Abstract:
The present invention relates to a drive train ( 50 ) of a hair cutting appliance ( 10 ) and to a self-aligning coupling linkage ( 66 ) for a drive train ( 50 ). The coupling linkage ( 66 ) comprises a driving shaft and a non-aligning output shaft ( 56 ), said coupling linkage ( 66 ) comprising: a first driving coupling element ( 78 ) arranged to be driven by a driving shaft ( 54 ), particularly by a motor shaft, a transmission shaft ( 70 ), particularly a rigid transmission shaft ( 70 ), comprising a first drivable coupling element ( 80 ) at a first end and a second driving coupling element ( 86 ) at a second end thereof, wherein first driving coupling element ( 78 ) engages the first drivable coupling element ( 80 ) for rotatingly driving the transmission shaft ( 70 ), thereby forming a first pivoting joint ( 76 ), wherein the second driving coupling element ( 86 ) is arranged to engage a second drivable coupling element ( 88 ) of an output shaft ( 56 ), wherein the first driving coupling element ( 78 ) and the first drivable coupling element ( 80 ) define a male connector comprising an external polygonal profile ( 90 ), viewed in a cross-sectional plane perpendicular to a longitudinal axis, and a female connector comprising an internal polygonal profile ( 102 ), and wherein the external polygonal profile ( 90 ) of the male connector, viewed in a longitudinal axial section, is at least sectionally provided with convexly shaped flanks ( 92 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The present disclosure relates to a hair cutting appliance, particularly to an electrically operated hair cutting appliance, and more particularly to a coupling linkage mechanism for a drive train of a hair cutting appliance. 
       BACKGROUND OF THE INVENTION 
       [0002]    WO 2013/150412 A1 discloses a hair cutting appliance and a corresponding blade set of a hair cutting appliance. The blade set comprises a stationary blade and a movable blade, wherein the movable blade can be reciprocatingly driven with respect to the stationary blade for cutting hair. The blade set is particularly suited for enabling both trimming and shaving operations. 
         [0003]    For the purpose of cutting body hair, there exist basically two customarily distinguished types of electrically powered appliances: the razor, and the hair trimmer or clipper. Generally, the razor is used for shaving, i.e. slicing body hairs at the level of the skin so as to obtain a smooth skin without stubbles. The hair trimmer is typically used to sever the hairs at a chosen distance from the skin, i.e. for cutting the hairs to a desired length. The difference in application is reflected in the different structure and architectures of the cutting blade arrangement implemented on either appliance. 
         [0004]    An electric razor typically includes a foil, i.e. an ultra-thin perforated screen, and a cutter blade that is movable along the inside of and with respect to the foil. During use, the outside of the foil is placed and pushed against the skin, such that any hairs that penetrate the foil are cut off by the cutter blade that moves with respect to the inside thereof, and fall into hollow hair collection portions inside the razor. 
         [0005]    An electric hair trimmer, on the other hand, typically includes generally two cutter blades having a toothed edge, one placed on top of the other such that the respective toothed edges overlap. In operation, the cutter blades reciprocate relative to each other, cutting off any hairs that are trapped between their teeth in a scissor action. The precise level above the skin at which the hairs are cut off is normally determined by means of an additional attachable part, called a (spacer) guard or comb. 
         [0006]    Furthermore, combined devices are known that are basically adapted to both shaving and trimming purposes. However, these devices merely include two separate and distinct cutting sections, namely a shaving section comprising a setup that matches the concept of powered razors as set out above, and a trimming section comprising a setup that, on the other hand, matches the concept of hair trimmers. 
         [0007]    Common electric razors are not particularly suited for cutting hair to a desired variable length above the skin, i.e., for precise trimming operations. This can be explained, at least in part, by the fact that they do not include mechanisms for spacing the foil and, consequently, the cutter blade from the skin. But even if they did, e.g. by adding attachment spacer parts, such as spacing combs, the configuration of the foil, which typically involves a large number of small perforations, would diminish the efficient capture of all but the shortest and stiffest of hairs. 
         [0008]    Similarly, common hair trimmers are not particularly suited for shaving, primarily because the separate cutter blades require a certain rigidity, and therefore thickness, to perform the scissor action without deforming. It is the minimum required blade thickness of a skin-facing blade thereof that prevents hair from being cut off close to the skin. Consequently, a user desiring to both shave and trim his/her body hair may need to purchase and apply two separate appliances. 
         [0009]    Furthermore, combined shaving and trimming devices show several drawbacks since they basically require two cutting blade sets and respective drive mechanisms. Consequently, these devices are heavier and more susceptible to wear than standard type single-purpose hair cutting appliances, and also require costly manufacturing and assembly processes. Similarly, operating these combined devices is often experienced to be rather uncomfortable and complex. Even in case a conventional combined shaving and trimming device comprising two separate cutting sections is utilized, handling the device and switching between different operation modes may be considered as being time-consuming and not very user-friendly. Since the cutting sections are typically provided at different locations of the device, guidance accuracy (and therefore also cutting accuracy) may be reduced, as the user needs to get used to two distinct dominant holding positions during operation. 
