Reliability   

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.

Reliability Metrics

Measuring Equipment Performance

Metrics help to focus efforts on the most critical equipment rather than reacting to the crisis de jour. They measure progress and help to adjust efforts accordingly. They are critical for identifying and resolving specific problems. Equipment metrics can be surprisingly simple. Only three data elements, collected for each machine and analyzed properly, are really necessary for most situations.

This discussion is about the metrics for machine performance. It does not include metrics of maintenance department productivity, budgeting or cost allocation. Such additional metrics are required to operate a maintenance department effectively.

All these metrics are most effective in graphical form. They are not very meaningful as individual numbers. However, in the context of past and future, trends, anomalies and patterns reveal themselves.

All of the first four metrics, the most useful, derive from three numbers. Assuming a calculation period of one week, the following questions must be answered:

• How many breakdowns (failures) did we have this week?
• How long did each breakdown take to repair?
• How many hours were scheduled for the equipment?

Calculations and tracking can be further simplified by assuming that each machine is scheduled for about the same production (say 40 or 80 hours) and simply using one week as the time bucket.

(Lambda)