Basic Factors to Consider for Equipment Reliability

F. Stanley Nowlan and Howard F. Heap in their 1978 book titled Reliability-Centered Maintenance (RCM) described in very simple language the objectives driving a maintenance program. In my view, the objectives apply to any physical item of significance. Specially when the item plays a designated function which we deem critical, maintaining the health of the item becomes relevant.

According to Nowland and Heap (1978, p. xvi), the objectives driving the need for a maintenance program are:

Realization of the inherent safety and reliability levels of the equipment.

Realization of the inherent safety and reliability levels of the equipment.

1. Restoring safety and reliability to their inherent levels when deterioration has occurred.

2. Realization of the inherent safety and reliability levels of the equipment.

3. Obtaining the information necessary for design improvement of those items whose inherent reliability proves inadequate.

4. Accomplishing these goals at a minimum total cost, including maintenance costs and the costs of residual failures.

The key takeaway from the above stated objectives is – to perform certain objectively evaluated tasks that would ensure that the equipment continues to deliver its specified functionalities without losing its designed safety and reliability, at a minimum possible cost.

Four Basic Condition Factors

In accordance with the doctrine of RCM, the assessment of appropriate tasks is guided by analyzing the expected consequences of failures. This is done via a detailed decision tree.

The decision-making process is obviously very rigorous when the equipment or the system under investigation is a complex one. A variety of statutory, technical and financial feasibility considerations come into play to arrive at a well-defined set of maintenance tasks to achieve the ultimate objectives.

At the same time, there are four important equipment related condition factors that deserve consideration while justifying the maintenance tasks. These factors are:

1. Cleanliness

2. Temperature

3. Looseness

4. Lubrication

The root causes of many failure modes are often found to be impacted by the existence of unfavorable states of the factors mentioned above. Many a times these factors are overlooked during task assessment.

Cleanliness

In order to ensure the restoration of the inherent functional safety and reliability, the need for ensuring a state of reasonable cleanliness is paramount. Numerous recorded failures have arisen from the lack of cleanliness. Examples of spilled flammable fluids causing fire hazards are common. Dust deposits on protection and interlocking electrical relay contacts have caused serious equipment malfunction. Unclean condensers deteriorate the efficiency of a refrigeration system, leading to additional stress on the compressor and potential compressor failure – to cite a few common examples.

Temperature

Likewise, inappropriate operating temperature plays a detrimental role in various mechanical and electrical/ electronic equipment and systems. High ambient temperature is detrimental to almost all types of devices utilizing electronic components. On the mechanical side, the viscosity of lubricants and hydraulic fluids are sensitive to temperature. It is essential that the cooling agent in heat exchangers of the lubrication/ hydraulic systems (mostly cooled water) is maintained at right temperature.

Looseness

Looseness is a major contributing factor causing the loss of functional safety and reliability. Loose foundation bolts, coupling bolts, bearing housings, loose electrical terminations, loose cable harness, etc., are a few instances. Significant and consequential failures have been attributed to the aspect of looseness.

Lubrication

It is hard to find mechanical equipment that does not feature some kind of movement. These movements occur between parts and components in contact. Applying the appropriate lubrication is mandatory in ensuring the inherent functional safety and reliability. Any lack of integrity in lubrication in terms of improper type and specification, the amount of lubricant required, contamination, etc., can seriously damage parts and components. As a result, it can jeopardize the inherent functional safety and reliability of the equipment or the system.

Recognizing the above factors and applying them adequately in the RCM task assessment is fundamental to restoring the inherent safety and reliability of the equipment. It can be ensured in a very cost-effective manner. This would ultimately help in preventing high maintenance cost and potential costs arising from collateral damages in the future.     

Reference: Nowlan, F.S. and Heap, H.F. (1978). Reliability-Centered Maintenance. Office of Assistant Secretary of Defense.

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