         [0010]    The above WO 2013/150412 A1 tackles some of these issues by providing for a blade set comprising a stationary blade that houses the movable blade such that a first portion of the stationary blade is arranged at the side of the movable blade facing the skin, when used for shaving, and that a second portion of the stationary blade is arranged at the side of the movable blade facing away from the skin when in use. Furthermore, at a toothed cutting edge, the first portion and the second portion of the stationary blade are connected, thereby forming a plurality of stationary teeth that cover respective teeth of the movable blade. Consequently, the movable blade is guarded by the stationary blade. 
         [0011]    This arrangement is advantageous insofar as the stationary blade may provide the blade set with increased strength and stiffness since the stationary blade is also present at the side of the movable blade facing away from the skin. This may generally enable a reduction of the thickness of the first portion of the stationary blade at the skin-facing side of the movable blade. Consequently, since in this way the movable blade may come closer to the skin during operation, the above blade set is well-suited for hair shaving operations. 
         [0012]    Aside from that, the blade set is also particularly suited for hair trimming operations since the configuration of the cutting edge, including respective teeth alternating with slots, also allows longer hairs to enter the slots and, consequently, to be cut by the relative cutting motion between the movable blade and the stationary blade. 
         [0013]    However, there is still a need for improvement in hair cutting appliances. This may particularly involve user comfort related aspects and performance related aspects. Particularly with hair cutting appliances comprising blade sets that are pivotably attached to the housing, the drive train and particularly the power transmission arrangement between a motor and the blade set needs to be adapted to the pivotable arrangement. Furthermore, since the housing portion may be adequately shaped so as to permit an appropriate ergonomic positioning and orientation of the appliance with respect to the skin of the user, the blade set may be arranged at an angle with respect to the housing. This may pose further challenges. 
       SUMMARY OF THE INVENTION 
       [0014]    It is an object of the present disclosure to provide a drive train, particularly a coupling linkage for a drive train of a hair cutting appliance that may enhance the operating performance of the appliance and contributes to a pleasant user experience. Furthermore, it is desired to provide a hair cutting appliance fitted with a respective drive train. It is particularly desired that the hair cutting appliance enables both shaving and trimming operations. Even more particularly, it is desired that the cutting appliance exhibits an advanced contour following capability. Preferably, the present disclosure may generally address at least some drawbacks inherent in known prior art hair cutting appliances as discussed above, for instance. It may be further desirable to provide a drive train that is particularly suited for angular offset compensation. It would be further preferred to diminish unpleasant emissions that may emerge from a drive train, e.g. running noises and vibrations. 
         [0015]    In a first aspect of the present invention a coupling linkage for a drive train of a hair cutting appliance comprising a driving shaft and a non-aligning output shaft is presented, said coupling linkage comprising:
       a first driving coupling element arranged to be driven by a driving shaft, particularly by a motor shaft,   a transmission shaft, particularly a rigid transmission shaft, comprising a first driveable coupling element at a first end and a second driving coupling element at a second end thereof,       
 
         [0018]    wherein the first driving coupling element engages the first driveable coupling element for rotatingly driving the transmission shaft, thereby forming a first pivoting joint, 
         [0019]    wherein the second driving coupling element is arranged to engage a second driveable coupling element of an output shaft, 
         [0020]    wherein the first driving coupling element and the first driveable coupling element define a male connector comprising an external polygonal profile, viewed in a cross-sectional plane perpendicular to a longitudinal axis, and a female connector comprising an internal polygonal profile, and 
         [0021]    wherein the external polygonal profile of the male connector, viewed in a longitudinal axial section, is at least sectionally provided with convexly shaped flanks 
         [0022]    This aspect is based on the insight that the coupling linkage may provide the drive train with the ability to span angular and/or parallel offsets between the driving shaft and the output shaft. More particularly, a hair cutting appliance fitted with the coupling linkage may be configured to make use of a housing that comprises a neck portion that is inclined with respect to a base portion or gripping portion thereof. This may significantly improve the visibleness of the appliance during operation, particularly when shaving. Consequently, handling the device may be improved. Furthermore, the coupling linkage may be provided with the ability to compensate angular deviations. Angular deviations may involve constant deviations but also variable deviations. This may be beneficial since in this way the drive train may become considerably tolerant to angular deviations between the driving shaft and the output shaft. This may reduce the need of providing high precision parts for the drive train and the housing of the hair cutting appliance. Consequently, manufacturing the drive train and the hair cutting appliance may be simplified. 
         [0023]    The coupling linkage may generally be referred to as coupling linkage mechanism, preferably as self-aligning coupling linkage mechanism. The coupling linkage may be arranged to replace a conventional universal joint. At least in some embodiments, the coupling linkage may be utilized to compensate parallel offset deviations between the driving shaft and the output shaft. 
         [0024]    In one embodiment, the first driveable coupling element at the first end of the transmission shaft is arranged as an axially extending recess comprising an internal polygonal profile, and wherein the first driving coupling element is arranged as a substantially axially extending external polygonal profile. Consequently, the driving coupling element and the driveable coupling element may be engaged basically without significant (circumferential) backlash or backlash-free. 
         [0025]    In one embodiment, the second driving coupling element and the second driveable coupling element form a second pivoting joint when brought into engagement, wherein the second driving coupling element at the second end of the transmission shaft is arranged as a male connector comprising an external polygonal profile, wherein the second driveable coupling element of the output shaft is arranged as a female connector comprising an axially extending recess comprising an internal polygonal profile. 
