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NEC, Birmingham(B40 1NT)

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Super precision ball bearings for high speed dental handpieces

Super precision ball bearings for high speed dental handpieces
Super precision ball bearings hold the key to meeting increasing requirements for higher reliability, longer operating life and reduced noise levels in high speed dental handpieces.

In terms of bearing selection, it doesn't get much more challenging than choosing a bearing for a high speed dental handpiece. As well as having to withstand the harsh operating environment in which the bearings are constantly bombarded with debris, the latest dental handpiece turbines typically have to operate at speeds of between 400,000 and 500,000rpm. The bearings also have to undergo repeated chemical or steam sterilisation cycles, which can strip away the bearing lubricant.

If that's not enough, the bearings must also remain cool enough to avoid injury or discomfort to the patient and the dental surgeon, whilst also offering the lowest possible noise and vibration levels in order to reduce patient anxiety and allow for a more relaxed, comfortable operating environment.
Wear on a dental handpiece is often limited to the bearing itself, which is typically located at the tip of the handpiece turbine. High speed operation, repeated sterilisation and the effects of operating debris can cause the bearing to fail prematurely. However, due to a unique combination of features designed to retain lubricant and reduce contamination, The Barden Corporation's X-life range of dental bearings offer a number of technical advantages over conventional bearings.

Barden's X-life branded products are premium quality bearings produced using state-of-the-art manufacturing technologies that enable a more uniform surface over the whole contact surface between the rolling elements and raceway. As a result, under identical load, there is a significant reduction in the stress conditions present on the rolling elements and raceway. This means reduced friction and lower bearing temperatures, less strain is placed on the lubricant, higher basic dynamic load ratings, and an increased basic rating life.

Robert Globe, Sales and Marketing Manager at Barden comments: "When it comes to dental turbine bearings, the main requirements are longer life, high speed, reduced noise, low temperature and resistance to sterilisation. We've developed new honing techniques which enable us to produce super precision deep groove or angular contact ball bearings with extremely tight controls on both the roundness and surface finish of the inner and outer ring raceways - two critical factors that affect the performance and life expectancy of a dental turbine bearing."

Hybrid Ceramic Bearings
An important bearing feature is the use of ceramic balls rather than steel balls. Ceramic balls are harder, lighter and more wear resistant than their steel counterparts. At speeds of around 450,000rpm, this means the ceramic balls generate less centrifugal force, which reduces wear and internal loads on the bearing. Lubricant life is also extended, since ceramic balls produce fewer wear particles than steel balls. 

Barden is one of the world's leading suppliers of ceramic hybrid bearings. The company also constantly develops and tests new cage designs and materials, as well as new coatings, surface treatments, seals and lubricants. Mark Pritchard, Senior Product Engineer at Barden comments: "We've developed new improved sealing designs in which the shield is incorporated into the outer ring. This reduces the critical gap between the integral shield and the bearing inner raceway by 60% compared to conventional shield-and-circlip designs. This provides significant advantages such as minimising the risk of shield ejection, preventing contamination and retaining the lubricant more effectively, which in turn results in a bearing with a much improved operational life."

In dental turbines, cage breakage is responsible for 90% of bearing-related failures. Turbine bearings do not fail due to fatigue: the cause of a cage breaking is normally due to cage wear and subsequent fracture. Most cages for deep groove ball bearings are snap type retainer cages. The opening for inserting the ball must be designed in such a way that the prongs do not break when pushing the balls in. To hold the balls in the pocket, a narrower width is desirable. Angular contact ball bearings have a 'halo' or window cage that is easier to manufacture and will not be ejected from the bearing when worn. This cage is stronger and less vulnerable to the effects of repeated sterilisation of the dental handpiece. Normally, for high speed bearing applications, phenolic resin is the chosen cage material. This offers modest, but sufficient tensile strength, low friction and is less sensitive to poor lubrication. However, the material does degrade when exposed to heat, particularly at the temperatures required for sterilisation.

Dental bearings need to be protected against contamination from the outside and to keep lubricant inside. Low noise and vibration are critical for good turbine operation. Not only does the air stream take lubricant along with it, it also results in air noise. Also, when the turbine is stopped and no air is flowing, a vacuum is created within the turbine and so outside air that contains all kinds of contaminants will rush in if there is inadequate shielding.

Normally, dental bearings use AISI440C stainless steel balls. These balls are between 1mm and 1.6mm diameter, depending on the type of bearing used. The ceramic ball first found use in dental bearings in the early 1990s due to drastic reductions in the cost of manufacturing ceramic balls and technological advancements. The advantages of using ceramic balls are their lower density (3,200kg/m3) compared to steel balls (7,800kg/m3). This results in lower centrifugal forces and therefore improved kinematics (spin, roll and ball excursion), reduced build up of heat, lower stress levels and reduced forces on the cage.

In addition, ceramic balls are tribo-chemically inert, which reduces adhesive wear and improves the life of the lubricant. Also, unlike steel balls, there is no risk of ceramic balls cold welding to the rings.
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