the kinematic pair called a sliding joint. The screw is usually identified as its own kinematic pair called a helical joint. Two levers, or cranks, are combined into a planar four-bar linkage by attaching a link that connects the output of one crank to the input of another. Additional links can be attached to form a six-bar linkage or in series to form a robot. Classification of machines The identification of simple machines arises from a desire for a systematic method to invent new machines. Therefore, an important concern is how simple machines are combined to make more complex machines. One approach is to attach simple machines in series to obtain compound machines. However, a more successful strategy was identified by Franz Reuleaux, who collected and studied over 800 elementary machines. He realized that a lever, pulley, and wheel and axle are in essence the same device: a body rotating about a hinge. Similarly, an inclined plane, wedge, and screw are a block sliding on a flat surface. This realization shows that it is the joints, or the connections that provide movement, that are the primary elements of a machine. Starting with four types of joints, the revolute joint, sliding joint, cam joint and gear joint, and related connections such as cables and belts, it is possible to understand a machine as an assembly of solid parts that connect these joints. Kinematic synthesis The design of mechanisms to perform required movement and force transmission is known as kinematic synthesis. This is a collection of geometric techniques for the mechanical design of linkages, cam and follower mechanisms and gears and gear trains. See also Linkage (mechanical) Cam and follower mechanisms Gears and gear trains Mechanism (engineering) Rolamite, the only elementary machine discovered in the 20th century References Mechanical engineering