What is a gear stage?

Gears transmit and convert torques, speeds, directions of rotation, and forces. This is achieved with machine elements such as gearwheels, shafts, belts, rods, etc. There are single- and multi-stage versions. Multi-stage gears have at least two stages. The gear stages are connected in series and change both the speed and the torque.

Adding more gear stages makes it possible to achieve higher gear ratios. Each additional stage, however, increases the number of components in the gearbox. This requires more space and results in higher weight and a higher cost of acquisition. Susceptibility to errors also increases, as each stage multiplies the number of potential errors (Gaussian error propagation).

Reducing or increasing gears?

Each gear stage is characterised by a particular transmission (reducing or increasing) ratio. This describes the ratio between the input and output speed. If the speed at the gear input shaft is higher than at the gear output shaft (i > 1), it is referred to as the speed reduction ratio. If the speed at the gear input shaft is lower than at the gear output shaft (i < 1), it is referred to as the speed increasing ratio.

Cycloidal gears and strain wave gears are reducing gears. This means they reduce the rotational speed of the motor and simultaneously increase the motor torque. Speed and torque are inversely proportional due to the reduction ratio, although efficiency is still very important in calculating the torque.

Reduction (i > 1)

• The speed at the gear output shaft is lower than at the gear input shaft
• The torque at the gear output shaft is lower than at the gear input shaft
• Speed reduction ratio

Increase (i > 1)

• The speed at the gear output shaft is higher than at the gear input shaft
• The torque at the gear output shaft is higher than at the gear input shaft
• Speed increasing ratio

A high output torque is advantageous in many cases. It enables the transmission of higher forces, facilitates handling of heavy loads, and helps to maximise the service life and reliability of the machine or robot.

Single-stage and multi-stage cycloidal gears 

Cycloidal gears are available as both single-stage and multi-stage gears. Single-stage cycloidal gears are equipped with internal eccentrics. They achieve gear reduction ratios up to i = 200, have zero backlash (because there are no geared elements), and are offered in very small sizes. However, their reduction ratio is limited. Another disadvantage: The large eccentrics with their accordingly high inertia rotate at the motor speed. This results in a relatively high vibration level and a low fatigue limit.

Our cycloidal gears have external eccentrics. This necessitates an upstream spur gear stage and requires a two-stage reduction principle (spur gear stage and eccentric stage). The solid shaft version therefore has two gear stages and the hollow shaft version has three gear stages. The total number of gear stages is four.

Two-stage reduction principle offers numerous advantages

The spur gear stage transmits the rotary motion of the drive or servo motor to the eccentric shafts. This reduces the speed corresponding to the reduction ratio of the input shaft to the spur gears. Due to the reduced speed, the eccentrics rotate more slowly and can be smaller, which significantly reduces vibrations and therefore improves the fatigue limit. While the geared elements in the first stage cause minimal backlash, this is negligible, since it is reduced in the cycloidal stage. The cycloidal stage is preloaded, which means it has zero backlash. The number of pins and the resulting reduction ratio of the cycloidal stage makes it possible to reduce the tooth flank backlash in the first stage.

The two-stage reduction principle not only reduces the vibrations and inertia, but also allows larger reduction ratios. It also results in excellent performance with respect to dynamics and smooth operation, as well as high repeatability and path accuracy. The two-stage reduction principle is also advantageous in terms of cost-effectiveness. Since the overall reduction can be controlled only by the spur gear stage, this eliminates the production of different costly gear housings.

Gear stages of cycloidal gears and planetary gears

Due to their speed step, planetary gears can achieve a maximum transmission ratio of i = 10 in one stage. If one compares the dimensions of planetary gears and cycloidal gears, one notices that the overall length of planetary gears increases substantially with higher reduction ratios. Multi-stage planetary gears are therefore much longer than cycloidal gears with the same reduction ratio.

The advantage of our multi-stage cycloidal gears with external eccentrics

• Very high reduction ratios in a compact space
• Flexible adaptation of the various reduction ratios through the first stage
• Price-independent, due to the elimination of different housing parts for the different reduction ratios
• Identical length with different reduction ratios
• Reduce vibration level and higher fatigue limit
• The lower number of gear stages reduces the complexity of the overall system and minimises susceptibility to faults
• High cost-effectiveness due to a product design that is compatible with batch production
• By design, cycloidal gears are more rigid and compact (about 50% shorter), and also lighter than multi-stage planetary gears

Strain wave gears and gear stages

Strain wave gears always have one stage. Nevertheless, they achieve high reduction ratios in a single gear stage, with 100 % zero backlash. Addition of gear stages is possible (see belt drives or bevel gear pre-stages for angled motor connection), although this generally cancels out the decisive advantage of zero backlash. An exception is the combined strain wave gear and cycloidal gear. This design has virtually zero backlash and enables high reduction ratios with maximum precision. To ensure zero backlash, however, we always recommend the most rigid connection possible between the motor shaft and the gear input, or a one-part solution for the motor and gear shaft.

The gear stages of our precision gears at a glance

Cycloidal gears:

• 2-stage i up to 320
• 3-stage i up to 500
• 4-stage i up to 7,000 and higher is possible

Strain wave gears:

• 1-stage i = 30-160
• Higher reduction ratios are generally possible, however this must be considered in close coordination with the quality requirements of our IATF certification

Would you like to learn more about the gear stages of our precision gears, or do you have questions relating to a specific project? Then don’t hesitate to contact our experts.