Tooth tip circle: The circle passing through all the tooth tips of a gear is called the tooth tip circle. Its radius is represented by ra, and its diameter is represented by da.

Root circle: The circle that passes through the bottom edge of all tooth slots becomes the root circle. Its radius is represented by rf, and its diameter is represented by df.

Dividing circle: In order to facilitate the calculation of the dimensions of various parts of the gear, a circle is selected on the gear as the reference for calculation, and this circle is called the dividing circle. The radius is represented by r, and the diameter is represented by d. The various parameters on the dividing circle do not have subscripts.

Tooth tip height: The part of the gear tooth between the indexing circle and the tooth tip circle is called the tooth tip, and its radial height is called the tooth tip height. Represented by ha.

Root height: The part of a gear tooth between the indexing circle and the root circle is called the root, and the radial height is called the root height. Represented by hf.

Total tooth height: The radial height of the gear teeth between the tooth crown circle and the tooth root circle is called the total tooth height, denoted by h.

Tooth thickness: On any circumference, the arc length between the tooth profiles on both sides of the gear teeth is called the tooth thickness on the modified circumference, expressed as sk.

Tooth groove width: The space between adjacent teeth of a gear is called tooth groove. On any circumference, the arc length of the tooth groove is called the width of the tooth groove on that circumference, represented by ek.

Tooth pitch: On any circumference, the length of the arc on the same side of two adjacent teeth is called the tooth pitch on that circumference, represented by Pk.

On the base circle, the length of the arc on the same side of two adjacent teeth is called the base circle pitch, expressed in Pb.

Normal tooth pitch: The distance between adjacent teeth on the same side in the normal direction is called the normal tooth pitch, expressed as Pn. According to the asymptotic property, Pn=Pb °