Correct Answer - Option 4 : permit relative motion
Explanation:
Kinematic pair:
A kinematic pair or simply a pair is a joint of two links having relative motion between them.
Kinematics pair can be classified according to:
- Nature of contact
- Nature of relative motion
- Nature of mechanical constraint
Kinematic pairs according to Nature of Contact:
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Lower pair
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Higher pair
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A pair of links having surface or area contact between the members is known as a lower pair.
The contact surface of the two links is similar.
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When a pair has a point or line contact between the links, it is known as a higher pair.
The contact surface of the two links is dissimilar.
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E.g Nut turning on a screw, shaft rotating in a bearing, all pairs of slider-crank mechanism, universal joint etc.
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E.g Wheel rolling on a surface, cam and follower pair, tooth gears, ball and roller bearing etc.
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Kinematic pair according to nature of relative motion:
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Sliding pair
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When two links are so connected that one is constrained to have sliding motion relative to another.
Eg. crosshead and guide, rectangular rod in a rectangular hole in a prism.
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Turning pair
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When two links are so connected that one is constrained to turn or revolve about a fixed axis of another.
Eg. crankshaft turning in a bearing.
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Rolling pair
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Two elements of a pair are connected in such a way that one rolls over another fixed link.
Eg. Ball and roller bearings, rolling wheel on a flat surface etc.
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Screw pair
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If two mating links have a turning as well as sliding motion between them, they form a screw pair. this is achieved by cutting matching threads on the two links.
Eg. the lead screw and the nut of a lathe is a screw pair and a bolt with nut
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Spherical pair
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Two elements of a pair are connected in such a way that one element (with the spherical shape) turns or swivels about the other fixed element.
Eg. the ball and socket joint, attachment of a car mirror, pen stand, etc
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According to the nature of mechanical constraint:
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Closed pair
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Unclosed pair
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When the elements of a pair are held together mechanically they constitute a closed pair.
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When two links of a pair are in contact either due force of gravity or some spring action, they constitute an unclosed pair.
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