What determines the mechanical advantage of a gear train?

• A. The ratio between the number of teeth on the drive gear and the driven gear.
• B. The diameter of the load gear.
• C. The total weight of gears.
• D. The number of gears in the train.

Answer: (A)

Mechanical advantage in a gear train comes from the gear ratio, which is set by how many teeth the gears have (or their pitch diameters). In a simple gear pair, the tangential speed at the tooth pitch must match, so the output speed relates to the input speed by the ratio of the gear teeth: the speed of the driven gear is the input speed times the drive teeth divided by the driven teeth. The torque relationship is the inverse: the output torque equals the input torque multiplied by the driven teeth divided by the drive teeth. So, when the driven gear has more teeth than the drive gear, you get more output torque (greater mechanical advantage) but slower output speed; when the driven gear has fewer teeth, you get less torque and faster speed. That’s why the gear ratio—the ratio of teeth counts between drive and driven gears—determines the mechanical advantage. The diameter of the load gear and the total weight don’t determine MA by themselves, and adding more gears changes possible ratios but not the fundamental determinant.