Page 273 - Intro Predictive Maintenance

264 An Introduction to Predictive Maintenance

Figure 12–3 Normal distribution of failures.

Once the breaking point is determined, a margin of safety should be added to account
for variations in individual components, environments, and operating conditions.
Depending on the severity of failure, that safety margin could be anywhere from one
to three standard deviations before the average failure point. As Figure 12–3 shows,
one standard deviation on each side of the mean will include 68 percent of all varia-
tions, two standard deviations include 95 percent, and three standard deviations is 98.7
percent. When the mission is to prevent failures, however, only the left half of the dis-
tribution is applicable. This single-sided distribution also shows that we are dealing
with probabilities and risk.

The earlier the threshold is set and effective preventive maintenance done, the greater
the assurance that it will be done before failure. If the mean-time-between-failures
(MTBF) is 9,000 miles with a standard deviation of 1,750 miles, then proper preven-
tive maintenance at 5,500 miles could eliminate almost 98 percent of failures. Note
the word proper, meaning that no new problems are injected. That also means,
however, that costs will be higher than necessary because components will be replaced
before the end of their useful life, and more labor is required.

Once the threshold set point has been determined, it should be monitored to detect
when it is exceeded. The investment in monitoring depends on the period over which
deterioration may occur, the means of detection, and the benefit value. Figure 12–4
illustrates the need for automatic monitoring.

If failure conditions build up quickly, a human may not easily detect the condition,
and the relatively high cost of automatic instrumentation will be repaid.

The monitoring signal may be used to activate an annunciator that rings a bell or lights
a red light. It may activate a feedback mechanism that reduces temperature or other
parameters. A thermostat connected to a heating and air-conditioning system provides
this feedback function to regulate temperature. The distinction between operational
controls and maintenance controls is not important because the result is a reduced