The reliability target, objective, mission, or goal is the statement that provides a design team with focus and direction. A well-stated goal will establish the business connection to the technical decisions related to product durability expectations, while providing clarity across the organization and enabling a common language for discussing design, supply chain, and manufacturing decisions.

Let’s explore the definition of a ‘well-stated reliability goal.’ First, it is not simple mean time between failure, ‘as good as or better than…,’ or ‘a 5-year product.’ These are common ‘goals’ found across many industries, yet none enable a clear technical understanding of the durability expectations for the product.

The common definition for reliability is “the ability or capability of the product to perform the specified function in the designated environment for a minimum length of time or minimum number of cycles or events.”

This definition has four elements: function, environment, duration, and probability. Let’s explore each of these elements in turn.

The function is what the product is to do or perform. For example, an emergency room ventilator provides assisted breathing for a person. This requires it to produce breathable air within a range of pressures and a prescribed cycle of respiration. The functionality may include requirements for filtering, temperature, and adjustments to pressure and timing of the cycle. Often, a product development team either develops or is given a detailed set of functional requirements.

The functional elements of a product are often directly measurable. Further, the quality assurance branch of most organizations verifies that the design and production units meet the functional requirements. When the product does not met the functional requirements, it is considered a product failure. Within the function definition are the most important functions—those that must not fail—as well as the functions that upon failure may simply degrade performance, if noticed by the customer at all.

The environment could be considered as the weather around the product when in use. In this case ‘weather’ can include temperature, humidity, UV radiation intensity, etc. It should also include environmental factors that provide destructive stresses, such as vibration, moisture, corrosive gases, and voltage transients.

Another element of the environment is the use profile of the product. Will the product be used once a day for a few minutes, such as a remote control for the stereo system, or does it operate 24/7, such as a server system processing transactions for a major online store? The profile may include details concerning human interactions, operating modes, shipping, storage, and installation. The environmental conditions need to detail how the product responds or degrades to the set of stresses it encounters, focusing on the drivers for the product’s most likely failure mechanisms.

The duration is the amount of time or number of cycles the product is expected to function. A computer printer may be expected to print for five years. A washing machine is expected to wash clothes for 10 years. An implanted hearing aid is expected to last the life of the patient.

The duration expectations may be defined by contracts, market expectations, or business decisions. The duration or life expectancy most likely is not the warranty period. For example, many personal computers have a 90-day or one-year warranty period, yet the product is expected to last at least two years or more with normal use.

Many products have multiple durations that are of interest. Three of the most basic of these are:

• Out of box,
• Warranty, and
• Design life.

The initial, out-of-box, or installation period is that duration when the customer is first setting up and using the product. Brand visibility is at its highest and the expectation that a new product will function as expected is very high. The types of failures that may occur include installation or configuration errors, mistaken purchase, shipping or installation damage, or simply buyer error. All of these ‘failures’ cost the company producing the product resources.

The warranty period is the duration associated with the producer’s promise to provide a product free of defects for a stated period of time. For example, a computer may have a one-year warranty period. During this year, if the product fails (usually limited to normal use and operating environment) the producer will repair or replace the product. Naturally, this will cost the producer resources.

The design life is the business- or market-expected product duration of function use. After the warranty period there isn’t an expectation for the producer to replace or repair the product, yet the customer may have a reasonable expectation that the product will function satisfactorily over the design life duration. For example, many cell phones have a 90-day warranty, yet as consumers we have an expectation that the phone will function for two years or more.

Marketing or senior management personnel may set the design life. They may want to establish a market position for the product related to reliability. One way is to design a very robust product with a long design life duration. For example, HP calculators often have only a 90-day or one-year warranty, yet many have lasted 10 or more years. These calculators are known for their robustness and often cost more to purchase, thus customer’s are willing to pay a ‘reliability premium.’

Each of the three durations often involves different risks related to the failure mechanisms. It is rare for bearings to wear out in the first 30 days, yet more likely for a 10-year design life. Establishing three or more durations within the product reliability goal enables the product designers to focus on and address the full range of product reliability risks.

The probability is the likelihood of the product to survive over a specified period of time. In the formal definition of reliability above, the phrase ‘ability or capability’ refers to the probability. This is the statistical part of the reliability goal and without it the goal is fairly meaningless. Furthermore, stating a probability without an associated duration and distribution is also meaningless in most cases.

What is the chance that a particular product will function as expected over the entire expected design life? How many of the installed units will be functional over the warranty period? Since each product and the associated environmental stresses vary, the use of statistics is unavoidable in describing product reliability. Even the definition of a product failure may vary by customer.

Although there are many common terms to convey the probability of survival, the use of a percentage surviving is the easiest to understand and the most easily applied across an organization. Stating that 95% of units are expected to survive over the 5-year design life means that 95 out of 100 units will function properly over that 5-year period. A similar statement is that not more than 5% of products will fail over the full five years. In other words, the failure rate per year is not more than 1%.

Finally, stating a separate failure probability for each duration of interest provides a set of
duration/probability couplets that enables a different focus for early or out-of-box failure risks versus longer term failure risks.

Some products have availability goals and undergo routine maintenance or repair. These products and many complex systems require additional complexity in their goal setting.

The point is that setting the reliability goal for a product is not as simple as stating a ‘five year life’:  It requires a clear statement with sufficient detail of each of the four elements: function, environment, duration, and probability. It should also typically include at least three duration/probability couplets. The goal establishes the direction or target for the entire design, supply chain, and manufacturing teams.

Bio:

Fred Schenkelberg is an experienced reliability engineering and management consultant with his firm FMS Reliability. His 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. If you enjoyed this articles consider subscribing to the ongoing series Musings on Reliability and Maintenance Topics.