The structure of a pneumatic control valve can be decomposed into two parts: the valve body (including the valve seat and valve hole, etc.) and the valve core. According to the relative position of the two, there are two types: normally closed and normally open. Valves can be classified structurally into three types: globe valves, sliding column valves, and sliding plate valves. （1） The structure and characteristics of a globe valve. The valve core of a globe valve moves axially along the valve seat to control intake and exhaust. The basic structure of a two-way globe valve is shown in Figure 4.2. In Figure 4.2a, after the working air pressure is input into the P port of the valve, the valve core is tightly pressed against the valve seat under the action of the spring and gas pressure, and compressed air cannot flow out from the A port; Figure 4.2b shows that when the valve stem is subjected to a downward force, the valve core moves downward and disengages from the valve seat, allowing compressed air to flow from port P to port A for output. This is the switching principle of globe valves. The valve shown in Figure 4.3 has a normally open structure. Figure 4.3a shows the initial state, which is opposite to Figure 4.2a. The valve core leaves the valve seat under the action of spring force, and compressed air flows from port P to port A for output. Figure 4.3b shows the working state. Under the upward force of the valve stem, the valve core is tightly pressed against the valve seat to close the valve port, and the flow channel is closed. There is no compressed air flowing out of port A.
