Abstract:
The invention relates to a dental preparation instrument that includes a tool that rotates in an alternately reversible manner. The preparation instrument has a drive for the tool. The drive provides a continuous fully rotational motion, and a gearing is arranged before the tool. Said gearing converts the continuous fully rotational motion of the drive into an alternately reversing rotational motion, which is continuous as viewed over several reversals.

Description:
FIELD OF THE INVENTION 
     The invention relates to a dental preparation instrument, comprising a reversibly rotating tool that is, for example, used in the treatment of root canals. The preparation instrument has a drive for the tool. 
     BACKGROUND OF THE INVENTION 
     A rotating handpiece for endodontics is known from EP 1 196 109 B1, wherein a file is rotated such that it rotates across a first angular range in a first direction for cutting or smoothing, and rotates over a second angular range opposite the first direction to remove the abraded material, wherein the first angular range is greater than the second angular range and lies within a range of 90-180° and 45-120°, respectively. 
     It is known from EP 2 438 884 A1 that the angular range for cutting or smoothing is less than the elastic torsion angle of the tool, and that the angular range for cutting or smoothing is 3 to 20 times greater than the angular range for removal. 
     A motor is used in each case that has a reversible rotational direction and can be programmed by means of a control device. The disadvantage is that the handpiece requires a motor with special electronics, and the handpiece therefore cannot be used in existing dental workplaces with an existing motor control for a conventional, fully rotational motor. 
     The object of the invention is to avoid this disadvantage. 
     BRIEF SUMMARY OF THE INVENTION 
     A dental preparation instrument according to the invention works with a tool that rotates in an alternately reversible manner. A drive for the tool is provided. The drive provides a continuous, fully rotational motion. A gearing is arranged upstream from the tool in the drive train and converts the continuous, fully rotational motion of the drive into a rotating motion that is alternately reversible and is continuous when viewed over several reversals. 
     By means of the arrangement of the gearing in the preparation instrument, it is possible to connect the preparation instrument to conventional workplaces with a control for fully rotational motors. In particular, mechanical gearing is possible as the gearing. 
     In one advantageous further development, the gearing has a rotatably mounted input shaft and a rotatably mounted output shaft. A fixed gear wheel is arranged in the gearing. A mount is provided on the input shaft for a rotatable gear wheel that meshes with a fixed gear wheel and is rotatably mounted relative to the mount at a distance R from the middle axis of the fixed gear wheel. The rotatable gear wheel is connected to a sliding block at a distance from its rotary axis. The sliding block is guided in a sliding block guide connected to the output shaft. 
     In the interplay with the sliding block guide, the change in distance of the sliding block from the rotational center of the input or output shaft generates an alternating positive and negative direction of rotation which, by appropriately choosing the geometric relationships, causes a change in the direction of rotation of the output shaft. 
     Advantageously, the sliding block can be arranged on a lever extending away from the rotary axis of the rotatable gear wheel at a distance D that is either:
         greater than the radius r of the rotatable gear wheel and less than the radius R of the rotary axis of the rotatable gear wheel about the central axis of the fixed gear wheel, when the radius r of the rotatable gear wheel is less than the radius R of the rotary axis of the rotatable gear wheel about the central axis of the fixed gear wheel, or   less than the radius r the rotatable gear wheel and greater than the radius R of the rotary axis of the rotatable gear wheel about the central axis of the fixed gear wheel, when the radius r of the rotatable gear wheel is greater than the radius R of the rotary axis of the rotatable gear wheel about the central axis of the fixed gear wheel.       

     Given these geometric relationships, a reversal of the direction of rotation can be ensured, wherein the corresponding elements of the two directions of rotation can be fixed by means of the geometric relationships. 
     Advantageously, the sliding block can be arranged on a lever extending away from the rotary axis of the rotatable gear wheel outside the circumference of the rotatable gear wheel. The lever is connected to the rotatable gear wheel. Consequently, establishment of the elements of the rotary angle is not dependent on the size of the gear wheel. 
     Advantageously, the sliding block can be arranged on the rotatable gear wheel within the circumference of the rotatable gear wheel. Additional components are thereby avoided. 
