LAYOUT PLANNING
After a suitable site for a plant is chosen as a result of location planning, the proper
locations of machines or work stations within the plant are determined by what is called
layout planning.
This chapter will focus on general issues pertaining to facilities layout rather than
determining the optimal locations
of machines or workstations within the plant.
The layout type pretty much depends on the type of the process to be used in the
plant. since the processing requirements will affect the configuration of work stations and
hence the plant layout, different processing types and their features are listed below.
Types of Processing
1. Repetitive Processing
High volume of production
Low unit costs
High utilization of labor and equipment
Highly standardized products
Highly repetitive operations
Low skill requirements
Inflexible
2. Intermittent Processing
Lower volume of production
Higher unit costs
Lower utilization of labor and equipment
More customized services
Production in batches or lots
Skilled or semi-skilled workers
More flexible
3. Projects
Complex
Non repetitive and unique activities
Very high skill requirements
The layout type suitable for each processing type listed above is shown below:
Types of Processing
Basic Layout Types
Repetitive Processing -------->Product Layouts
Intermittent processing ------>Process Layouts
Projects ------------------------> Fixed-position Layouts
Combination Layouts
In case of repetitive mass production, highly standardized items such as radios are
produced. All the workstations or machines are located in the plant to permit efficient and
smooth movement of the parts or materials used in the standard item that is produced.
Hence, the whole layout is focused on the product and product flow; therefore, it is called
product layout.
Intermittent processing is characterized by general purpose equipment such as
drilling equipment, lathes, etc. that can handle a variety of different processing requirements
that involve a variety of different, nonstandard products. Hence, the layout is based on
different processing requirements and it is called process layout. The drilling equipments
are grouped in one department, lathes are grouped in another department and so on. Each
department has a special function and therefore, this type of layout is also called functional
layout.
Projects involve non repetitive, one-time unique activities of complex jobs. An good
example would be a dam construction. It is analogous to one-time production of a very
major item. Since the production item is very big such as a dam, the product would not
move from work station to work station. Instead, materials, equipment, and workers are
moved to the location of the project. Since the location of the project remains fixed or
stationary, the corresponding layout type is called fixed position layout.
The three basic layout types discussed above are ideal models. In real life, it is
common to see layouts that represent some combination of these pure layouts.
Product layouts are suitable when rapid and smooth flow of high volume of
standardized products is the major goal as in the case of radio production; usually only one
or a few similar standardized items are produced.
Process layouts, on the other hand, are more suitable for producing a larger number
of different products or customized products through the use of flexibility provided by the
general purpose machinery. Even though the process layouts have the advantage of
flexibility, such layouts involve complicated scheduling problems due to the multiple paths
for parts to different functional departments.
Cellular manufacturing is a technique that attempts to take the advantage of the
rapid, smooth, and efficient flow of production layout and flexibility advantage of the
process layout to produce a variety of items in smaller amounts. In this type of layout,
machines are grouped together to produce a set of similar items; such a grouping of
machines is called a cell. For different dissimilar sets, we may have other machine
groupings or cells. Grouping of the items is also necessary apart from the grouping of the
machines. Grouping of items that have similar design or manufacturing characteristics is
called group technology and it is essential for effective cellular manufacturing.
Automated versions of cellular manufacturing that involve computer controls in
starting the work at each machine and in transferring the parts from machine to machine
with little or no human intervention are called flexible manufacturing systems.
In product layouts, the tasks performed in producing a certain standardized item
must be assigned to the workstations in a way to achieve a smooth workflow. There are
many tasks in the production line that must be assigned to work stations. The determination
of which tasks to assign to which work stations along the production line, known as line
balancing will have an impact on the configuration of the work stations and hence the plant
layout.
The configuration of the workstations must be such that the idle time is minimized in
the workstations along the production line. This can be achieved through line balancing.
An example illustrates the line balancing technique.
Example:
In a production line, 320 units are produced in an 8-hour day. There are four tasks:
Task
a
b
c
d
Time(min)
Follows
1.0
0.7
0.5
0.2
2.4
a
b, c
Compute the cycle time, minimum number of stations required, and assign tasks to
stations.
Solution:
The precedence diagram on the basis of the above information
is depicted as follows.
a
c
b
d
The cycle time and minimum number of stations computations are as follows:
Cycle Time = operating time / output = 480 min. / 320 units = 1.5 min. /unit. The cycle
time of 1.5 minutes indicates that one unit of an item is produced every 1.5 minutes; i.e., one
unit comes off the production line every 1.5 minutes.
Minimum number of stations = total time / cycle time = 2.4 / 1.5 = 1.6 ≈ 2 stations
(always round up no matter how small the fraction after the decimal point is; otherwise the
job won't be complete).
Since the minimum number of stations is 2 we first draw two empty squares or rectangles
to represent these two stations. Then, we assign the tasks in such a way as to preserve the
order depicted in the precedence diagram and to ensure that the station times would not
exceed the cycle time of 1.5.
Alternative 1.
Station 1
Station 2
1.0
a
0.7
aa
b
0.5
0.2
d
c
Station Time 1.5
Idle Time
0
0.9
0.6
OR
Alternative 2
Station 1
Station 2
1.0
0.7
a
b
0.5
0.2
d
c
Station Time
Idle Time
1.0
0.4
1.4
0
After the assignment of tasks to the workstations, the new revised cycle times must
be computed where the cycle time is longest station time; the cycle time is not necessarily
the same as the previous one as our assignment may be different the present arrangement
that the production line has.
In Alternative 1, the revised cycle time is still 1.5 whereas in Alternative 2 the
revised cycle time is 1.4 as the actual cycle time is the longest station time. The idle times
are computed by finding the differences between the revised cycle time and the station
times. Alternative 2 is preferred because the percentage of idle time is smaller than that of
Alternative 1 indicating that Alternative 2 is more balanced.
The percentage of idle time is given by:
% idle time = idle time per cycle / # of stations x cycle time.
For Alternative 1, % idle time = (0 + 0.6) / 2 x 1.5 = 0.2 or 20%.
For Alternative 2, % idle time= (0 + 0.4) / 2 x 1.4 = 0.14 or 14%
The lower the percentage of idle time the more balanced is the production line; a idle time
percentage of zero indicates that the line is perfectly balanced.