Thomas Industrial Library

The next step is to predict the expected performance of your system, comparing those results with your desired performance. Start by reviewing the individual components, obtaining reliability information of MTBF and MTTR for such units used in similar environments and duty cycles as intended with your design. This information is best if it comes from similar equipment being used in like conditions to your own factory. Other sources of information include similar industries, common reliability charts of general equipment, vendor literature, and other experienced users. Be careful, however, when you obtain and use this information; your specific conditions and use often vary from that of other sources. For critical parts with unknown performance, reliability quantification testing (RQT) may be appropriate.

Assume you have researched the components and determined that MTTR equals 0.5 hours or less and MTBF is represented by the table in figure 7-8. By summing failure rate ( l ) for the individual components, you

can derive the system failure rate, which in this case l equals 0.03803. In turn, the inverse of the system failure rate, or 26.3 hours, is the system MTBF.

Now compare the predicted value to the desired value. In this example, you will see that the proposed system's MTBF of 26.3 is less than half of 62.1, the requested system MTBF. The proposed system's performance does not meet your desired standard.

This information is very powerful because you have not yet committed either the business case or design dollars. Without this exercise, the project might have proceeded, locking you into a system that would either under perform for its entire life or require unexpected investment to increase capability to expectations.

By comparing desired and predicted results, individual item performance gaps quickly reveal areas for reinvestigation. These gaps are dramatically revealed by looking at the difference in failure rate between the desired and the predicted for each item. Dividing each component failure rate difference by the desired system failure rate (0.01610) and expressing the number as a percentage helps target improvement opportunities. These calculations are represented in figure 7-9.

As indicated by the table, the delivery pump has a large negative impact on the proposed system. The project team now has several options: purchase a more reliable pump, consider redundancy, or use a different technology such as overhead installation with gravity feed. All of this effort now takes place in the strategy, concept, and initial design stages of the project—prior to commitment of significant system configurations and project dollars. The results of successful reviews strongly support design for reliability projects.

Once the acceptable configuration and design system are derived, the individual component results form an important part of the purchase specifications; they must be included on the purchase orders. Each item can be specified for MTBF and MTTR requirements, with identification of any performance testing prior to acceptance. If you understand each component's expected performance, then you can establish the proper operating and maintenance strategy. This strategy can be documented and applied at startup so that high performance is obtained at initial operation. This approach supports square starts for new projects.

References:

Ishikawa, Kaoru. Guide to Quality Control. White Plains , New York : Quality Resources, Asian Productivity Organization, 1991.

Moubray, John. Reliability-centered Maintenance. 2^nd Edition, New York, New York: Industrial Press, 1997.

Shingo, Shigeo. A Revolution in Manufacturing: The SMED System , Cambridge, Massachusetts: Productivity Press, 1985.

Moore, Ron. MAKING COMMON SENSE COMMON PRACTICE models for manufacturing excellence. Houston , Texas : Gulf Publishing Company, 1999.