Refrigeration & Equipment Engineering

Unit 7 - Power transmission elements

7.3.Kinematic link or Element
Each part of a machine which moves relative to some other part is known as kinematic link.

Kinematic pairs
The two links or elements of a machine when in contact with each other are said to form a pair if the relative motion between them is completely fixed.

Types of kinematic pairs:

• Sliding pair: when the nature at contact between the elements of a pair is such that it can only slide relative to other, the pair is known as sliding pair. The piston and cylinder, cross head and guides of a reciprocating steam engine ram and it guides, tail stocks on the Lathe bed etc. are the examples. A sliding pair has a completely constrained motion.
• Turning pair: When the nature of contact between elements of a pair is such that it can only turn or revolve about a fixed axis. A shaft with collars at both ends fitted in to a circular hole, the crankshaft in a journal bearing in an engine, lathe spindle supported in head stock, cycle wheels turning over their axles etc. A turning pair also has a completely constrained motion.
• Screw pair: When the nature of contact between the elements of a pair is such that one element can turn about the other by screw threads, it is thus known as screw pair. The lead screw of a lathe with nut and bolt with a nut are the examples of a screw pair.
• Spherical pair: When the nature of contact between the elements of a pair is such that one element with spherical shape turns or swivels about the fixed element, the pair formed is called a spherical pair. The ball and socket joint, pen supporters joint in a pen stand etc. are the examples of a spherical pair.
• Lower pair: When the two elements of a pair have a surface contact when relative motion takes place and the surface of one element slides over the surface of other, the pair formed is known as lower pair: turning pair, screw pair form lower pairs.
• Hither pair: when the relative motion between the elements of a pair is partly turning and partly sliding, then the pair is known as higher pair. The elements of higher pairs, generally have line or point contact. A pair of friction discs, toothed gearing, belt and rope arises ball and roller bearings and can and follower are the examples of higher pairs.
• Forced closed pair: When the elements of a pair are kept in contact by the action of external forces, the pair is said to be a forced closed pair. The can and the follower is an example of forced closed pair, as it is kept in contact by the forces exerted by spring and gravity.