Simply stated, torque is a unit of work. As rotary instruments, dental handpieces can generate tremendous amounts of torque to perform the work needed. As such, torque can have a great impact on how effective your handpiece is. But what affects torque?
First of all, the construction of the dental handpiece can influence its performance. (Check out Practice Tips issue #22 on Highspeed Handpiece Design for the fundamentals of how dental handpieces are constructed.) The basic construction of a turbine is illustrated below. The turbine depicted is that for a Midwest Quiet Air™ standard chuck highspeed handpiece, but most turbines are similar in design and will use the same types of components.
The impeller catches the drive air causing the turbine to spin. Naturally, the more air the impeller can catch, the more energy from the drive air is transferred into torque. As they can catch more air, usually larger impellers = more torque. Of course, engineers spend countless hours designing the impellers of dental handpieces to capture as much air as possible while keeping size to a minimum. So size is far from the only consideration (and is not an absolute either- size doesn’t overrule all other considerations by any means). The number of vanes, the shape of the impeller, using twin impellers or a number of other engineering tweaks and tricks can all result in more efficient impeller designs without increasing size.
Next, we have the bearings. The bearings reduce friction and allow the turbine to spin. Without bearings, most turbines would not spin at all (or not without the application of tremendous force). Given their role, the bearings used can also have a tremendous influence on torque. Cage types, materials, shielding, (just to mention a few factors) will all have an effect on bearing performance and thus, torque. Larger bearings often go hand-in-hand with large impellers, but again, don’t put too much confidence in size. This is only one method of potentially increasing torque.
The key for most practitioners is to keep your bearings in good shape so they can do their job. Most practitioners will notice a degradation of performance as bearings wear. Worn bearings increase friction requiring more energy to spin the turbine. With extreme wear, concentricity can also be affected further reducing torque as energy can be diffused by lateral movement (i.e. bur wobble) in addition to further increased friction. So, to get maximum torque, it’s important to keep your bearings in good shape through good routine maintenance (as discussed in Practice Tips #9).
Bearings simplify rotation of the turbine as the impeller catches the drive air, but the drive air itself is providing the motive force, so of course, drive air will have a tremendous impact on torque.
You need clean dry air to run your handpieces. Debris in the air can clog the lines impeding air flow and reducing pressure. Debris can also wreak havoc on the delicate components of your dental handpieces (particularly the bearings). While having a lesser impact, the same can be said of water in the air lines. Make certain your air is properly filtered and the filters are well maintained. Make sure to check your compressor regularly (see issue #34 On Routine Office Maintenance) and be attentive to the quality of air coming out of your unit. Small problems can turn into big ones if ignored for too long; just as a small cavity can lead to a root canal if ignored for too long.
Air pressure will also influence torque, but more is not necessarily better. Once again, dental handpieces are precision-engineered instruments designed to operate under very specific conditions. While turning up your air pressure may give you more torque, this may also stress the components giving you shorter turbine life. In addition, under extreme pressure turbines can stall out (analogous to flooding the engine of your car). Just as many cars are designed to get their best mileage at a certain speed, many dental handpieces are designed to yield the best performance at a certain air pressure. Check with the manufacturer of your dental handpiece for their recommendations.
Drive air pressure (& thus, torque) can also be affected by exhaust. If your dental handpiece isn’t properly exhausting, this can cause back-pressure reducing the net pressure on the impeller (or even counter pressure) having a significant effect on torque. Most dental units ("bracket tables") and dental handpieces in the United States use a standard ISO-B (or ISO-C) coupler (see Issue Practice Tips #36 “ABC’s of ISO” for more information on couplers) which will allow the exhaust to flow back into the unit. Make certain that this line is kept clear and open and is free of kinks or obstructions. Air which is properly exhausted will flow over the impeller better, also yielding more torque.
While the impeller and bearings can have an impact on torque, the other components of the turbine should not have much impact. The spindle (central shaft) of the turbine simply provides a location to mount the bearings and impeller and holds the chuck which in turn holds the bur. A worn spindle could affect the balance of the turbine which would result in a loss of torque (and wear to the bearings), but a spindle in good condition should have almost no impact. As the chuck resides in the center of the spindle, it has just as much effect on torque as the spindle. The main impact on performance from the chuck would be poor bur retention. While this wouldn’t directly affect torque generated, this would impact the transference of torque to the bur (as the bur slips) so you would have a net reduction of applied torque.
Test for good bur retention every time you use your handpiece (just tug on the bur a little). You should also clean the chuck (if a push button or lever handpiece) as part of routine maintenance. If using a chuck with a wrench (aka standard chuck), you can unscrew the chuck from the back of the turbine to inspect it. A properly functioning chuck should have no jagged or irregular edges and should be split evenly at the end. With wear, the chuck will often lose a piece at the end (which will affect bur retention).
As you can see, a number of factors can impact the torque of your dental handpiece. Keep an eye on your air, make certain to perform routine maintenance, and always test for good bur retention and you should get great performance from your dental handpieces.