Statistical quality control and reliability engineering pdf
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Reliability Engineering pp Cite as. Statistical quality control and reliability tests are performed to estimate or demonstrate quality and reliability characteristics figures on the basis of data collected from sampling tests. Estimation leads to a point or interval estimate of an unknown characteristic, demonstration is a test of a given hypothesis on the unknown characteristic in an acceptance test. Estimation and demonstration of an unknown probability is investigated in Section 7. Basic models for accelerated tests and for goodness-of-fit tests are considered in Sections 7.
Statistical process control
Statistical process control SPC is a method of quality control which employs statistical methods to monitor and control a process. This helps to ensure that the process operates efficiently, producing more specification-conforming products with less waste rework or scrap.
SPC can be applied to any process where the "conforming product" product meeting specifications output can be measured.
Key tools used in SPC include run charts , control charts , a focus on continuous improvement , and the design of experiments. An example of a process where SPC is applied is manufacturing lines.
SPC must be practiced in 2 phases: The first phase is the initial establishment of the process, and the second phase is the regular production use of the process. An advantage of SPC over other methods of quality control, such as " inspection ", is that it emphasizes early detection and prevention of problems, rather than the correction of problems after they have occurred.
In addition to reducing waste, SPC can lead to a reduction in the time required to produce the product. SPC makes it less likely the finished product will need to be reworked or scrapped. SPC was pioneered by Walter A. Shewhart at Bell Laboratories in the early s. Shewhart developed the control chart in and the concept of a state of statistical control. Shewhart consulted with Colonel Leslie E. Simon in the application of control charts to munitions manufacture at the Army's Picatinny Arsenal in Department of Agriculture and served as the editor of Shewhart's book Statistical Method from the Viewpoint of Quality Control which was the result of that lecture.
Deming was an important architect of the quality control short courses that trained American industry in the new techniques during WWII. The graduates of these wartime courses formed a new professional society in , the American Society for Quality Control, which elected Edwards as its first president. However, he understood that data from physical processes seldom produced a normal distribution curve that is, a Gaussian distribution or ' bell curve '. He discovered that data from measurements of variation in manufacturing did not always behave the way as data from measurements of natural phenomena for example, Brownian motion of particles.
Shewhart concluded that while every process displays variation, some processes display variation that is natural to the process " common " sources of variation ; these processes he described as being in statistical control. Other processes additionally display variation that is not present in the causal system of the process at all times " special " sources of variation , which Shewhart described as not in control.
The application of SPC to non-repetitive, knowledge-intensive processes, such as research and development or systems engineering, has encountered skepticism and remains controversial. In his seminal article No Silver Bullet , Fred Brooks points out that the complexity, conformance requirements, changeability, and invisibility of software   results in inherent and essential variation that cannot be removed.
This implies that SPC is less effective in the domain of software development than in, e. In manufacturing, quality is defined as conformance to specification. However, no two products or characteristics are ever exactly the same, because any process contains many sources of variability.
In mass-manufacturing, traditionally, the quality of a finished article is ensured by post-manufacturing inspection of the product. Each article or a sample of articles from a production lot may be accepted or rejected according to how well it meets its design specifications , SPC uses statistical tools to observe the performance of the production process in order to detect significant variations before they result in the production of a sub-standard article.
Any source of variation at any point of time in a process will fall into one of two classes. Most processes have many sources of variation; most of them are minor and may be ignored. If the dominant assignable sources of variation are detected, potentially they can be identified and removed. When they are removed, the process is said to be 'stable'. When a process is stable, its variation should remain within a known set of limits. That is, at least, until another assignable source of variation occurs.
When the package weights are measured, the data will demonstrate a distribution of net weights. If the production process, its inputs, or its environment for example, the machine on the line change, the distribution of the data will change.
For example, as the cams and pulleys of the machinery wear, the cereal filling machine may put more than the specified amount of cereal into each box.
Although this might benefit the customer, from the manufacturer's point of view it is wasteful, and increases the cost of production. If the manufacturer finds the change and its source in a timely manner, the change can be corrected for example, the cams and pulleys replaced.
