Brevini scores at Wembley

“Brevini has a worldwide reputation for the reliability and light-weight design of its planetary gear reduction technology. This is an ideal example of where a high reduction ratio and high torque for a relatively slow moving application are required, but without the normal weight penalty of a helical or worm and wheel gear reduction arrangement. It is also a good example of the level of cooperation that we have with our customers, starting from the design brief and working through to the finished engineering project.” Comments Jon Snaith, Joint MD of Brevini UK.

Wembley is the world’s largest covered stadium, and it cost £778 million to build. The most innovative aspect of the engineering design is the retractable roof which, when open, lets the sun reach all corners of the field. If needed, the roof can close in just 15 minutes to protect the more than 90,000 spectators seated in the stadium from poor weather. The roof is supported by an imposing metallic arch that weighs 1,750 tons, is 133 meters tall, and has a span of 315 meters.

The fixed part of the roof is attached to one side of the arch soaring over the stadium. On the other side are seven sliding panels that can open and close high above the stadium, moving on roof trusses that are 160 meters long and supplied with tracks. The panels move at a speed of 30 mm per second. Thirty-four Brevini planetary gear units make the seven panels slide perfectly into place. The panels each have a different shape and weight ranging from 63 to 330 tons each, for a total of 1200 tons.

The planetary design is ideal for this application; although the roof sections are heavy, minimising weight in the suspended structure was a major consideration. The small size and modular construction of planetary gearboxes means that they provide high reduction capability from a highly compact package. As such, planetary gearboxes are frequently used in areas such as slewing drives for turning or moving large cogged items that require slow movement at very high loads. In this type of application more traditional ‘worm and wheel’ gear units would simply be too large or too heavy to use.

When compared to some larger worm gear units with high reduction ratios that may be only 60% efficient, the planetary solution is also very much more efficient, at 98% efficiency per stage. This increased efficiency is based on reduced friction; a planetary gearbox is driven by a centre shaft that is connected to a carrier plate carrying three smaller gear wheels. The gear wheels run inside a toothed outer ring and drive a central gear cut around the central output shaft. This means that the load is spread among many contact points around each gear. Many, lighter contact points mean less friction, hence increased efficiency. These attributes also contribute significantly to the gearboxes inherent reliability and long operating life.

Brevini planetary gearboxes can be assembled with multi stages of reduction, transmitting torques from 1.6kNM to 1100kNm and crucially, reduction ratios from 3:5:1 to 20,000:1 in a compact staged construction. The high reduction ratios mean that smaller drive motors than would be used with other gearbox configurations can be specified, thereby reducing weight even further.