Abstract:
A dentists&#39; preparation instrument ( 1 ) comprises a turbine ( 4 ) for driving a tool ( 3 ) using compressed air, and a rotor ( 6 ) that is arranged in a turbine compartment ( 5 ), rotates about a longitudinal axis ( 7 ), and has blades ( 11 ) extending to a face ( 13 ) of the rotor ( 6 ). The turbine compartment ( 5 ) has a discharge port ( 12 ) for the compressed air towards a returning air duct ( 8 ). Said discharge port ( 12 ) is disposed in such a way that both the face ( 13 ) of the rotor ( 6 ) and parts of the blades ( 11 ) sweep past the discharge port ( 12 ) as the rotor rotates about the longitudinal axis ( 7 ). For deceleration purposes, the rotor ( 6 ) has a deceleration contour ( 9 ) on the face ( 13 ), said deceleration contour being formed by projections ( 14 ), and the turbine compartment ( 5 ) has a deceleration contour chamber ( 15 ) inside which the deceleration contour ( 9 ) is accommodated. The discharge port ( 12 ) in part also extends across the deceleration contour chamber ( 15 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage entry of International Application No. PCT/EP2013/065057, filed Jul. 17, 2013, which claims priority to German Patent Appln. No. 10 2012 212 480.2 filed Jul. 17, 2012. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a dentists&#39; preparation instrument having a turbine for driving a tool by means of compressed air. A rotor disposed in a turbine compartment and rotating about a longitudinal axis has blades extending to a face of the rotor. The turbine compartment has a discharge port for the compressed air to a return air duct, wherein the discharge port is disposed in the turbine compartment in such a way that the face of the rotor passes by the discharge port laterally in rotation about the longitudinal axis. 
     Description of the Related Art 
     A dental turbine handpiece is known from EP 0 974 308 B1 with which, after the turbine has been shut down, because of the reduced pressure that develops due to after running in a turbine compartment upstream from the discharge port, a curtain flow is created for the purpose of preventing intake or reverse draw. This takes place due to the fact that a flow web protruding into the flow path deflects the flow from the discharge port inward. A flow groove may be placed upstream from the flow web disposed in the turbine compartment to make the flow step greater and to increase the efficiency. The compressed air flows out on one face of the blades and into an annular chamber provided in the turbine wheel, where it loses pressure. Both the annular chamber and parts of the blades lie opposite the discharge port. During functional operation, however, there is a break in the curtain flow because of the pressure gradient. 
     DE 100 60 152 B4 describes a dental turbine handpiece in which a rotor is equipped with a first and a second turbine wheel to increase the torque, and connecting ducts are provided to deflect the pressure medium from the first turbine wheel to the second turbine wheel. In one embodiment, the flow passes twice through the second turbine wheel, which decelerates the rotor and reduces the rotational speed. 
     The object of the invention is to achieve a reduction in the idling speed in order to improve the lifetime and noise behavior without restricting the effective power to an extent that would be of practical relevance. 
     BRIEF SUMMARY OF THE INVENTION 
     A dentists&#39; preparation instrument having a turbine for driving a tool by means of compressed air according to the invention comprises a rotor disposed in a turbine compartment and rotating about a longitudinal axis, having blades that extend to a face of the rotor wherein the turbine compartment has a discharge port for the compressed air to a return air duct. The discharge port is disposed in the turbine compartment in such a way that the face of the rotor passes by the discharge port in rotation about the longitudinal axis and parts of the blades also pass by. The rotor has a disruptive contour formed by projections on the face and the turbine compartment has a disruptive contour space in which the disruptive contour is accommodated. The discharge port also extends partially beyond the disruptive contour space. 
     The disruptive contour on the turbine wheel in combination with a return air duct at the level of the disruptive contour creates a deceleration force that depends greatly on the rotational speed by utilizing the available airflow. A significant reduction in the rotational speed with only minor loss of torque is therefore achieved because the deceleration power in the third order is a function of the rotational speed, which means that only a low deceleration power is achieved at a low rotational speed and a disproportionately higher deceleration power is achieved at a higher rotational speed. Ribs or webs disposed on the face may be considered as projections in particular, an intermediate space being opened radically toward the discharge port between the projections. 
     To increase the deceleration effect, an eddy chamber which is connected to the discharge port may be provided in the return air duct, the width of this eddy chamber across the longitudinal axis and across the direction of flow of the return air duct amounting to at least 1.2 to 3 times a reference variable a, and its length LW in the direction of flow of the return air duct amounting to at least 0.3 to 1.5 times reference variable a, where the reference variable a is expressed either as a range from 0.5 to 1.5 times the smallest width w or as a range from 0.5 to 1.5 times the height HLA of the blades  11  of the rotor  6 . 
