seated (shut). The action of the poppet is similar to that of the valves in an automobile engine. In most  valves  the  poppet  is  held  in  the  seated position  by  a  spring. The  valve  consists  primarily  of  a  movable poppet which closes against the valve seat. In the closed  position,  fluid  pressure  on  the  inlet  side tends  to  hold  the  valve  tightly  closed.  A  small amount of movement from a force applied to the top  of  the  poppet  stem  opens  the  poppet  and allows  fluid  to  flow  through  the  valve. The use of the poppet as a-valving element is not  limited  to  directional  control  valves. Rotary Spool The   rotary   spool   directional   control   valve (fig.  6-22)  has  a  round  core  with  one  or  more passages  or  recesses  in  it.  The  core  is  mounted within a stationary sleeve. As the core is rotated within  the  stationary  sleeve,  the  passages  or recesses connect or block the ports in the sleeve. The  ports  in  the  sleeve  are  connected  to  the appropriate  lines  of  the  fluid  system. Sliding spool The   operation   of   a   simple   sliding   spool directional control valve is shown in figure 6-23. The  valve  is  so-named  because  of  the  shape  of  the valving element that slides back and forth to block and  uncover  ports  in  the  housing.  (The  sliding element is also referred to as a piston.) The inner piston areas (lands) are equal. Thus fluid under pressure  which  enters  the  valve  from  the  inlet  ports CHECK  VALVE Figure 6-22.—Parts of a rotary spool directional control valve. Figure  6-23.—Two-way,  sliding  spool  directional  control valve. acts  equally  on  both  inner  piston  areas  regardless of  the  position  of  the  spool.  Sealing  is  usually accomplished  by  a  very  closely  machined  fit between the spool and the valve body or sleeve. For valves with more ports, the spool is designed with  more  pistons  or  lands  on  a  common  shaft. The sliding spool is the most commonly used type of  valving  element  used  in  directional  control valves. Check  valves  are  used  in  fluid  systems  to permit flow in one direction and to prevent flow in  the  other  direction.  They  are  classified  as one-way  directional  control  valves. The   check   valve   may   be   installed   inde- pendently in a line to allow flow in one direction only,  or  it  may  be  used  as  an  integral  part  of globe,  sequence,  counterbalance,  and  pressure- reducing  valves. Check  valves  are  available  in  various  designs. They are opened by the force of fluid in motion flowing in one direction, and are closed by fluid attempting to flow in the opposite direction. The force of gravity or the action of a spring aids in closing  the  valve. 6-16