         [0026]    In one embodiment, the coupling linkage further comprises a biasing element, particularly a spring element, wherein the biasing element is interposed between the first driveable coupling element and the first driving coupling element. The biasing element may be configured to compensate length deviations in the drive train. Since the involved components may be at least slightly preloaded, particularly axially preloaded, running noises may be further reduced. 
         [0027]    In one embodiment, the biasing element is arranged in an axially extending recess at the transmission shaft, wherein the biasing element urges the first driveable coupling element and the first driving coupling element substantially in an axial longitudinal direction of the transmission shaft. Consequently, the biasing element is covered and protected by the transmission shaft. 
         [0028]    In one embodiment, the biasing element is coupled to a push rod slidably arranged at the transmission shaft, wherein the push rod is arranged to contact the first driving coupling element. 
         [0029]    In one embodiment, the external polygonal profile of each male connector is at least sectionally provided with spherical flanks Consequently, the respective shafts may swivel with respect to each other, thereby altering their relative angular orientation. The spherical flanks may comprise spheric or globular surfaces that may ensure that the respective driving coupling elements and driveable coupling elements are in close contact for transmitting rotation. This may further reduce running noises. The spherical flanks may roll with respect to their counterparts upon relative angular motion between neighboring shafts. 
         [0030]    In one embodiment, the internal polygonal profile of each female connector comprises a number of circularly arranged protrusions alternating with indentations disposed therebetween, wherein the protrusions and the indentations define contact flanks arranged to contact the flanks of the polygonal profile of the respective male connector. The contact flanks may comprise a basically planar shape. Consequently, manufacturing the internal polygonal profile may be simplified. 
         [0031]    In one embodiment, the number of contact flanks of the female connector is adapted to the number of flanks of the male connector. In one embodiment, the female connector and the male connector of at least one of the first pivoting joint and the second pivoting joint are configured in such a way that the pivoting joint exhibits a defined circumferential backlash. This may simplify inserting the external polygonal profile into the internal polygonal profile. However, it may be preferred that a minimized circumferential backlash is provided at the pivoting joint. 
         [0032]    In a further aspect of the present invention a drive train for a cutting head of a hair cutting appliance is presented, the drive train comprising a driving shaft, an output shaft and a coupling linkage in accordance with at least some embodiments of the present disclosure, wherein the driving shaft and the output shaft are arranged at an angular offset, wherein the coupling linkage connects the driving shaft and the output shaft, and wherein the output shaft comprises an eccentric portion arranged to engage a transmitting member of a blade set for driving a movable cutter blade thereof in a reciprocating manner. 
         [0033]    In a further aspect of the present invention a hair cutting appliance is presented, particularly an electrically operable hair cutting appliance, said hair cutting appliance comprising a housing, a cutting head attached to said housing, and a drive train comprising a driving shaft, an output shaft and a coupling linkage in accordance with at least some embodiments of the present disclosure, wherein the cutting head comprises a blade set comprising a movable cutter blade and a stationary blade, wherein the movable cutter blade is movable with respect to the stationary blade, and wherein the drive train is arranged to actuate the movable cutter blade when the cutting head is attached to the housing. 
         [0034]    It is further preferred that the hair cutting appliance is selectively operable in a shaving mode and a trimming mode. It is further preferred in this context that the cutting head is operable in a contour following mode, particularly when shaving. The contour following mode may involve a pivotable mounting of the blade set such that the blade set may be adapted to an actual skin surface. Furthermore, it may be preferred that the cutting head is operable in a trimming mode. The trimming mode may involve a fixed angular swivel orientation of the blade set with respect to the housing. 
         [0035]    Preferably, the cutting head is releasably attached to said housing. Consequently, the drive train may comprise an interface where respective components may be disengaged and separated when the cutting head is released. 
         [0036]    In one embodiment of the hair cutting appliance, the movable cutter blade is arranged in a guide slot and laterally movable with respect to the stationary blade, wherein the second pivoting joint defines an releasable coupling interface, wherein the output shaft and the second driveable coupling element are detachable from the second driving coupling element and the transmission shaft when the cutting head is released from the housing, thereby releasing the second driveable coupling element from engagement with the second driving coupling element. 
         [0037]    In one embodiment, the hair cutting appliance may further comprise a main portion formed by the housing, and a neck portion, wherein the main portion houses a motor, wherein the blade set is attached to the neck portion, and wherein the neck portion is oriented with an angular offset with respect to a main orientation of the main portion. In other words, the neck portion may be inclined with respect to the main portion of the housing. 
