Patent Publication Number: US-6219920-B1

Title: Dry shaving apparatus

Description:
This case is a divisional case of Ser. No. 09/052,617, filed on Mar. 31, 1998, now U.S. Pat. No. 6,151,780. 
    
    
     This invention relates to a dry shaving apparatus, with an electric motor arranged in a housing and with at least one shaving head comprising two inner cutters which are operatively associated with a common outer cutter and are arranged on a common guide element so as to be drivable by a drive element in relative opposite directions against the force of at least one spring element. 
     From JP 53-63610 (A) a shaving head for a dry shaving apparatus is known in which two inner cutters are slidably guided on two guide elements and drivable in relative opposite directions by an eccentric device disposed between adjacent end walls of the inner cutters, said inner cutters being held in engagement with the eccentric device by means of two spring elements provided outside the contour of the inner cutters. 
     It is an object of the present invention to improve upon a dry shaving apparatus of the type initially referred to, in particular to provide a dry shaving apparatus in which vibration and running noise are largely reduced and which affords economy of manufacture. 
     According to the present invention, this object is accomplished in a dry shaving apparatus of the type initially referred to by the features indicated in claim  1 . 
     The drive mechanism of the present invention which sets the two inner cutters in vibration has a plurality of advantages. The drive mechanism is a self-contained power system which, by reason of its arrangement relative to the housing in a manner imposing zero force, is prevented from transmitting vibrations to the housing. Being completely counterbalanced, the drive mechanism operates in vibration-free fashion. The drive mechanism with its components is not affected by manufacturing tolerances and wear. The spring element(s) acting on the inner cutters, in conjunction with the drive elements driven by the electric motor, provide for permanent compensation for tolerances, converting the kinetic energy of the inner cutters into useful potential energy. In consequence, the drive mechanism consumes less energy than known drive mechanisms setting inner cutters in motion. In cases where a rotary motor is used, the cutting frequency of the inner cutters can be doubled when used in combination with an elliptically shaped rotor. In addition, the drive mechanism is characterized by significantly reduced noise because the spring elements provided also compensate for wear occurring on movable components. Another advantage of the present invention resides in that it is suitable for use in a plurality of dry shavers with different configurations of shaving heads or cutter assemblies. The drive mechanism of the present invention is suited for use in dry shaving apparatus equipped with both one shaving head and several shaving heads. The shaving heads as such may be configured exclusively as short-hair cutters or as long-hair trimmers, and they may be used as a combination of short-hair cutter and long-hair trimmer and driven by the drive mechanism of the present invention. Moreover, it is possible for the shaving heads to be arranged in both fixed and pivotal and floating fashion on the housing of a dry shaving apparatus or in a shaving head frame mountable on the housing of a dry shaver. 
     In a further configuration of the present invention, provision is made for the guide element to be formed by an axle. In a preferred embodiment of the present invention, the guide element is formed by a single axle. In an embodiment of the present invention, the axle forming the guide element is preferably carried with the inner cutters in two bracket elements. The use of bracket elements for carrying the axle of the guide element admits of a plurality of different embodiments within the scope of the present invention. An embodiment of the present invention provides for the axle of the guide element to be movably carried in the bracket elements. Movable within the meaning of this invention is understood to mean any direction of movement and any type of movement. 
     In a preferred embodiment of the present invention, the axle of the guide element is carried in the bracket elements so as to be movable in and in opposition to a vertical direction. This vertical movability of the axle makes it possible, for example, to control the contact pressure necessary to achieve an optimum cutting action as the inner cutters engage the outer cutter, or in another embodiment, to mount the shaving head or any of the shaving heads provided in a floating fashion in and in opposition to the vertical direction. 
     In a further embodiment of the present invention, the bracket elements are spring-mounted, making it possible to control either the contact pressure of the inner cutters against the outer cutter as described in the foregoing, or a floating movement of the shaving head(s) in and in opposition to a vertical direction. 
     In an embodiment of the present invention affording relative ease and economy of manufacture, the bracket elements are fixedly arranged on the housing. 
     In a preferred embodiment of the present invention, a wall element upon which a spring element takes support is provided in each of the inner cutters. In a preferred embodiment of the present invention, a spring element acting in opposition to the driving motion of the drive elements is provided between the two inner cutters. In another embodiment of the present invention, each inner cutter is associated with a spring element resting with one end against a wall element of the inner cutter while its other end is in frictional or positive engagement with the guide element. In a further aspect of this embodiment, the frictional engagement of the spring element with the guide element is made by a stop on the guide element. By means of the frictional engagement of the spring element with the guide element directly, or indirectly by a stop on the guide element, it is ensured that vibrations of the reciprocated masses of the inner cutters are not transmitted to the housing of the dry shaving apparatus. In order to positively preclude any transmission of vibrations to the housing, a further aspect of this embodiment makes provision for a spacing between each bracket element and the adjacent stop on the guide element, thus precluding contact of the stop with the bracket element. 