     Advantageously, the fixed gear wheel and rotatable gear wheel can be a spur gear such that the roll-off path of a point on the rotatable gear wheel is an epicycloid. 
     Advantageously, the fixed gear wheel can be a sun gear, and the rotatable gear wheel can be a spur gear such that the roll-off path of a point on the rotatable gear wheel is a hypocycloid. 
     Advantageously, an angle piece can be available to accommodate the tool, a motor can be available as the drive for the tool, and the gearing can be part of the angle piece, or part of the motor, or part of an additional connecting piece between the motor and angle piece. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A device according to the invention is explained on the basis of the drawings. In the drawings: 
         FIG. 1  shows a dental preparation instrument with a motor, an angle piece, and a connecting piece with gearing; 
         FIG. 2  shows a dental preparation instrument with a motor, an angle piece, and integrated gearing; 
         FIG. 3  shows a dental preparation instrument with a motor with integrated gearing and an angle piece; 
         FIG. 4  shows a section through a gearing with a spur gear-spur gear toothing; 
         FIG. 5  shows a plan view of the gearing from  FIG. 4 ; 
         FIG. 6  shows a plan view of a gearing with a sun gear-spur gear toothing; 
         FIG. 7  shows a longitudinal section of another gearing with a sun gear-spur gear toothing; 
         FIG. 8  shows a schematic path of the rotary angle at the output over the rotary angle on the drive. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  schematically portrays a dental preparation instrument  1  comprising a drive in the form of a motor  2  and an angle piece  3 , wherein a connecting piece  4  with a gearing  43  is placed between the motor  2  and the angle piece  3 . The motor  2  is connected to a hose (not shown) through which supply lines are guided, for example, to ensure electrical supply to the motor, but also possibly to provide media such as light, water or air. 
     The angle piece  3  has a head  5  in which a tool  6  is rotatably mounted. The rotational motion of the motor  2  depicted by the arrow  7  is transmitted to the connecting piece  4  and is converted into an alternately reversing rotational motion designated by the double arrow  8  by the gearing G placed there which is also provided with reference number  43 . This alternately reversing rotational motion is transmitted by the angle piece  3  to the tool  6 . 
       FIG. 2  shows a dental preparation instrument  11 , wherein the gearing  43  is integrated in the angle piece such that, at the input to the angle piece  13 , a fully rotational motion originating from a motor  2  can be received. 
       FIG. 3  shows a dental preparation instrument  21 , wherein the angle piece  3  from  FIG. 1  can be connected to a motor  22  that is provided with a gearing which converts the fully rotational motion of the motor into an alternately reversing rotational motion depicted by the double arrow  8  and the arrow  7 . 
       FIG. 4  shows a section through a gearing  43  with spur gear-spur gear toothing. The fully rotational motion about a central axis  10  provided by a motor (not shown) indicated by the arrow  7  is converted to a desired speed range by means of a reduction gear  41  such that there is a fully rotational motion about the central axis  10  by an input shaft  42  of the gearing  43 . The gearing  43  has a rotatably mounted output shaft  44  which executes an alternately reversing rotational motion about the central axis  10  indicated by the arrow  8 . 
     The fully rotational motion of the drive is converted into the alternately reversing rotational motion of the input shaft  44  by using a sliding block control in which a sliding block  45 , executing a fully rotational motion, is guided in a sliding block guide  46  connected to the output shaft  44 . The sliding block  45  is connected for conjoint rotation to a rotatable gear wheel  47  that is rotatably mounted and whose rotary axis  11  maintains a distance R to the central axis  10  of the input shaft  42  and engages with a fixed gear wheel  48  in a spur gear-spur gear connection between the gear wheels  47 ,  48 , and wherein the fixed gear wheel  48  is placed inside, and wherein the rotatable gear wheel  47  rolls along the outer circumference of the fixed gear wheel  48 . 
     The input shaft  42  has a mount  49  in which a bearing arrangement  50  is provided for rotatably bearing the rotatable gear wheel  47  about the rotary axis  11 . 
