Actuators Linear Actuators 12-33 Figure 12-52.  Linear actuator. When the selector valve is placed in the ON or EXTEND  position ,  fluid is admitted under pressure to the left-hand  chamber of the actuating cylinder.  [Figure 12-53]  This  results in the piston being forced toward the right. As the  piston moves toward the right, it pushes return fluid out of  the right-hand chamber and through the selector valve to the  reservoir. When the selector valve is placed in its RETRACT  position, as illustrated in  Figure 12-50,  fluid pressure enters  the right chamber, forcing the piston toward the left. As the  piston moves toward the left, it pushes return fluid out of the  left chamber and through the selector valve to the reservoir.  A single-action actuating cylinder is illustrated in  Figure 12-52A.  Fluid under pressure enters the port at the  Actuating cylinders are of two major types: single action  and double action. The single-action (single port) actuating  cylinder is capable of producing powered movement in  one direction only. The double-action (two ports) actuating  cylinder is capable of producing powered movement in two  directions. Seals are used to prevent leakage between the piston and the  cylinder bore and between the piston rod and the end of the cylinder. Both the cylinder housing and the piston rod have  provisions for mounting and for attachment to an object or  mechanism that is to be moved by the actuating cylinder. A double-action (two ports) actuating cylinder is illustrated  in  Figure 12-52B.  The operation of a double-action actuating  cylinder is usually controlled by a four-way selector valve.  Figure 12-53  shows an actuating cylinder interconnected  with a selector valve. Operation of the selector valve and  actuating cylinder is discussed below. An actuating cylinder transforms energy in the form of fluid  pressure into mechanical force, or action, to perform work.  It is used to impart powered linear motion to some movable  object or mechanism. A typical actuating cylinder consists of  a cylinder housing, one or more pistons and piston rods, and  some seals. The cylinder housing contains a polished bore  in which the piston operates, and one or more ports through  which fluid enters and leaves the bore. The piston and rod  form an assembly. The piston moves forward and backward  within the cylinder bore, and an attached piston rod moves  into and out of the cylinder housing through an opening in  one end of the cylinder housing.  left and pushes against the face of the piston, forcing the  piston to the right. As the piston moves, air is forced out of  the spring chamber through the vent hole, compressing the  spring. When pressure on the fluid is released to the point  it exerts less force than is present in the compressed spring,  the spring pushes the piston toward the left. As the piston  moves to the left, fluid is forced out of the fluid port. At  the same time, the moving piston pulls air into the spring  chamber through the vent hole. A three-way control valve is  normally used for controlling the operation of a single-action  actuating cylinder. A B Fluid inlet Mount Piston seal Spring chamber Cylinder housing Hydraulic fluid pressure Fluid return to reservoir Hydraulic fluid pressure Fluid return to reservoir Air vent Piston rod Rod wiper seal Piston Extend Retract Fluid extension/sping return Double-action actuator Single-action actuator