Failure models and throughput rate of transfer lines

This paper addresses the impact of failure models on the performance of transfer lines composed of N machines separated by N − 1 buffers. The two most used failure models, operation-dependent failures (ODF) and time-dependent failures (TDF), are considered. A machine subject to TDF can fail even it is not working on a part. A machine subject to ODF cannot fail if it is not working on a part. A transfer line considered in this paper contains both machines subject to TDF and machines subject to ODF. Up times, repair times and processing times are continuous random variables of general distribution. We prove that modelling a machine subject to ODF of a transfer line as a machine subject to TDF, as usually done in the literature, leads to under-estimation of the throughput rate if the time between failures of the related machine is exponentially distributed while all other times are general random variables. We then propose a timed Petri net approach for performance evaluation and simulation of transfer lines with both machines subject to TDF and machines subject to ODF. Numerical results show that this result holds under more general conditions.