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Assembly

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  • Assembly

  • 1. Fundamentals of Manual Assembly Lines 2. Analysis of Single Model Assembly Lines 3. Line Balancing Algorithms 4. Mixed Model Assembly Lines 5. Workstation Considerations 6. Other Considerations in Assembly Line Design 7. Alternative Assembly Systems

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  • Manual Assembly Lines

    Most consumer products are assembled on manual assembly lines

    Factors favoring the use of assembly lines: High or medium demand for product

    Identical or similar products

    Total work content can be divided into work elements

    It is technologically impossible or economically infeasible to automate the assembly operations

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  • Why Assembly Lines are so Productive

    Specialization of labor Learning curve

    Interchangeable parts Components made to close tolerances

    Assemblies do not need fitting of mating components

    Work flow principle Products are brought to the workers

    Line pacing Workers must complete their tasks within the cycle

    time of the line

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  • Typical Products Made on Assembly

    Lines Automobiles

    Cooking ranges

    Dishwashers

    Dryers

    Furniture

    Lamps

    Luggage

    Microwave ovens

    Personal computers

    Power tools

    Refrigerators

    Telephones

    Toasters

    Trucks

    Video DVD players

    Washing machines

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  • Manual Assembly Line Defined

    A production line consisting of a sequence of workstations where assembly tasks are performed by human workers as the product moves along the line Work systems consisting of multiple workers organized to

    produce a single product or a limited range of products Each product consists of multiple components joined together

    by various assembly work elements Total work content - the sum of all work elements required to

    assemble one product unit on the line

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

    powered tools

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  • Manual Assembly Line

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    Configuration of a manual assembly line with n manually operated workstations

  • Manual Assembly Line

    The production rate of an assembly line is determined by its slowest station.

    Assembly workstation: A designated location along the work flow path at which one or more work elements are performed by one or more workers

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    Manufacturing System

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

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    Manufacturing System

  • Manual Assembly Line

    Products are assembled as they move along the line

    At each station a portion of the total work content is performed on each unit

    Base parts are launched onto the beginning of the line at regular intervals (cycle time)

    Workers add components to progressively build the product

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  • Assembly Workstation

    Typical operations performed at manual assembly stations

    Adhesive application

    Sealant application

    Arc welding

    Spot welding

    Electrical connections

    Component insertion

    Press fitting

    Riveting

    Snap fitting

    Soldering

    Stitching/stapling

    Threaded fasteners

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    A designated location along the work flow path at which one or more work elements are performed by one or more workers

  • Manning level

    There may be more than one worker per station.

    Utility workers: are not assigned to specific workstations.

    They are responsible for helping workers who fall behind,

    relieving for workers for personal breaks,

    maintenance and repair

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    Manufacturing System

  • Manning level

    Average manning level:

    where M=average manning level of the line, w = number of workers on the line wu=number of utility workers assigned to the system,

    n=number of workstations, wi=number of workers assigned specifically to station i

    for i=1,,n

    n

    ww

    n

    wM

    n

    i

    iu

    1

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    Manufacturing System

  • Work Transport Systems

    Two basic categories: Manual

    Mechanized

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  • Manual Work Transport Systems

    Work units are moved between stations by the workers without the aid of a powered conveyor

    Types: Work units moved in batches

    Work units moved one at a time

    Problems: Starving of stations

    Blocking of stations

    No pacing

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  • 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.

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    Manufacturing System

  • Mechanized Work Transport Systems

    Work units are moved by powered conveyor or other mechanized apparatus

    Categories: Work units attached to conveyor

    Work units are removable from conveyor

    Problems Starving of stations

    Incomplete units

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  • Types of Mechanized Work Transport

    Continuous transport Conveyor moves at constant speed

    Synchronous transport/intermittent transport Work units are moved simultaneously with stop-

    and-go (intermittent) motion to next stations

    Asynchronous transport Work units are moved independently between

    workstations Queues of work units can form in front of each

    station

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  • Continuous Transport

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    Conveyor moves at constant velocity vc

  • Synchronous Transport

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    All work units are moved simultaneously to their respective next workstations with quick, discontinuous motion

  • Asynchronous Transport

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    Work units move independently, not simultaneously. A work unit departs a given station when the worker releases it. Small queues of parts can form at each station.

