DESIGN FOR THE LIFE CYCLE

Kenneth Crow
DRM Associates

© 2002 DRM Associates  All rights reserved. May be used with attribution. Other use prohibited.

For many durable goods, there are a variety of other design considerations related to the total product life cycle. For consumable products, some of these life cycle factors may be of lesser importance. Life cycle factors that may need to be addressed during product design include:

• Testability/Inspectability
  
• Reliability/Availability
  
• Maintainability/Serviceability
  
• Design for the Environment
  
• Upgradeability
  
• Installability
  
• Safety and Product Liability
  
• Human Factors

The relative importance of these factors and their orientation will vary from industry to industry and product to product. However, there are general design principles for these life cycle requirements that will be generally applicable to many items. A basic integrated product development concept is the parallel design of support processes with the design of the product. This parallel design requires early involvement and early consideration of life cycle factors (as appropriate) in the design process. However, in many organizations, consideration or design of the support processes is an after-thought and many of these developmental activities are started after the design of the product is well under way if not essentially complete.

DESIGN FOR TESTABILITY / INSPECTABILITY

Test and inspection processes can consume a significant amount of effort and the development or acquisition of test equipment can require considerable time and expense with some products. Early involvement of the test engineering or quality assurance functions can lead to design choices that can minimize the cost of developing or acquiring necessary equipment and the effort to test or inspect the product at the various stages of production. A starting point is to establish a common understanding between Engineering, their customers, and other functional departments regarding the requirements for product qualification, product acceptance after manufacture, and product diagnosis in the field. With this understanding, a design team can begin to effectively design products and test and inspection processes in parallel.

Increasingly complex and sophisticated products require capabilities and features to facilitate test and acceptance of products and diagnosis products if a defect is identified. Specific principles which need to be understood and applied in the design of products are:

• Use of Geometric Dimensioning and Tolerancing (GD&T) to provide unambiguous representation of design intent
  
• Specification of product parameters and tolerances that are within the natural capabilities of the manufacturing process (process capability index Cp and Cpk)
  
• Provision of test points, access to test points and connections, and sufficient real estate to support test points, connections, and built-in test capabilities
  
• Standard connections and interfaces to facilitate use of standard test equipment and connectors and to reduce effort to setup and connect the product during testing
  
• Automated test equipment compatibility
  
• Built-in test and diagnosis capability to provide self test and self-diagnosis in the factory and in the field
  
• Physical and electrical partitioning to facilitate test and isolation of faults

In addition, test engineering should be involved at an early stage to define test requirements and design the test approach. This will lead to the design or specification of test equipment that better optimizes test requirements, production volumes, equipment cost, equipment utilization, and testing effort/cost. Higher production volumes and standardized test approaches can justify development, acquisition, or use of automated test equipment. The design and acquisition of test equipment and procedures can be done in parallel with the design of the product which will reduce leadtime. Design of products to use standardized equipment can further reduce the costs of test equipment and reduce the leadtime to acquire, fabricate, and setup test equipment for both qualification testing and product acceptance testing.

DESIGN FOR RELIABILITY

Reliability consideration has tended to be more of an after-thought in the development of many new products. Many companies' reliability activities have been performed primarily to satisfy internal procedures or customer requirements. Where reliability is actively considered in product design, it tends to be done relatively late in the development process. Some companies focus their efforts on developing reliability predictions when this effort instead could be better utilized understanding and mitigating failure modes, thereby.developing improved product reliability. Organizations will go through repeated (and planned) design/build/test iterations to develop higher reliability products. Overall, this focus is reactive in nature, and the time pressures to bring a product to market limit the reliability improvements that might be made.

In a integrated product development environment, the orientation toward reliability must be changed and a more proactive approach utilized. Reliability engineers need to be involved in product design at an early point to identify reliability issues and concerns and begin assessing reliability implications as the design concept emerges.

