Bearing Design Considerations for Medical Devices – Part 1

Bearings are essential components used in applications throughout the medical industry. Designers and engineers are faced with important decisions when specifying bearings used in medical equipment. In this technical article, AST^® explores five critical design elements of bearings used in medical device applications, starting with materials.

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, non-magnetic, 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. 

In the next installment of this series, AST addresses lubrication and precision level options for bearings used in medical equipment. Look for Part 2 of Bearing Design Considerations for Medical Devices, coming February 14, 2024.