The task of gears is to transmit torques and to change the speed of rotation or the torque between the input and the output. When it is necessary to increase torque, planetary gears are often employed, particularly in the automobile industry. An example is their use in transfer cases in four-wheel-drive vehicles. Planetary gears are installed in the hubs of the driven wheels of lorries and buses as well as construction and agricultural vehicles, where they enable the vehicles to change down to slow speeds. An early application of planetary gears is still sometimes found today in the form of bicycle hub gears. Thanks to their compact construction, they are effective in increasing torque, despite the relatively small space of the wheel hub. Planetary gears are characterised by its high efficiency with a high transfer of torque even in very small construction spaces. They are suitable for clockwise and counter-clockwise use as well as in alternating, constant or intermittent operation.

Structure of planetary gears

Planetary gears are also referred to as epicyclic gear trains. They contain an input and an output shaft in a coaxial arrangement. They consist of a number of gear wheels arranged around a central gear. Since this arrangement is reminiscent of planets orbiting the sun, the central wheel is also referred to as the sun gear, while the surrounding ones are called planet gears. They are located within an external gear ring. The sun gear is driven by a motor and transmits its motion to the surrounding planet gears, which roll against the surrounding gear ring and thus orbit the sun gear evenly. The planet gears are mounted on a joint carrier that is connected to the output shaft. The rotational speed of the output is lower than that of the drive, and the torque of the output is higher in reverse proportion. The more gear wheels mesh with each other, the higher the torque. In general, one planetary gear system contains three to four planet gears.

Planetary gears commonly comprise a combination of several gear wheels toggled within and behind each other, on the basis of this simple basic form. Several simple planet gear sets can be merged into a multiple gear unit. This enables a hub gear changer in a bicycle to have in excess of the three speeds usually achieved with a simple gear set.

Operating modes of planetary gears

As a rule, a planetary gear comprises three shafts – the sun gear, the planet carrier, and the external ring gear. Often, though, the motion of one of the shafts is arrested, so that the input and output are on the other shafts. Which shaft or wheel is held, which serves as the input and which as the output depends on the constructive tasks. In certain cases, three-shaft operation is also used, in which either two shafts are driven and one is the driver (summation gears) or vice versa (distribution gears). Temporary three-shaft operation is also possible. Here, the third shaft is fixed by a frictional coupling. Under load, two-shaft operation can be interrupted and then restored by uncoupling and coupling.

Planetary gears in two-shaft operation

In principle, when it comes to selecting which two of the three shafts to use and the turnaround between the driving and driven shaft, there are six possible combinations. In two-shaft operation, a distinction is made between standing transmission and revolving transmission. With standing transmission, the shafts of the sun gear and external ring are in motion. The planet carrier is immobilised and is either fixed to the casing or halted by a brake.  The sun gear meshes with several planet gears, which transmit their combined force to the outer ring.

With revolving transmission, the external ring gear is immobilised and forms the frame. The sun gear and planet carrier shaft form the input and output. This operating mode enables higher transmission ratios than with standing transmission, which is why most industrial planetary gears are of this construction.

Planetary gears in three-shaft operation

In three-shaft operation, the planetary gear functions as a summation or distribution gear.  The distribution gear has one driving shaft and two driven shafts. The speed ratio of the two output shafts must be defined. A classic application is the distribution of power to two wheels in the drive axle of a car (differential gears with front-wheel drive). With four-wheel drives, two axle differentials are augmented by a centre differential. In hybrid electric vehicles, the power of the motor is distributed to the wheels and an electric generator. Summation gearboxes are also employed in hybrid vehicles to merge the power of the combustion engine and the electric motor (parallel hybrid).

Advantages of planetary gears

  • High efficiency
  • Compact construction
  • High power density

Planetary gears are characterised by their high level of efficiency; they also display a high power density in comparison to other types of gears, due to their compact construction. The use of the external ring gear in particular reduces both the volume and mass of this type of gear. It also enables high torque transmission in a small construction space thanks to the use of multiple parallel tooth combinations with several orbiting wheels. As the torque is distributed over several gear wheels, the tooth forces are smaller than in other types of gear. Moreover, no synchronisation is necessary, which means that it is possible to change gear without interrupting the traction. As all gear wheels constantly mesh with each other, the planetary gear operates at a low noise level.

Disadvantages of planetary gears

  • Complex construction
  • Higher power dissipation than with spur gears
  • Complex bearings

The disadvantages of planetary gears are their complex construction and higher power loss compared to spur gears. As the power is transmitted through at least two meshed teeth, the power dissipation is double that of a simple spur gear. Moreover, this type of gear requires complex bearings, especially if it is to be used as a three-shaft gear.

Planetary gears from Harmonic Drive®

The compact and precise Harmonic planetary gears were designed to satisfy high precision requirements down to a reduction range of less than 45:1. Planetary gears from Harmonic Drive AG offer unbeaten compactness, no increase in play throughout their service life, and excellent repeat precision in a range of ±20 angular seconds.