     In a preferred embodiment of the present invention, each inner cutter is associated with a spring element resting with one end against a wall element of the inner cutter while its other end bears against a common supporting element. The supporting element of the present invention is adapted to be coupled to the guide element in self-centering fashion. In a preferred embodiment of the present invention, the supporting element is a yoke structure slidably coupled to the guide element by means of bearings provided in the two yoke ends. This embodiment of the present invention is eminently suited to ensure at all times self-centering of the inner cutters set in vibration utilizing the spring element(s) acting on the inner cutters as well as the drive elements having one end thereof in engagement with the wall elements of the inner cutters while their other ends engage the expansion means driven by the electric motor. 
     In an embodiment of the present invention, a wall element for the drive element is provided in each of the inner cutters. Preferably, a wall element of an inner cutter is adapted to be acted upon by the drive element on the one hand and by the spring element on the other hand. Using a wall element as supporting element for the spring element and simultaneously as abutment for the drive element has the effect of reducing the mass of the two inner cutters to a relatively low level. 
     In an embodiment of the present invention, the spring element acting on the inner cutters is configured as a compression spring. In another embodiment of the present invention, the spring element acting on the inner cutters is configured as an extension spring. In all embodiments of the invention, the spring elements serve as energy storage converting the kinetic energy of the inner cutters into usable potential energy following motion reversal of the two inner cutters. In addition, the spring tension, in particular a given bias of these spring elements, makes sure that tolerances are permanently compensated for. 
     The movement in relative opposite directions of the inner cutters along the axle serving as guide element against the force of at least one spring element is effected by drive elements which admit of a variety of embodiments. In one embodiment, the drive element is configured as a single-armed lever arranged for pivotal movement about a pin. In another embodiment, the drive element is configured as a double-armed lever arranged for pivotal movement about a pin. A feature common to these two embodiments is that the pin of each pivotally mounted drive element is essentially provided on a vertical line determined by an imaginary extension of a plane wall surface of a wall element of an inner cutter, which wall surface is acted upon by a roll-off cam. 
     In order to largely preclude friction in the transmission of motion from the drive elements to the inner cutters, the drive element is configured as a lever acting via a roll-off cam on a wall element of the inner cutter. In a preferred embodiment of the present invention, the roll-off cam is provided on the drive element configured as lever. In an alternative aspect of this embodiment, the roll-off cam is provided on the wall element of the inner cutter. 
     In an embodiment of the present invention, the roll-off cam is of a symmetrical configuration. In a further embodiment of the present invention, the roll-off cam is of an asymmetrical configuration. The respective curve shape of the roll-off cam is influenced by an expansion means driven by the electric motor and releasing the lever action of the drive elements. In cases where a rotor with an elliptically shaped outer wall is utilized for control of the reciprocating motion of the drive levers acting on the two inner cutters, a symmetrical roll-off cam is preferred at the ends of the drive elements engaging the inner cutters. 
     A further advantageous embodiment of the present invention is characterized in that the drive element is configured as an oscillatory bridge structure. In a further aspect of this embodiment, the drive element is formed by at least two oscillatory bridge structures of identical construction having vibration elements operating in relative opposite directions. For manufacture of the oscillatory bridge structures setting the inner cutters in motion in relative opposite directions, only one injection mold is necessary by reason of the selection of an identical construction, enabling the cost of manufacture to be kept low. 
     In another advantageous embodiment, an oscillatory bridge structure having at least two vibration elements movable in relative opposite directions is provided. Further advantageous configurations of the oscillatory bridge structures suitable for use as drive elements including their associated expansion means are indicated in claims  32  to  36 . 
     In a preferred embodiment of the present invention, an expansion means driven by an electric motor is provided between two adjacent ends of the drive elements pivotally mounted about their respective pins. The electric motor is preferably a direct-current motor. In a preferred embodiment of the present invention, the expansion means is formed by a rotor having an elliptical guideway for the transmission of motion to the drive elements. An alternative aspect of this embodiment is characterized in that the expansion means is formed by a crank drive mechanism. 
     To ensure a good engagement of the inner cutters with the outer cutter and hence obtain good cutting results, the inner cutters are held against the outer cutter by means of at least one spring element. In a further aspect of this approach, the inner cutters are held in engagement with the outer cutter by spring-mounting the bracket elements. 
