Gearbox problems solved
Published:  22 June, 2007

Yorkshire Water's Rawcliffe Sewage Treatment Works processes waste water for the equivalent of 45,000 homes, treating up to a maximum flow rate of 23,600m3 per day and adhering to strict standards for safe disposal. The problem Dave Milburn, Rawcliffe"s site manager faced was, up until recently, this treatment capacity was an 'ideal’ rather than a reality. Due to gearboxes breaking down on the sites ten large aerators, all ten were never working at the same time, until Brevini designed a reliable replacement drive system employing WIMES spec EFF1 efficiency motors, inverter drives and high-toque Brevini planetary reduction gears.

Jon Snaith, senior brevini engineer in charge of the project commented: "We faced a number of challenges on this project, including having to design a retrofit solution that would fit in place of what was already there with little or no additional engineering. We first looked at the failure mode of the existing gearboxes, then designed and engineered a system to solve these issues.”

Two aspects were key; the first was the physical weight of the aerator paddles, up to half a tonne each with the majority of the weight sitting on the gearbox end bearings. The second was vibration, caused by the fact that the paddles were not perfectly balanced to start with and compounded by the effects of subsequent uneven corrosion and weathering that is typical of these items.

Snaith continued: “Vibration is the bane of all mechanical systems and in extreme cases such as this it can weaken mountings, wear out gears and cause bearings to collapse. We started from the mounting upwards, selecting special Anti-vibration washers and nylon-sleeve locking bolts to reduce the effects of vibration, balancing the paddles and fitting large taper roller bearings to the final output stage of the gearboxes that could take far more punishment, up to ten tonnes thrust load in this case.”

All ten of the aerator drives have now been replaced, with an estimated payback period of just two years for the entire project. The wear life even in constant operation is a predicted 100,000 hours with just simple and inexpensive oil changes required periodically.

The Brevini gearboxes used were designed specifically for arduous, continuous applications such as this, the planetary design having inherent advantages over conventional helical gearbox designs because there is an equilibrium of forces on individual gears so the planet carrier is in charge of transmitting torque only. Conventional Helical gearboxes employ a system of bearing supported gear carrying shafts where forces are concentrated. Planetary gearboxes have a complete gear system that is in-line, so there are no high torsional loads on the bearings and gear elements - the result in an extremely long wear-life.

Sensors that monitor the oxygen content of the wastewater trigger the aerators. The 5.5kW motors supplied are inverter driven for maximum efficiency, the drives also allow smooth ramp-up and ramp-down and an optimised 1440rpm motor speed. The motors drive two planetary stages at 20:1 reduction ratio to achieve 70 rpm at the output shaft, rated for a continuous output torque of 13,000nm and a 20,000nm peak intermittent load.

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