Planetary Gears – Advantages, Disadvantages and Applications

Planetary gears are one of the most commonly used gears in industrial transmission systems. They are considered to have high transmission efficiency, diverse transmission ratios and large torque.

In this article, Nam Phuong Viet will introduce you in detail about this type of planetary gear structure.

What is planetary gear?

Planetary gears are a combination of many gears of different sizes arranged in a special way, similar to the structure of a miniature solar system.

A planetary gear system can transmit with a high transmission ratio in a compact space. And can create large torque, meeting different transmission needs. The special thing is that this system can operate in many different modes, depending on which components in the system are held fixed or rotated freely.

This gear structure is widely used in mechanical machines, cars, and gearboxes.

Structure and operating principle of planetary gears

A basic planetary gear system consists of four main components, including:

• The sun gear is in the centre
• The planetary gears revolve around the sun gear
• The outer ring gear is a large internal gear that surrounds the planetary gears
• The planetary carrier

All these components are scientifically assembled to form a powertrain with very high performance.

Working principle

The principle of operation of a planetary gear system is the interaction of the components: the sun gear, the planet gear, and the outer ring. Depending on whether each component is fixed or moved, the planetary gear system can create different gear ratios, increase torque, or change rotational speed.

• If the outer ring is fixed and power is applied to the sun gear, the planetary gear carrier will rotate faster than the sun gear, creating a gear ratio greater than 1 (increase speed).
• If the sun gear is fixed and power is applied to the outer ring, the planetary gear carrier will rotate slower than the outer ring, creating a gear ratio of less than 1 (decrease speed).
• If the planetary gear carrier is fixed and power is applied to either the sun gear or the outer ring, the remaining gear will rotate in the opposite direction to the input gear, reversing the direction of rotation.

Advantages and disadvantages of planetary gear structure

Advantages of planetary gears:

• Large transmission ratio in small space
• High transmission efficiency
• Uniform load distribution
• High durability and long service life
• Smooth, quieter and smoother operation than other transmission systems
• Flexible in design

Disadvantages of planetary gears:

• Complex structure, requiring synchronization between many gears operating at the same time
• Difficult to disassemble, repair, or replace components
• High production and maintenance costs
• The assembly process requires very high precision

How to calculate planetary gear ratio

The gear ratio of a planetary gear system depends on which components in the system are fixed and which components participate in the transmission. To calculate the gear ratio, it is necessary to determine the position of each component.

When the outer ring is fixed

At this time the sun gear drives and the rack rotates, the transmission ratio is determined by the formula:

i = 1 + Z_ring/Z_sun

In which:

• Z_ring: Number of teeth of the outer ring.
• Z_sun: Number of teeth of the sun gear.

When the planetary holder is fixed

At this time the sun gear and outer ring gear rotate, and the transmission ratio is determined as follows:

i = – Z_ring/Z_sun

Fixed sun gear case

The outer ring gear and the planetary rack rotate, the transmission ratio will be determined by the formula:

i = – Z_sun/(Z_ring – Z_sun)

When no component is fixed, a general formula can be used to calculate the gear ratio.

i = n_in/n_out = (1 + (Z_ring/Z_sun))/(1 + (Z_ring/Z_sun) x (n_ring/n_sun))

In there:

• n_in: Input member rotational speed (sun gear or outer ring gear).
• n_out: The rotational speed of the output component (rack or other gear).

Applications of planetary gears in industry

Planetary gear systems are commonly used in applications such as automatic transmissions, robots, industrial machinery, and other types of mechanical equipment. Specifically:

• Automatic transmissions in automobiles
• Gear reduction gearboxes in industrial machinery such as cranes, presses, crushers, tractors, and heavy mechanical equipment.
• Wind turbine systems
• In robot drives
• In medical equipment such as MRI machines, CT machines, etc.
• Machine tools and processing equipment such as CNC machines, lathes, milling machines
• Electric and hybrid vehicle drive systems
• Conveyor systems
• In aircraft drive systems, especially in jet engines and wing control systems

Planetary gear sets help optimize speed, torque, and stable operation in complex systems and heavy loads.

You have just learned about planetary gears and related aspects such as structure, and principles, … with Nam Phuong Viet. Hopefully, the information provided above will be useful to you. If you have any questions, please contact us for answers.