     In yet another embodiment, each inner cutter is held in engagement with the outer cutter by means of a spring element resting against a housing wall. In an embodiment of the present invention, the outer cutter is movably mounted. The movability of the outer cutter may be achieved by movably securing the outer cutter in a shaving head frame mountable on the housing of the dry shaver, or by movably arranging it in an outer cutter frame, or by movably arranging an outer cutter frame with the outer cutter. In a further aspect of this embodiment, the outer cutter is pivotally mounted. In a still further embodiment, the outer cutter is floatingly mounted in and in opposition to the vertical direction. Mounting the outer cutter movably ensures either a good engagement of the outer cutter with the inner cutters, that is, a good cooperation between these two cutting members, or the formation of movable shaving heads such that these shaving heads are pivotal about a pivot axis or, in another variant, are movable up and down in the shaving head frame or on the housing of the dry shaving apparatus in and in opposition to a vertical direction. 
     In a preferred embodiment of the present invention, the spring element is arranged on the guide element formed by an axle. This arrangement provides for clearly defined allocation and supporting of the spring element relative to the drive mechanism and the reciprocable inner cutters. In a further embodiment of the present invention, at least one guide element configured as an axle is held in a frame carried in the bracket elements. In another aspect of this embodiment, the frame is pivotally mounted in the bracket elements. In another embodiment of the present invention, at least one axle is floatingly carried in the frame. 
    
    
     Some preferred embodiments of the present invention will be described in more detail in the following with reference to the accompanying drawing. In the drawing, 
     FIG. 1 is a sectional view of the upper part of a dry shaving apparatus with two inner cutters arranged on a guide element and drivable in relative opposite directions by two pivotally mounted drive elements under the action of an expansion means driven by an electric motor against the force of a spring element, with the guide element being mounted in bracket elements fixedly disposed on the housing; 
     FIG. 1.1 is a view of an expansion means configured as a rotor; 
     FIG. 2 is a sectional view of a dry shaving apparatus of FIG. 1, showing the guide element carried in spring-mounted bracket elements; 
     FIG. 3 is a view of a dry shaving apparatus of FIG. 2, showing drive elements arranged crosswise and a spring element configured as an extension spring acting on the two inner cutters; 
     FIG. 4 is a view of a dry shaving apparatus of FIG. 2, showing a guide element, movably mounted bracket elements and spring elements acting on the two inner cutters; 
     FIG. 5 is a view of a dry shaving apparatus of FIG. 3, showing inner cutters having their respective ends acted upon by a spring element; 
     FIG. 6 is a view of a dry shaving apparatus substantially according to FIG. 5, showing two inner cutters having their respective ends acted upon by a spring element; 
     FIG. 7 is a view of a dry shaving apparatus of FIG. 5, showing two inner cutters and a U-shaped supporting element embracing the inner cutters, and spring elements disposed between the supporting element and the respective inner cutter; 
     FIG. 8 is a view of a dry shaving apparatus of FIG. 2, showing an expansion means configured as a crank drive mechanism acting on the drive elements; 
     FIG. 9 is a view of a dry shaving apparatus with bracket elements provided on the housing and carrying a frame pivotal about a pivot axis Z, with the relatively movable inner cutters being held in the frame by means of a guide element; 
     FIG. 10 is a view of a dry shaving apparatus substantially according to FIG. 9, showing drive levers acting on wall elements of the inner cutters and transmitting by means of a crank drive mechanism the driving motion to the inner cutters; 
     FIG. 11 is a sectional view of a dry shaving apparatus equipped with a shaving head having a foil movable up and down on the shaving head frame in and in opposition to the vertical direction; 
     FIG. 12 is a sectional view of the upper portion of a dry shaving apparatus of FIG. 9, showing two shaving heads pivotally mounted about a pivot axis, with the parallel inner cutters being drivable by means of a common drive lever utilizing an expansion means; 
     FIG. 13 is a perspective view of the upper part of a dry shaving apparatus showing the shaving head removed; 
     FIG. 14 is a perspective view of an oscillatory bridge structure; and 
     FIG. 15 is a perspective view of two drive elements configured as oscillatory bridge structures which are coupled to an electric motor via crank elements and a double eccentric device. 
