VDW Project of the Year validates safety of standard PLC control yet again

Source: Gerda Kneifel, VDW

Sep 27, 2017

Safety standards now have a study proving their effectiveness. As part of the Safety Day at the EMO Hannover 2017, the VDW-Research Institute, the research community of the VDW (German Machine Tool Builders’ Association), awarded the prize for the “Project of the Year.” The award aims to honor outstanding research projects conducted by production-engineering academics that are built to close knowledge gaps concerning certain specifics of machine tools, production technology and engineering.

The winner of this year’s prize is Dipl.-Ing. Nika Nowizki working for the Institute for Machine Components (IMA) of the University of Stuttgart. Nowizki’s study examined the field-evidenced reliability of automatic multi-spindle lathes from the Index company, with a view to determine the dependability of the ‘workpiece clamping’ safety function, which is actuated by a standard PLC.

“The characteristic reliability measures determined in the statistical analysis of the field data obtained are in some cases significantly higher even than those stipulated in the relevant standard,” said Nowizki.

According to VDW, providing proof of machine tool safety is a relatively difficult process, because hazardous scenarios are relatively rare, but are of utmost importance because of the potential fatal effects.

Many lathes (including their safety functions) are controlled by means of standard PLCs from prestigious manufacturers of control systems, some of which were installed as a standard feature even before the ISO 13849-1 safety standard was introduced about ten years ago. Machine tool manufacturers still work with this type of control system today, because of its proven field reliability. One important foundation for this dependability is the cascading of safety measures in the product safety standards, in this case the ISO 23125 Lathe Safety Standard, which specifies the standard for safety functions and has a sophisticated operating-mode system.

“Both manufacturers and users have hitherto always felt that the machines built to standard are very safe – without any scientific evidence to that effect,” Nowizki said. “And now it’s more than just a feeling. We can provide statistically corroborated evidence confirming that the workpiece clamping function implemented with the standard PLC on the lathes examined complies with the stipulations laid down in the safety standards ISO 23125 and ISO 13849-1, and under certain circumstances even outperforms these.”

In her study, Nowizki examined the running times of 578 multi-spindle lathes with a total of 3,951 spindles. The machines were controlled with a standard PLC of identical type. The mechanical engineer analyzed data reaching back as far as 1992 and her estimates covered accumulated operating hours for the machines examined of at the very least over 93 million, during which not a single safety-relevant accident had happened. The performance levels (PLs) rate a safety function’s contribution towards risk reduction. The value(s) stipulated in ISO 23125 of PL = b, and PL = c for workpiece clamping, were unambiguously met in this study.

“This important finding, meaning proof of field-proven reliability for the current state of the art of workpiece clamping on lathes, should be taken into due account during the most recently initiated revision of the ISO 23125 standard,” said Heinrich Mödden, consultant for machine safety in the VDW. “We are also taking this opportunity to issue an invitation to work on this revised standard. Of the most important global producers of lathes, last time it was mainly manufacturers from Japan and Germany who got involved. But also other other major lathe-producing countries such as China and Korea should become active.”

Comparable studies, says Nowizki, are also possible with other machine tools. And there are already specific plans ongoing to conduct them. At the beginning of next year, the Institute for Machine Elements at the German Federation of Industrial Research Associations (AiF) will submit an application for an even larger project with different types of machine.

A VDMA Position Paper on workpiece clamping was presented during the EMO Safety Day, which for the first time laid down regulations for responsible handling of actuator-driven workpiece clamping devices at component suppliers, machinery manufacturers and operators.

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