Planetary Gears

A planetary gear is a type of epicyclic gear composed of a central gear, called a sun gear, used as an input gear. The gear has three or more gears that rotate around it, called planets. The planetary gear meshes with the ring gear, which surrounds the sun gear and the planetary gear in the shape of an internal spur gear. The design of planetary gears makes them particularly sturdy and easy to convert to different transmission ratios.

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What is Planetary Gear?

A planetary gear, also known as an epicyclic gear, is a type of gear system that consists of one or more outer gears (called planet gears) revolving around a central gear (called the sun gear) within an outer ring gear (called the ring gear or annulus). The arrangement of gears resembles a solar system, hence the name “planetary.”

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Structure of Planetary Gears

planetary gear parts

The structure of planetary gears consists of the following main components:

  • Sun Gear: The sun gear is the central gear of the planetary gear system, located at the center of the planetary gear assembly. It is usually a small gear with internal racks.
  • Ring Gear/Annulus: A ring gear is the external gear of a planetary gear system, surrounded by the sun and planet gears. It has an external rack.
  • Planet Gear: A planet gear is a gear located between the sun gear and the ring gear. Usually, there are multiple planetary gears connected together through a planetary carrier (planet carrier). Planetary gears also have internal racks.
  • Planetary Carrier: The planetary carrier is the central part that connects the planetary gears. It is usually a circular or bracket like structure used to support planetary gears and enable them to rotate around the sun gear.

Types of Planetary Gears

Single-Stage Planetary Gear Set

Single-Stage Planetary Gear Set

A single-stage planetary gear set consists of a sun gear, three planet gears, a carrier, and the ring gear.

Multi-Stage Planetary Gear Set

Multi-Stage Planetary Gear Set

Multi-stage planetary gear sets have two or more planetary gears to achieve the required speed and torque.

In-Line Planetary Gear Set

With an in-line planetary gear set, the input and output shafts are in a straight line and consist of a single sun gear and multiple planet gears contained in a ring gear.

Offset Planetary Gear Sets

Offset planetary gear sets are known as parallel shaft planetary gears. They have the input and output shafts placed in an offset or parallel position.

Coaxial Planetary Gear Set

A coaxial planetary gear set refers to a configuration of planetary gears where the sun gear, ring gear, and planet gears are aligned on the same central axis. In this arrangement, the gears are coaxial, meaning they share a common central axis of rotation

Right-Angle Planetary Gear Sets

With right angle planetary gear sets, the input and output shafts are arranged at right-angles to each other.

Harmonic Drive Planetary Gear Set

also known as strain wave gearboxes and wave generator gearboxes. They use a flexible metal cup and circular spline to transfer torque.

Differential Planetary Gear Sets

With a differential planetary gear set, every gear rotates, which is a key factor in the gear set for driving the wheels of a car.

Simpson Planetary Gear Set

A Simpson planetary gear set has a single sun gear and two or three sets of planetary carriers that are connected in a series. The structure of a Simpson planetary gear set has three forward gears, one reverse gear, a neutral, and is used in a three or four ratio automatic transmission.

Ravigneaux Planetary Gear Set

A Ravigneaux planetary gear set is an improvement of the Simpson planetary gear set and consists of two sun gears, two ring gears, and two sets of planet gears on a single carrier. The two suns are centrally located along a common rotating axis and are of different sizes.

Uses for Planetary Gear

agricultural gearbox manufacturer

The widespread use of planetary gear sets is due to their size, weight, and compact design. They act as reducers, causing the motor to slow down and increase torque. Planetary gears are used in multiple industries because they can generate torque and share loads with multiple gears because there are more contact surfaces and areas between the gears. The load distribution is uniform, making the gears more resistant to damage.
Planetary gear sets are used in rugged and durable applications because their load distribution is uniform, and their rugged and power intensive design can handle high torque and deceleration. Due to their self-alignment characteristics, they can withstand high impact loads and overhanging loads.


Track Drives

Track drivers  are used to  locomotives, fixed electric motors, self-propelled lawn mowers, and tanks. The switching gears in the planetary gearbox increase the torque required for acceleration and braking of the transmission system.


Wheel Drives

Planetary gear sets are used in wheel transmissions because they have lower induction losses and fewer gear changes.  Planetary gear sets are used with feed machine tools, presses, and conveyors.

Gearbox for Wind Turbines

Wind Turbines

Wind turbines require gearboxes capable of increasing rotor speed. Planetary gearboxes are preferred due to their high-power density and concentric input and output. The input from the blades provides the largest load that a planetary gear system has to endure.

BPG high precision planetary gearbox

High Precision Planetary Gearbox

The structure of a High Precision Planetary Gearbox is usually similar to traditional planetary gearboxes, but has higher accuracy and performance requirements

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