The 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. A well-stated goal provides clarity across the organization and permits a common language for discussing design, supply chain, and manufacturing decisions.
Let’s explore the definition of a ‘well-stated reliability goal.’ First, is it not simple MTBF, “as good as or better than…” or ‘a 5-year product’. These are common ‘goals’ found across many industries, yet none permit a clear technical understanding of the durability expectations for the product.
The common definition for reliability is
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 a minimum number of cycles or events.
(Ireson, Coombs et al. 1995)
Note this definition has four elements:
• Function
• Environment
• Duration
• Probability
Function
The function is what the product is to do or perform. For example, an emergency room ventilator is to provide assisted breathing for a person. This requires the ventilator to produce breathable air within a range of pressures, within a prescribed cycle of respiration. It may include requirements for filtering, temperature, and adjustments to pressure and timing of the cycle, etc. 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 function of most organizations verifies that the design and production units meet the functional requirements. When the product does not meet the functional requirements, it is considered a product failure. Within the function, definition is the most important functions, which must not fail, as well as the functions that upon failure may simply degrade performance, if noticed by the customer at all.
Environment
The environment could be considered the weather around the product when in use. ‘Weather’ includes such things as temperature, humidity, UV radiation intensity, etc. It should also include environmental factors that provide destructive stresses, such as vibration, moisture, corrosive gasses, voltage transients, and more.
Another element of the environment is the use of the product. What is the use profile? Maybe once a day for a few minutes, like a remote control for the stereo system. Or perhaps its a 24/7 operation, such as for 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 the product encounters. The environmental conditions focus on the drivers for the product’s most likely failure mechanisms.
Duration
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 ten years. An implanted hearing aid is expected to last the life of the patient; if the patient is a child, this expectation may be more than 70 years.
The duration expectations may be defined by contract, market expectations, or by a business decision. The duration or life expectancy most likely is not the warranty period. For example, many personal computers have a three month or 1 year warranty period. The product is expected to last at least two years or more with normal use.
Many products have multiple durations that are of interest.
• Out of box
• Warranty
• 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 1-year warranty period. During this one 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 3-month warranty, yet as consumers, we have an expectation that the phone will function for two years or more.
Marketing or senior management 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. HP calculators often have only a 3-month or 1-year warranty, yet many have lasted 10 or more years. These calculators are known for their robustness and often cost more to purchase – 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 permits the design team to focus on and address the full range of product reliability risks.
Probability
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.
While there are many common terms to convey the probability of survival, the use of a percentage surviving is the easiest understood and most easily applied across an organization. Stating that 95% of units are expected to survive over the 5-year design life, means 95 out of 100 units will function properly over that 5-year period. A similar statement is that not more than 5% of products fail over the full five years. Or, it may be stated as not more than a 1% failure rate per year.
A common probability statement is the inverse of the failure rate or MTBF. The 95% reliability over five years (t) becomes approximately 100 years MTBF (θ). This does not mean the product will last 100 years; it does mean that 95% of the products are expected to last five years.
$$ \displaystyle\large R\left(t\right)=e^{\frac{t}{\theta}}=e^{\frac{5}{100}}=0.95 $$
Finally, stating a separate failure probability for each duration of interest provides a set of
duration/probability couplets that permit different focus for early or out of box failure risks, versus the longer-term failure risks.
Sometimes the product has a specific mission time, like an aircraft with an expected 12-hour mission over a 20-year serviceable life period. The probability of success for the 12-hour mission time is maybe set relatively high. It may also have a conditional probability, considering the number of missions since the last major service. Some products have availability goals and undergo routine maintenance or repair. These products and many complex systems require additional complexity in their goal setting. For the purpose of this discussion, we are considering simple products that are not normally repaired or products where the main interest is in the time to the first failure.
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 team.
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