When you come in to land after a shortÂ flight, probably the last thing on your mindÂ 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 rawÂ material and ends with a high heat treatmentÂ of the end product.
âAt the start of the process, we use conventionalÂ textile processing technology,â says KeithÂ Williams, Principal Carbon Materials Engineer.Â âAerospace customers are very risk averseÂ so we donât change anything in a qualifiedÂ product unless we have to. Changes can mean re-qualification and certification isÂ highly expensive.â
(1) Raw material arrives and is fed on toÂ the start of production line: the aerospaceÂ acrylic fibre consists of thousands ofÂ individual filaments.
(2) The fibre is crimped and chopped beforeÂ being fed into a carding machine where aÂ series of wire-coated rollers comb and alignÂ the fibres.
(3) The chopped fibres are amalgamated withÂ continuous fibre to create a layered web ofÂ felted fabric
(4) Rolls of two-foot wide fabric are fed into aÂ continuous 30-foot long furnace at more than 1,000Â°C to create a carbonised fibre byÂ âdrivingâ off the non-carbon components. The inert atmosphere prevents burning.Â 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 theÂ end of the line.
(6) 15% of discs are built up by hand fromÂ smaller fabric segments, a structure that gives the disc improved strength and lower wear. âThis is time consuming so weâre nowÂ looking at how to automate this process,â says Richard Gorman, Module Manager at Meggitt’s Coventry facility.
(7) Layers of laid-up fabric move to theÂ 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 isÂ stored offsite in triplicate for 25 years to complyÂ 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 theÂ right thickness. It can take several weeks to create sufficient quantity of discs for theÂ carbon infiltration process in the furnace andÂ several days to load the discs into theÂ 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 1,000Â°C.
(9) Jigs are loaded into the 30-foot deep furnaceÂ according to a precise laboratory plan. DiscsÂ are oriented according to size and shape soÂ that each one achieves a similar densityÂ during infiltration.Â Multi-stage steam vacuum ejectors suckÂ air from the furnace creating a soft vacuum.Â The combination of heat and vacuum breaksÂ the carbon atoms out of the natural gasÂ (methane or CH4) molecules, allowing themÂ to infiltrate the compressed fibres in the jigs.
(10) After a cooling period, the furnace is openedÂ and unloaded. The first-fire discs areÂ unloaded from the jigs, weighed and thenÂ tested before being reloaded for their secondÂ firing. Second-fire discs are unloaded andÂ proceed to the next phase.
(11) Discs are machined into their final shapeÂ after undergoing graphitisation, a heat treatment process which transforms the highly disordered carbon atom structures into near perfect three dimensionalÂ crystals of pure graphite. Every single discÂ then undergoes a thermal conductivity test toÂ assure highest quality.
(12) Discs are then painted to reduce catalyticÂ oxidation from contaminates like runwayÂ de-icing agents and to prevent thermalÂ oxidation at the high temperatures theyÂ experienceâduring a rejected take off, brakesÂ can reach temperatures of around 2,000Â°C.
(13) Last but not least, u-shaped clips are fittedÂ to protect rotor discs as the wheel surrounding them rotates.
Itâs an extraordinary process from start to finish.Â So next time you touch down after a short flight,Â spare a thought for the journey your brake discsÂ made before you even took off.
Concorde was the first commercial aircraftÂ to use carbon brakes, designed andÂ manufactured at Meggittâs Coventry siteÂ more than 40 years ago.
Today, more than 30,000 aircraft makeÂ 15 million landings on Meggitt wheels andÂ brakes.