Bearings meet tough demands of medical and dental applications

Paul Boughton

The demands made upon bearings in medical and dental applications are such that developments are always at the leading edge of technology, explains Nick Dowding

In medical applications, it doesn’t get any tougher for bearings than in an X-ray machine. Here, special X-ray tube bearings are used to support the spinning X-ray anode, operating at speeds in excess of 10,000 rpm under harsh conditions. In addition to withstanding the passage of high voltage, the bearings must also operate in a vacuum environment down to 10-8torr and at temperatures of 400 to 500˚C.

Barden’s special design for bearings used under these operating conditions is a cartridge style full complement one, incorporating a flanged shaft with integral races to which the target anode is attached. A separate flange manufactured of lower thermal conductivity material, which can be welded to the shaft in order to reduce heat transfer from the anode, complements this arrangement. The bearings also have controlled axial clearance in order to compensate for thermal growth at the elevated temperatures at which they operate. In addition, the design meets specific application requirements with either solid or spring preloaded spacers that separate the bearing outer rings.

In order to provide effective lubrication under extreme X-ray conditions, Barden utilises advanced surface engineering technologies such as plasma and ion-beam assisted deposition. Working closely with specialist organisations in these fields, Barden is developing a range of advanced solid lubricants, some, 2000 times thinner than a human hair, to complement its high temperature X-ray bearing materials.

Flexible surgical arms

Super precision bearings are also helping surgeons to perform intricate laser surgery on patients.

All surgical flexible laser arm units have a similar basic structure. The flexible arm is connected to a vertical pillar. This in turn is connected to the main body of the system, which contains the controls and display panel, as well as a firing tube system that contains the CO2 molecules to be 'lased'. The flexible arm normally requires two or three articulated joints to give it the freedom of motion it requires. A set of precision mirrors are positioned at each articulated joint and the pillar, which direct the laser beam in a unidirectional, straight line, from the firing tube at the base of the system, through to the end of the flexible arm, where the hand-piece is located, which in turn focuses the laser beam onto the patient.

The surgeon requires complete freedom of movement of the hand-piece, which is provided by the articulated joints in the flexible arm. However, this flexibility creates a potential problem for the laser beam, which must travel in a straight line at all times throughout the system, regardless of the movements made by the surgeon. Typical surgical arms are around 2-3m in length and so the laser has to maintain its position at all times. The slightest deviation of the laser beam from the centre of the arm will cause operational problems at the hand-piece. A small angular deviation of more than 1mm over a 2-3m length can cause large errors in accuracy.

Precision bearings are critical in ensuring that the laser beam maintains its position throughout the length of the system. A typical surgical arm will require bearings for articulated joints that guarantee no more than 5 microns of radial run out. These bearings therefore need to be manufactured to at least ABEC 7 standards, sometimes to ABEC 9 (ISO P2). Rotational accuracy of the bearings is also important, which means providing a smooth running, low friction, low noise bearing that is lubricated for life.

Surgical cutting and drilling tools

As well as flexible arms, the latest handheld cutting and drilling tools used in surgical procedures also require super precision bearings in order to ensure that they meet their performance standards and expected life.

These tools are now more sophisticated, more compact and ergonomic in their design. The type of power tool – whether electric, air or motor-driven – that is used by doctors today during surgical procedures, will normally require super precision ball bearings, often custom engineered to suit the particular demands of the application.

In surgical procedures, the operator of the sawing tool will undoubtedly have to put a relatively large load on the ball bearings. Since the tool might be used to saw through bone or to cut through tendons, muscles, cartilage and other bodily tissues, the operating environment for the bearings is very harsh. Typically, the bearings are located close to the end of the cutting tool, often working inside the patient’s body and so resistance to acids and other corrosive media is critical.

Power tool manufacturers therefore require bearings that are both compact enough to fit into the slender design of the tool, but also able to cope with the high loads and relatively high operating speeds (up to 80,000rpm) involved. In addition, the bearings will have to withstand the corrosive, slightly acidic operating environment inside the patient’s body.

Resistance to temperature is also critical. As surgical tools require regular sterilisation in autoclaves, the bearings must be designed to withstand temperatures up to around 140°C. Most bearing designs for surgical cutting tools are manufactured in corrosion-resistant 440C stainless steel, which provides sufficient resistance to cleaning chemicals and other aggressive acidic media. The bearings, which are supplied as either deep groove or angular contact configurations, are normally provided with phenolic cages to provide further resistance to repeated sterilisation cycles.

The harsh operating environment for the bearings also means that protection is required against any fine particles of soft tissue and bone that may penetrate the contact regions between the balls and the bearing raceways, which would then lead to high stress concentrations and could lead to the eventual failure of the bearing.

In order to counter these problems, Barden can provide integral shields that are designed to help retain the lubricant whilst preventing the ingress of contaminants. This improved sealing design reduces the critical gap between the integral shield and the bearing inner ring to 60% of that when compared to conventional shield and circlip designs. This results in a reduction in operating noise, greater lubricant retention and improved protection from contaminants – which leads to a longer life of the cutting tool.

High-speed dental bearings

High speed turbine bearings for dental hand-pieces must also operate under arduous conditions. As well as being required to operate at speeds up to 500,000 rpm, repeated sterilisation of the hand-piece and the continued build up of lubrication and operating debris can cause these bearings to fail more quickly than expected.

In order to address these adverse operating conditions, Barden has developed miniature bearings that employ ceramic (Silicone Nitride) balls. As ceramic balls are 50 per cent lighter than steel balls, the centrifugal forces generated at operating speeds up to 500,000rpm in a Dental Turbine are significantly reduced, providing lower stress levels and operating temperatures in the ball to race rolling contact zone.

The unique properties of silicone nitride ceramic balls drastically reduce the predominant cause of raceway surface wear. In conventional bearings, microscopic surface aspirates on the balls and races will ‘cold weld’ or stick together even under normal lubrication and load conditions, resulting in wear particles remaining in the bearing rolling contact zone. This results in adhesive wear.

Lubricant life is also increased if ceramic balls are used, primarily because less wear particles are produced and held in the rolling contact zone. These particles cause the lubricant to degrade particularly under the marginal lubricant condition that often exists in dental bearings from repeated sterilisation and the build up of debris.

Finally, cage to ball loads are significantly reduced with ceramic balls, under the conditions of outer ring misalignment, that exist, to some degree, in the majority of dental bearings mounted in O-rings. As a result, stress on the bearing cage is lower and wear on the ball pocket and cage outside diameter are reduced. These factors are particularly beneficial in prolonging bearing life under repeated Autoclave and Chemclave sterilisation processes, both of which reduce bearing cage strength.

Nick Dowding is Business Development Manager at Barden UK, Estover, Plymouth, Devon, UK.

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