With single spur gears, a pair of gears forms a gear stage. If you connect several gear pairs one after another, this is known as a multi-stage gearbox. For each gear stage, the direction of rotation between the drive shaft and the output shaft is reversed. The overall multiplication factor of multi-stage gearboxes is calculated by multiplying the ratio of each gear stage.
The drive speed is reduced or increased by the factor of the gear ratio, depending on whether it is a ratio to slow or a ratio to fast. In the majority of applications ratio to slow is required, since the drive torque is multiplied by the overall multiplication factor, unlike the drive speed.
A multi-stage spur gear can be realized in a technically meaningful way up to a gear ratio of approximately 10:1. The reason for this lies in the ratio of the number of teeth. From a ratio of 10:1 the driving gearwheel is extremely small. This has a negative effect on the tooth geometry and the torque that is being transmitted. With planetary gears a multi-stage gearbox is extremely easy to realize.
A two-stage gearbox or a three-stage gearbox can be achieved by simply increasing the length of the ring gear and with serial arrangement of several individual planet stages. A planetary gear with a ratio of 20:1 can be manufactured from the individual ratios of 5:1 and 4:1, for example. Instead of the drive shaft the planetary carrier contains the sun gear, which drives the following planet stage. A three-stage gearbox is obtained by means of increasing the length of the ring gear and adding another planet stage. A transmission ratio of 100:1 is obtained using individual ratios of 5:1, 5:1 and 4:1. Basically, all individual ratios can be combined, which results in a large number of ratio options for multi-stage planetary gearboxes. The transmittable torque can be increased using additional planetary gears when doing this. The direction of rotation of the drive shaft and the output shaft is always the same, provided that the ring gear or housing is fixed.
As the number of gear stages increases, the efficiency of the overall gearbox is reduced. With a ratio of 100:1 the efficiency is lower than with a ratio of 20:1. In order to counteract this situation, the fact that the power loss of the drive stage is low must be taken into consideration when using multi-stage gearboxes. This is achieved by reducing gearbox seal friction loss or having a drive stage that is geometrically smaller, for example. This also reduces the mass inertia, which is advantageous in dynamic applications. Single-stage planetary gearboxes are the most efficient.
Multi-stage gearboxes can also be realized by combining different types of teeth. With a right angle gearbox a bevel gear and a planetary gearbox are simply combined. Here too the overall multiplication factor is the product of the individual ratios. Depending on the type of gearing and the type of bevel gear stage, the drive and the output can rotate in the same direction.
Advantages of multi-stage gearboxes:
- Wide range of ratios
- Continuous concentricity with planetary gears
- Compact design with high transmission ratios
- Combination of different gearbox types possible
- Wide range of uses
Disadvantages of multi-stage gearboxes (compared to single-stage gearboxes):
- More complex design
- Lower degree of efficiency