    
    
     Referring now to FIG. 1 of the drawing, there is shown a dry shaving apparatus with an electric motor  11  received in a housing  10  and with at least one shaving head SK comprising two inner cutters  1  and  2  operatively associated with a common outer cutter  13  and arranged to be driven by a drive element  5  and  6 , respectively, in relative opposite directions and against the force of at least one spring element  4 . The inner cutters  1  and  2  are slidably guided on a common guide element  3  in and in opposition to a horizontal direction B—directions of arrow B—by means of two wall elements  21  and  210  and, respectively, by means of two wall elements  20  and  200 . The guide element  3  is configured, for example, as an axle  33  and, for the purpose of ensuring a perfect guiding function for the two inner cutters  1  and  2 , is passed through bearing bores  37  provided in the wall elements  20 ,  21 ,  200  and  210 . A spring element  4  configured as a compression spring is disposed between the two opposite wall elements  20  and  21  of the inner cutters  1  and  2 . The drive elements  5  and  6  configured as double-armed levers have roll-off cams  31  integrally formed thereon through which they act on the side of the wall elements  20  and  21  facing away from the spring element  4 . The curvature of the roll-off cam  31  is of such shape that no relative sliding motion occurs on the wall upon which it acts. The two double-armed drive elements  5  and  6  are mounted for pivotal movement about pins  8  and  9 , respectively, which are provided on a pin support  34 . An expansion means  7  admitting of a variety of embodiments is provided between the ends of the drive elements  5  and  6  remote from the inner cutters. The embodiment of FIG. 1 comprises a rotor  53  which is fastened to the motor shaft  55  of an electric motor  11  and whose outer wall engaged by the drive elements  5  and  6  is of an elliptical shape as illustrated in FIG. 1.1. The outer wall of the rotor  53  which serves as a guideway and is in abutment with a respective lever end of the drive elements  5  and  6  is designated by F. FIG. 1.1 is a view of the rotor  53  as seen when rotated through 45° from a mid-position ML. This position of the rotor  53  is maintained in all embodiments shown in which a rotor  53  is provided. This position corresponds to the mid-position of the inner cutters  1 ,  2  in the course of their oscillation in the horizontal direction B. 
     The guide element  3  configured as axle  33  is movably carried in bearings  16  and  17  provided in two bracket elements  14  and  15  integrally formed on the housing  10 , with the bearings  16  and  17  of the axle  33  in the bracket elements  14  and  15  being formed of elongate recesses  35  and  36  extending in a vertical direction A—direction of arrow A. Because the axle  33  is movable within the elongate recesses  35  and  36 , an optimum engagement of the two inner cutters  1  and  2  with the outer cutter  13  is ensured, assisted by the action of the spring elements  18  and  19  bearing against the inner cutters. In addition, the up and down movement of the axle  33  with the inner cutters  1  and  2  slidably arranged thereon in the directions B, which movement is made possible by means of the elongate recesses  35  and  36 , in combination with a movably disposed outer cutter  13 , may be utilized for a floating shaving head SK. 
     For such an embodiment it is only necessary for the outer cutter  13  to be secured in the shaving head frame  12  detachably mounted on the housing  10  such as to be movable in the vertical direction A, or for the outer cutter  13  to be provided in an outer cutter frame that is vertically movably arranged in the shaving head frame  12 . 
     Provided on the guide element  3  or axle  33  are stops  22  and  23  which, upon assembly of the drive mechanism, are spaced from the two bracket elements  14  and  15  at a predetermined distance S. This spacing S is dimensioned such as to preclude contact of the stops  22  and  23  with the respective bracket elements  14  and  15  during operation of the dry shaving apparatus. 
     In the embodiment of FIG. 1, the two drive elements  5  and  6  pivotally mounted about their pins  8  and  9 , respectively, have a respective lever end thereof in engagement with the elliptically shaped outer wall F of the rotor  53 , while their roll-off cams  31  formed on the opposite lever ends engage the wall elements  20  and  21  of the inner cutters  1  and  2 , zero play of this latter engagement being ensured by virtue of the biasing force of the compression spring element toward the wall elements  20  and  21 . The respective pins  8  and  9  of the pivotally mounted drive elements  5  and  6  are each arranged on a vertical line L determined by an imaginary extension of a plane wall surface of the wall elements  20  and  21  of the inner cutters  1  and  2 , which wall surface is acted upon by the roll-off cam  31  and has an orientation perpendicular to the direction of movement of the inner cutters. By means of such a structural design and cooperative relationship of the individual components of the drive mechanism, the two inner cutters  1  and  2  movable in relative opposite directions on the axle  33  serving as guide element  3  are combined to form a self-contained power system which in operation does not permit any contact with the stops  22  and  23  and/or the bracket elements  14  and  15  and, in consequence, does not permit the transmission of any vibration of the oscillating inner cutters to the housing  10  after they are set in operation. This self-contained power system centers itself automatically on the axle  33  forming the guide element  3 , ensuring by means of the biased spring element  4  that any manufacturing tolerances of the components of the drive mechanism are compensated for, consequently causing the drive mechanism to operate without producing any rattling noise. 
