Reliability HotWire

Issue 93, November 2008

Hot Topics
Interaction Between the House of Quality (within Design for Six Sigma) and the FMEA (within Design for Reliability)

The terms quality and reliability are often used interchangeably, but while they are strongly connected, they are not the same. While reliability is concerned with the performance of a product over its entire lifetime, quality is concerned with the performance of a product at one point in time, usually at the end of the manufacturing phase, to assure conformance to specifications. Reliability assures that components, equipment and systems function without failure for desired periods during their whole design life [1]. One can say that reliability can be perceived as the continuation of quality over time. However, the underlying goal of quality and reliability systems is the same: to achieve customer satisfaction [2].

Design for Six Sigma (DFSS) has been widely employed as the framework to ensure that an organizations quality objectives are met and Design for Reliability (DFR) is becoming increasingly accepted as the best practice approach for ensuring reliability within the design. The two processes are complementary and there are many correlations between the tools employed by each. In this article, we present a brief summary of a comprehensive Design for Reliability process and we explore the relationship between two tools that are employed in the early product definition stage: the House of Quality (HoQ), which is part of the Quality Function Deployment (QFD) techniques used in Design for Six Sigma and the FMEA, which is part of the Design for Reliability process.

DFSS processes are widely known within industry, so in this article we will assume that the reader has adequate exposure in that area. We will just focus on providing an overview of DFR process in order to familiarize the reader with this framework before proceeding to discuss two specific tools in more detail.

What is Design for Reliability (DFR)?
Design for Reliability refers to the process of designing reliability into products in order to ensure that customer expectations for reliability are fully met while minimizing costs and increasing profit margins. In simple terms, whereas individual reliability analysis methods enable the computation of the reliability of an item, Design for Reliability provides a process for assuring that the optimum/desired reliability is designed into the item. This process encompasses multiple tools and practices in order to drive reliability into products.

The three basic underlying concepts that should be kept in mind under a DFR framework are:

  1. Reliability must be designed into products and processes, using the best available science-based methods.

  2. Knowing how to calculate reliability is important, but knowing how to achieve reliability is equally, if not more, important.

  3. Design for Reliability practices must begin early in the design process and must be well integrated into the overall product development cycle.

The main DFR strategies are to:

  • Design out failure mechanisms.

  • Reduce variation in product strength.

  • Reduce the effect of usage/environment.

  • Increase design margins.

ReliaSoft has proposed a comprehensive DFR process in order to enable organizations to achieve their reliability goals. It is summarized in the following chart: *

[Click to Enlarge]

Design for Six Sigma QFD and the House of Quality
In the world of quality and Six Sigma, Quality Function Deployment (QFD) techniques are widely known and used. QFD is a methodology that systematically translates the "voice of the customer" (VOC) into functional requirements for the product. QFD helps transform customer needs into engineering characteristics for a product or service, prioritizing each product or service characteristic while simultaneously setting development targets for a product or service. Through a planning matrix named the "House of Quality" (HoQ), QFD transforms customers wants into designs, manufacturing processes and production control requirements. The transformation of customer requirements will help develop part characteristics, process requirements and product standards necessary for the product development phase [3, 4].

In Figure 1, we illustrate a simple base structure for the House of Quality [5]. Keep in mind that there is no "standard" format for the HoQ, because it can and should be customized to serve the specific application needs. It would be different for mapping out a new product, redesigning an existing product or improving an existing service. But the underlying concept is always mapping customer requirements through the voice of the customer (VOC) to technical requirements; in other words, matching the "whats" to the "hows."

Figure 1: The House of Quality

Failure Modes and Effects Analysis (FMEA)
FMEA is an analytical engineering technique to ensure that all the potential failures of a product have been considered and analyzed in terms of failure modes, related causes and possible effects on the customer. The FMEA may be employed at various stages of the DFR process and may also be incorporated into DFSS and other activities as well.

The FMEA inputs can be briefly demonstrated through a simple example for the tires in a vehicle:

  • Function: Maintain the proper level of inflation (e.g. 30 psi in front and 29 psi in back).

  • Failure: Insufficient inflation (flat tire).

  • Effects: Inability to drive, damage to rims, loss of control, etc.

  • Causes: Puncture, worn sides, improper maintenance, etc.

  • Controls: Check tire pressure when filling tank, rotate and replace tires according to manufacturers guidelines, etc.

The key to a successful FMEA is to be "risk conscious" and to keep the team focused on risk. Two commonly used approaches for risk assessment with the FMEA framework are Risk Priority Numbers (Severity x Occurrence x Detection) and Criticality Analysis.

Interaction Between the House of Quality and FMEA
It can be said that the House of Quality (as a part of a DFSS process) and the FMEA (as part of a DFR process) are essentially two technical tools dealing with the same issue, namely the customers satisfaction, with each operating from a different point of view. QFD can be viewed as the defender of the customer needs, while FMEA is the defender of the voice of the engineer [3].

As quality is deployed into the design through the use of tools such as the House of Quality, at the same time the DFR process begins with input from the HoQ. Through the HoQ, functional specifications are fed into the FMEA, which can then be used to identify design risks and evaluate whether functional requirements can be met. At the same time, the results of the FMEA can be fed back into the HoQ in order to close the gaps between the customer requirements and the achievable reliability characteristics that will enable the specific product attributes to be matched to product requirements. Identified design improvements are also reflected back to the technical requirements.

In conclusion, it can be stated that the two tools are complementary, and when used in sync can be very beneficial to map out product requirements and close the gap with the reliability characteristics of a new product.

[1] Madu, C. N., "Reliability and Quality Interface," International Journal of Quality & Reliability Management vol. 16, no. 7, pp. 691 698, 1999.
[2] ReliaSoft Publications, "Design for Reliability: Overview of the Process and Applicable Techniques", Reliability Edge, Volume 8, Issue 2.
[3] Braglia, M., Fantoni, G. and Frosolini, M., "The House of Reliability," International Journal of Quality and Reliability Management vol. 24, no. 4, pp. 420-440, 2007.
[4] Wolfe, P., Planning for Failure: HOQ vs. FMEA, QFD Online,, 2008.
[5] Benbow, D. W., and Kubiak, T. M., The Certified Six Sigma Black Belt Handbook, Milwaukee, WI: ASQ Quality Press, 2005.

*If you want to learn more about DFR, ReliaSoft offers the training course D560 - Design for Reliability (DFR) Program Planning and Implementation, as part of the company's training curriculum. This course provides the length and breadth of the most effective Design for Reliability (DFR) methods, including an overview of the tools that support DFR. Also included is instruction in developing a successful DFR program for your company or organization.

Copyright 2008 ReliaSoft Corporation, ALL RIGHTS RESERVED