Theory of Machine
When the elements of a pair are kept in contact by the action of external forces, the pair is said to be a

Self-closed pair
Force-closed pair
Lower pair
Higher pair

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

Moved by the cam from the instant the follower begins to rise, till it reaches its highest position
Of rotation of the cam for definite displacement of the follower
Through which the cam rotates during the period in which the follower remains in the highest position
During which the follower returns to its initial position

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Theory of Machine
The contact ratio is given by

(Length of the arc of approach)/cosφ
(Length of path of recess)/(Circular pitch)
(Length of the arc of contact)/(Circular pitch)
(Length of the path of approach)/(Circular pitch)

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