     The embodiment of the dry shaving apparatus of FIG. 2 differs from the embodiment of FIG. 1 substantially in that the two bracket elements  14  and  15  are spring-mounted in the housing  10  of the dry shaving apparatus. For this purpose, a respective stop  24  and  25  is provided on the bracket elements  14  and  15  projecting into the housing  10  by means of rods  42  and  43 , such that the bracket elements  14  and  15  are movable in and in opposition to a vertical direction A, under the action of springs  38  and  39  bearing with one end against the stops  24  and  25  while their other ends take support upon an inner wall of the housing  10 . To ensure a parallel guiding of the two rods  42  and  43  and hence of the bracket elements  14  and  15 , the rods  42  and  43  are guided in bearing bores  44 ,  45 ,  46 ,  47  correspondingly provided in two adjacent walls of the housing  10 . For the purpose of limiting the movability of the guide elements  14  and  15  in and in opposition to the vertical direction A, further stops  48  and  49  are provided on the rods  42  and  43 , respectively, their relative distance to the stops  24  and  25  determining the maximum amount of spring travel taking into account the wall thickness of the housing  10  into engagement with which all the stops  24 ,  25 ,  48  and  49  are moved. The axle  33  forming the guide element  3  is carried in bearings  16  and  17  in the two bracket arms  14  and  15 , the bearings being designed to admit of no movement of the axle  33  in and in opposition to the vertical direction A. Consequently, the springs  38  and  39  acting on the bracket elements  14  and  15  via the stops  24  and  25  can be utilized for urging the two inner cutters  1  and  2  against the outer cutter  13 . In cases where the outer cutter  13  is movably arranged in the shaving head frame  12 , the springs  38  and  39  may be used both for urging the two inner cutters  1  and  2  into engagement with the outer cutter  13  and for providing a floating bearing for a shaving head comprising an outer cutter  13  and inner cutters  1  and  2  in and in opposition to the vertical direction A. 
     The embodiment of a dry shaving apparatus of FIG. 3 differs from the embodiment of FIG. 2 essentially in that the drive elements  5  and  6  are mounted on the pin support  34  for pivotal movement about the pins  8  and  9  in a manner crossing each other. For motion transmission from the drive elements  5  and  6  to the inner cutters  1  and  2 , wall elements  200  and  210  are provided in the inner cutters  1  and  2 , such that under the action of the spring element  4  configured as an extension spring and arranged to be in positive engagement with the two wall elements  20  and  21  of the inner cutters  1  and  2 , engagement of the roll-off cams  31  of the drive elements  5  and  6  with the wall elements  200  and  210  is ensured. In view of the relatively close proximity of the wall elements  20 ,  200  and  21 ,  210  of the two inner cutters  1  and  2  and making allowance for the longitudinal dimensions of the two inner cutters  1  and  2 , a further wall element  300  and  310 , respectively, is provided in the inner cutters  1  and  2 , thereby ensuring good guidance and sliding motion of the two inner cutters on the axle  33  forming the guide element  3 . 
     Similar to the spring element  4  of the embodiments of FIGS. 1 and 2 which is configured as a compression spring, the spring element  4  connected to the inner cutters  1  and  2  and configured as an extension spring ensures permanent relative engagement of all movable components of the drive mechanism, that is, engagement of the drive elements  5  and  6  with the wall elements of the inner cutters  1  and  2  and with the rotor  53  driven by the electric motor  11 . This means that self-centering of the power-controlled system is ensured also in this embodiment by reason of the symmetry of all forces acting from outside. 
     The embodiment of the dry shaving apparatus of FIG. 4 largely corresponds to the embodiment of FIG.  2 . In contrast to the embodiment of FIG. 2, in the embodiment of FIG. 4 the two inner cutters  1  and  2  are held in engagement with the outer cutter  13  by means of spring elements  18  and  19 . The spring characteristic of the spring elements  18  and  19  differs from the spring characteristic of the springs  38  and  39 . The spring elements  18  and  19  are weaker than the springs  38  and  39 . As a result, the springs  18  and  19  bearing against a wall of the housing  10  effect a good engagement of the inner cutters  1  and  2  with the outer cutter  13  by exerting a low spring force on the inner cutters  1  and  2 , in addition to substantially reducing the friction of the inner cutters  1  and  2  in their sliding movement on the axle  33 . The springs  38  and  39  which are slightly stiffer serve to provide a floating bearing for a shaving head SK comprising the two inner cutters  1  and  2  and the outer cutter. The floating movement of the shaving head SK in and in opposition to the vertical direction A results from the action of contact forces applied to the outer cutter  13  and their transmission via the outer cutter  13  to the inner cutters  1  and  2  therewith engaged and onwards via the axle  33  carrying the two inner cutters  1  and  2  to the bracket elements  14  and  15 , including the action of the springs  38  and  39  bearing with one end against the housing  10  and with their other end against the bracket elements. 
     The embodiment of the dry shaving apparatus of FIG. 5 corresponds largely to the structural design of the dry shaving apparatus of FIG. 3 from which it is distinguished by incorporating two spring elements in the form of compression springs acting on the inner cutters  1  and  2 , in lieu of a single spring element  4  configured as an extension spring. Provided on the inside of the inner cutters are, for example, four wall elements  20 ,  21  and  200 ,  210 , of which the wall elements  20  and  21  are acted upon by the drive elements  5  and  6  arranged crosswise. 
