Links and joints: detailed classification#
Types of closure in kinematic pairs#
For a joint to work, contact between the links must be guaranteed. This can be achieved in three ways:
Form closure: Contact is ensured by the geometry of the two members of the pair itself (e.g. cylinder-piston).
Force closure: Contact is ensured by the force exerted by an external element, such as a spring (e.g. cam-follower).
Chain closure: Contact is ensured by means of another member of the same mechanism (e.g. meshing of two gear wheels).
Classification of links#
By material behavior:
Rigid: Assumed not to deform. This is the main assumption in this course.
Elastic: Their deformation is relevant (e.g. springs, flexible beams).
Fluid: The link is a fluid (e.g. hydraulic cylinder).
By type of motion (in four-bar mechanisms):
Crank: If it can make full turns (360°) about a fixed axis.
Rocker: If it rotates about a fixed axis but without completing full turns.
Connecting rod or coupler: A link with no end connected to the ground.
Slider: Slides along a guide.
Classification of pairs by degree of freedom#
The degree of a kinematic pair is the number of degrees of freedom it allows in the relative motion.
Degree I pairs (1 d.o.f.)
A point of the moving link traces a curve.
Revolute or pin joint:
(R)
Prismatic or slider:
(P)
Helical:
(H)
Degree II pairs (2 d.o.f.) A point of the moving link traces a surface.
Cylindrical:
(C)
Rotation + Translation:
Degree III pairs (3 d.o.f.) A point of the moving link traces a volume.
Spherical or ball joint:
(S)
Planar:
(Pl)