<1> Mechanisms may be categorized in several different ways to emphasize their similarities and differences. One such grouping divides mechanisms into planar, spherical, and spatial categories. All three groups have many things in common; the criterion which distinguishes the groups, however is to be found in the characteristics of the motions the links.www.mapeng.net马棚网机械资讯

<2> A planar mechanism is one in which all particles describe plane curves in space and all these curves lie in parallel planes; i.e. the loci of all points are plane curves parallel to a single common plane. This characteristic makes it possible to represent the locus of any chosen point of a planar mechanism in its true size and shape on a single drawing or figure. The motion transformation of any such mechanism is called coplanar. The plane four-bar linkage, the plate cam and follow, and the slider-crank mechanism are familiar examples of planar mechanism. The vast majority of mechanism in use today is planar.www.mapeng.net马棚网机械资讯

<3> Planar mechanisms utilizing only lower pairs are called planar linkages; they may include only revolve and prismatic pairs. Although a planar pair might theoretically be included, this would impose no constraint and thus be equivalent to an opening in the kinematics chain. Planar motion also requires that axes of all prismatic pairs and all revolute axes be normal to the plane motion.www.mapeng.net马棚网机械资讯

<4> A spherical mechanism is one in which each link has some point which remains stationary as the linkage moves and in which the stationary point of all links lie at a common location; i.e. the locus of each point is a curve contained in a spherical surface, and the spherical surface defined by several arbitrarily chosen points are all concentric. The motion of all particles can therefore be completely described by their radial projections, or “shadows”, on the surface of a sphere with properly chosen center.www.mapeng.net马棚网机械资讯

<5> Spherical linkages are constituted entirely of revolute pairs. A aspheric pair would produce no additional constrains and would thus be equivalent to an opening in the chain, while all other lower pairs have nonspheric motion. In aspheric linkages, the axes of all revolute pairs must intersect at a point. www.mapeng.net马棚网机械资讯

<6> Spatial mechanisms, on the other hand, include no restrictions on the relative motions of the particles. The motion transformation is not necessary coplanar, nor must it be concentric. A spatial mechanism may have particles with loci if double curvature. Any linkage which contains a screw pair, for example, is a spatial mechanism, since the relative motion within a screw pair is helical.www.mapeng.net马棚网机械资讯