Assembly line balancing
An assembly line consists of a series of workstations each with a uniform time interval that is referred to as a takt time (which is also the time between successive units coming off the end of the line) At each workstation work is performed on a product by adding parts and/or by completing an assembly operation. The work performed at each station is made up of many tasks (also referred to as elements, or work units). Such tasks are described by in option-time analysis. Generally, they are groupings that cannot be subdivided on the assembly line without paying a high penalty in extra motions. The total work to be performed at a workstation is equal to the sum of the tasks assigned to that workstation. The assembly line balancing problem is one of assigning all of the tasks required to a series of workstations so that the time required to do the work at each station does not exceed the takt time, and at the same time. the unassigned (i.e., idle) time across all workstations is minimized. An additional consideration in designing the line is to assign the tasks as equitably as possible to the stations.
The problem is further complicated by the relationships among tasks imposed by product design and process technologies. This is called the precedence relationship which specifies the order in which the tasks must be performed in the assembly process.
Steps in assembling line balancing.
The sequence of steps required to balance an assembly line is straightforward:
I. Specify the sequential relationship among tasks using a precedence diagram. The diagram consists of circles and arrows. Circles represent individual tasks; arrows indicate the order of task performance.
2. Determine the required takt time (T). using the following formula: Production time per day T = Output per day (in units)
3. Determine the theoretical minimum number of workstations (NI) required to satisfy the takt time constraint using the following formula:
Sum of task times (S)
N =--------
I Takt time (T)
4. Select a primary rule by which task are to he assigned to workstations, and a secondary rule to break ties.
5. Assign tasks. one at a time. to the fir~t workstation until the ~UI11of the task times is equal to the takt time, or no other tasks are feasible because of time or sequence restrictions.
1.Repeat the process for Workstation
2. Work-ration
3. and so on. until all tasks are assigned.
6. Evaluate the efficiency of the resulting assembly line using the following formula:
Sum of ta-k times (S) Efficiency = ~-~--:---=---:----:----------
Actual number of workstations (N,,) '< Takt time (T)
7. If efficiency is unsatisfactory, rebalance the hue using a Jltkrent decision rule.
A toy company produces a Model J Wagon that is to be assembled on a conveyor belt. Five hundred wagons are required per day. The company is currently operating on a one-shift, eight-hour-a-day schedule, with one hour off for lunch (i.e., net production time per day is seven hours). The assembly steps and times for the wagon are given in Exhibit 8.10. Assignment: Find the balance that minimizes the number of workstations subject to takt time and precedence constraints..
