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 315m span roof which, when open, lets the sun reach all corners of the field. If needed, the roof can close in just 15 minutes.
The fixed part of the roof is attached to one side of an arch that soars 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 160m long. The various panels weigh a total of 1,200 tons and are driven at a speed of 30mm/s via 34 Brevini planetary gear units.
While 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 within a compact envelope. Planetary gearboxes are frequently used in areas such as slewing drives for turning or moving large, heavy loads at a slow pace. In this type of application, the more traditional 'worm and wheel' gear unit 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 alternative is very much more efficient (98% per stage), thanks to multiple gear contact points and correspondingly reduced friction. These attributes also contribute significantly to the inherent reliability and long operating life of this type of gearbox.
Brevini's units can be assembled in several stages to achieve reduction ratios of up to 20,000:1, and torque transmission capabilities from 1.6kNm to 1,100kNm. High reduction ratios mean that smaller motor drive can be specified, reducing weight even further.