A gearbox can multiply torque or multiply speed — never both at once. The product of torque and speed (power) stays essentially fixed, minus small losses, no matter how the gear ratio is chosen.
Machine design applies mechanical engineering principles to the selection and sizing of components — gears, shafts, bearings, springs, fasteners — that make up mechanical systems. Gear selection is a foundational example of a recurring theme in machine design: physical tradeoffs that no clever design choice can eliminate, only redistribute.
Power transmitted through an ideal (lossless) gear train stays essentially constant — power equals torque times angular speed. A gear reduction that increases torque by a factor (say, 4×) must decrease output speed by that same factor to keep the power roughly balanced (accounting for real, generally small, mechanical losses). This isn't a design limitation to be engineered around — it's a direct consequence of energy conservation applied to a torque-and-rotation system.
A machine design engineer selecting a gear ratio is fundamentally choosing where on the torque-speed tradeoff curve a given application needs to sit — a winch needs high torque at low speed to lift heavy loads, while a cooling fan needs high speed with comparatively little torque. Choosing the wrong ratio doesn't just under-perform; a torque-speed mismatch with the actual load can stall a motor, waste energy, or fail to move the load at all.
Real gear trains aren't perfectly lossless — friction between meshing teeth, bearing friction, and lubricant drag all consume some input power as heat, so real output power is always somewhat less than input power (gear train efficiency is typically in the 90–98% range per stage for well-designed gearing). This is why real torque-speed calculations include an efficiency factor rather than assuming perfectly conserved power.
No — for a fixed input power, increasing output torque through gear reduction necessarily decreases output speed by roughly the same factor (and vice versa for gear-up/overdrive configurations), since power is conserved (minus small mechanical losses). No gear arrangement can multiply both simultaneously without an additional power source.
Lifting a heavy load requires high torque at relatively low speed — a high gear reduction ratio takes a motor's comparatively high speed, low torque output and converts it into the low speed, high torque output the lifting application actually needs, at the cost of the motor having to run much faster than the load actually moves.
No — different gear types (spur, helical, worm, bevel) have meaningfully different typical efficiencies due to how their teeth engage and how much sliding versus rolling contact occurs. Worm gears, for instance, typically have lower efficiency than spur or helical gears due to significant sliding contact, which is an important consideration when selecting gear type for a given power transmission application.
Try our STEM Learning Studio
More calculators, simulators, and guides for this discipline.