Metal spraying maximises the lifetime of generator rotor

Paul Boughton

RFI Screening Limited, specialist surface treatment engineers, has opted for Metallisation MK61 flamespray equipment to metal spray a 40 tonne generator rotor with fine silver.  

The generator rotor, which is a combination of electrical, mechanical and manufacturing components, is regularly exposed to hard mechanical stress and high temperatures, while being subjected to electrical voltage and current. Over time this stress can lead to breakdown, costly repair and maintenance and possibly inconvenient downtime. To maximise the lifetime of the generator rotor, RFI Screening was contracted to metal spray it, using the flamespray equipment.

Prior to metal spraying the generator rotor, the surface was grit blasted, to Standard SA 2.5, to clean and prepare the shaft. The shaft was also pre-heated by induction to prevent coating shrinkage issues and enhance the adhesion of the metal spray. The generator rotor was then sprayed with a 25 micron coating of Molybdenum, which is a bonding material, to optimise the adhesion of the top coat of silver. A final coating of silver was then added to achieve the specified outside diameter on the shaft. Silver is chosen due to its high levels of conductivity compared to other materials such as steel and even copper. In a typical application, a coating 250 microns thick of silver would be sprayed.

The flamespray system was chosen instead of arcspray due to the higher efficiency that can be achieved with a more controllable application process. The flamespray system with small diameter wire will also produce a fine, close structured coating that will maximise the conductivity of the applied silver, which is the main purpose of the coating.

The Metallisation MK61 is an oxygen-acetylene fuelled flamespray system, primarily for spraying engineering coatings of steels, bronzes, copper and molybdenum. The lightweight, robust pistol offers the possibility to reclaim damaged parts for small to medium sized applications, as it has a continous nozzle set up, ideal for engineering coatings. Molybdenum can also be sprayed with the MK61 as a soft bond coat through to a very hard, wear resistant coating.

The MK61 is also suitable for use with RFI Screening’s mobile metal spraying unit, as, although the pistol is lightweight, it is robust, sturdy and well balanced making it very easy to handle.  Requiring only gases and compressed air, it also lends itself to site applications where 3 phase electricity may not be available. It comes with a variety of hose lengths to suit a wide range of applications where difficult access may be an issue.

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