Mechanisms

“A machine is a combination of resistant bodies so arranged that by their means the mechanical forces of nature can be compelled to do work accompanied by certain determinate motions.”

— Franz Reuleaux, The Kinematics of Machinery, 1875, translated by Alexander B. W. Kennedy, 1876

The Machine Elements part sizes the components a machine is built from, the gears, bearings, bolts and springs that are bought from a catalogue or made to a standard. This part is about how such parts are arranged to produce a motion or a force: the latch that snaps shut, the clamp that holds a workpiece with a tenth of the force it exerts, the lamp arm that stays where it is put, the ratchet that turns one way only. A mechanism is a way of arranging links, joints and elastic members, and the same handful of arrangements recurs across products that look nothing alike.

The tools are those of the earlier parts. The kinematics of rigid bodies describes how the links move, virtual work gives the relation between the forces at the input and the output without a free body diagram for every link, potential energy decides which positions are stable, and a motion solver checks the result for the full assembly, as SolidWorks Motion does. Each chapter ends in a design rule that can be printed and tested.

Linkages and mobility counts the degrees of freedom of a mechanism and classifies the four-bar linkage, and the crank-slider turns rotation into translation. Simulating mechanisms in Python sets up the motion solvers the later chapters use for checking. Over-centre mechanisms explains latches, toggle clamps and the dead centre, and why a cargo door whose latches stopped short of it was lost. Straight-line mechanisms and parallelograms, pantographs and scissors guide a point or a platform, and spring balancing makes a heavy arm weightless. Cams and followers produce any prescribed motion, ratchets, freewheels and indexing produce motion in one direction or in steps, and self-locking mechanisms hold by friction. Compliant mechanisms replace the joints by flexing parts, the natural choice for a printed design.