Unit 2

Management of Conversion System

Chapter 7: Layout designs

Lesson 22 - Design of product and process layouts

Learning Objectives

After reading this lesson you will be able to understand

Designing of product layout

Line balancing

Cycle time and theoretical minimum

Designing of process layout

This lesson introduces you to the concept of design of product as well as process layout.

You learn to appreciate the meaning and significance of line balancing, cycle time and

the allied concepts. The behavioral dimension is also stressed upon.

Dear students, by now all of us are familiar with the process of layout planning, which we

discussed in the previous lesson. Design of product as well as process layout is an

intricate as well as fascinating process, as you shall soon realize. Let’s start now.

Design of product layouts

In product layout, equipment or departments are dedicated to a particular product line,

duplicate equipment is employed to avoid backtracking, and a straight-line flow of

material movement is achievable. Adopting a product layout makes sense when the batch

size of a given product or part is large relative to the number of different products or parts

produced

Assembly lines are a special case of product layout. In a general sense, the term

assembly line refers to progressive assembly linked by some material handling device.

The usual assumption is that some form of pacing is present and the allowable processing

time is equivalent for all workstations. Within this broad definition, there are important

differences among line types. A few of these are material handling devices (belt or roller

conveyor, overhead crane); line configuration (U-shape, straight, branching); pacing

(mechanical, human); product mix (one product or multiple products); workstation

characteristics (workers may sit, stand, walk with the line, or ride the line); and length of

the line (few or many workers). The range of products partially or completely assembled

on lines includes toys, appliances, autos, clothing and a wide variety of electronic

components. In fact, virtually any product that has multiple parts and is produced in large

volume uses assembly lines to some degree.

A more-challenging problem is the determination of the optimum configuration of

operators and buffers in a production flow process. A major design consideration in

production lines is the assignment of operation so that all stages are more or less equally

loaded. Consider the case of traditional assembly lines illustrated in Figure 7.3. In this

example, parts move along a conveyor at a rate of one part per minute to three groups of

workstations. The first operation requires 3 minutes per unit; the second operation

Figure 7.3 Traditional assembly line

Requires 1 minute per unit; and the third requires 2 minutes per unit. The first

workstation consists of three operators; the second, one operator; and the third, two

operators. An operator removes a part from the conveyor and performs some assembly

task at his or her workstation. The completed part is returned to the conveyor and

transported to the next operation. The number of operators at each workstation was

chosen so that the line is balanced. Since three operators work simultaneously at the first

workstation, on the average one part will be completed each minute. This is also true for

other two stations. Since the parts arrive at a rate of one pr minute, parts are also

completed at this rate.

Assembly-line systems work well when there is a low variance in the times required to

perform the individual subassemblies. If the tasks are somewhat complex, thus resulting

in a higher assembly-time variance, operators down the line may not be able to keep up

with the flow of parts from the preceding work station or may experience excessive idle

time. An alternative to a conveyor-paced assembly line is a sequence of workstations

linked by gravity conveyors, which act as buffers between successive operations. An

example is shown in Figure 7.4. There are two sources of delays in this type of system –

Fig 7.4 Nonconveyor –paced assembly line with buffers

flow-blocking delay and lack-of-work delay. Flow-blocking delay occurs when a

production stage completes a unit but cannot release it because the in-process storage at

the next stag is full. This operator must remain idle until storage space becomes available.

Lack-of-work delay occurs whenever one stage completes work and no units are awaiting

processing from the previous stage.

Well, I guess that was quite a eye-opener. But that’s not the end of it. Let us quickly

move over to the concept of line balancing.

Line balancing

Assembly-line balancing often has implications for layout. This would occur when, for

balance purposes, workstation size or the number used would have to be physically

modified.

The most common assembly line is a moving conveyor that passes a series of

workstations in a uniform time interval called the workstation cycle time (which is also

the time between successive units coming off the end of the line). At each workstation,

work is performed on a product either by adding parts or by completing assembly

operations. The work performed at each station is made up of many bits of work, termed

tasks, elements, and work units. Such tasks are described by motion-time analysis.

Generally, they are grouping that cannot be subdivided on the assembly line without

paying a penalty in extra motions.

The total work to be performed at a workstation is equal to the sum of the tasks assigned

to that workstation. The line balancing problem is one of assigning all tasks to a series of

workstations so that each workstation has no more than can be done in the workstation

cycle time, and so that the unassigned (idle) time across all workstations is minimized.

