Table of Contents
What Is Pitch Diameter?
The pitch diameter (PD) is the diameter of the theoretical pitch circle on which gear teeth, screw threads, or pulley teeth are spaced. For gears, it is the circle at which two meshing gears effectively contact each other. For threads, it is the diameter where the thread width and groove width are equal.
The pitch diameter is the most critical dimension in gear and thread design because it determines the gear ratio, center distance between mating gears, and thread fit class. Accurate pitch diameter measurement is essential for proper meshing and load distribution.
Pitch Diameter Formulas
Gear Pitch Reference
| Diametral Pitch | Tooth Size | Common Applications |
|---|---|---|
| 3-5 | Coarse | Heavy machinery, mining equipment |
| 6-10 | Medium | General industrial, automotive |
| 12-20 | Fine | Instruments, small machines |
| 24-48 | Very Fine | Watches, precision instruments |
| 64-120 | Ultra Fine | Micro-mechanisms |
Common Thread Pitch Diameters
| Thread Size | TPI | Major Dia | Pitch Dia |
|---|---|---|---|
| 1/4"-20 UNC | 20 | 0.2500" | 0.2175" |
| 5/16"-18 UNC | 18 | 0.3125" | 0.2764" |
| 3/8"-16 UNC | 16 | 0.3750" | 0.3344" |
| 1/2"-13 UNC | 13 | 0.5000" | 0.4500" |
| 3/4"-10 UNC | 10 | 0.7500" | 0.6850" |
| 1"-8 UNC | 8 | 1.0000" | 0.9188" |
Frequently Asked Questions
What is the difference between diametral pitch and circular pitch?
Diametral pitch (DP) is the number of teeth per inch of pitch diameter. Circular pitch (CP) is the distance along the pitch circle between corresponding points on adjacent teeth. They are inversely related: CP = pi / DP. DP is the standard in the US, while metric gears use module (m = 1 / DP in inches, or m = 25.4 / DP in mm).
How do I find the center distance between two gears?
The center distance equals the sum of the two pitch diameters divided by 2. For example, if gear A has a PD of 3" and gear B has a PD of 6", the center distance is (3 + 6) / 2 = 4.5 inches.
Why is the pitch diameter important for threads?
Thread pitch diameter determines the fit class (how tightly a bolt fits in a nut). It is where the thread flanks of the bolt and nut contact each other. Measuring pitch diameter with thread wires or a thread micrometer is the standard method for verifying thread accuracy in manufacturing.