Ch03&Ch04-Work Flow, Batch Processing, Assembly Lines

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Work Systems and the Methods, Measurement, and Management of Workby Mikell P. Groover, ISBN 0-13-140650-7.

©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.

Work Flow and Batch Processing

Sections:1. Sequential Operations and Work Flow2. Batch Processing3. Defects in Sequential Operations and

Batch Processing4. Work Cells

Chapter 3



Some Definitions

� Sequential operations (versus unitoperations): series of separate processingsteps that are performed on each work unit� Operations performed sequentially

� Work flow: physical movement of work unitsthrough the sequence of unit operations

� Batch processing: processing of work units in(finite) quantities or amounts



Sequential Operations in Industry

� To complete a work unit, the operations areperformed sequentially (rather thansimultaneously)� Manufacturing� Assembly� Construction� Mortgage applications� Medical services� Education� Transportation



Work Flow Patterns

� Pure sequential: all work units follow thesame exact sequence of operations andworkstations� Work flow is identical for all work units

� Mixed sequential: different work units areprocessed through different operations� Different work flows for different types of work units



Network Diagram

Indicates any of several possible quantitativerelationships among operations in a multi-stationwork system

Possible variables in a network diagram:

Quantities moving between operations

Flow rates of materials

Distances between work stations



Bottlenecks in Sequential Operations

� Bottleneck = slowest operation in thesequence

� The bottleneck operation limits the productionrate for the entire sequence

Rps=Min{Rpi} for i=1, 2, . . . , nwhereRps=overall production rate of the system, pc/hrRpi=production rate of operation i, pc/hrn=the number of operations in the sequence

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1 2 43 5

3 pc/min 3 pc/min 3 pc/min3 pc/min 3 pc/min

1 2 43 5

3 pc/min 3 pc/min 3 pc/min2 pc/min 3 pc/min

Tot. capacity

3 pc/min

Tot. capacity

2 pc/min




� Slowest process limits the output of the otheroperations in the sequence.

� Blocking: production rate(s) of one or moreupstream operations are limited by the rate of adownstream operation� Accumulating work-in-process (WIP) inventory before

the bottleneck station makes no sense. Theupstream operations must produce at a rate that isless than or equal to the bottleneck operation

2 3

3 pc/min 2 pc/min




� Slowest process limits the output of the otheroperations in the sequence.

� Starving: production rate(s) of one or moredownstream operations are limited by the rateof an upstream operation� The downstream operations can work no faster than

the rate at which the bottleneck feeds work units tothem.

43

3 pc/min2 pc/min



Why a workstation is the slowest?

� Technological factors: limits on the speed ofthe equipment (feed rate restricts workpieceprogresion)

� Work allocation decisions: the ways in whichthe total work content in the sequence isdivided among the stations

� Ergonomic limitations: the physical or mentalrestrictions of the human worker at theworkstation



Batch Processing

� Batch processing: processing of work units in finitequantities or amounts

� Work units can be materials, products, information, orpeople

� Batch processing is common in production, logistics, andservice operations� Batch production in manufacturing� Passenger air travel� Cargo transport� Book publishing� Payroll checks� Laundry� Grading of student papers



Types of Batch Processing

� Sequential batch processing: members ofthe batch are processed one after the other

� Simultaneous batch processing:members of the batch are all processed atthe same time

SequentialProduction machiningBatch assembly

Book printingPayroll checks

Grading of student papers

SimultaneousChemical batch processesHeat treating of multiple

partsPassenger air travel

Cargo transportLaundry

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Batch Production

Alternating cycles of setup and production runexperienced by a work system engaged in batchproduction (cycles are not necessarily identical)

Production is intermittent . This meansequipment are not kept continuously runningand work units are not continuously worked on.



Motivations of Batch Processing

� Work unit differences: different types of workunits must be processed separately

� Learning curve effect: cycle time per workunit decreases as batch quantity Q increases(applies only to sequential batch production)

� Equipment limitations: limits on the quantitiesthat can be processed

� Material limitations: the material must beprocessed as a unit



Disadvantages of Batch Processing

� Setup: Interruption due to changeovers betweenbatches (tool change, loading, unloading etc.)

