The following is the third entry in a four-part series by John Wallace (AST’s Vice President of Operations) focused on bearing choices in medical technology.
Look for the final update soon, which will focus on laboratory and diagnostic equipment.
Surgical / Dental Tools
Some of the more demanding medical applications include surgical and dental tools (drills and saws), laboratory and diagnostic equipment, and imaging equipment. Bearings of special design, or catalog bearings with modifications or enhancements, are typically required.
These hand held tools, particularly dental drills, generally operate at very high speeds – rotational speeds of 400,000 RPM or greater are common. “Low” speed is 125,000 RPM, which is quite fast. High precision, ABEC 5 and 7, miniature and instrument series ball bearings are used.
However, for ultra-high speeds, these bearings are modified further and have improved raceway surface finishes. In addition, the raceways and surfaces that guide the retainer have tighter dimensional and geometrical tolerances, in some cases ABEC 9. More expensive, angular contact designs are often recommended for their high speed stability, and they also allow for the use of full machined type retainers. This, again, enhances speed capability.
Running at high speeds also presents challenges with instrument noise levels and heat generation. High audible noise during a dental procedure is a problem for both the dentist and the patient. When bearings are assembled, it is necessary to have a certain amount of internal clearance, or radial play, built in. This allows for one bearing race to move both radially and axially relative to the other.
Application of a preload across a pair of bearings is recommended. Preload can be defined as the application of an axial load across a pair of bearings to force the rolling elements to assume a contact angle for the purpose of removing the internal clearance. The result is constant ball to race contact. This reduces ball skidding, vibration and noise. However, disadvantages of preload include, torque, heat, and reduction in fatigue life. Preload determination is a balancing act where the goal is to apply the least amount of axial preload force possible while meeting the performance requirements of the instrument.
During surgery, the bearings in tools are regularly exposed to harsh conditions and liquids, including blood and saline, as well as particulate debris. When space permits, shielded bearings should always be used. The speed of these tools is generally too high for seals, but when conditions permit seals should be used. Seals are the best option for keeping foreign debris out of the interior of the bearing and keeping lubricant in.
To read the article in full, click the link below and the article begins on page 97.