Bearing Design Considerations for Medical Devices – Part 3
We have come to the final installment of our feature article on design considerations for bearings used in medical devices. We hope the previous posts have provided you with a clear understanding of the materials, lubrication, and precision levels to evaluate when specifying a bearing for your medical application. In this final entry, we examine two undesirable conditions bearings may experience: contamination and noise / vibration.
The following is Part 3 of a three-part post. You can read Part 2 here and Part 1 here.
Noise and Vibration
Noise and vibration in a rotating bearing is an undesirable condition. Bearings manufactured to meet low noise and vibration requirements are an important consideration, particularly in very high-speed applications over 100,000 RPM, such as dental or surgical drills. Noise is the audible component of vibration and a function of rotational speed. Vibration is caused by several factors, including rough or damaged ring and ball surfaces, poor geometric tolerances, contamination, improper lubrication, incorrect radial play, and improper shaft and housing fits.
Most manufacturers of miniature and instrument bearings produce bearings that operate at very low noise levels, referred to as EMQ, or electric motor quality. An EMQ bearing is manufactured by superfinishing the rolling contact surfaces to a mirror-like finish, which provides very smooth rotation. Equally important is maintaining tight control of the geometric tolerances of the bearing components. It is important to note that unlike ABEC tolerance classes, EMQ standards are not uniform between bearing manufacturers.
Contamination
Medical devices and instruments are often exposed to various contaminants when in use, such as blood, saline solutions, and antiseptic fluids. Sterilization of equipment can introduce rinsing liquids and high-pressure steam. Diagnostic equipment, such as hematology analyzers, use a variety of fluids and reagents during operation. Miniature and instrument ball bearings are available with different types of closures, called shields and seals. Closures can extend bearing life by preventing contaminants from reaching the critical surfaces inside the bearing, while at the same time limiting the loss of lubricant. The metallic shield is the most common closure and highly recommended for almost all applications. It is manufactured from 300 series stainless steel, which has a maximum operating temperature of 600 °F.
Since a shield makes no contact with the bearing inner ring, it has no appreciable impact on torque or speed. Molded rubber seals are recommended for more highly contaminated environments. The most common bearing seal material is nitrile rubber, also known as NBR or Buna-N. This type of seal consists of a rubber profile bonded to a steel insert and has a maximum operating temperature of 240 °F. A seal is typically fixed into the bearing outer ring and contacts the inner ring, providing better protection than a shield. This seal contact results in an increase in rotational torque and reduces the maximum speed capability of the bearing, however. Still, this limitation is usually seen as a design trade-off made to improve bearing life. Light contact seal versions are available if the increased torque from the seal lip is of concern.
Nitrile rubber reacts negatively with certain fluids and lubricants and therefore may not be suitable for certain applications. Alternate materials include fluoroelastomers, such as Viton, which have good chemical resistance and a maximum operating temperature of 400 °F. FDA-approved food-grade silicone rubber, which has an operating temperature range from –80 to 450 °F, is another good option for extreme temperature applications. One of the best sealing solutions for bearings used in medical applications (particularly those in the surgical suite) is the glass-reinforced PTFE or Teflon seal. Like the molded rubber and Viton seals, this type of seal contacts the inner ring, providing better protection than a metal shield in contaminated environments. These materials have outstanding chemical resistance, can withstand high and low temperatures, and produce less torque than rubber seals. These seals are not as robust as those made from other materials, however, and therefore may not be an effective solution.
Summary
Selecting suitable bearings for medical devices is crucial in ensuring their reliability and performance. Factors such as material choice, lubrication, precision and noise levels, and protection from contamination should be carefully considered to meet the demanding requirements of the healthcare industry. By prioritizing these design considerations, manufacturers can create safe and effective medical devices that improve patient outcomes and enhance the overall quality of healthcare.