 Gear efficiency

Gear efficiency is the fraction of mechanical input power delivered as useful mechanical output power over a stated operating condition and system boundary:For steady forward operation with positive input power, $P_{\mathrm{loss}}=P_{\mathrm{in}}-P_{\mathrm{out}}$ accounts for dissipation within that boundary. Tooth-contact friction is one contribution; a complete gearbox measurement can also include other rotating supports and lubricant losses. Citizen Micro explicitly treats its quoted stage efficiencies as reference values that depend on transmitted torque and speed.^[ref]

If the positive reduction gear ratio is $i=|{\omega }_{\mathrm{in}}|/|{\omega }_{\mathrm{out}}|$, thenAn increase in torque therefore does not itself measure gear efficiency: reduction increases torque while decreasing speed. For an original example, let input torque be $2\mathrm{N}\mathrm{m}$ at $1200$ revolutions per minute, and output torque be $7.6\mathrm{N}\mathrm{m}$ at $300$ revolutions per minute. Then $i=4$ andThe angular-speed conversion factors cancel in the ratio but must be included when computing power in watts.

A bench test places torque and rotational-speed measurements on both sides of the chosen transmission, compares synchronized measurements after reaching steady operation, and reports load, speed and temperature. During acceleration, a difference between input and output power can partly represent changing kinetic energy, rather than loss. Near zero useful load, approximately fixed losses can also produce a much smaller efficiency than at a loaded operating point. For successive stages with consistent boundaries and no extra power path, efficiencies multiply; three hypothetical $0.95$ stages yield $0.95^3\approx 0.8574$, rather than $0.95$.