scope:

Spherical four-bar crank mechanisms consist of four links connected together by four revolute joints. The axes of rotation of all joints intersect at a single point. Having a degree of freedom (mobility) F = +1, spherical four-bar crank mechanisms, like planar four-bar crank mechanisms, are constrained mechanisms. They derive from a fourbar kinematic chain with revolute joint axes intersecting at a single point (Figure 1) when one of the links is selected as the frame. (Technical principles of mechanisms are covered in standard VDI 2127.)

All link points move on concentric spherical shells with the common mid-point O. In the borderline case, spherical crank mechanisms pass into planar mechanisms when the sphere radii are infinitely large and the axes of rotation thus run parallel to each other. There are many analogies between spherical and planar four-bar crank mechanisms (see standard VDI 2145) and these are taken into account in this standard by the corresponding nomenclature for the spherical mechanisms.

If the axes of rotation do not all intersect at a single point but at least two axes are out of square with each other then we have a (basic) spatial mechanism.