Free Quality - Line Balancing - Rex Webb

8/14/2019 Free Quality - Line Balancing - Rex Webb

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Assembly-Line Balancing:A Valuable Tool for

Increasing Efficiency

By Rex Webb


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Overview1. What is assembly-line balancing?

2. How can assembly-line balancing be used in your organization?

3. What are the steps in balancing an assembly line?4. Example of assembly-line balancing.5. Assembly-line balancing exercise.6. Summary.7. Suggested reading list.


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What is Assembly-Line Balancing?Assigning each task to a workstation within an assembly

line in order to meet the required production rate and to achieve a minimum amount of idle time.

A

H

I

F

G J K

C

E

B

D

Example of 11 tasks assigned to 5 workstations.


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Steps in Balancing an Assembly Line

1. List the sequential relationships among tasks and thendraw a precedence diagram

2. Calculate the required workstation cycle time3. Calculate the theoretical minimum number of

workstations4. Choose a primary rule that will determine how tasks

are to be assigned to workstations


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Steps in Balancing an Assembly Line

5. Beginning with the first workstation, assign each task,

one at a time, until the sum of the task times is equal tothe workstation cycle time or until no other tasks canbe assigned due to sequence or time restrictions

6. Repeat step 5 for the remaining workstations until all

the tasks have been assigned to a workstation7. Evaluate the efficiency of the line balance8. Rebalance if necessary


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Example of Assembly-Line Balancing

Scenario : The Model Z Bicycle is assembled in anassembly line. Four hundred bicycles are requiredeach day. Production time per day is 420 minutes.

Find the balance that minimizes the number of

workstations, that stays within the workstationcycle time limitation, and that complies with taskprecedent constraints.


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Business View

Task Task Time(in seconds)

Task Description Tasks thatmust precede

A 50 Connect the front tire to the bicycle frame.

B 16 Insert the handle bar. A

C 14 Tighten handle bar with two screws and nuts. B D 55 Connect the rear tire to the bicycle frame.

E 20 Position chain mechanism to the frame. D

F 17 Attach right hand brake to handle bar. C

G 17 Attach left hand brake to handle bar. C

H 17 Attach right side pedal. E

I 17 Attach left side pedal. E

J 13 Position chain onto chain mechanism. F,G,H,I

K 14 Attach seat post. J

250

Building the Model Z Bicycle: Assembly Steps and Times


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1. List the sequential relationships among tasksand then draw a precedence diagram

Tasks Tasks thatmust precede



A E D I E

B A F C J F,G,H,I

C B G C K J

D H E


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1. List the sequential relationships among tasksand then draw a precedence diagram

H

G

14 sec.

55 sec.

50 sec.

A

16 sec.

B

20 sec.

E

14 sec.

C

17 sec.

17 sec.

17 sec.

17 sec.

F

I

13 sec.

J K

D


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3. Calculate the theoretical minimumnumber of workstations

Sum Totalof Task Times 250 seconds

Number of = = = 3.97 = 4 (rounded up)workstations Cycle Time 60 seconds


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4. Choose a primary rule that will determine howtasks are to be assigned to workstations

For this example, our primary ruleis to prioritize tasks based on thelargest number of following tasks.

If there is a tie, our secondaryrule is to prioritize tasks in theorder of the longest task time.

In general, select rules thatprioritize tasks according to thelargest number of followers or based on length of time.

TaskNumber ofFollowing Tasks

A 6

B or D 5

C or E 4

F, G, H, or I 2J 1

K 0


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5. Beginning with the first workstation, assigneach task, one at a time, until the sum of the task times is equal to the workstation

cycle time or until no other tasks can beassigned due to sequence or timerestrictions

6. Repeat step 5 for the remainingworkstations until all the tasks have beenassigned to a workstation


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Precedence Graph for Model Z BicycleThe five workstations are identified by color

14 sec.

H G 55 sec.

50 sec.

A

16 sec.

B

20 sec.

E

14 sec.

C

17 sec.

17 sec.

17 sec.

17 sec.

F

I

13 sec.

J K

D


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7. Evaluate the efficiency of the line balance

Sum Totalof Task Times 250

Efficiency = = = 0.83 or 83%

Actual Number (5)(60)of WorkstationsX Cycle Time


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Assembly-Line Balancing Exercise

Based on the task information on the previous screen:

1. Draw a precedent diagram2. Determine the required cycle time to meet a demand

of 540 units per day based on nine-hour work day.3. Determine the minimum number of workstations using

the information above in number 2.4. Balance the line using the longest task time as your

primary rule5. Calculate the efficiency of your line balance.


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Exercise Solutions1. Draw a precedent diagram

32 sec.

F

H

18 sec.

33 sec.

A

16 sec.

D

40 sec.

C

11 sec.

G

30 sec.

28 sec.

25 sec. 16 sec.

I

E

42 sec.

J

K

B


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Exercise Solutions3. Determine the theoretical minimum number of workstations using

the information in the previous slides.

Sum Totalof Task Times 291 seconds

Number of = = = 4.85 = 5 (rounded up)

workstations Cycle Time 60 seconds


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4. Balance the line using the longest task time as your primary rule

Stations FeasibleTasks

Task Task Time (seconds)

RemainingUnassignedTime

Station 1 A,BB,D

AB

3318

279

Station 2 C,DD

CD

4016

204

Station 3 EF

EF

2511

3524

Station 4 G,HH,I

GH

2830

322

Station 5 I,JJ

IJ

4216

182

Station 6 K K 32 28


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Exercise Solutions5. Calculate the efficiency of your line balance.

Sum Totalof Task Times 291

Efficiency = = = 0.81 or 81%

Actual Number (6)(60)of WorkstationsX Cycle Time


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Suggested Reading List1. Bayhan, G. Mirac and Kilincci, Ozcan, A Petri Net Approach for

Simple Assembly Line Balancing Problems, International Journal of Advanced Manufacturing Technology , vol. 30, pgs. 1165-1173,1 Nov. 2006.

2. Amen, M., Cost -oriented Assembly Line Balancing: ModelFormulations, Solution Difficulty, Upper and Lower Bounds,European Journal of Operational Research , vol. 168, pgs. 747-770, 1 Feb. 2006.

3. Boysen, Nils et. al., A Classification of Assembly Line BalancingProblems, European Journal of Operational Research, vol. 183,pgs. 674-694, 1 Dec. 2007.

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Free Quality - Line Balancing - Rex Webb