Reliability And Quality Are Related

Reliability and quality cannot be treated as separate entities and both are affected throughout the entire product life cycle.

According to the American Society for Quality (ASQ), Milwaukee, (asq.org): “Reliability has sometimes been classified as ‘how quality changes over time.’

The difference between quality and reliability is that quality shows how well an object performs its proper function, while reliability shows how well this object maintains its original level of quality over time, through various conditions.”

Reliability and quality are interrelated. For high quality you need high data integrity. Operations require reliable power. Proper skill level enables precision maintenance, which ensures longer machine reliability. Standardized maintenance improves the repeatability of best work practices.

Weibull analysis can be used to monitor reliability growth and identify warranty times. If you buy a product and it breaks within the warranty period, you’re consoled that the repair/replacement cost is covered. Either the manufacturer has not performed a good warranty break-even calculation, or a replacement is less expensive than the repair. After the warranty period, the product may break more often, and repairs will be at your expense. Understanding and managing these relationships supports higher reliability, quality, and customer satisfaction.

When purchased, an automobile that is comfortable, easy to drive, gets expected mileage, and is safe and dependable is considered high quality. Similarly, a new machine that consistently produces within specification and has no downtime is also considered high quality. Quality refers to the inherent capability (how well it conforms to specifications/requirements) at the start of machine or product use. Reliability is what happens after that with daily use.

As explained in Practical Reliability Engineering, (Patrick D. T. O’Connor, John Wiley & Sons Ltd, 2002, Fourth Edition):

“…. times to occurrence can seldom be forecast accurately. Reliability is therefore an aspect of engineering uncertainty. Whether an item will work for a particular period is a question that can be answered as a probability. This results in the usual engineering definition of reliability as: The probability that an item will perform a required function without failure under stated conditions for a stated period of time.”

The relationship of reliability and quality to producing good product is also evident in the OEE (Overall Equipment Effectiveness) calculation. OEE of a process/manufacturing facility is calculated as the product of three components:

OEE = Availability x Performance x Quality

Availability is the percentage of scheduled time the machine or operation is available. It includes downtime losses and uptime. Performance is the speed at which the machine or operation functions, as a percentage of its designed speed. Quality is the number of good units produced as a percentage of the total units started.

Reliability has a probability component and comprises various elements such as availability (as described above), quality over time, and durability. Reliability and quality are affected throughout the entire product life cycle. All persons in these areas need to understand and be involved, but it needs to be managed holistically at the systems level.

During these challenging times I’ve noticed more quality issues in various companies, things such as lax processes, poor follow-up, poor craftsmanship, and insufficient quality control. This is not the time to let reliability and quality slip. “A company cannot buy its way into quality—it must be led into quality by top management.” (W. Edward Deming, Out of Crisis, MIT, 1986). EP