How does a cycloid gear work 1

Cycloid and planetary gears are used in numerous different areas of application. The range of different gear types and designs from leading manufacturers is correspondingly wide. For applications that require particularly high precision - be it in plant construction, robotics, machine tool construction or medical technology - robust precision gears with a long service life are in demand. With planetary and cycloid gears, two different types have established themselves here. In many cases, design engineers are therefore faced with the challenge of recognizing and weighing up the respective advantages and disadvantages of these types of gearbox in order to obtain the optimal reduction solution.

Planetary gears distribute the load

Planetary gears essentially consist of three basic elements: a central so-called sun gear, three or more satellite or planetary gears and a ring gear. In a typical planetary gearbox, the sun gear transmits motion to the satellites.

These then roll off in the static ring gear. The planet gears are mounted on the planet carrier, which then transmits the speed to the output shaft. By adding one or more preliminary stages, the overall reduction can be increased further.
In contrast to simple reduction solutions such as spur gears, planetary gears have lower forces acting on the gears, since the torque is always distributed over several gears. In addition, this distribution results in low-noise operation without interrupting the power flow. The reduction in a planetary stage is 3: 1 to 10: 1. Multi-stage gears are required for larger reductions.


Cycloid gear with two-stage reduction

Cycloid gears, on the other hand, rely on two-stage drive components. They mainly consist of four components: a drive shaft, two to three eccentric shafts, two cam disks (RV disks) and an output shaft that runs slowly according to the reduction ratio. First, in a first stage, the rotary motion of the servomotor is transmitted to the spur gears via the input shaft. The speed is reduced according to the reduction ratio of the input shaft to the spur gears. The spur gears sit on eccentric shafts, each of which has three eccentrics offset by 120 °. The two cams are driven by needle bearings on the eccentrics. The three eccentric shafts are mounted in the one of the output shaft.

On the inside of the housing, a curved profile corresponding to the RV panes is incorporated. These form the second reduction stage. The curve profile in the housing has one more recess than the RV disks. Bolts, which are arranged between the RV disks and the cam profile in the housing, transfer the rotary motion to the output shaft in a rolling manner. When the eccentric shafts complete a full turn, the cam disks turn off-center by one pitch. All the curves of the cam touch the bolts and roll on them.

High torque with the highest precision

A so-called tooth engagement is ensured over the entire circumference of the cam profile, since the two cam disks are shifted by 180 ° to one another on the eccentrics. In this way, very high torques can be transmitted with the highest precision and smooth running, and high reduction ratios can be achieved due to the two reduction levels. Cycloid gears allow reductions from 30: 1 to over 300: 1 - without additional preliminary stages, which are otherwise necessary with planetary gears.


Image source: © Nabtesco Precision Europe GmbH

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