Determining the optimum time for wear replacement

The Project

Salzgitter Flachstahl, the largest steel subsidiary in the Salzgitter Group, is one of Europe’s leading steel manufacturers. “For the manufacture of high-quality steel products, we rely on state-of-the-art production technologies for manufacturing and further processing,” says maintenance engineer Rebecca Dittkrist. 
They operate, for instance, a 42-m³ main gearbox of a rolling mill. The single-stage gearbox features a nominal load of 14 MW and a nominal speed of 32 revolutions per minute. Condition monitoring systems use vibration measurement to detect wear at an early stage. However, measuring vibration acceleration only provides reliable results at speeds above 60 revolutions per minute. The gearbox in Salzgitter remains almost 50% below this limit, but only in rolling operation.

The solution to a complex problem

A system with these specifications is a challenge for any condition monitoring system. Process data processing is vital for reliable operation. The fact that the relevant information in this application is distributed exclusively to different operating modes poses a problem for conventional condition monitoring systems. iba AG’s solution: subtrends. They enable data to be collected for different operating conditions, separated from each other and also separately alarmed. In concrete terms, this means that this system assesses both situations with low speeds and high load and situations with higher speeds without load. All the characteristic values are determined separately for each operating mode.

Only four vibration sensors are installed in the enormous main gearbox. One sensor each on the top, bottom, drive and output end, with dual sensors fitted to the locating bearings, which measure in both radial and axial directions. However, the number of sensors does not depend on the number of monitored components, but rather on the dimensions of the gearbox since vibrations cannot propagate through material unhindered. “The monitoring unit installed here differs from commercially available systems in that it communicates directly with the ibaPDA process data recording system,” explains Herwig Eichler, Sales Manager for Condition Monitoring at Hainzl Industriesysteme. Over the last 15 years, ibaPDA has become almost a benchmark in the metal industry. This is not least due to the wealth of interfaces with automation systems. This connection is essential, because without the process data it would be very difficult to distinguish between rolling and calibration operation.

The perfect time for gearbox replacement

Just under two years before final replacement of the gearbox, the iba system provided the first signs of gearbox wear. This was initially unbelievable since the gearbox is designed for a service life of well over 20 years and had not yet been in operation for six years at the time first signs of wear were found. For this reason, comparative measurements were also made with a commercially available system. Wear on the bearing of the fast running drive shaft was also detected by this system and also rated as ‘noticeable’, but not ‘critical’. However, this system failed to detect much more critical wear on the rolling element of the slowly rotating output shaft. In the iba system, however, the signs of wear were clearly visible and increased steadily. 

For other units, the decision would probably be made to replace them during the next scheduled shutdown. However, it would take ten working days to change this gearbox during a scheduled shutdown. On the other hand, gearbox failure during production would mean a much longer, unscheduled downtime, as necessary spare parts and equipment would first have to be procured. The current condition was assessed in weekly meetings and the assessment for the remaining service life was continuously updated. “We consulted experts from our dedicated network of vibration specialists time and again. They actively supported us in determining the right time for replacement,” says Günter Spreitzhofer and adds: “In October 2017, we jointly decided that continued operation until May 2018 is possible for the time being.”

Seven months later, it was evident that wear and tear was progressing continuously, but very slowly. It was therefore decided in May 2018 to postpone the replacement by another six months. This decision was also based on continuous monitoring of wear development and the option to bring forward the half-year shutdown should critical changes occur. In September, all the experts involved concluded that the gearbox would definitely have to be replaced during the next shutdown, Figure 4. As forecast, the now heavily worn rolling bearing continued working for another two months. Eventually it was replaced during the next scheduled shutdown. The gearbox was then inspected at the workshop to determine whether it was actually damaged. When the bearing was dismantled, the damaged rolling element broke and the damage to the inner ring of the drive shaft bearing was also clearly visible. This was confirmation that the time for replacement had been perfectly chosen. The basis for this success is the powerful condition monitoring system which provides reliable and timely alarms thanks to consideration of relevant process data, combined with the expertise of seasoned specialists in the field of vibration diagnostics and maintenance planning.

Günter Spreitzhofer and Herwig Eichler agree: “The Salzgitter case is a showcase example of a perfect maintenance strategy. All the stakeholders are working hand in hand and communication between mechanical maintenance and the vibration experts involved is exemplary. Only under these conditions can a condition monitoring system play to its strengths and provide the optimum benefit.”