Assembly Line Balancing The process of equalizing the amount of work at each work station on an assembly line.

How to Balance a Line Specify the task relationships and their order of precedence. Draw and label a precedence diagram. Calculate the desired cycle time (Cd). Calculate the theoretical minimum number of workstations (N). Group elements into workstations recognizing cycle time & precedence. Evaluate the efficiency of the line (E). Repeat until desired line efficiency is reached. Specify task relationships and order of precedence – this determines what order the tasks need to be done in to complete one product. Draw and label precedence diagram – this creates a visual layout of the process. Work elements are usually represented by circles or nodes and connected by arrows to show precedence. Desired cycle time – calculates the maximum amount of time the product can stay at a workstation in order to meet a targeted production amount. Theoretical minimum number of work stations –the sum of the task times divided by the desired cycle time. Group elements into work stations – this is the trial and error portion of the process, begin arranging the tasks into workstations to determine the most efficient line. Evaluate efficiency – this calculates the sum of the task times divided by the number of workstations times the actual cycle time. Repeat – if you do not get a line with your desired efficiency the process must be repeated. Next few slides will run audience through how to balance a line by hand.

Order of Precedence Joe’s Sub Shop Specify the task relationships and their order of precedence. Joe’s Sub Shop Task Work Element Precedence Time (min) A Receive Order — 2 B Cut Bread 1 C Prepare Toppings D Assemble Sandwich B,C 3 E Wrap Sandwich F Deliver Sandwich This is an example of a precedence chart for making a sub sandwich. Each step in the process of making a sandwich is listed out, then task precedence is determined, in addition to the time it takes to complete each task. Next you will see how to draw a precedence diagram.

The Precedence Diagram Draw and label a precedence diagram. B 1 min A 2 min Here is the start to the precedence diagram. Each task is labeled then connected to each preceding task.

The Precedence Diagram Draw and label a precedence diagram. B 1 min D 3 min E 1 min F 3 min A 2 min This shows the completed precedence diagram for a sub sandwich. C 2 min

Cycle Time Calculate the desired cycle time (Cd). Cd = Cd = If Joe’s Sub Shop has a demand of 100 sandwiches per day. The day shift lasts 8 hours. Cd = production time available desired units of output Next the desired cycle time must be calculated. To calculate desired cycle time, you divide the production time available by the number of units desired. If Joe’s Sub Shop has a demand of 100 sub sandwiches per day, and the day shift is 8 hours long, what is the desired cycle time? The desired cycle time is 4.8 minutes. This means that the sandwich can only stay 4.8 minutes at each workstation in order to meet Joe’s demand of 100 sandwiches per day. Cd = 8 hours x 60 minutes/hour 100 sandwiches Cd = 4.8 minutes

 ti Minimum Work Stations j Calculate the theoretical minimum number of workstations (N). If Cd = 4.8 minutes N =  ti Cd j ti = completion time for task i j = number of tasks Cd = desired cycle time Next the theoretical minimum number of workstations must be calculated. If Joe’s Sub Shop has a desired cycle time of 4.8 minutes, what is the theoretical minimum number of workstations possible? i =1

 ti Minimum Work Stations j Calculate the theoretical minimum number of workstations (N). If Cd = 4.8 minutes N =  ti Cd j N = 2 + 1 + 2 + 3 + 1 + 3 4.8 The theoretical minimum number of workstations is 2.5. This means that the sandwich can can theoretically have 2.5 workstations in order to meet Joe’s demand of 100 sandwiches per day. Since it is not possible to have half of a work station, we will round this up to 3 workstations. i =1 N = 2.5 workstations  3 workstations

Workstation Time (min) Order Work Stations Group elements into workstations recognizing cycle time & precedence. Joe’s Sub Shop Workstation Task Element Time (min) Workstation Time (min) 1 A 2 3 B C D 4 E F Here is a completed chart of possible workstations. Notice that no workstation time exceeds the desired cycle time of 4.8 minutes. Now lets calculate the efficiency of the line.