     Parallel to and at a predetermined distance from the wall elements  20  and  21  are two further wall elements  200  and  210  which are acted upon by a respective spring element  40  and  41  configured as a compression spring. The ends of the spring elements  40  and  41  on the side remote from the wall elements  200  and  210  act upon a respective stop  22  and  23  provided on the guide element  3  or the axle  33 . The axle  33  is movably carried in bearings  16  and  17  formed as bearing bores in the bracket arms  14  and  15 . A spacing S is maintained between the adjacent walls of the bracket arms  14  and  15  and the stops  22  and  23 , its dimension being such as to preclude any contact of the stops  22  and  23  with the bracket arms  14  and  15  when the shaver is in operation. 
     The stops  22  and  23  represent only one embodiment of a support for the spring elements  40  and  41  on the guide element  3  or axle  33 . In another embodiment, this support may also be obtained by connecting an end of the spring elements  40  and  41  to the guide element  3  or axle  33 —not shown. When the electric motor is started, the rotary motion of the motor shaft  55  is transmitted via the rotor  53  to the two drive elements  5  and  6  pivotal about the pins  8  and  9 , and via the roll-off cams provided on the drive elements  5  and  6  to the wall elements  20  and  21  of the inner cutters  1  and  2 , thereby causing movement of the inner cutters  1  and  2  mounted on the guide element  3  against the pressure of the spring elements  40  and  41  in the direction of the bracket arms  14  and  15  until the motion is reversed as predetermined by the rotor  53 , the energy stored in the spring elements  40  and  41  being then released after the motion reversal of the inner cutters  1  and  2  has taken place. 
     The embodiment of the dry shaving apparatus of FIG. 6 corresponds largely to the embodiment of the dry shaving apparatus of FIG.  5 . The embodiment of the dry shaving apparatus of FIG. 5 is distinguished from the embodiment of FIG. 6 in that the embodiment of FIG. 6 incorporates spring elements  40  and  41  in the form of extension springs in lieu of the two spring elements  40  and  41  of FIG. 5 which are configured as compression springs. The spring element acting as extension spring element  40  has one end fixedly connected to the wall element  210  of the inner cutter  1  and its other end to the stop  23 . By contrast, the spring element  41  acting as extension spring has one end thereof fixedly connected to the wall element  200  of the inner cutter  2  and its other end to the stop  22  of the guide element  3 . The embodiment of FIG. 6 is further distinguished by incorporating, in lieu of the crosswise drive elements  5  and  6  of FIG. 5, two double-armed drive elements extending parallel to each other which are pivotally mounted on the pin support  34  by means of pins  8  and  9 . The roll-off cams  31  of the drive elements  5  and  6  roll along the respective insides of the wall elements  20  and  21 . 
     While the embodiment of a dry shaving apparatus of FIG. 7 corresponds largely to the embodiment of FIG. 5, its essential difference resides in the provision of a supporting element  26  in lieu of the provision of stops  22  and  23  on the guide element  3 . The supporting element  26  is a yoke structure embracing both inner cutters  1  and  2  and having in its yoke ends  27  and  28  bearings  29  and  30  configured as plain bearings by means of which the supporting element  26  is horizontally slidably arranged on the guide element  3  formed by an axle  33  in and in opposition to the directions B. To ensure the transmission of motion from the rotor  53  via the double-armed drive elements  5  and  6  to the wall elements  20  and  21  of the inner cutters  1  and  2 , an opening  56 , for example, is provided in the common supporting element  26  of the springs  40  and  41 , through which opening the drive elements  5  and  6  are passed. In the embodiment of FIG. 7, the two spring elements  40  and  41  rest with one end against the wall elements  200  and  210  provided in the inner cutters  1  and  2  while their other ends take support upon the two yoke ends  27  and  28  of the supporting element  26 . The guide element  3  formed by the axle  33  is passed through all wall elements  20 ,  200 ,  21 ,  210  of the inner cutters  1  and  2  and through the bearings  29  and  30  of the yoke ends  27  and  28  of the supporting element  26 , and is movably held in the bracket elements  14  and  15 . The spacing S maintained between the yoke ends  27  and  28  of the substantially U-shaped supporting element  26  and the two bracket elements  14  and  15  ensures an automatic self-centering of the supporting element  26  and the two inner cutters  1  and  2  on the guide element  3 , compensating at the same time for any manufacturing tolerances of the components of the drive mechanism. The spacing S is of such dimension that it precludes any contact of the yoke ends  27  and  28  of the supporting element  26  with the bracket elements  14  and  15  when the shaving apparatus is in operation. 
