The component importance measure is an index of how much or how little an individual component contributes to the overall system reliability. It is useful to obtain the reliability importance measure or value of each component in the system prior to investing resources toward improving specific components. This is done to determine where to focus resources in order to achieve the most benefit from the improvement effort. The reliability importance measure of a component can be determined based on the failure characteristics of the component and its corresponding position in the system.
Once the reliability of a system has been determined, engineers are often faced with the task of identifying the least reliable components in the system in order to improve the design. For example, in a series system, the least reliable component has the biggest effect on the system reliability. If the reliability of the system needs to be improved, then efforts should first be concentrated on improving the reliability of the component that has the largest effect on reliability. (The cost of improving reliability is not considered in this article. However, this can be done using more complex algorithms available in ReliaSoft’s BlockSim software.) In simple systems such as a series system, it is easy to identify the weak components. However, this becomes more difficult in more complex systems. Therefore, a mathematical approach is needed to provide the means of identifying and quantifying the importance of each component in the system.
Calculating Reliability Importance
The reliability importance, I, of component i in a system of n components is given by:
Equation 1
where,
Rs(t) is the system reliability, and
Ri(t) is the component reliability.
The value of the reliability importance given by this equation depends both on the reliability of a component and its corresponding position in the system.
Static Reliability Importance
Consider a series system of three components, with reliabilities of 0.7, 0.8, and 0.9 at a given time, t. Using Eqn. (1), the reliability importance in terms of a value for each component can be obtained. The reliability importance values for these components can be calculated using ReliaSoft’s BlockSim. By using the BlockSim plot option and selecting a Static Reliability Importance plot, the graph in Figure 1 can be obtained.
Figure 1: Static Reliability Importance Plot
The values shown for each component were obtained using Eqn. (1). The reliability equation for this series system is given by:
Taking the partial derivative of Eqn. (2) with respect to R1 yields:
Thus the reliability importance of Component 1 is 0.72. The reliability importance values for Components 2 and 3 are obtained in a similar manner.
Time-Dependent Reliability Importance
The reliability importance of a component can be calculated at a specific point in time or over a range of time. In the previous example, time-dependency was not considered. However, as demonstrated in Eqn. (1), the reliability importance of a component is a function of time. Another way to look at it is to generate a plot of Reliability Importance vs. Time. With this plot, the reliability importance of the component as a result of the behavior of its entire failure distribution can be observed rather than the importance relating to just one point on the distribution. For example, Figure 2 illustrates the reliability importance vs. time for a four-component system. In this figure, it can be seen that at 400 hours, Component 4 has a higher reliability importance than Component 1 and at 1200 hours this is reversed. Therefore, the measure will vary depending on the time of interest to the analyst.
Figure 2: Reliability Importance vs. Time
Application to a Complex System
Consider the system shown in Figure 3. All components have the same reliability of 90% at a given time. The equation for system reliability obtained from BlockSim is given by Eqn. (3).
Figure 3: System Reliability Block Diagram and Reliability Importance Plot
Using Eqn. (1), the reliability importance was calculated and the results were plotted in Figure 3. Although the components are identical, their reliability importance is different. This is due to their unique positions within the system. When calculating the reliability importance of a component, its failure properties as well as its system properties are considered.
When you ask front line supervisors or team leaders if all people in their teams are performing to the same standards or if some are doing more work and achieving more results than others, you will often get the same answer. All over the world, the most common answer, after some analysis, verifies that about 30% of the people do 70% of the work.
When you ask front line supervisors or team leaders if all people in their teams are performing to the same standards or if some are doing more work and achieving more results than others, you will often get the same answer. All over the world, the most common answer, after some analysis, verifies that about 30% of the people do 70% of the work.
Unfettered expression and spiritual satisfaction? How does this relate to managing a maintenance department, especially one in the U.S. Postal Service? Open your mind. Take a page from the Zen Buddhist monks who preach: When you are quiet and listen, you become aware of sounds not normally heard. USPS maintenance leaders are listening and beginning to understand that maintenance success doesn't come through closed minds and closed doors.
Unfettered expression and spiritual satisfaction? How does this relate to managing a maintenance department, especially one in the U.S. Postal Service? Open your mind. Take a page from the Zen Buddhist monks who preach: When you are quiet and listen, you become aware of sounds not normally heard. USPS maintenance leaders are listening and beginning to understand that maintenance success doesn't come through closed minds and closed doors.
It is not uncommon that many reliability and maintenance improvement initiatives fail to deliver expected results. Why is it so? Some of the most common causes I have observed include:
It is not uncommon that many reliability and maintenance improvement initiatives fail to deliver expected results. Why is it so? Some of the most common causes I have observed include:
Why do improvement efforts fail or perhaps not sustain the gains? There are many reasons, but those most often stated are “lack of commitment” and not “following the process”. But why is there lack of commitment, and why aren’t processes followed? Here are a few of the reasons that I’ve seen:
Why do improvement efforts fail or perhaps not sustain the gains? There are many reasons, but those most often stated are “lack of commitment” and not “following the process”. But why is there lack of commitment, and why aren’t processes followed? Here are a few of the reasons that I’ve seen:
When a piece of production machinery broke down at the Whirlpool plant in Findlay, Ohio, several years back, it was accepted practice for the machine operator to call maintenance and then sit back and wait for the problem to be fixed. Critical information and knowledge was not shared between the operator and maintenance technician. Like many companies, these workers were stuck in traditional roles - operators run the machines, maintenance fixes the machines, and the two do not cross. As a result, productivity opportunities were missed.
When a piece of production machinery broke down at the Whirlpool plant in Findlay, Ohio, several years back, it was accepted practice for the machine operator to call maintenance and then sit back and wait for the problem to be fixed. Critical information and knowledge was not shared between the operator and maintenance technician. Like many companies, these workers were stuck in traditional roles - operators run the machines, maintenance fixes the machines, and the two do not cross. As a result, productivity opportunities were missed.
Many managers are unaware that best-in-class companies routinely design-out maintenance at the inception of a project. That, clearly, is the first key to highest equipment reliability and plant profitability. Whenever maintenance events occur as time goes on, the real industry leaders see every one of these events as an opportunity to upgrade. Indeed, upgrading is the second key, and upgrading is the job of highly trained, well-organized, knowledgeable reliability professionals.
Many managers are unaware that best-in-class companies routinely design-out maintenance at the inception of a project. That, clearly, is the first key to highest equipment reliability and plant profitability. Whenever maintenance events occur as time goes on, the real industry leaders see every one of these events as an opportunity to upgrade. Indeed, upgrading is the second key, and upgrading is the job of highly trained, well-organized, knowledgeable reliability professionals.
The true translation — might it be proper to say a new and improved translation? — is being used today by Cervecería Cuauhtemoc Moctezuma, one of the largest brewers of beer in Latin America. Known throughout this company as Mantenimiento Alto Desempeño (MAD), or translated as High-Performance Maintenance, the concept of TPM is alive and well at the company's six plants in Mexico. Perhaps the best example is at CCM's brewery in Tecate, located a short drive from the U.S.-Mexico border on the Baja California peninsula.
The true translation — might it be proper to say a new and improved translation? — is being used today by Cervecería Cuauhtemoc Moctezuma, one of the largest brewers of beer in Latin America. Known throughout this company as Mantenimiento Alto Desempeño (MAD), or translated as High-Performance Maintenance, the concept of TPM is alive and well at the company's six plants in Mexico. Perhaps the best example is at CCM's brewery in Tecate, located a short drive from the U.S.-Mexico border on the Baja California peninsula.
