A planetary gearbox is a compact gear system often used in robotics to increase torque while reducing motor speed. Its parts fit around one central axis, which makes it useful when a robot joint, wheel hub, or actuator must stay small and strong. The main pieces are the sun gear, planet gears, planet carrier, and ring gear.
Because several planet gears share the load, the gearbox can transmit high torque for its size.
In a common reduction setup, the motor drives the sun gear, the ring gear is fixed, and the planet carrier becomes the output. The planet gears roll between the sun and ring, causing the carrier to rotate more slowly than the motor but with greater torque. The gear ratio depends mainly on the numbers of teeth on the sun and ring gears.
This coaxial arrangement keeps the input and output shafts aligned, which simplifies robotic arm joints and compact drive modules.
Key Facts
- Main parts: sun gear, planet gears, planet carrier, ring gear, and housing.
- For fixed ring, sun input, carrier output: gear ratio = 1 + Nr/Ns.
- Speed reduction means output speed is lower than input speed: omega_out = omega_in/gear ratio.
- Ideal torque increase is proportional to gear ratio: tau_out = tau_in × gear ratio.
- Power is approximately conserved except for losses: P_in ≈ P_out + losses.
- Planetary gearboxes are compact and coaxial because the input and output rotate around the same centerline.
Vocabulary
- Sun gear
- The central gear in a planetary gearbox that commonly receives input torque from the motor.
- Planet gear
- A gear that meshes with both the sun gear and the ring gear while orbiting around the center.
- Ring gear
- The outer gear with internal teeth that surrounds the planet gears.
- Planet carrier
- The rotating frame that holds the planet gear axles and often serves as the output in a reduction gearbox.
- Gear ratio
- The ratio that compares input speed to output speed and determines the ideal torque multiplication.
Common Mistakes to Avoid
- Using the normal external gear formula for the ring gear, because a ring gear has internal teeth and changes the motion relationship.
- Forgetting which part is fixed, because the gear ratio changes depending on whether the sun, ring, or carrier is held stationary.
- Assuming torque increases without speed decreasing, because gearboxes trade speed for torque while approximately conserving power.
- Ignoring efficiency losses, because friction, bearing drag, and gear tooth contact reduce the real output torque below the ideal value.
Practice Questions
- 1 A planetary gearbox has a sun gear with 20 teeth and a fixed ring gear with 60 teeth. For sun input and carrier output, calculate the gear ratio.
- 2 A motor spins at 3600 rpm and drives a 5:1 planetary gearbox. What is the ideal output speed at the carrier?
- 3 Explain why a planetary gearbox is useful in a robotic arm joint where the motor shaft and output joint axis must stay aligned.