A production line is typically associated with continuous or flow production system. Production lines are particularly appropriate for high volume operations. In a production line work is divided into individual tasks and assigned to consecutive workstations on the line. In mass production on progressive assembly line the workload between various machines or workstations should be balanced. The need for balancing the line becomes obvious when it is considered that the output to be received from the line is determined by the maximum time involved in the performance of work at one particular workstation. The imbalances existing in the line would lead to wastage of time at all other work stations when one work station holds up the total output rate. Therefore, it is necessary to level out or balance the cycle times at each workstation.
Line balancing refers to the apportionment of sequential work activities into workstations in order to achieve maximum possible utilization of facilities and to minimize idle time. In case of wholly automated operations, line balancing is largely achieved through engineering design. In other cases balancing of equipment capacities poses a problem. If the time requirements at one workstation are very large in comparison with other stations, the tasks at the station may have to be further subdivided or additional personnel may be added to the station. Alternatively, a parallel section may be provided so that two or more units may be worked on simultaneously. The speed of an assembly line is determined by the desired rate of output, spacing of products on the line, time requirements of workstations, and pace considerations appropriate to the workers.
Many simulation and heuristic models are used for balancing assuming that workers have constant operation times. These models do not necessary result in mathematically provable optimal balances. But reasonably good solution can be found. Let us describe certain terms used in these techniques.
- Task: Task is the natural minimum element of work beyond which assembly work cannot be divided rationally without creating unnecessary work. The tasks are usually denoted as Ui.
- Total work content: This is the aggregate amount ofwork of the total assembly. Thus if t i denotes the performance time of the i’th task, the sum of the performance time is the total work content if N represents the total number of tasks.
- Station work content: This is the time required to perform the work content at the given station. This is also called operation time.
- Cycle time: This is the maximum operation time and determines the rate of output.
- Balance delay time: It is the amount of idle time on the line due to the imbalance, if any. Balance delay is the ratio between the total idle time and the total time spent by the product in moving from the beginning to the end of the line.
The balancing restrictions are constraints imposed on the order or time sequence in which work elements have to be performed and these arise from technological precedence relationships or zoning constraints or due to the nature of the tasks. Balancing delay of a line (d) can be expressed mathematically as a ratio:
D = NC – [ T/NC ]
- D is the balance delay ratio
- N is the number of work stations
- C is the maximum operation time, and
- T is the total work content.
NC may be minimized by (a) working from a given production rate and equivalently a cycle time ‘C’ to determine the lowest vale of ‘N’ of (b) working from a given number of work stations and equivalently by a given value of ‘N’ to determine the lowest value of ‘C’ the operation time to maximize production rate consistent with the time and ordering constraints.
Line balancing requires accounting for operator time variability and grouping work activities so that they most efficiently balance the production line. Simulation has proved to be a useful technique for studying the efforts of variable performance times. It reveals worker idle time, waiting time of parts, length of waiting line and average output.
Heuristic methods are widely used for grouping assembly line activities into the optimum number of work stations. One heuristic method of balancing involves drawing a precedence diagram complete with activity times and then grouping the activities into work station zones that do not exceed the specified time availability per station. Assuming that activities may be combined within a given zone so long as precedence relationships are maintained, the work zones can be designated on the precedence diagram. Then appropriate components into preceding zones are moved until there is maximum possible use of the times.
In order to balance the production line grouping of work activities may become necessary. Line balancing activities are usually undertaken to meet a specified output. For example, if a conveyor speed is 4 feet per minute and units are placed at 4 feet intervals, the output rate will be one unit per minute. Each workstation will have one minute of available time. If the output is to be two units per minute either the conveyor speed should be doubled or a 2 feet spacing should be used. In both the cases the time per unit available at each workstation would be reduced to half a minute. In order to produce at a specified rate the assembly sequence must be carefully delineated and the time requirements for each assembly task must be known. An efficient balance among the activities will complete the required work while maintaining the specified sequence and minimizing the idle time.
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