DescriptionModule 6 – Layout Decisions (Lesson: Assembly-Line Balancing)

Read pages 393-396

Developing a precedence diagram for an assembly line

Cycle time = operation time to complete customer demands.

Formula:

Cycle time = Production time available per day

Units required per day

Theoretical minimum number of workstations = Sum of all Tasks time

Cycle time

Example 1:

Problem 1: Boeing wants to develop a precedence diagram for an electrostatic wing component

that requires a total assembly time of 66 minutes. The staff gather tasks, assembly times, and

sequence requirements for the component as shown below. Boeing determines that there are 480

productive minutes of work available per day. The production schedule requires that 40 units of the

wing component be completed as output from the assembly line each day. It now wants to group

the tasks into workstations.

Task

A

B

C

D

E

F

G

H

I

Assembly Time (minutes)

10

11

5

4

12

3

7

11

3

66

Immediate Predecessor

A

B

B

A

C, D

F

E

G, H

Question 1. DRAW THE PRECEDENCE DIAGRAM.

Question 2. Compute the cycle time.

Cycle time = Production time available per day

Units required per day

= 480 minutes

40 units

= 12 minutes/unit

Question 3. Compute the theoretical minimum number of workstations.

Theoretical minimum number of workstations = Total task time

Cycle time

= 66

12

= 5.5 or 6 stations

The division of workstations is shown in Figure 9.14, page 396,

Question 4. Determine the line efficiency (page 397).

Efficiency =

Total task times

(Actual number of workstations) x (Largest assigned cycle time)

Efficiency =

66 minutes

(6 stations) x (12 minutes)

= 66 x 100

72

= 91.7%

NOTE: Opening a seventh workstation, for whatever reason,

would decrease the efficiency of the balance to 78.6%

(assuming that at least one of the workstations

still required 12 minutes):

Efficiency =

66 minutes

(7 stations) x (12 minutes)

= 66 x 100

84

= 78.6%

Example 2:

Problem 2: Yanbu appliances wants to establish an assembly line to manufacture its new product,

the Mini-Me Microwave Oven. The assembly line will operate 7.5 hours per day. The goal is to

produce 50 ovens per day. The tasks, task time, and immediate predecessor for producing 1 oven

are as follows.

Task

A

B

C

D

E

F

Assembly Time (minutes)

5

4

4

4

4

4

Immediate Predecessor

A

A, B

B, C

C

D, E

Question 1. DRAW THE PRECEDENCE DIAGRAM.

Question 2. Compute the cycle time.

Question 3. Compute the theoretical minimum number of workstations.

Question 4. Determine the line efficiency.

Module 6 – Layout Decisions (Lesson: Work Cell)

Read Figure 9.10 – Improving Layouts by Moving to the Work Cell Concept on page 389

and page 390.

Staffing and Balancing Work Cells

Takt time = pace of production to meet customer demands.

Cycle time = operation time to complete customer demands.

Lead time = time from customer’s request of orders up to delivery.

Formula:

Takt time = Total work time available/Units required

Workers required or Staff needed = Total operation time required/Takt time

Example 1:

Problem 1: Stephen Hall’s company in Dayton makes auto mirrors. The major customer is the

Honda plant nearby. Honda expects 600 mirrors delivered daily, and the work cell producing the

mirrors is scheduled for 8 hours. Hall wants to determine the takt time and the number of workers

required.

Solution 1:

Takt time = 8 hours x 60 minutes x 60 seconds = 28,800 seconds = 48 seconds

1 hour

1 minute

600 units

unit

THIS MEANS: Customer requirement is 1 mirror every 48 seconds.

Example 2:

In relation to problem 1, additional data is given below. These are operations time.

Type of Operation

Assemble

Paint

Test

Label

Pack for shipping

Time required (in seconds)

50

45

10

20

15

140

Solution 2:

Workers required = Total operation time required/Takt time

= 140

48

= 2.92

THIS MEANS: To produce 1 unit every 48 seconds will require 2.92 people.

THEREFORE, with 3 workers, they can produce 617 units per day. How?

Takt time per 3 workers = 140

3

= 46.67 seconds

Takt time = Total work time available

Units required

46.67 seconds = 28,800 seconds

Units required

Units required = 28,800 seconds

46.67 seconds

= 617.09

= or 617

Problem 3

If testing time is expanded to 20 seconds, what is the staffing requirement? [Answer: 3.125

employees]

What is the Takt time if 3 workers? [Answer: 50 seconds]

What is the number of Units required if 3 workers? [Answer: 576]

What is the Takt time if 4 workers? [Answer: 37.5 seconds]

What is the number of Units required if 4 workers? [Answer: 768]

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