What Is Pitch Circle Diameter (PCD) in Gears?

1. What Is Pitch Circle Diameter?

The pitch circle is an imaginary theoretical circle used to describe the meshing relationship between two gears. It is not a physical edge or surface on the gear and therefore cannot be directly observed or measured with a standard caliper.

During gear transmission, the pitch circles of two correctly meshing gears are considered to roll against each other at the same linear velocity. This theoretical rolling relationship forms the basis for calculating the transmission ratio and center distance.

Simply put, the pitch circle is the theoretical reference circle used to describe how a gear transmits motion and power. The diameter of this circle is called the Pitch Circle Diameter (PCD).

For a standard metric gear, the PCD can be calculated using:

PCD = m × z

Where:

m = Module

z = Number of Teeth

2.PCD Is Not the Same as Gear Outside Diameter

It is important to distinguish PCD from the gear's outside diameter.

A gear has several theoretical reference circles, including the pitch circle, addendum circle (outside circle), dedendum circle (root circle), and base circle.

The addendum circle is located outside the pitch circle. Therefore, for a standard gear, the outside diameter is normally larger than the PCD.

For example, for a standard gear with a module of 2 and 20 teeth:

PCD = 2 × 20 = 40 mm

The corresponding standard outside diameter is typically 44 mm.

Conclusion

Pitch Circle Diameter (PCD) is the diameter of the theoretical pitch circle of a gear. It is a fundamental parameter in gear design, transmission calculations, machining, and assembly. Understanding the difference between PCD and outside diameter is particularly important when interpreting engineering drawings or preparing CNC machining processes.