Dental Handpiece Bearings basically are the heart and soul of any handpiece.
Dentists around the world use handpiece every day. It’s one of the most important pieces of equipment in the dental office, but how exactly does it work?
As you are probably aware, a typical highspeed handpiece uses air to rotate a cutting bur at about 400,000 rpm. These instruments are among the fastest spinning turbines in the world.
Well-known brands such as KaVo, Midwest, W&H, Bien Air, NSK, and Sirona own a big share of the handpiece market although there are over 100 different models available from various manufacturers throughout the world. Nonetheless, all of these manufacturers use the same basic design.
Understanding how high speed handpieces work
The handpiece itself is just a handle to provide a means of controlling the turbine. In fact, a handpiece without a turbine is called a “shell”- as it’s really just a hollow case. The heart and soul of the handpiece is the turbine ( the combination of rotor and dental handpiece bearings )
The typical handpiece is connected to an air hose. As you press on the foot control, air enters the handpiece through the back. The air moves up the body of the handpiece within a small tube. The air then enters the head of the handpiece where the turbine ( rotor and 2 dental bearings ) is housed. It is the turbine that performs all the work of the handpiece.
High speed handpieces are assembled under very tight tolerances all of which are to assure concentricity is maintained. All of the causes of failure will negatively impact concentricity in one way or another. Ultimately, the bearing components are very small and actually fairly delicate, particularly when you consider the speed of more than 300,000 rpm. It doesn’t take much stress before they can be damaged.
Understanding how dental bearings work
In a handpiece there’s only one moving part; that’s all there is to fix: I am talking about dental handpiece bearings. You can have a chucking issue where the bur is being released, which leads to a patient safety issue.
Bearings are little metal balls. Those balls are what allow the turbine to spin (by reducing friction). The bearing consists of several components. At the heart of the assembly are the actual ball bearings. These can be stainless steel, although most high-end manufacturers (including us, yes!) use ceramic dental bearings. The bearings ride around an inner ring called the inner race. There is a slight groove (raceway) on the outside of this ring that the ball bearings roll in. The inner race is generally the strongest part of the bearing assembly as it is what is pressed onto the spindle.
There is then a cage that holds the balls at equidistant intervals around the inner race. This keeps the turbine well balanced.
The entire bearing assembly is then closed off with a shield. The shield is designed to keep debris from entering the cage and getting onto the bearings. Naturally, if debris gets onto the dental handpiece bearings it can throw the entire turbine out of balance,
Dental Handpiece Bearings technology
In terms of dental handpiece 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 bearings typically have to operate at speeds of between 300,000 and 350,000rpm. The bearings also have to undergo repeated chemical or steam sterilization cycles, which can strip away the bearing lubricant.
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Dental handpiece bearings` state-of-the-art manufacturing technologies enable a more uniform surface over the whole contact surface between the rolling elements and raceway. As a result, under a similar load, there is a significant reduction in the stress conditions present on those parts above. This means reduced friction and much lower bearing temperatures, less strain is placed on the lubricant oil, higher basic dynamic load ratings, and increased life span.
Important dental handpiece bearings feature is the use of ceramic balls rather than normal steel balls. Ceramic balls are harder, lighter, and more wear-resistant than their steel balls. At speeds of around 300,000rpm, 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.
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.
Ceramic Ball Bearings Vs. Steel Ball Bearings
Dental Ceramic Bearings are typically constructed with a ferrous inner and outer ring or race with ceramic balls in the place of steel. Ceramic bearings offer many advantages over all-steel bearings, such as higher speed and acceleration capability, increased stiffness, lower friction, and more. Ceramic balls are also non-conductive. Ceramic bearing balls require less lubricant and exhibit less lubrication degradation, which results in increased bearing life. Ceramic bearings manufactured from Si3N4 can operate at temperatures up to 1600F. Ceramics also are resistant to oxidation.
Ceramic bearings balls are typically made from (Si3N4) ceramic silicon nitride and have a greater hardness than steel balls resulting in longer ball life. Ceramic bearing balls have smoother surface finishes than most steel bearing balls. Thermal properties are also better steel balls which result in less heat generation due to friction at high speeds.
To manufacture an extra fine surface finish on ceramic balls, the balls are elevated with a magnetic field and then polished with a plasma stream. Ceramic bearings balls are rated at higher spin rates than steel bearing balls.
Ceramic bearing balls can weigh up to 40% less than steel ones, depending on size and material. This reduces centrifugal loading and skidding, so hybrid ceramic bearings can operate 20% to 40% faster than conventional bearings. This means that the outer race groove exerts less force inward against the ball as the bearing spins. This reduction in force reduces friction and rolling resistance. The lighter balls allow the bearing to spin faster and uses less energy to maintain its speed.
