In this laboratory we will investigate the kinematics of some simple mechanisms used to convert rotary motion into oscillating linear motion and vice-versa. The first of these is the slider-crank - a mechanism widely used in engines to convert the linear thrust of the pistons into the useful rotary motion of the drive-shaft. In this lab you will measure the linear acceleration of the piston of a lawn mower engine at various rotation rates of the drive shaft. The results exemplify a simple relation between speed and acceleration for kinematically restricted motions, which you will discover. An adjustable slider-crank apparatus and a computer simulation will show you some effects of changing the proportions of the slider- crank mechanism on piston velocity and acceleration. Other linkages and cam mechanisms may also be used for linear-rotary motion conversion and some of these will be included in the lab.
Linear momentum balance allows us to relate the forces acting on a body to its acceleration (F = ma). Thus a knowledge of the acceleration a of a piston permits analysis of the total force F acting on it. Knowledge of these forces is crucial if one is to choose the right material, proportions, and operating conditions for a new design