Engineering Insight

By: EMDT Staff

In terms of bearing selection, it doesn’t get much more challenging than choosing a bearing for a high-speed dental handpiece. In addition to withstanding a harsh operating environment in which the bearings are constantly bombarded with debris, dental handpiece turbines typically must operate at speeds between 400,000 and 500,000 rpm. The bearings also must undergo repeated chemical or steam sterilisation cycles, which can strip away the bearing lubricant. They also must 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 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. Owing to a unique combination of features designed to retain lubricant and reduce contamination, however, the X-life range of dental bearings from the Barden Corp. Ltd (Plymouth, UK; www.bardenbearings.co.uk) offer a number of technical advantages over conventional bearings.

 

The X-life bearings’ design provides uniformity 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 translates to reduced friction and bearing temperatures, less strain placed on the lubricant, higher basic dynamic load ratings and an increased basic rating life, when comparing performance with conventional bearings.

 

When it comes to dental turbine bearings, the main requirements are long life, high speed, minimal noise, low temperature and resistance to sterilisation,” comments Robert Globe, Sales and Marketing Manager at Barden. “Over the last five years, our production plant in the United Kingdom has invested significant amounts of money in new production machinery. We’ve developed honing techniques that 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.”

 

“Whilst we can provide suitable replacement bearings for any make or model of dental handpiece, very often there is a need for us to customise the bearing to provide an optimised design for a particular application,” Globe says. “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 on very short notice, which, again, we are able to provide.”

 

An important bearing feature for this application 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 around 450,000 rpm, 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.

 

“Unlike other suppliers of dental bearings, we have our own manufacturing facilities for producing ceramic balls. Based in the United States, Winsted is part of the Schaeffler Group, which means that we are able to control the quality and specify the roundness of the ceramic balls.” On the R&D side, Barden constantly develops and tests new cage designs and materials, as well as new coatings, surface treatments, seals and lubricants.

 

“We’ve developed improved sealing designs in which the shield is incorporated into the outer ring,” comments Mark Pritchard, Senior Product Engineer at Barden. “This reduces the critical gap between the integral shield and the bearing’s inner raceway by 60% 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 a much-improved operational life.”

 

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 3.175 mm and an outside diameter of 6.35 mm.

 

“The machinery we have here in the United Kingdom enables us to produce bearings with a typical roundness of less than 0.5 µm, says Pritchard. “Special attention is also paid to ensure that the ring harmonic levels are kept low. The rate of change and the magnitude and number of lobes are carefully controlled.”

 

“All races are double honed and super finished, typically in the region of 0.01 µm Ra. Our dental bearings are supplied with the bore calibrated in either 2.5 or 1.0 µm steps, which provides a more-accurate assembly,” he adds.

 

In dental turbines, cage breakage is responsible for 90% of bearing-related failures. Turbine bearings do not fail owing to fatigue; the cause of a cage breaking is normally a result of 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 and low friction. It also is less sensitive to poor lubrication. The material, however, 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. Thus, recent cage designs use graphite or PTFE fillers. Used predominantly in angular-contact bearings, these materials are more sensitive to poor lubrication than phenolic resin.

 

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. As a result, 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 1 and 1.6 mm diam, depending on the type of bearing used. The ceramic ball first found use in dental bearings in the early 1990s. This breakthrough was made possible owing to a dramatic reduction in the cost of manufacturing ceramic balls. Ceramic balls are advantageous because of their low density (3200 kg/m³) relative to steel balls (7800 kg/m³). This results in lower centrifugal forces and, therefore, improved kinematics (spin, roll and ball excursion), reduced heat buildup, lower stress levels and reduced forces on the cage. In addition, ceramic balls are tribo-chemically inert, which reduces adhesive wear and improves lubricant life. Also, unlike steel balls, there is no risk of ceramic balls cold welding to the rings.

 

Assembly and cleanliness are important in dental bearings. “All Barden dental bearings are assembled in Class 10,000 cleanrooms in the United Kingdom,” Pritchard explains. “Actual assembly takes place at Class 100 benches under laminar flow conditions.”

 

Dental bearings can be supplied with a number of different lubrication options. Bearings can be oiled, where the customer uses his or her own lubrication, greased with Barden’s standard grease or with a type of grease selected by 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 for use as a lubricant.