Consider a pinion driving wheel as shown in figure. When the pinion rotates in clockwise, the contact between a pair of involute teeth begins at *K* (on the near the base circle of pinion or the outer end of the tooth face on the wheel) and ends at *L* (outer end of the tooth face on the pinion or on the flank near the base circle of wheel).

*MN *is the common normal at the point of contacts and the common tangent to the basecircles. The point *K* is the intersection of the addendum circle of wheel and the common tangent. The point *L* is the intersection of the addendum circle of pinion and common tangent.

The length of path of contact is the length of common normal cut-off by the addendum circles of the wheel and the pinion. Thus the length of part of contact is *KL* which is the sum of the parts of path of contacts *KP* and *PL*. Contact length *KP* is called as **path of approach** and contact length *PL* is called as **path of recess**.

*r*_{a}* = O*_{1}*L *= Radius of addendum circle of pinion,and

*R *_{A}* = O*_{2}*K *= Radius of addendum circle of wheel

*r = O*_{1}*P *= Radius of pitch circle of pinion,

and *R = O*_{2}*P* = Radius of pitch circle of wheel.

Radius of the base circle of pinion = *O*_{1}*M = O*_{1}*P cos**f* *= r cos**f*

and radius of the base circle of wheel = *O2N = O2P cos* *f* *= R cos**f*

From right angle triangle *O*_{2}*KN*

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