The Toyota Production System, and later on the concept of Lean, was developed around eliminating the three types of deviations that shows inefficient allocation of resources. The three types are Muda (無駄, waste), Mura (斑, unevenness), and Muri (無理, overburden).
Muda (無駄)
Muda means wastefulness, uselessness and futility, which is contradicting value-addition. Value-added work is a process that adds value to the product or service that the customer is willing to pay for. There are two types of Muda, Type 1 and Type 2. Muda Type 1 includes non-value-added activities in the processes that are necessary for the end customer. For example, inspection and safety testing does not directly add value to the final product; however, they are necessary activities to ensure a safe product for customers. Muda Type 2 includes non-value added activities in the processes, but these activities are unnecessary for the customer. As a result, Muda Type 2 should be eliminated.
There are seven categories of waste under Muda Type 2 that follow the abbreviation TIMWOOD. The seven wastes are (1) Transport i.e. excess movement of product, (2) Inventory i.e. stocks of goods and raw materials, (3) Motion i.e. excess movement of machine or people, (4) Waiting, (5) Overproduction, (6) Over-processing, and (7) Defects.
Mura (斑)
Mura means unevenness, non-uniformity, and irregularity. Mura is the reason for the existence of any of the seven wastes. In other words, Mura drives and leads to Muda. For example, in a manufacturing line, products need to pass through several workstations during the assembly process. When the capacity of one station is greater than the other stations, you will see an accumulation of waste in the form of overproduction, waiting, etc. The goal of a Lean production system is to level out the workload so that there is no unevenness or waste accumulation.
Mura can be avoided through the Just-In-Time ‘Kanban’ systems and other pull-based strategies that limits overproduction and excess inventory. The key concept of a Just-In-Time system is delivering and producing the right part, at the right amount, and at the right time.
Muri (無理)
Muri means overburden, beyond one’s power, excessiveness, impossible or unreasonableness. Muri can result from Mura and in some cases be caused by excessive removal of Muda (waste) from the process. Muri also exists when machines or operators are utilized for more than 100% capability to complete a task or in an unsustainable way. Muri over a period of time can result in employee absenteeism, illness, and breakdowns of machines. Standardize work can help avoid Muri by designing the work processes to evenly distribute the workload and not overburden any particular employee or equipment.
Relationship between Muda, Mura and Muri
Muda, Mura, and Muri are interrelated. Eliminating one of them will affect the other two. For example, a firm that needs to transport 6 tons of materials to a customer has several options (Lean Enterprise Institute, 2016).
The first option is to load one truck with all 6 tons and make a single trip. However in this example, it would be considered Muri due to the overburden of the truck. This excess load can lead to a breakdown.
The second option is to divide the transportation into two trips. One with two tons and the other with four tons. This would be considered Mura since the unevenness of the arrival of materials to the customer can lead to problems at the receiving dock. In the first trip, the delivery may be too little for the production necessary on-site. In the second trip, the amount of delivered material may be too much for on-site storage and material handling. This leads to Muri since one of the truck is overburden and the receiver is also overburden for that delivery. Additionally, Muda can be seen from the uneven workload. This can cause employees who receive the materials to wait around.
The third option is to load two tons on each truck and make three trips. Even though this option has no Mura and Muri, it has Muda since the truck would not be fully loaded on each trip. Each truck can carry up to 3 tons of material and this option makes one unnecessary trip.
The fourth option is to deliver the materials with two trucks each with 3 tons. In this example, this would be the optimal level that minimizes Muda, Mura, and Muri. Muda does not exist because the trucks are carrying the loads at their maximum capacity. There is no excess capacity nor unnecessary trips with this strategy. Mura does not exist because the workload between the two deliveries are uniform. As a result, there is no unevenness. And finally, Muri is absent from this option because both the truck and the operators are not working beyond their capacity.
In real world applications of Lean, it is not always easy or possible to find an optimal solution. Reducing Muda can lead to Muri. The existence of Mura can be seen as a waste in Muda. And finally Muri can lead to a breakdown in the system that will result in a large amount of Muda and Mura. Since real world problems are dynamic and the needs of customers are always changing, our work processes must also change as well. As we design our processes and standardize our work, we must look at the resulting system from the lens of these three concepts. Only by considering the impacts of Muda, Mura, and Muri and optimizing our production strategy can we develop an efficient Lean system.
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