Doublecrank

Doublecrank, in mechanical engineering, refers to a four-bar linkage configuration in which both the input and the output links are cranks, meaning each can undergo continuous rotation about its fixed pivots. This arrangement contrasts with crank-rocker (one crank and one rocker) and double-rocker (two rockers) configurations. A doublecrank mechanism typically consists of a fixed frame and three moving links, with the two end links attached to the frame as cranks and the middle link acting as a coupler that transmits motion between the cranks.

Kinematically, a doublecrank provides a single degrees of freedom for the whole linkage, since the relative positions of the cranks determine the entire configuration. The design aims to allow both ends to rotate through a range of motion, with the coupler converting rotary motion from one crank to the other. Practical implementations must consider potential dead-center positions where motion transmission can momentarily fail, as well as the speed- and torque transmission characteristics that depend on link lengths and pivot spacing.

Design considerations for a doublecrank include satisfying length conditions that permit full rotation of both cranks under operating constraints, commonly discussed in the context of Grashof-type four-bar linkages. Engineers must account for mechanical interference, manufacturing tolerances, and dynamic effects such as acceleration-induced loads on the coupler. Applications range from compact actuators and precision positioning systems to educational demonstrations of planar kinematics.

History and study of four-bar linkages, including the doublecrank configuration, trace to foundational work in kinematics by figures such as Watt and Reuleaux, and remain a staple topic in mechanical design and robotics education. See also four-bar linkage, crank-rocker, and linkage synthesis.