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Line of Balance
The Line Of Balance (LOB) process is employed when a repetitive
process exists within the contract's work scope. The manufacturing of parts
and the assembly of units in the factory are two candidates for the use of LOB.
Please print and/or refer to Exhibit 1.
Line Of Balance (LOB) is a management control process for collecting, measuring
and presenting facts relating to time, cost and accomplishment - all measured
against a specific plan. It shows the process, status, background, timing and
phasing of the project activities, thus providing management with measuring
tools that help:
- Comparing actual progress with a formal objective plan.
- Examining only the deviations from established plans, and gauging their
degree of severity with respect to the remainder of the project.
- Receiving timely information concerning trouble areas and indicating
areas where appropriate corrective action is required.
- Forecasting future performance.
The "Line of Balance" itself is a graphic device that enables a manager
to see at a single glance which of many activities comprising a complex operation
are "in balance" - i.e., whether those which should have been completed
at the time of the review actually are completed and whether any activities
scheduled for future completion are lagging behind schedule. The Line of Balance
chart comprises only one feature of the whole philosophy which includes numerous
danger signal controls for all the various levels of management concerned.
History: LOB was devised by the members of a group headed by George E. Fouch.
During 1941, the Goodyear Tire & Rubber Company monitored production with
LOB. It was successfully applied to the production planning and scheduling of
the huge Navy mobilization program of World War ll. LOB proved to be a valuable
tool for expediting production visibility during the Korean hostilities. During
this period, defense suppliers used LOB.
LOB application has been further expanded, making it suitable now across a whole
spectrum of activities ranging from research and development through job shop
and process flow operations.
Specific forms and reports will be found to differ in detail, but the basic
pattern and symbology are quite uniform throughout industry.
Standard Symbols: All LOB chart use standard symbols, as shown in the
lower right hand corner of Exhibit 1. They identify the "sensors"
(milestones), i.e., readily identifiable stages of development or control point
in the process designating completion of specific activities or clusters of
activities.
Application to Production: Exhibit 1 is a simplified example of a LOB
Chart for a hypothetical fabrication and assembly operation and demonstrates
the original application in monitoring and controlling production. The finished
LOB chart displays first, the OBJECTIVE (the required delivery schedule), as
shown in the upper left hand portion. Second, there is a clearly defined PLAN
for meeting that objective, indicating interrelationships, and how each part
of component fits into the assembly process, as well as the exact point in the
cycle when each one is required to be available. This is shown in the graphing
of sensors, using standard symbols, in the lower half of the chart. The bottom
scale is the number of working periods (in this case, the measure is in days),
counting backwards from total completion, when each component must be finished.
Third, there is an appraisal of the progress that has been achieved, given by
the vertical bars in the PROGRESS chart in the upper right hand portion. Finally,
also in the upper right hand portion, there is the LINE OF BALANCE, (i.e., a
measure of the level of progress that must have been reached if the objective
is to be met on schedule, according to the established plan). These four basic
elements are vital ingredients of any effective management system. Together
they will provide for the continuous exercise of authority and create a balanced
and integrated operation out of a large number of individual and uncoordinated
transactions.
The Objective curve is a plot of schedule cumulative deliveries against calendar
dates. In this instance, the curve tells us that a total of ninety units are
scheduled for delivery between November 1 and June 30. The dotted curve indicates
that actual deliveries have fallen below the required number, reaching only
thirty-eight units by May 10, whereas forty-eight had been planned.
The Operating (manufacturing) Plan is represented by the series of interconnecting
horizontal lines, seen in the lower portion of the Line of Balance Chart. Along
these lines are the sensors indicating identifiable stages of development and
control points. These control point are numbered consecutively from left to
right across the schematic diagram, and from top to bottom wherever two or more
points have a common position along the horizontal axis. As will be seen later,
each of these control sensors is keyed by a corresponding number to a bar graph
in the Progress portion of the LOB chart. The Operation Plan illustrated has
an established cycle of twenty-four days per unit. It indicates the manner in
which the several types and kinds of parts and components are joined to form
the completed product.
To restrict the number of sensor points to a minimum (no more than fifty), certain
conventions have been introduced. One convention is to develop a separate chart
for each of two or more categories of parts (such as, purchased, company made,
major components, customer furnished parts, etc.). In any case, there always
remains the requirement for summary of the whole to indicate the overall program
state. A Summary Chart generally is made by selecting key control points from
each of the supporting charts, and having each such point represent a number
of subordinate sensors.
A similar device frequently is adopted in the treatment of complex products
consisting of a large number of parts. This expedient calls for each sensor
to represent an association of parts (for example, a so called "family
group" of items on an indented parts list). Under such conditions the symbol
should be positioned for the earliest required part. All other related data
(such as stock status) should be representative of the least favorable condition
obtaining within the particular family group at the time of the survey.
