Reliability Theory's Role in Total Productive Maintenance

Reliability & Reliability Centered Maintenance

Total Productive Maintenance (TPM)

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Beginning in World War II, the War Department sponsored a new science called Reliability. Reliability is the science of maintenance. It uses statistics and failure theory to measure, understand and improve the performance of equipment and maintenance. Reliability theory can guide engineers as they design and test new equipment. After equipment has been in service, reliability data tells the maintenance engineer how to improve its performance.

As the Gulf Wars demonstrated, this science has produced outstanding results in defense. Regrettably, little of this knowledge has found its way into industry. Most maintenance operations still operate on the principal of "if it ain't broke, don't fix it".

Failure Modes

Failures occur in one of several modes. Understanding modes and what mode is the likely cause for specific failures is important because different approaches or strategies may be more or less effective on the various modes. Table 1 summarizes the various failure modes and illustrates their characteristic failure rates over time.

Reliability Metrics

Reliability uses many metrics for evaluating equipment and systems. The original metric, Mission Reliability, answered the question of figure 4. For industrial maintenance, the metric of Failure Rate is usually more relevant. Failure rate is the number of failures per 1000 hours of operation. It can apply to a complex system such as a machine tool or it can apply to a large number of simple components such as light bulbs. This discussion focuses on individual units of complex equipment.

Mission Reliability

Figure 4 Mission Reliability

Question: If we dispatch 1000 heavy bombers for an 8-hour mission, what percentage will complete the mission without mechanical failure?

Table 1 Failure Modes & Characteristics

Failure Mode Failure Rate Curve

Early Life

These occur when the equipment is placed in service and are caused by sub-standard components and/or improper installation. Early life failures occur frequently when the equipment is first placed in service and then rapidly decline.

Random Failures

These result from variations in both the load imposed on any given component and the variations in strengths of supposedly identical components. Random failure rates are essentially constant over the life span of the equipment and are normally small being overshadowed in most practical cases by other failure modes.

Major Wearout

This occurs when major sub-systems or structures become worn or weakened to the point that proper repair is impossible or impractical. Failure rate begins to rise sharply and the only solution is major overhaul or replacement.

Minor Wearout

Most complex equipment requires regular replacement of various components as each component reaches its individual wearout life. Since the many components have different lives and are changed at different times, the failure rate tends to be relatively constant overall and mimics the random failure rate curve.

Early Life, Random & Wearout

When the previous failure modes are combined, the result is the "Bathtub Curve", familiar to many.

Design Deficiency

This type of failure is the result of design error and shows up as a series of wearout failures. This type of failure does not occur on equipment that has been extensively tested and developed. It is inevitable on new designs which have not been thoroughly tested and on the "special" machines which are often used in industry.

The worst problems will normally be corrected early on until the failure rate is reduced to a tolerable level. At that point, remaining design deficiencies are indistinguishable from minor wearout failure.

All Modes Combined

With all modes combined, the failure curve is the familiar bathtub but with spikes of increased failures at irregular time. Determining the mode for specific failures requires additional investigation and cannot be determined from the failure curve alone.

More About...

TPM Introduction
Reliability Theory
Equipment Metrics
Maintenance Strategies
Pillars of TPM
TPM Dynamics
TPM In A Nutshell
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