Bathtub curve to analyze the failure patterns of Equipment
A bath tub curve is used to observe, analyze and express the failure pattern of an equipment over its functional life. The different sections of a bath tub curve is as following-
FAILURE PATTERNS Section A – B
This portion is the “early failure or infant mortality” area. Failure rates are high and are caused by design and manufacturing problems. The remedy is redesign, and improved quality control. After these problems have been overcome the problem remains although at a reduced failure rate level. In this case the failures are caused by faulty maintenance practices, errors during re-assembly of the component and installation into the system.
FAILURE PATTERNS – Section B – C
Having passed point B the failure rate becomes substantially constant, and lower than in the A – B area. Failures which occur in the B – C area are known as “chance or random failures”, and do not exhibit any fixed pattern.
The almost constant failure rate in section B – C is of great importance in Reliability schemes. Failures which do occur in section B – C are brought about by random occurrences, such as unexpectedly high transient voltage, vibration etc. Depending on the nature of the component, section B – C may be long or short.
FAILURE PATTERNS – Section C – D
This section shows a rapid increase in failure rates compared to B – C as the component is entering the “wear out” phase, and a fixed pattern of failures can be expected. Note that the “random failure” mechanism will still occur as it did in B – C and A – B, so there may be some failures which do not fit the emerging pattern. The establishment of point C is extremely important in Reliability schemes.
It is now obvious that the optimum time for overhaul is just prior to point C. We may be familiar with a regulated escalation process using a method, whereby we inch our way along section B – C hoping to stumble on point C by inspecting samples. The establishment of point C by other methods, for some classes of components is another important aspect of Reliability schemes.