fluidpowerworld.com and read them first to gain the basics required for this article. If your basics are sound, however, carry on learning the intricacies of industrial directional valve symbology.
manufacturer specific, so ensure you use their documentation when interpreting symbols. What matters is that the a actuator operates the a envelope, for example. The valve ports are listed both above and below the center condition envelope. This not only provides clarity on the port locations, but also defines the at-rest position of the valve. For example, a valve may not have a neutral center condition, but could be at rest in the a or b envelopes, such as with a 2-position valve. Regardless, I’ve only ever seen the display order of A B on top with P T underneath.
I’m separating the lessons on directional valves between industrial, mobile and cartridge valve technologies. Each uses symbology differently because their valves are manufactured and applied individually based on the needs of their industries. Through-center valves are not used with industrial applications, and 3-way valves are only common with cartridge valves, for example. This article covers the basics of industrial valve symbols, while Hydraulic Symbology 202 will cover the compound symbols used with industrial stack valves.
The above symbol shows the typical 3-position directional valve with the internal lineation removed. Each of the square boxes depicts one of the three functional envelopes of which the valve is capable. The center condition is referenced as position 0 because it’s the natural valve state prior to any actuation. The center condition is important to consider because the type of pump needed is dictated by center flow conditions. If the valve allows flow in neutral, a fixed displacement pump is ideal, while a center condition blocking incoming flow requires a pressure compensated, variable displacement pump. The remaining two squares depict the operational envelopes of the second and third valve positions, which are the functions of the valve. Envelopes are referenced with a or b to define the envelope pertaining to the given actuation method. The reference can be located on either side of the valve symbol, and is generally 24
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The actuators, in function, are the electric or mechanical devices that shift the valve out of neutral to any of the operational envelopes. There are myriad actuators, although with electronic technology advancing so quickly, the forms of mechanical actuation are becoming rarer. Electric solenoid operation is by far the most common for industrial valves, and is depicted by the same basic actuator rectangle with a diagonal line. Although a line leaning to the left is meant to represent the coil pulling on the plunger, the rightleaning line showing a coil pushing on the plunger is more common, regardless of axial direction. The blunted actuator representing a physical actuation is the most generic representation of manual operation. This symbol can technically represent any form of physical actuation but is nondescriptive. It’s preferred to reserve this for only the manual override of a valve, and is often used in conjunction with the solenoid operator (see below). For the strictly manually operation, the lever actuator makes sense, and can be seen in forms varying from the one above.
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12/16/19 1:45 PM
