From Ventilators to Surgical Robotics: Bearing Design Considerations for Medical Devices

Healthcare is undergoing a great transformation, largely driven by the innovation of medical devices. These devices play an increasingly pivotal role in diagnosing, treating, and improving the lives of patients worldwide. Reliability is paramount, and many devices depend on the use of precision ball bearings to operate effectively under demanding conditions.

Surgical power tools, dental drills, pumps, and ventilators are just a few of the medical applications that require the use of miniature and instrument ball bearings. Bearings are also critical components in a wide variety of diagnostic and imaging equipment. Material, lubrication, precision level, low noise, and protection from contamination are key attributes that designers must properly determine to ensure the optimal performance of bearings used in medical devices.

Materials

Direct exposure to patient tissue and bodily fluids, sterilization requirements, and regulatory compliance all drive the need for appropriate material selection when specifying bearing components. Bearings used in medical devices must be manufactured from rings and balls produced with high-purity materials. Refined martensitic stainless steel is recommended for most medical applications. This alloy, known by the trade names KS440, ACD34, and X65Cr13, offers the same corrosion resistance as conventional AISI 440C but contains lower carbon and chromium content.

This chemistry results in fine, evenly dispersed carbides after heat treatment, producing lower noise and vibration characteristics during bearing operation. This is highly desirable for high-speed medical instruments such as dental drills and surgical handpieces. For higher corrosion resistance, nitrogen-enhanced martensitic stainless steel can be used. This steel is more expensive than 440C but offers five times the corrosion resistance, which is beneficial for use in environments such as exposure to blood. This alloy also exhibits extended fatigue life and very low noise levels.

Balls produced from ceramic materials such as silicon nitride provide great benefits for some applications. Ceramic balls are lightweight, nonmagnetic, and resistant to attack from most liquids and chemicals. They also greatly improve the limiting speed of the bearing, which is ideal for handpieces spinning at very high revolutions. While ceramic balls have an impressive list of beneficial characteristics, contact stress is greater due to the high ball hardness and the fatigue life of the bearing is compromised. Steel balls are a better option when the typical bearing failure mode is characterized by fatigue.

Full ceramic bearings, in which both the rings and balls are manufactured from ceramic material, offer the advantage of being completely non-magnetic. These bearings are ideally suited for use in imaging equipment. Full ceramic bearings cannot be produced to the same precision levels of typical steel bearings, however, and can be cost prohibitive. Titanium and 300 series stainless steel are considered biocompatible options but are not commonly used due to a reduction in load capacity and a large increase in cost.

The retainer, or ball separator, is also an important bearing component and its material selection should not be overlooked. Retainers influence the speed capability of a bearing, as well as the torque and noise levels produced. Retainers used in bearings typical of medical applications are made from 300 series stainless steel. In high-speed applications, however, it is often necessary to use a plastic or phenolic resin crown-style retainer. For extremely high-speed rotation, an angular contact bearing with a full-machined, one-piece type retainer should be used. This style of retainer provides increased stability at higher speeds. A wide array of plastic materials is available for producing retainers, which are lightweight, resistant to temperatures up to 500 °F, and autoclavable. Phenolic resin cages have a porous structure and can be impregnated with oil for better lubricity in the ball pockets. Some materials, such as polyamide-imides, contain additives like graphite or PTFE, which improve lubricity properties.

Lubrication

Proper lubricant selection — meaning both type and amount — is critical to bearing life and performance but is often overlooked. Operating temperature is the primary consideration when selecting a lubricant. Temperature directly affects the viscosity of the lubricant base oil, which in turn impacts the lubricant’s ability to support application loads. In medical applications, bearing lubricants are subjected to numerous demanding environmental factors including sterilization, temperature extremes, high-speed rotation, saline wash down or irrigation, reagents, blood, and exposure to radiation.

Lubricant selection not only depends on the operating conditions of the bearing; it may also be subject to regulatory requirements. Manufacturers of medical devices are often required to use NSF-certified lubricants (e.g., food-grade H1 or H2) or biocompatible lubricants according to ISO 10993. In surgical tool applications involving patient contact, medical-grade silicone or mineral oil-based lubricants are commonly used. These lubricants must be able to withstand high speeds and be resistant to steam and water washout from the sterilization process. Solid film lubrication is another option but is generally used only in situations where an oil or grease would likely fail. These include high- or low-temperature extremes, exposure to radiation, or vacuum environments where outgassing is a concern.

Solid films are highly engineered and usually added at the component level prior to bearing assembly. Common examples of dry films are graphite, molybdenum disulfide, tungsten disulfide, or gold. Solid films are subject to wear and therefore should be used in only low-speed, lightly loaded applications. The bearing manufacturer (or a lubrication specialist) should be consulted during lubricant selection due to the large number of lubricants commercially available across a wide range of prices.

Precision Level

The precision level, or tolerance class, of a ball bearing should be carefully considered when specifying for use in a medical device. While all bearings are extremely precise mechanisms, thought must be given to the different precision level options available and their effect on bearing performance and lifespan. Raceway parallelism, for example, can affect bearing torque, so a nonparallel raceway condition will result in torque spikes during rotation. In high-speed applications, excessive bearing runout can result in an imbalance in the rotating mass. Both conditions can lead to unpredictable performance and premature failure.

Bearing tolerance classes are specified in the United States by the American Bearing Manufacturers Association (ABMA) and are defined as ABEC 1, ABEC 3, ABEC 5, ABEC 7, and ABEC 9. The higher the tolerance class number, the tighter the tolerances. ABEC ratings specify tolerances of size and form for the individual inner and outer rings. Tolerances of size refer to the basic boundary dimensions: the inner and outer diameters and the ring widths. Tolerances of form include roundness, taper, runout, and parallelism. Bearings with higher tolerance classes are intended for use in precision applications that require high running accuracy, high speed rotation, and/or low torque. In handheld instruments, such as surgical or dental drills, higher precision bearings help reduce noise, vibration, and heat generation, making tool use more comfortable for the operator. The downside of using higher precision bearings is price: the higher the ABEC level, the more expensive the bearing.

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 is comprised 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.

This article was written by Mark Manegold, Technical Marketing Manager, AST Bearings, Parsippany, NJ. Manegold is a degreed mechanical engineer with more than 20 years of experience with application analysis, design, and new product development in the bearing industry. For more information, e-mail This email address is being protected from spambots. You need JavaScript enabled to view it. or visit here .

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