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New angles on gearbox efficiency

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Even the best ideas can take years to achieve success. Sometimes specifiers come to appreciate outstanding performance characteristics only after some spectacular examples of efficiency in action.

A case in point is planetary gearboxes. Engineers have known for a long time that these radically different gearboxes an produce outstanding power transmission efficiencies - with typical losses of only 3% per stage - all in a remarkably compact configuration.

This kind of efficiency, remarkable in gearbox design, ensures a high proportion of energy generated by motors to drive machinery is not gobbled up by mechanical losses within the gearbox that is multiplying and transmitting the torque produced.

In fact, planetaries are often a brilliant choice for applications requiring high torque in intermittent operation. Compared with a parallel shaft arrangement, a planetary gearbox in high torque/intermittent operations can often achieve the same ratio with one fewer reduction stage, with cost and dimension savings.

This is not to say that planetaries are the ideal gearbox in every situation. As a producer of a full range of technologically advanced gearboxes - from the smallest worm boxes up to the biggest 550,000Nm planetaries - Bonfiglioli appreciates that there are horses for courses.

But, properly specified, planetaries are eminently suited for many static industrial plant applications throughout Australia and New Zealand, including agitators, conveyor feeders, pumps, mixers, stirrers, scrapers and settling pond thickeners, for example. They also have applications in mobile plant, including cranes, drilling rigs and drives for hoists and luff and slew equipment.

Reliability of this calibre has given the lie to detractors who erroneously believe that older, more conventional layouts are more reliable and that - in the event of failure - the older designs can be brought back into service more quickly by temporary welding or hand dressing of gears. In fact, the best planetaries now produced are of advanced modular construction that can relatively effortlessly be configured to changing needs.

Possibly it is their radically different configuration that has caused some designers to approach planetaries with caution. But it is this planetary construction that gives them their unique compactness and power.

Planetary construction dispenses altogether with the traditional arrangement of a pinion driving one large gear on a parallel shaft. Instead, the planetary gearbox surrounds the pinion (called a "sun gear") with three or more smaller planetary gears mounted in a planetary carrier (which transfers drive to the next stage of the gearbox or to its output).

This arrangement of several smaller planetary gears around the input pinion offers distinct advantages for certain applications:

* High efficiency. The spur gears used in planetary gearboxes are inherently of high efficiency (97-98 per cent per stage).

* Space savings. The planetary arrangement facilitates multiple reduction stages in a very compact space. These can be achieved because each reduction stage adds only a small increase in dimension. These advantages are even greater when the reduction ratio is increased, and they translate into an economical purchase price and installation.

* High torque capacity. Because the torque being transmitted at any time is shared between multiple sets of teeth on the primary drive pinion, torque capability is greatly increased.

* Increased reliability - and higher radial loads permissible on the gearbox's output shaft. Reliability and radial load capacity benefit because the shaft itself is not carrying any radial loads induced by the gears themselves (unlike parallel shaft systems). Benefits of reduced radial loads include extended bearing life, a critical component of machinery reliability. Reduced radial loads within the gearbox also permit it to tolerate higher radial loads from the equipment it is driving.

* Suitability for shaft mounting. Benefits of shaft mounting, as discussed previously, include elimination of the cost and complexity of couplings and elimination of the time and labour involved in ensuring correct alignment of the gearbox and the plant it is driving. Direct drive through shaft mounting also avoids radial loads imposed by chaindrives.

* Flexibility of power input. This allows the gear units to be combined with a wide range of electric motors and other types of gearbox in the vast Bonfiglioli range, including helical and worm. Various types of hydraulic motors too can be utilised together with plain high speed input shafts.

*Malcolm Lewis is managing director of Bonfiglioli Transmission (Australia) and Neil Pollington is Bonfiglioli's New Zealand manager.

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