Product layouts arrange resources in sequence so the product can be made as efficiently as possible. This type of layout is used when producing a large volume of one standardized product and there is repetition of the process. In product layouts the material moves continuously and uniformly through a sequence of operations until the work is completed.
Look at Figure 5.10 and notice that there is simultaneous work going on at each workstation. This is different from having one person do all the work beginning to end. This type of process allows a large volume to be produced. When designing product layouts, the objective is to decide on the sequence of tasks to be performed by each workstation. To accomplish this we need to consider the logical order of the tasks to be performed and the time required to perform each task. Also, we need to consider the speed of the production process, which specifies how much time each workstation has to perform the assigned tasks. This entire process is called line balancing. Below are the steps that must be followed in designing product layouts:
STEP 1. IDENTIFY TASK TIMES AND PRECEDENCE RELATIONSHIPS
The first step in line balancing is to identify the tasks that must be performed in order to produce the product, their duration and the sequence in which they need to be performed. This is called a precedence relationship.
Figure 5.11 illustrates tasks, task times, and the immediately preceding task for a pizza assembly. Notice that some tasks cannot be performed until other tasks are completed. For example, the dough must be rolled before the sauce can be added.
However, sausage, pepperoni, or mushrooms are added only after the cheese. This precedence relationship is shown in diagrammatic form in Figure 5.12.
STEP 2. DETERMINE CYCLE TIME
Recall that the product being produced on the assembly line moves in a conti- nuous fashion. This means that each workstation has exactly the same amount of time to complete their assigned tasks. The maximum amount of time each station on the assembly line has to complete its assigned tasks is calledcycle time ortakt time. The actual work time assigned to each station cannot exceed the assigned time, or cycle time. Otherwise, there would not be enough time to complete the work. This is also the frequency with which each unit is produced.
Therefore, cycle time is directly related to the number of units produced.
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The total task time to produce one pizza is 245 seconds, if a single person singlehandedly carries out all the tasks from beginning to end without a break. In such a case, the maximum output in one hour would be:
Maximum outputẳ3;600seconds=hour
245seconds=hour ẳ14:7 pizzas=hour
However, if we want to produce more pizzas than that, we will have to divide the work among a number of people working simultaneously at workstations to achieve the desired output rate. The first thing to compute is the cycle time:
Cycle timeẳAmount of time allowed to complete work at each station Cycle timeẳAvailable production time perday
Desired number of units perday
Notice that the time periods given to compute cycle time are per day, but they can also be given per hour or another time period, as long as the time periods are consistent. Remember that operations run continuously for eight hours per day. This means that the numerator—available production time per day—can be assumed to be 480 minutes per day or 28,800 seconds per day.
FIGURE 5.11 Precedence relationships for pizza assembly.
Task Description Task time (sec) Preceding task
A Roll dough 60 -
B Add pizza sauce 40 A
C Sprinkle cheese 35 B
D Add sausage 25 C
E Add pepperoni 35 C
F Add mushrooms 25 C
G Shrinkwrap pizza 15 D,E,F
H Add label 10 G
Total time 245 sec
FIGURE 5.12
Precedence diagram of pizza assembly.
A B C
D
E
F
H G
25 sec
60 sec 35 sec 15 sec 23 sec
22 sec 30 sec 35sec
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The denominator—the desired number of units—is the amount we wish to produce in a certain time period. Let’s say we want to produce 300 pizzas per day. We can then compute the cycle time needed.
Cycle timeẳ28;800seconds per day
300pizzas per day ẳ96 seconds per workstation
STEP 3: DETERMINE THE THEORETICAL MINIMUM NUMBER OF WORKSTATIONS
The goal in line balancing is to assign tasks to workstations to achieve the best efficiency. It is usually helpful to have a baseline of what is theoretically the minimum possible number of station we will need, although the balanced line may have more stations. The theoretical minimum number of stations computes the number of stations needed if we were 100% efficient. Remember that when determining the number of stations, round up to the next whole number as you can never have a partial station.
Theoretical minimum no: of stationsðNị ẳSum of the task times Cycle time
The numerator—sum of the task times—is the total task time if one person was doing all the tasks. In our case it is 245 seconds. The denominator is the cycle time we wish to produce at. In our case it is 96 seconds per workstation.
Nẳ 245seconds
96seconds per stationẳ2:55 stationsẳ3 stations
STEP 4: ASSIGN TASKS TO WORKSTATIONS
We can now proceed to assign tasks to workstations. Notice that we have to follow our precedence relationships to ensure the right sequence. For example, this means that you cannot put the cheese on before adding the sauce.
As we assign tasks to workstations, following the precedence, we have to make sure that the sum of the task times at any station does not exceed the cycle time.
Otherwise, the person working at that station will not have time to complete his or her task. Also, sometimes we have a choice of tasks to assign to a station. For example, we can decide whether to put either pepperoni or mushrooms or sausage next in the sequence. There are two rules we can follow here to make it easier:
& Select the task with the longest tasks time (Longest Task Time Rule).
& Select the task with the most number of followers (Number of Followers Rule).
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These are just rules of thumb, or what we call ‘‘heuristics.’’ They will not necessarily result in the optimal solution, but will give us a good solution. In our examples we will use the longest task time rule. However, when assigning tasks in large and complex processes multiple rules are used to balance the line, all with the goal of providing the highest efficiency.
Figure 5.13 shows the assignment of tasks to workstations using the longest task time rule. Notice that the line is not balanced very well, as the last station has much idle time—73 seconds—compared to the other stations. Once we have made these initial assignments we can go back and modify the tasks or the sequence in order to balance out the workload at each station. There are many strategies that can be used. For example, the last task, placing the label, may be modified and simplified to cut down on the time. Another option might be to give additional tasks to that station that were not originally included, such as doing a quality check.
STEP 5. COMPUTE EFFICIENCY
Once we have assigned tasks to workstations on the line we need to compute our efficiency:
Efficiencyẳ Sum of task times
Number of workstationsCycle time
ẳ 245seconds
4stations96seconds=stationẳ63:8%
FIGURE 5.13 Assigning tasks to workstations.
Note: Cycle time is 96 seconds/station Station
Eligible task
Assigned
task Time
Cumulative time
Idle time
1 A A 60 60 36
B B 30 90 6
2 C C 35 35 61
D, E, F E 35 96 0
3 D, F D 25 25 71
F F 22 47 24
G G 15 62 9
4 H H 23 23 73
There isn’t enough time remaining to add more tasks to this station.
Start another station.
Selection of tasks is made using the longest task time rule.
J J
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Our efficiency could be improved with better utilization of the line. One option may be to split tasks into smaller work elements or change technology to reduce the time required. More workers also can be assigned to the bottleneck tasks, which is task A in our case. This could reduce the time it takes and allow us to balance the line more efficiently.