Theory of Machine
The two elements of a pair are said to form a higher pair, when they

Have a surface contact when in motion
Permit relative motion
Are kept in contact by the action of external forces, when in motion
Have a line or point contact when in motion

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Theory of Machine
Angle of action of cam is defined as the angle

Through which the cam rotates during the period in which the follower remains in the highest position
Moved by the cam from beginning of ascent to the termination of descent
Of rotation of the cam for a definite displacement of the follower
Moved by the cam from the instant the follower begins to rise, till it reaches its highest position

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