1. Always assume the worst will eventually happen. This applies especially to critical parts and assemblies. Know what is critical especially to the use and customers. What are the critical parts? How will the customer abuse the assembly?
2. Always check for tolerance stack up problems. Parts in tolerance today may not be in the future. Don’t assume stability from the suppliers or that wear can not occur. Most of the time we do not know the relationship between being in specification and the ultimate reliability. A DOE (Design of Experiments) would help here.
3. Metal inserts in plastic parts are hard to mold well. This may lead to problems in use because of residual stress and will eventually cause problems through tool wear and/or part cracking.
4. Always maximize the radii that are present. Small radii lead to high stress concentrations and failure prone places. Harden these areas or use harder metals where possible when the radii can not be increased.
5. Use as few connections as possible. This includes connectors, wire connections such as welds or solder joints, crimps and material connections and seals. Remember all connections are potentially weak points that will fail given time and stress.
6. All seals fail given time and stress. You need at least two levels of sealing to ensure the product will last as long as the customer expects. Remember that some materials diffuse through others. Perhaps three levels of seal are required.
7. Threads on bolts and screws shouldn’t carry shear loads. Remember they need preloads and/or stretches to ensure proper loading initially. Metal stretches, fractures and corrodes as well as developing high stress concentrations in use under tension. Be sure to allow for this.
8. Use as few nuts, bolts and screws as possible. While these are convenient temporary connection methods, it is 100 year old technology. Lock washers and locktite have been developed to slow down the rate of loosening. All will eventually come loose anyway when there is stress, temperature or vibration present.
9. Belts and chains will stretch and/or slip when use to deliver power. Remember these types of parts need constant tension devices to aid their reliability. Again this is old technology that can be made reliable by careful application. (Note, this is one of the biggest field problem with snow-throwers.)
10. Avoid set screws as these easily come loose because of their small sizes. Even when used on a flat, set screws are only “temporary connection” mechanisms. Locktite only makes “the temporary” a little longer in the presence of stress.
11. Watch the use of metal arms to carry loads. They often deflect in an imperceptible manner. This is especially true when loads are dynamic.
12. Integrate as many mechanical functions as possible. Use as few separate and distinct mechanical parts that are joined as possible. Joints are usually unreliable.
Each of these common mechanical design problems is used in common “every day life” situations where 10% failures per year might be acceptable or near the limit of technology (washing machines, other appliances, many instruments and even some cars). The same standard designs will not work well in high reliability applications where only 1 or 2% failures per year are desired or acceptable (aerospace, military, medical devices etc.). Remember the difference between the two applications when designing.
By: by James McLinn CRE, Fellow ASQ JMREL2@aol.com
Published in Mechanical Design Reliability Handbook: Simplified Approaches and Techniques ISBN 0277-9633 February 2010 (available as free download for ASQ Reliability Division Members)
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