Module 3 - Manual Assembly Lines
Sections:
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
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
 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
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
 Most consumer products are assembled on manual
assembly lines
Why Assembly Lines are so Productive
 Specialization of labor
 Learning curve
 Interchangeable parts
 Components made to close tolerances
 Work flow principle
 Products are brought to the workers
 Line pacing
 Workers must complete their tasks within the cycle time
of the line
Manual Assembly Line
Configuration of a manual assembly line with n manually
operated workstations
Typical Products
Made on Assembly Lines
Automobiles
Personal computers
Cooking ranges
Power tools
Dishwashers
Refrigerators
Dryers
Telephones
Furniture
Toasters
Lamps
Trucks
Luggage
Video DVD players
Microwave ovens
Washing machines
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
Assembly Workstation
It is a designated location along the work flow path at which
one or more work elements are performed by one or more
workers
Typical operations performed at manual assembly stations
Adhesive application
Electrical connections
Snap fitting
Sealant application
Component insertion
Soldering
Arc welding
Press fitting
Stitching/stapling
Spot welding
Riveting
Threaded fasteners
Assembly Workstation
 For manual assembly line, manning level (M) of
workstation i, symbolized by Mi, is the number of workers
assigned to that station, where i =1, 2 ….n, and n is
number of workstations on the line
 For larger products such as assembly of a truck, Mi >1
 Average manning level of manual assembly line is:
𝑤
𝑀=
𝑛
where w is the number of workers on the assembly line
Assembly Workstation
 However, manual assembly lines often include utility
workers that are not assigned to any specific workstation
 Utility workers help workers who fall behind, relieve
workers for personal breaks, and perform material handling
and maintenance tasks
 Including utility workers we have:
𝑀=
𝑛
𝑖=1 𝑤𝑖
𝑤𝑢 +
𝑛
where 𝑤𝑢 is the number of utility workers assigned to the
system
Work Transport Systems
 Two basic categories:
 Manual
 Mechanized
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
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
Types of Mechanized Work Transport
 Continuous transport
 Conveyor moves at constant speed
 Synchronous transport
 Work units are moved simultaneously with stop-andgo (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
Continuous Transport
Conveyor moves at constant velocity vc
Synchronous Transport
All work units are moved simultaneously to their
respective next workstations with quick, discontinuous
motion
Asynchronous Transport
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
Synchronous transport
Asynchronous transport
Overhead trolley conveyor
Belt conveyor
Drag chain conveyor
Walking beam
Rotary indexing machine
Power-and-free conveyor
Cart-on-track conveyor
Automated guided vehicles
Power and free conveyor
 The defining feature of a power and free conveyor is that it
consists of two tracks — an upper track powered by a chain,
and the lower track is unpowered.
 Trolleys, which carry the load, run on the lower track,
supported by rolling wheels.
 Pusher dogs on the powered track engage with the trolleys to
move them and disengage with the trolleys to stop them
 Unlike traditional linear conveyors, power and free versions
can allow products to accumulate, or “stack up” by holding
trolleys stationary while other trolleys “catch up” and join
them.
Cart-on-track conveyor
 Cart-On-Track Conveyor has a track on it and the cart can
be driven on the track.
 These carts are transported by rotating a tube.
 There is drive wheel connected to each cart that rests on the
tube and that is used to vary the speed of the cart .
 These carts are independently controlled by tubes and
multiple carts can be located on the tube.
 Accumulation is achieved by maintaining the drive wheel
parallel to the tube.
Automated guided vehicles
 Computer-controlled and wheel-based, automatic guided
vehicles (AGV) are load carriers that travel along the floor of a
facility without an onboard operator or driver.
 Their movement is directed by a combination of software and
sensor-based guidance systems.
