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Bearing Design Considerations for Medical Devices – Part 2

Cutaway view of a dentail handpiece. Bearings can be seen in several locations supporting the shafts.

Bearing Design Considerations for Medical Devices – Part 2

In the previous installment of this technical article, we explored different materials to consider for bearings used in medical equipment. Here we will examine lubrication and precision level options. The following is Part 2 of a three-part post. You can read Part 1 here.

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.

In the next installment of Bearing Design Considerations for Medical Devices, AST will discuss two undesirable conditions bearings may experience in medical device applications: contamination and noise / vibration. Look for Part 3, coming February 28, 2024.