When producing sugar from sugar beet, the beet is first washed and then sent to slicers that operate in a similar way to a kitchen grater, but on an industrial scale. The beet is cut into thin vshaped strips, known as cossettes, at a rate of up to 520 tonnes an hour. These cossettes then go into diffusion towers where they are mixed with hot water, at about 70°C, to extract the sugar from the plant cells.
This diffusion process generates two products – the sugar dissolved in hot water, known as “raw juice”, and all that is left of the cossettes – hot, wet pulp.
At one of British Sugar’s plants, this pulp is then dried in a two-stage process. In the first stage, a horizontal twin-screw press removes about 75% of the water mechanically, before it moves to hot driers for the final stage of the process. The dried pulp is then turned into pellets for animal feed.
The twin-screw press is driven via a huge dual-output gearbox, weighing 29 tonnes and standing 2.5m high. The gearbox was installed originally in 1993 and had operated trouble-free for more than 20 years, but recently needed a thorough service.
The gearbox was transported on a low loader to SEW-Eurodrive’s plant in Normanton, West Yorkshire. On arrival it was driven into the factory and lifted using two overhead gantry cranes and a specialist lifting frame.
As with other gearboxes arriving at Normanton for repair, the sugar plant gearbox was first inspected and a detailed report with photography of worn and damaged parts was sent to the customer with a quotation. The service engineers used microscopic cameras that can reveal detailed damage to gearbox components. The box was given a unique identification number and the related documents were scanned and saved to provide full traceability.
SEW’s technical department provides root-cause analysis for failures and CAD drawings for modifications and retrofits. If modifications are made, or bespoke parts manufactured, full engineering drawings are produced.
The steel case of the sugar factory's gearbox was in three sections secured by M56 case bolts. These bolts had corroded so badly that a specialist torque wrench capable of delivering 14kNm of torque was needed to remove them. The three casing sections were then subjected to highpressure steam-cleaning to remove years of accumulated debris.
Analysis using a magnetic particle inspection (MPI) technique, which detects sub-surface discontinuities, revealed damage to some of the internal gears and shafts, and it was decided that they would need to be replaced. The service personnel used specialist tooling and equipment, including oil-injection tools operating at pressures of up to 4,000 psi, to remove the bearings and the gears. They also had access to a 400-tonne press the size of a single decker bus, and a 150-tonne ram.
Rebuilding the gearbox with new gears and shafts from the original manufacturer, and with bearings and seals supplied by SEW, involved the resetting of end floats and careful checking of gear contact marks to ensure full-face contact across the teeth.
A specialist metrology company was brought in to check the precise alignment of the gear casing and bore tolerances using a laser-tracking technology. Retro-reflectors were positioned carefully at points on the gearbox casing to reflect a laser beam back to the tracker, which allowed key coordinates to be measured. Analysis of the measurements revealed that the alignment of the casing and bores were satisfactorily within tolerances.
After the serviced gearbox had been returned to the British Sugar site, an SEW engineer helped to commission and test it. SEW says that that the repaired box is as good as new. It is backing this claim with a 12-month warranty, and expects the gearbox to give at least another 20 years of trouble-free service.