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Maintaining older gear units: part one

World Cement,


The history of industrial cement production is directly related to the improvement of grinding mills and therefore also the invention of driving systems. Although the ability to form a cementitious material with hydraulic properties has been around since the mid-19th century, it took more than half a century until the first industrial cement production lines were established. Gearboxes for grinding mills have been in use as a core component of these production lines since the 1920s.

In addition to the continuous invention of new geared solutions, one of FLSmidth MAAG Gear’s major efforts is to keep long-serving gears running for even longer.

Durable gear design

Alongside the development of the rotary kiln and the addition of gypsum to control the setting of cement, the improvement in raw material and clinker grinding enabled by the development of ball mills made it possible to produce Portland cement on an industrial scale. The invention of the mill drive system went hand in hand with this development.

FLSmidth started to produce gearboxes for the central drives of tube mills (SymetroTM gearboxes) in 1926. A large number of these gearboxes are still running in cement production lines today, performing their duty for more than 80 years.

‘Symetro’ is so named for the symmetric splitting of the motor torque to two transmission paths inside the gearbox. The high-speed coupling connects the motor shaft to the high-speed pinion where the driving torque is divided and transmitted to the two intermediate wheels. The equal torque split is ensured through the free floating balancing wheel, the location of which is controlled through the combination of the guide rings, the position of the machined teeth between the intermediate pinion and the balancing wheel, the corrected and marked tooth for timing, and the tension of the balance bearing. In the second gear stage the torque is again brought together using the intermediate pinions and the balancing wheel. Finally the torque is transmitted through the low speed coupling to the mill. Both the low and high-speed couplings are flexible membrane couplings, where the membranes consist of several thin steel plates, allowing for minor angular deviation to compensate for small misalignment caused by thermal expansion of the gear and mill casings. Compared to similar design from other gear suppliers, the Symetro gear transmits the torque with only two gear stages and a reduced number of bearings. This gives better overall efficiency.

This is part one of a three-part article written for World Cement’s December issue and abridged for the website. Subscribers can read the full issue by signing in, and can also catch up on-the-go via our new app for Apple and Android. Non-subscribers can access a preview of the December 2015 issue here.

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