adjustment  is  made,  the  locknut  is  tightened against  the  connector  to  prevent  the  connector from  turning.  The  other  end  of  the  connector  is attached,  either  directly  or  through  additional mechanical  linkage,  to  the  unit  to  be  actuated. In order to satisfy the many requirements of fluid  power  systems,  piston-type  cylinders  are available in various designs. Single-Acting Cylinder The single-acting piston-type cylinder is similar in  design  and  operation  to  the  single-acting ram-type  cylinder.  The  single-acting  piston-type cylinder uses fluid pressure to provide the force in   one   direction,   and   spring   tension,   gravity, compressed air, or nitrogen is used to provide the force in the opposite direction. Figure 10-5 shows a  single-acting, spring-loaded,   piston-type actuating cylinder. In this cylinder the spring is located  on  the  rod  side  of  the  piston.  In  some spring-loaded  cylinders  the  spring  is  located  on the  blank  side,  and  the  fluid  port  is  on  the  rod side  of  the  cylinder. A   three-way   directional   control   valve   is normally  used  to  control  the  operation  of  the single-acting piston-type cylinder. To extend the piston  rod,  fluid  under  pressure  is  directed through the port into the cylinder (fig. 10-5). This pressure acts on the surface area of the blank side of  the  piston  and  forces  the  piston  to  the  right. This  action  moves  the  rod  to  the  right,  through the end of the cylinder, thus moving the actuated unit  in  one  direction.  During  this  action,  the spring is compressed between the rod side of the piston and the end of the cylinder. The length of the stroke depends upon the physical limits within the  cylinder  and  the  required  movement  of  the actuated unit. To   retract   the   piston   rod,   the   directional control  valve  is  moved  to  the  opposite  working position,  which  releases  the  pressure  in  the Figure 10-5.—Single-acting, spring-loaded, piston-type actuating cylinder. cylinder. The spring tension forces the piston to the left, retracting the piston rod and moving the actuated unit in the opposite direction. The fluid is free to flow from the cylinder through the port, back through the control valve to the return line in  hydraulic  systems  or  to  the  atmosphere  in pneumatic  systems. The end of the cylinder opposite the fluid port is  vented  to  the  atmosphere.  This  prevents  air from being trapped in this area. Any trapped air would   compress   during   the   extension   stroke, creating  excess  pressure  on  the  rod  side  of  the piston.  This  would  cause  sluggish  movement  of the piston and could eventually cause a complete lock,  preventing  the  fluid  pressure  from  moving the  piston. The  spring-loaded  cylinder  is  used  in  arresting gear systems on some models of carrier aircraft. To raise (retract) the arresting hook, fluid pressure is  directed  through  the  arresting  hook  control valve  to  the  rod  side  of  the  cylinder.  This  force moves  the  piston,  which,  through  the  rod  and mechanical linkage, retracts the arresting hook. The arresting hook extends when fluid pressure is  released  from  the  rod  side  of  the  cylinder, allowing  the  spring  to  expand. Leakage  between  the  cylinder  wall  and  piston is  prevented  by  adequate  seals.  The  piston  in figure  10-5  contains  V-ring  seals. Double-Acting  Cylinder Most   piston-type   actuating   cylinders   are double-acting,   which   means   that   fluid   under pressure can be applied to either side of the piston to  apply  force  and  provide  movement. One  design  of  the  double-acting  cylinder  is shown in figure 10-6. This cylinder contains one piston and piston rod assembly. The stroke of the piston  and  piston  rod  assembly  in  either  direction is  produced  by  fluid  pressure.  The  two  fluid  ports, one near each end of the cylinder, alternate as inlet and  outlet  ports,  depending  on  the  direction  of Figure  10-6.-Doub1e-acting  piston-type  actuating  cylinder. 10-4