TechArchive

Original article date: January 1998

Why hasn’t planetary gearing been adopted more in the UK? It’s also normally possile to replace an existing gearbox with a planetary equivalent. Here’s the case in favour of this type of motion!

As buyers of foreign machinery will notice, planetary gearing has a far higher market penetration elsewhere in the world, whilst British manufacturers of hydrostatic mobile plant have widely adopted this drive. But UK designers of industrial machines and other static plant, however, have been more reluctant to abandon conventional worm and parallel shaft gear units in favour of planetary power transmission.

With output torques up to 2MNm, planetary gearing is well suited for a wide range of applications, particularly at the low speed, low power, high torque end of the spectrum.

Although often regarded as complicated, planetary boxes actually consist of a basic gear train with two spurs and an idler in-between. The input gear is known as thesun, the output gear is inverted to form an internally-toothed fixedannulusand the idler becomes three or moreplanetgears mounted on a carrier, from which the drive is taken, thereby spreading the torque transmitting capacity across multiple teeth.

The modular planetary gearing stages provide ratio building blocks that can be combined together to form an almost infinite number of ratio permutations, with a technically unlimited upper ceiling. Furthermore, the compact design means increasing the ratio from, say, 1000:1 to 10 000:1 adds only millimetres to the length of the box.

Weight savings over a conventional worm box can be as high as 60%, which translates into smaller support structures, less mechanical stress on the machinery, lower freight charges and the option of mounting on the drive shaft. Above ground, less weight is critical for such applications as wind generators. Elsewhere it simply means a lighter box.

Compared to alternative transmissions that can lose half of the input power, planetary gearboxes are upwards of 90% efficient, even when running at ultra-low speeds. This is a more dramatic economy than the 2-3% savings achievable from energy-efficient motors.

Planetary gears are typically half the size of many conventional boxes, which means more compact drive trains, easier access for routine maintenance and an absence of pits, gangways and structural work. They lend themselves to neat design solutions – the rounded housing is synergistic with the lines of rotating machinery and conveyors. Designers of mobile plant, generators, winches and antennae value to compact dimensions. Thanks to the modular construction method, planetary units were the first fully flexible gearing system suitable for an extensive range of applications. Design variants are many, with a choice of foot-mounted, flange-mounted and shaft-mounted housings, input connections for hydraulic and electric motors and flexible couplings, as well as a variety of male and female output shafts.

Case-hardened, correctively ground gears, computer-optimised tolerances and the distribution of torque between multiple gear teeth all mean that planetary units can be specified for the life of a machine, with no maintenance other than routine oil changes.

Suggested quote:

“Compared to alternative transmissions that can lose half of the input power, planetary gearboxes are upwards of 90% efficient, even when running at ultra-low speeds. This is a more dramatic economy than the 2-3% savings achievable from energy-efficient motors.”

• Lohmann + Stolterfoht
• 19 Queensbrook
• Spa Road
• Bolton BL1 4AY
• Tel: 01204 534083
• Fax: 01204 534084

Pic to come

January 1998