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Line Balancing
Chapter · January 2015
DOI: 10.1002/9781118785317.weom100058
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Line balancing is a technique used in connection
with the design of PRODUCT LAYOUT or ‘‘lines’’.
The term ‘‘balancing’’ is used because
one of its main objectives is to minimize the idle
time and spread it as evenly as possible across
the workstations.
When balancing a line, the following factors
need to be taken into account:
- the required output rate or cycle time (which
depends on the demand for the product);
- precedence constraints (these are restrictions
on the order in which tasks can be done; in
other words, certain tasks will have ‘‘predecessor
tasks’’ that must be done first);
- zoning constraints (these are restrictions on
where certain tasks or combinations of tasks
should, or should not, take place);
- whether there is a need for workstation duplication
or replication (this would be the case
when any task takes longer than the available
cycle time).
The line-balancing problem comprises two
aspects: (i) determination of the required number
of stations and (ii) the assignment of tasks to each
station with the objective of maximizing efficiency
(by minimizing idle time and spreading it
evenly across workstations).
The effectiveness of the balance decision is
measured by the ‘‘balance loss’’ of the line.
The balance loss is the time invested in
making one product that is lost
through imbalance, expressed as a percentage
of the total time investment. For a paced n
stage line, the time lost through imbalance is
the cumulative difference between the stations’
allocated work times and the cycle time allowed
by the pacing of the line. For unpaced lines, it is
the cumulative difference between each stage’s
work time and that of the stage with the largest
work time (this effectively governs the cycle time
of the whole line).
A very simple line-balancing problem may be
solved by ‘‘trial and error’’. Most practical problems,
however, are extremely complex, requiring
thousands of tasks to be assigned across hundreds
of workstations and with numerous precedence
and zoning constraints to be taken into account.
To solve such problems, a large number of
heuristic algorithms have been developed such as
the Kilbridge and Wester method and the ranked
positional weights technique. Being based on
heuristics, or ‘‘rules’’ that have been tested
empirically, such techniques can provide good,
although not necessarily optimal, results. More
recently, simulation has grown in popularity as
an approach to balancing lines, and a visual interactive
simulation can allow the line designed to
immediately see the effect of any modifications
made.
Product layouts have traditionally been used
to produce highly standardized products, but
today the demand is for a greater variety of products
or models. Therefore, two types of line are
now in widespread use and require a modification
to the traditional line-balancing approach. These
are multimodel lines, where the line is reorganized
periodically to produce different models or
variants, and mixed-model lines, where the line
is designed to allow simultaneous production of
any model or variant without reorganization.
The aim in multimodel line balancing should
be to minimize total production cost, taking
account of the additional factor of changeover
costs. For very large batches, the problem degenerates
into the successive application of single model
line balancing.
The main costs of an operator changing
from one product to another are connected
with reallocation of inventory and equipment to
workstations and LEARNING CURVES of operatives
in new jobs. To reduce these costs, the
number of stations and location of equipment
should be constant whenever possible, and work
elements common to more than one model
should always be performed by the same operator.
As work content and production requirements
vary between models, the cycle times are
the best factors to manipulate in reducing idle
time, but balancing efficiency may be sacrificed
for compatibility. The total balance loss will be
the average per model, weighted in proportion
to production ratios. A sensible ploy is to balance
the line for the most popular model and to adjust
this basic arrangement by empirical methods for
the other models. If this is unsatisfactory, the
steps may be repeated but centred on the model
of second highest production volumes, etc.
For very small batches, the problem is akin
to the mixed-model line. Here, achieving a good
long-term balance is more difficult and depends
on the sequencing of model types proceeding
down the line. One approach is to balance the
line using a range of task times for each activity.
Bibliography
Bartholdi, J.J. and Eisenstein, D.D. (1996) A production
line that balances itself. Operations Research, 44 (1),
21–35.
Bollinger, S. (1998) Fundamentals of Plant Layout, Society
of Manufacturing Engineers in Association with
Richard Muther and Associates, Dearborn, MI.
Ghosh, S. and Gagnon, R. (1989) A comprehensive
literature review and analysis of the design, balancing
and scheduling of assembly systems. International
Journal of Production Research, 27 (4), 637–670.
Gunther, R.E., Johnson, G.D. and Peterson, R.S. (1983)
Currently practiced formulations for the assembly line
balance problem. Journal of Operations Management,
3 (4), 209–221.
Sule, D.R. (1994) Manufacturing Facilities: Location,
Planning and Design, PWS, Boston.
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