The circle that generates the involute curve of the tooth flank, with a diameter equal to pitch diameter times the cosine of the pressure angle. No involute exists inside it, so the part of the tooth below the base circle is a non-working fillet. The line of action is tangent to the base circles of both gears, and base pitch, the spacing of teeth measured along that line, must match on mating gears.
Base circle sits above the root on this pinion, so anything below it is fillet, don't try to read profile down there.
Also heard as
- base diameter
Related terms
Involute
The curve traced by the end of a taut string unwound from a circle, the shape of almost every gear tooth flank.
Pressure angle
The angle between the line of tooth contact force and the tangent to the pitch circle, usually 20 degrees.
Line of action
The straight path along which the point of tooth contact travels, tangent to both base circles.
Pitch diameter
The diameter of the pitch circle, the imaginary rolling circle on which tooth size and gear ratio are based.