Recent expansions in the aircraft industry have led to new and interesting projects for Mannings Thermal Engineers of Southport who have been able to apply their specialist knowledge and experience of heat treatment processes to the latest innovations in manufacture and maintenance in this demanding industry.
An advanced heating system supplied by Mannings is used during production of the new Eurofighter and this has enabled them to become part of Europe's biggest military aircraft programme. The equipment was supplied for BAe Systems Limited of Salmesbury, Lancashire and is designed to heat a special cast aluminium vacuum-forming tool used in the manufacture of the engine bay doors of the fighter. Heating in, this case is required to achieve rapid curing of a special adhesive compound which bonds two layers of the door together.
Mannings worked closely with the toolmaker during the project and had to satisfy strict criteria. The tool is approximately two tonnes in weight and needed to be uniformly heated to a temperature of 500C. To meet the requirements of the application Mannings provided specially designed low voltage heating elements, which had to be inserted in machined recesses inside the tool itself. Temperature control is carried out in six heating zones and probes accurately measure the temperature of each zone. The heating system, which is an integral part of the tool, reduces the cure time of the adhesive from around 3 days to approximately six hours. Bob Cross who is Development Engineer for BAe systems said "because of our tight production schedule it is necessary to complete this operation as quickly as possible and the improved curing time which we have achieved using the Mannings heating system has helped our overall schedule".
A similar approach has been applied to the heat treatment required following engine maintenance procedures. Modern jet engines are incredibly reliable, and many precautions are taken to ensure that they remain in prime condition. Every engine is regularly overhauled and subjected to stringent tests and examinations by trained specialists. Even very small flaws in engine casings are repaired and expertly stress relieved by heat treatment before being allowed back into service. In an industry which demands the highest technical standards, this stress relieving process must be carried out under carefully controlled and monitored conditions.
For this reason GE Aircraft Engine Services Ltd of Nantgarw, Cardiff have installed a Mannings purpose build electric heat treatment centre which can stress relieve several very small areas of metal components simultaneously at high temperatures in confined spaces. The maximum temperature which can be achieved is 10000C and the heated zone can be kept to a minimum size. Before being awarded the contract, Mannings carried out a study of the technical requirements and prepared a report of their recommendations.
In this case the heat treatment is used to reduce internal stresses which can be induced during weld repairs and it can also improve elasticity, ductility and strength. The process involves heating the affected area and maintaining a predetermined temperature for a fixed period before cooling at a controlled rate.
The heat treatment unit is housed in a portable, skid mounted steel cabinet. It has a separate programmer for each of twelve independent heating zones and is fitted with all the necessary monitoring instruments, indicators and alarms. Two chart recorders with selectable channels allow a permanent record of up to 48 separate temperatures to be obtained from sensing elements fixed to the work piece. A feature of the new system is the array of heating elements provided with the unit. These are critical to the stress relieving process since they provide the energy at the location where it is required. The elements are marked with the engine repair type so that similar heat treatments can be easily replicated.
The latest technology can also be applied to traditional heat treatment equipment. Mannings demonstrated this by the successful completion of a project at the BAe Systems Limited facility in Brough, East Yorkshire, where automation and modern techniques were used in the design of a new Salt Bath Solution Treatment Plant.
The salt bath process is carried out in three stages and is used for the treatment of aluminium alloy airframe component to improve malleability so that the parts can be pressed into shape during later stages of manufacture. The components are placed in a specially designed workbasket and immersed in a bath of molten chemical salt where they are uniformly heated at a temperature of 5000C. After a period of 30 minutes in the salt bath the basket is automatically transferred to a temperature controlled air agitated polymer quench tank where the final metal properties are obtained. The workbasket is then moved to a rinse tank before being finally set on a loading/unloading station.
The temperature of the molten salt is maintained to within 50C by burst firing thyristors in two vertical zones an independent over-temperature interlock has been installed for additional safety. Once the components have been heated for the required time period, the pneumatically operated lid of the salt bath opens automatically and a robotic Basket Transfer Machine swings into action. Rapid transfer of the work basket from the salt bath to the quenching tank is essential to ensure that the desired mechanical properties are produced in the components and this transfer can be completed within 7 seconds.
The transfer machine has two counter balanced parallel arms which are positioned on each side of the two tanks. Steel pins, on the end of each arm, locate on a "flight bar" on the basket as the arm rotates through 360 degrees, picking up the basket and moving it in a semi-circular arc to set it down in the quench tank. The design ensures that the transfer is completed smoothly and that the basket remains vertical at all times.
The whole operation is carried out inside an enclosed force-ventilated room and the entire process can be viewed at all times through observation windows in the sidewalls. Control and monitoring, takes place from a panel outside the room. For safety reasons personnel must not be allowed inside the room while the treatment is in process - and this is achieved in a simple but ingenious way. The room door can be locked from the outside by the same key, which is used to activate the control panel. While the room is occupied the key is held captive in the door lock and the control panel cannot be activated. The key can only be released after the door is locked.
Mannings apply their designs to a wide range of heating problems in the aircraft industry and they have been able to revolutionise many traditional procedures to improve quality, save time and reduce running costs.
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