     The function of the eddy chamber consists of creating turbulence in the exhaust air at the outlet to thereby intensify the interaction with the disruptive contour. 
     The turbine compartment may advantageously have a sharp discharge edge at the discharge port toward the eddy chamber. The intensification of the deceleration effect is achieved by the fact that discharge eddies are created at the discharge edge by the flow disruption, and are given space in the eddy chamber and also penetrate into the disruptive contour space, where they interact with the disruptive contour. 
     An advantageous refinement consists of the fact that the discharge port completely covers the disruptive contour in its height. This facilitates penetration of the return air into the disruptive contour space. 
     The discharge port may advantageously cover the blades over at least 70% of their height, preferably completely. 
     The face of the blades of the turbine may advantageously be covered by a disk, and the disruptive contour may protrude over the disk. This achieves a clear separation of function between the blades of the turbine and the disruptive contour, and only the disruptive contour space with the disruptive contour therein is acted upon by the exhaust air from the discharge port and/or from the eddy chamber. This has the advantage that, due to a clear separation of function, the design of the disruptive contours can be optimized with respect to the increase in flow resistance, whereas the blades can be designed optimally with regard to the increase in power. The disruptive contour can be designed so that it has a particularly high drag coefficient, also known as the cW value. 
     The disruptive contour may advantageously have a height of at least 5% of the height of the blades in the direction of the longitudinal axis, preferably approx. 20% and max. 50%. 
     An advantageous refinement may consist of the disruptive contour having ribs that are curved in the opposite direction with respect to the blades of the turbine. Therefore, the flow resistance in the disruptive contour is increased and thus the deceleration power is also increased. 
     For ease of production, it may be advantageous if the eddy chamber is free of additional guide devices. 
     The eddy chamber may advantageously extend over a range of max. 120°, but it is always disposed between the discharge port and the return air duct. 
     The eddy chamber may have rounded discharge edges in the return air duct to prevent flow losses at the typical operating rotational speeds. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A device according to the invention is explained on the basis of the drawings, which show: 
         FIG. 1  a dentists&#39; preparation instrument having a turbine for driving a tool by means of compressed air with a rotor having a disruptive contour in a side view; 
         FIG. 2  the turbine compartment with the rotor having the disruptive contour from  FIG. 1  in a side view in detail; 
         FIG. 3  the turbine compartment with the rotor having a disruptive contour from  FIGS. 1 and 2  in a view of the disruptive contour in detail; 
         FIG. 3A  a rotor having an opposite disruptive contour in a side view; 
         FIG. 3B  the rotor from  FIG. 3A  in a view from above; 
         FIG. 4  an embodiment of the turbine compartment with the rotor having a disruptive contour from  FIG. 2 , with an additional eddy chamber in a side view in detail; 
         FIG. 5  the embodiment according to  FIG. 4 , where the rotor has the disruptive contour from  FIG. 3  in a view of the disruptive contour in detail. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows, schematically in a partially cutaway side view, a dentists&#39; preparation instrument  1 , having a turbine  4  that is acted upon by compressed air to drive a driven tool  3  that is mounted in a head part  2 . The turbine  4  comprises a turbine compartment  5 , in which a rotor  6  is mounted to rotate about a longitudinal axis  7 . The bearing of the rotor  6  in the head part  2  itself and the cooperation of the rotor  6  with the tool  3  are not shown here; reference is made in this regard to the prior art, from which a wide variety of approaches are known from the documents cited in the introduction, for example. 
     In addition to a compressed air supply (not shown), a return air duct  8 , which carries the compressed air out of the turbine compartment  5 , is provided in the head part  2 , so that compressed air is passed through a handle part to a coupling part of the preparation instrument, as is also known from the prior art. 
       FIG. 1  already shows that the rotor  6  is provided with a disruptive contour  9 , which is disposed together with the rotor  6  in the turbine compartment  5 . The functioning of this disruptive contour  9  and the interaction with the other functional modules are illustrated in  FIGS. 2 and 3 . 
       FIG. 2  shows the turbine compartment  5  with the rotor  6  in detail, this still being a basic diagram, just as before, in which the bearing of the rotor  6 , for example, has been omitted for reasons of simplicity. The rotor  6  has blades  11 , which deflect the compressed air provided for the drive of the turbine to improve efficiency. The compressed air flowing away from the blades  11 , also referred to as exhaust air, leaves the turbine compartment  5  through a discharge port  12 , which connects the turbine compartment  5  to the return air duct  8 , and thus enters the return air duct  8 . 
