MTBF

The mean number of hours between two consecutive system failures in a long run. MTBF (mean time between failures) includes both operational and downtime hours between two consecutive failures, and it is the average distance in hours between two failures in a long run. On the Advanced page in the RBD calculation properties, you specify the run length for Number of system failures to reach steady state.

MTTF

The expected or average time to failure in hours. For repairable systems, MTTF (mean time to failure) is the average time to reach the first system failure. Hence, it is also called MTTFF (mean time to first failure). If you select Account for repair in reliability on the General page in the RBD calculation properties, reliability is calculated using the failure and repair information supplied in the calculation properties for blocks, and this reliability value is then used to calculate MTTF.

MTTR

MTTR (mean time to repair) is the average time it takes to repair a failed system and return it to an operational state, based on the repair time distributions for its repairable components or blocks. MTTR can be calculated by considering the frequency of each failure scenario and its respective average repair times.

Optimal number of spares

The number of spares that minimizes or maximizes a specified objective function or the goal. If the goal is minimizing the total cost, then the optimal number of spares represents the number of spares corresponding to the minimum total cost. Similarly, if the goal is maximizing the mean availability, then the optimal number of spares is the number of spares that maximizes the mean availability. The optimal number of spares is calculated by evaluating the function specified in the goal with varying number of spares. For example, if the goal is minimizing the total cost, then the total cost is calculated for different numbers of spares. The optimal number of spares corresponds to the minimum value of the total cost.

Optimal maintenance intervals

The value of the periodic maintenance interval that minimizes or maximizes a specified objective function or the goal. If the goal is minimizing the total cost, then it is the value of the maintenance interval corresponding to the minimum total cost. Optimal maintenance intervals are calculated using optimization algorithms integrated within the simulation engine.

Optimal repair resources

The number of the repair resources that minimizes or maximizes a specified objective function or the goal. If the goal is minimizing the total cost, then it is the number of repair resources corresponding to the minimum total cost. The optimal repair resources are calculated using optimization algorithms integrated within the simulation engine.

Reliability

The probability that a system remains operational until a specified time. Reliability is a time-based probability value, so it is always a metric between 0 and 1. A reliability of 0 means that the system always fails before the specified time or never functions until the specified time. A reliability value of 1 indicates that the system is always successful or never fails during the specified time. The system reliability value is calculated using both the system configuration specified in the block diagram and the failure distribution specified in the calculation properties of the blocks. If all blocks are in series, and all of them follow exponential failure distributions, then the system itself follows an exponential failure distribution, and the failure rate is the sum of the failure rates of its components. If you select Account for repair in reliability on the General page in the RBD calculation properties, reliability is calculated using the failure and repair information supplied in the calculation properties for blocks, and this reliability value is then used to calculate MTTF.

Unreliability

The probability that a failure occurs during a given time period.

Failure Rate

The chance of failure at a specified time given that the system is not yet failed. It also represents the expected number of system failures per unit time given that no failure has occurred in the system until the specified time. It is a time-based metric and also known as instantaneous failure rate. The failure rate function can be used to study the behavior of the system's failure over time. Furthermore, the failure rate function, (t), is an important representation in the lifetime modeling of the system due to its intuitive interpretation as the amount of risk of failure associated with the system at time t. If you select Account for repair in reliability on the General page in the RBD calculation properties, reliability is calculated using the failure and repair information supplied in the calculation properties for blocks, and this reliability value is then used to calculate failure rate.

Availability

The probability that the system is operating properly at a specified time. Availability, like reliability, is a time-based probability metric, so it is always a metric between 0 and 1. Hence, it is also called point availability or instantaneous availability. When availability is calculated without considering logistic delays, then it is called inherent availability. When logistic delays are considered in the availability calculation, then it is called operational availability. Windchill RBD can calculate either availability or operational availability in a single calculation. Availability is a function of both reliability (how quickly the system fails) and maintainability (how quickly the system is repaired). A common term, "five nines", refers to a system which has an availability of 0.99999, which means that it is operational 99.999% of the time.

Mean availability

The average availability over a specified time interval. Mean availability is the ratio of the mean total uptime to the total time over a specified interval.

Steady state availability

The availability in a long run, or as time tends to infinity. Steady state availability is the ratio of the total uptime to total time in a long run. On the Advanced page in the RBD calculation properties, you specify the run length for Number of system failures to reach steady state.

Unavailability

The probability that, at a given time, the system does not function, due to either a failure or a repair.

Total downtime

The total downtime of the system during the specified time interval.