 Because they move on a predictable path with precisely
controlled
acceleration
and
deceleration,
automatic obstacle detection bumpers.
and
include
Line Pacing

A manual assembly line operates at a certain cycle time 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
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
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 worker’s 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
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
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
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
Analysis of single model assembly lines
 Product demand is usually expressed as annual quantity
which can be reduced to an hourly rate
 Assuming that the plant operates N weeks per year, the
required hourly production rate, 𝑅𝑝 is given by:
𝐷𝑎
𝑅𝑝 =
𝑁𝑆𝑤 𝐻𝑠ℎ
where 𝐷𝑎 is annual demand for a single product, 𝑆𝑤 is
number of shifts per week, 𝐻𝑠ℎ is number of hours per shift
 𝑅𝑝 must be converted to a cycle time 𝑇𝑐 which is the time
interval for operating the line
60𝐸
𝑇𝑐 =
𝑅𝑝
where 𝑇𝑐 is cycle time of the line in mins/cycle, E is the line
efficiency, and the constant 60 converts hourly production rate
to cycle time in mins
 Cycle time 𝑇𝑐 establishes the ideal cycle rate, 𝑅𝑐 of the line
60
𝑅𝑐 =
𝑇𝑐
 𝑅𝑐 must be > 𝑅𝑝 since a line may not be 100% efficient
𝑅𝑝 𝑇𝑐
𝐸=
=
𝑅𝑐 𝑇𝑝
 This is due to lost production time from equipment breakdowns,
power outages, quality problems, labour problems etc.
 An assembled product requires a certain total amount of time to
build called work content time (𝑇𝑤𝑐 )
 To determine the number of workers, w, on the production line:
𝑊𝐿
𝑤=
𝐴𝑇
60𝐸𝑇𝑤𝑐
and 𝑊𝐿 = 𝑅𝑝 𝑇𝑤𝑐 or 𝑊𝐿 =
𝑇𝑐
where WL is workload to be accomplished over a period of
time, and AT is the available time in the period.
 The theoretical minimum number of workers, 𝑤 ∗ is computed as
𝑇𝑤𝑐
∗
𝑤 = 𝑀𝑖𝑛𝑖𝑚𝑢𝑚 𝑖𝑛𝑡𝑒𝑔𝑒𝑟 ≥
𝑇𝑐
 Achieving the theoretical minimum value in practice is difficult
because of repositioning losses and line balancing problem (it
is difficult to divide the work content time evenly amongst all
workstations
Repositioning losses
 Repositioning losses on a production line occur because
some time is required each cycle to reposition the worker or
the work unit or both
 Repositioning time, (𝑇𝑟 ) must be subtracted from 𝑇𝑐 to
obtain the available time remaining to perform assembly
task at each station as the service time, 𝑇𝑠𝑖
 Maximum service time occurs at bottleneck station and can
be described as:
𝑀𝑎𝑥 𝑇𝑠𝑖 ≤ 𝑇𝑐 − 𝑇𝑟
Components of Cycle Time Tc
𝑇𝑠
𝐸𝑟 =
𝑇𝑐
 Components of cycle time at several workstations on a
manual assembly line. At the bottleneck station, there is no
idle time.
 where 𝐸𝑟 is the repositioning efficiency
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
Line Balancing Problem
 Total work content time 𝑇𝑤𝑐
𝑛𝑒
𝑇𝑤𝑐 =
𝑇𝑒𝑘
𝑘=1
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
𝑇𝑠𝑖 =
𝑇𝑒𝑘
𝑘𝜀𝑖
 The station service times must add up to the total work
content time
𝑛
𝑇𝑤𝑐 =
𝑇𝑠𝑖
𝑖=1
Measures of Balance Efficiency
 It is almost impossible to obtain a perfect line balance due to
differences in minimum rational work element times and