� Setup times are lost productive time� Setup changeovers in batch production� Airplanes at a terminal unloading and loading

passengers

� Work-in-process: inventory accumulated during theproduction� Multiple batches competing for the same equipment� Queues of work units form in front of each

workstation, resulting in large inventories of partiallyprocessed units



Work Cells

� Work cell: a group of workstations dedicatedto the processing of a range of work unitswithin a given type

� Part family: the range of work units that areprocessed� Members of the part family are similar but not

identical

� Mixed sequential work flow system

� Work cells and part families are associatedwith group technology



Group Technology

� An approach to manufacturing in which similar parts areidentified and grouped together to take advantage oftheir similarities in design and production

� Instead of processing each part types in batches, thework units are processed individually and continuously,without the need for time-consuming changeoversbetween part types

� This is enabled by� The similarity of parts within a part family� The adaptibility and flexibility of the workers and

equipments in the cell that can accomodate the moderatedifferences among part family members



Work Cell Layouts

� In-line: straight line flow of work units� high speed� meaningfull only if all work units follows the same

sequence of operations� consume large space

� U-shaped: shape of work flow is “U”� Similar to in-line except for shape – saves space� Better communication among workers

� promotes teamwork� Better in realizing mixed sequential flow

� Loop: continuous flow of work units around a loop layout

� Rectangular: similar to loop layout

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In-Line Work Cell



U-Shaped with Manual Handling



U-Shaped with Mechanized Handling



Work Cell with Loop Layout



Work Cell with Rectangular Layout



Manual Assembly Lines

Sections:1. Fundamentals of Manual Assembly

Lines2. Analysis of Single Model Assembly

Lines3. Line Balancing Algorithms4. Other Considerations in Assembly

Line Design5. Alternative Assembly Systems

Chapter 4

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Manual Assembly Lines

� Work systems consisting of multiple workers organizedto produce a single product or a limited range ofproducts

� Assembly workers perform tasks at workstations locatedalong the line-of-flow of the product� Usually a powered conveyor is used� Some of the workstations may be equipped with portable

powered tools.

� Factors favoring the use of assembly lines:� High or medium demand for product� Products are similar or identical� Total work content can be divided into work elements� To automate assembly tasks is impossible



Why Assembly Lines are Productive

� Specialization of labor

� When a large job is divided into small tasks and eachtask is assigned to one worker, the worker becomeshighly proficient at performing the single task(Learning curve)

� Interchangeable parts

� Each component is manufactured to sufficiently closetolerances that any part of a certain type can beselected at random for assembly with its matingcomponent.

� Thanks to interchangeable parts, assemblies do notneed fitting of mating components



Some Definitions

� Work flow

� Each work unit should move steadily along the line

� Line pacing

� Workers must complete their tasks within a certaincycle time, which will be the pace of the whole line



Manual Assembly Line

� A production line that consists of a sequence ofworkstations where assembly tasks areperformed by human workers

� Products are assembled as they move alongthe line� At each station a portion of the total work content is

performed on each unit

� Base parts are launched onto the beginning ofthe line at regular intervals (cycle time)� Workers add components to progressively build the

product



Manual Assembly Line

•Configuration of an n-workstation manual assembly line

•The production rate of an assembly line is determined byits slowest station.

�Assembly workstation: A designated location along thework flow path at which one or more work elements areperformed by one or more workers



Two assembly operators working on an engine assembly line (photo courtesy of Ford Motor Company)

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Manning level

� There may be more than one worker perstation.

� Utility workers: are not assigned tospecific workstations.

