Torque multiplication: A planetary gearset provides a gear reduction from input to output. If you use the sun and ring gears with different numbers of teeth, the output shaft can rotate more slowly than the input but with greater torque. The overall torque gain is roughly the gear reduction ratio, minus losses.
Typical arrangement: In a common planetary gearbox, output torque is increased when the gearbox is used in a reduced-speed, higher-torque configuration. The reduction ratio is set by the number of teeth on the sun gear, planet gears, and ring gear.
Power conservation: Ideally, power in equals power out (P_in ≈ P_out, minus losses). Since power is torque × angular speed, a decrease in speed with an increase in torque can preserve the same overall power.
Efficiency and losses: Real-world gears have friction, windage, and bearing losses. Planetary systems are efficient but not 100%. Typical efficiency might be in the 70–95% range depending on design, lubrication, and load.
Advantages of planetary gears:
High torque in a compact package due to multiple planets sharing load.
Good stiffness and load distribution.
Compact, lightweight relative to the torque they provide.
Limitations:
Some speed reduction is traded for torque; not suitable if you need high speed and high torque simultaneously without compromise.
Complexity and cost are higher than simple gear trains.
If you want a quick rule of thumb: the torque multiplication factor is approximately equal to the gear reduction ratio (minus minor losses). For example, a 5:1 planetary reduction will, ideally, give about 5× more output torque than input torque (under ideal conditions). In practice, you'd multiply by the ratio and then subtract about 5–25% to account for losses, depending on lubrication and efficiency.
If you have a specific gearbox (gear ratio, input speed, expected efficiency), I can run a quick calculation to estimate the output torque.