     FIG. 8 shows an embodiment of a dry shaving apparatus corresponding largely to the embodiment of FIG.  2 . The embodiment of FIG. 8 differs from the embodiment of FIG. 2 in that the two double-armed drive elements  5  and  6  are driven by the motor shaft of an electric motor  11  by means of a crank drive mechanism  54 . The motor shaft  55  of the electric motor  11  is connected to a double eccentric device  58  having eccentric cams  57  and  59  to which respective crank levers  70  and  71  are hingedly connected, the drive elements  5  and  6  being connected by means of drive pins  72  and  73  engaging the crank levers  70  and  71 . 
     The dry shaving apparatus of the embodiment illustrated in FIG. 9 corresponds to the embodiment of FIG. 1 as regards the drive means connected to the electric motor  11  for driving the two inner cutters  1  and  2  slidably mounted on the axle  33  forming the guide element  3  against the pressure of a spring element  4 . In contrast to the embodiment of FIG. 1, a frame  60  is provided in which the guide element  3  is movably held in bearings  16  and  17 . Spring elements  18  and  19  bearing with one end against a wall of the frame  60  and with their other end against the inner cutters  1  and  2  urge the inner cutters  1  and  2  into engagement with the outer cutter  13 . In the embodiment of FIG. 9, the bracket elements  14  and  15  constitute part of the housing  10 . Alternatively, however, they may also be detachably secured to the housing  10 —not shown. Provided at the upper end of the bracket arms  14  and  15  are journal bearings  61  and  62 . By means of journals  63  and  64  in the journal bearings  61  and  62 , the frame  60  is mounted for pivotal movement about a pivot axis Z. As illustrated in FIG. 12, for example, the possibility exists to arrange in the frame  60  several shaving heads SK each having two inner cutters movable in relative opposite directions and operatively associated with at least one outer cutter, together with the spring elements  18  and  19 . The frame  60  largely encompasses the drive mechanism for the inner cutters  1  and  2  formed of the drive elements  5  and  6  and the rotor and connected through at least one opening at least to the motor shaft  55  of the electric motor  11 . Attached to the frame  60  is the pin support  34  carrying the pins  8  and  9  for the drive elements  5  and  6 . 
     FIG. 10 shows an embodiment of a dry shaving apparatus corresponding largely to the embodiment of FIG. 9, the difference being that the inner cutters  1  and  2  arranged in a pivotal frame  60  for sliding movement on a guide element  3  under the action of a spring element  4  are set in a reciprocating movement by single-armed drive elements  5  and  6 . The drive elements  5  and  6  configured as single-armed levers are pivotally mounted on the pins  8  and  9  of a pin support  34  fastened to the housing  10 . The ends of the drive elements  5  and  6  remote from the pins  8  and  9  are provided with a respective roll-off cam  31  acting on a respective wall element  20 ,  21 . The drive pins  72  and  73  are provided on the drive elements  5  and  6  approximately half-way between the axle  33  and the pins  8  and  9  and are connected to the crank levers  70  and  71  of a crank drive mechanism  54 , as explained in the foregoing with reference to the embodiment of FIG.  8 . 
     FIG. 11 shows a side view in section of a dry shaving apparatus according to FIG.  1  and further embodiments previously described. Detachably or fixedly secured to the housing  10  is a shaving head frame  12 . The shaving head frame includes an outer cutter  13  movably mounted in a vertical direction A by means of fastening elements  80  and  81 . Alternatively, the outer cutter  13  may also be fixedly secured to the shaving head frame  12  by means of the fastening elements  80  and  81 . The inner cutter  2  slidably arranged on a guide element  3  or an axle  33  is urged into engagement with the outer cutter  13  by means of a spring element  18 —see FIG.  1 . The axle  33  is passed through a bearing bore  37  provided in the wall element  20  of the inner cutter  2 . The double-armed drive element  6  bears with a lever end against the wall element  20  of the inner cutter  2  to displace the inner cutter  2  against the action of a spring element  4 —see FIG.  1 . The double-armed drive element  6  is pivotally mounted on a pin  9 , such that its end remote from the inner cutter  2 , which is set in motion by a motor shaft of an electric motor  11 , is in abutment with the expansion means  7  configured as rotor  53 . The pin  9  is provided on a pin support  34  secured to a wall of the housing  10  by means of bracket arms  340 ,  341 . 
     FIG. 12 shows a side view of a dry shaving apparatus with a shaving head assembly pivotally mounted about a pivot axis Z and including, for example, two shaving heads SK 1  and SK 2  configured as short-hair cutters arranged parallel to each other in a frame  60 . The frame  60  is connected to a shaving head frame  12  in which, for example, an outer cutter  13  embracing the two inner cutters  2  is fixedly or movably secured in tension. The embodiment of the shaving head assembly of FIG. 12 presents essentially a duplication of the shaving head assembly of FIG. 11, FIG. 12 illustrating and describing a side view of the embodiment of FIG.  9 . In a modification of the representation of FIG. 12, the outer cutter  13  may also be split in the middle, that is, beneath the pivot axis Z, being accordingly of a two-part configuration. 