  • Material Handling Equipment for

    Mechanized Work Transport Continuous transport

    Overhead trolley conveyor Belt conveyor Roller conveyor Drag chain conveyor

    Synchronous transport Walking beam transport equipment Rotary indexing mechanisms

    Asynchronous transport Power-and-free overhead conveyor Cart-on-track conveyor Automated guided vehicle systems Monorail systems Chain-driven carousel systems

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  • Line Pacing

    A manual assembly line operates at a certain cycle time to achieve the required production rate of the line On average, each worker must complete his/her assigned task within this cycle time

    Pacing provides a discipline for the assembly line workers that more or less guarantees a certain production rate for the line

    Several levels of pacing: 1. Rigid pacing 2. Pacing with margin 3. No pacing

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  • Rigid Pacing

    Each worker is allowed only a certain fixed time each cycle to complete the assigned task Allowed time is set equal to the cycle time less

    repositioning time Synchronous work transport system provides rigid

    pacing

    Undesirable aspects of rigid pacing: Incompatible with inherent human variability Emotionally and physically stressful to worker Incomplete work units if task not completed

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  • Pacing with Margin

    Worker is allowed to complete the task within a specified time range, the upper limit of which is greater than the cycle time

    On average, the workers average task time must balance with the cycle time of the line

    How to achieve pacing with margin: Allow queues of work units between stations Provide for tolerance time to be longer than cycle

    time Allow worker to move beyond station boundaries

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  • No Pacing

    No time limit within which task must be completed

    Each assembly worker works at his/her own pace

    No pacing can occur when: Manual transport of work units is used

    Work units can be removed from the conveyor to perform the task

    An asynchronous conveyor is used

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  • Coping with Product Variety

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

    Batch model assembly line (BMAL) Hard product variety

    Products must be made in batches

    Mixed model assembly line (MMAL) Soft product variety

    Models can be assembled simultaneously without batching

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  • MMAL vs. BMAL

    Advantages of mixed model lines over batch models lines:

    No lost production time between models

    High inventories typical of batch production are avoided

    Production rates of different models can be adjusted as product demand changes

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  • MMAL vs. BMAL

    Difficulties with mixed model line compared to batch model line

    Line balancing problem more complex due to differences in work elements among models

    Scheduling the sequence of the different models is a problem

    Logistics is a problem - getting the right parts to each workstation for the model currently there

    Cannot accommodate as wide model variations as BMAL

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  • Alternative Assembly Systems

    Problems on high production assembly lines Complain about the monotony of the repetitive tasks that

    must be performed and the unrelenting pace that must be maintained when a moving conveyor is used

    Poor quality workmanship Sabotage of the line equipment

    Alternative assembly systems Single station manual assembly cells Assembly cells based on worker teams

    Single station manual assembly cell with multiple workers Moving the product through multiple workstations, but having

    the same worker team follow the product Automated assembly systems

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  • Design for Assembly (DFA)

    The key to successful DFA Design the product with as few parts as possible Design the remaining parts so they are easy to

    assemble

    General principles of DFA Use the fewest number of parts possible to reduce the

    amount of assembly required Reduce the number of threaded fasteners required Standardize fasteners Reduce parts orientation difficulties Avoid parts that tangle

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  • Analysis of Single Model Lines The formulas and the algorithms in this section are developed for single

    model lines, but they can be extended to batch and mixed models.

    The assembly line must be designed to achieve a production rate sufficient to satisfy the demand.

    Demand rate production rate cycle time Annual demand Da must be reduced to an hourly production rate Rp

    where Da = annual demand Rp = hourly production rate Sw = number of shifts/week Hsh = number of hours/shift

    shw

    ap

    HS

    DR

    52

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    Manufacturing System

  • 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 will be lost

    due to equipment failures power outages, material unavailability, quality problems, labor problems.

    Line efficiency (uptime proportion): only a certain proportion of 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

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    Manufacturing System

  • Determining Cycle Time

    Rc = cycle rate for the line (cycles/hour)

    Tc = cycle time of the line (minutes/cycle)

    Rp = required production rate (units/hour)

    E = line efficiency

    Rc must be greater than Rp because E is less than 100%

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    c

    p

    R

    RE

  • Number of Stations Required

    EAT

    T

    ETWL

    TRWL

    AT

    WLw

    c

    wc

    wcp

    60

    60

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  • Number of Stations Required

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

    The theoretical minimum number of stations that will be required to on the line to produce one unit of the work unit, w*:

    where Twc = work content time, min; Tc = cycle time, min/station If we assume one worker per station then this gives the minimum

    number of workers

    c

    wc

    T

    Tw intmin*

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    Manufacturing System

  • Theoretical Minimum Not Possible

    Repositioning losses Some time will be lost at each station every cycle

    for repositioning the worker or the work unit; thus, the workers will not have the entire Tc each cycle

    Line balancing problem (imperfect balancing): It is not possible to divide the work content time

    evenly among workers, and some workers will have an amount of work that is less than Tc

    Task time variability Quality problems

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  • Repositioning Losses

    Repositioning losses occur on a production line because some time is required each cycle to reposition the worker, the work unit, or both

    On a continous transport line, time is required for the worker to walk from the unit just completed to the the upstream unit entering the station

    In conveyor systems, time is required to remove work units from the conveyor and position it at the station for worker to perform his task.

