Speed reduction and torque multiplication
Gears transmit torque and motion between shafts using mechanical advantage: a small pinion (fewer teeth) meshing with a large gear (more teeth) reduces output speed by the ratio of tooth counts while multiplying torque by the same factor. A 3:1 reduction means the output shaft rotates 1/3 as fast as the input but delivers 3 times the torque, assuming 100 percent efficiency. Multiple reduction stages (compound gearing) achieve very high reductions in compact envelopes, common in automotive transmissions and industrial reducers.
Power flow and efficiency trade-offs
Power input equals power output in an ideal gearbox (torque times angular velocity is constant), but real gears lose 3 to 15 percent to friction and churning, depending on lubrication quality, speed, and material. Helical gears (teeth angled along the axis) run more quietly and smoothly than spur gears, but generate thrust loads requiring additional bearing support. Planetary gears (sun, planet, ring arrangement) offer high reductions and compact layouts but higher complexity.
Ratio selection and application limits
Machine tool spindles need high speed and low torque, so the motor directly drives the spindle or through small reductions. Cranes and winches need massive torque and low speed, so large reductions are essential. Automotive transmissions vary gearing to optimize fuel consumption across a range of road speeds. Each application balances input motor characteristics, output load requirements, and cost, often using shifting mechanisms (manual, automatic, continuously variable) to adapt the ratio dynamically.