In involute gearing, the contact point between a pair of teeth moves along a straight line tangent to the base circles of both gears, inclined at the pressure angle. The usable length of this line, bounded by the two tip circles, is the length of action, and dividing it by base pitch gives the contact ratio. The tooth load always acts along it, which is why bearing loads have a fixed direction.
Length along the line of action is short on this pair, so contact ratio is barely 1.2, every handoff is going to bang.
Also heard as
- path of contact
- pressure line
Related terms
Contact ratio
The average number of tooth pairs in contact at once, which largely sets how smoothly and quietly a pair runs.
Involute
The curve traced by the end of a taut string unwound from a circle, the shape of almost every gear tooth flank.
Base circle
The circle from which the involute tooth profile is unwound, smaller than the pitch circle.
Pressure angle
The angle between the line of tooth contact force and the tangent to the pitch circle, usually 20 degrees.