Page 57 - PNEUMATIC SYSTEM INSTRUMENT

FOUR-WAY AND FIVE-WAY, THREE-POSITION VALVES

A family of three-position industrial pneumatic four-way valves (in

both four and five-ported versions) are readily available. In these

valves, the two extreme positions are directly related to the cylinder’s

direction of motion. They are the power positions of the valve,

controlling the cylinder’s movement, first in one direction, then in the

other — just as a two-position, four-way valve does.

The centre position of a three-position valve satisfies other system

requirements. (The centre position also is commonly referred to as the

neutral condition.) There are a variety of centre conditions available

with four-way directional valves, Figure 7.6. The most common centre

conditions are pressure centre (with both cylinder ports open to

pressure), exhaust centre (with both cylinder ports open to exhaust

and pressure blocked), and blocked centre (with all ports blocked).

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