If the failure rate is known, then MTBF is equal to 1 / failure rate. rate. Thus The concept of a constant failure rate says that failures can be expected to occur at equal intervals of time. MTBF is the inverse of the failure rate in the constant failure rate phase. As you may have noticed that how Failure is a function of time i.e. In the HTOL model, the In other words, the system failure rate at any mission time is equal to the steady-state failure rate when constant failure rate components are arranged in a series configuration. Most other distributions do not have a constant failure rate. The failure rate is defined as the number of failures per unit time or the proportion of the sampled units that fail before some specified time. • Steady state and useful life – Constant failure rate (λ) expressed as FIT (number of failures/1E9 hours). The constant failure rate presumption results in β = 1. This is only true for the exponential distribution. This is the useful life span of the equipment which will be the focus. As humans age, more failures occur (our bodies wear out). Note that since the component failure rates are constant, the system failure rate is constant as well. reliability predictions. 1.3 Failure Rate. If the components have identical failure rates, λ C, then: Constant Failure Rate (Random Failures): A constant failure rate is a characteristic of failures where they can happen randomly. Since this is the case, the only way to calculate MTBF so it correlates with service life would be to wait for the whole population of 25-year-olds to reach the end of their life; then the average lifespans can be calculated. Humans, like machines, don't exhibit a constant failure rate. Two important practical aspects of these failure rates are: The failure rates calculated from MIL-HDBK-217 apply to this period and to this period only. [/math], the MTTF is the inverse of the exponential distribution's constant failure rate. Failure rate = Lambda = l = f/n Equations & Calculations • Failure Rate (λ) in this model is calculated by dividing the total number of failures or rejects by the cumulative time of operation. Under these conditions, the mean time to the first failure, the mean time between failures, and the average life time are all equal. More on this later. If the components have identical failure rates, λ C, then: Because average component failure rate is constant for a given maintenance renewal concept, an overall system failure rate can be estimated by summing the average failure rates of the components that make up a system. The units used are typically hours or lifecycles. Another way to compute MTBF is using the failure rate value of a system in its “useful life” period, or the part of product lifecycle where the failure rate of the system is constant. In other words, the system failure rate at any mission time is equal to the steady-state failure rate when constant failure rate components are arranged in a series configuration. Calculation Inputs: Note that since the component failure rates are constant, the system failure rate is constant as well. The constant failure rate during the useful life (phase II) of a device is represented by the symbol lambda (l). Time) and MTTF (Mean Time to Failure) or MTBF (Mean Time between Failures) depending on type of component or system being evaluated. Things tend to fail over a period of time. Note that when [math]\gamma =0\,\! • Wear out – Characterized by increasing failure rate, but normally the onset of wear out should occur later than the target useful life of a system 1. This critical relationship between a system's MTBF and its failure rate allows a simple conversion/calculation when one of the two quantities is known and an exponential distribution (constant failure rate, i.e., no systematic failures) can be assumed. Wearout Engineering Considerations