Protecting production in Prachovice

Cemex operates the second largest cement plant in the Czech Republic in Prachovice, about 100 km east of Prague. Up to 5000 tpd of clinker is ground into ready-to-use Portland cement there using two identical ball mills 15 m in length. Each of these ball mills is moved by a girth gear. On each mill, two 2250 kW asynchronous motors transfer power via gear units to two pinions with separate bearings. The pinions engage with the girth gear and apply torque to it. A mounting flange and bolted fixings form the connection between the girth gear and the circumferential surface of the ball mill.

A diagnostic check of the mill drives in spring 2017 revealed that both girth gears and all four pinions were already showing significant signs of wear. Continuing to run a system with weakened gearing that exhibits an advancing damage pattern can lead to a failure of the drive and thus the entire system. This quickly results in long downtimes, which in turn leads to bottlenecks in delivery and costly production losses.

To ensure the future availability of the cement mills, Cemex opted to replace both girth gears, all four driving pinions, and both girth gear covers. Due to their low level of wear, the gear units were classified as not critical and therefore not replaced.

From the very start of the project, SEW-Eurodrive assisted the cement manufacturers with any unresolved issues. Questions of material selection, design/dimensioning, project planning, lubrication, all the way through to construction and commissioning were discussed as a team. The company has many years of experience using girth gears in a wide variety of industries. Moreover, the SEW-Eurodrive portfolio covers the entire spectrum of drive technology – from the motor and gear units to drive electronics and controllers. The comprehensive support provided throughout the entire design phase ultimately prompted the Cemex-maintenance management team to continue working with SEW-Eurodrive in the later stages of the project.

Cutting-edge material technology

When manufacturing girth gears, SEW exclusively uses high-strength austempered ductile iron (ADI). The properties of this material are superior to those of traditional girth gear materials, such as cast steel. For instance, the ADI used by SEW has a minimum tensile strength of 1000 MPa and a minimum elongation of 5%. Heat treatment is required to achieve the necessary mechanical properties. This treatment encourages the desired microstructure to form and ensures the material exhibits high permissible strength values. The material’s contact fatigue strength, which comes as a result of its excellent cold work hardening properties, makes the girth gears very durable and practically wear-free, if dimensioned, loaded, and lubricated correctly. Moreover, the girth gear can be designed to be significantly more compact and lighter when using ADI instead of the traditional solution.

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Read the article online at: https://www.worldcement.com/special-reports/11102023/protecting-production-in-prachovice/