         [0038]    In one embodiment of the hair cutting appliance, the main portion houses a driving shaft, wherein the neck portion houses an output shaft, wherein the driving shaft and the output shaft are arranged at an overall offset angle δ, wherein the transmission shaft of the coupling linkage couples the driving shaft and the output shaft, wherein the transmission shaft is arranged at a partial offset angle α with respect to the output shaft, wherein the partial offset angle α comprises an angular dimension being substantially half the size of an angular dimension of the overall offset angle δ. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]    These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the following drawings 
           [0040]      FIG. 1  shows a schematic perspective view of an exemplary electric hair cutting appliance comprising a cutting head which is shown in a detached state; 
           [0041]      FIG. 2  is a perspective rear view of a drive train for a hair cutting appliance, the drive train comprising a coupling linkage mechanism; 
           [0042]      FIG. 3  shows an exploded perspective rear view of the coupling linkage mechanism in accordance with the embodiment illustrated in  FIG. 2 ; 
           [0043]      FIG. 4  is a cross-sectional side view of the drive train illustrated in  FIG. 2 ; 
           [0044]      FIG. 5  is a cross-sectional view of the of a coupling linkage mechanism of the drive train shown in  FIG. 4  taken along the line V-V in  FIG. 4 ; 
           [0045]      FIG. 6  is another cross-sectional view of the of the coupling linkage mechanism of the drive train shown in  FIG. 4  taken along the line VI-VI in  FIG. 4 ; 
           [0046]      FIG. 7  is yet another cross-sectional view of the of the coupling linkage mechanism of the drive train shown in  FIG. 4  taken along the line VII-VII in  FIG. 4 ; 
           [0047]      FIG. 8  is still another cross-sectional view of the of the coupling linkage mechanism of the drive train shown in  FIG. 4  taken along the line VIII-VIII in  FIG. 4 ; 
           [0048]      FIG. 9  is still yet another cross-sectional view of the of the coupling linkage mechanism of the drive train shown in  FIG. 4  taken along the line IX-IX in  FIG. 4 ; 
           [0049]      FIG. 10  is a perspective top view of a blade set for a hair cutting appliance, the blade set being attached to a contour following mechanism including a hinged pivoting mechanism, the blade set being arranged to be driven by a driving shaft; and 
           [0050]      FIG. 11  is a perspective exploded bottom view of the blade set illustrated in  FIG. 10 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0051]      FIG. 1  shows schematically illustrates a hair cutting appliance  10 , particularly an electric hair cutting appliance  10 . The hair cutting appliance  10  may include a housing  12  which may house a motor for driving the hair cutting appliance  10 . The housing  12  may further house a battery, such as, for instance, a rechargeable battery, a replaceable battery, etc. However, in some embodiments, the hair cutting appliance  10  may be provided with a power cable for connecting a power supply. A power supply connector may be provided in addition or in the alternative to an (internal) electric battery. The housing  12  may further comprise an operating switch  34 . 
         [0052]    The housing  12  of the hair cutting appliance  10  may generally comprise a main portion  14  and a neck portion  16 . As shown in  FIG. 1  in a detached state, the neck portion  16  may be associated with a cutting head  18  and, at least in some embodiments, with a receiving portion  28  for the cutting head  18  at the housing  12 . 
         [0053]    The cutting head  18  may comprise a blade set  20 . The blade set  20  may comprise a stationary blade  22  and a movable blade  24 . The stationary blade  22  and the movable blade  24  may be operated so as to generate relative motion therebetween. By way of example, the movable blade  24  may be reciprocatingly driven with respect to the stationary blade  22 . Consequently, the stationary blade  22  and the movable plate  24  may cooperate to cut hair. An exemplary embodiment of a cutting head  18  including a particular embodiment of a blade set  20  will be further illustrated and described in  FIGS. 10 and 11  hereinafter. 
         [0054]    As shown in  FIG. 1 , it may be preferred that the cutting head  18  is releasable or detachable from the housing  12  of the hair cutting appliance  10 . In other words, the cutting head  18  may be releasably attached to the housing  12 . Consequently, respective interfaces have to be provided at the cutting head  18  and the housing  12 . By way of example, the cutting head  18  may comprise a mounting portion  26  which is configured to engage the receiving portion  28  or to be received by the receiving portion  28  at the housing  12 . Since basically a driving motor is provided at the housing  12  and may be therefore associated with the main portion  14 , and since the drivable blade set  20  is provided at the cutting head  18  and therefore associated with the neck portion  16 , also a respective drive train needs to comprise a respective interface. In other words, when a user attaches or detaches the cutting head  18  to the housing  12 , also the drive train needs to be coupled and decoupled, respectively. 
         [0055]    The blade set  20  illustrated in  FIG. 1  is shown in a hidden edge mode wherein at least some hidden edges are visible, particularly for illustrative purposes. As can be further seen from  FIG. 1 , the housing  12  may comprise a protruding driving member  30  which is configured to engage and cooperate with a transmitting member  32  at the cutting head  18 . By way of example, the driving member  30  may comprise a driving shaft including an eccentric coupling member which may rotate about an axis of the driving shaft. Consequently, the eccentric member may engage a respective drivable slot at the transmitting member  32  for reciprocatingly driving the transmitting member  32  and, consequently, the movable blade  24  of the blade set  20 . For improving the cutting performance of the hair cutting appliance  10 , it may be advisable to arrange the blade set  20  in a particular orientation with respect to a main orientation of the housing  12 . This may involve arranging the blade set  20  at an angle with respect to the housing  12 . Such an angled or inclined arrangement may enhance the visibility of the blade set when the hair cutting appliance  10  is used for cutting hair. Furthermore, manually grasping and handling the device which may involve moving the hair cutting appliance  10  through hair in a desired manner may be simplified when the blade set  20  is arranged to assume a predefined orientation with respect to the housing  12 , particularly to a handling portion thereof. 
         [0056]    As used herein, the main portion  14  and the neck portion  16  do not necessarily have to be strictly associated to respective components of the hair cutting appliance  10  as indicated in  FIG. 1 . There may be alternative embodiments of hair cutting appliances  10  that may be modified with respect to the position of the “interface” between the housing  12  and the cutting head  18 . As exemplarily shown in  FIG. 1 , the main portion  14  may comprise at least a substantial portion of the housing  12 . The neck portion  16  may involve the cutting head  18 , particularly a housing thereof, and at least a small portion of the appliance&#39;s base housing  12 . 