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    Manufacturing System

  • Repositioning Losses

    Repositioning time = time available each cycle for the worker to position = Tr

    Service time = time available each cycle for the 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

    c

    rc

    c

    sr

    T

    TT

    T

    TE

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    Manufacturing System

  • Cycle Time (Tc) on an Assembly Line

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    Components of cycle time at several stations on a manual assembly line. At the bottleneck station, there is no idle time.

    Tsi = service time, Tr = repositioning time

  • Line Balancing Problem

    Given: 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

    Problem: To assign the individual work elements to

    workstations so that all workers have an equal amount of work to perform

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  • Assumptions About Work Element

    Times Element times are constant values

    But in fact they are variable

    Work element times are additive The time to perform two/more work elements in

    sequence is the sum of the individual element times

    Additivity assumption can be violated (due to motion economies)

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  • Work Element Times

    Total work content time Twc where Tek = work element time for element k

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

    The station service times must add up to the total work content time

    Manufacturing System

    en

    k

    ekwc TT1

    ik

    eksi TT

    n

    i

    siwc TT1

  • Constraints of Line Balancing Problem

    Different work elements require different times. When elements are grouped into logical tasks

    and assigned to workers, the station service times, Tsi, are likely not to be equal.

    Simply because of the variation among work element times, some workers will be assigned more work.

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

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  • Precedence Constraints

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    Restrictions on the order in which work elements can be performed

    Precedence diagram

  • Line Balancing Algorithms

    Largest Candidate Rule Assignment of work elements to stations based on amount

    of time each work element requires Kilbridge and Wester Method

    Assignment of work elements to stations based on position in the precedence diagram

    Elements at front of diagram are assigned first Ranked Positional Weights

    Combines the two preceding approaches by calculating an RPW for each element

    COMSOAL Random Sequence Generation Sequences are generated by selecting at random from the

    set of available tasks and placing that task next in sequence

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  • Example:

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    Grommet : sealant like ring

  • Example:

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    Grommet : sealant like ring

  • Example: A problem for line balancing

    Given: The previous precedence diagram and the standard times. Annual demand=100,000 units/year. The line will operate 50 wk/yr, 5 shifts/wk, 7.5 hr/shift. Uptime efficiency=96%. Repositioning time lost=0.08 min.

    Determine a) total work content time, b) required hourly production rate to achieve the annual

    demand, c) cycle time, d) theoretical minimum number of workers required on the

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

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  • Example: Solution

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

    Twc = 4.0 min

    The hourly production rate

    Manufacturing System

    units/hr 33.53)5.7)(5(50

    000,100pR

    en

    k

    ekwc TT1

    shw

    ap

    HS

    DR

    50

  • Example: Solution

    The corresponding cycle time with an uptime efficiency of 96%

    The minimum number of workers: w* = (Min Int 4.0 /1.08=3.7) = 4 workers

    The available service time Ts = 1.08 - 0.08 = 1.00 min

    Manufacturing System

    min08.133.53

    )96.0(60cT

    pc

    R

    ET

    60

    c

    wc

    T

    Tw *

    rcs TTT

  • Measures of Balance Efficiency

    It is almost imposible to obtain a perfect line balance

    Line balance efficiency, Eb:

    Perfect line: Eb = 1

    Balance delay, d:

    Perfect line: d = 0

    Note that Eb + d = 1 (they are complement of each other)

    Manufacturing System

    s

    wcb

    wT

    TE

    s

    wcs

    wT

    TwTd

  • Overall Efficiency

    Factors that reduce the productivity of a manual line

    Line efficiency (Availability), E,

    Repositioning efficiency (repositioning), Er,

    Balance efficiency (balancing), Eb,

    Overall Labor efficiency on the assembly line =

    Manufacturing System

    br EEE

    c

    rc

    c

    sr

    T

    TT

    T

    TE

    s

    wcb

    wT

    TE

    p

    cR

    ET

    60

  • Worker Requirements

    The actual number of workers on the assembly line is given by:

    where

    w = number of workers required

    Rp = hourly production rate, units/hr

    Twc = work content time per product, min/unit

    Manufacturing System

    c

    rc

    c

    sr

    T

    TT

    T

    TE

    s

    wcb

    wT

    TE

    sb

    wc

    cbr

    wc

    br

    wcp

    TE

    T

    TEE

    T

    EEE

    TRw

    60intmin

    p

    cR

    ET

    60

  • Continously moving conveyors -

    Workstation considerations Number of stations:

