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Design News, May 2013

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M E D I CA L surgical and dental tools (drills and saws), laboratory and diagnostic equipment, and imaging equipment. Bearings of special design, or catalog bearings with modifications or enhancements, are typically required. These handheld tools, particularly dental drills, generally operate at very high speeds — rotational speeds of 400,000 RPM or greater are common. "Low" speed is 125,000 RPM, which is quite fast. High precision, ABEC 5 and 7, miniature and instrument series ball bearings are used. For ultra-high speeds, these bearings are modified further and have improved raceway surface finishes. In addition, the raceways and surfaces that guide the retainer have tighter dimensional and geometrical tolerances, in some cases ABEC 9. More expensive, angular contact designs are often recommended for their high-speed stability, and they also allow for the use of full machined type retainers. This, again, enhances speed capability. Running at high speeds also presents challenges with instrument noise levels and heat generation. High audible noise during a dental procedure is a problem for both the dentist and the patient. When bearings are assembled, it is necessary to have a certain amount of internal clearance, or radial play, built in. This allows for one bearing race to move both radially and axially relative to the other. Application of a preload across a pair of bearings is recommended. Preload can be defined as the application of an axial load across a pair of bearings to force the rolling elements to assume a contact angle for the purpose of removing the internal clearance. The result is constant ball to race contact. This reduces ball skidding, vibration, and noise. However, disadvantages of preload include, torque, heat, and reduction in fatigue life. Preload determination is a balancing act where the goal is to apply the least amount of axial preload force possible while meeting the performance requirements. During surgery, the bearings in tools are regularly exposed to harsh conditions and liquids, including blood and saline, as well as particulate debris. When space permits, shielded bearings should always be used. The speed of these tools is generally too high for seals, but when conditions permit, seals should be used. Seals are the best option for keeping foreign debris out of the interior of the bearing and keeping lubricant in. Laboratory and Diagnostic Equipment The laboratory work that goes into the testing of blood, urine, tissue, and other specimens is critical for the timely diagnosis and treatment of millions of patients every day. Thousands of tests are prescribed in hematology, immunochemistry, and histology every hour around the world. In the area of hematology, samples are typically subjected to a variety of conditions during testing and analysis. This includes light scatter analysis techniques for counting cells, mechanical motion and agitation, controlled temperature and humidity cycles, and the addition of reagents. Medical technologists use advanced laboratory and diagnostic equipment to conduct and catalog these tests and results. Due to the high volume of tests and the requirements for reliability, these test systems are often highly automated, programmable, and have full data management and storage capabilities. In addition, they can handle hundreds of samples; often times open vials, and conduct multiple tests during an automated cycle. Bearings for these applications should be manufactured from the type of martensitic stainless steels described previously. The bearings are often exposed to high humidity or moisture resulting from condensation. Sealed bearings should be considered whenever the potential for contamination exists. The most common bearing seal material is a nitrile rubber. However, this may not be well suited, or permitted due to regulatory requirements. Teflon seals are often used in medical devices. They have outstanding chemical resistance, high and low temperature capability, and exhibit less torque than nitrile rubber seals. Viton is also available when a more robust seal is required. The seals found on most types of bearings are not designed for immersion and fluid penetration will eventually take place. They offer excellent protection from particulate contaminants or a fluid splash and wipedown situation. These systems move test samples, most often vials, to various locations within the machine for scanning, testing, or the addition of a reagent prior to analysis. In addition, samples may be spun, shaken, or otherwise agitated for various reasons. These movements and motions are then repeated over thousands of cycles. To achieve the precise positioning and repeatability requirements, in most cases, bearings should be ABEC 3 or better. Housing and shaft design should allow for very precise fitting to minimize any eccentricities or the chance for slippage or fretting. When fitting bearings that have thin cross sections, such as miniature bearings, line-toline fits are commonly specified. Interference fits can reduce the internal clearance in the bearings. If this reduction is excessive, bearing life will be compromised. In these applications, where positional accuracy must be controlled to precise levels, the radial (and axial) play in the bearing is usually unacceptable. Application of a preload, described previously, is recommended. The principle benefits are precise shaft positioning (no free motion), control of axial and radial compliance, and shared loading between bearings. In addition, shaft rotational accuracy is greatly improved, minimizing runout characteristics. John Wallace is the vice president of operations at AST Bearings LLC. For more information, go to: www.astbearings.com. T16 T REND WAT C H : M E 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 [www.desig nnews .com] ES245456_DNTW1305_T16.pgs 05.02.2013 23:50 UBM

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