13.11 Line of Balance (LOB)

Line of Balance (LOB) is a production control technique which combines features from a critical path scheduling time chart with a required delivery schedule, and presents in graphic form information relating to time and accomplishment of production. It shows the delivery objective, sequence and duration of all activities required to produce a product, a progress chart of the current status of production items, and, from these charts, an LOB to show the relationship of actual component production to schedule.

LOB is most appropriate for assembly operations involving a number of discrete components and has proven most useful in production programs from the point when raw materials or incoming parts arrive, to the shipment of the end product.

Without a computer controlled production process, Line of Balance does not lend itself readily to day-by-day updating, but a weekly or monthly check is usually frequent enough to keep the process on schedule. If the project falls behind schedule, management will know it, and know why, far enough in advance to make smooth adjustments.

Reporting to customers or top management is quick, inexpensive and graphic. The charts used for analysis and troubleshooting are suitable for at-a-glance status reporting. A set of clear, simple charts is easier to understand than a list of facts and figures, and charts are faster and more reliable than oral reports.

A Line of Balance study has four elements:

0. The objectives of the program (Objective Chart);
0. The production plan, and a schedule for achieving it;
0. The current program status; and
0. A comparison between where the program is and when it's supposed to be.

The first step in using LOB is to gather and organize the needed material for the three charts which comprise an LOB report. Once this is done you can "strike the line of balance" whenever necessary to keep track of the program.

The objective chart is designed to display planned and actual deliveries in cumulative and items per unit of time.  In Figure 13-10, for example, the delivery schedule calls for three items in December, five in January, seven more in February and five each month thereafter through June. The delivery schedule should realistically reflect attainable production capability taking into account learning associated with a new product (if this is an initial production activity) anticipated methods improvements, or other factors expected to influence productivity.

The other curve on the Objective Chart shows actual delivery of parts. The horizontal difference shows how far actual deliveries lag behind scheduled deliveries in terms of time, the vertical difference shows the variance, in numbers of units, from schedule.

Figure 13-10  Line of Balance Objectives Chart (A), Production Plan (B) and Program Status (C)

Following the development of the objectives, the second step is to chart the planned process of production. The production plan is a graphic flow chart of the operations required to complete a unit. Selected production activities are plotted against the lead time required before shipment. For example, Figure 13-10 illustrates the key plant operations in the manufacturing sequence of a rocket.

The production plan is developed by setting down the selected events and operations in their proper sequence, commencing at the point of delivery and moving backward through the entire production process. The control points are numbered from left to right and from top to bottom as shown in Figure 13-10. This will usually result in four or more general sequential phases as follows: the final assembly process, preceded by major subassembly work, proceeded by manufacture of parts, proceeded by acquisition and preparation of raw materials and purchased parts. 

In Figure 13-10, the receipt of purchased parts identified as event 1 must start 24 working days in advance of final delivery for that unit. The gyro components must enter the production stream at control point 2 on day 22, as must the guidance and control components at control point 3 in order to assure start of the assembly at the guidance section (event 5) on day 16.  If the required material or number of parts is not at each control point or any critical event in the production flow of a unit is not started on time (or completed on schedule), the delay is symptomatic of a problem which should be investigated; corrective action should be taken to forestall continuing delays and late deliveries.

The progress chart, example shown in Figure 13-10, pertains to the status of actual performance and comprises a bar chart which shows the quantities of materials, parts, and subassemblies available at the control points at a given time. Production progress is depicted in terms of quantities of materials, parts and subassemblies which have passed through the individual check points or control points of the production plan, including those contained in end items already completed. This information is derived from production records or accumulated by a physical inventory for each control point.

Development of the objective chart, the production plan, and program progress chart completes the accumulation of physical information. There remains the task of relating the facts already gathered. This is accomplished by striking a "Line of Balance, (LOB)" which is the basis to be used for comparing the program progress to the objective.

The balance line quantity depicts the quantities of end item sets for each control point which must be available as of the date of the study to support the delivery schedule. In different words, it specifies the quantities of end item sets for each control point which must be available in order for progress on the program to remain in phase with the objective. Figure 13-10 is illustrative of the procedure for striking the LOB.

The balance line quantity depicts the quantities of end item sets for each control point which must be available at the end of the reporting period to support the delivery schedule. The required quantities are then compared with the actual completions by control point. Where the actual completions are less than the required quantity, this would indicate that there is a strong probability that deliveries will not be met at some future point. The timing of the potential delivery shortfall can be determined from the lead time data displayed in the LOB. If the behind schedule control point is 20 weeks flow time prior to final delivery, we would expect to see the impact in 20 weeks if corrective action is not taken.