� They are responsible for

(1) helping workers who fall behind,

(2) relieving for workers for personal breaks,

(3) maintenance and repair



Manning level

�Average manning level:

whereM=average manning level of the line,

wu=number of utility workers assigned to the system,n=number of workstations,

wi=number of workers assigned specifically to stationi for i=1,…,n

n

wwM

n

iiu ∑

=+

= 1



Work Transport Systems-Manual Methods

� Manual methods� Work units are moved between stations by the

workers (by hand) without powered conveyor

� Problems:� Starving of stations

� The assembly operator has completed the assignedtask on the current work unit, but the next unit has notyet arrived at the station

� Blocking of stations� The operator has completed the assigned task on the

current work unit but cannot pass the unit to thedownstream station because that downstream workeris not yet ready to receive it.



Work Transport Systems-Manual Methods

� To reduce starving,� use buffers

� To prevent blocking,� provide space between upstream and downstream

stations.

� But both solutions can result in higher WIP,� which is economically undesirable.



Work Transport Systems-Mechanized Methods

� Continuously moving conveyor: operates at constant velocity1. Work units are fixed to the conveyor

� The product is large and heavy� Worker moves along with the product

2. Work units are removable from the conveyor� Work units are small and light� Workers are more flexible compared to synchronous lines, less flexible than

asynchronous lines

� Synchronous transport (intermittent transport – stop-and -goline): all work units are moved simultaneously between stations.� Problem:

� Task must be completed within a certain time limit. Otherwise the lineproduces incomplete units;

� Excessive stress on the assembly worker.� Not common for manual lines (variability), but often ideal for automated

production lines

� Asynchronous transport : a work unit leaves a given station whenthe assigned task is completed.� Work units move independently, rather than synchronously (most flexible one).� Variations in worker task times� Small queues in front of each station.



Coping with Product Variety

� Single model assembly line (SMAL)� Every work unit is the same

� Batch model assembly line (BMAL ) – multiple modelline� Two or more different products� Products are so different that they must be made in

batches with setup between batches

� Mixed model assembly line (MMAL)� Two or more different models� Differences are slight so models can be made

simultaneously with no setup time (no need for batchproduction)

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Coping with Product Variety

� Advantages of mixed models over batch order models

� No production time is lost during changeovers

� High inventories due to batch ordering are avoided

� Production rates of different models can be adjusted asproduct demand changes.

� Disadvantages of mixed models over batch order models

� Each station is equipped to perform variety of tasks (costly)

� Scheduling and logistic activities are more difficult in thistype of lines.



Analysis of Single Model Lines

�The formulas and the algorithms in this section aredeveloped for single model lines, but they can beextended to batch and mixed models.

�The assembly line must be designed to achieve aproduction rate sufficient to satisfy the demand.�Demand rate → production rate→ cycle time�Annual demand Da must be reduced to an hourlyproduction rate Rp

whereDa = annual demandRp = hourly production rateSw = number of shifts/weekHsh = number of hours/shift

shw

ap HS

DR

52=



Determining Cycle Time

�Now our aim is to convert production rate, Rp, to cycle time,Tc.

�One should take into account that some production time willbe lost due to

� equipment failures� power outages,� material unavailability,� quality problems,� labor problems.

�Line efficiency (uptime proportion): only a certain proportionof the shift time will be available.

where production rate, Rp, is converted to a cycle time, Tc,accounting for line efficiency, E.

pc R

ET

60=



Number of Stations Required

�Work content time (Twc): The total time of all workelements that must be performed to produce one unitof the work unit.

�The theoretical minimum number of stations that willbe required to on the line to produce one unit of thework unit, w*:

w* = Minimum Integer ≥

whereTwc = work content time, min;Tc = cycle time, min/station

If we assume one worker per station then this gives theminimum number of workers

c

wc

TT



Theoretical Minimum Not Possible

� Repositioning losses: Some time will be lostat each station every cycle for repositioning theworker or the work unit; thus, the workers willnot have the entire Tc each cycle

� Line balancing problem (imperfectbalancing): It is not possible to divide the workcontent time evenly among workers, and someworkers will have an amount of work that isless than Tc



Repositioning Losses

�Repositioning losses occur on a productionline because some time is required eachcycle to reposition the worker, the work unit,or both

� On a continuous transport line, time is requiredfor the worker to walk from the unit justcompleted to the the upstream unit entering thestation

� In conveyor systems, time is required toremove work units from the conveyor andposition it at the station for worker to performhis task.

