Bearing Design Considerations in Medical Technology - Part 2
Surgical and dental tools Some of the more demanding medical applications include surgical and dental tools (drills and saws), laboratory and diagnostic equipment, and imaging equipment. Bearings of special design, or catalogue bearings with modifications or enhancements, are typically required. These handheld tools, particularly dental drills, generally operate at very high speeds — rotational speeds of 400,000RPM or higher are common. Low speed is 125,000RPM, which is quite fast. High-precision (ABEC 5 and 7) miniature and instrument series ball bearings are used.
However, 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 of the instrument.
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 in hospitals and laboratories 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 catalogue these tests and results.
Due to the high volume of tests and the requirements for reliability, these test systems are often highly automated and programmable and have full data management and storage capabilities. In addition, they can handle hundreds of samples (oftentimes open vials) and conduct multiple tests during an automated cycle. These systems have demanding positioning requirements and utilize a variety of different types of bearings, including linear, angular contact, thin section, and miniature and instrument ball bearings.
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. In addition, they are exposed to fluids during testing that include blood and the reagents used for the test, so good fatigue life is critical.
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 and high- and low-temperature capability, and they 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-to-line 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, radial (and axial) play in the bearing is usually unacceptable. Application of a preload, described previously, is recommended. The principal 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.
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. Bearing manufacturers conduct continuous research into materials for components such as retainers, special lubricants, and optimization of the bearing geometry to in order to meet the ever-increasing demands of the industry. In addition to superb product quality, bearing suppliers must offer a flexible approach to problem solving.
John Wallace – vice president of operations at AST Bearings