Reliability in Design and Development

The Downside of a Fault Tolerant System

Maintaining high reliability or availability is a marked advantage for any system. A system that achieves the ability to avoid system downtime due to a single failure event, is essential in many applications. Yet, the fault tolerant capability comes at a price.

A system that achieves the ability to avoid system downtime due to a single failure event, is essential in many applications. Yet, the fault tolerant capability comes at a price.

Here is a short list and brief description of fault tolerant design disadvantages:

Masking or obscuring low-level failures

The nature of a fault tolerance design is to continue to operate normally even with a component failure.

Thus if the ability to detect a component failure relies on a loss of function or capability, it may be difficult to detect the failure. This sets the stage for a second component failure to cause a system downing event. [Read more…]

by Fred Schenkelberg Leave a Comment

Solving a Reliability Optimization Example

In the previous article, What is Reliability Optimization, we defined the concept. One of the elements of optimization is identifying which elements of a system to focus improvement efforts on.

Simply improving every element of a design may provide an overall improvement of reliability performance.

Given constraints such as time or funding, selecting the specific few elements that would provide the most improvement is key. [Read more…]

by Fred Schenkelberg 2 Comments

What is Reliability Optimization?

Delivering the best reliability performance within the various constraints imposed.

Without constraints such as budget, time to market, customer expectation, product functional capabilities, and product weight, you certainly could design and deliver a highly reliable product.

There always are constraints.

In the Oliver Wendall Holmes poem, The One Hoss Shay, the deacon procures the strongest oak, the supplest leather, and the best of best materials. Cost was not a constraint. And the shay lasted 100 years to the day.

If the technology permits there may be stronger or more durable components available for a price, yet cost is often a limiting factor. [Read more…]

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Deciding What Should Have Fault Tolerance

In some circumstances, it is desirable to ensure the system continues to operate even if there is an internal failure. An aircraft navigation system should be able to operate even if an internal dc-dc regulator fails, for example.

Not everything within some systems benefits by being fault tolerant.

For example, a failure of a cabin reading light over a passenger seat is not critical to the safe operation of the aircraft, thus is likely not created to be fault tolerant. One criterion to determine what should be fault tolerant is the criticality of the function the system provides.

This also applies to specific subsystems within a system allowing some elements to be created fault tolerant and others within the system not. [Read more…]

by Fred Schenkelberg Leave a Comment

Fault Tolerance Basics

Fault tolerance is a system that is reliant to the failure of elements within the system. It also may be called a fail safe design.

A fault tolerant system may continue to operate just fine, after one of the power supplies fails, for example. Or it may operate in a reduced or degraded state.

Other systems may have a ‘limp home’ condition, allowing the system to save critical data or allowing you to drive to a safe place to change a flat tire. [Read more…]

by Fred Schenkelberg 4 Comments

Tolerance Intervals for Normal Distribution Based Set of Data

This is not the same as a confidence interval. For a mean or standard deviation, we can calculate the likelihood that the true parameter is within a range of values — confidence interval concerning a parameter.

A tolerance interval applies to the individual readings, not the statistics. The interval contains a certain proportion of the values within the distribution of individual data points. The endpoints are tolerance limits. [Read more…]

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Reliability Block Diagrams Overview and Value

A reliability block diagram is a graphical and mathematical model of the elements of a system permitting the calculation of system reliability given the reliability of the elements.

The model reflects the reliability performance structure including series, parallel, standby and other arrangements of system elements.

Each block in an RBD represents a component or subsystem of the system. The organization and connecting lines represent the reliability structure of the system and may or may not be representative of the system’s functional block diagram. [Read more…]

by Fred Schenkelberg 24 Comments

Root Sum Squared Tolerance Analysis Method

The root sum squared (RSS) method is a statistical tolerance analysis method.

In many cases, the actual individual part dimensions occur near the center of the tolerance range with very few parts with actual dimensions near the tolerance limits. This, of course, assumes the parts are mostly centered and within the tolerance range.

RSS assumes the normal distribution describes the variation of dimensions. The bell-shaped curve is symmetrical and fully described with two parameters, the mean, μ, and the standard deviation, σ. [Read more…]

by Fred Schenkelberg 2 Comments

Safety Factor

Products that fail may create an unsafe situation.

For catastrophic failure mechanisms, the design team may consider establishing a safety factor or margin of safety policy. This provides the design team guidance as they size structures, select components, and evaluate performance and reliability.

A safety factor or margin are measures of the separation of the stress and strength for a specific failure mechanism. If something has a 2x safety factor it implies the element is twice as strong as the expected stress. [Read more…]

by Fred Schenkelberg 2 Comments

Why do Tolerance Analysis

The short answer is, everything varies.

The longer answer involves the agreement between what is possible and what is desired.

If we could design a product and it could be replicated exactly, including every element of the product, we would not need tolerances. Any part would work with any assembly. We would simply specify the dimensions required. [Read more…]

by Fred Schenkelberg 2 Comments

Worst Case Tolerance Analysis

Worst case tolerance analysis is the starting point when creating a tolerance specification. It is a conservative approach as it only considers the maximum or minimum values of part variation, whichever leads to the worst situation.

The worst case method simply adds the dimensions using the extreme values for those dimensions. Thus if a part is specified at 25mm +/- 0.1mm than use either 25.1mm or 24.9mm, whichever leads to the most unfavorable situation. [Read more…]

by Fred Schenkelberg 4 Comments

Common Mode Failures

Common mode or common cause failures related to redundant systems where one cause can lead to the failure of otherwise redundant elements leading to system failure.

Elements which should fail independently are under some circumstances dependent.

When considering the probability of individual paths in a complex redundant system, take due care to consider the common mode failures which may have a higher probability than any single path in the system. [Read more…]

by Fred Schenkelberg 11 Comments

Fault Tree Analysis 8 Step Process

Fault Tree Analysis (FTA) provides a means to logically and graphically display the paths to failure for a system or component. One way to manage a complex system is to start with a reliability block diagram (RBD). Then create a fault tree for each block in the RBD.

Whether a single block or a top level fault for a system the basic process to create a fault tree follows a basic pattern. [Read more…]

by Fred Schenkelberg 6 Comments

Benefits of Fault Tree Analysis

Is a logical, graphical diagram that organizes the possible element failures and combination of failures that lead to the top level fault being studied.

The converse, the success tree analysis, starts with the successful operation of a system, for example, and examines in a logical, graphical manner all the elements and combinations that have to work successfully.

With every product, there are numerous ways it can fail. Some more likely and possible than others. The FTA permits a team to think through and organize the sequences or patterns of faults that have to occur to cause a specific top level fault. The top level fault may be a specific type of failure, say the car will not start. Or it may be focused on a serious safety related failure, such as the starter motor overheats starting a fire. [Read more…]

by Fred Schenkelberg 5 Comments

10 Steps of FMEA

In Chapter 8 of The Basics of FMEA by Robin E. McDermott, et. al. discusses the ten steps for an FMEA. I find it to be an excellent summary for describing and conducting a failure mode and effect analysis.

Therefore based on the work of McDermott and others, plus my own experience here are the ten steps with my descriptions. [Read more…]