A multi-stage gearbox – what is that?
Multi-stage means that several pairs of gears are connected in series within a gearbox. This way you get a higher gear ratio.
In the case of a spur gearbox, the direction of rotation of the input and output shafts changes automatically with each additional stage. The situation is different with multi-stage planetary gearboxes - there, the direction of rotation remains the same.
The total ratio of a multi-stage gearbox depends on the ratios of the individual gearbox stages.
How is the gear ratio calculated?
The ratios of the individual gearbox stages are multiplied. This is because the gear ratio acts in such a way that the input speed in the multi-stage gearbox is reduced or increased by exactly the factor of the gear ratio. Usually, however, a reduction of the speed is required - i.e. a conversion to a slower speed because the torque is multiplied in the process.
What gear ratios are possible?
- Let's take a single-stage spur gearbox as a comparison. Here, gear ratios of about 10:1 or 1:10 are reasonably feasible. Otherwise, the driving or driven gear will be very small. This in turn has a negative effect on the maximum transmittable torque.
- In comparison, planetary gearboxes in particular can be easily designed as multi-stage gearboxes. Two-stage or three-stage gearboxes are feasible by extending the ring gear (inside the gearbox case) and arranging individual planetary stages in series.
- There are ratios that are better than others in terms of efficiency. In the case of two-stage planetary gearboxes, for example, this applies to a gear ratio of i=20. Because this achieves a higher efficiency than a two-stage ratio of i=100.
- Multi-stage gearboxes usually combine single-stage ratios from 3 to 10. This means that with a two-stage gearbox, ratios of i=9 (3*3) to i=100 (10*10) are possible without any problems.
- Another interesting and practical feature here is that you can combine many individual gear ratios in a multi-stage gearbox. This provides maximum flexibility in the transmission of torques or speeds.
How good is the efficiency?
The more stages a gearbox has, the lower its efficiency. To counteract this, care is taken to ensure that the power loss of the input stage is low. This can be done, for example, by reducing friction losses in the gearbox seal. Alternatively, the input stage can also be made physically smaller. This reduces the inertia and gives the gearbox greater dynamics. In general, however, the following still applies: single-stage planetary gearboxes have the best efficiency.
Special case of right-angle gearboxes
Multi-stage gearboxes are also created when different types of gearing are combined. This is the case, for example, with right-angle gearboxes. This is because they usually have a bevel gearbox and a planetary gearbox working together.
The advantages of a multi-stage gearbox:
- They offer a wide range of gear ratios.
- They can be built to be coaxial throughout.
- They are compact, but still provide high gear ratios.
- Combining different types of gearboxes is easily feasible.
- This allows many possible applications.
The disadvantages of a multi-stage gearbox (compared to single-stage gearboxes):
- Their design is relatively complex.
- As the number of stages increases, the efficiency decreases.
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