Gear Train Mechanical Efficiency
Gear Train Mechanical Efficiency
No gear train transmits power without some loss — friction at the tooth mesh, bearing drag, and lubricant churning all consume a portion of the input power before it reaches the output shaft. Mechanical efficiency, the simple ratio of output power to input power, is the standard single-number way to quantify how much of that input power actually does useful work downstream.Efficiency losses compound through a gear train with multiple stages, since each mesh multiplies rather than adds its own efficiency to the overall figure — a four-stage gearbox with 97% efficiency per stage ends up around 88% overall, a meaningfully different number than the per-stage figure might suggest, which matters directly for motor sizing and for the heat the gearbox has to dissipate.
The mechanical efficiency of the gear train is eta_g = P_out/P_in, the output power delivered divided by the input power supplied. where P_in is the input power supplied to the gear train and P_out is the output power delivered at the output shaft, giving the resulting mechanical efficiency eta_g.
Inputs
P_in := 5 kW=5000 W
P_out := 4.6 kW=4600 W
Dividing the delivered output power by the supplied input power directly gives the fraction of power that made it through the gear train without being lost to friction and drag.
eta_g := P_out/P_in=0.92
Results
An efficiency of 92% here is a reasonable value for a single or two-stage gearbox using rolling-element bearings and well-lubricated gears; efficiency drops further for worm gear sets (sometimes well below 90% per stage) because of their inherently higher sliding friction at the tooth mesh compared to spur or helical gears. The remaining 400 W of lost power (5 kW minus 4.6 kW) does not disappear — it converts to heat that the gearbox housing has to dissipate, which is why efficiency is not just an energy-cost consideration but also a thermal design input for gearbox cooling. Because efficiency multiplies across stages in a multi-stage train, a designer comparing gearbox options needs the overall efficiency across all stages, not just the headline efficiency of the highest-efficiency individual mesh.
Assumptions & limitations
- Steady-state power flow assumed — efficiency can vary somewhat with load and speed away from the rated condition.
- Represents overall efficiency of the gear train as a single lumped figure, not broken down by individual stage.
- Assumes the input and output power are measured at the same instant under steady operation (no transient losses).
- Bearing, seal, and lubricant churning losses are all lumped into this single efficiency figure, not separated out.