Factory Physics Glossary
Availability: The availability of a single station is the long run percentage of time a station is up. It is defines as the ration between the mean time to repair and the sum of the mean time to failure and the mean time to repair. Click here for the mathematical formula.
Bottleneck: The station in the line with the highest long-term utilization. In simple flow lines without yield loss, where the arrival rate of jobs to every station is the same, the bottleneck is the station with the slowest average rate. However, in systems where yield loss, rework, or complex routings make the arrival rate different at different stations, the slowest station may not be the busiest. Since it is the busiest station that acts as a primary constraint on system performance, we designate the highest utilization station as the bottleneck.
Buffer: Space to store jobs waiting to be processed at a station or to be moved to another station. The size of a buffer (i.e., how many jobs it can hold) can be governed by physical limitations (e.g., available space or storage devices) or logical limitations (e.g., kanban cards).
Coefficient of Variation: A unitless measure of the variability of a random variable, defined as the ratio of the standard deviation to the mean. It is denoted by CV. Frequently one used the square of this quantity know as the squared co-efficient of variability and denoted as SCV.
Cycle Time: The average time between the release and completion of a job along a given routing.
Exponential Distribution: A particular type of random variable, characterized by being memoryless. That is, if the life of a component is exponentially distributed, then the expected time until failure is constant regardless of the age of the component.
Make-to-Order: A system in which work releases are triggered by customer orders or forecasts. A push system (e.g., material requirements planning) is make-to-order, at least in theory.
Make-to-Stock: A system in which work releases are triggered by voids in stock levels. A two-bin inventory system (i.e., a system in which two bins of a component are maintained, and a new bin is ordered each time the first one becomes empty), and most pull systems (e.g., kanban systems) are make-to-stock.
Lead Time: The time designated for a job to traverse a designated portion of the production process. Customer lead times are the times allotted to fill a customer request. Notice that lead times are management constants (i.e., set by policy), while cycle times are attributes of the system itself.
Random Variable: A repeated numeric measure that takes on unpredictable values that follow a specified distribution. Examples of random variables of interest to manufacturing systems include process times, times to failure, times to repair, yield loss fraction, and demand for a part/product during a specified time interval.
Raw Process Time: The average time for a single job to traverse a routing (line). Notice that this includes an average amount of failure time, setup time, and other non-value-added time. To calculate raw process time, we first compute the effective process time (i.e., as adjusted for failures, setups, etc.) at each station, and then add these times along the routing.
Routing: The sequence of workstations passed through by a job. Routings begin at a stockpoint (raw material or an intermediate crib) and end at a stockpoint (an intermediate crib or finished goods inventory).
Throughput: The average output of a production process (station, line, plant) per unit time. At the firm level, throughput is defined as production per unit time that is sold. However, since plants frequently do not control what is sold, a more conventional definition at the plant level is the average number of good (non-defective) parts per unit time.
Traffic Intensity: A measure of the utilization of a station. This is given by the ratio of the raw process time of the station to the average inter arrival time to the station.
Station Capacity: The capacity of a single station is defined as the long-tern rate of production if materials were always available. Note that we must account for failures, setups, and other detractions when computing capacity. Click here for an example.
Stand Alone Availability: The stand alone availability of a single station is the long run percentage of time a station is up. It is defines as the ration between the mean time to repair and the sum of the mean time to failure and the mean time to repair. Click here for the mathematical formula.
Utilization: The utilization of a station is defined as the ratio of the rate into the station and the station capacity. Click here for an example.
Variance: A measure of variability (spread) of a random variable (see any introductory statistics text for a formal definition).
WIP (Work in Process): Inventory between the start and end points of a routing.
System Classification