precedence constraints among elements
 Line balance efficiency, 𝐸𝑏 :
𝑇
𝑤𝑇𝑠
𝐸𝑏 = 𝑤𝑐
where 𝑇𝑠 is maximum available service time on line 𝑀𝑎𝑥 𝑇𝑠𝑖
 Perfect line is when 𝐸𝑏 = 1
 Balance delay, d :
𝑑=
𝑤𝑇𝑠 −𝑇𝑤𝑐
𝑤𝑇𝑠
 Perfect line: d = 0
 Note that 𝐸𝑏 + d = 1 (they are complement of each other)
 Overall Labor efficiency on the assembly line = 𝐸𝐸𝑟 𝐸𝑏
 The challenge lies with establishing the relationship
before the line is built and operated
Line Balancing Algorithms
 The objective in line balancing is to distribute the total
work content on the assembly line as evenly as possible
among the workers
𝑀𝑖𝑛𝑖𝑚𝑖𝑠𝑒 𝑤𝑇𝑠 − 𝑇𝑤𝑐 𝑜𝑟 𝑀𝑖𝑛𝑖𝑚𝑖𝑠𝑒 𝑤
𝑖=1 𝑇𝑠 − 𝑇𝑠𝑖
subject to
𝑇𝑒𝑘 ≤ 𝑇𝑠
𝑘𝜀𝑖
 𝐴𝑙𝑙 𝑝𝑟𝑒𝑐𝑒𝑑𝑒𝑛𝑐𝑒 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑚𝑒𝑛𝑡𝑠 𝑎𝑟𝑒 𝑜𝑏𝑒𝑦𝑒𝑑
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
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
Determining number of workers in Mixed Model
Assembly Lines
 To determine the number of workers, w, for mixed model
𝑊𝐿
assembly line: 𝑤 =
and
𝐴𝑇
𝑃
𝑊𝐿 =
𝑅𝑝𝑗 𝑇𝑤𝑐𝑗
𝑗=1
where WL is workload to be accomplished over a scheduled
period of time, and AT is the available time per worker in the
period. P is number of models to be produced and j is used to
identify the model
 The theoretical minimum number of workers, 𝑤 ∗ is computed as
𝑊𝐿
∗
𝑤 = 𝑀𝑖𝑛𝑖𝑚𝑢𝑚 𝑖𝑛𝑡𝑒𝑔𝑒𝑟 ≥
60𝐸
𝐴𝑇 = 60𝐸𝐸𝑟 𝐸𝑏
where E is line efficiency, 𝑬𝒓 is repositioning efficiency, 𝑬𝒃 is
balance efficiency.
Mixed Model Assembly Line Balancing Objective
 To distribute the total work content on the assembly line as
evenly as possible among the workers
 The objective can be expressed as:
𝑤
𝑀𝑖𝑛𝑖𝑚𝑖𝑠𝑒 𝑤𝐴𝑇 − 𝑊𝐿 𝑜𝑟 𝑀𝑖𝑛𝑖𝑚𝑖𝑠𝑒
𝐴𝑇 − 𝑇𝑇𝑠𝑖
𝑖=1
where w is number of workers assuming that Mi =1, so that n =
w. AT is available time in the period of interest (min), and 𝑇𝑇𝑠𝑖 is
total service time at station i to perform its assigned portion of
the workload in mins.
𝑃
𝑊𝐿 =
𝑅𝑝𝑗 𝑇𝑤𝑐𝑗
𝑗=1
Total time per element, 𝑇𝑇𝑘 is:
𝑃
𝑇𝑇𝑘 =
𝑅𝑝𝑗 𝑇𝑒𝑗𝑘
𝑗=1
 Total service time at each station is computed as:
𝑃
𝑇𝑇𝑠𝑖 =
𝑇𝑇𝑘
𝑘𝜀𝑖
where 𝑇𝑇𝑠𝑖 is total service time at station i
 Balance efficiency, 𝐸𝑏 is computed as
𝑊𝐿
𝐸𝑏 =
𝑤 𝑀𝑎𝑥 𝑇𝑇𝑠𝑖
where 𝑀𝑎𝑥 𝑇𝑇𝑠𝑖 is the maximum value of total service time
among all stations
Precedence Constraints
 Restrictions on the order in which work elements can be
performed
Precedence diagram
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
Model launching in Mixed Model Lines
 In a single-model line, this time interval is constant and set
equal to the cycle time Tc.
 The same applies for a batch-model line, but Tc is likely to
differ for each batch because the models are different and
their production requirements are probably different.
 In a mixed-model line, model launching is more complicated
because each model is likely to have a different work content
time, which translates into different station service times.
 Thus, the time interval between launches and the selection of
which model to launch are interdependent.
Model launching in Mixed Model Lines
 For example, if a series of models with high work content
times are launched at short time intervals, the assembly line
will quickly become congested (overwhelmed by too much
work).
