ST 002-10 Lohr, 2010-02-18

Safety Risk of Blanket Reliability Characteristics: Anomalous MTTFd Values are a Quality Feature

Ever since Machinery Directive 2006/42/EC came into force, designers have had to make allowances for the failure probabilities of all safety-related components and incorporate this information into the Performance Level (PL) calculations. Most suppliers are already making the relevant statistical data available. Dr. Alexandre Orth, who is responsible for Reliability & Maintainability (R&M) in the Quality Methods department at Bosch Rexroth AG, explains why users should not accept unverified blanket values and how following this course of action can help prevent systematic errors.

"There's MTTFd and there's MTTFd. The deviation of individual model ranges from the EN ISO 13849 MTTFd blanket values actually constitutes a quality feature attesting to the careful and responsible determination of these vital reference figures. The difference between general blanket values and the actual MTTFd value determined for the component series in use can, with more complex circuitry in particular, have a significant impact on achievement of the necessary Performance Level (PL). If there is any suggestion of doubt, the machine manufacturer must question the blanket values specified by manufacturers for entire product classes and verify the values himself.

It is inevitable that MTTFd values determined in line with standards and calculated individually will often differ markedly from blanket values, for example with hydraulic valves: The value specified in the standard (150 years) only applies to stand-alone valves. However, pilot-operated valve solutions consisting of two valves are often used in practice, for example. In accordance with the "Parts count procedure" specified by EN ISO 13849, the MTTFd value for model ranges of this kind must always be fewer than 150 years, as there are now two error sources to take into consideration. In this instance, the user is able to discern the quality and validity of a supplier's MTTFd values at first glance.

The EN ISO 13849 standard also stipulates compliance with safety-related design principles. For instance, components must be able to enter a safe state in the event of a power failure. Any components that do not satisfy this safety requirement are unsuitable for safety-related applications, despite good reliability.

In such cases, accepting blanket values may result in a systematic distortion of Performance Level (PL) calculation and cause serious errors in the implementation of standardized safety concepts. The same is true of PFH values for electrical control systems and B10 values for pneumatic components."

Bosch Rexroth AG is one of the world’s leading specialists in the field of drive and control technologies. Under the brand name of Rexroth the company supplies more than 500,000 customers with tailored solutions for driving, controlling and moving. Bosch Rexroth is a partner for industrial applications and factory automation, mobile applications and using renewable energies. As The Drive & Control Company, Bosch Rexroth develops, produces and sells components and systems in more than 80 countries. In 2008 Bosch Rexroth, part of the Bosch Group, achieved sales of around 5.9 billion Euro with 35,300 employees.

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