The following is the first entry in a four-part series by John Wallace (AST’s Vice President of Operations) focused on bearing choices in medical technology. Look for future updates, including lubrication, surgical/dental tools and laboratory/diagnostic equipment.
All bearings should be manufactured using rings produced from high purity material. For most medical applications, martensitic stainless steel, similar to AISI 440C, is recommended and often required due to regulatory requirements. This material, which can be specified differently depending on its manufacturer, provides for good corrosion resistance and has fine, evenly dispersed carbides which result in lower noise and vibration levels than 440C. This type of stainless steel is very desirable, particularly in high speed devices.
Nitrogen enhanced martensitic stainless steel is also available, and while it is more expensive it offers up to 5 times the corrosion resistance when compared to traditional “440C type” materials. Other benefits of this material are very low noise levels, extended fatigue life due to its fine structure that contains smaller chromium nitrides (as opposed to chromium carbides) and a high resistance to corrosion resulting from exposure to blood.
In some cases, balls produced from ceramic materials, such as silicon nitride, can prove beneficial. Ceramic balls are light weight, highly polished, non-magnetic, exhibit high hardness, and resistant to attack from most liquids and chemicals. Ceramic balls greatly improve the high speed capability of the bearing. Bearings made of steel rings and ceramic balls are commonly called hybrid bearings. While ceramic balls have an impressive list of beneficial characteristics for bearing applications, they are not a “cure all” medicine. Due to the high hardness of the ball, contact stress is increased so fatigue life is compromised. So, when the typical failure mode is characterized by fatigue, it’s usually best to stick with steel balls.
Retainers, or ball separators, are typically produced from a 300 series stainless steel. In high speed applications, it is often necessary to use a plastic or phenolic resin “snap-in,” or crown, style retainer. For very high speeds, an angular contact bearing with a full machined retainer is recommended. These types of retainers provide increased stability at higher speeds.
Phenolic resin cages have a porous structure and can be impregnated with oil for additional lubricity. Some of the plastic materials, such as polyamide-imides, contain additives such as graphite and Teflon for additional lubricity in emergency running conditions. A wide array of lightweight plastic material are available, which, can handle temperatures up to 500F and are autoclavable.
An autoclave is a device used to sterilize surgical tools, dental drills, or other devices by subjecting them to high pressure saturated steam for around 20-30 minutes depending on the size of the load. This is a common practice that can have a negative impact on the bearing materials and lubrication.
To read the article in full, click the link below and the article begins on page 97.