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Process Layout

Chapter 8Chapter 8

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Designing Line-Flow Layouts

Line balancing is the assignment of work to stations in a line so as to achieve the desired output rate with the smallest number of workstations.

Work elements are the smallest units of work that can be performed independently.

Immediate predecessors are work elements that must be done before the next element can begin.

Precedence diagram allows one to visualize immediate predecessors better; work elements are denoted by circles, with the time required to perform the work shown below each circle.

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Line Balancing Example 8.3Example 8.3

Green Grass, Inc., a manufacturer of lawn & garden equipment, is designing an assembly line to produce a new fertilizer spreader, the Big Broadcaster. Using the following information, construct a precedence diagram for the Big Broadcaster.

© 2007 Pearson Education© 2007 Pearson Education

Line Balancing Green Grass, Inc.Green Grass, Inc.

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AA Bolt leg frame to hopperBolt leg frame to hopper 4040 NoneNoneBB Insert impeller shaftInsert impeller shaft 30 30 AACC Attach axleAttach axle 5050 AADD Attach agitatorAttach agitator 4040 BBEE Attach drive wheelAttach drive wheel 66 BBFF Attach free wheelAttach free wheel 2525 CCGG Mount lower postMount lower post 1515 CCHH Attach controlsAttach controls 2020 D, ED, EII Mount nameplateMount nameplate 1818 F, GF, G

TotalTotal 244244

WorkWork Time Time Immediate ImmediateElementElement DescriptionDescription (sec) (sec) Predecessor(s) Predecessor(s)

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Desired Output and Cycle Time

Desired output rate, r must be matched to the staffing or production plan.

Cycle time, c is the maximum time allowed for work on a unit at each station: 1

rc =

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Theoretical Minimum

Theoretical minimum (TM ) is a benchmark or goal for the smallest number of stations possible, where total time required to assemble each unit (the sum of all work-element standard times) is divided by the cycle time. It must be rounded up

Idle time is the total unproductive time for all stations in the assembly of each unit.

Efficiency (%) is the ratio of productive time to total time.

Balance Delay is the amount by which efficiency falls short of 100%.

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Output Rate and Cycle TimeExample 8.4

Green Grass, Inc. Desired output rate, r = 2400/week Plant operates 40 hours/week r = 2400/40 = 60 units/hour

Cycle time, c = 1/60

= 1 minute/unit

= 60 seconds/unit

1

r

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Calculations for Example 8.4 continued

Theoretical minimum (TM ) - sum of all work-element standard times divided by the cycle time.

TM = 244 seconds/60 seconds = 4.067 It must be rounded up to 5 stations

Cycle time: c = 1/60 = 1 minute/unit = 60 seconds/unit

Efficiency (%) - ratio of productive time to total time.

Efficiency = [244/5(60)]100 = 81.3%

Balance Delay - amount by which efficiency falls short of 100%.

(100 − 81.3) = 18.7%

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Line Balancingcc = 60 seconds/unit = 60 seconds/unitTM = 5 stationsTM = 5 stationsEfficiency = 81.3%Efficiency = 81.3%

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S3 B,F,G B 30 30D,E,F,G F 55 5

S4 D,E,G D 40 20E,G G 55 5

S5 E,I I 18 42E E 24 36H H 44 16

Cumm IdleStation Candidate Choice Time Time

Green Grace, Inc.Green Grace, Inc.

© 2007 Pearson Education© 2007 Pearson Education

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Green Grass, Inc. Green Grass, Inc. Line Balancing SolutionLine Balancing Solution

cc = 60 seconds/unit = 60 seconds/unitTM = 5 stationsTM = 5 stationsEfficiency = 81.3%Efficiency = 81.3%

The goal is to cluster the work elements into 5 workstations so that the number of work-stations is minimized, and the cycle time of 60 seconds is not violated. Here we use the trial-and-error method to find a solution, although commercial software packages are also available.

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Line Balancing

cc = 60 seconds/unit = 60 seconds/unitTM = 5 stationsTM = 5 stationsEfficiency = 81.3%Efficiency = 81.3%Production = 2400Production = 2400

S2S3

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Work Work Station Work stationStation Elements Time Idle Time

Line CapacityLine CapacityFastest cycle time Fastest cycle time = 55 seconds/unit= 55 seconds/unitProduction = (40 hours x 60 minutes/hr x 60 seconds/hr)/55 = Production = (40 hours x 60 minutes/hr x 60 seconds/hr)/55 = 26182618Efficiency at capacity = 244/(5x55) = .887 = 88.7%Efficiency at capacity = 244/(5x55) = .887 = 88.7%

Green Grace, Inc.Green Grace, Inc.

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Application 8.3

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Application 8.3

© 2007 Pearson Education

Application 8.3

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Application 8.4Finding a Solution

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Other Considerations

In addition to balancing a line, managers must also consider four other options:

1. Pacing: The movement of product from one station to the next as soon as the cycle time has elapsed.

2. Behavioral factors of workers.3. Number of models produced: A mixed-model line

produces several items belonging to the same family.

4. Cycle times depend on the desired output rate, and efficiency varies considerably with the cycle time selected. Thus exploring a range of cycle times makes sense.

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Solved Problem 2

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Solved Problem 2Precedence Diagram

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© 2007 Pearson Education

Solved Problem 2Line Balancing Process

© 2007 Pearson Education

Solved Problem 2Line Balancing Solution

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