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A coaxial gearbox - what is that?

Coaxial, in the case of gearboxes, simply means that the input shaft and the output shaft of the gearbox are on the same axis of rotation.

At the same time, there are many forms of coaxial gearboxes. For example, single-stage planetary gearboxes that operate coaxially, but multi-stage planetary gear units are also coaxial gearboxes. Likewise, shaft gearboxes and eccentric gearboxes (cycloidal gearboxes) can be designed in this way. The same applies to spur gearboxes.

Here, the input and output shafts are usually connected via two pairs of gears and, thanks to the appropriate arrangement, can then lie on one axis of rotation.

Most coaxial gearboxes are planetary gearboxes

The most widely used coaxial gearbox is the planetary gearbox. It can be designed as a single-stage gearbox or also as a multi-stage planetary gearbox. The operating principle of such a coaxial gearbox is quickly explained:

  • In a coaxial planetary gearbox, several spur gears (the planet gears) run in an internally toothed ring gear.
  • In doing so, they describe a concentric circular path.
  • The so-called planet carrier ensures that the spur gears are held on their path.
  • The planet gears are driven by a centrally located and externally toothed spur gear, the so-called sun gear.
  • This principle thus resembles a solar system - hence the names sun gear, planet gear and planetary gearbox.

How does a coaxial gearbox convert the torque?

All coaxial gearboxes convert speed and torque through their gear ratio. For example, let's take a ratio i=10:1. In this case, the speed in the gearbox is reduced by a factor of 10. The torque is increased by the same factor of 10.

It is interesting to note here that both single-stage and multi-stage gearboxes are possible. Even in one case. To achieve this, several planetary stages are coupled one after the other. The output of the first stage becomes the input for the next stage. In this way, large ratios can be achieved in coaxial planetary gearboxes (up to i > 500). This design possibility is a real strength of planetary gearboxes and it is only possible in their coaxial design.

Where is the ring gear fixed in the planetary gearbox?

In most cases, the force at the output acts directly via the rotating planet carrier, which is integrated into the output shaft. The ring gear is usually fixed and does not rotate.
However, there are also solutions in which the ring gear is not fixed, but the planet carrier is. We all know one of these solutions: the hub gears on a bike. It, too, is nothing other than a coaxial gearbox in which the torque is not transmitted via the planet carrier but via the ring gear.

The direction of rotation of coaxial gearboxes

Whether a coaxial gearbox rotates in the same direction or in the opposite direction is, in principle, quite simply a question of design. With the right design, you can control this individually depending on the requirements of the machine design and the application. In most cases, however, coaxial gearboxes rotate in the same direction.

Where are coaxial gearboxes used?

Coaxial planetary gearboxes are found everywhere in machinery where drive technology and gearboxes interact. Coaxial shaft or eccentric gearboxes are found in robotics for the most part. Generally because high ratios are required in a small space. At the same time, designers in robotics rely on the extremely high stiffness of such coaxial gearboxes.

What are the main advantages of coaxial gearbox?

  • They have a very good efficiency.
  • They exhibit a very high power density.
  • They can be built very compactly.
  • They are created for very many applications.
  • It is also easy to build multi-stage coaxial gearboxes and thus use high transmission ratios.
  • Coaxial gearboxes are also available in the Hygienic Design.

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