         [0057]    It may be generally preferred that the neck portion  16  of the hair cutting appliance  10  is at least slightly curved or inclined with respect to a main orientation of the main portion  14 . A main orientation of the main portion  14  is exemplified in  FIG. 1  by an arrow indicated by reference numeral  44 . An orientation of the neck portion  16  or, rather, the cutting head  18  is exemplified in  FIG. 1  by an arrow indicated by reference numeral  42 . 
         [0058]    Consequently, the main portion  14  and the neck portion  16  may be arranged at an angle of inclination with respect to each other, refer also the angle δ (delta) shown in  FIG. 2 . 
         [0059]    Generally, a driving motor may be arranged in the housing having a main orientation that is basically parallel to the main orientation  44  of the housing. Consequently, also a motor shaft may be basically aligned with the main orientation  44 . Hence, there might be the need to “bridge” an angular offset between the orientation of the main portion  14  and the orientation of the neck portion  16 . Angular offset compensation may require complicated mechanisms which may increase the costs of the hair cutting appliance  10 . Furthermore, there might be the adverse effect that a conventional angular offset compensating drive train causes vibration and noise which might interfere with the desired user sensation and cutting performance. 
         [0060]    With reference to  FIGS. 2 to 9 , an exemplary embodiment of a drive train arrangement  50  for a hair cutting appliance will be illustrated and further described. The drive train arrangement or drive train  50  may be arranged to compensate considerable angular offsets between an input and an output member. The drive train  50  may generally be referred to as self-aligning drive train  50 . 
         [0061]    With particular reference to the perspective views of  FIGS. 2 and 3 , a general layout of the drive train  50  will be elucidated. The drive train  50  may comprise or be coupled to a motor  52 , particularly an electric motor  52 . The motor  52  may be arranged at a housing  12  of a hair cutting appliance  10  at an exemplary orientation which may be basically aligned with the main orientation  44 . The motor  52  may comprise a driving shaft  54  which can be rotated by the motor  52 . An orientation of the driving shaft  54  and, consequently, of the motor  52 , is indicated by an axis  58  in  FIG. 2 . At an output end thereof, the drive train  50  may further comprise an output shaft  56 . A main orientation of the output shaft  56  is indicated in 
         [0062]      FIG. 2  by an axis  60 . The axis  58  and  60  may be arranged at an overall offset angle δ (delta). At an output end of the output shaft  56 , an eccentric portion  62  may be provided. The eccentric portion  62  may be arranged to rotate about a central axis of the output shaft  56 . Consequently, rotation of the output shaft  56 , refer to the curved arrow indicated by  64  in  FIG. 2 , may be converted into a reciprocating motion which may be used to drive the movable blade  24  of the blade set  20 . 
         [0063]    It is worth mentioning in this context that it might be advantageous to attach the cutting head  18 , particularly the blade set  20  thereof, in a pivotable manner at the housing  12  of the hair cutting appliance  10 . This may have the advantage that the contour following capability of the hair cutting appliance  10  may be significantly increased. This may greatly improve the cutting performance, particularly the shaving performance of the hair cutting appliance  10 . Reference in this regard is made to  FIG. 10 . 
         [0064]    At least in some embodiments, pivotably arranging the blade set  20  may also require pivotably arranging the output shaft  56 . Consequently, in these embodiments, the overall offset angle δ may be regarded as a variable angle. However, at least in some alternative embodiments, the angular orientation of the output shaft  56  with respect to the driving shat  54  may be substantially fixed. Consequently, a relatively constant angular offset may be present. It is particularly preferred that the drive train  50  is capable of “bridging” variable (unstable) overall offset angles δ. To this end, the drive train  50  may comprise a coupling linkage mechanism  66  which is capable of compensating angular offsets when transmitting rotations. 
         [0065]    The coupling linkage mechanism  66  may further comprise a transmission shaft  70  which is interposed between the driving shaft  54  and the output shaft  56 . A general orientation of the transmission shaft  70  may be illustrated by an axis indicated by reference numeral  72  in  FIG. 2 . The transmission shaft  70  may be coupled, at a first end thereof, to the driving shaft  54 . The transmission shaft  70  may be further coupled, at a second end thereof, to the output shaft  56 . Generally, the transmission shaft  70  may be configured to transmit torque or a rotational motion. Consequently, the transmission shaft  70  may be arranged to be driven by the driving shaft  54 . Furthermore, the transmission shaft  70  may be arranged to drive the output shaft  56 . The transmission shaft  70  may be arranged at an angle α with respect to the driving shaft  54 . The transmission shaft  70  may be further arranged at an angle β with respect to the output shaft  56 . The angles α and β may be regarded as partial offset angles. Needless to say, a sum of the angles α and β may basically correspond to the overall offset angle δ. Furthermore, each of the partial offset angles α and β could comprise half the size of the overall offset angle δ. It is worth mentioning in this context that the axes  58 ,  60  and  72  are shown in an offset state with respect to the respective shafts  24 ,  56  and  70  in  FIG. 3 , primarily for illustrative purposes. 
