When you come in to land after a short flight, probably the last thing on your mind is how the brake discs on the wheels below you were made.
Read on and get the full story.
In the UK, the process starts with the raw material and ends with a high heat treatment of the end product.
“At the start of the process, we use conventional textile processing technology,” says Keith Williams, Principal Carbon Materials Engineer. “Aerospace customers are very risk averse so we don’t change anything in a qualified product unless we have to. Changes can mean re-qualification and certification is highly expensive.”
(1) Raw material arrives and is fed on to the start of production line: the aerospace acrylic fibre consists of thousands of individual filaments.
(2) The fibre is crimped and chopped before being fed into a carding machine where a series of wire-coated rollers comb and align the fibres.
(3) The chopped fibres are amalgamated with continuous fibre to create a layered web of felted fabric
(4) Rolls of two-foot wide fabric are fed into a continuous 30-foot long furnace at more than 1,000°C to create a carbonised fibre by ‘driving’ off the non-carbon components. The inert atmosphere prevents burning. The process is designed for maximum commercial efficiency and minimum environmental impact:
(5) 85% of discs are built up from fabric cut into thin annular layers: each layer is orientated
to obtain the strength and wear characteristics required for each brake disc. A robot cuts the layers and places them on a very accurate scale. Discs are carefully weighed at every stage of the process to ensure the correct density is achieved at the end of the line.
(6) 15% of discs are built up by hand from smaller fabric segments, a structure that gives the disc improved strength and lower wear. “This is time consuming so we’re now looking at how to automate this process,” says Richard Gorman, Module Manager at Meggitt’s Coventry facility.
(7) Layers of laid-up fabric move to the next stage in the process. Each disc has a quality history ID with a unique serial number referencing raw material batch, weight and the operators who have worked on it at each stage of the process. Data is stored offsite in triplicate for 25 years to comply with Meggitt quality control.
(8) Discs are laid up on graphite jig plates to be compressed. Spacers are inserted to ensure discs are compressed to the right thickness. It can take several weeks to create sufficient quantity of discs for the carbon infiltration process in the furnace and several days to load the discs into the furnace.
Next, each disc undergoes a process known as chemical vapour deposition or infiltration (CVD or CVI), spending several months in the furnace at more than 1,000°C.
(9) Jigs are loaded into the 30-foot deep furnace according to a precise laboratory plan. Discs are oriented according to size and shape so that each one achieves a similar density during infiltration. Multi-stage steam vacuum ejectors suck air from the furnace creating a soft vacuum. The combination of heat and vacuum breaks the carbon atoms out of the natural gas (methane or CH4) molecules, allowing them to infiltrate the compressed fibres in the jigs.
(10) After a cooling period, the furnace is opened and unloaded. The first-fire discs are unloaded from the jigs, weighed and then tested before being reloaded for their second firing. Second-fire discs are unloaded and proceed to the next phase.
(11) Discs are machined into their final shape after undergoing graphitisation, a heat treatment process which transforms the highly disordered carbon atom structures into near perfect three dimensional crystals of pure graphite. Every single disc then undergoes a thermal conductivity test to assure highest quality.
(12) Discs are then painted to reduce catalytic oxidation from contaminates like runway de-icing agents and to prevent thermal oxidation at the high temperatures they experience—during a rejected take off, brakes can reach temperatures of around 2,000°C.
(13) Last but not least, u-shaped clips are fitted to protect rotor discs as the wheel surrounding them rotates.
It’s an extraordinary process from start to finish. So next time you touch down after a short flight, spare a thought for the journey your brake discs made before you even took off.
Concorde was the first commercial aircraft to use carbon brakes, designed and manufactured at Meggitt’s Coventry site more than 40 years ago.
Today, more than 30,000 aircraft make 15 million landings on Meggitt wheels and brakes.