Dental machining device and method for monitoring sealing elements in dental machining devices

A dental machining device (1) having at least one shaft (2) which projects into a machining chamber (4) and is displaceable in both axial and radial directions includes a sealing element (7) having at least two axially spaced sealing lips (8, 9) bearing against the shaft (2) in its peripheral direction and enclosing an annular chamber (10) with the shaft (2). At least one fluid conduit (11) opens into the annular chamber (10) to supply a fluid (16) thereto. A pressure sensor (14) is provided for detecting a state variable of the fluid, which is detected and held ready for transmission to a control unit (14″).

BACKGROUND OF THE INVENTION

The invention relates to a dental machining device that includes at least one axially and radially displaceable shaft projecting into a machining chamber and having a sealing element for sealing the shaft. The invention further relates to a method for monitoring such sealing elements.

DESCRIPTION OF THE PRIOR ART

Shaft sealing elements that are designed for movements in a single dimension, for translatory or rotary movements of a shaft, are known in the prior art. Known sealing elements for translatory movements include, but are not limited to, gland seals, piston rings, and bellows. Sealing elements for rotary movements include, but are not limited to, radial shaft seals, axial shaft seals, axial face seals, and rotary turrets.

A disadvantage of the sealing elements known in the prior art is that they wear quickly with alternating or superimposed translatory and rotary movements and lose or never even attain the required degree of tightness.

Worn sealing elements pose a significant problem for a dental machining device, especially when involving machining of ceramic materials by grinding or milling. The resultant grinding dust can penetrate through defective seals into the bearing area of a shaft of the machining device and lead to irreparable damage of the bearings in a very short time.

It is therefore an object of the invention to provide a dental machining device and a method for monitoring the sealing elements in a dental machining device that permit optimal, low-wear sealing in the case of radial as well as axial motion, in order to prevent damage from grinding dust penetrating into the shaft bearings.

SUMMARY OF THE INVENTION

In the dental machining device of the invention which includes at least one axially and radially displaceable shaft projecting into a machining chamber and having a sealing element for sealing said shaft, the sealing element has at least two axially spaced sealing lips bearing peripherally against the shaft and forming an annular space therewith. At least one fluid conduit opening into said annular space is provided for supplying a fluid. Furthermore, means for determining a state variable of the fluid are provided.

The efficacy of the seal during operation can be verified by said means.

Advantageously pressurizing device for delivering the fluid, say, a pump, a compressor, or a pressure reservoir, is connected to the fluid conduit.

The fluid in the annular space can advantageously be under excess pressure, preferably ranging from 1 and 500 mbar, and the front sealing lip can advantageously be biased to such an extent that said sealing lip also bears against the shaft under excess pressure.

The means for determining a state variable of the fluid can advantageously comprise a pressure sensor or a flowmeter.

The means for determining a state variable of the fluid can advantageously have an computer interface, by means of which information on state variables of the fluid can be transmitted.

The sealing element comprising the at least two sealing lips can advantageously be designed as a single unit.

The state variable can advantageously be selected from the pressure and/or the rate of volumetric flow of the fluid.

The fluid used is advantageously a liquid, preferably water, or a gas, preferably air.

Another object of the invention is to provide a method for monitoring sealing elements for axially and radially displaceable shafts in a dental machining unit, in which the sealing element comprises at least two axially spaced sealing lips bearing peripherally against the shaft and forming an annular space therewith. The method comprises the following steps: charging the annular space with a fluid via a fluid conduit, determining a state variable of the fluid, and holding a value corresponding to one of the state variables of the fluid or to a change in a state variable of the fluid in readiness for transmission to a control unit.

A conclusion regarding the condition of the sealing element can thus be made on the basis of the state of the fluid.

The annular space is advantageously charged with the fluid by means of a pressurizing device.

The fluid can be advantageously conducted into the annular space under excess pressure, preference being given to an excess pressure ranging from 1 to 500 mbar, and the front sealing lip can advantageously be biased to such an extent that said sealing lip also bears against the shaft under excess pressure.

It is especially advantageous for a warning signal to be emitted when a pressure drop occurs which exceeds a first threshold value but is lower than a second threshold value, and when a further pressure drop occurs which exceeds the second threshold value, grinding of a workpiece currently being machined is completed followed by a shutdown of the machining device.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1shows a portion of a dental machining device1. The machining device1has a hollow shaft2which projects into a grinding chamber4from an inner bearing chamber3.1exhibiting bearings3, which inner bearing chamber is delimited by the shaft2and a bearing mount3.2. The grinding chamber4and an internal chamber5of the grinding machine are separated from each other by a partition6of the housing, see alsoFIG. 3.