The data from measurements of variations at points on the process map is monitored using control charts. Control charts attempt to differentiate "assignable" "special" sources of variation from "common" sources.
Using control charts is a continuous activity, ongoing over time. When the process does not trigger any of the control chart "detection rules" for the control chart, it is said to be "stable". A process capability analysis may be performed on a stable process to predict the ability of the process to produce "conforming product" in the future. A stable process can be demonstrated by a process signature that is free of variances outside of the capability index.
A process signature is the plotted points compared with the capability index. When the process triggers any of the control chart "detection rules", or alternatively, the process capability is low , other activities may be performed to identify the source of the excessive variation. The tools used in these extra activities include: Ishikawa diagram , designed experiments , and Pareto charts. Designed experiments are a means of objectively quantifying the relative importance strength of sources of variation.
Once the sources of special cause variation are identified, they can be minimized or eliminated. Steps to eliminating a source of variation might include: development of standards, staff training, error-proofing, and changes to the process itself or its inputs.
When monitoring many processes with control charts, it is sometimes useful to calculate quantitative measures of the stability of the processes. These metrics can also be viewed as supplementing the traditional process capability metrics.
Several metrics have been proposed, as described in Ramirez and Runger. Digital control charts use logic-based rules that determine "derived values" which signal the need for correction. For example,. From Wikipedia, the free encyclopedia. Main article: Common cause and special cause statistics. Edwards, Lectures on statistical control of quality. Edwards and Dowd S. SPC Press, Inc. British Deming Association. Quality Engineering. This article includes a list of general references , but it remains largely unverified because it lacks sufficient corresponding inline citations.
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Statistical Quality Control and Reliability Tests
Many companies have tried to upgrade their quality, adopting programs that have been staples of the quality movement for a generation: cost of quality calculations, interfunctional teams, reliability engineering, or statistical quality control. Few companies, however, have learned to compete on quality. Part of the problem, of course, is that until Japanese and European competition intensified, not many companies seriously tried to make quality programs work even as they implemented them. But even if companies had implemented the traditional principles of quality control more rigorously, it is doubtful that U. To get a better grasp of the defensive character of traditional quality control, we should understand what the quality movement in the United States has achieved so far.
Used with permission. We have made it easy for you to find a PDF Ebooks without any digging. Introduction to reliability Portsmouth Business School, April 3 Bath tub curve Infant Mortality : This stage is also called early failure or debugging stage. Reliability engineering is a well-developed discipline closely related to statistics and probability theory. Generally defined as the ability of a product to perform, as expected, over certain time. Arizona State University's Master of Engineering in quality, reliability, and statistical engineering offers specialized courses founded on basic engineering and statistics principles that are central to improving quality, reliability, and achieving meaningful results in today's modern business organizations. I did not think that this would work, my best friend showed me this website, and it does!
What is Reliability?
It was found that all other firms have within the past five years followed the industry leader in implementing statistical quality control, vendor management and reliability engineering. The study found that the dollar amount spent on quality assurance capital equipment, the amount of training, the proportion of engineering personnel and the sophistication of the data processing capability to rank correlate well with the product quality. In terms of quality costs, the inverse relationship between the traditional ASQC quality costs and product quality was observed. It was also observed that the amount devoted to the capital equipment in the quality assurance budget grew both proportionally and absolutely. Finally, it was observed that in terms of relative quality, the costs of the engineering personnel and market research personnel were higher for firms with high product quality.
Dripper testing: Application of statistical quality control for measurement system analysis. Hermes S. Patricia A. Marques 2. Antonio P.
Statistical process control SPC is a method of quality control which employs statistical methods to monitor and control a process. This helps to ensure that the process operates efficiently, producing more specification-conforming products with less waste rework or scrap. SPC can be applied to any process where the "conforming product" product meeting specifications output can be measured. Key tools used in SPC include run charts , control charts , a focus on continuous improvement , and the design of experiments.
Save extra with 2 Offers. Gupta Book Summary: This is a textbook-cum-working manual. It deals, primarily, with various types of Shewhart control charts and with various types of scientific acceptance sampling systems and procedures.