COMSOALComputer Method for Sequencing Operations for Assembly Lines

Lindsay McClintockOPERMGT 345 – 004May 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

Joe’s Sub ShopTask Work Element Precedence Time (min)

A Receive Order — 2B Cut Bread A 1C Prepare Toppings A 2D Assemble Sandwich B,C 3E Wrap Sandwich D 1F Deliver Sandwich E 3

Specify the task relationships and their order of precedence.

The Precedence Diagram Draw and label a precedence diagram.

A2 min

B1 min

The Precedence Diagram Draw and label a precedence diagram.

A2 min

B1 min

C2 min

D3 min

E1 min

F3 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 =production time available

desired units of output

Cd =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

N = ti

Cd

j

i =1

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

N = ti

Cd

j

i =1 N =2 + 1 + 2 + 3 + 1 + 3

4.8

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 3B 1

2 C 2 23 D 3 4

E 14 F 3 3

Line Efficiency Evaluate the efficiency of the line (E).

If Ca = 4 minutes and n = 4 work stations.

E = ti

nCa

j

i =1

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.

E = ti

nCa

j

i =1 E =2 + 1 + 2 + 3 + 1 + 3

4 * 4

E = 75.0% effective

Trial and Error Repeat until desired line efficiency is reached.

Joe’s Sub ShopWorkstation Task Element Time

(min)Workstation Time (min)

1 A 2 4C 2

2 B 1 4D 3

3 E 1 4F 3

E = 100.0% effective

An Exercise

Task Precedence Time (min)

A — 3

B A 5

C — 2

D B,C 4

E D 2

A sample precedence chart

An Exercise Draw and label a precedence diagram.

A3 min

B5 min

An Exercise Draw and label a precedence diagram.

A3 min

B5 min

C2 min

D4 min

E2 min

Calculate the desired cycle time (Cd). If, there is a demand for 100 units to be

produced every 12 hours.

An Exercise

Cd =production time available

desired units of output

Cd =12 hours x 60 minutes/hour

100 units

Cd = 7.2 minutes

An Exercise Calculate the theoretical minimum number

of workstations (N). If Cd = 7.2 minutes

N = ti

Cd

j

i =1

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

N = ti

Cd

j

i =1N =

2 + 5 + 2 + 4 + 27.2

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).

E = ti

nCa

j

i =1

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 3 5C 2

2 B 5 53 D 4 6

E 2

E = 83.3% effective

The Real World

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

A real world precedence chart

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