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Repositioning Losses

�Repositioning time = time available eachcycle for the worker to position = Tr

�Service time = time available each cycle forthe worker to work on the product = Ts

�Service time Ts = Max{Tsi} ≤Tc – Tr

where Tsi= service time for station i, i=1,2,..,n

�Repositioning efficiency Er =c

rc

c

s

TTT

TT −=



Cycle Time on an Assembly Line

•Components of cycle time at several stations ona manual assembly line

Tsi=service time, Tr=repositioning time



Line Balancing Problem

Given:� The total work content consists of many distinct

work elements� The sequence in which the elements can be

performed is restricted� The line must operate at a specified cycle time

(=service time + repositioning time)The Problem:� To assign the individual work elements to

workstations so that all workers have an equalamount of work to perform



Assumptions About Work Element Times

1. Element times are constant values

� But in fact they are variable

2. Work element times are additive

� The time to perform two/more work elements insequence is the sum of the individual element times

� Additivity assumption can be violated (due tomotion economies)



Work Element Times

�Total work content time Twc

Twc =

where Tek = work element time for element k

�Work elements are assigned to station i that add up tothe service time for that station

Tsi =

�The station service times must add up to the total workcontent time

Twc =

∑=

en

kekT

1

∑∈ik

ekT

∑=

n

isiT

1



Constraints of Line Balancing Problem

� Different work elements require different times.

� When elements are grouped into logical tasks andassigned to workers, the station service times, Tsi, arelikely not to be equal.

� Simply because of the variation among work elementtimes, some workers will be assigned more work.

� Thus, variations among work elements make it difficult toobtain equal service times for all stations.

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Precedence Constraints

� Some elements must be done before theothers.

� Restrictions on the order in which workelements can be performed

� Can be represented graphically (precedencediagram)



Example:

Grommet : sealant like ringGrommet : sealant like ringGrommet : sealant like ringGrommet : sealant like ring



Example:

Grommet : sealant like ringGrommet : sealant like ringGrommet : sealant like ringGrommet : sealant like ringWork Systems and the Methods, Measurement, and Management of Work

by Mikell P. Groover, ISBN 0-13-140650-7.©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.

Example: A problem for line balancing

� Given: The previous precedence diagram and thestandard times. Annual demand=100,000 units/year. Theline will operate 50 wk/yr, 5 shifts/wk, 7.5 hr/shift. Uptimeefficiency=96%. Repositioning time lost=0.08 min.

� Determine(a) total work content time,(b) required hourly production rate to achieve the annualdemand,(c) cycle time,

(d) theoretical minimum number of workers required onthe line,

(e) service time to which the line must be balanced.



Example: Solution

(a) The total work content time is the sum of the work element times given in the table

Twc=4.0 min(b) The hourly production rate

(c) The corresponding cycle time with an uptime efficiency of 96%

(d) The minimum number of workers:w* = (Minimum Integer ≥ 4.0 /1.08=3.7)=4 workers(e) The available service time

Ts=1.08-0.08=1.00 min

units/hr 33.53)5.7)(5(50

000,100 ==pR

min08.133.53

)96.0(60 ==cT

∑=

=en

kekwc TT

1

shw

ap HS

DR

50=

pc R

ET

60=

c

wc

T

Tw =*

rcs TTT −=Work Systems and the Methods, Measurement, and Management of Work


Measures of Balance Efficiency

�It is almost imposible to obtain a perfect line balance

�Line balance efficiency , Eb:

