Reference Circle
The reference circle with diameter d = m·z is the base circle of the individual gear. Module and circular pitch are defined on it; it separates the tooth addendum from the dedendum. Unlike the operating pitch circle, it is a property of the single gear and does not change with pairing parameters.
Definition and Significance
The reference circle diameter is given by d = m·z (module times number of teeth). The reference circle is the circle on which the circular pitch p = π·m is measured and on which the basic rack profile (DIN 867) of the gear lies. Addendum and dedendum are measured from the reference circle (addendum = 1·m, dedendum = 1.25·m for standard gearing). The reference circle is thus a fixed, purely geometric property of the individual gear.
Distinction from the Operating Pitch Circle
While the reference circle is a property of the individual gear, the operating pitch circle is a pairing-dependent quantity. For standard gearing with standard center distance, reference circle and operating pitch circle coincide. With a profile shift or altered center distance, the operating pitch circle moves outward — the reference circle, however, remains unchanged. The operating pressure angle then deviates from the standard pressure angle of 20°.
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