Watts Linkage

The Watts Linkage mechanism creates nearly straight line motion using pivot links. The small square in the diagram follows a nearly vertical path as the links rotate. This mechanism can be used to constrain motion to a linear path without the need for rails or rods to slide on.

The Watt's linkage was invented by James Watt, originally for use in steam engines. The Watt's linkage is used, for example, in the rear axle of some car suspensions to prevent relative sideways motion between the axle and body of the car or to prevent axle movement in the longitudinal direction of racing cars.