What is Reliability

What is Reliability


Reliability, maintainability, availability, or the ‘ilities’ are common in our language with reference to products, services, equipment, and people. Joe is regularly available for the meeting; We can count on (depend or rely) Sara to finish the report on time; My car starts every morning without fail; and many more. What is meant with these concepts and specifically how do we manage achieving and sustaining business objectives related to these ‘ility’ concepts? The purpose of this short paper is to provide an introduction to key concepts and approaches commonly used for reliability and maintainability management.

With some common sense, an appreciation of the goals, understanding of expected or past failures, and the proper application of reliability engineering tools, you can manage to improve profitability, increase throughput, or enhance a brand image. With a sound design, robust supply chain, consistent manufacturing, and adequate maintenance nearly any product or complex system can meet or exceed their reliability or maintainability goals.

Reliability and Quality

Reliability is a quality aspect of a product. Or, as some like say, reliability is quality over time. Either way, the basic definition we will use here is reliability is the probability of a product successfully functioning as expected for a specific duration of time within a specified environment. For example, a TV remote control has a 98% probability of successfully controlling the associated television (change volume, channels, etc.) for two years in a North American home environment.

There are four elements to the reliability definition: 1) Function, 2) Probability of success, 3) Duration, and, 4) Environment.

Maintainability is related to reliability, as when a product or system fails, there may be a process to restore the product or system to operating condition. Maintainability is a characteristic of design, assembly, and installation that is the probability of restoration to normal operating state of failed equipment, machine or system within a specific timeframe, while using the specified repair techniques and procedures. We often consider two other ‘ilities’ with maintainability: 1) serviceability or the ease of performing inspections, diagnostics and adjustments; and, 2) reparability or the ease of restoring functionality after a failure.


Closely related to reliability and maintainability is availability. Availability is a characteristic of a system (piece of equipment or product) to function as expected on demand. One way to measure availability is the percentage of time the system is functioning per year. An example may be the cable TV service to a home is 95% available, meaning that for the 100 hours of desired TV entertainment per year the system functioned for only 95 hours, and was not functioning (under maintenance, power outage, etc.) for 5 hours.

The fundamental idea from a customer’s point of view is the product works as expected. The car starts, the bottling machine fills the bottles accurately and quickly, the printer just works. When asked a customer does not want any failures, especially with the specific product that they purchase. They do want to enjoy the benefit provided by the functioning product. Unfortunately the variability of materials, assembly techniques, environments, and expectations do lead to failures. Failure happens.

In one sense, reliability and maintainability management is the management of failure. The specific approaches and tools available to the R&M manager permit the optimization of the problem to finding a cost effective solution to the design, assembly and use of a product. Reliability and maintainability engineering pulls resources and skills from across many fields including design, materials, finance, manufacturing, environmental, and statistics. R&M engineers are often asked in one form or another only two questions: 1) What will fail? and, 2) When will it fail? For each specific situation (i.e. satellite or game controller) the R&M engineer assesses risk, balances the probability and consequence of failure with value, and negotiates with development, manufacturing, suppliers and customers to deliver a reliable and maintainable solution. It is often an exciting, rewarding and challenging role.

About Fred Schenkelberg

I am an experienced reliability engineering and management consultant with my firm FMS Reliability. My passion is working with teams to create cost-effective reliability programs that solve problems, create durable and reliable products, increase customer satisfaction, and reduce warranty costs.

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