Ball Bearings for High-Speed Dental Handpieces

Barden has introduced the X-Life range of super-precision ball bearings designed to enhance reliability, lengthen operating life and reduce noise levels in high-speed dental handpieces. In terms of bearing selection, it does not 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 extremely high speeds of between 400,000 and 500,000rev/min.

The bearings also have to undergo repeated chemical or steam sterilization cycles, which can strip away the bearing lubricant. In addition, the bearings much also remain cool enough to avoid injury or discomfort to the patient and the dental surgeon, while 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 sterilization and the effects of operating debris can cause the bearing to fail prematurely. However, due to a combination of features designed to retain lubricant and reduce contamination, Barden’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 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 reduction in the stress conditions present on the rolling elements and raceway.

This means reduced friction and lower bearing temperatures, less strain placed on the lubricant, higher basic dynamic load ratings, and an increased basic rating life. Robert Globe, sales and marketing manager at Barden, said that while Barden can provide suitable replacement bearings for any make or model of dental handpiece, very often there is a need for the company to customise the bearing in order to provide an optimised design for a particular application. The ability to custom engineer a bearing for an OEM is important, because OEMs are continually improving their handpiece designs, so the bearing has to be constantly improved too.

OEMs also need samples and prototypes of the bearing at short notice, which Barden is also able to provide. 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 450,000rev/min, 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, as a company, is in complete control of the quality, cost and delivery of the ceramic balls and has its own manufacturing facilities for producing them.

On the research and development side, Barden 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, said: ‘We have 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 per cent compared with 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 much improved operational life,’ he added.

A typical dental handpiece bearing and turbine assembly has not changed all that much over the last 50 years. Most dental turbine designs are based on a miniature Pelton wheel. The Borden rotor, which was introduced in 1957, was the prototype of the modern air turbine. Turbine bearings are extremely small, with most having a bore size of 3.175mm and an outside diameter of 6.35mm. Pritchard said: ‘The machinery we have here in the UK enables us to produce bearings with a typical roundness of less than 0.5um. ‘Special attention is also paid to ensure that the ring harmonic levels are kept low.

‘Both the rate of change, the magnitude and the number of lobes are carefully controlled. ‘All races are double-honed and super-finished, typically in the region of 0.01um Ra. ‘Our dental bearings are supplied with the bore calibrated in either 2.5um or 1.0um steps, which provides a more accurate assembly,’ he added. In dental turbines, cage breakage is responsible for 90 per cent 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. This type of cage has been used for decades and is still being used today. 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. Sterilising the dental handpiece after every patient requires a strong heat-resistant cage material and so more recent cage designs use graphite or PTFE fillers. Used predominantly in angular contact bearings, these materials are more sensitive to poor lubrication compared with phenolic. 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 the 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 with 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. Assembly and cleanliness are also important in dental bearings. Pritchard said: ‘All Barden dental bearings are assembled in Class 10,000 clean rooms in the UK. ‘Actual assembly takes place in laminar flow clean air, Class 100 benches, with the number and size of airborne contaminants strictly controlled,’ he added.

Barden dental bearings can be supplied with a number of different lubrication options. Bearings can be oiled, where the customer uses their own lubrication, or greased with Barden’s own standard grease or with a type of grease agreed with the customer. Some dental bearings are supplied dry, when the bore is glued onto the rotor shaft. Lubricants must not contain toxic chemicals or carcinogens and must be safe for human contact. This means that there are only a small number of greases that are both safe and effective as a lubricant.

Article Resource: http://factory-automation.blogspot.com/2010/10/ball-bearings-for-high-speed-dental.html

More related post: http://news.thomasnet.com/fullstory/Precision-Ball-Bearings-suit-high-speed-dental-handpieces-586355

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