Crown gearboxes were already used in technical systems a century ago and can still be seen today in historic mills. Over the course of time, they were displaced by bevel gears because these were easier to calculate; the crown gear disappeared from the market. However, this changed when the focus shifted to energy-efficient drive technology in industry and to smaller drives. After all, the right combination of efficient motors, gearboxes and control systems enables high energy savings in the power range below 250 W.
The efficiency is also in the range of 90% at high reductions.
As decentralized drive technology has become more important in machine and plant engineering due to its flexibility, there is an increasing number of such drives. Transmissions that operate at the highest possible efficiency are therefore in demand, and this is where crown gearbox technology can score points. The gearbox specialist Zeitlauf, which is now part of ebm-papst, recognized this early on and re-defined traditional crown gearhead technology with EtaCrown and EtaCrownPlus (Fig. 1).
State-of-the-art crown gearboxes of various sizes cover the reductions in the single-stage range up to 10:1, two-stage to 113:1 and three-stage to 289:1. In the single-stage range, they achieve efficiency levels of more than 90% and can be manufactured more economically than bevel gears. Due to their principle, crown gearboxes also place fewer demands on the center distance adjustment than bevel gears and even achieve a reduction ratio of 10:1 from a single stage (bevel gear: 5:1).
Great efficiency with high reduction
If you take a closer look at the design of the various crown gearboxes, you will understand quickly why crown gearboxes are a good choice for many applications: Gears of bevel gears have a conical shape. The gear function is only perfect if the center lines of the bevel gears cut at exactly one point. As a result, even thermal expansion can impair the function. Worm gears are less sensitive, but with them the direction of the force flow is very unfavorable. The torque results primarily in a tensile or pressure load on the worm pinion.
Since the pinion slides on the worm gear, depending on the reduction, up to 2/3 of the input drive energy is converted into heat. The motor and the transmission must therefore be dimensioned larger than actually necessary to achieve the desired power. In addition, the axial load on the worm gear has to be absorbed by the correspondingly dimensioned axial bearings.
With the modern EtaCrown and EtaCrownPlus crown gearboxes from the ebm-papst modular drive system, this is different (Fig. 2): The involute drive pinion builds in cylindrical form and the contact between the pinion and the output gear is a roller contact; hardly any friction losses occur. The efficiency is therefore also in the range of 90% at high reductions. The motor power is thus almost fully available for the drive task. 10% power loss compared to up to 75% for conventional worm gear designs is an enormous potential for savings even in small drives. It is often also possible to dimension the drive motor smaller and save space and costs. As the motor with drive pinion and the output axle are in one plane on the EtaCrown, the transmission can also be installed easily mirrored, which reduces storage and simplifies logistics.
Advantages in the application
There are many applications for which crown gearboxes are recommended and in which they have proven effective for many years. In addition to the high efficiency levels, typical reasons for using them are the high transmittable torque and the lack of self-locking. In the case of barriers and access monitoring systems, for example, the transmissions can be turned back manually even in the event of a high reduction without the need for components for decoupling to protect the drive against damage.
Due to the offset-free design, the motor-transmission combination can also be integrated easily into door profiles (Fig. 3). There are no motor protrusions. This avoids recesses or additional spacers for bypassing distances. The symmetry in the gear design makes versions for left or right stops unnecessary. The brake can be mounted directly on the drive shaft or, as usual, on the B side of the motor, simplifying the design and making the drive system more compact.
Crown gearboxes impress with their great reliability, smooth operation and low heating.
Other applications for the versatile crown gearboxes include lift trucks and automated guided vehicles (Fig. 4). High transmittable torques, durability and compact dimensions speak for the use of a crown gearbox. In medical technology, crown gearboxes impress with their great reliability, smooth operation and low heating.
EtaCrown and EtaCrownPlus are part of the ebm-papst modular system and can be combined with all DC and EC motors as well as brakes and encoders. Customized complete drive solutions from a single source with perfectly matched components can be combined easily in the online portal; thanks to defined preferred types, selected drive configurations are ready for shipment within 48 hours. which means that sampling, for example, can be done very quickly.