Mean Time To Failure / Mean Time To Dangerous Failure

MTTF (Mean Time to Failure) describes the average operating time of a device or component until the first failure. In addition to the MTTF value, there is often mention of MTTF_D (Mean Time To Dangerous Failure), which refers to the operating time until a dangerous fault occurs. Both MTTF and MTTF_D originate from the field of risk assessment. MTTF_D is particularly relevant in the context of EN ISO 13849-1, as the standard allows the performance level to be determined based on this value and other factors such as safety principles and the diagnostic coverage. In short, MTTF and MTTF_D help users with risk analysis and, in the case of mechanical components such as brakes, with maintenance planning.

• MTTF indicates the time until a component fails
• MTTF_D as the basis for calculating the performance level
• Value relevant for brakes as safety-critical components 

Why is the MTTF value important for brakes? 

Whether in AGVs, robotics or conveyor technology – wherever KEB brakes are used, safety and reliability are key priorities. A high MTTF value is therefore an important criterion when selecting the appropriate brake. It should be noted that the MTTF value depends on the switching frequency. This means that brakes have different MTTF values depending on the application, but are not necessarily of different quality. In addition, MTTF or MTTF_D are required to certify a machine as safety-relevant in accordance with standards. Users can use the value to estimate the probability of a brake failure and thus better calculate spare parts strategies and life cycle costs.

Difference: MTTF and MTTF_D

While MTTF provides a basic indication of the average service life of a component, MTTF_D (Mean Time To Dangerous Failure)refers to the operating time until a dangerous failure occurs. Against this background, MTTF_D values are particularly important in the area of functional safety, as only safety-critical failures are considered. 

According to the DIN EN 13849-1 standard, the MTTF_D value is classified as low, medium or high. A low rating is therefore given for an MTTF_D value of between three and ten years. A value of ten to 30 years is classified as medium. A high rating is given for a service life of 30 to 100 years or more. 

A non-hazardous brake failure occurs when the brake no longer performs at full capacity but does not pose a danger to people or machinery. This is the case, for example, when a brake no longer opens completely, i.e. remains slightly closed. As a result, the machine can still move, but with increased friction. The brake does not suddenly lock or fail, but the malfunction leads to increased wear, energy loss and increased noise.