 ti Line Efficiency j Evaluate the efficiency of the line (E). nCa If Ca = 4 minutes and n = 4 work stations. E =  ti nCa j ti = completion time for task i j = number of tasks Ca = actual cycle time n = actual number of workstations This is the equation for calculating the assembly line efficiency. It is the sum of the task times divided by, the number of workstations times the actual cycle time. The actual cycle time is the maximum workstation time on the line. The actual cycle time for this problem is 4 minutes. i =1

 ti Line Efficiency j Evaluate the efficiency of the line (E). nCa If Ca = 4 minutes and n = 4 work stations. E =  ti nCa j E = 2 + 1 + 2 + 3 + 1 + 3 4 * 4 The efficiency of the assembly line, using this group of workstations, is 75.0%. i =1 E = 75.0% effective

Workstation Time (min) Trial and Error Repeat until desired line efficiency is reached. Joe’s Sub Shop Workstation Task Element Time (min) Workstation Time (min) 1 A 2 4 C B D 3 E F Because Joe’s Sub Shop wants a higher rate of efficiency than 75.0%, we must continue to try different combinations of workstations until a desired line efficiency is reached. (click to 2nd part of slide) Notice how task B was moved from workstation 1 to workstation 2 and C was moved to workstation 1. (Click to next part of slide) This combination is 100% effective. Now let’s try working a simple problem together. E = 100.0% effective

Class Exercise A sample precedence chart A — 3 B 5 C 2 D B,C 4 E Task Time (min) A — 3 B 5 C 2 D B,C 4 E This is a sample precedence chart. We are going to balance this line by hand. Write this chart down in your notes.

Class Exercise Draw and label a precedence diagram. A 3 min B 5 min Here is the start of the precedence diagram. Now you try to draw the rest of the diagram. (Allow time for audience to complete diagram)

Class Exercise Draw and label a precedence diagram. A 3 min B 5 min D Here is the completed precedence diagram. Is this what you came up with? Now we need to calculate the desired cycle time and the theoretical minimum number of workstations. C 2 min

Class Exercise Calculate the desired cycle time (Cd). Cd = Cd = If, there is a demand for 100 units to be produced every 12 hours. Cd = production time available desired units of output Now calculate the desired cycle time if there is a demand for 100 units to be produced every 12 hours. (Allow time for the audience to calculate) The desired cycle time is 7.2 minutes. If you got 0.12 as an answer you forgot to convert the number of available hours into minutes. Cd = 12 hours x 60 minutes/hour 100 units Cd = 7.2 minutes

Class Exercise Calculate the theoretical minimum number of workstations (N). If Cd = 7.2 minutes N =  ti Cd j ti = completion time for task i j = number of tasks Cd = desired cycle time Next the theoretical minimum number of workstations must be calculated. If the desired cycle time of 7.2 minutes, what is the theoretical minimum number of workstations possible? i =1

Class Exercise Calculate the theoretical minimum number of workstations (N). If Cd = 7.2 minutes N =  ti Cd j N = 2 + 5 + 2 + 4 + 2 7.2 The theoretical minimum number of workstations is 2.08. Since it is not possible to have part of a work station, we will round this up to 3 workstations. Now comes the fun part… i =1 N = 2.08 workstations  3 workstations

Workstation Time (min) Class Exercise Group elements into workstations recognizing cycle time & precedence. Workstation Task Element Time (min) Workstation Time (min) ? A 3 B 5 C 2 D 4 E Now try to fill in the number of work stations and the time for each workstation. Remember that no workstations time can exceed 7.2 minutes but you can change the order of the work elements as long as you keep precedence in mind. (Give the audience plenty of time to try and figure this out)

 ti Class Exercise j Evaluate the efficiency of the line (E). nCa ti = completion time for task i j = number of tasks Ca = actual cycle time n = actual number of workstations Now calculate the efficiency of the line you created. Remember, the actual cycle time is the maximum workstation time on the line. (Give audience time to calculate) What did everyone come up with? i =1

Workstation Time (min) Class Exercise The most efficient set up of the line Workstation Task Element Time (min) Workstation Time (min) 1 A 3 5 C 2 B D 4 6 E This is the most efficient set up of the line. E = 83.3% effective