Manufacturing Faults

Early failures may occur almost immediately, and the failure rate is determined by manufacturing faults or poor repairs. Random failures are due to mechanical or human failure, while wear failure occurs mainly due to mechanical faults as the equipment becomes old. One of the techniques used by maintenance engineers is to record the mean time to failure (MTF) of equipment items to find out in which period a piece of equipment is likely to fail. This provides some of the information required to determine an appropriate maintenance strategy tor each equipment item. 

End-product testing (quality control) is toe reliance only on appropriate analytical tests to demonstrate toe quality of a medicinal material. End-product testing alone is considered as testing quality into the product. You only accept those materials that pass specification. This is not adequate to ensure that toe medicine is free from all manufacturing faults, that test methods employed are adequate to establish its purity and that a small proportion of defective materials would be detected. I will give several examples to iUustrate this point. 

PFR went critical for the first time in March 1974 and commissioning of the steam generators followed. Up to 1976 there were failures in gas-space leaks in one evaporator, two superheaters and one reheater. These early failures are believed to have been due to manufacturing faults. 

Examination of the failed modules has shown that the most likely reason of a sodium-water reaction is a manufacturing fault failed to be found during anufacturer s tests using applicable inspection methods.Over a total period of operation a series of the following activities on the steam generators has been fulfilled aiming at improvement of their reliability ... 

There is therefore a need to use plastics parts with a higher level of fire safety meeting the requirements of UL94 Vertical tests so as to ensure adequate consumer protection. External fire sources, inadequate design, manufacturing faults or defects due to simple wear and tear, plus consumer misuse may lead to flashover and fully developed fires in a very short time if housings and backplates are not flame retarded to meet the vertical test criteria. 

Design error Inappropriate material Manufacturing fault Incorrect installation Unexpected service conditions Deliberate or accidental misuse. 

Figure 2. Attributes influencing manufacturing fault trigger.

Suppose that the subassembly has the worst possible configuration across Quality Control, Process Novelty and Manufacturer Status i.e. vector of attribute levels (1, 1, 1), which means no existing process standards, new manufacturer, no quality control. This configuration results in the highest probability of a manufacturing fault. 

Figure 6. Relative risk reduction achieved by moving each attribute affecting the Manufacturing Fault trigger from the best to the worst level configuration, whilst the other attributes remain fixed to a mid-level.

Our expert panel also provided individual probability judgments about the chance of each of the trigger probabilities such as a manufacturing fault for a subassembly produced by a new manufacturer who had not employed process standards and not used any form of quality control. That is, a worst case scenario as shown in Table 2. 

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