Theory of Machine
In a Hartnell governor, the stiffness of the spring is given by (where S1 and S2 = Spring forces exerted on the sleeve at max. and min. radii of rotation, and h = Compression of the spring)

(S₁ - S₂) / h
(S₁ - S₂) / 2h
(S₁ + S₂) / h
(S₁ + S₂) / 2h

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Theory of Machine
In gears, interference takes place when

The tip of a tooth of a mating gear digs into the portion between base and root circles
Pitch of the gears is not same
Gears do not move smoothly in the absence of lubrication
Gear teeth are undercut

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