Full definition
Toothed wheel transmitting power and motion between shafts through direct meshing of teeth, providing precise speed ratios with high efficiency. Major types: spur (parallel shafts, simplest, noisy at high speed), helical (parallel shafts, quieter, axial thrust), bevel (intersecting shafts, straight or spiral), worm (90° non-intersecting, high reduction 5:1-100:1, self-locking). Standards: AGMA 2001-D04 (rating), AGMA 2015 (accuracy), ISO 1328 (tolerances). Materials: through-hardened steel (30-40 HRC), case-hardened steel (58-62 HRC surface), cast iron, bronze (worm wheels), engineering plastics (light duty). Efficiency: spur/helical 96-99%, bevel 95-98%, worm 45-90%. Module (metric) or diametral pitch (imperial) defines tooth size.