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 SINUMERIK: Calculating the PFHD value of a holding brake
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 QUESTION: In the case of a vertical axis, how can one regard the holding brake of the 1FT6, 1FT7 and 1FK6 servo motors - according to EN ISO 13849 - in a safety function? ANSWER: If a machine is built according to the specifications of EN ISO 13849, machine manufacturers are requested to define safety functions for their machines and to calculate the failure probability of these safety functions. Especiallly when one has to consider vertical axes in a safety function, a brake must be included in the consideration of the safety aspects. To calculate the PFHD value of the integrated holding brake of servo motors 1FT6, 1FT7 and 1FK7, one can assume the following: 1) The B10 value of the holding brake is 10.000.000 if the number of emergency stops does not exceed the prescribed number. 2) A brake test checking the specified nominal holding torque of the brake for "No motion" is assigned a DC value of 90%. (DC = diagnostic coverage). 3) Depending on the application and the structure of the safety function, one has to consider the test frequency (test interval), demand rate, demand time as well as usage time apart from the DC when determining the PFH/PL (Probability of Failure per Hour/Performance Level). 4) To safeguard against the wear in connection with emergency stops, one must ensure in any case that the brake test is performed independently of the brake test interval and additionally directly after each emergency stop. In the case of the integrated holding brake, one can assume a category 2 structure according to EN ISO 13849-1 if the test frequency is correspondingly. With the help of the category, the diagnostic coverage and the MTTFD value, one can then determine the corresponding PFHD value by consulting Appendix K of the EN ISO 13849-1. * See Configuration Manual for number of emergency stops
 Entry ID:48182838 Date:2011-01-28
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