The almost 100% contact within the cycloidal pin design and the even force distribution within the gearbox allow a high load with low backlash.

© Nabtesco

A cam profile corresponding to the cam disks is incorporated on the inside of the housing. This forms the second reduction stage. The cam profile in the housing has one more recess than the cams. Pins arranged between the cams and the cam profile in the housing transmit the rotary movement to the output shaft in a rolling motion. When the eccentric shafts complete a full rotation, the cams rotate off-center by one pitch. All cams of the cam disk touch the pins and roll on them.

So-called tooth engagement is ensured over the entire circumference of the cam profile, as the two cam disks on the eccentrics are shifted by 180° in relation to each other. This means that torques of up to 5 times the nominal torque can be transmitted with a precision of less than one angular minute with extremely smooth running and high reduction ratios can be achieved thanks to the two reduction stages. Specifically, cycloidal gears allow reduction ratios from 30:1 to over 300:1 - without additional preliminary stages, which are otherwise necessary with standard planetary gears.

The gearboxes in the RV-C series from Nabtesco have a hollow shaft that allows supply and data cables, wires or lasers to be routed through the gearbox.

© Nabtesco

"The power transmission via pins and rollers, which is typical of cycloidal gears, ensures high efficiency, a long service life and extremely low backlash in the gearbox," says Marcus Löw, highlighting the main advantages over planetary gears. "In addition, the rolling friction of all elements involved in the power transmission guarantees a very low breakaway torque - 0.1 Nm to around 5.9 Nm, depending on the size and transmission ratio."

The two-stage reduction principle reduces vibrations and mass inertia and allows larger reduction ratios. The almost complete contact within the cycloidal pin design and the even force distribution within the gearbox also allow a high load with low backlash.

Thanks to this special design, the gearboxes achieve high repeatability and path accuracy. In the zero crossing of the hysteresis, for example, the opening of the RV gearboxes from Nabtesco is significantly less than one angular minute. The symmetrical design and the roller bearing support of all shafts also ensure constant operating behavior over the entire service life. They also allow high load peaks of up to five times the nominal torque, as can occur in emergency stop situations, for example.

The decision criteria

When it comes to deciding between cycloidal and planetary gearboxes in the design phase, the first thing to consider is the precision required in the application: "If play and positioning accuracy are essential, cycloidal gearboxes consistently offer the better performance data," says Marcus Löw. In other words: "Cycloidal gears have a more robust design than planetary gears and therefore have a longer service life. Even over a long period of time, the backlash increase in cycloidal gears is extremely low." Due to their design, they are significantly stiffer, more compact (around 50 % shorter) and lighter than multi-stage planetary gearboxes. They also offer 500 % higher overload safety. Another important criterion is the required reduction ratio. With reduction ratios of 3:1 to 100:1, planetary gearboxes offer a high torque density. "Even if it is possible in principle, the design of cycloidal gears makes it difficult to achieve reduction ratios below 30:1 without an additional preliminary stage," says Marcus Löw. "However, if the required reduction ratios are above 30:1, cycloidal gears have an advantage, as they do not require any additional preliminary stages compared to planetary gears."

When comparing the dimensions of the two gearbox types, it is noticeable that the overall length of planetary gearboxes increases noticeably with increasing reduction ratios. For example, multi-stage planetary gearboxes are around 30 to 40 % longer than cycloidal gearboxes with identical reduction ratios. Cycloidal gears, on the other hand, have a diameter that is around 10 % larger with a shorter overall length.

According to Löw, in order to find the optimum gearbox in each case, it is necessary to consider all the requirement parameters of the individual work cycles with regard to service life and emergency stop situations. Special attention must therefore be paid to the specifications of the individual gearboxes: "The design of an application should always be based on the gearbox in order to achieve the best possible economic performance," advises the expert. This makes it easier to design the entire application.

Cycloidal gearboxes such as those from Nabtesco are primarily used in applications that require fast and precise positioning movements with high loads without resonance. Optional gearboxes allow motors to be mounted at right angles and also enable larger reduction ratios than those already offered by the factory of up to i = 300. With other gearboxes, these can only be realized via several planetary stages.

The RV-E series combines the RV gearboxes with large integrated angular contact ball bearings. They are used to absorb external forces and torques; an additional external bearing is unnecessary. This results in precision gearboxes in a compact design with extreme rigidity and long, constant operating behavior.

Author:
Axel Hahne is a technical editor from Buchholz.