On the classification of spherical four-bar linkages

Spherical four-bar linkages can be classified according to the rotatability, i.e. the capability of performing complete revolutions, of one link relative to the other three links. For a spherical four-bar linkage, two link lengths may be replaced at a time by their respective supplementary arc lengths. This reduction process is carried out until the sum of all four link lengths is a minimum. Grashof's rule may then be applied to test the rotatability of the linkage. The shortest link of a Grashof linkage is capable of making complete revolutions. A Grashof linkage may be a crank-rocker, a drag-link or a double-rocker. A non-Grashof linkage is always a double-rocker. Examples are given.