The diametral pitch is the ratio of the number of teeth to the number of millimeters of pitch diameter while the normal diametral pitch is the diametral pitch as calculated in the normal plane and is equal to the diameter pitch divided by the cosine of helix angle. The effective face width is a portion of the face width that actually comes into contact with mating teeth, as occasionally one member of a pair of gears may have a greater face width than the other. The actual torque ratio of a gear set divided by its gear ratio is called efficiency. The face of tooth is that surface of the tooth which is between the pitch circle to the top of the tooth. The face width is the length of the teeth in axial plane. Fillet curve is the concave portion of the tooth profile where it joins the bottom of the tooth space. A surface which is between the pitch circle and the bottom land is called flank tooth. The flank includes the fillet. The helical overlap is the effective face width of a helical gear divided by the gear axial pitch; also called the face overlap. The helix angle is the angle that a helical gear tooth makes the gear axis. The highest point of single tooth contact is the largest diameter on a spur gear at which a single tooth is in contact with the mating gear.
An internal diameter is the diameter of a circle coinciding with the tops of the teeth on an internal gear. An internal gear on the other hand is a gear with teeth on the inner cylindrical surface. The