Law of gearing says that the common normal at the point of contact between a pair of teeth must always pass through the pitch point.

The theoretical minimum value for the contact ratio is one, that is there must always be at least one pair of teeth in contact for continuous action.

Interference may only be prevented, if the addendum circles of the two mating gears cut the

The two non-intersecting and non-parallel i.e. non-coplanar shaft connected by gears. These gears are called skew bevel gears or spiral gears and the arrangement is known as skew bevel gearing or spiral gearing. This type of gearing also have a line contact, the rotation of which about the axes generates the two pitch surfaces known as hyperboloids.

ηmax = cos (θ + φ ) + 1/ cos (θ − φ ) + 1

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Diametral pitch, pd = T/D = π/pc

The two parallel and co-planar shafts connected by the gears. These gears are called spur gears and the arrangement is known as spur gearing. These gears have teeth parallel to the axis of the wheel as shown in Fig. 12.1. Another name given to the spur gearing is helical gearing, in which the teeth are inclined to the axis.

The fundamental condition of correct gearing is the common normal at the point of contact between a pair of teeth must always pass through the pitch point.