Practice Tips #22: High-speed Handpiece Design

High-speed Handpiece Design

You use your high-speed handpiece every day, but do you know their design? It’s one of the most important pieces of equipment in the dental office, but how does it work? We answer both of those questions for you.

As you probably know, a typical highspeed handpiece uses air to rotate a cutting bur at about 350,000 rpm’s. These instruments are among the fastest spinning turbines in the world.

Midwest, Star, KaVo, NSK and Lares own about 90% of the US handpiece market although there are over 50 different makes and models available from various manufacturers throughout the World. Nonetheless, all of these manufacturers use the same basic design.

PHOTO: Various Makes of Turbine

How Do They Work?

The typical handpiece connects 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 sits. It is the turbine that performs all the work of the handpiece. The handpiece itself is just a handle to provide a means of controlling the turbine (as well as serving as a conduit for air to drive the turbine and air and water to cool the surface being cut). In fact, in manufacturer circles, 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.

Standard Components of a Turbine:

Typical Turbine Components

Spindle

At the center of the turbine is the spindle (#1 in the diagram above). This is the shaft that spins.

Chuck

Inside the spindle is the chuck (#2). This hollow tube holds the bur, which of course does the cutting. As you can see in the diagram, the chuck has little slits at the end. These slits allow the chuck to compress as it moves forward inside the spindle to grip the bur.

Standard Chuck Handpieces (shown above): has a threaded chuck (like the inside of the spindle). A bur wrench helps to rotate the chuck, which screws it farther into the spindle and compresses the end.

Autochuck Handpieces (e.g. push button): the chuck threads too, but rather spring loaded. It includes a mechanism that allows manipulation of the chuck forward or back. This opens and closes the end to grip (or release) the bur.

Impeller

Placed on the middle (from front to back) of the spindle (pressed in place) is the impeller (#4). This catches the air, which causes the turbine to spin. As you can see the spindle and impeller are fairly simple solid pieces of metal (usually stainless steel or aluminum) and thus, rarely fail. Chucks will occasionally fail as they can fracture where the slits are, but this is still relatively uncommon.

Bearings

On either side of the impeller are bearings (#3 and #5 above). But aren’t those solid cylinders in the diagram? Bearings are little metal balls, aren’t they? Well, yes and no. Technically, what we have are bearing assemblies; these are what allow the turbine to spin (by reducing friction). The following diagram is a cross-section of a typical bearing assembly:

PHOTO: Bearing Cross-section

As you can see, the bearing assembly, or “bearing” as referred to in the handpiece world, consists of several components (including the balls you expect).

1. Ball Bearings: These are generally stainless steel, but could be ceramic bearings.
2. Inner Ring: The bearings ride around an inner ring called the inner race. The ball bearings roll in a slight groove (raceway) on the outside of this ring. It is generally the strongest part of the bearing assembly and presses onto the spindle.
3. Ball Cage: Holds the balls at equidistant intervals around the inner race. This keeps the turbine balanced. Example: An old washing machine with the clothes all on one side causes an erratic rotation due to improper weight distribution. It keeps the turbine spinning smoothly, as it maintains an evenly distributed load.
   
   Bearing cages can be made of a variety of materials, generally, they are a polymer (i.e. plastic) and are usually the most fragile component of the entire turbine. The plastics used are very resistant to heat and have very low-friction, as they directly contact the actual bearings.
4. Shield: The entire bearing assembly is then closed off with a shield. It keeps debris from entering the cage and getting onto the bearings. If debris gets in, it throws the turbine out of balance. This puts stress on the cage, which then fails and allows the bearings to “clump.” This causes the turbine to spin erratically and eventually stop spinning completely.

O-rings

Outside of the cage is the outer race. Just as with the inner race, there is a groove in which the bearings ride. The outer race rests on o-rings (two #8’s in the diagram), which in turn rest against the inside of the handpiece head.

TECH TIP: All of these components are extremely small and thin. Whenever replacing a turbine, make certain you have removed all of these components of you old turbine from the handpiece. It’s very common for an o-ring, outer race, or shield to adhere to the inside of the handpiece head (or end cap). 

NOTE: All of these components press together using a bearing press and held together with friction. This is why a press is required if you wish to repair your own handpiece, saving you money.

Wave (Thrust) Washer

A wave (thrust) washer (#9) are also called a loading spring. The Quiet Air turbine diagram shown above is one of the only standard chuck turbines to use a washer.

Washers are normally only used on autochuck handpieces to provide lateral tension. This keeps the turbine pressed against the end cap, which facilitates actuating the chuck. If you have trouble after installing a turbine that uses a washer, sometimes you can remove the washer and have the turbine function well.

It is not uncommon (particularly with age) to get a slight build-up within the handpiece head. You might have the push button or other mechanism that actuates the chuck start to vary from the original specs, so the washer can impede proper function. The washer above comes bent to provide additional tension. Sometimes the washers are flat and simply serve as spacers.

End Cap

Technically, the end cap (#7) is not part of the turbine. The end cap threads onto the back of the handpiece head to hold the turbine in. The end cap also includes the button (or lever) used to activate the chuck on autochuck turbines. The back bearing will usually seat into the end cap of the turbine.

To simplify installation, some manufacturers include end caps with their turbines, providing them already seated on the rear bearing of the turbine. It’s a matter of convenience and it can impact the price, of course.

