For some years now, the foundry Ferry Capitain of Joinville in Northeast France, a manufacturer of girth gears for cement mills, has been offering replacement ball mill girth gears made of ductile cast iron (Fig. 1). Thanks to their short delivery times and the fact that they allow repair welding, these girth gears are a reliable solution in the case of repairs or replacement of components. Ferry Capitain is a member of Compagnie Industrielle et Financière de Bussy (Groupe CIF).
EN 1560 specifies that ductile cast iron must have a carbon content of more than 2.06 % and that the carbon must be present in the form of tiny nodules. The carbon nodules make the cast iron ductile weldable. At the same time, it possesses mechanical properties similar to those of steel. Today, ductile cast iron is an alternative material to steel. The use of this material substantially reduces delivery times and costs.
The following case study provides a good example for these advantages. The Wotan cement factory in Üxheim-Ahütte in the Eifel region of Germany had ordered a replacement girth gear made of cast steel G34CrMo4 and a replacement pinion made of forged 42CrMo4 from an Eastern European supplier for their cement mill no. 2 (diameter 3.2 m, drive power 1600 kW). After startup and commissioning with these new components, Wotan found that the vibration amplitude was even higher than it had been with the old girth gear, which was, however, worn-out and thus unusable.
Experts from Ferry Capitain first carried out a comprehensive inspection of the new components. This revealed that the Eastern European supplier had not manufactured the tooth geometry in compliance with the relevant standard. The pressure angle of pinion and wheel were different, which prevented correct tooth meshing.
At the wish of the client the teeth of the girth gear and pinion were first recut by a sister company of Ferry Capitain in Groupe CIF, namely FCMD GmbH in Hattingen/Ruhr (Fig. 2). After reinstallation of the components, the cement mill ran with normal vibration values and with a correct tooth contact pattern.
Then, a few weeks later, the vibrations started to increase again. This time, the main reason for the rough running was abnormal wear. Within a period of only two months, this was comparable with the normal wear after five years‘ operation.
After provisional weld repair, the girth gear was recommissioned. “Quick running-in” with a special lubricant brought down the vibrations again. However, in expectation of a low general life expectancy of this girth gear from Eastern Europe, the client requested a permanent solution.
For the performance of this order, Ferry Capitain profited from its long-time experience with the manufacturing of girth gears made of high-grade ductile cast iron, a material which is already in widespread use in the minerals industry. In addition to the short production time compared to cast steel, the biggest advantage is the material quality: immediately after casting and a minimal cooling-down time, the ductile cast iron component is available – free from cavities, without cracks and with a clean surface. The material requires no thermal post-treatment.
The mechanical properties of ductile cast iron are very similar to those of cast steel, even at identical hardness values. Of the more than 700 ductile cast iron girth gears manufactured so far by Ferry Capitain, not a single one has failed due to tooth damage. In recent years, numerous tests with ductile cast iron materials have been performed in cooperation with the Centre Technique des Industries Mécaniques (CETIM), of the French Technological Research Institute for the Mechanical Engineering Industry in Senlis. These tests proved that ductile cast iron has significantly better fatigue limit in surface compression than cast steel. The strength of ductile cast iron tooth roots is very similar to that of cast steel. Another advantage of ductile cast iron is its superior vibration dampening properties.
These properties allow the simple replacement of old cast steel girth gears – and with fully identical dimensions, which was a decisive point for Wotan-Zement.
On the basis of this successfully executed spare-parts order and the good quality of the delivered girth gear, Wotan awarded Ferry Capitain another order for a ductile cast iron girth gear some years later for use on a similar cement mill.
New specific welding tests on gear teeth made of ductile cast iron have shown that it is possible to perform a repair at the plant site with minor loss of fatigue strength. This minimizes the production downtime.
Since the beginning of 2012, the technological centre of one of the world’s leading suppliers of cement has approved ductile cast iron girth gears for use on its kilns and mills.