While ceramic hybrid bearings use ceramic balls in place of steel ones, they are constructed with steel inner and outer rings; hence the hybrid designation. While the ceramic material itself is stronger than steel, it is also stiffer, which results in increased stresses on the rings, and hence decreased load capacity. Ceramic balls are electrically insulating, which can prevent ‘arcing’ failures if current should be passed through the bearing.
The state of the art: Angular Contact Bearings
The main difference between radial ball bearings and angular contact ball bearings is the retainer type: radial bearings include a crown retainer (fig. 1) whereas angular contact bearings include a full retainer (fig. 2).
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 sterilization of the dental handpiece.
Dental handpiece bearings need to be protected against contamination from the outside and to keep the lubricant inside. Low noise and vibration are critical for good turbine operation. Not only does the air stream take lubricant along with it, but 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.
Last, dental ceramic balls are 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.
Radial Bearings Vs. Angular Bearings
Basically, there are two general types of dental handpiece bearings depending on the type of work that the dental tools are created for: radial bearings and angular bearings. Both of them are used in modern dental high speed handpiece and have their own purpose, thus advantages.
The first type (radial dental handpiece bearings, Fig.1) includes rolling-element bearings that use balls to maintain the separation between the bearing races, their purpose being to reduce rotational friction and support radial and axial loads. It’s designed for working in deep grooves and for angled dental surfaces. Radial bearings have an inner ring, an outer ring, a series of balls, and typically a cage to contain the balls.
The second type of dental handpiece bearings is the angular bearing ( Fig2). Such dental bearings are used for side or thrust loads. Thrust bearings use balls, rollers, or needles. In addition to these working surfaces they also typically have a ring and a seating surface. Here, the material plays an important role – using ceramic bearings in your dental handpiece will allow it to last up to 30% longer, compared to standard steel bearings. Reference
In open bearings ( Standard Radial contact ) the lubricant tends to migrate out of the bearing during the autoclave cycle (sterilization process). Single-shielded, double-shielded, and integral-shielded bearings also known as Angular Contact Bearings (fig. 4) are better equipped to protect against external contamination, lubricant migration, and autoclave steam.
What causes the failure of high speed handpieces?
As already discussed, generally it is the dental handpiece bearings (specifically the bearing cage) that will fail first in a typical turbine. Let`s list the typical causes of bearing failures:
Build-up of debris
Debris will accumulate on the dental handpiece bearings causing the turbine to become imbalanced, stressing the cage and other bearing components leading to failure. The shield helps protect the bearings but debris can still accumulate on the outside of the turbine and imbalance it as well. Regular and proper maintenance is crucial to handpiece performance and longevity.
Excessive air pressure
Most manufacturers recommend running their handpieces at 35-40 psi (for high speed). Generally, anything in excess of 45 psi will cause damage to the dental handpiece bearings. Some practitioners like to use air pressure over 50 PSI to achieve better cutting power. Remember, these dental handpiece bearings are very, very small and their components (especially the cage) are fragile.
Everyone must decide the value of increased speed vs. shortened turbine life for their method of practicing. While higher pressure will normally result in higher speed, it will also usually result in shorter bearing life.
Excessive temperatures during sterilization
All dental handpieces are made to be heat sterilized at a maximum of 275° F (135° C). Dry heat sterilizers operate at much higher temperatures and should never be used on handpieces.
Side load stress
“Side load” is pressure applied to the inside of the bearing assembly in a perpendicular direction. Side load is usually the result of cutting with the side of the bur. When cutting with the side of the bur is an everyday occurrence and should not damage your turbine or dental handpiece bearings, keep in mind that burs have a limited life span because can lose cutting power fairly soon.
Also, burs after repeated use can lose the original balance, thus their rotation is not correct as it was supposed to be. Dull or overused burs won’t cut as well, so they are more prone to induce side load. When side-cutting, always try to use a sharp and fresh bur or diamond.
Use of bent burs or a bur that isn’t fully seated.
Naturally, a bent bur will not maintain concentricity and will wobble, stressing the bearings.
Using a bur that isn’t fully seated can cause side load as the weight at the front of the turbine is less than that at the back. This will not have as much of an impact on balance, but can still stress the front bearing – which is the one that makes the most of the work.
In conclusion, dental handpiece bearings are the first feature you should take care of when considering to invest in your new high speed handpiece.
For some, saving a minute on a crown prep is worth the cost of more frequent turbine replacements, this is something you need to decide for yourself.