Theory of Machine
In locomotives, the ratio of the connecting rod length to me crank radius is kept very large in order to

Minimize the effect of secondary forces
Have perfect balancing
Minimize the effect of primary forces
To start the locomotive quickly

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Theory of Machine
The contact ratio is given by

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

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