A queue is the number of hours that a work order sits in front of a work center before it starts. It also

includes the move time required to physically move partially completed items from the previous

operation to the next work center1. A queue is used to provide a buffer between the time that a

component arrives at a machine or work center and when the operation actually starts2. A queue

can help to neutralize delays in previous operations and to reduce the waiting time and flow time of

the work orders3.

An order that moves into a work center on schedule following completion of a previous operation

will move into the queue in priority sequence. This means that the order will be placed in the queue

according to its priority level, which is determined by the priority control technique used for the

production system. A priority control technique is a method of determining the sequence and timing

of production orders in a manufacturing system4. Some examples of priority control techniques are

first-come, first-served (FCFS), shortest processing time (SPT), earliest due date (EDD), and critical

ratio (CR). Depending on the priority control technique, an order may move to the beginning, end, or

middle of the queue.

An order will not necessarily move to the end or the beginning of the queue, as these positions

depend on the priority level of the order and the other orders in the queue. An order will not move

into the queue when there is capacity available, as this implies that there is no queue at all. Capacity

is the amount of time or resources available for production at a work center5. Capacity can be

affected by factors such as machine availability, labor skills, setup time, and maintenance. Capacity

planning is a function of determining and adjusting the optimal level of resources needed to meet

the demand5.