Cycloidal gears vs. helical gear units

High reduction ratios & compact design

Cycloidal gears versus helical gear units

In most cases, helical gear units are not in competition with cycloidal gears – they differ too much in regard to technical performance specifications, as well as their specific advantages and typical applications. However, there are a few fields of application where switching from  helical gear units to cycloidal gears could well be worthwhile. Examples include welding positioners and the printing industry, as well as conveyor systems for packaging technology and food production, which require precise or intermittent transport to the various stations.

Minimal backlash throughout the entire lifetime

High reduction ratios in a compact design

Low inertia for high dynamics

Cycloidal gears: Maximum precision at all reduction ratios

Due to their high backlash, helical gear units are not suitable for precision applications. Cycloidal gears are clearly superior in this case. With a hysteresis loss of only 0.5 to 1 arcmin throughout the entire lifetime, they ensure precisely executed motions and high-precision positioning. The two-stage reduction principle (spur gear stage and eccentric stage) enables maximum precision even at very high reduction ratios.

Another advantage: Cycloidal gears have a large speed step of 1:40 or even higher. Even high reduction ratios require only two or three gear stages. In helical gear units, on the other hand, the transmissible moment is limited by the tooth engagement. A single stage can achieve only very small reduction ratios. Higher reduction ratios therefore require more gear stages. This results in a larger number of components in the gearbox and also requires more space. The large speed step of cycloidal gears thus has two advantages: a more compact design and increased safety. The lower number of gear stages reduces the complexity of the overall system and minimises susceptibility to faults.

The very compact design allows users to build smaller and lighter machines. That saves installation space and resources. The lower inertia of cycloidal gears also enables faster acceleration. While helical gear units are less expensive, this advantage must be seen in relation to the lower power density, especially in the case of high torques and high reduction ratios.

By the way: Cycloidal gears also have a spur gear stage. It is upstream of the eccentric stage and transmits the rotary motion of the drive or servo motor to the eccentric shafts.  Helical gear units backlash here is negligible, however, since it is reduced by the cycloidal stage.

The advantages of cycloidal gears over helical gear units:

  • Lower backlash
  • Maximum precision at all reduction ratios
  • Better dynamics
  • Lower inertia
  • Higher power density (torque/weight ratio)
  • High reduction ratios from 30:1 to more than 300:1 with only two or three gear stages
  • More compact design
  • Higher rigidity
  • Lower weight

Helical gear units

Helical gear units have a very simple design. The power is transmitted by straight or helical spur gears to parallel axes, with the option of multiple gear output shafts. For higher reduction ratios, several gearwheels and shafts are arranged in succession. The backlash of the individual stages in the helical gear unit is cumulative, so that the total backlash of the gearbox continues to increase. In addition, the efficiency of the helical gear unit decreases with each additional stage. The module (size of a gearwheel) increases, as does the size and weight of the gearbox. With each stage, the rotary direction of the gear output shaft changes.

Helical gear units feature a robust design, high efficiency, and high speeds. They are extremely versatile and among the most common types of gears. Especially applications requiring high load capacities and a robust design, such as cranes, construction machines and conveyor systems, benefit from the special properties of helical gear units. They are also used in turbines and propellers, where their high speeds are advantageous. Automotive transmissions are likewise helical gear units.

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