     The discharge port  12  is disposed in the turbine compartment  5  in such a way that both the face  13  of the rotor  6  and also parts of the blades  11  pass laterally by the discharge port  12  in rotation about the longitudinal axis  7 . 
     The rotor  6  has the disruptive contour  9 , which is formed by projections  14 , on the face  13 , so that, from the standpoint of function, a disruptive contour space  15 , in which the disruptive contour is accommodated, is formed within the turbine compartment  5 . It is important that the discharge port  12  also extends partially over the disruptive contour space  15 . 
     As a result, the discharge port  12  has a discharge cross section of such dimensions that a portion of the blades  11  as well as at least a portion of the disruptive contour  9  pass by the discharge port. 
     The cross section of the discharge port  12  may correspond to the cross section of the return air duct  8 , but there may also be a reduction in cross section from the cross section of the discharge port  12  to the cross section of the return air duct  8 . 
     Fundamentally, it should first be pointed out that the exhaust air flowing around the axis of the rotor changes directions in the transition to the discharge duct. The speed of the exhaust air, which is initially directed tangentially, develops progressively a velocity component directed radially to the rotor axis immediately before entering the discharge duct. The velocity component directed in the circumferential direction of the exhaust air decreases by comparison. Therefore, a relative velocity is established between the exhaust air and the rotor. This velocity difference creates an area acted upon by pressure on the downstream-facing side of the disruptive contour. The deceleration pressure created in this way depends on the rotational speed in the second order. 
     The face  13  of the blades is covered by a disk  16  and the disruptive contour  9  protrudes beyond the disk  16 . The flow path is such that the compressed air deflected by the blades flows into the discharge port  12 , where some of it also enters the disruptive contour space  15 , from which it can then escape again through the same discharge port  12  if allowed by the pressure difference. Other clearance losses also result in compressed air penetrating into the disruptive contour space. On the whole, however, it is true that the disruptive contour is exposed to only a substream of the compressed air. 
     The projections  14  on the disruptive contour  9  may also be formed by ribs or webs protruding beyond the face, as shown by the view of the disruptive contour in  FIG. 3 . The projections are disposed on the outside circumference, where an intermediate space between the projections is open radially toward the discharge port. 
     The projections  14  of the disruptive contour may also be embodied in the form of ribs  17 , which are curved in the opposite direction from the blades  11  of the rotor to increase the cW value, shown schematically in  FIGS. 3A and 3B . Here again, the face  13  of the blades  11  of the rotor  6  is covered with a disk  16 . 
     The number of projections, when they are embodied in the form of webs or ribs extending radially or with a radial curvature away from the longitudinal axis, may correspond to the number of blades, the rotor having seven blades in many cases, all of them disposed in a uniform distribution over the circumference. Preferably, however, at least two and at most twice as many as the number of blades are provided. 
       FIG. 4  shows in a side view one embodiment of the turbine compartment  5  with the rotor  6  having the disruptive contour  9  disposed on the disk  16  from  FIG. 2  with an eddy chamber  21  disposed between the turbine compartment  5  and the return air duct  8 . The discharge port  12  of the turbine compartment  5  develops into the eddy chamber  21 , the height H of which in the direction of the longitudinal axis  7 , as shown, can cover only a portion of the blades  11  but may also go over the total height of the turbine compartment  5 , represented as a dotted line  21 ′ and having the height H′. 
     The interaction with the disruptive contour  9  is intensified because the relative velocity between the exhaust air and the rotor  6  becomes greater the greater the velocity difference between the velocity vectors of the exhaust air and of the rotor  6  at the discharge port  12  upstream from the return air duct  8 . This difference can be intensified by the fact that turbulence is created in the exhaust air directly at the discharge port  12  and upstream from the return air duct  8 , so that portions of the exhaust air may even flow in the opposite direction from the direction of rotation of the rotor, which is not shown here. 
     The eddy chamber  21 , which is disposed upstream from the return air duct  8  in the direction of flow in the head housing, has a length LW in the direction of flow of the return air duct  8 , indicated by the arrow  22 . The length LB is defined by the radial extent of the disruptive contour, projected onto a plane running through the longitudinal axis  7  and the projections  14  on the disruptive contour  9  itself and should correspond at least to the radial extent of the disruptive contour on the rotor which cooperates definitively with an eddy developing in the eddy chamber  21 . This eddy is not shown for reasons of simplicity. The total length LG having a deceleration effect available to the eddy is given by the length LW of the eddy chamber and the length LB of the disruptive contour. 
       FIG. 5  shows the eddy chamber  21  in a view of the disruptive contour  9 , which is disposed on the disk  16 . It can be seen here that the eddy chamber  21  is connected to the discharge port  12  of the turbine compartment  5  and has a width B across the longitudinal axis  7  and across the direction of flow of the return air duct  8 , represented by the arrow  22 . 
