Base Circle - Gear Nomenclature
Many gears have tooth forms made up of involute curves called involute tooth forms. The circle that serves as the basis for drawing the involute curve is the base circle (d b) which size is determined by the pressure angle (α) and the pitch circle diameter (d ).
d b = d cosα
For example as shown below, when a cord is wound on a circular tube, then unwound while held in tension, the trace drawn by the tip of the cord is the involute curve. The tube in this case represents the base circle.
