Mechanism (engineering)

In engineering, a mechanism is a device that transforms input forces and movement into a specific set of output forces and movement. While we often use the word "machine" to describe a complete device, a mechanism is usually just one part of a larger mechanical system. For example, a car contains a steering mechanism, and a wristwatch contains a winding mechanism. When multiple mechanisms work together, they form a machine. A mechanism essentially manages power to achieve a desired physical result.

To study how these devices work, engineers use a model consisting of links and kinematic pairs. Links are the individual components, which are modeled as rigid bodies. This means that the distances between points on a link do not change during movement; the link does not flex. Kinematic pairs, also called joints, are the connections between these links. These joints provide ideal constraints that dictate how the links can move relative to one another.

Kinematic pairs are categorized into two main types: lower pairs and higher pairs. A lower pair is an ideal joint characterized by surface contact between the two elements. Examples include the revolute pair, which is a hinged joint that allows rotation, and the prismatic joint, which acts as a slider. Other lower pairs include cylindrical joints, which combine rotation and sliding, and spherical joints, also known as ball joints. A screw joint is another lower pair, but it has only one degree of freedom because its sliding and rotational motions are linked by a helix angle.

In contrast, a higher pair involves contact at only a single line or a single point. A common example is the cam and follower mechanism. In this setup, a driving link called the cam makes direct contact with a driven link called the follower. The specific shape of the cam determines the movement of the follower. Another example of a higher pair is the contact between the meshing teeth of two gears.

Historically, the study of mechanisms has evolved significantly. From the time of Archimedes through the Renaissance, people viewed mechanisms as combinations of simple machines like levers, pulleys, and screws. However, the German scientist Franz Reuleaux changed this perspective in the late 1800s. He focused on the relationship between links and their joints. Reuleaux defined a machine as a combination of resistant bodies arranged to compel the mechanical forces of nature to do work through determinate motion.

Mechanisms are also classified by the type of space in which they move. Planar mechanisms are constrained so that all movement occurs within a single plane. These can be analyzed using plane geometry and are often represented by a skeleton-like kinematic diagram. Spherical mechanisms, such as a gimbaled gyroscope or a robotic wrist, move around a fixed center point. Finally, spatial mechanisms allow a body to move through general three-dimensional space.

Examples of spatial mechanisms include robot arms, humanoid robotic systems, and the Stewart platform.

Linkages are a specific type of mechanism made of a collection of links connected by joints. One of the most useful examples is the planar four-bar linkage. There are many specialized linkages, such as Watt's linkage, which helps generate an approximate straight line. This was vital for the early steam engine and is still used in vehicle suspensions. More recent inventions include the Klann and Jansen linkages, which are designed to create walking movements.

Modern engineering also utilizes compliant mechanisms. Unlike traditional mechanisms that use rigid bodies, these consist of rigid bodies connected by flexible or "compliant" elements. These systems offer several advantages, such as a reduced part count and lower maintenance. Because they do not rely on traditional joints, they do not require lubrication and experience less mechanical wear. They also eliminate "slop," which is the unwanted parasitic motion caused by gaps between moving parts.