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Sweet smell of success

04 January 2017

When selecting a gear drive solution, the obvious choice is not necessarily the most appropriate. Here, we offer an overview of typical gearbox arrangements together with a recent case study from Brevini UK whose engineering expertise helped solve a long-term odour problem

The most straightforward gear arrangement is the spur gear comprising a disk or cylinder with teeth projecting radially aligned parallel to the axis of rotation. While these gears offer very high efficiencies they have disadvantages, particularly at higher speeds where the sudden meeting of teeth causes stress and noise, making this solution most suitable for low-speed applications.

The helical gear addresses this problem, to some extent. Curved, with a tooth shape describing a segment of a helix, it can be meshed in parallel or crossed orientations. Angled teeth engage gradually, hence a smoother and quieter operation. For many applications, though, helical gears are unsuitable due to the axial thrust inherent in the design which requires appropriate thrust bearings to be incorporated, impacting on lubricant requirements. This can either be accommodated or addressed, using gearboxes built around helical stages, with the helix angle of one being the negative of the other. 

Bevel gears are another option, best-suited to lower speeds. These are gears where the two shafts intersect and the tooth-bearing faces are cone-shaped. While this design offers flexibility in the angle between the shafts, the gears must be precisely-mounted and the axes capable of supporting significant forces, with noise again a consideration. A spiral bevel gear addresses some of these issues, offering similar advantages to helical gears, with gradually-engaging teeth to deliver smoother, quieter transmission and higher efficiency.

A planetary gearbox consists of one or more outer gears (planets) revolving around the central (sun) gear within an outer ring (annulus) which meshes all. The axes of all gears are parallel, hence input and output shafts are always inline. Highly efficient even at low speeds, they can transmit three times the torque of a similarly-sized conventional spur gearbox. Planetary gearboxes can be combined with bevel or helical gears, e.g. the Brevini Posiplan range, combining technologies to achieve an extremely compact, efficient solution.  

Efficiency factors

Gearbox efficiency is determined by a whole host of factors, including: mounting arrangements; construction materials; duty cycles; location and environment; maintenance requirements and required lifespan. Hence, a holistic approach – combining engineering expertise and access to a broad product portfolio – is hugely beneficial, as demonstrated by Brevini UK’s recently-completed commission at Saltend Waste Water Treatment Works, where long-standing issues around odours had disturbed the local community.

Gearbox efficiency is determined by a whole host of factors

For this major project, Brevini was commissioned to work on the Morgan Sindall Sweco joint venture, to replace the original EIMCO rake drive system in the lamella tanks – a significant part of the whole waste water treatment plant at Saltend.

Generally, water industry applications tend towards high output torque and low (or very low) output speed requirements, which make planetary drives the optimal solution. For the Saltend project, most gear unit suppliers would consider providing a simple gear unit as this would minimise the risk of problems with specification, design and manufacture, with an inline planetary unit seeming the most obvious selection. However, the gear unit is only a small part of the system.

Brevini’s proposed drive arrangement was deemed to be far more efficient, generating the required output from a much smaller prime mover. The Brevini drive has an electric motor, a planetary gear unit, a final-stage slew-pinion and bearing, plus supportive steelwork. With no separate method of torque limitation or monitoring specified, Brevini proposed to utilise the motor supply current as the generator of the torque level. 

While this is, technically, the least accurate position, the total combined drive efficiency of 86% is an improvement over the original drive. Brevini offered this option in part due to the application requirement, where a normal torque-limiter wouldn’t have been suitable, in case the system might stall or be disengaged. Once Brevini’s solution was approved, the new drives were installed safely to MS2JV requirements – delivered, installed, completed and works-tested ahead of schedule.  

Key Points

  • While spur gears offer high efficiencies, at higher speeds the sudden meeting of teeth causes stress and noise
  • Axial thrust inherent in the helical design means appropriate bearings have to be incorporated, impacting on lubricant requirements
  • While bevel gears offer flexibility in the angle between the shafts, the gears must be precisely-mounted

 
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