The shaft2can serve as a holder for a workpiece to be machined (not shown) or as a guide for a machining tool (not shown), and it is axially displaceable and rotatable about a central axis2.1.

A sealing unit comprising a sealing element7with two sealing lips8,9is provided for sealing the bearings3of the shaft2against the grinding chamber4. Together with the shaft2, the two sealing lips8,9form an annular space10radially surrounding said shaft2.

The sealing element7is a single piece and is manufactured by machining and in-mold embossing. It is preferably made of a shape-memorizing polymer. A sealing element manufactured in this way will suffer less wear than a standard spring-loaded seal.

A fluid channel11extending to the outer circumference of the sealing element for charging the same with the supplied fluid16opens into the annular space10. The fluid16used is basically a liquid or a gas. Air16is used in the pre-sent embodiment.

The air16is conducted to the fluid channel11through a connection12in the bearing mounting3.2. The air16is supplied at a pressure of 300 mbar by a pressurizing device, which in this case is a compressor13. The bias on the sealing lip8is set so that said front sealing lip8does not lift from the shaft2in spite of the excess pressure in the annular space10, but instead sealingly bears against said shaft2.

As another component of the sealing unit, a pressure sensor14is provided in the conduit system downstream of the compressor1, which sensor emits a voltage signal as a function of the prevailing pressure3. Said voltage signal is transmitted by an interface14′ to a control unit14″ for regulating the machining device1, and the state can be shown on a display or analyzed.

The pressure sensor can be disposed in the annular space10, the fluid channel11, the connection12, or the conduit leading to the compressor13.

The sealing lips8,9are shaped so that they point toward the grinding chamber4and form a cone-shaped boundary thereto, and the sealing lips have a certain degree of flexibility. The sealing lip9has an angled end area9.1in order to affix, say, a lock washer for the application of a spring bias. The sealing lips8,9are shaped so that the excess pressure produced by the compressor13causes the sealing lip9to press against the shaft2while the sealing lip8is freed from pressure without, however, lifting from said shaft2.

In the event of damage occurring to the front sealing lip8or, less frequently, to the rear sealing lip9, a pressure drop exceeding the normal loss of pressure that always occurs during operation is caused by air16escaping from the annular space10. This additional pressure drop is registered by the pressure sensor14and transmitted to the control unit14″. If need be, the control unit14″ can stop the grinding machine1, depending on the pressure values, and thus protect the bearings from intrusive grinding dust. The defective sealing element7can be replaced by a technician. Furthermore, in the case of a damaged front sealing lip8the blocking air16under excess pressure has the function of blowing intrusive particles back out into the machining chamber4by means of an air stream flowing away from the annular space10. The penetration of particles into the inner bearing chamber3.1is thus effectively avoided.

FIG. 2shows a magnified view of a detail ofFIG. 1, in which the front sealing lip8of the sealing element7is worn, i.e., said sealing lip8no longer abuts against the shaft2because of the wear. In this case, there is the hazard of intrusion of grinding dust particles15into the annular space10unless countermeasures are taken. The excess pressure of the air16in said annular space10, however, creates an air flow that blows at least some of the grinding dust particles15away from the defective site on said front sealing lip8. Furthermore, the second sealing lip9prevents intrusion of the fluid as well as of any particles into the inner bearing chamber3.1and the interior space5.

The pressure conditions in the annular space10are changed when the sealing lip8lifts from the shaft. The outflowing air stream leads to a drop in pressure, which is detected by the pressure sensor14(not shown inFIG. 2) in the annular space10.

Depending on the pressure drop determined, various procedures can be carried out. A warning signal with the request to prepare for a seal replacement can be emitted with a pressure drop exceeding a first threshold value but lower than a second threshold value. In spite of the slightly increased loss of pressure in the seal, normal operation of the machine can continue until the seal is replaced.

With an increased loss of pressure in which the second threshold value is exceeded, a block that is in the process of being machined can be finished followed by a shutdown of the machining device, in order not to jeopardize production as well as to protect the machining device from greater damage.

The machining device1shown inFIG. 3has machining tools31,32arranged in a machining chamber4for machining a workpiece33, which is held in a chuck34. Said machining chamber4and the interior space5of the grinding machine are separated from each other by a partition6of the housing. The sealing element of the shafts2,2′, which project into the machining chamber4as supports for the machining tools31,32, is shown as Detail A in the preceding figures.