    Total length of the assembly line

    where L = length of the assembly line, m: Lsi = length of station i, m

    Manufacturing System

    n

    iis

    LL1

    M

    wn

  • Continously moving conveyors -

    Workstation considerations 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

    Manufacturing System

  • Continously moving conveyors -

    Tolerance Time Defined as the time a work unit spends inside the

    boundaries of the workstation

    Provides a way to allow for product-to-product variations in task times at a station where Tt = tolerance time, min; Ls = station length, m (ft); vc = conveyor speed, m/min (ft/min)

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    c

    st

    v

    LT

  • Continously moving conveyors -Total

    Elapsed Time The time a work unit spends on the assembly

    line. where ET = total elapsed time, min; Tt = tolerance time, min; L = length of the assembly line, m (ft); vc = conveyor speed, m/min (ft/min)

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    t

    c

    nTv

    LET

  • Line Balancing Objective

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

    Minimize (wTs Twc) or Minimize Subject to: (1) (2) all precedence requirements are obeyed

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    w

    i

    sis TT1

    s

    ik

    ek TT

  • Largest Candidate Rule

    List all work elements in descending order based on their Tek values; then, proceed three-step procedure:

    Start at the top of the list and selecting the first element that satisfies precedence requirements and does not cause the total sum of Tek to exceed the allowable Ts value When an element is assigned, start back at the top of the list and repeat

    selection process

    When no more elements can be assigned to the current station, proceed to next station

    Repeat steps 1 and 2 until all elements have been assigned to as many stations as needed

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  • Example:

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    Grommet : sealant like ring

  • Solution for Largest Candidate Rule

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  • Solution for Largest Candidate Rule

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  • Solution for Largest Candidate Rule

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  • Solution for Largest Candidate Rule

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    Physical layout of workstations and assignment of elements to stations using the largest candidate rule

  • Kilbridge and Wester Method

    Selects work elements for assignment to stations according to their position in the precedence diagram

    Work elements in the precedence diagram are arranged into columns

    The elements can be organized into a list according to their columns, with the elements in the first column listed first

    If a given element can be located in more than one column, then list all of the columns for that element

    Proceed with same steps 1, 2, and 3 as in the largest candidate rule

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  • Kilbridge and Wester Method

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  • Kilbridge and Wester Method

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  • Kilbridge and Wester Method

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  • Ranked Positional Weights Method

    A ranked position weight (RPW) is calculated for each work element

    RPW for element k is calculated by summing the Te values for all of the elements that follow element k in the diagram plus Tek itself

    Work elements are then organized into a list according to their RPW values, starting with the element that has the highest RPW value

    Proceed with same steps 1, 2, and 3 as in the largest candidate rule

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  • Solution for Ranked Positional Weights

    Method

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  • Solution for Ranked Positional Weights

    Method

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  • Balance efficiency

    Largest Candidate Rule

    Kilbridge and Wester Method

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    8.0

    15

    4bE

    8.0

    15

    4bE

  • Balance efficiency

    Ranked Positional Weights Method

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    6.5796.060

    601

    6060

    108.092.0

    87.092.05

    4

    ERR

    TR

    TTT

    E

    cp

    c

    c

    rsc

    b

  • Mixed Model Assembly Lines

    A manual production line capable of producing a variety of different product models simultaneously and continuously (not in batches)

    Problems in designing and operating a MMAL: Determining number of workers on the line

    Line balancing - same basic problem as in SMAL except differences in work elements among models must be considered

    Model launching - determining the sequence in which different models will be launched onto the line

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  • Other Considerations in Line Design

    Line efficiency Management is responsible to maintain line

    operation at efficiencies (proportion uptime) close to 100% Implement preventive maintenance Well-trained emergency repair crews to quickly fix

    breakdowns when they occur Avoid shortages of incoming parts to avoid forced

    downtime Insist on highest quality components from suppliers

    to avoid downtime due to poor quality parts

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  • Other Considerations - continued

    Methods analysis To analyze methods at bottleneck or other

    troublesome workstations

    Subdividing work elements It may be technically possible to subdivide some

    work elements to achieve a better line balance

    Sharing work elements between two adjacent stations

    Alternative cycles between two workers

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  • Other Considerations - continued

    Utility workers To relieve congestion at stations that are

    temporarily overloaded

    Changing workhead speeds at mechanized stations Increase power feed or speed to achieve a better

    line balance

    Preassembly of components Prepare certain subassemblies off-line to reduce

    work content time on the final assembly line

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  • 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 large

    task 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 stocks

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  • Other Considerations - continued

    Zoning constraints - limitations on the grouping of work elements and/or their allocation to workstations

    Positive zoning constraints Work elements should be grouped at same station

    Example: spray painting elements

    Negative zoning constraints Elements that might interfere with each other

    Separate delicate adjustments from loud noises

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  • Other Considerations - continued

    Position constraints Encountered in assembly of large products such as

    trucks and cars, making it difficult for one worker to perform tasks on both sides of the product

    To address, assembly workers are positioned on both sides of the line

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