201302 06 SiMan Assembly
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Transcript of 201302 06 SiMan Assembly
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Assembly
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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
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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
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Two assembly operators working on an engine assembly line (photo courtesy of Ford Motor Company)
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Manufacturing System
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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
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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
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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
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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
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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
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Synchronous Transport
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All work units are moved simultaneously to their respective next workstations with quick, discontinuous motion
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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.
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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
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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
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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
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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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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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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
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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
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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
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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
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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
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Example:
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Grommet : sealant like ring
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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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Solution for Largest Candidate Rule
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Physical layout of workstations and assignment of elements to stations using the largest candidate rule
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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
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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
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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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