         [0066]    The driving shaft  54  and the transmission shaft  70  may define a first pivoting joint  76 . The first pivoting joint  76  may comprise a first driving coupling element  78  and a first drivable coupling element  80 . The transmission shaft  70  and the output shaft  56  may define a second pivoting joint  84 . The second pivoting joint  84  may comprise a second driving coupling element  86  and a second drivable coupling element  88 . 
         [0067]    As can be best seen in  FIGS. 2 and 3 , at least in some embodiments, the first driving coupling element  78  may be arranged as an engaging driving coupling element which may also be referred to as male driving coupling element. Consequently, the first drivable coupling element  80  may be arranged as a receiving drivable coupling element which may also be referred to as female drivable coupling element. 
         [0068]    Furthermore, the second driving coupling element  86  may be arranged as an engaging driving coupling element which may also be referred to as male coupling element. The second drivable coupling element  88  may be arranged as receiving drivable coupling element which may also be referred to as female drivable coupling element. However, in the alternative, male and female coupling elements may be exchanged, at least in some embodiments. 
         [0069]    Generally, the first driving coupling element  78  may be arranged at the driving shaft  54 . The first driving coupling element  78  may be fixedly attached to or fixed at the driving shaft  54 . The first drivable coupling element  80  may be arranged as the transmission shaft  70 . The second driving element  86  may be arranged at the transmission shaft  70 . The second drivable coupling element  88  may be arranged at the output shaft  56 . 
         [0070]    The first pivoting joint  76  and the second pivoting joint  84  may be basically arranged as rotary coupling joints. Consequently, the coupling linkage mechanism  66  may transfer rotational motion. It is particularly preferred that, with respect to the rotation transmission, at least one of the first pivoting joint  76  and the second pivoting joint  84  is arranged with low backlash or, more preferably, basically backlash-free. This may basically allow smooth running of the coupling linkage mechanism  66 . Running noises, shocks, vibrations and jolts may be prevented or, at least, significantly reduced. This is advantageous since the driving shaft  54  and, consequently, the output shaft  56  may be rotated at high speed when the hair cutting appliance  10  is operated. The first pivoting joint  76  and the second pivoting joint  84  may be arranged to allow swiveling movement, particularly angular offset compensation movement between the respective coupled elements, particularly between the driving shaft  54  and the transmission shaft  70 , at the first pivoting joint  76 , and between the transmission shaft  70  and the output shaft  56 , at the second pivoting joint  84 . 
         [0071]    With particular reference to the exploded view representation of the coupling linkage mechanism in  FIG. 3 , and with further reference to the sectional illustrations in  FIGS. 4 to 9 , an exemplary embodiment of the coupling linkage mechanism  66 , particularly of the pivoting joints  76 ,  84  thereof, will be further illustrated and described. The cross-sectional view shown in  FIG. 5  is basically perpendicular to a central axis of the driving shaft  54 , refer to the line V-V in  FIG. 4 . The cross-sectional views shown in  FIGS. 6, 7 and 8  are basically perpendicular to a central axis of the transmission shaft  70 , refer to the respective lines VI-VI, VII-VII, and VIII-VIII in  FIG. 4 . The cross-sectional view shown in  FIG. 9  is basically perpendicular to a central axis of the output shaft  56 , refer to the line IX-IX in  FIG. 4 . 
         [0072]    As can be best seen in  FIGS. 3 and 5 , the first driving coupling element  78  may comprise an external polygonal profile  90 . Similarly, as can be best seen in  FIGS. 3 and 8 , the second driving coupling element  86  may comprise an external polygonal profile  190 . The polygonal profiles  90 ,  190  may comprise respective driving flanks  92 ,  192 . By way of example, the external polygonal profile  90  may comprise a basically triangular profile. It goes without saying that respective edges of the polygonal profile  90  may be rounded or chamfered. The flanks  92  of the external polygonal profile  90  may basically extend between respective edges of the polygonal profile  90 . It goes without saying that, in some embodiments, the polygonal profile  90  may be provided with four, five or even further flanks  92 . It is particularly preferred that the polygonal profile  90  is at least sectionally arranged as a spherical polygonal profile. It is particularly preferred that the driving flanks  92  are convexly shaped. As can be best seen in  FIG. 4 , the flanks  92  of the polygonal profile  90  may also comprise a convex axial extension. It is particularly preferred that the polygonal profile  90  is, at least at the flanks  92 , spherically curved such that a defined contact with the first drivable coupling element  80  is achieved even when a considerable angular offset (refer to the angle α in  FIG. 2 ) between the driving shaft  54  and the transmission shaft  70  is present. This may permit a smooth run of the first pivoting joint  76 . 
