CHAPTER EIGHT - CAMS AND VALVE GEAR

Abstract

This chapter focuses on cams and valve gear. The possible combinations of the cam profile and the type of follower are numerous, consisting of those that give a desired motion but are uneconomical to produce and those that are relatively cheap to produce but do not fully satisfy the ideal motion. In the case of cams of a specified profile, the kinematics of a follower—such as a roller or flat faced tappet—can be determined by either analytical or vectorial methods. In the analytical method, an expression relating the displacement, or lift, of the follower in terms of the constant cam and follower dimensions and a parameter defining the cam position is obtained. The displacement equation can be successively differentiated to give corresponding equations for velocity and acceleration. When contact is between the roller and cam nose, the acceleration is negative, that is, downward, and at the change over points, there are sudden changes of acceleration. When the acceleration is positive, the inertia of the tappet and other moving parts of the valve gear produces an inertia force acting in a direction opposite to the acceleration, that is, downward. The inertia force then tends to keep the tappet in contact with the cam profile. When there is contact with the nose, the inertia force acts upward, and the tappet tends to leave contact with the cam.