The next step in our example is to cause a visual combination of the data displayed
in the Objective and the Plan portions of the chart. This will be used to establish
a gauge for measuring the performance requirements that will be necessary to
meet the prescribed delivery goal under operating conditions established by
the Manufacturing Plan. This combination of elements is known as the LINE OF
BALANCE, the feature that gives its name to the technique.
Deriving the Line Of Balance Referring to Exhibit 1, note that the date of the
progress review is May 10. This now becomes the date for all reference purposes.
The delivery requirements at any time will be found by erecting a perpendicular
at the point corresponding to the date in question, and extending it to intersect
the cumulative delivery curve. The value of the ordinate at that point represents
the required TOTAL DELIVERIES for that time. In the case illustrated, the curve
shows that by May 10 a total of 48 units should have been shipped. In the Line
Of Balance, the 48 units relate to sensors 24 and 25, the events that take place
at the time of delivery.
For CURRENT needs to insure FUTURE deliveries, consider sensors Numbers 1 and
2. These actions indicate initiation of the manufacturing cycle and are slated
for accomplishment 24 days prior to delivery of the finished unit. On May 10,
we must have completed not only the 48 end items sets of items 1 and 2 required
for delivery on that date, but must also have completed an additional quantity
sufficient to meet the shipping needs 24 working days later. The precise level
of this requirement can be found by erecting a perpendicular at the calendar
date that is 24 working days after May 10, that is, June 13. The cumulative
delivery curve at that point calls for 78 finished units, showing that a total
of 78 end item sets of items 1 and 2 should have been completed (or have been
available for use on May 10). The Line of Balance is drawn at this level in
the Progress Chart. Similarly, sensor 3, which is slated for 23 days prior to
the delivery date, must provide for requirements for June 12, namely, 76 units,
which is its Line of Balance.
Now, consider sensors numbers 4, 5, and 6, all of which are required 21 working
days in advance of shipment. The May 10 level of requirements for these items
is represented by the value of the ordinate at the point corresponding to June
10, 72 units. For sensor number 7, scheduled for accomplishment 18 working days
in advance of shipment, a requirement for 66 end item sets is shown by the Objective
curve value for June 5.
By following the same principle of construction, requirement levels for all
other elements are established, culminating in a 48 unit delivery schedule by
May 10, the date of the study, and providing for planned future deliveries.
The end result is the characteristic step down contour of a Line of Balance.
Properly constructed, this invariably will step downward from a high point on
the left to the level indicated for cumulative deliveries on the date of the
study. By comparing the Line of Balance with the record of completed sensors
of each item, management is afforded a graphic portrayal of program status and
an accurate forecast of shipping capability.
The vertical bars in the Progress chart are typical LOB representation of the
progress being made on a program. As was mentioned earlier, each sensor in the
Operating Plan is keyed by an identifying number to a bar graph display. The
length of this bar represents the number of end item sets that have been completed
or are available for use, as read off the vertical scale used for the Objective
curve. It will be noted that because of the manner in which the chart was constructed,
the bar graphs with the lowest numbers relate to the events that occur earliest.
This automatically points out the priority of corrective action. Also, because
progress is reported in terms of END ITEM SETS, the inventory count is translated
into the capability of delivery of finished units. That is to say, if the end
product is a bicycle, the bar graph for wheels will be on a length that is equivalent
to the total number of wheels that have been completed (or are available for
use) divided by two. The results show how many finished bicycles can be delivered
out of the current stock level of wheels.
All the sensor that are behind schedule are indicated by bar graphs that fail
to meet the Line of Balance. The first of these is sensor number 8, complete
fabrication of part "D". Sensor number 8 is a "make" assembly
which is manufactured relatively early in the factory cycle. To the extent that
supporting sensors 5 and 6 are on schedule, evidently some problem exists in
the fabrication process. The effects of this difficulty have been transmitted
throughout subsequent operations as may be seen by the bar graphs for 10, 15,
16, 18, 19, 21, 22, 23, 24, and 25. It may be concluded that the fault for shipping
only 38 instead of 48 units lies almost entirely with the failure to complete
the required quantity of part "D". The chart also reveals the presence
of a problem area in the operation represented by sensor 13 and 15. Even if
the troubles with part D were cleared up, the deliveries would be limited to
only 51 units as shown by the height of bar graph 15.
This rudimentary example serves to illustrate the application of this technique
to a simple process of fabrication and assembly. Line of Balance can be applied
to all other manufacturing or production operations, whether they are job shop
or flow shop. Although more than some fifty years have elapsed since Line of
Balance was first introduced, it is still considered to be most effective for
control of production.
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