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Categories: Theory of Constraints
Tools to Identify Constraints

Tools to Identify Constraints

By Bob Sproull

Review

In my last post, we completed our discussion on the various types of constraints that exist within many manufacturing facilities. We also presented a brief description of each of the five primary types of constraints that exist within any management system and how to deal with each type.

In today’s post, we will discuss some of the actions and tools that can be used to identify the constraints that might exist. There is a wonderful book written by Boaz Ronen and Shimeon Pass entitled, Focused Operations Management—Achieving More with Existing Resources [1], that I highly recommend for everyone trying to identify, remove or reduce the impact of constraints.

Actions to Identify Constraints

The identification of constraints can be completed by several simple actions that involve both the leadership and frontline workers.

  • Walk the complete process from the entry of raw materials into the primary process all the way to the storage of finished goods. What you’re looking for is an excess accumulation of work-in-process (WIP) inventory. Because the constraint is many times the slowest step in the process, WIP inventory will accumulate directly in front of the constraint, if the constraint is a physical one.
  • Completely review your company’s policies, procedures and performance metrics. For example, if your company is using manpower efficiency and/or equipment utilization, then you probably have excessive amounts of inventory clogging the flow of products through your processes.
  • Talk to your frontline workers. The workers know better than anyone does where your constraint or bottleneck is. They will use their experience and intuition to pinpoint the location of your constraint(s).

Tools Used to Identify Your Constraint(s)

There are tools you can use to identify and locate your constraint. These tools will also help to determine the impact magnitude of relieving your constraint. Additionally, they will indicate what to look for as you work to remove or reduce the constraint’s magnitude. The tools Ronen and Pass recommend using are:

  • Process-flow diagram
  • Time analysis
  • Load analysis
  • Cost-utilization diagram

Let’s look at each one of these tools in more detail.

Process-Flow Diagram

A process-flow diagram is a basic flowchart that describes the workflow in the system, the steps of the process, and any decision points as the product flows. The authors recommend that in order to facilitate constraint identification, keep the process-flow diagram simple, reflecting only the main work process. The figure below is an example of a simple process-flow diagram with each of the individual processing times listed for each step in this process. Because step 4 has the longest processing time at 20 minutes, it is designated as the constraint in this process. In order for this process to operate most efficiently and effectively, steps 1, 2 and 3 should operate at about the same rate (i.e. 20 minutes) as the system constraint (step 4). If they do not, the process will become clogged with excessive amounts of WIP in front of the constraint. In addition, if this happens, lead times become extended, on-time delivery rates will deteriorate and customer loyalty will be jeopardized.

The process-flow diagram helps improve communications within the organization because it is a simple visual aid that helps people understand workflow.

Time Analysis

Analyzing the processing times of each step enables easy identification of the workstation where the part spends the longest amount of time. As previously stated, step 4 is our designated constraint since it requires 20 minutes to complete this step. Once the part leaves step 3, it must wait in queue in front of the constraint, which many times accounts for a long time interval. In our process-flow diagram example, prior to arrival at the constraint, the part spends roughly 13 minutes of wait time after processing parts from step 2.  I always recommend combining the time analysis with the process-flow diagram to simplify the constraint identification.

Load Analysis (Capacity Utilization)

Load analysis is another simple tool, which can be used to locate the system constraint.  Ronen and Pass [1] tell us that to determine the load on resources in the system during the time of analysis several data must be examined:

  • Total number of labor hours during the time period analyzed
  • The overall planned work—the number of parts planned to work on (i.e. produce, plan, etc.) with a specific quantity by type of customer, product, etc.)
  • A table describing the effect of the labor hours that each resource requires to produce each product

The authors [1] have provided a good example of how this technique works as follows:

A manufacturing company has received orders for four of their products, and the question is whether the company can produce the entire order. The plant is working 190 hours per month. The orders for the next month consist of 100 units of product A, 50 of product B, 25 of product C, and 200 of product D. The plant consists of four (4) production stations which are summarized in the following table:

Labor hours per unit per station

Product

Station 1

Station 2

Station 3

Station 4

A

0.6

0.15

0.73

-

B

0.35

0.72

1.18

0.5

C

1.6

-

1.36

2.56

D

0.2

0.06

0.44

0.41

The table lists the number of labor hours required to process each product at each station. In the table below, we see the load analysis data. The first step is to multiply the planned quantity of each product by the labor hours required of each station for each unit of product. As a second step, calculate the load: add all the labor hours expended at each station for each of the products, then divide by the total number of labor hours per month (190 hours). The load analysis demonstrates that station 3 cannot perform all of its tasks to meet market demand (i.e. the load is greater than 100 %). It is the most heavily utilized station and is, therefore, the system constraint.

Load Analysis

Product

Planned Qty

Station 1

Station 2

Station 3

Station 4

A

100

100 x 0.6 = 60

100 x 0.15=15

100 x 0.73=73

-

B

50

50 x 0.35=17

50 x 0.72=36

50 x 1.18=59

50 x 0.5=25

C

25

25 x 1.6=40

-

25 x 1.36=34

25 x 2.56=64

D

200

200 x 0.2=40

200 x 0.06=12

200 x 0.22=88

200 x 0.41=82

Total Hours

 

157

63

254

171

Load

 

83%

33%

134%

90%

Next Time

In my next post, we will complete this discussion of tools to use for identifying your system constraint by laying out the basics of the Cost-Utilization Diagram. As always, if you have any questions or comments about any of my posts, leave a message and I will respond.

 

Until next time.

 

Bob Sproull

References:

[1] Boaz Ronen and Shimeon Pass, Focused Operations Management –Achieving More with Existing Resources published by John Wiley & Sons, Inc., 2008

Bob Sproull

About the author

Bob Sproull has helped businesses across the manufacturing spectrum improve their operations for more than 40 years.

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