Assembly Line Balancing

1
Assembly Line Balancing

• Jaime Joo
• MBA 530 Section 1
• Brigham Young University

2
Outline

• What is Assembly Line Balancing?
• How can Assembly Line Balancing benefit your
  operations?
• Classic approach to ALB
• Lets practice!
• ALB in the real world
• Conclusions

3
What is Assembly Line Balancing (ALB)?

• ALB is the procedure to assign tasks to
  workstations so that
• Precedence relationship is complied with
• No workstation takes more than the cycle time to
  complete
• Operational idle time is minimized

4
How can Assembly Line Balancing benefit your
operations?

• A balanced line
• Promotes one piece flow
• Avoids excessive work load in some stages
  (overburden)
• Minimizes wastes (over-processing, inventory,
  waiting, rework, transportation, motion)
• Reduces variation

5
Unbalanced Line
!?
Zzz
Zzz
?
10 sec
40 sec!
20 sec
15 sec
Overproduction! Generates waste
Undesirable waiting
6
Balanced Line
?
?
?
?
25 sec
25 sec
20 sec
15 sec

• Promotes one piece flow
• Avoids overburden
• Minimizes wastes
• Reduces variation

7
Line Balancing prerequisites

• Prior to balancing a line we must
• Determine the required workstation cycle time (or
  TAKT time), matching the pace of the
  manufacturing process to customer demand
• Standardize the process

8
Classic approach to ALB

• Also known as SALBP (Simple Assembly Line
  Balancing Problem), the classic approach to ALB
  is an heuristic process to optimize assembly
  lines simplifying the problem to a basic level of
  complexity

Dubbed SALBP by Becker and Scholl (2004)
9
Example

• The next table shows the tasks performed in a
  production line. Our goal is to combine them into
  workstations. The assembly line operates 8 hours
  per day and the expected customer demand is 1000
  units per day. Balance the line and calculate the
  efficiency and theoretical minimum number of
  workstations.

10
Example (cont.)
Task Task Time (sec) Preceding Task
A 13 -
B 11 A
C 15 A
D 20 B
E 12 B
F 13 C
G 13 C
H 18 D, E
I 17 F, G
J 15 H, I
K 9 J
Total Time 156
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Example (cont.)

• Step 1 Draw a precedence diagram according to
  the given sequential relationship

20 sec
D
11 sec
18 sec
B
H
12 sec
13 sec
9 sec
15 sec
E
A
K
J
13 sec
17 sec
F
15 sec
I
C
13 sec
G
12
Example (cont.)

• Step 2 Determine Takt time or Workstation Cycle
  Time
• CProduction time per day / Customer demand (or
  output per day)
• C 28800 sec (8 hours) / 1000 units 28.8
• Step 3 Determine the theoretical number of
  workstations required
• N Total Task Time / Takt time
• N 156 / 28.8 5.42 (6 workstations)

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Example (cont.)

• Step 4 Define your assignment rules. For this
  example our primary rule will be number of
  following tasks and the secondary rule will be
  longest operation time

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Example (cont.)

• Step 5 Assign tasks to workstations following
  the assignment rules and meeting precedence and
  cycle time requirements
• To form Workstation 1

11 sec
13 sec
B
Following tasks 5
A
Lot 15gt11!
15 sec
C
Following tasks 5
WS1 AC28 sec Cycle Time met!
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Example (cont.)

• Forming Workstation 2

BDgtCycle time!
11 sec
13 sec
B
LOT_FGgtE
A
15 sec
C
WS2 Operation time24 sec (ltC)
Arbitrarily choose F
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Example (cont.)

• Following the same criteria we achieve our
  balancing with 7 workstations

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Example (cont.)

• Step 6 Calculate Efficiency
• Efficiency Total Task Time / (Actual number of
  workstations Takt Time)
• Efficiency 156 / (728.8) 77
• How to interpret this efficiency?
• Is this the best efficiency achievable?

18
Lets Practice

• We have found a new market for our product. This
  market is less demanding so we have decided not
  to include a particular feature, specifically the
  feature added by task I. As a consequence, task
  time in F drops to 5 seconds and task time in G
  drops to 8 seconds. Balance the line according to
  the other conditions.

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Lets practice (cont.)
Task Task Time (sec) Preceding Task
A 13 -
B 11 A
C 15 A
D 20 B
E 12 B
F 13 5 C
G 13 8 C
H 18 D, E
I 17 F, G
J (new I) 15 H, F, G
K (new J) 9 I
Total Time 156 126
20
Lets practice (cont.)

• Lets take some time to solve this new problem.
  This time we will calculate keeping the primary
  and secondary rules as in the original problem.

21
Lets practice (solution)

• Precedence diagram

20 sec
D
11 sec
18 sec
B
H
12 sec
13 sec
9 sec
15 sec
E
A
J
I
5 sec
F
15 sec
C
8 sec
Previously J K respectively
G
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Lets practice (solution)

• Takt time
• C 28,800 sec / 1000 units 28.8
• Theoretical number of workstations
• N 126/28.8 4.38 (5 workstations)
• Primary rule number of following tasks
• Secondary rule longest operation time

23
Lets practice (solution)

• Following the rules and observing cycle time and
  precedence we obtain

24
Lets practice (solution)

• Efficiency 126/(628.8) 73
• Challenge Is this the best efficiency
  achievable? Try to solve with LOT as the primary
  rule and you will obtain a 5 workstations
  balance, increasing efficiency to 87

25
ALB in the real world

• The simple ALB problem approach is limited by
  some constraints
• Balance on existing and operating lines
• Workstations have spatial constraints
• Some workstations cannot be eliminated
• Need to smooth workload among workstations
• Multiple operators per workstation
• Different paces among operators, different lead
  times within the same workstation

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ALB in the real world (cont.)

• Operator spatial constraints
• Different workstation imposed working positions
• More than one task to be performed in what should
  be the space for one task
• Multiple Products
• Coping with different products, some operations
  are needed for some products but not for others
• Some products can introduce peak times in some
  workstations
• Different task times performed in different
  shifts
• Particularly when introducing new employees or
  workers with some degree of incapacity

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Conclusion

• Simply Assembly Line Balancing is a valid method
  to optimize assembly lines. However, many
  variables found in real operating lines increase
  the complexity of the problem. More complex
  algorithms have been developed to solve the
  difficult task of balancing large scale
  industrial lines. Some of them are commercially
  available in software.

28
References

• F. Robert Jacobs Richard B. Chase Operations
  and Supply Management, The Core
  McGraw-Hill/Irwin First Edition
• Emanuel Falkenauer Line Balancing in the real
  world Optimal Design
• Paul Swift. http//www.beyondlean.com/line-balanci
  ng.html