TECH TIP: As mentioned previously, components of worn turbines or bearings get stuck in the end cap, so always inspect the end cap thoroughly when replacing a turbine. Also, many end caps (like the Quiet Air) have a groove into which one of the o-rings seat. Always replace this o-ring when installing a new turbine. You will probably need to use a scaler or explorer to remove the old o-ring.

As you can see, turbines are fairly complex assemblies, particularly the bearings. Now that we know the design of a turbine – what does this mean to you? Are you able to do your own handpiece repair?

Bearing Failures

With the insight we have into design, now we can look at what causes failure. As already discussed, generally it is the bearings (specifically the bearing cage) that fails first in a typical turbine. What causes bearing failures are as follows (in order of frequency/likelihood):

1. Build up of debris
2. Excessive air pressure
3. Excessive temperatures during sterilization
4. Side load stress (we mentioned load above)
5. Use of bent burs or a bur that isn’t fully seated

Side Load Stress

“Load” as the name implies, consists of its weight or pressure. You get “side load” when you apply pressure to the inside of the bearing assembly in a perpendicular direction. This happens when you cut with the side of the bur. “But I cut with the side of the bur all the time. Can’t you do a crown prep?” Now we get into a more subjective area.

Naturally, cutting with the side of a bur happens and should not damage your turbine or bearings. Excessive side load will not necessarily lead to immediate bearing failure. The key is that you allow the bur to cut. 

• If you find yourself leaning into the bur, putting a lot of pressure on the side of the bur (say 5-10 pounds or so), this creates side load.
• If you press on the end of the bur, the opposite end (inside your turbine) presses in the opposite direction (like a lever). This pushes on the inner brace of the bearing, compressing one side of the bearing assembly creating localized increased friction, which leads to imbalanced rotation which stresses the cage ultimately causing fracture and failure.
• The more you press on your bur, the more you press on your bearings. Try to keep it fairly light to avoid stressing your bearings.
• Of course, worn or dull 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.

Occasional pressure is expected and should not have a dramatic effect on turbine life. However, constant or frequent pressure certainly will. For some, saving a minute on a crown prep is worth the cost of more frequent turbine replacements.

Generally, side load has a greater impact on the rear bearing than the front bearing. If you replace your own turbines and notice that it’s usually the rear bearing that falls out in pieces (while the front bearing appears ok), you might consider trying a lighter touch when cutting with the side of your bur.

Bur Quality and Installation

Using a bur that is bent or that is not fully seated into the turbine can also cause excessive side load. 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 (as it is normally doing most of the “work”). On standard chuck handpieces, this can also cause the chuck to fracture.

Sterilizing Temperatures Too High?

Excessive temperature should be self-explanatory. All dental handpieces are now made to be heat sterilized in accordance with the CDC. They should be sterilized in a steam or chemical vapor sterilizer at a maximum of 275° F (135° C). Dry heat sterilizers operate at much higher temperatures and should never be used on handpieces. You should check your sterilizer regularly for overheating with a lag thermometer. Generally, if your sterilizer is overheating, you will probably have damage to other items in the office, but it is not unusual to notice it in your handpieces first.

Insufficient Handpiece Pressure?

Most manufacturers recommend running their handpieces at about 35 psi (for highspeeds). Generally, anything in excess of 40 psi causes damage to the bearings. Always check with the manufacturer for their recommendations, of course, and try to stay within 5 psi. Some practitioners like to use air pressure over 40 PSI to achieve better cutting power.

Remember, these bearings are very, very small and their components (especially the cage) are fragile. While the cage may not fail right away, you could experience premature failures, shortening the excepted life of the cartridge if you use higher air pressure than recommended. As with side loads, 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 rpm’s, it will also usually result in shorter bearing (turbine) life.

Debris Build-up?

Last of all, debris buildup inside of the bearings can cause premature failures. Debris will accumulate on the 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. Please see practice tip #9, practice tip #45, and practice tip #65 devoted to maintenance for more information.

Turbines 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. Once again, the bearing components are very small and actually fairly delicate, particularly when you consider the speed of 350,000 rpm (or more). It doesn’t take much stress before they can be damaged.

A Few Other Tips:

1. Never run cold water over handpieces to achieve rapid cool downs after sterilization. Rapid cool downs could cause warping of handpiece components and you could also introduce contaminants onto your sterilized instruments.
2. You should never run your handpieces without burs. Doing so could cause damage to the spindle/chuck assemblies. This is more important with standard chuck turbines.
3. For autochuck handpieces (push button or lever) lubricate the chuck at least once a week to keep it clean and functioning well. Debris can clog the chuck and interfere with holding the bur.

A Quick Note on Electric Handpieces:</h3> >Most electric handpieces use similar designs to slow-speed attachments. That is, there is a drive shaft which meshes directly with a gear in the turbine assembly, rather than an impeller that catches air. They still have bearings and, as discussed above, bearings are what typically fail. The same precautions apply to electric handpieces as pneumatic handpieces when operating (other than air pressure, of course). Beware of side loads, keep them clean, don’t use bent burs etc.

With electric handpieces, it’s even more important to avoid damaging your bearings as the electric motor will continue to yield the same rpm’s and torque at the bur, even if your bearings have failed. To maintain this performance, however, an electric handpiece will need to work harder and can overheat. See the FDA's letter on "Patient Burns from Electric Dental Handpieces" for more detail.  Be sure to handle your electric handpiece with care too.