 On the other hand, if a series of models with low work
content times are launched at long time intervals, then
stations will be starved for work (with resulting idleness).
 Neither congestion nor idleness is desirable.
 Determining the time interval between successive launches is
referred to as the launching discipline, 2 approaches are:
Variable rate launching
 Variable rate launching – the time interval between the
launching of the current base part and the next is set equal
to the cycle time of the current item
𝑇𝑤𝑐𝑗
𝑇𝑐𝑣 𝑗 =
𝑤𝐸𝑟 𝐸𝑏
where 𝑻𝒄𝒗 𝒋
is time interval before the next launch in
variable-rate launching, w is number of workers on the line,
𝑻𝒘𝒄𝒋 is work content time of the product just launched
(model j)
in mins, 𝑬𝒓 is repositioning efficiency, 𝑬𝒃 is
balance efficiency.
Variable rate launching
 The advantage of VRL is that units can be launched in any order
without causing idle time or congestion at w/s, as long as the
specified model mix is achieved by the end of the shift.
 The model mix can be readily adjusted to adapt to changes in
demand for the various products made on the line.
 However, the work carriers on a moving conveyor are usually
located at constant intervals along its length and so the work units
must be attached only at these positions.
 This is not compatible with VRL which presumes that work units
can be attached at any location along the conveyor corresponding
to the VRL interval Tcv for the preceding model.
Fixed rate launching for two models
 In Fixed rate launching, the time interval between 2
consecutive launches, 𝑻𝒄𝒇 is constant.
 The launching discipline is usually set by the speed of a
conveyor and the spacing between work carriers.
 The schedule must also be consistent with the time and
manpower available on the line, so 𝐸𝑟 and line 𝐸𝑏 must be
figured in.
1 𝑃
𝑅𝑝 𝑗=1 𝑅𝑝𝑗 𝑇𝑤𝑐𝑗
𝑇𝑐𝑓 =
𝑤𝐸𝑟 𝐸𝑏
Where 𝑻𝒄𝒇 is in mins, 𝑹𝒑𝒋 is production rate of model j in units/hr,
𝑻𝒘𝒄𝒋 is work content time of model j in min/unit, 𝑹𝒑 is total
production rate of all models, P is number of models produced,
𝑬𝒓 is repositioning efficiency, 𝑬𝒃 is balance efficiency.
Fixed rate launching for two models
 Congestion and idle time can be identified in each successive
launch as the difference between the cumulative fixed rate
launching interval and the sum of the launching intervals for the
individual models launched on the line.
𝑚
𝐶𝑜𝑛𝑔𝑒𝑠𝑡𝑖𝑜𝑛 𝑡𝑖𝑚𝑒 𝑜𝑟 𝑖𝑑𝑙𝑒 𝑡𝑖𝑚𝑒 =
𝑇𝑐𝑗ℎ − 𝑚𝑇𝑐𝑓
ℎ=1
where 𝑻𝒄𝒇 is fixed rate launching interval in mins, m is launch
sequence during the period, h is launch index number
for
summation purposes, 𝑻𝒄𝒋𝒉 is cycle time associated with model j in
launch position h, calculated as: 𝑇𝑐𝑗ℎ =
𝑇𝑤𝑐𝑗
𝑤𝐸𝑟 𝐸𝑏
Fixed rate launching for two models
 Congestion is realised when the previous equation on
“ 𝐶𝑜𝑛𝑔𝑒𝑠𝑡𝑖𝑜𝑛 𝑡𝑖𝑚𝑒 𝑜𝑟 𝑖𝑑𝑙𝑒 𝑡𝑖𝑚𝑒 “ yields a positive difference,
indicating that the actual sum of task times for the models thus
far launched (m) exceeds the planned cumulative task time
 Idle time yields a negative difference, indicating that the actual
sum of task times for the models is less than the planned time
for the current launch (m)
 It is desirable to minimize both congestion and idle time, so
𝑚
𝐹𝑜𝑟 𝑒𝑎𝑐ℎ 𝑙𝑎𝑢𝑛𝑐ℎ 𝑚, 𝑠𝑒𝑙𝑒𝑐𝑡 𝑗 𝑡𝑜 𝑚𝑖𝑛𝑖𝑚𝑖𝑠𝑒
𝑇𝑐𝑗ℎ − 𝑚𝑇𝑐𝑓
ℎ=1
2
Fixed rate launching for three or more models