     By means of journals  63 ,  64  in the bracket elements  14 ,  15 , whereof the journal  64  and the bracket element  15  are illustrated in FIG. 12, the frame  60  is mounted for pivotal movement about the pivot axis Z. The axles  33  forming the guide elements  3  extend through the bearing bore  37  in the wall elements  20  of the inner cutters  2  and have their ends carried in the frame  60 —see FIG.  9 . The fork-shaped drive element  6  which is mounted so as to be pivotal about a pin  9  engages the wall elements  20  of the inner cutters  2  to set these in motion. The end of the double-armed drive element  6  remote from the two inner cutters has a bearing surface LF extending transversely to the vertical center line M for engagement with a rotor  53  driven by a motor shaft  55  of an electric motor  11 . The lateral extent of the bearing surface LF is dimensioned such as to ensure driving of the inner cutters via the drive elements  5  and  6  in the course of the entire pivotal motion of the pivotally mounted shaving head assembly about the pivot axis Z. The embodiment of FIG. 12 shows clearly that further shaving heads SK may be provided in a frame  60  and driven by the drive mechanism described solely by the provision of additional fork ends to the drive elements  5  and  6 . 
     FIG. 13 is a perspective view of the upper part of a dry shaving apparatus, showing a shaving head frame  12  to which an outer cutter  13  is secured removed from the housing  10 . The embodiment of the dry shaving apparatus of FIG. 13 differs from the embodiments of FIGS. 2 and 8 essentially in that the drive elements acting on the inner cutters  1  and  2  are formed by at least one oscillatory bridge structure  500  having at least two vibration elements  550  movable in relative opposite directions. Together with the spring element  4  acting on them, the inner cutters  1  and  2  operatively associated with the common outer cutter  13  are movably arranged on a common guide element  3 . The guide element  3  is carried in bracket elements  14 ,  15  which are either spring-mounted in the housing  10  so as to be movable in and in opposition to the direction of the arrow A—see FIG. 8, or immovably arranged—see FIG.  1 . Holding elements  520  for the spring elements  18  and  19  are provided on the vibration elements  550 , movable in relative opposite directions, of the oscillatory bridge structure  500  in order to maintain the inner cutters  1  and  2  in engagement with the common outer cutter  13  with the requisite contact pressure. 
     Further details of the oscillatory bridge structure  500  will be explained with reference to FIGS. 14 and 15. 
     FIG. 14 shows an oscillatory bridge structure  500  essentially comprising two vibration planes  551  and  552  extending in the horizontal direction of vibration of the inner cutters  1  and  2 , the planes being formed by a step  501  and rigid bridge members  573 ,  574 ,  575  having vibration arms  570 ,  571  and  572  integrally formed thereon. The bridge members  573  and  574  with the vibration arms  570  and  571  are formed on the upper vibration plane  551 , while the bridge member  575  with the vibration arm  572  is formed on the vibration plane  552 . To secure the oscillatory bridge structure  500  directly to the housing  10  or indirectly to another component of the housing  10 , holding elements  576 ,  577  and  578  are formed at the respective ends of the vibration arms  570 ,  571  and  572 , such as to ensure vibration of the vibration planes  551 ,  552  in a plane. A respective recess  553  and  554  is provided in either longitudinal side of the bridge member forming the vibration plane  552 . The dimensions of these lateral recesses  553  and  554 , the relative distance L of the vibration arms  570  and  571 , and the width K of the vibration arm  572  are dimensioned in such manner that two such oscillatory bridge structures  500  can be assembled together via these recesses and relative distances to form an oscillatory bridge unit—see FIG.  15 . 
     Integrally formed on the vibration plane  551  of the oscillatory bridge structure  500  on the side close to the inner cutter are a holding element  520  for a spring element  18  or a spring element  19 , and a motion-transmitting element  505  having a roll-off cam  31  for transmitting motion to an inner cutter and a U-shaped recess  507  for passage of a guide element carrying the inner cutters and the spring element. 
     FIG. 15 shows two oscillatory bridge structures  500  of the type described with reference to FIG. 14 in assembled condition, each of these oscillatory bridge structures  500  being driven by an electric motor  11  and an expansion means  7 . The expansion means  7  is, for example, a double eccentric device  700  having eccentric cams  701  and  702  in mutually opposing arrangement which are connected, through a respective crank element  703 , to the lower vibration plane  552 , closer to the electric motor  11 , of the respective oscillatory bridge structure  500  in order to subject the vibration planes  551  and  552  to oscillation in relative opposite directions, said planes vibrating in a horizontal plane because of the steps  501 . 
     In all embodiments illustrated and described, the transmission of the drive forces of the drive elements  5  and  6  to the inner cutters via the roll-off cam is symmetrical to the axis of symmetry of the inner cutters.