seated (shut). The action of the poppet is similarto that of the valves in an automobile engine. Inmost valves the poppet is held in the seatedposition by a spring.The valve consists primarily of a movablepoppet which closes against the valve seat. In theclosed position, fluid pressure on the inlet sidetends to hold the valve tightly closed. A smallamount of movement from a force applied to thetop of the poppet stem opens the poppet andallows fluid to flow through the valve.The use of the poppet as a-valving element isnot limited to directional control valves.Rotary SpoolThe rotary spool directional control valve(fig. 6-22) has a round core with one or morepassages or recesses in it. The core is mountedwithin a stationary sleeve. As the core is rotatedwithin the stationary sleeve, the passages orrecesses connect or block the ports in the sleeve.The ports in the sleeve are connected to theappropriate lines of the fluid system.Sliding spoolThe operation of a simple sliding spooldirectional control valve is shown in figure 6-23.The valve is so-named because of the shape of thevalving element that slides back and forth to blockand uncover ports in the housing. (The slidingelement is also referred to as a piston.) The innerpiston areas (lands) are equal. Thus fluid underpressure which enters the valve from the inlet portsCHECK VALVEFigure 6-22.—Parts of a rotary spool directional controlvalve.Figure 6-23.—Two-way, sliding spool directional controlvalve.acts equally on both inner piston areas regardlessof the position of the spool. Sealing is usuallyaccomplished by a very closely machined fitbetween the spool and the valve body or sleeve.For valves with more ports, the spool is designedwith more pistons or lands on a common shaft.The sliding spool is the most commonly used typeof valving element used in directional controlvalves.Check valves are used in fluid systems topermit flow in one direction and to prevent flowin the other direction. They are classified asone-way directional control valves.The check valve may be installed inde-pendently in a line to allow flow in one directiononly, or it may be used as an integral part ofglobe, sequence, counterbalance, and pressure-reducing valves.Check valves are available in various designs.They are opened by the force of fluid in motionflowing in one direction, and are closed by fluidattempting to flow in the opposite direction. Theforce of gravity or the action of a spring aids inclosing the valve.6-16
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