CHAPTER 10ACTUATORSOne of the outstanding features of fluid powersystems is that force, generated by the powersupply, controlled and directed by suitablevalving, and transported by lines, can be con-verted with ease to almost any kind of mechanicalmotion desired at the very place it is needed.Either linear (straight line) or rotary motion canbe obtained by using a suitable actuating device.An actuator is a device that converts fluidpower into mechanical force and motion.Cylinders, motors, and turbines are the mostcommon types of actuating devices used in fluidpower systems.This chapter describes various types ofactuating cylinders and their applications,different types of fluid motors, and turbines usedin fluid power systems.CYLINDERSAn actuating cylinder is a device that convertsfluid power to linear, or straight line, force andmotion. Since linear motion is a back-and-forthmotion along a straight line, this type of actuatoris sometimes referred to as a reciprocating, orlinear, motor. The cylinder consists of a ram orpiston operating within a cylindrical bore. Actuat-ing cylinders may be installed so that the cylinderis anchored to a stationary structure and the ramor piston is attached to the mechanism to beoperated, or the piston or ram may be anchoredto the stationary structure and the cylinderattached to the mechanism to be operated.Actuating cylinders for pneumatic andhydraulic systems are similar in design andoperation. Some of the variations of ram- andpiston-type actuating cylinders are described inthe following paragraphs.RAM-TYPE CYLINDERSThe terms ram and piston are often usedinterchangeably. However, a ram-type cylinder isusually considered one in which the cross-sectionalarea of the piston rod is more than one-half thecross-sectional area of the movable element. Inmost actuating cylinders of this type, the rod andthe movable element have equal areas. This typeof movable element is frequently referred to asa plunger.The ram-type actuator is used primarily topush rather than to pull. Some applicationsrequire simply a flat surface on the external partof the ram for pushing or lifting the unit tobe operated. Other applications require somemechanical means of attachment, such as a clevisor eyebolt. The design of ram-type cylinders variesin many other respects to satisfy the requirementsof different applications.Single-Acting RamThe single-acting ram (fig. 10-1) applies forcein only one direction. The fluid that is directedinto the cylinder displaces the ram and forces itoutward, lifting the object placed on it. Since thereFigure 10-1.—Single-acting ram-type actuating cylinder.10-1
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