operations management notes 2

OPRE 6260 OPERATIONS MANAGEMENT

Notes 2

2001

Line Balancing

The process of assigning tasks to workstationsin such a way that the workstations have ap-proximately equal time requirements.

COMSOL : Computer Method of SequencingOperations Assembly Line (Chrysler Corpora-tion)

CALB : Computer Assembly Line Balancing

ALPACA : Assembly Line Planning and Con-trol Activity (GM).

0.1 Min 0.7 Min 1.0 Min 0.5 Min 0.2 Min✲ ✲ ✲ ✲

Figure 1:

OT = Operating time per day

CT = Cycle time

One shift per day; One shift is 8 hours.

OT = 8× 60 = 480 minutes per day

Cycle Time = CT = 1.0 min

Output =OT

CT=

480

1.0= 480 units per day

Cycle Time = CT = 2.5 min

Output =OT

CT=

480

2.5= 192 units per day


Figure 2:


CT = Cycle time

D = Desired output rate


OT = 8× 60 = 480 minutes per day

D = 480 units per day

CT =OT

D=

480

480= 1.0 Minute


Figure 3:

Number of Workstations Need

Nmin =

∑t

CT

Nmin = Theoretical min number of stations

CT = Cycle Time∑

t = Sum of task time

Nmin =

∑t

CT=

2.5

1.0= 2.5 stations

Nmin = 3

.

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Figure 4: Precedence Diagram


CT = Cycle time



Example A: OT = 8× 60 minutes per day


CT =OT

D=

480

96= 5 Minutes

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CT = Cycle time



Example B: OT = 8× 60 minutes per day


CT =OT

D=

480

80= 6 Minutes

Heuristic (intuitive) rules

1. Assign tasks in order of most following tasks.

2. Assign tasks in order of greatest positionalweight. Positional weight is the sum of eachtask’s time and the times of all following tasks.

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0.1 Min

0.7 Min

1.0 Min

0.5 Min 0.2 Min

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Figure 6: Example 1

Heuristic (intuitive) rule : Most number of follow-ers.

Cycle Time = CT = 1.0 Min

Workstation Time Eligible Assign StationRemaining Task Idle Time

D = 560 units per week

OT = 40 hours per week

CT =OT

D=

40

560= 0.07 Hours

N =

∑ti

CT=

0.26

0.07= 3.7 = 4

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Heuristic (intuitive) rule : Most numberof followers.

Cycle Time = CT = 1.0 Min


1 1.0 a,c a0.9 c c0.2 none - 0.2

2 1.0 b b 0.03 1.0 d d

0.5 e e0.3 - - 0.3

0.5

Two widely used measures of effective-ness : .

1. Balance Delay : The percentage of idletime of the line.

Nactual = actual number of stations.

Percentage of idle time = Idle time per cycle×100Nactual×cycle time

Percentage of idle time =0.5

3× 1.0× 100

Percentage of idle time = 16.7%

2. Efficiency:

Efficiency = 100− percent idle time

Efficiency = 100− 16.7 = 83.3%

Line Balancing Procedure :

1. Identify the cycle time and determine the minimumnumber of workstations.

2. Make assignments to workstations in order, beginningwith Station 1. Tasks are assigned to workstations mov-ing from left to right through the precedence diagram.

3. Before each assignment, use the following criteria todetermine which tasks are eligible to be assigned to aworkstation :

a. All preceding tasks in the sequence have been assigned.

b. The task time does not exceed the time remaining atthe workstation.

4. After each task assignment, determine the time re-maining at the current workstation by subtracting thesum of times for tasks already assigned to it from thecycle time.

5.Break ties that occur using one of these rules:

a. Assign the task with the longest task time.

b. Assign the task with greatest number of followings

If there is tie, choose one task arbitrarily.

6. Continue until all tasks have been assigned to work-stations.

7. Compute appropriate measures (e.g., percent idle time,

Designing Process Layout

The main issue in design of process layouts con-cerns the relative positioning of the departmentsinvolved.

Departments must be assigned to locations.

The problem is to develop a reasonably goodlayout; some combinations will be more desir-able than others.

Some departments may benefit from adjacentlocations.

