Condition Monitoring - Bearing down on stoppages01 October 2004

A machine tool spindle condition monitoring unit (CMU) at car manufacturer BMW's plant at Steyr, Austria, has reduced the number of bearing failures from 21 per year to zero. The Multilog CMU, supplied by bearings manufacturer SKF, is an on-line surveillance system that collects and evaluates vibration and process machinery data round the clock from permanently installed sensors, then automatically captures alarms as they occur. The CMU records data on a scheduled basis, which allows immediate notification of any impending problems. It can be used in harsh, remote, unsafe or difficult-to-reach locations.

SKF has also created proprietary algorithms to adapt to the complex nature of the machine tool process. Advanced triggering capabilities enable the Multilog system to dwell, set priorities for collection and check the machine state (working or idle) when the data was collected. The CMU includes pre-emptive logic gating to enable the precise timing of data collection to match specific points in the machining cycle of single machining centres. And SKF has also developed very small accelerometers with frequency capabilities that are tuned to the particular machine tool application.

Steyr produces around two-thirds of all BMW four- and six-cylinder petrol and diesel engines - around 600,000 per year, or 90 per hour. The plant runs drilling, turning and milling machines at speeds from 1,000 to 12,000 rpm. The spindles on these machines are a mixture of belt-driven and directcoupled.

In December 1999, a pilot project was started to investigate bearing spindle failures on these machines. Across two of the lines at Steyr, BMW installed two local monitoring units (LMUs) and a total of 56 sensors, to monitor 27 separate spindles. Using SKF's software to evaluate bearing vibration trends, plant engineers at BMW discovered an alarming increase in spindle failures over the millennium shutdown period. The data pointed to outer raceway damage as the primary cause.

According to SKF's Marty Herzog: "Maintenance spent most of the two-week shutdown period washing down the production lines, which was where the damage [corrosion] came from. Oxidation of the outer raceway was caused by the ingress of emulsion during washdown of the machines."

The pilot study also highlighted sudden vibration increases when new grinding spindles were fitted to machines. The cause of this was that the grinding wheel itself was unbalanced.

Herzog says: "Before the pilot project in 1999, BMW Steyr reported a total of 21 bearing failures per year which caused spindle crash. This resulted in an average of ten hours per incident for repairs."

The pilot was completed in May 2001. Herzog explains: "At the two monitored lines, the number of bearing failures was reduced to zero; bearing damage was detected early on 38 separate occasions on one line, and 14 times on the other. But unplanned downtime was reduced to zero."

In 2002, Steyr added SKF's Multilog system which included adding six extra LMUs. The system comprises 240 on-line sensors and eight LMUs. This year, BMW plans to expand this system to 700 sensors and a CMU for each new production line.

Other companies using SKF's CMU include Volvo, Audi, Bosch, Renault, Ford, Saab and VW.

SOE

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