the dental handpiece shown in fig1 comprises a holding sleeve 1 ( for the user ) and a head sleeve 2 connected to sleeve 1 and inclined relative to the longitudinal axis a of the sleeve 1 . a two - part drive shaft 5 is mounted in the handpiece with the aid of bearings 3 , 4 , and is transversely divided in the zone of a bending location 6 between sleeves 1 and 2 . the two drive shaft elements 5a and 5b thus formed are drivingly interconnected by a ball - type planetary transmission 7 housed within the connecting zone ( 6 ). the holding sleeve 1 and the head sleeve 2 constitute two independent elements releasably connected with each other by a screw thread 8 . the drive shaft element 5a is the driven element and is mounted in the head sleeve 2 for rotation by the ( driving ) drive shaft element 5b via the transmission 7 , for driving a dental instrument 9 coupled therewith , for example a drill . for this purpose , the drive shaft element 5a engages , via a toothed - wheel or bevel gear 10 , with a drive sleeve 11 holding the instrument 9 and arranged at right angles to the drive shaft element 5a , bearings 12 and 13 for the drive sleeve 11 being arranged in an angle head 11a . the transmission 7 may be designed in different ways , as shown diagrammatically in fig4 to 7 . the illustration of transmission according to fig4 corresponds , in its structure , to the embodiment according to fig1 wherein step - up transmission takes place in the ratio 1 : 2 . 7 from the driving shaft element 5b to the driven shaft element 5a whereas fig5 shows an embodiment of stepping - down transmission . in each case , a cage - like end 14 facing the transmission 7 of one of the drive shaft elements ( 5b in fig1 and 7 or 5a in fig5 and 6 ) is designed in the manner of a cage open at its end facing the other drive shaft element , in the form of a round fork . through the agency of webs 15 of the cage , the end 14 engages between the balls 16 of the ball - type planetary transmission 7 ( cf . also fig2 showing the transmission 7 in cross section ). the balls 16 are mounted , with frictional engagement , between an inner ring 17 and an outer ring 18 . one of the rings 17 , 18 is non - rotatably arranged , ( outer ring 18 in fig1 and 5 and inner ring 17 in fig6 and 7 ). the other ring ( in fig1 and 5 the inner ring 17 and in fig6 and 7 the outer ring 18 ) is mounted for rotation on and with a respective drive shaft element . thus , inner ring 17 is rotatable respectively with shaft element 5a in fig1 and 4 , and shaft element 5b in fig5 whereas outer ring 18 is rotatable respectively on shaft element 5b in fig6 and 5a in fig7 . one of the rings 17 , 18 , i . e . in each particular instance the ring rotatable with the drive shaft element 5a in fig1 and 7 , or with shaft element 5b in fig5 and 6 , not engaging with a balls 16 , i . e . in fig1 and 5 the inner ring 17 and in fig6 and 7 the outer ring 18 , is axially displaceable and rotatable under the influence of a coupling element in the form of displacement ring 19 . the ring 19 is shown only diagrammatically in fig4 to 7 , but is shown in detail in fig1 in which it is held fast by means of a transverse pin 20 on the drive shaft element 5a . this applies also to fig1 and 7 , but in fig5 and 6 the ring 19 will be secured to shaft element 5b by pin 20 . for this purpose , the displacement ring 19 according to fig1 and 3 is provided with helical gear teeth 22 having straight and / or curved tooth profiles 21 and meshing with corresponding helical toothing 23 provided on the associated axially displaceable ring ( inner ring 17 in fig1 and 5 or outer ring 18 in fig6 and 7 ). in the case of the helical toothings 22 , 23 , the cone angle is larger than the friction angle . during normal operation of the instrument 9 , the helical toothings 22 , 23 serve substantially only for transmitting rotational movement between the axially displaceable ring 17 and the displacement ring 19 , and therewith the drive shaft element 5a in fig1 and 7 or shaft element 5b in fig5 and 6 . during idle travel , the balls 16 are driven by the cage webs 15 . due to the centrifugal force of the balls 16 , the latter are pressed against the outer ring 18 . the movement , reciprocating relative to the balls 16 , of the cage webs 15 presses the balls in the direction of the ends of the webs ; thus , the webs 15 endeavour to press the balls 16 out between the inner ring 17 and the outer ring 18 . thereby , also in cooperation with annular grooves 25 formed in the inner ring 17 and the outer ring 18 ( see fig1 ), frictional engagement is produced so that the inner ring 17 is driven . via the tooth profiles 21 , 24 of the helical toothings 22 , 23 , the drive shaft element 5a is set in rotation . the friction in the bearings , 3 , 12 and 13 produces the result that the drive shaft element 5a rotates relative to the drive shaft element 5b and thereby , via the profiles 21 , 24 , the inner ring 17 is pressed only slightly axially but adequately for rotation transmission against the balls . therewith , transmission of rotation is guaranteed . when the instrument 9 is loaded , in that its rotation is prevented or retarded for example at a tooth to be treated , the relative rotation thereby occurring of the two drive shaft elements 5a and 5b , and therewith of the displacement ring 19 and of the inner ring 17 in fig1 and 5 or the outer 18 in fig6 and 7 , produces the result that the tooth profiles 21 , 24 of the two helical gearings 22 , 23 slide against each other in the sense of a displacement , taking place in the axial direction , of the inner ring 17 in fig1 and 5 or of the outer ring 18 in fig6 and 7 , so that automatic adjustment takes place of the balls 16 mounted in the annular grooves 25 of the inner ring 17 and of the outer ring 18 so as to increase the transmission of torque between the shaft elements 5a and 5b . the teeth of the helical gearings 22 , 23 are uniformly distributed over the periphery of the displacement ring 19 and the inner ring 17 in fig1 and 5 or the outer ring 18 in fig6 and 7 , and are so designed that no automatic locking can occur . in the case of the embodiments according to fig1 and 7 , the displacement ( caused by the rotation of the displacement ring 19 ) of the inner ring 17 in fig1 and 4 or of the outer ring 18 in fig7 takes place relative to the driven shaft element 5a which consequently directly influences the loading of the instrument 9 , whereas in the case of the embodiments according to fig5 and 6 this occurs at the driving shaft element 5b . in the case of the embodiments according to fig4 to 7 , the drive shaft elements 5a and 5b are shown coaxial as for application to straight ( non - angled ) handpieces ; however , they could , as in the case of the embodiment according to fig1 be inclined at an obtuse angle relative to each other for use with handpieces having a head sleeve ( 2 ) inclined relative to the holding sleeve ( 1 ). referring to fig1 the inner ring 17 is fast with a sleeve 17a extending towards the displacement ring 19 and at which the helical toothing 23 is provided , the sleeve 17a being arranged to be axially displaceable and rotatable on the drive shaft element 5a . similarly , although not shown , where appropriate the outer ring 18 may have a sleeve coupled therewith and having helical teeth for cooperation with helical teeth of the displacement ring .
a dental handpiece comprises a holding sleeve , a head sleeve connected to the holding sleeve for mounting a dental instrument , a connecting zone providing an interconnection between the holding sleeve and the head sleeve , a two - part drive shaft housed within the handpiece , a first part of the shaft being arranged within the holding sleeve and a second part of the shaft being arranged within the head sleeve , and a ball - type planetary transmission housed within the connecting zone and providing a drive connection between the first and second parts of the shaft . the handpiece may be a straight handpiece or the head sleeve may be inclined to the holding sleeve to provide an angled handpiece .