COMSOAL
Computer Method for
Sequencing Operations for
Assembly Lines
Lindsay McClintock
OPERMGT 345 – 004
May 6, 2003
Today’s Topics

Assembly Line Balancing


By Hand

Overview

Example

Exercise
By Computer
 Using
COMSOAL
Assembly Line Balancing

The process of equalizing the
amount of work at each work
station on an assembly line.
How to Balance a Line

Specify the task relationships and their order
of precedence.

Draw and label a precedence diagram.

Calculate the desired cycle time (Cd).

Calculate the theoretical minimum number of
workstations (N).

Group elements into workstations recognizing
cycle time & precedence.

Evaluate the efficiency of the line (E).

Repeat until desired line efficiency is reached.
Order of Precedence

Specify the task relationships and their
order of precedence.
Joe’s Sub Shop
Task Work Element
Precedence Time (min)
A
Receive Order
—
2
B
Cut Bread
A
1
C
Prepare Toppings
A
2
D
Assemble Sandwich
B,C
3
E
Wrap Sandwich
D
1
F
Deliver Sandwich
E
3
The Precedence Diagram

Draw and label a precedence diagram.
B
1 min
A
2 min
The Precedence Diagram

Draw and label a precedence diagram.
B
1 min
D
3 min
A
2 min
C
2 min
E
1 min
F
3 min
Cycle Time

Calculate the desired cycle time (Cd).


If Joe’s Sub Shop has a demand of 100
sandwiches per day.
The day shift lasts 8 hours.
Cd =
Cd =
production time available
desired units of output
8 hours x 60 minutes/hour
100 sandwiches
Cd = 4.8 minutes
Minimum Work Stations

Calculate the theoretical minimum number
of workstations (N).

If Cd = 4.8 minutes
j
N=
 ti
i =1
Cd
ti = completion time for
task i
j = number of tasks
Cd = desired cycle time
Minimum Work Stations

Calculate the theoretical minimum number
of workstations (N).

If Cd = 4.8 minutes
j
N=
 ti
i =1
N=
2+1+2+3+1+3
4.8
Cd
N = 2.5 workstations
 3 workstations
Order Work Stations

Group elements into workstations
recognizing cycle time & precedence.
Joe’s Sub Shop
Workstation
Task
Element
Time (min)
Workstation
Time (min)
1
A
2
3
B
1
2
C
2
2
3
D
3
4
E
1
F
3
4
3
Line Efficiency

Evaluate the efficiency of the line (E).

If Ca = 4 minutes and n = 4 work stations.
j
E=
 ti
i =1
nCa
ti = completion time for
task i
j = number of tasks
Ca = actual cycle time
n = actual number of
workstations
Line Efficiency

Evaluate the efficiency of the line (E).

If Ca = 4 minutes and n = 4 work stations.
j
E=
 ti
i =1
E=
2+1+2+3+1+3
4*4
nCa
E = 75.0% effective
Trial and Error

Repeat until desired line efficiency is reached.
Joe’s Sub Shop
Workstation
Task
Element Time
(min)
Workstation
Time (min)
1
A
2
4
C
2
B
1
D
3
E
1
F
3
2
3
E = 100.0% effective
4
4
An Exercise

A sample precedence chart
Task
Precedence
Time (min)
A
—
3
B
A
5
C
—
2
D
B,C
4
E
D
2
An Exercise

Draw and label a precedence diagram.
A
3 min
B
5 min
An Exercise

Draw and label a precedence diagram.
A
3 min
B
5 min
D
4 min
C
2 min
E
2 min
An Exercise

Calculate the desired cycle time (Cd).

If, there is a demand for 100 units to be
produced every 12 hours.
Cd =
Cd =
production time available
desired units of output
12 hours x 60 minutes/hour
Cd = 7.2 minutes
100 units
An Exercise

Calculate the theoretical minimum number
of workstations (N).

If Cd = 7.2 minutes
j
N=
 ti
i =1
Cd
ti = completion time for
task i
j = number of tasks
Cd = desired cycle time
An Exercise

Calculate the theoretical minimum number
of workstations (N).

If Cd = 7.2 minutes
j
N=
 ti
i =1
N=
2+5+2+4+2
7.2
Cd
N = 2.08 workstations
 3 workstations
An Exercise

Group elements into workstations
recognizing cycle time & precedence.
Workstation
Task
Element
Time (min)
Workstation
Time (min)
?
A
3
?
?
B
5
?
?
C
2
?
?
D
4
?
?
E
2
?
An Exercise

Evaluate the efficiency of the line (E).
j
E=
 ti
i =1
nCa
ti = completion time for
task i
j = number of tasks
Ca = actual cycle time
n = actual number of
workstations
An Exercise

The most efficient set up of the line
Workstation
Task
Element
Time (min)
Workstation
Time (min)
1
A
C
B
D
3
2
5
4
5
E
2
2
3
E = 83.3% effective
5
6
The Real World

A real world precedence chart
Task
Precedence
Time (min)
A
—
3.25
B
A
4.50
C
—
12.00
D
B,C
1.25
E
D
5.00
F
A
0.50
G
C
1.50
H
D,F,G
25.50
I
H
3.25
J
I
6.00
K
A,G
1.25
COMSOAL



Computer Method for
Sequencing Operations for
Assembly Lines
Developed by IBM
Fast and Easy
How it Works

5 Common Heuristics Used

Ranked positional weight

Longest operation time (LOT)

Shortest operation time (SHOT)

Most number of following tasks

Least number of following tasks
How it Works

The COMSOAL program proceeds in 6
steps as follows:

STEP 1: For each task, identify those
tasks which immediately follow it in
precedence order.

STEP 2: Place in LIST A for each task in
the assembly, the total number of tasks
which immediately precede it in the
precedence diagram.

STEP 3: From LIST A, create LIST B
composed of the tasks which have zero
predecessors. If no task remain
unassigned to stations, then stop.
How it Works (con’t)

STEP 4: From LIST B, create LIST C
composed of the tasks whose performance
times are no greater than the available time
at the station. If LIST C is empty, open a
new station with the full cycle time available
and go through STEP 4 again.

STEP 5: Randomly select from LIST C a
task for assignment to the station.

STEP 6: Update the time available at the
station and LIST B to reflect the time
consumed and the completed predecessors
at this stage. If LIST B is empty update
LIST A and return to STEP 3 otherwise
return to STEP 4.
Why COMSOAL?



Simplifies complex assembly line
balancing problems
Faster, easier, and more accurate than
calculating by hand
Saves time and money
References


Russell, Roberta S. and Bernard W. Taylor
III. Operations Management. 4th ed. New
Jersey: Prentice Hall, 2003.
Graves, Robert, Dr. “Perspectives on
Material Handling Practice.”
http://www.mhia.org/bs/pdf/75021.pdf