     The sliding block  45  with its central axis  12  is situated at a distance D to the rotary axis  11  of the rotatable gear wheel  47 , wherein the sliding block  45  is at an axial distance to the rotatable gear wheel  47  such that the rotatable gear wheel  47  is placed on one side of the mount  49 , and the sliding block  45  is placed on the other side of the mount  49 . To balance the weight of the sliding block  45 , a counterweight  51  is provided on the mount  49 . Instead of the counterweight  51 , the sliding block control can be designed in duplicate and especially symmetrically in order to bring about a balance of weight. 
       FIG. 5  schematically illustrates the geometrical relationships. Proceeding from the mount  49  on which the counterweight  51  is placed at a distance from the central axis  10 , the two gear wheels  47 ,  48  are depicted which are located behind the plane of the drawing and are actually covered by the mount  49 . The rotatable gear wheel  47  supported in the mount  49  has a radius r and rotates about its rotary axis  11  which is situated at a distance R to the central axis  10 , wherein it meshes with the fixed gear wheel  48  and rolls thereupon when the mount  49  rotates in the direction of the arrow  7 . 
     Together with the rotatable gear wheel  47 , the sliding block  45 , which is situated on the rotatable gear wheel  47  at a distance D to the rotary axis  11  of the rotatable gear wheel  47 , also rotates via a lever  53  which is indicated by the arrow of  52  on the rotatable gear wheel  47 . During this, the sliding block is located outside of the circumference of the rotatable gear wheel  47 . Since the sliding block  45  is guided in a longitudinally displaceable and rotatable manner in a sliding block guide  46  connected to the output shaft (not shown), the sliding block guide  46  follows the rotational motion of the sliding block  45  about the rotary axis  11  placed eccentric to the central axis  10 , and in so doing moves about the central axis  10 , indicated by the arrow  8 . 
     The distance D of the sliding block to the rotary axis  11  of the rotatable gear wheel  47  in the depicted case is greater than the radius r of the rotatable gear wheel  47  and less than the radius R of the rotary axis  11  of the rotatable mounted gear wheel  47  about the central axis  10  of the fixed gear wheel  48 , since the radius r of the rotatable gear wheel  47  is less than the radius R of the rotary axis  11  of the rotatable gear wheel  47  about the central axis  10  of the fixed gear wheel  48 . 
       FIG. 6  shows that a sun gear-spur gear-toothing of the fixed gear wheel  48  with the rotatable gear wheel  47  can be provided instead of the spur gear-spur gear toothing. This does not alter any of the kinematics of the motion, however. 
       FIG. 7  shows a structural design of an alternative sun gear-spur gear toothing which is more compact in regard to the structural length. The fixed gear wheel  48  surrounds the rotatable gear wheel  47  which is supported on a mount  49  of the input shaft  42  by means of a bearing  50 , and its rotary axis  11  lies at a distance R to the central axis  10  of the input shaft  42 . The sliding block  45  is attached to the rotatable gear wheel  47  at a distance D to the rotary axis  11  and engages in the sliding block guide  46  connected to the output shaft  44 . The sliding block  45  is situated on the rotatable gear wheel  47  within the circumference of the rotatable gear wheel  47 . 
     In this case, the distance D of the sliding block  45  to the rotary axis  11  of the rotatable gear wheel  47  is less than the radius r of the rotatable gear wheel  47 , and greater than the radius R of the rotary axis  11  of the rotatable gear wheel  47  about the central axis  10  of the fixed gear wheel  48 . The radius r of the rotatable gear wheel  47  is then larger than the radius R of the rotary axis  11  of the rotatable gear wheel  47  around the central axis  10  of the fixed gear wheel  48 . 
       FIG. 8  shows a schematic path of the rotary angle Phi 2  at the output over the rotary angle Phi 1  on the drive. It can be seen that the continuous, fully rotational motion of the drive is converted into an alternately reversing rotational motion of the output which is continuous as viewed over several reversals and therefore progresses as viewed collectively. 
     Given a corresponding design of the gear wheels  47 ,  48 , a transmission ratio can also be achieved between the drive and output if this is desired. 
     In principle, friction wheels can be used in the gearing instead of toothed wheels in order to provide the gearing function.