Use of computer-aided engineering (CAE) analysis and simulation tools at an early stage in the design can improve product reliability more inexpensively and in a shorter time than building and testing physical prototypes. Tools such as finite element analysis, fluid flow, thermal analysis, integrated reliability prediction models, etc., are becoming more widely used, more user friendly and less expensive. Design of Experiments techniques can provide a structured, proactive approach to improving reliability and robustness as compared to unstructured, reactive design/build/test approaches. Further, these techniques consider the effect of both product and process parameters on the reliability of the product and address the effect of interactions between parameters. Finally, the company should begin establishing a mechanism to accumulate and apply "lessons learned" from the past related to reliability problems as well as other producibility and maintainability issues. These lessons learned can be very useful in avoiding making the same mistakes twice.

Specific Design for Reliability guidelines include the following:

• Design based on the expected range of the operating environment.
  
• Design to minimize or balance stresses and thermal loads and/or reduce sensitivity to these stresses or loads.
  
• De-rate components for added margin.
  
• Provide subsystem redundancy.
  
• Use proven component parts & materials with well-characterized reliability.
  
• Reduce parts count & interconnections (and their failure opportunities).
  
• Improve process capabilities to deliver more reliable components and assemblies.

DESIGN FOR MAINTAINABILITY / SERVICEABILITY

Consideration of product maintainability/serviceability tends to be an after-thought in the design of many products. Personnel responsible for maintenance and service need to be involved early to share their concerns and requirements. The design of the support processes needs to be developed in parallel with the design of the product. This can lead to lower overall life cycle costs and a product design that is optimized to its support processes.

When designing for maintainability/serviceability, there needs to be consideration of the trade-offs involved. In high reliability and low cost products or with consummable products, designing for maintainability/serviceability is not important. In the case of a durable good with a long life cycle or a product with parts subject to wear, maintainability/serviceability may be more important than initial product acquisition cost, and the product must be designed for easy maintenance. In these situations, basic design rules need to be considered such as:

• Identify modules subject to wear or greater probability of replacement. Design these modules, assemblies or parts so that they can be easily accessed, removed and replaced.
  
• Use quick fastening and unfastening mechanisms for service items.
  
• Use common handtools and a minimum number of handtools for disassembly and re-assembly.
  
• Minimize serviceable items by placing the most likely items to fail, wear-out or need replacement in a small number of modules or assemblies. Design so that they require simple procedures to replace.
  
• Use built-in self-test and indicators to quickly isolate faults and problems.
  
• Eliminate or reduce the need for adjustment.
  
• Use common, standard replacement parts.
  
• Mistake-proof fasteners so that only the correct fastener can be used in re-assembly. Mistake-proof electrical connectors by using unique connectors to avoid connectors being mis-connected.

Design for Maintainability guidelines have much in common with Design for Manufacturability guidelines.

In addition, service and support policies and procedures need to be developed, service training developed and conducted, maintenance manuals written, and spare parts levels established. As these tasks are done in parallel with the design of the product, it reduces the time to market and will result in a more satisfied customer when inevitable problems arise with the first delivery of a new product.

ABOUT THE AUTHOR

Kenneth A. Crow is President of DRM Associates, a management consulting and education firm focusing on integrated product development practices. He is a distinguished speaker and recognized expert in the field of integrated product development. He has over twenty years of experience consulting with major companies internationally in aerospace, capital equipment, defense, high technology, medical equipment, and transportation industries. He has provided guidance to executive management in formulating a integrated product development program and reengineering the development process as well as assisted product development teams applying IPD to specific development projects.

He has written papers, contributed to books, and given many presentations and seminars for professional associations, conferences, and manufacturing clients on integrated product development, design for manufacturability, design to cost, product development teams, QFD, and team building. Among many professional affiliations, he is past President and was on the Board of the Society of Concurrent Engineering and is a member of the Product Development Management Association and the Engineering Management Society. He is a Certified New Product Development Professional. For further information, contact the author at DRM Associates, 2613 Via Olivera, Palos Verdes, CA 90274, telephone (310) 377-5569, fax (310) 377-1315, or email at kcrow@aol.com.