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Design News, May 2013

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M E D I CA L DESIGN APPLICATIONS Bearing Design Considerations in Medical Technology Pharmaceutical, dental, and medical device applications present many challenges for bearings. These include high speeds, low noise, long service life, and resistance to harsh environments or aggressive chemicals or fluids. BY JOHN WALLACE, AST BEARINGS I It is estimated there are more than 500,000 medical products on the market today. Many of these devices rely on rolling element bearings to achieve required performance levels. Engineers and designers are faced with unique challenges developing these products, and bearings play a key role in simple and complex devices. Bearing Materials All bearings should be manufactured using rings produced from high purity material. For most medical applications, martensitic stainless steel, similar to AISI 440C, is recommended and often required due to regulatory requirements. This material, which can be specified differently depending on its manufacturer, provides for good corrosion resistance and has fine, evenly dispersed carbides, which result in lower noise and vibration levels than 440C. This type of stainless steel is very desirable, particularly in high-speed devices. Nitrogen-enhanced martensitic stainless steel is also available, and while it is more expensive it offers up to five times the corrosion resistance when compared to traditional "440C type" materials. Other benefits of this material are very low noise levels, extended fatigue life due to its fine structure that contains smaller chromium nitrides (as opposed to chromium carbides), and a high resistance to corrosion resulting from exposure to blood. In some cases, balls produced from ceramic materials, such as silicon nitride, can prove beneficial. Ceramic balls are lightweight, highly polished, non-magnetic, exhibit high hardness, and resistant to attack from most T14 T REND WAT C H : ME D I C A L / A S U P P L E M E N T T O D E S IGN NE WS MAY 2013 magenta cyan yellow black liquids and chemicals. Ceramic balls greatly improve the high-speed capability of the bearing. Bearings made of steel rings and ceramic balls are commonly called hybrid bearings. While ceramic balls have an impressive list of beneficial characteristics for bearing applications, they are not a "cure all" medicine. Due to the high hardness of the ball, contact stress is increased so fatigue life is compromised. So, when the typical failure mode is characterized by fatigue, it's usually best to stick with steel balls. Retainers, or ball separators, are typically produced [www.designnews .com] ES245455_DNTW1305_T14.pgs 05.02.2013 23:50 UBM

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