Machine Design
An open belt drive is used when

Shafts are arranged parallel and rotate in the same directions
Driven shaft is to be started or stopped whenever desired without interfering with the driving shaft
Shafts are arranged parallel and rotate in the opposite directions
Shafts are arranged at right angles and rotate in one definite direction

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Machine Design
The contact ratio is the ratio of

Length of arc of recess to the circular pitch
Length of arc of approach to the circular pitch
Length of arc of contact to the circular pitch
Length of path of contact to the circular pitch

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