Formula One gearboxes
Formula One transmissions are the most advanced in the racing world, with an unparallel combination of performance, weight and packaged size. A key part of the gearbox is the casing, as it forms not only a housing for the gearbox internals but also acts as an integral part of the car's structure. Here I will look at some of the construction methods and materials used to create these casing.
Magnesium has long been a favoured material for gearbox casings, thanks to its useful mechanical properties. Magnesium alloys are about 35% lighter than aluminium alloys, and certain alloys can be heat-treated to UTS values approaching 43 ksi, making them attractive because of their high strength:weight ratio.
The stiffness of magnesium is generally only about 63% of aluminium alloys, however, so components being switched from aluminium to magnesium will need larger cross-sections and section moduli to achieve the same stiffness as the aluminium part, but can still give a weight saving of 20-25% depending on the design. Magnesium can also absorb shock loadings better than aluminium, making it ideal where durability is need. However, in recent years the emergence of new casting techniques for aluminium has toppled magnesium from its position as the material of choice.
In the past, the major factor governing the weight of aluminium gearbox casings was the casting wall thickness. Traditional casting methods did not allow thin wall sections to be cast reliably, thus limiting weight savings irrespective of the actual component strength. However, new casting methods that allow for very thin wall sections to be created mean engineers can now create casings that are marginally lighter than a magnesium counterpart, while still retaining the same strength characteristics.
As a result of these new casting methods, combined with advances in FEA simulation, the benefits of an aluminium casing now outweigh those of a magnesium unit, although the production costs are considerably higher. As a point of note, the manufacturer of the aluminium gearbox used by the HRT and Virgin Racing teams quotes the weight of the unit as being "in the region of 40 kg".
The first titanium gearbox to appear in Formula One was produced by Ferrari in 1997, and was fabricated as opposed to cast. Although it was exceptionally lightweight compared to a magnesium or aluminium unit, the difficulties of fabricating titanium meant the production costs were astronomical.
It was not until the Minardi team began experimenting with a cast titanium unit in 2000 that the material became a viable option for casing construction. The use of a rapid casting process, with the extensive use of rapid prototyped patterns (created using a selective laser sintering process), meant that not only did the production cost of the transmission fall considerably, but the entire design could be optimised to a greater degree. This was thanks to the complex patterns that could be created through rapid prototyping, allowing for internal structures that were not possible with traditional casting methods.
The end result in the case of the Minardi gearbox was a weight saving of 25% and a reduction in package size of 20% over the previous magnesium casing. Titanium is still used extensively in Formula One transmissions, although the current trend is to combine it with composite components.
The first carbon fibre Formula One gearboxes appeared in the late 1990s, but considerable problems were experienced in relation to the heat rejection properties of the material. Today, the most common use of carbon in transmissions is in conjunction with metallic - usually titanium - structural components.
One of the first teams to take this approach was Renault with its R23 car in 2003, which used a casing with a titanium lower section and carbon fibre upper section. However, the design was dropped the following year in favour of a 100% titanium unit. The current trend is now for the structural 'bulkheads' of the gearbox to be fabricated from titanium, with the rest of the structure moulded from composite material, providing a very light yet exceptionally stiff structure.
Fig. 1 - The fully cast titanium gearbox produced for the Minardi team provided significant weight savings
Written by Lawrence Butcher