Assembly Line Balancing

What is assembly line balancing?

Assembly line balancing is the assignment of work to stations in a production line so as to achieve the desired output rate (Takt Time) with the smallest number of workstations.  Normally, one worker is assigned to a station.  Thus the line that produces at the desired pace with the fewest workers is the most efficient one.  Assembly line balancing must be performed when a line is initially set up, when a line is rebalanced to change its hourly rate, or when product/process changes via engineering change order.  The goal is to obtain workstations with well-balanced workloads.

The analysis starts with separating the work into work elements, the smallest unit or work that can be performed independently.  Once the work elements are identified, a time study is used to determine the work time standard.

Line Balancing Steps:

  1. Identify Task Times and Precedence Relationships
  2. Determine Cycle Time
  3. Determine Theoretical Minimum Number of Workstations
  4. Assign Tasks to Workstations
  5. Compute Efficiency

Task List with Time and Precedence Diagram:  Most lines must satisfy some technological precedence requirement or order of assembly.  However, most lines also allow for some latitude and more than one sequence of operations.  To help visualize immediate predecessors better, constructing a precedence diagram.

identifytasktimesprecedencediagram

Cycle Time: After determining the desired output rate for an assembly line, the analyst can calculate the cycle time.  The cycle time is the maximum time allowed for work on a unit at each station.  If  the time required for work elements at a station exceeds the cycle time, the station will be a bottleneck, preventing the line from reaching its desired output rate.

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Theoretical Minimum:  To achieve the desired output rate, managers use line balancing to assign every work element to a station, making sure to satisfy all precedence requirements and to minimize the number of stations formed.  If each station is operated by a different worker, minimizing the number of workstations maximizes the efficiency of the worker.  Perfect balance is achieved when the sum of the work elements times at each station equals the cycle time and no station has any idle time.

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Efficiency: The ratio of productive time to total time, expressed as a percentage.

efficiency