- Module (m)Module
- The pitch diameter in millimetres divided by the number of teeth. Two gears mesh only when their modules are identical.
- Pitch diameterPitch diameter
- The reference circle on which tooth thickness and space are equal; all geometry ratios start from it.
- Pressure anglePressure angle
- The angle between the line of action and the common tangent at the pitch point; 20° is standard, 25° carries more load.
- BacklashBacklash
- The clearance between meshing teeth when the drive flank is in contact; too little causes seizure, too much causes noise.
- AddendumAddendum
- The radial height of a tooth above the pitch circle, normally one module for a standard full-depth tooth.
- Case depthCase depth
- The hardened carburised layer measured to 550 HV; it must exceed the depth of maximum shear stress.
- Core hardnessCore hardness
- Hardness below the case, typically 30–42 HRC; it supports the case and resists tooth breakage under shock.
- Helix angleHelix angle
- The angle of the tooth relative to the shaft axis; it raises overlap and lowers noise but adds axial thrust.
- Root filletRoot fillet
- The curved transition at the tooth root; its radius governs bending fatigue life more than any other single feature.
- Grade (GB/T, DIN, AGMA)Accuracy grade
- A single number describing allowed tooth deviations. GB/T 6 is roughly DIN 6 and AGMA 10–11 in common sizes.
