“Caveat Emptor” – Let the Buyer Beware
The Latin phrase “Caveat Emptor” describes the unfortunate situation where Design Engineering practices produce an environment that favors the seller over the buyer. To reverse that trend, Design Engineers need to reduce the amount of non-conforming product as well as confusion in the procurement process, by doing a better job of considering performance variations due to Initial, Environmental and End-of-Life conditions in the product’s life cycle.
Each parameter in a product’s design specification has both a nominal value as well as a tolerance. It is common design practice to associate the tolerance around the nominal value at the time of manufacture. These Initial tolerances might be due to mechanical dimensions or to variations in material properties such as electrical resistance, heat treatment or chemical properties (Figure 1).
Figure 1: Mechanical Dimension
When design engineers predict their product’s performance during the design process, it is good practice to consider the Initial tolerances of the individual components and combine them with all the Initial tolerances of the other components that make up the overall design of the product. After considering all of these combined Initial tolerances, the product must still be able to meet its performance requirements. There are numerous techniques that can be used to analyze the results of combined tolerances from multiple components. Those methods might include Worst-Case Analysis, RSS Analysis, Sensitivity Analysis, and Monte Carlo Analysis. Each of these methods has advantages and disadvantages when compared to the other methods. The selection of the appropriate analysis technique is the subject of another article.
In addition to these Initial tolerances, all components have a second and completely separate variation associated with that component’s performance in different environments. We call these Environmental variations. The magnitude of these variations can greatly exceed those associated with the Initial tolerances. One example of an Environmental variation is the change in mechanical dimension with change in temperature due to the material’s coefficient of thermal expansion. Other examples include the change in wire resistance due to temperature, the change in magnetic flux from a magnet or the voltage from a battery that also change with temperature. Other environments to consider include shock and vibration, solar radiation, the visual impact of fog and clouds, EMI, and humidity. Each of these individual environments will produce corresponding variation in components that impact the product’s overall performance. In addition, it is common for the product to experience more than one of these environments at the same time. All of these Environmental variations should be predicted during the design process and their effects combined with other Environmental variations in sensible combinations. After considering exposure to all of these combined Environmental variations, the product must still satisfy its performance requirements. Just as with Initial tolerances, there are appropriate engineering tools that can be used to determine the combined effects from multiple environmental sources.
All products have a lifetime over which they are expected to perform according to their agreed specification. To protect buyers and also to reduce the liability of suppliers, warranties are often specified for products, for periods of time from months to 10 or even 20 years. It is important, therefore, for the design engineer to consider the performance variations associated with each component of the design as the product is used and as it ages near the end of its life. We call these End-of-Life variations. Examples of End-of-Life variations include changes in friction as lubricants dry out, or corrosion that impacts not only visual appearance but can also seriously impact the structural strength of metals. As mechanisms operate during use, bearing friction and relative component alignment may change as bearings wear out. My personal experience of designing products to survive the corrosive effects associated with salt-fog in a marine environment was a prime example to me of the effects of both the environment as well as the passage of time.
As with Initial tolerances and Environmental variations, it is important that the product also meet its performance requirements through the product’s specified lifetime. What this means is that the design must be robust enough to maintain at least its minimum performance as bearings wear, as materials age, as lubricants dry, as capacitors age, battery capacity drops, and metals corrode. It is important to recognize that the buyer expects the product to meet its performance specification throughout its entire life. However, since it is difficult for engineers to quantify the degradation of performance as products age and are used over time, they often fail to perform the required analyses, and thus End-of-Life performance suffers. Even so, there are numerous examples of products that perform to their End-of-Life expectancy and even well beyond as shown in Figure 2.
Figure 2: 68 Year-Old Drill Press and 40 Year-Old Calculator
These three bands, Initial tolerances, Environmental variations, and End-of-Life variations can be illustrated in Figure 3. It is easy to see that if a product is designed to achieve desirable performance over all three of these bands, the initial performance, at the time of manufacture, will likely exceed the product’s minimum performance requirements. Unfortunately, some suppliers take advantage of this wide performance window by selling parts that minimally meet expectations now, while ignoring the degradations that are sure to happen over time, or in non-ideal environmental conditions.
Figure 3: Tolerance and Variation Bands
The buyer, on the other hand, assumes that the product’s performance will always achieve its minimum specified performance even at the End-of-Life. This major disconnect in buyer/supplier expectations often produces a product that may achieve its performance specification at the time of manufacture, but which degrades both over the environment as well as overtime. This disconnect produces a product that does not meet the performance for which the buyers pay, and which perpetuates a Caveat Emptor environment.
Make no mistake about it, the engineer’s responsibility is to make sure that the product functions desirably at the beginning and the end of its life. Some engineers misunderstand what this means and test their new products according to end of life performance requirements. Instead, engineers should test their new products according to more stringent initial performance requirements so that by the end of a product’s life – after ware, degradation, and during exposure to variable environments – the product will still function desirably.
All three of these tolerance and variation bands discussed above should be considered by the design engineer during the design process to ensure that the product’s performance requirements are achieved not only at the time of its manufacture and delivery to the customer, but also during its use over its specified environment as well as at the end of its expected lifetime. All of the variation and tolerances from all the individual components should be combined using an appropriate analysis technique to ensure compliance of the product in any sensible combination of components and expected environments over the life of the product. If the design engineers follow these steps, and perform the required tolerance analyses and performance testing, they can be confident that the product will satisfy its overall performance requirements to the satisfaction of a weary buyer.
