Lean Sigma: A Practitioners Guide

2017-07-07 02:10:07

24. Load Chart

Overview

A Load Chart is used to visually represent two primary project objectives:

Understand the ability of a process to meet the pace of Customer demand

Evaluate the distribution of labor content in the process

The Chart itself is comprised of two elements, as shown in Figure 7.24.1. The horizontal line represents the pace of Customer demand in the form of the Takt Time. For more details see "TimeTakt Time" in this chapter. In effect this is the time that the process has available to process one entity. For some processes, a differing number of entities progresses through different steps; thus, the line might be staggered and not perfectly horizontal.

Figure 7.24.1. A Load Chart as applied to the operators in a process.

The second set of elements in the Load Chart is the vertical bars representing the Cycle Time of each operator in the process. For more details see "TimeIndividual Step Cycle Time" in this chapter. If an operator bar rises above the Takt line, then the operator is cycling slower than the Takt Time; therefore, it is not cycling fast enough to keep up with Customer demand. The highest bar in the Load Chart is the bottleneck in the process. If any bar remains well below the Takt line, then that step cycles quicker than Takt and the process generates excess inventory or sits idle from time to time.

The goal clearly is to ensure that the process steps all meet Takt and all the bars sit just below the Takt line. In the goal scenario, there is no internal bottleneck and the Customer demand is the determining factor.

Logistics

Constructing a Load Chart doesn't have to be a team sport, but it does require a significant data capture, which might require Team help. For more information see "TimeTakt Time" and "TimeIndividual Step Cycle Time" in this chapter.

After the data capture is complete it is a simple matter to construct the Load Chart in a spreadsheet, taking perhaps 1015 minutes.

Roadmap

The steps to constructing the Load Chart are as follows (note that the first two steps have more than likely been completed earlier in the project roadmap):

Step 1.	Identify the entity types flowing through the process and the volume of demand of each type in a given period, usually a month.
Step 2.	Identify the steps in the process (usually done in a Process Map) and group the steps by Operator so that all steps done by Operator 1 are together, and so on (see Figure 7.24.1).
Step 3.	Calculate the Takt Time for each Operator (see the "TimeTakt Time" in this chapter). This is done by identifying the Available Work Time for the Operator and dividing it by the demand (number of entities) for the Operator, both for the given time period. This becomes the height of the horizontal line in the Load Chart.
Step 4.	Calculate the Individual Step Cycle Time for each step, that is how long it takes the Operator on average to complete the process step. Total the Individual Step Cycle Times for each operator for the steps they undertake. For example, Operator B in Figure 7.24.1 completes operations 4 and 6. Operation 4 takes the Operator 60 seconds to complete and Operation 6 takes 28 seconds to complete. The Total Cycle Time for Operator B is 60 + 28 = 88 seconds. This becomes the bar height in the Load Chart.
Step 5.	Draw the Load Chart using the numbers generated in Steps 3 and 4.

Interpreting the Output

Figure 7.24.2 shows a typical Load Chart for a process prior to improvement. Each worker performs a fixed operation at each workstation and Cycle Times are not balanced. Operators 2 and 4 are cycling slower than Takt and the process cannot meet Customer demand. More than likely there is expediting or overtime involved in reducing backlog on this process.

Figure 7.24.2. Load Chart for an imbalanced process (line).

Likewise, Operators 1, 3, 5, and 6 are cycling below Takt and thus are generating inventory or sitting idle or some combination of the two. It is common in a situation like this to see Operator 1 processing at full speed and a stockpile of inventory sitting in front of Operator 2. Operator 2 might receive criticism for not working hard enough and probably is a source of defects due to the hurried nature of the work. Operator 3 might appear to be the "star" of the line. Operator 3 appears completely on top of the work and can relax and still create a stockpile in front of Operator 4, the bottleneck in the process. Operator 4 is working flat out and still can't keep up and probably is another source of defects. Operators 5 and 6 are idle for a large portion of their time and grab and process everything they can from Operator 4 as soon as it is ready.

The solution lies in spreading the workload. Initially, the primary focus is initially on any bars that rise above the Takt Line. For these Operators, the work needs to be broken down and any NVA elements removed. If the bars are still above the line, then work needs to be offloaded to adjacent operators, or better still a complete redistribution of labor undertaken.

Figure 7.24.3 shows the resulting situation after the balancing has taken place. All Operators workloads are slightly below the Takt Line to balance output to demand.

Figure 7.24.3. Load Chart after line balancing.

It is possible to determine the number of Operators required in the process based on the Takt Time and the work content (or Cycle Time) of the steps, as follows:

Of course, before doing this it is best to remove any NVA work in the process steps to minimize the total work done in the process and reduce the number of Operators required.

Other Options

Load Charts can be used on a larger scale by applying them to areas, rather than to individual Operators. For example, in a hospital Surgery Center, it is possible to have an individual bar for

Pre-admission testing

Intake and preparation

Operating suite

Post-anesthesia care

Recovery

In this example, each area can be examined to determine if there is available capacity to increase throughput.

The difference in construction in this case is that the height of the bar would be the Process Cycle Time for that area, that is the speed at which a single patient can be processed. The Takt line would be calculated in the same way as before from the working hours of the area and the patient demand or the area.

Load Charts can also be applied to machines or equipment as shown in Figure 7.24.4. In this case, the bars comprise:

Load time per cycle

The machine Cycle Time (for a single cycle)

Unload time per cycle

Setup time averaged for a single cycle (i.e., if a 20-minute setup were required every 10 units, then 2 minutes would be applied as the Setup for a single cycle)

Figure 7.24.4. Application of a Load Chart to machines or equipment versus Operators.

The initial improvement targets are usually the Loads, Unloads, and Setups because these are NVA by definition.

By examining Available Machine Work Time and machine demand for a given period, the Takt Line is calculated in the same way as before.