Ten reasons to go planetary

Brevini puts forward 10 reasons why a planetary type gearbox should be considered instead of a conventional worm and wheel or spur gear unit.

The overriding requirement for mechanical reliability and durability in equipment for the water supply and treatment industry can militate against the introduction of new technology.

In the case of reduction gearboxes, one company, Brevini believes this is the case and has put forward 10 reasons why a planetary type gearbox should be considered instead of a conventional worm and wheel or spur gear unit.

Gearboxes are used throughout the industry in filtering, pumping, clarifying, thickening and aerating equipment, often in remote locations or large outdoor processing sites, hence the need to be ultimately reliable.

Planetary gearboxes are now offered with a ten-year minimum life expectancy and have a proven track record of dependable performance.

So much so, that the largest hydraulic wheel ever built, a recent investment in 'green' power generation, used a two stage planetary gearbox on the water turbine.

The main reasons for the planetary choice, after reliability, were high efficiency and the compact lightweight nature of the design, allowing very high torque loads in a confined space.

The typical efficiencies of the unit are 96%, ensuring that the energy being generated is not being consumed by mechanical losses.

With today's increased focus on energy efficiency, the knowledge that a single stage planetary gearbox unit is around 98% efficient even at very low speeds can be useful.

Especially when compared to some traditional worm gears that can be as low as 50% efficient.

Getting more performance from a smaller working envelope and saving weight and cost in the process are objectives shared by most engineers, certainly those who design and maintain equipment in the water industry such as pumps, agitators and scrapers.

These devices are prime areas where planetary gearboxes can demonstrate the benefits of alternative gearing technology.

Planetary gearboxes are hardly new, but for some reason a surprising number of designers and engineers regard them with suspicion.

Why this is so is still a mystery, but one theory is that prejudice against their use could result from the innovative design that makes them so effective.

The traditional (and erroneous) thinking is that older, more conventional, gearing layouts are more reliable.

Also, in a failure situation they can be brought back into service quicker (albeit temporarily) by welding or hand dressing of gears.

If this is indeed the line of thinking then it overlooks certain crucial facts.

Firstly, computerised selection as used by companies such as Brevini, can ensure that a planetary gearbox will last the lifetime of the machine.

Secondly, planetary gearboxes are perfectly balanced units and are extremely reliable in service.

Finally, other than routine oil changes, no maintenance is required on Brevini planetary gearboxes throughout their design life.

Reliability apart, what are the other major considerations that a designer or engineer takes into account when selecting one type of gearbox in preference to another? Certainly, in terms of planetary technology, the task has, until now, been complicated by the fact that, no formalised list of the advantages of the technology has been produced.

In an effort to rectify this situation, and to make the case for planetary technology, Brevini has now produced such a list (see end of article).

It compares planetary gearboxes with conventional gear units and highlights ten areas where planetary units can provide real advantages to the OEM and user, both in specifying and using the gearbox.

One application that proves conclusively the reliability aspects of planetary technology is the long-term use of Brevini gearboxes to provide high torque and low speed operation on the drive heads of scrapers and thickeners in the water treatment industry.

In this extremely arduous application the Brevini gearbox rotates a drive head mechanism at speeds of as low at 0.05rev/min, 24 hours a day, 365 days a year.

To drive the rakes through the settled solids, could be anywhere 1000 to 50,000Nm depending upon tank size and other factors.

Neither are there any lubrication problems arising from the drive operating at low speeds, a condition which can reduce the reliability of conventional gear units.

Clarifiers with a skimmer, a sludge scraper and a flocculation process arm, and thickeners with sludge scrapers driven by a central drive can be fitted with Brevini Standard series shaft mounted gearboxes with grease protection or special double sealing.

Accessories include a torque limiter to prevent damage on the racks in case of blockage or snaring.

When driven by a peripheral drive mounted at the rake arm edge a central drive can be used via a slew pinion and geared bearing using a Brevini Standard series and an RPR slew drive.

By mounting a surface aerator turbine directly onto the gearbox output shaft of a Standard series drive or an MDU mixer drive, size and weight advantages can allow for a smaller footprint, easier handling, lower installation costs and all round savings on fabrication and bridge design.

Brevini also offers a drive package for the water industry.

The drive itself comprises a compact electric motor, multi-stage planetary gearbox and a mechanical torque limiter located in the bell housing acting as a limit sensor switch, disconnecting the drive in the event of an overload.

The inherent capacity of planetary gear trains to deliver high reduction ratios in small packages, typically 10,000:1 enables them to transmit three times the torque of similarly sized, conventional spur gear units.

The following are Brevini's ten reasons to go planetary.

1 - Compact size - Brevini planetary gearboxes can be half the size of conventional gear units, yet transmit the same or higher levels of torque.

2 - Low weight - Weight savings can be as much as 60%, which can mean lighter and smaller support structures which, in turn, are easier to handle.

3 - Superior efficiency - While some transmissions can be less than 50% efficient, planetary units are 98% per stage of reduction.

4 - Low maintenance - Other than routine oil changes, no maintenance is required through the design life, not even replacement bearings.

5 - Extended life - Using computerised selection, based on ISO standards, Brevini technical staff are able to select planetary units to last the machines total design life.

6 - Design flexibility - The modular construction of Brevini planetary units, together with Brevini's expertise, offers solutions to any nonstandard application.

7 - Cost effective - Compared with equivalent conventional gearboxes, volume production of standard planetary stages enables significant economies of scale and highly competitive pricing.

8 - Fully reversible - The construction of a planetary gear stage allows it to be equally efficient in either direction.

9 - Coaxial arrangement - Brevini's completely sealed universal adapter system allows any standard type and make of motor to be direct mounted to the gear unit.

10 - Short deliveries - Brevini's modular construction means that standard parts are held in UK stock to allow short lead times for a larger number of variants and sizes.

All of these units are supported by extensive UK technical expertise.

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