Theory of Machine
A reed type tachometer use the principle of

Torsional vibration
Transverse vibration
Longitudinal vibration
Damped free vibration

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Theory of Machine
The displacement of the reciprocating roller follower, when it has contact with the straight flanks of the tangent cam, is given by (where r₁ = Minimum radius of the cam, r₂ = Radius of the roller follower, and θ = Angle turned by the cam from the beginning of the follower displacement)

(r₁ - r₂) (1 - cosθ)
(r₁ + r₂) (1 + cosθ)
(r₁ - r₂) [(1 - cosθ)/cosθ]
(r₁ + r₂) [(1 - cosθ)/cosθ]

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