         [0073]    As can be best seen in  FIG. 3  and  FIG. 4 , the coupling linkage mechanism  66  may further comprise a push rod  96  which may be configured to apply a defined contact force to the driving shaft  54  and the transmission shaft  70 , particularly a substantially axial contact force. By way of example, the push rod  96  may be coupled to a biasing element  94 , particularly to a spring or, more explicitly, to a helical spring. The push rod  96  may be arranged at a recess  98  at the transmission shaft  70 . In other words, the transmission  70  may house the push rod  96 . Also the biasing element  94  may be arranged at the recess  98 . The recess  98  may be arranged as an axially extending recess  98  at the transmission shaft  70 . The biasing element  94  may be arranged in the recess  98  between the push rod  96  and the transmission shaft  70 . Consequently, the biasing element  94  may urge the push rod  96  into contact with a contact front face  100  of the first driving coupling element  78 . Furthermore, the biasing element  94  may urge the transmission shaft  70 , particularly the second driving coupling element  86  thereof, into contact with the second drivable coupling element  88 . Consequently, the drive train  50 , particularly the coupling linkage mechanism  66  thereof, may be axially preloaded which may further contribute to the smooth-running capability of the drive train  50 . As used herein, the term “axially preloaded” shall not be construed as requiring a perfect axial alignment of the respective shafts  54 ,  56  and  70 . Generally, the biasing element  94  and the push rod  96  may compensate length deviations in the coupling linkage mechanism  66 . 
         [0074]    It goes without saying that, at least in some embodiments, the push rod  96  and the biasing element  94  also might be arranged at the driving shaft  54  or at the output shaft  56 . Furthermore, the push rod  96  might also be arranged as an external push rod  94  externally mounted to the transmission shaft  70 . 
         [0075]    The external polygonal profile  90  of the first driving coupling element  78  may engage a corresponding internal polygonal profile  102  at the first drivable coupling element  80 . In the same way, the external polygonal profile  190  at the second driving coupling element  86  may be arranged to engage a corresponding internal polygonal profile  202  at the second drivable coupling element  88 . The internal polygonal profile  102  may be adapted to the external polygonal profile  90  for rotational entrainment. The internal polygonal profile  102  may comprise a plurality of drivable flanks  104  which may be adapted to the number of the driving flanks  92  of the external polygonal profile  90 . However, as can be best seen in  FIG. 7 , the internal polygonal profile  102  may comprise further flanks which are not arranged as drivable flanks  104 . Generally, the internal polygonal profile  102  may comprise (internal) protrusions  106  that alternate with (internal) indentations  108 . The drivable flanks  104  may be arranged at the protrusions  106 . As exemplarily shown in  FIGS. 5, 6 and 7 , the internal polygonal profile  102  may be arranged to receive the triangularly shaped external polygonal profile  90 . However, in the alternative, the internal polygonal profile  102  may also be modified so as to receive a respective external polygonal profile  90  that comprises four, five or even more driving flanks  92 . Also the internal polygonal profile  202  of the second drivable coupling element  88  may comprise respective drivable flanks  204 . The internal polygonal profile  202  may be provided with (internal) protrusions  206  that are alternating with (internal) indentations  208 , wherein the drivable flanks  204  are provided at the protrusions  206  to be contacted by respective driving flanks  192  of the external polygonal profile  190  of the second driving coupling element  86 . Reference is made in this regard to  FIG. 8  and to  FIG. 9 . 
         [0076]    The second pivoting joint  84 , particularly the external polygonal profile  190  and the internal polygonal profile  202  may be shaped and arranged much like the first pivoting joint  76 , particularly the external polygonal profile  90  and the internal polygonal profile  102  thereof. This does not necessarily require that the first pivoting joint  76  and the second pivoting joint  84  need to have similar or equal oval dimensions. As can be seen from  FIGS. 2 to 9 , the first pivoting joint  76  and the second pivoting joint  84  may be different in size. However, it may be also envisaged to provide a respective coupling linkage mechanism  66  with two pivoting joints  76 ,  84  having basically the same size. 
         [0077]    As a result of the at least partially spherical shape of the external polygonal profiles  90 ,  190 , particularly due to the curved (or convexly curved) axial extension of the driving flanks  92 ,  192  thereof, the external polygonal profiles  90 ,  190  may basically swivel with respect to—or roll at—the drivable flanks  104 ,  204  of the internal polygonal profiles  102 ,  202  when the respective coupled shafts  54 ,  56 ,  70  alter their relative angular orientation. Consequently, the drive train  50  may be regarded as self-aligning drive train  50 . 
         [0078]    With further reference to  FIGS. 10 and 11 , an exemplary beneficial embodiment of a blade set  20  for a cutting head  18  will be further illustrated and described. Advantageously, a drive train  50  in accordance with at least some aspects of the present disclosure may be operatively coupled to the blade set  20  for reciprocatingly driving a movable blade  24  thereof with respect to a stationary blade  22 .  FIG. 10  shows a perspective top view of an arrangement of a cutting head  18  wherein a blade set  20  is attached to a contour following mechanism  150 .  FIG. 11  shows a perspective exploded bottom view of the blade set  20 . 
         [0079]    The stationary blade  22  may comprise at least one toothed leading edge  110 , particularly a first toothed leading edge  110  and a second toothed leading edge  110  opposite to the first toothed leading edge  110 . At the at least one toothed edge  110 , a plurality of teeth  112  may be provided. Between respective teeth of the plurality of teeth  112 , tooth slots may be arranged through which hairs may enter the blade set  20  to be cut in a joint operation by the stationary blade  22  and the movable blade  24 . The blade set  20  and a hair cutting appliance  10  fitted with the blade set  20  can be moved through hair in a moving direction  118  to cut hair. The blade set  20  is particularly configured to enable shaving and trimming operations. Shaving may be regarded as cutting hair very close to a user&#39;s skin level. Trimming may be regarded as cutting hair at a desired predefined length with respect to the skin level. 