Eb = Perfect line: Eb = 1

�Balance delay , d:

d = Perfect line: d = 0

�Note that Eb + d = 1 (they are complement of eachother)

s

wc

wTT

s

wcs

wTTwT −

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Overall Efficiency

�Factors that reduce the productivity of amanual line

� Line efficiency (Availability), E,

� Repositioning efficiency (repositioning), Er,

� Balance efficiency (balancing), Eb,

�Overall Labor efficiency on the assembly line = br EEE ⋅⋅

c

rc

c

sr T

TT

T

TE

−==s

wcb wT

TE =

pc R

ET

60=



Continously moving conveyors- Workstation considerations

�Total length of the assembly line

where L=length of the assembly line, m: Lsi=length of station i, m� Constant speed conveyor: (if the base parts remain fixed during their assembly)�Feed rate

fp=1/Tc

where fp=feed rate on the line, products/min�Center-to-center spacing between base parts

sp=vc/fp=vcTc

where sp= center-to-center spacing between base parts, m/part and vc=velocity of the conveyor, m/min

∑=

=n

iisLL

1



Continously moving conveyors- Tolerance Time

� Defined as the time a work unit spends inside theboundaries of the workstation

� Provides a way to allow for product-to-product variationsin task times at a station

� Tt =

where

Tt = tolerance time, min;Ls = station length, m (ft);

vc = conveyor speed, m/min (ft/min)

c

s

vL



Line Balancing Objective

� To distribute the total work content on the assembly lineas evenly as possible among the workers

� Minimize (wTs – Twc)or

� Minimize

Subject to:(1)

(2) all precedence requirements are obeyed

sik

ek TT ≤∑∈

( )∑=

−w

isis TT

1



Line Balancing Algorithms – Heuristics

1. Largest candidate rule

2. Kilbridge and Wester method

3. Ranked positional weights method, alsoknown as the Helgeson and Birne method

� In the following descriptions, assume oneworker per workstation



Largest Candidate Rule� List all work elements in descending order based on their

Tek values; then,

1. Start at the top of the list and selecting the first elementthat satisfies precedence requirements and does notcause the total sum of Tek to exceed the allowable Tsvalue

When an element is assigned, start back at the top of thelist and repeat selection process

2. When no more elements can be assigned to the currentstation, proceed to next station

3. Repeat steps 1 and 2 until all elements have beenassigned to as many stations as needed

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Solution for Largest Candidate Rule



Example:










•Physical layout of workstations and assignmentof elements to stations using the largestcandidate rule



Ranked Positional Weights Method

� A ranked position weight (RPW) is calculated for eachwork element

� RPW for element k is calculated by summing the Te

values for all of the elements that follow element k in thediagram plus Tek itself

� Work elements are then organized into a list according totheir RPW values, starting with the element that has thehighest RPW value

� Proceed with same steps 1, 2, and 3 as in the largestcandidate rule

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Solution for Ranked Positional Weights Method



Example:






Other Considerations in Line Design

� Methods analysis� To analyze methods at bottleneck or other troublesome

workstations� improved motions,� better workplace layout,� special tools to facilitate manual work elements� product design

� Utility workers� To relieve congestion at stations that are temporarily

overloaded

� Preassembly of components� Prepare certain subassemblies off-line to reduce work

content time on the final assembly line



Other Considerations - continued

� Storage buffers between stations� To permit continued operation of certain sections of

the line when other sections break down

� To smooth production between stations with largetask time variations

� Parallel stations� To reduce time at bottleneck stations that have

unusually long task times

� Worker (Labor) Shifting with crosstraining� Temporary (or periodic) relocation to expedite or to

reduce subassembly stocksWork Systems and the Methods, Measurement, and Management of Work


Most Follower Rule

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6

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19191919

Item iMost Follower

1 9

2 5

3 4

4 4

5 4

6 4

7 3

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9 2

10 1

19/19 16/19 15/19 10/19

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We omit

� Worker Requirements in 4.2.2

� 4.3.2 Kilbridge and Western Method

� 4.5 Alternative Assembly Systems

Documents

Ch03&Ch04-Work Flow, Batch Processing, Assembly Lines