Theory of Machine
When the addenda on pinion and wheel is such that the path of approach and path of recess are half of their maximum possible values, then the length of the path of contact is given by (where r = Pitch circle radius of pinion, R = Pitch circle radius of wheel, and φ = Pressure angle)

[(r + R) cosφ]/2
[(r² + R²) cosφ]/2
[(r² + R²) sinφ]/2
[(r + R) sinφ]/2

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Theory of Machine
The relation l = (2/3).(j + 2) apply only to kinematic chains in which lower pairs are used. This may be used to kinematic chains in which higher pairs are used, but each higher pair may be taken as equivalent to

One lower pair and two additional links
Any one of these
Two lower pairs and one additional link
Two lower pairs and two additional links

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