Example:

Sharing expensive tools or equipments.

Sequence of operations.

Some departments should be separated.

Example: A lab with delicate equipment wouldnot be located near a department that had equip-ment with strong vibrations.

Sand blasting department and painting depart-ment.

be assigned1

2

3

4

5

6

D E F

A B C

Figure 8: Process Layout

6! = 1× 2× 3× 4× 5× 6 = 720

14 departments need to be arranged in 14 loca-tions.

14! = 1× 2× 3 . . . 12× 13× 14 = 8.7178× 1010

More than 87 billion different ways to arrange.

Measures of Effectiveness

Process layout : a variety of processing require-ments.

Customers or materials in these systems requiredifferent operations and different sequences ofoperations.

Transportation costs or time can be significant

One of the major objectives in process layoutis to minimize transportation cost, distance, ortime.

This is usually accomplished by locating depart-ments with relatively high interdepartmental workflow as close together as possible.

Information Requirements

The design of process layout requires the follow-ing information:

1. A list of departments or work centers to be ar-ranged, their approximate dimensions, and thedimensions of the building or buildings that willhouse the departments.

2. A projection of future work flows betweenthe various work centers

3. The distance between locations and the costper unit of distance to move loads between lo-cations

4. The amount of money to be invested in thelayout.

5. A list of any special considerations (e.g., op-erations that must be close to each other or op-erations that must be separated).

The activity relationship chart for ABC machine shop is shown in

the figure below. Arrange the eight departments in a 2× 4 gird.

A: Absolutely necessary

E: Especially important

I: Important

O: Ordinary closeness OK

U: Unimportant

X: Undesirable

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Computer-Aided Layout:

The size and complexity of process layout problems have led to the

development of a number of computerized packages.

Well-known packages:

ALDEP (Automated Layout DEsign Program)

It uses an initial random selection of a department and location and

then a search of remaining departments’ closeness rating with the

chosen department in order to assign locations. This procedure is

continued until all departments have been assigned. The completed

layout is evaluated and scored according to how well the preference

have been satisfied. The process is repeated a number of times, and

the best layout is identified.

CORELAP (COmputerized Relationship LAyout Planning)

CORELAP processes location assignments using the preference rat-

ing to order the choice of departments. Hence, it begins with a de-

partment pair that has an A rating, then another A pair, and son on

until all a relationship have been assigned. Next, the E relationship

pairs are assigned, and son on down the line until all departments

have been assigned to locations.

Both ALDEP and CORELAP use the preference rating (A-E-I-O-

U-X).

CRAFT uses From-To-Chart.

CRAFT (Computerized Relative Allocation of Facilities Technique)

CRAFT seeks to minimize material flow cost. It requires informa-

tion on material flow rates between departments, unit distance trans-

portation costs and initial layout. It exchanges pairs until no further

improvements can be obtained. The output is a printout of a rect-

angular layout.

Other Software packages

MICRO-CRAFT

BLOCPLAN uses a relationship chart as well as a from-to-chart as

input data for flow. Layout cost can be measured either by distance-

based objective or the adjacency-based objective.

MULTIPLE (LAYOPT)

Information about Software packages

Article : “Points to consider in Selecting Facilties Planning Soft-

ware,” Quarterman Lee, Industrial Engineering, IIE Solution, pp.

42-48, January, 1998.

Methods

1. Activity Relationship Diagram

2. Graph-Based Process

The process of constructing an activity relationship diagram often

requires compromises, especially when not all closeness rating can be

satisfied.

Planer Graph

A graph is planner if it can be drawn so that its vertices are points in

the plane and each edge can be drawn so that it intersects no other

edges.

Dual Graph

To construct the dual of a planar graph, place a dual node in each

face of the primal planner graph. Whenever two faces shares an edge

in their common boundary, join the nodes of the corresponding faces

by an edge crossing the edge representing the common boundary.

Reference:

Production/Operations Management byWilliam J. Steven-son, Sixth Edition, Irwin/McGraw-Hill, 1999.

Facility Layout and Location: An analytical Approachby Richard L. Francis, Leon F. MaGinnis, Jr., John A.White, Second Edition, Irwin/McGraw-Hill, 1992.

operations management notes 2

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