The problem is complicated by the relationships among tasks imposed by product design

and process technologies. This is called the precedence relationship, which specifies the

order in which tasks must be performed in the assembly process.

The steps in balancing an assembly line are:

1. Specify the sequential relationships among tasks using a precedence diagram.

2. Determine the required workstation cycle time ©, using the formula

C = )(Re

Prinunitsutperdayquiredoutp

meperdayoductionti

3. Determine the theoretical minimum number of workstations (Nt) required to

satisfy the workstation cycle time constraint using the formula

Nt = )(

)(CCycletimeTimesSumoftaskt

4. Select a primary rule by which tasks are to be assigned to workstations, and a

secondary rule to break ties.

5. Assign tasks, one at a time, to the first workstation until the sum of the task times

is equal to the workstation cycle time, or no other tasks are feasible because of

time or sequence restrictions. Repeat the process for Workstation 2, Workstation

3, and so on until all tasks are assigned.

6. Evaluate the efficiency of the balance derived using the formula

Efficiency = )()(ln

)(CeoncycletimxWorkstatiNakstationsumberofworActua

TimesSumoftaskt

7. If efficiency is unsatisfactory, rebalance using a different decision rule.

It will be clearer if we take an example now.

Example The MS 800 car is to be assembled on a conveyor belt. Five hundred cars are

required per day. Production time per day is 420 minutes, and the assembly steps and

times for the wagon are given below. Find the balance that minimizes the number of

workstations, subject to cycle time and precedence constraints.

Task Task Time

(in seconds)

Description Tasks that must

precede

A 45 Position rear axle support and hand fasten -

B

C

D

E

F

G

H

I

J

K

11

9

50

15

12

12

12

12

8

9

four screws to nuts

Insert rear axle

Tighten rear axle support screws to nuts

Position front axle assembly and hand

fasten with four screws to nuts

Tighten front axle assembly screws

Position rear wheel 1 and fasten hubcap

Position rear wheel 2 and fasten hubcap

Position front wheel 1 and fasten hubcap

Position front wheel 2 and fasten hubcap

Position car handle shaft on front axle

assembly and hand fasten bolt and nut

Tighten bolt and nut

A

B

-

D

C

C

E

E

F, G, H, I

J

Solution

1. Draw a precedence diagram as follows

G

F

KJ

I

HE D

CB

A

2. Determine workstation cycle time. Here we have to convert production time to

seconds because our task times are in seconds

C = )(Re

Prinunitsutperdayquiredoutp

meperdayoductionti

= carsx

500sec60min420

= 500

25200 = 50.4

3. Determine the theoretical minimum number of workstations required (the actual

number may be greater)

Nt = CT =

ondsonds

sec4.50sec195 = 3.87 = 4 (rounded up)

4. Select assignment rules.

a. Prioritize tasks in order of the largest number of following tasks

Task Number of following tasks

A

B or D

C or E

F, G, H, or I

J

K

6

5

4

2

1

0

Our secondary rule, to be invoked where ties exist from our primary rule, is

b. Prioritize tasks in order of longest task time. Note that D should be

assigned before B, and E assigned before C due to this tiebreaking rule.

5. Make task assignments to form Workstation 1, Workstation 2, and so forth until

all tasks are assigned. It is important to meet precedence and cycle time

requirements as the assignments are made.

Station Task Task Remaining Feasible Task with Task with

time (in

sec)

unassigned

time (in

sec)

remaining

tasks

most

followers

longest

operation

time

Station 1 A 45 5.4 idle None

Station 2 D 50 0.4 idle None

Station 3 B

E

C

F

11

15

9

12

39.4

24.4

15.4

3.4 idle

C, E

C, H, I

F, G, H, I

None

C, E

C

F, G, H, I

E

F, G, H, I

Station 4 G

H

I

J

12

12

12

8

38.4

26.4

14.4

6.4 idle

H, I

I

J

None

H, I H, I

Station 5 K 9 41.4 idle None

6. Calculate the efficiency.

Efficiency = NaCT =

4.505195x

= .77 or 77%

7. Evaluate the solution. An efficiency of 77 percent indicates an imbalance or idle

time of 23 percent (1.0 - .77) across the entire line.

Is a better balance possible? Try balancing the line with rule b and breaking ties with rule

a.

In addition to balancing a line for a given cycle time, managers must also consider four

other options: pacing, behavioural factors, number of models produced, and cycle times.

