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What is a Pitch Circle?

Jean Marie Asta
Jean Marie Asta

"Pitch circle" is a term used when discussing the mechanics of gears. It is the imaginary circle that passes through the points where two interlocking gears touch. Knowing the pitch circle diameter of a gear is important when calculating several measurements related to gear mechanics.

Two gears can interlock in a variety of ways. One gear can rotate inside of another gear, outside of another gear or even perpendicular to another gear. The interlocking gears transfer power from one part of a machine to another piece of the machine. To work properly, the system has to be designed so that the gears are the appropriate size and diameter in order to transfer the right amount of power at the desired speed.


To make sure that the power transfers as efficiently as possible, gear systems have to be carefully designed so that the teeth of one gear will turn the teeth of the next gear. The pitch circle diameter is required to make some of these calculations. When two gears engage effectively, the pitch circles of the two gears must be tangent to each other.

The pitch circle can be divided into further important components. The circular pitch, or pitch, is the distance between the teeth of a gear. For two gears to interlock, the distance from one tooth along the pitch circle to the next tooth must be the same in both gears. Otherwise, they will not be synchronized when they rotate. The area of a gear tooth between the pitch circle and the top of the gear tooth is called the addendum and the area from the pitch circle to the bottom of the gear tooth is called the dedendum.

After the pitch circle has been drawn, the pitch circle diameter and pitch circle radius of each gear can be determined. The ratio of the first gear's pitch circle diameter to that of the second gear's pitch circle diameter must be the same as the ratio of the number of teeth in the first gear to the number of teeth in the second gear. So, by knowing the ratio of the two pitch circle diameters, the engineer also knows how many teeth each gear must have per square inch or square centimeter. The ratio of the gears' pitch circle diameters is also equal to the velocity ratio, which is the ratio of the speed of rotation of the input gear to the speed of rotation of the output gear. Therefore, knowing the diameter of a gear pitch circle is vital when determining whether one gear will work with the next one in a gear system.

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