     The eddy chamber  21  itself has rounded discharge edges  24  at its transition to the return air duct  8  in order to reduce the flow resistances in the return air duct itself. 
     One possible extent of the eddy chamber  21  is described as follows with reference to  FIGS. 4 and 5 , wherein both the smallest width w of the return air duct  8  or a measure HLA based on the height of the blades  11  of the rotor  6 , approximately at least half the height HLA and max. the double height HLA, can be selected as the reference variable a. The smallest width w is understood to be the greatest measure a sphere may have in order to pass through the cross section. In the case of a circular hole, this is the diameter of the hole, or in the case of a cube, it is the short side of the cross section. There may also be a largest width W, which is shown schematically for a rectangular cross section of the return air duct  8 . 
     Thus, the reference variable a may be expressed either as a range from 0.5 to 1.5 times the smallest width w or as a range from 0.5 to 1.5 times the height HLA of the blades  11  of the rotor  6 . 
     A range of 0.5*a&lt;H&lt;2*a, preferably H=a or H=HBK+HLA, has proven especially suitable for the height H of the eddy chamber  21 . The length LW of the eddy chamber  21  is in a range of 0.3*a&lt;LW&lt;1.5*a, preferably LW=a. The width B of the eddy chamber  21  is in a range 1.2*a&lt;B&lt;3*a, preferably B=1.2*a. The length ratio of the length LB of the disruptive contour to the length LW of the eddy chamber  21  is preferably 1/2, so that a ratio of the length LB of the disruptive contour to the total length LG of 1/3is obtained for a total length LG. 
     The range of 0.05*HLA&lt;HBK&lt;0.5*HLA has proven to be especially suitable as the height HBK of the disruptive contour, again based on the height HLA of the blades  11  of the rotor  6 , preferably HBK=0.2*HLA. 
     The length LB of the disruptive contour  9  is defined by the radial extent of the disruptive contour  9  projected onto a plane, which runs through the longitudinal axis  7  and the projections  14  of the disruptive contour  9  itself and lies in a range of 0.1*R&lt;LBK&lt;0.7*R, preferably LBK=0.3*R, based on the radius R of the rotor  6 . 
     The width BBK of the disruptive contour  9 , i.e., the extent of a projection  14  in the circumferential direction, lies in a range of 0.05 mm&lt;BBK&lt;3.0 mm, preferably BBK=0.1 mm. 
     The disruptive contour  9  corresponds in diameter to the rotor  6  but may also be reduced if required by the spatial conditions and the fine tuning of the deceleration power by means of the lever arm and the deceleration surface. 
     The position of the eddy chamber  21  relative to the disruptive contour  9  is also important. To illustrate this,  FIG. 4  shows that the projections  14  of the disruptive contour  9  have a top edge  25  facing away from the blades  11 . 
     The eddy chamber  21  is defined in the axial direction of the rotor  6  by the bordering surfaces  26  and  27 . The distance S 1 , also given as the height HSP, between the top edge  25  of the disruptive contour  9  and the top bordering surface  27  of the eddy chamber  21  is at least 0.1 mm and max. 10 times the height HBK of the disruptive contour, preferably 2×HBK. The distance S 2  between the top edge  25  and the lower bordering surface  26  is at least 0.5*a and max. 3*a, ideally 1.5*a. 
     In addition, the position of the eddy chamber  21  relative to the return air duct  8  is relevant. The distance s 3  between the upper bordering surface  27  and the top side  28  of the return air duct  8  may be at least zero and max. s 3 &lt;(s 1 +HBK+HLA) and s 3 =s 1 +HBK is particularly suitable. 
     The eddy chamber  21  itself is free of additional guide devices and extends in the direction of rotation of the rotor  6  over an angle range alpha in the present case of approx. alpha equal to 45°, represented in  FIG. 5 . 
     It is true of all embodiments that the turbine compartment may have a sharp discharge edge  23  at the discharge port  12  to the return air duct  8  or to the eddy chamber  21 , as shown in  FIGS. 3 and 5 , so that there is a strong discharge eddy that cooperates with the disruptive contour  9 . 
     It has been found that the flow conditions at the discharge port cooperate with the disruptive contour in such a way that a deceleration is induced at higher rotational speeds, whereas there is only an insignificant loss of torque at lower rotational speeds. 
     For an understanding of the invention it is pointed out that other fluids may also be used instead of compressed air. In addition, compressed air is also mentioned in general when a partial depressurization has taken place and the pressure difference from atmospheric pressure is only minor, as is the case with the exhaust air in the return air duct  8 , for example.