Pacing is the movement of product from one station to the next after the cycle time has

elapsed. Paced lines have no buffer inventory. Unpaced lines require inventory storage

areas to be placed between stations.

Let us now focus on the relevant Behavioral aspects.

Behavioral factors.

The most controversial aspect of product layout is behavioural response. Studies have

shown that paced production and high specialization lower job satisfaction. One study

has shown that productivity increased on unpaced lines.

Many companies are exploring job enlargement and rotation to increase job variety and

reduce excessive specialization. For example, New York Life has redesigned the jobs of

workers who process and evaluate claims applications. Instead of using a production line

approach with several workers doing specialized tasks, New York Life has made each

worker solely responsible for an entire application. This approach increased worker

responsibility and raised morale.. In manufacturing, at its plant in Kohda, Japan, Sony

Corporation dismantled the conveyor belts on which as many as 50 people assembled

camcorders. It set up tables for workers to assemble an entire camera themselves, doing

everything from soldering to testing. Output per worker is up 10 percent, because the

approach frees efficient assemblers to make more products instead of limiting them to

conveyor belt’s speed. And if something goes wrong, only a small section of the plant is

affected. This approach also allows the line to match actual demand better and avoid

frequent shutdown because of inventory buildups.

Number of models produced. A mixed-model line produces several items belonging to

the same family. A single-model line produces one model with no variations. Mixed-

model production enables a plant to achieve both high-volume production and product

variety. However, it complicates scheduling and increases the need for good

communication about the specific parts to be produced at each station.

Cycle Times. A line’s cycle time depends on the desired output rate (or sometimes on the

maximum number of workstations allowed). In turn, the maximum line efficiency varies

considerably with the cycle time selected. Thus, exploring a range of cycle times makes

sense. A manager might go with a particularly efficient solution even if it does not match

the output rate. The manager can compensate for the mismatch by varying the number of

hours the line operates through overtime, extending shifts, or adding shifts. Multiple lines

might even be the answer.

Dear students, after we have been able to understand and appreciate the product layout

design and the related concepts, now are the time to focus on process lay out design.

Process layout design

The analysis involved in the design of production lines and assembly lines relates

primarily to timing, coordination, and balance among individual stages in the process.

For process layouts, the relative arrangement of departments and machines is the critical

factor because of the large amount of transportation and handling involved.

Procedure for designing process layouts

Process layout design determines the best relative locations of functional work centers.

Work centers that interact frequently, with movement of material or people, should be

located close together, whereas those that have little interaction can be spatially

separated. One approach of designing an efficient functional layout is described below.

1. List and describe each functional work center

2. Obtain a drawing and description of the facility being designed

3. Identify and estimate the amount of material and personnel flow among work

centers

4. Use structured analytical methods to obtain a good general layout

5. Evaluate and modify the layout, incorporating details such as machine orientation,

storage area location, and equipment access.

The first step in the layout process is to identify and describe each work center. The

description should include the primary function of the work center )drilling, new

accounts, or cashier_; its major components, including equipment and number of

personnel; and the space required. The description should also include any special

access needs (such as access to running water or an elevator) or restrictions (it must

be in a clean area or away from heat).

For a new facility, the spatial configuration of the work centers and the size and shape

of the facility are determined simultaneously. Determining the locations of special

structures and fixtures such as elevators, loading docks, and bathrooms becomes part

of the layout process. However, in many cases the facility and its characteristics are a

given. In these situations, it is necessary to obtain a drawing of the facility being

designed, including shape and dimensions, locations of fixed structures, and

restrictions on activities, such as weight limits on certain parts of a floor or

foundation.

To minimize transport times and material-handling costs, we would like to place

close together those work centers that have the greatest flow of materials and people

between them. To estimate the flows between work centers, it is helpful to begin by

drawing relationship diagram as shown in the figure below.

Work center A

Work center B

Work center C

Work center F

Work center D

Work center E

Very high flow High flow Moderate flow Low flow

Fig Relationship flow diagram

For manufacturing systems, material flows and transporting costs can be estimated

reasonably well using historical routings for products or through work sampling

techniques applied to workers or jobs. The flow of people, especially in a service

system such as a business office or a university administration building, may be

difficult to estimate precisely, although work sampling can be used to obtain rough

estimates.

The amounts and/or costs of flows among work centers are usually presented using a

flow matrix, a flow-cost matrix, or a proximity chart.

