You have just been assigned the job of setting up an electric fan assembly line
ID: 375399 • Letter: Y
Question
You have just been assigned the job of setting up an electric fan assembly line with the following tasks and precedence relationships:
Task
Time (minutes)
Description
Predecessors
A
2.8
Assemble frame
None
B
2.1
Assemble motor housing
None
C
1.1
Mount motor housing in frame
A,B
D
0.5
Attach blade
C
E
1.8
Assemble and attach safety grill
D
F
1
Test
E
The assembly line will be running 8 hours per day and 5 days per week. The demand forecast is 500 items per week. Your goal is to minimize the number of workstations necessary to complete enough units to satisfy the demand forecast.
a) Draw the precedence diagram.
b) What is the takt time (in minutes) that we need to produce the required number of units?
c) What is the theoretical minimum number of workstations that you will need?
d) Assign the tasks to the workstations to balance the line using the longest operating time rule (meaning the takt time you calculated above).
e) What is the efficiency of the balanced line? Since you have a balanced line with workstations, the takt time will be equal to the maximum total time among your workstations.
An engineer finds an alternative way of assembling and attaching the safety grill. It would take a little longer and increase the task time of task E to 3 minutes, but it could be done right after the frame is assembled. Tasks B, C, and D could be done before or after the safety grill is attached. Task F must still be the last task in the production process.
f) Would you benefit from redesigning the production process and using the new method? Why? What is the efficiency of this line?
Task
Time (minutes)
Description
Predecessors
A
2.8
Assemble frame
None
B
2.1
Assemble motor housing
None
C
1.1
Mount motor housing in frame
A,B
D
0.5
Attach blade
C
E
1.8
Assemble and attach safety grill
D
F
1
Test
E
Explanation / Answer
Please find answer to first 5 questions : a to e.1:
Takt time
= Available hours / Demand forecast
= 8 hours/ day x 5 days per week /500 items = 40 hours/500 item = 2400 minutes/ 500 = 4.8 minutes
TAKT TIME = 4.8 MINUTES PER ITEM
Total duration of all tasks together = 2.8 + 2.1 + 1.1 + 0.5 + 1.8 +1 = 9.3 minutes
Theoretical minimum number of workstations
= Total duration of all tasks together / Takt time
= 9.3/4.8
= 1.9375 ( 2 rounded to next higher whole number )
THEORETICAL MINIMUM NUMBER OF WORKSTATIONS = 2
Assignment of tasks to workstations using longest operating time rule is being done in such a way so that cumulative duration of activities under any workstation not to exceed Takt time
WORKSTATION
TASKS
CUMULATIVE DURATION ( MINS)
IDLE TIME = 4.8 MINUTES – CUMULATIVE DURATION
1
A,C,D
2.8 + 1.1 + 0.5 + 4.4
0.4
2
B
2.1
2.7
3
E,F
1.8 + 1 = 2.8
2
Efficiency of the balanced lined
= Total duration of all tasks / ( actual number of workstations x Takt time) x 100
= 9.3 / ( 3 x 4.8 ) x 100
= 64.58%
TAKT TIME = 4.8 MINUTES PER ITEM
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