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Helical pair has degree of freedom_____.
1. 1
2. 2
3. 3
4. 4

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Correct Answer - Option 1 : 1

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:

  1. Nature of contact
  2. Nature of relative motion
  3. Nature of mechanical constraint

Helical Pair (H-pair): A helical pair has one degree of freedom and permits rotational and translational motion. These motions are, however, related by the pitch value of the helical pair. A helical pair is often called a screw pair. Example: Motion between the bolt and fixed nut

1) Kinematic pairs according to Nature of Contact:

Lower pair

Higher pair

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.

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.

E.g Nut turning on a screw, shaft rotating in a bearing, all pairs of slider-crank mechanism, universal joint, etc.

E.g Wheel rolling on a surface, cam and follower pair, tooth gears, ball and roller bearing etc.

2) Kinematic pair according to nature of relative motion:

Sliding pair

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.

Turning pair

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.

Rolling pair

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.

Screw pair

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

Spherical pair

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

3) According to the nature of mechanical constraint:

Closed pair

Unclosed pair

When the elements of a pair are held together mechanically they constitute a closed pair.  

When two links of a pair are in contact either due to force of gravity or some spring action, they constitute an unclosed pair.

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