Design and use crown gears correctly

Gearbox Design and use crown gears correctly

2024-12-02 From Sarah Maul, mechanical engineering technician and authorized signatory at Maul Konstruktionen | Translated by AI 3 min Reading Time

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The principle of crown gears has been used for a long time, but there are still few companies specialized in this technology. Learn about the special features and how to use the technology correctly.

Crown gears can be found in many old mills. The modern version of this gearing is used in helicopters, cars, industrial gearboxes, and even electric toothbrushes.(Image: Almgren - stock.adobe.com) — Crown gears can be found in many old mills. The modern version of this gearing is used in helicopters, cars, industrial gearboxes, and even electric toothbrushes.
(Image: Almgren - stock.adobe.com)

Bevel gears owe their name to their resemblance to the headgear of ancient emperors and kings and have been fundamentally present since antiquity. Simple variants are still in use today in historical mills and are often used as a drive for a king pin. This special gearing is used in a wide range of applications: from helicopter drives, to car differentials (for example at Audi), industrial catalog gearboxes (EBM Papst), and even electric toothbrushes (Patent US6178579B1).

Milled crown gear, helical with 25 teeth.(Image: Maul Konstruktionen) — Milled crown gear, helical with 25 teeth.
(Image: Maul Konstruktionen)

As is often the case in gearing technology, the origins of gear design can be traced back decades: in the 1950s in the USA and in the 1990s in the Netherlands (Crown Gear company), intensive research was conducted to further develop crown gear technology. However, there are still few companies that design gearing, software that enables this, and specialized manufacturers. Currently, the most well-known is probably ASS AG in Switzerland.

Advantages and use of crown gear teeth

The special feature of crown gear teeth is that the teeth are located on the face perpendicular to the tooth axis. The crown gear has a constant module, while the pressure angle changes continuously from heel to toe across the tooth width. A regular spur gear meshes with the crown gear.

Representation of the axis angle from the spur gear to the crown gear.(Image: Maul Konstruktionen) — Representation of the axis angle from the spur gear to the crown gear.
(Image: Maul Konstruktionen)

The axial position of the pinion is arbitrary as long as the crown gear teeth remain fully in contact with the pinion teeth. This can be used in a sliding gear shift to change the rotation direction or transmission ratio. The axial movability of the pinion is also the greatest advantage over bevel gearing: only four instead of five tolerances need to be maintained. The distance from the pinion axis to the crown gear's top plane does not need to be very precise, but the tooth contact pattern must be considered. A disadvantage compared to bevel gears is the slightly lower load capacity with the same design size. An additional advantage of crown gearing is that the crown gear can be easily molded, and a standard gear can be used for the pinion. Crown gear transmissions, like all angular gearings, are used where torque needs to be transmitted around a corner.

Design variants of crown gear teeth

Pinion with a small number of teeth—Evoloid crown gear stage Z5 Z26.(Image: Maul Konstruktionen) — Pinion with a small number of teeth—Evoloid crown gear stage Z5 Z26.
(Image: Maul Konstruktionen)

According to Karlheinz Roth, who was a professor and head of the Institute for Design Engineering, Machine, and Precision Elements at the TU Braunschweig from 1965 to 1988, crown gear gearing is classified as a special form of bevel gearing with an axis angle of 90 degrees. They can be designed with helical or straight teeth and with or without radial axis offset. An application with few teeth on the pinion (evoloid gear) is also possible. Here, the helix overlap helps bring the total overlap over one. When several pinions are mounted on a long continuous shaft, each driving a crown gear, it is referred to as the previously mentioned king pin.

Manufacture crown gears and find specialists

Eroded crown gears have a significantly better surface than milled components.(Image: Maul Konstruktionen) — Eroded crown gears have a significantly better surface than milled components.
(Image: Maul Konstruktionen)

Crown gears can be produced by machining with a special shaping wheel, as well as through primary forming processes such as powder metallurgy or plastic injection molding. In prototyping, samples can be eroded, five-axis milled, or additively manufactured.

Risk of confusion with similar designs

In English, crown gear drives are often referred to as face gears, and they can be confused with bevel gears, which are sometimes called crown gears. Visually, straight-toothed crown gears resemble one side of the Hirth coupling. However, the latter is usually designed more simply, without an involute. (dm)

Gearbox specialist ceases operations

A company that knows suppliers for all processes related to crown gear transmissions and can design them for specific applications, including optimizing backlash or acoustic optimization, is Maul Konstruktionen. The specialist in gear and gearing design has designed crown gears for various applications and manufacturing processes and has mediated manufacturers. Thus, they are familiar with the challenges of this non-standardized gearing and, based on their experience, additionally examine the tooth contact pattern in 3D to assess the gear's running behavior, alongside the calculated safeties. However, the company had to cease operations at the end of October 2024. The reason was the overall economic situation, particularly in the automotive sector. "We regret this and thank all customers and suppliers for the long-standing cooperation," announced attorney Sarah Maul.

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