Page 20 - Machinery Component Maintenance

6 Machinery Component Maintenance and Repair

placement or overhaul. After maintenance the future trend in operating
cost is known or at least anticipated. Such a deterministic situation is il-
lustrated in Figure 1-3.
In probabilistic or indeterminate component life problems, the timing
and result of maintenance may depend on chance. In the simplest situa-
tion a piece of machinery can be described as being “good” or “failed.”
From a frequency distribution of the time elapsed between maintenance
activity and failure it is possible to determine the variations in the proba-
bility of failure with elapsed time. These relationships are thoroughly
dealt with in Reference 1.
Wi: saw from Figure 1-2 that inspection, overhaul, repair and finally
replacement are common to all maintenance strategies. The basic pur-
pose of inspection is to determine the condition of our equipment. All
machinery inspection should be based on these considerations:

1. Expected failure experience:
Deterministic
Probabilistic
2. Inspection cost.
3. Probability and risk of failure.
4. Probable consequences of failure, Le., safety-health, and business
loss.
5. The risk that inspection will cuuse a problem4.
6. The quality of on-stream condition monitoring results.

I : time

Figure 1-3. Deterministic trend in costs.