 Welcome to gear nomenclature. Gears are used to transmit power positively from one shaft to another by means of successively engaging teeth. The parts of a spur gear are important to identify for proper selection, replacement, and maintenance. In this learning activity you will examine various components of gears and the terms used to describe these components. Gear. This is the larger of the two gears of a simple speed reduction gear train. It is usually the output of the gear train. Pinion. The pinion is the smaller of the two gears of a simple speed reduction gear train. Usually the driver is connected to the motor. Tooth face. The face of the gear tooth is the contacting surface of the tooth. Circular pitch. The circular pitch is the distance from a point on one tooth to a corresponding point on the next tooth measured on the pitch circle. Clearance. Clearance is the distance between the top of one tooth and the bottom of the mating tooth space. Backlash. The backlash is the space between the faces of two gear teeth. This space causes a delay in the response of the driven gear by the driver gear, which is known as play in the mechanical system. Pitch circle. The pitch circle is a circle drawn around the gear through the center of the gear teeth. Addendum. The addendum or the top of the tooth is the height of the gear tooth above the pitch circle. It is the radial distance between the pitch circle and the outside diameter. Dedendum. The dedendum or the bottom of the tooth is the radial distance from the pitch circle to the bottom of the tooth. It is equal to the addendum plus the clearance. Hole depth. Hole depth is the full depth of the tooth or the distance equal to the addendum plus the dedendum. Root circle. The root circle is the circle formed by the bottoms of the gear teeth. Outside diameter. The outside diameter is the total diameter of a gear from the top tooth 180 degrees to tooth top. Line of action. The line of action is the initial point of contact that is associated with the pressure angle. The pressure angle is formed by a line through the point of contact of two mating teeth and tangent to the base circles. Pressure angle. The pressure angle is the angle of contact of a gear tooth. It is usually 14.5 degrees or 20 degrees. This angle is formed by a line through the point of contact of two mating teeth and a line at right angles to the center line of the gears. All mating gears must be of the same pressure angle in order to mesh properly. Congratulations! You have completed this learning activity gear nomenclature. If you like this video be sure to subscribe to our channel.