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"Diametral pitch," sometimes called "diametrical pitch," is a term used to classify different types and sizes of gears. Gears are precision instruments that are classified by a variety of different parameters. The number of gear teeth, the shape and size of the gear teeth, the gear hub design and the way that the gear is attached to the shaft are all ways to classify gears. Diametral pitch is the most common way of classifying gears.
To understand diametral pitch, it is necessary for one to know what a pitch circle is. In any gear system, the pitch circle is the imaginary circle that connects the points on the gear where two interlocking gears meet. The pitch circle divides the gear's tooth into the top of the gear tooth, or addendum, and the bottom of the gear tooth, called the dedendum. At any point where two gears touch, their pitch circles will be tangent to one another if the gear system is designed correctly.
Diametral pitch, then, is a function of the diameter of the gear's pitch circle. It is equal to the number of teeth of the gear per inch or per centimeter of its diameter, depending on which measuring system is used. For example, if a gear has 32 teeth and a diameter of 8 inches (20 cm), the diametral pitch is four teeth per inch or 1.6 teeth per centimeter. When a consumer purchases or orders a gear, a manager would tell his gear salesperson or mechanical engineer the diametral pitch of the gear needed in order to make sure that the proper type of gear is ordered.
When a gear system is first designed, diametral pitch is important because it helps determine what size and type of gear is needed to interlock with any other gear. A gear is designed to transfer power from one section of a machine to another section of the machine. Two gears that will interlock successfully need to have proportional measurements or they will not work properly together and the power will not be transferred. For example, the ratio of the number of teeth on one gear to the second gear needs to be the same as the ratio between the first gear's diametral pitch to that of the second gear's.
The diametral pitch helps determine how fast a gear can move in a machine as well. The velocity ratio of a gear is defined as the ratio of the first gear's rotation speed to the ratio of the second gear's rotation speed. This same ratio also needs to apply to the diametral pitches of the two gears for the system to function properly.