1. Flow Matrix

A flow matrix is a matrix of the estimated amounts of flow between each pair of work

centers. The flow may be materials (expressed as the number of loads transported) or

people who move between centers. Each work center corresponds to one row and one

column, and the element fij designates the amount of flow from work center (row) I

to work center (column) j. Normally, the direction of flow between work centers is

not important, only the total amount, so fij and fji can be combined and the flows

represented using only the upper righ half of a matrix.

Table Flow Matrix

Work Center

A B C D E F G H I

A

B

C

D

E

F

G

H

I

-

-

-

-

-

-

-

-

-

25

-

-

-

-

-

-

-

-

32

20

-

-

-

-

-

-

-

0

10

0

-

-

-

-

-

-

80

30

10

35

-

-

-

-

-

0

75

50

0

20

-

-

-

-

30

0

45

25

80

0

-

-

-

5

7

60

90

0

150

50

-

-

15

10

0

120

70

20

45

80

-

Daily

flows

between

work

centers

2. Flow –cost matrix

A basic assumption of facility layout is that the cost of moving materials or people

between work centers is a function of distance traveled. Although more complicated

cost functions can be accommodated, often we assume that the per unit cost of

material and personnel flows between work centers is proportional to the distance

between the centers. So for each type of flow between each pair of departments, I and

j, we estimate the cost per unit per unit distance, cij.

Table Flow-Cost Matrix

Work Center

A B C D E F G H I

A

B

C

D

E

F

G

H

I

-

-

-

-

-

-

-

-

-

25

-

-

-

-

-

-

-

-

32

40

-

-

-

-

-

-

-

0

10

0

-

-

-

-

-

-

80

90

10

35

-

-

-

-

-

0

75

50

0

20

-

-

-

-

30

0

45

50

80

0

-

-

-

5

7

60

90

0

150

150

-

-

15

10

0

240

70

20

45

80

-

Daily

cost for

flows

between

work

centers

(s per

day per

100 ft)

3. Proximity Chart

Proximity charts (relationship charts) are distinguished from flow and flow-cost

matrices by the fact that they describe qualitatively the desirability or need for work

centers to be close together, rather than providing quantitative measures of flow and

cost. These charts are used when it is difficult to measure or estimate precise amounts

or costs of flow among work centers. This is common when the primary flows

involve people and do not have a direct cost but rather an indirect cost, such as when

employees in a corporate headquarters move among departments (payroll, printing,

information systems) to carry out their work.

POM in practice 7.1 –

Well, now is the time to look back and systematically analyze what we have gathered

from the foregoing discussions and there is no better way to do it than with the help of a

practical case study.

The following case exposes you to the issues involved in process design and helps you

translate the textbook concept into practical reality.

Design of a University Library Workroom*

The workroom in the Southern Technical Institute Library processed about 8000 new

books each year. All library books must pass through the workroom in order to be

prepared for shelving in the library’s stacks. When the library was built, only about 3000

books were processed each year. Since no major additions or remodeling changes were

possible, this increase necessitated an evaluation of the existing layout.

A flow-process chart for the production process of cataloging new books is given

in Figure 7.6. Figures 7.7 and 7.8 illustrate the current layout and work flow in the

workroom. As you can see from the flow diagram, there is unnecessary movement back

and forth across the workroom. In addition, the main storage shelves constitute a major

barrier to the effective low of materials. From these observations, a new layout was

propose, which results in a much more orderly flow, shorter distance traveled, and more

book storage space. By moving the major storage shelves out of the center of the room

and off to one side, but the density of books in a rack can be increased by decreasing the

shelf height from 24 to 12 inches. This corresponds to a 367 percent increase in available

shelf space. This is illustrated in Figures 7.9 and 7.10.

Description Present/proposed

1. Receive books from

outside

2. Move to check-in area 33 / 12 feet

3. Check if duplication of

existing book

4. Move to catalog research

area

28 / 8 feet

5. Catalog-Library of

Congress class

5a. If no catalog

number, move to

storage area

26 / 26 feet

5b. Store until number

available

5c. Move to catalog

research area

20 / 26 feet

5d. Catalog

6. Move to verification area 28 / 8 feet

7. Verify

8. Move to pocket and call

number application area

18 / 10 feet

9.Install pocket and date

due slip

10. Apply call number

11. Store until shelving

Figure 7.6 Flow process chart for library workroom