Table of Contents
Transmission Mechanisms
Transmission mechanisms are the system that transmits, modifies, or transforms that motion.
Passively transmits energy from actuator to output.
Modifies speed, torque, direction, or type of motion
Planetary Gearbox Working Principle
The planetary gearbox operates on the principle of distributing power through multiple gears to achieve speed reduction and torque multiplication. In this 4:1 configuration, the ring gear is fixed, the sun gear is driven by a NEMA17 stepper motor, and the output is obtained from the planet carrier. As the sun gear rotates, it drives the planet gears that revolve around it while meshing with the stationary ring gear. This motion causes the carrier, which holds the planet gears, to rotate at a reduced speed with increased torque, providing a smooth and efficient power transmission.
Planetary Gearbox Configurations
Fixed Ring Gear
The highest transmission ratio of the planetary gearbox is obtained when the gearbox is driven by the sun gear while the ring gear is fixed. In this configuration, the output is taken from the planet carrier.
When the ring gear is fixed, and the sun gear serves as the input while the carrier provides the output, the gearbox delivers its highest reduction ratio .
The transmission ratio is calculated as:
Hence, the speed reduction ratio is
4:1
, and the input and output rotate in the
same direction
.
If the input and output are reversed (i.e., driving through the carrier and taking output from the sun), the ratio becomes
1:4 (0.25)
, maintaining the same rotation direction.
Other Gear Configuration
Fixed Carrier
If the carrier is held stationary, the sun gear drives the ring gear, resulting in a reverse motion between input and output.
The transmission ratio is given by:
Thus, the gearbox produces a 3:1 ratio with opposite rotation directions between input and output.
When the input and output are interchanged, the ratio becomes -1/3 , still maintaining the reverse rotation (a configuration commonly used to achieve reverse drive in gear systems).
Fixed Sun Gear
When the sun gear is fixed and the ring gear is used as the input, the output is obtained from the carrier.
The transmission ratio can be found using:
This gives a 1.33:1 ratio with the same direction of rotation .
If the input and output are reversed, the ratio becomes 0.75 (1/1.33) .
Direct Drive
In a direct drive configuration, all components: the sun gear, planet carrier, and ring gear, are locked together, rotating as a single unit.
This results in a transmission ratio of
indicating equal speed and same rotational direction .
This configuration is typically used in multi-speed gear systems (such as 3-speed hubs) to achieve neutral or mid-range gear operation.
Herringbone Gears
To minimize backlash and ensure smooth power transmission, double helical gears (herringbone gears) are used for the sun gear, planet gears, and ring gear instead of conventional spur gears. Double helical gears offer higher load-carrying capacity, reduced noise and vibration, and eliminate the axial thrust generated in single helical or spur gears, resulting in more efficient and precise motion.
Hero Shots
Testing and Control Setup
The NEMA17-driven planetary gearbox was tested using an Arduino microcontroller and a TB6600 stepper driver, powered by a programmable power supply. The gearbox was modeled in Fusion 360 and fabricated using a Prusa i3 3D printer.
Advantages of Planetary Gearbox
High Torque Density: Can transmit high torque within a compact structure.
Efficiency: Low energy loss due to balanced power distribution.
Durability: Capable of handling heavy loads and long operating hours.
Versatility: Suitable for various industrial and automation applications.
Compact Design: Saves space while delivering superior performance.
Smooth Operation: Reduces vibration and noise.
Disadvantages of Planetary Gearbox
Planetary gearbox has a higher noise level.
Load sharing will not be good if the gearbox is not accurate.
Planetary gearboxes must be grease-lubricated to prevent wear.
Cost can be higher for low backlash and longevity.