         [0080]    The stationary blade  22  of the blade set  20  may comprise a first wall portion  122  (refer to  FIG. 10 ) and a second wall portion  124  (refer to  FIG. 11 ). The first wall portion  122  and the second wall portion  124  may be at least partially offset from each other to define a guiding slot therebetween. In the guiding slot, the movable blade  24  may be slidably arranged. The first wall portion  122  may be regarded as a skin-facing wall portion. At the first wall portion  122 , a top surface  126  may be provided. Particularly when a hair cutting appliance  10  fitted with the blade set  20  is used for shaving, the top surface  126  may contact the to-be-shaved skin. 
         [0081]    The first wall portion  122  and the second wall portion  124  may be mutually connected at the at least one toothed leading edge  110  to define the plurality of teeth  112  at the at least one toothed leading edge  110 . Consequently, the teeth  112  may comprise a basically U-shaped cross section, wherein a first leg is formed by the first wall portion  122  and a second leg is formed by the second wall portion  124 . A connector portion between the first leg and the second leg of the U-shaped section may define tips of the teeth  112 . Between the first and the second leg, respective teeth  134  of the movable blade  24  may be arranged. Consequently, the stationary blade  22  may guard or cover the teeth  134  of the movable blade  24 . More particularly, the stationary blade  22  may cover the teeth  134  of the movable blade  24  at a skin-facing side (first wall portion  122 ) and at a side facing away from the skin (second wall portion  124 ). 
         [0082]    In some embodiments, the stationary blade  22  may be formed as a metal-plastic composite stationary blade. Consequently, the stationary blade  22  may comprise a plastic component  130  and a metal component  132 , refer to  FIG. 11 . The plastic component  130  and the metal component  132  may jointly define the shape of the stationary blade  22 . In one embodiment, at least a substantial portion of the first wall portion  122  is formed by the metal component  132 . Furthermore, at least a substantial portion of the second wall portion  124  may be formed by the plastic component  130 . The plastic component  130  and the metal component  132  may be mutually connected at the at least one leading edge  110 . However it is not required that the first wall portion  100  is exactly formed by the metal component  132  and that the second wall portion  124  is exactly formed by the plastic component  130 . By contrast, the plastic component  130  may entirely form the second wall portion  124  and, furthermore, form at least a minor portion of the first wall portion  122 . 
         [0083]    It is particularly preferred that the metal component  132  is arranged as an insert, particularly a sheet metal insert. Consequently, the stationary blade  22  may be obtained from an insert-molding process. This may involve that the plastic component  130  is formed from an injection-moldable plastic material. Molding the plastic component  130  may involve bonding the plastic component  130  to the metal component  132 . The movable blade  24  may be arranged in the guide slot defined by the first wall portion  122  and the second wall portion  124  of the stationary blade  22  in a reciprocatingly movable manner. Reference in this regard is made to the double-arrow denoted by reference numeral  136  in  FIG. 11  indicating the reciprocating motion of the movable cutting blade  24 . Generally, the orientation of the reciprocating motion  136  may be basically perpendicular to the assumed moving direction  118 . 
         [0084]    For driving the movable blade  24 , a transmitting member  138  may be provided. 
         [0085]    The transmitting member  138  may comprise a reciprocating member  140  and a contact bridge  142 . Generally, as shown in  FIG. 10 , the transmitting member  138  may be engaged by the eccentric portion  62 , particularly by an eccentric driving pin  62 , of an output shaft  56  in accordance with at least some aspects of the present disclosure. Rotation of the output shaft  56  (refer to the curved arrow  64  in  FIG. 10 ) may be converted via the eccentric portion  62  and the transmitting member  138  into a reciprocating motion  136 , particularly a rectilinear reciprocating motion, of the movable blade  24 . 
         [0086]    In some embodiments, the eccentric portion  62  may engage at the reciprocating member  140  which may define a guide slot for the eccentric portion  62 . The transmitting member  138  may be further provided with the contact bridge  142  which may be arranged between the reciprocating member  140  and the movable blade  24 . More particularly, the contact bridge  142  may be configured to contact the movable blade  24  for driving the movable blade  24 . By way of example, the contact bridge  142  may be bonded to the movable blade  24 , particularly laser-bonded. 
         [0087]    As can be best seen in  FIG. 10 , the blade set  20  may be attached to a contour following mechanism  150 . To this end, a connector portion  148  may be provided at the stationary blade  22 , particularly at the second wall portion  124  thereof, refer also to  FIG. 11 . The connector portion  148  may comprise at least one snap-on element. The blade set  20  can be detachably attached to the contour following mechanism  150 . The contour following mechanism  150  may comprise at least one hinged pivot mechanism  152 . The at least one hinged pivot mechanism  152  may be arranged at a four bar linkage mechanism. The hinged pivot mechanism  152  may provide the blade set  20  with an improved contour following capability. In other words, the blade set  20  may swivel or pivot with respect to a base of the contour following mechanism  150 . A pivoting motion is illustrated in  FIG. 10  by a curved double arrow indicated by reference numeral  154 . Being provided with the capability to swivel or pivot, the blade set  20  may adapt its actual orientation to the skin shape which may further improve the cutting performance, particularly the shaving performance, of the hair cutting appliance  10 . 
         [0088]    While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. 
         [0089]    In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. 
         [0090]    Any reference signs in the claims should not be construed as limiting the scope.