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Quiet and durable even at high radial loads
Planetary gear with double bearing mounted drive shaft
These days small motors are responsible for generating movement in many devices. Their high output demands high speeds, making it necessary to install transmissions for appropriately reducing speeds. In many fields planetary gears have gained acceptance as compact, long-lasting components. On disadvantage with this solution is the conventional bearing of the output shaft. Owing to the compact structure, the shaft is usually only bearing-mounted on the output side. In this case, heavy radial loads cause the shaft to tilt and thus also the planet carrier with the planet gear wheels. This means a lot of running noise and a high degree of wear in the output stage. A different design principle eliminates this deficit. Two bearings with a larger distance between them take up high radial loads and the gear wheels mounted between the planet carriers are uncoupled from the radial load. Reliability and service life are increased and the operating noise reduced.
Compact drives are popular in small apparatus and office machinery construction. The most important specifications are: high torque in the smallest possible construction space, thermal stability, optimized efficiency and low noise. Compared to worm or spur gears, high performance planetary gears have excellent speed reduction properties with compact dimensions and they are very low in terms of wear. The high packing density of the gear wheels that are arranged to fit into each other permits great transmittable torques with the smallest of dimensions. The adapted gear geometry prevents local overload and enables the forces that arise to be better distributed on the tooth flanks of the gear wheel. This excludes harmful, local stress peaks. ZEITLAUF® makes use of these advantages for its Performax® gears in combination with a modular gear structure that is reasonable in terms of the manufacturing price. They unit high performance and smooth running. With a new bearing concept for the output shaft that is integrated within an additional module, it is now possible for high radial loads to be borne without forfeiting the length of service life.
Compact and strong
The gears provide very high power density and excellent smooth running at speed reductions of up to 17:1 in a single stage. They make it possible to use single-stage gears in applications where other gear concepts have to be designed with two stages. Two-staged versions of this design type can achieve speed reductions of up to I = 204:1. Torque distribution over a large number of planets is favourable in this case; furthermore, the individual gear wheels and planet carriers “build into one another” saving space and enhancing the power density. Since planetary gears are very compact, it is possible to install several speed reduction stages in series in a very small space. At the input on the motor side the speeds are high and the torque low. However, each additional stage increases the load on the tooth flanks. At high speed reduction ratios and with the corresponding torque load the heavily loaded end stage is therefore equipped with steel planetary gears on needle bearings.
Radial load deficit overcome
Besides the load from the output torque, planetary gears must be able to take up radial loads on the output shaft. Conventional designs only provide for one bearing at the gear output. This gives rise to a breaking torque that causes the shaft to tilt. This in turn causes the carrier disc, which is connected to the output shaft and guides the planet wheels, to also tilt. The planet wheels are supported on one side in the internal ring gear resulting in an uneven load and a varied diametral clearance of the wheels. This causes increased running noise and a shorter service life. Smooth running and reliability of the drives installed in particular in medical devices and office machinery, or sophisticated service devices such as door openers, are absolutely essential. Design engineers of printers, copiers and scanners and for other applications with small motors like to use smooth-running belt drives or friction wheels. However, to be able to transmit the drive output without any slip, comparatively high radial prestressing forces are required. The consequence is greater running noise and premature gear wear.
- Figure 2: Radial loads/speed diagram; long service life even with heavy radial load
The drive specialists with over 50 years’ experience in the field of gear engineering have now developed a new final gear stage with double bearing for the output shaft specifically for applications of this kind. The Performax® HRL (High Radial Load) component can be used as a an extension module combined with the Performax® series. The basis is formed by a robust cage that guides the planet wheels of the final gear stage with precision. The cage, positively and non-positively connected to the output shaft is supported not only on the output side. On the output side the cage is centred by another ball bearing (Fig. 1). Thanks to the increased bearing clearance and the moveable bearing of the planet wheels, depending on the speed, radial forces up to four times higher than with the bearing design deployed to date are permissible. Depending on the speed, radial forces of up to 2000 N are handled without difficulty (Fig.2)
- Figure 3: Two-stage version; the extension module can be combined with all Performax® stages
In order to safely control the radial forces that arise and to integrate the output stage with the same dimensions into the Performax® series, the cage, the gear housing with the internal ring gear and the HRL intermediate stage are made from stable die-casting zinc alloy that is designed as the bearing block. At the same time, the dual-sided fixing of the planet wheels on needle bearings with the same dimensions increases the overload relief in contrast to the normal version. Nevertheless, the heavy duty gear versions are by no means less efficient in terms of smooth running than those known as quiet runners, even under maximum loads. Far from it: under radial load they work even more quietly than standard gears. Yet this drastically increased radial load application requires very little additional space. At 63 mm the external diameter of the gear remains just as small as for the standard versions. 67.2 mm only the dimensional length of the HRL output stage is somewhat larger and exactly as long as the two-stage standard version produced to date (Fig. 3). In terms of dimensional length, the two-stage HLR version corresponds to the length of the three-stage standard version. However, the interface to the customer application remains the same.
The new heavy-duty gear stage provides the user with further possibilities. It makes it possible for compact drives for conveyor or door technology, office machinery and reprographic technology to perform even more quietly and reliably. Good overload relief and minimal wear even under heavy loads serve to reduce repair and maintenance costs. Low operating noise improves user convenience in environments that are sensitive to noise. The quiet drive module perform reliably even under a heavy radial load.
Text:Friedrich Obermeyer, graduate engineer (technical college), Head of Development/Design Engineering at ZEITLAUF® GmbH antriebstechnik & Co KG, Andreas Zeiff, chemistry graduate, editorial office Stutensee
