Theory of Machine
In a disc clutch, if there are n₁ number of discs on the driving shaft and n₂ number of discs on the driven shaft, then the number of pairs of contact surfaces will be

n₁ + n₂
n₁ + n₂ - 1
n₁ + n₂ + 1
n₁ + n₂ - 2

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Theory of Machine
The inversion of a mechanism is

Obtained by reversing the input and output motion
Obtained by fixing different links in a kinematic chain
Turning its upside down
Changing of a higher pair to a lower pair

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Theory of Machine
The contact ratio is given by

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

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