 There is an additional term for three or more models, which is
𝑅𝑝𝑗
𝑄𝑗𝑚
where 𝑹𝒑𝒋 is the quantity of model j to be produced during the
period, 𝑸𝒋𝒎 is quantity of model j units remaining to be
launched during the period as m (number of launches)
increases, units/hr
 The fixed rate launching procedure can be expressed as:
𝑚
𝑅𝑝𝑗
2
𝐹𝑜𝑟 𝑒𝑎𝑐ℎ 𝑙𝑎𝑢𝑛𝑐ℎ 𝑚, 𝑠𝑒𝑙𝑒𝑐𝑡 𝑗 𝑡𝑜 𝑚𝑖𝑛𝑖𝑚𝑖𝑠𝑒
𝑇𝑐𝑗ℎ − 𝑚𝑇𝑐𝑓
+
𝑄𝑗𝑚
ℎ=1
 The effect of the additional term is to reduce chances that a unit
of any model j will be selected for launching as the number of
units of that model already launched during the period
increases.
Workstation Considerations
 A workstation is a position along the assembly line where
one or more workers perform assembly tasks
𝑤
𝑛=
𝑀
 A workstation has a length 𝐿𝑠𝑖 where i denotes station i
 Total length of assembly line, L in m or feet is
𝑛
𝐿=
𝐿𝑠𝑖
𝑖=1
 A constant speed conveyor is a common transport system
in manual assembly lines
 Base parts are launched at the beginning of the line at a
constant pace equal to 𝑇𝑐
Workstation Considerations
 The feed rate of the conveyor is simply the reciprocal of 𝑇𝑐
1
𝑓𝑝 =
𝑇𝑐
where 𝑓𝑝 is feed rate on the line, in products/min
 A constant feed rate on a constant speed conveyor provides
a centre to centre distance, 𝑠𝑝 between base parts
𝑣𝑐
𝑠𝑝 = = 𝑣𝑐 𝑇𝑐
𝑓𝑝
where 𝑣𝑐 is velocity of conveyor in m/min or ft/min
Workstation Considerations
 Tolerance time, is the time a work unit spends inside the
boundaries of the workstation
𝐿𝑠
𝑇𝑡 =
𝑣𝑐
where 𝑇𝑡 is tolerance time in min/part, assuming that all
station lengths are equal
 If stations have different lengths, identified by 𝐿𝑠𝑖 then the
tolerance time will differ proportionally since 𝑣𝑐 is constant
𝑛
𝐿
𝐸𝑇 = =
𝑇𝑡𝑖
𝑣𝑐
𝑖=1
where ET is elapsed time a work unit (specifically the base
part) spends on the conveyor during assembly in mins
 If all tolerance times are equal 𝐸𝑇 = 𝑛𝑇𝑡
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
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
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
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
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
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
Alternative Assembly Systems
 Single-station manual assembly cell
 Worker teams
 Automated assembly systems
Single-Station Manual Cell
 A single workstation in which all of the assembly work is
accomplished on the product or on some major
subassembly
 Common for complex products produced in small
quantities, sometimes one-of-a-kind
 Custom-engineered products
 Prototypes
 Industrial equipment (e.g., machine tools)
Assembly by Worker Teams
 Multiple workers assigned to a common assembly task
 Workers set their own pace
 Examples
 Single-station cell with multiple workers
 Swedish car assembly (job enlargement) - product
is moved through multiple workstations by AGVS,
but same worker team follows it from station to
station
Reported Benefits of Team Assembly
 Greater worker satisfaction
 Better product quality
 Increased capability to cope with model variations
 Greater ability to cope with problems that require more
time rather than stopping the entire production line
 Disadvantage:
 Team assembly is not capable of the high production
rates of a conventional assembly line