Hair-cutting unit with hair-entry slot having hair-retaining structure - Patents.com

Abstract

A hair-cutting unit 9a has an external cutting member 11a and an internal cutting member 13a rotatable relative to the external cutting member about a central axis. The external cutting member has a hair entry slot 35 with a slot angle α in the range of 0° to 20° at each radial position of the hair entry slot. The internal cutting member has cutting elements 59a, 59b, 59c each having a cutting edge 45a, 45b, 45c with a cutting edge angle β in the range of 55° to 85°. A rear edge 43 of each hair entry slot includes a hair retaining structure 47 having an array of hair abutment surfaces 53 extending transversely thereto and facing the central axis 19.

Description

[Technical field]

[0001] The present invention relates to a hair-cutting unit having a central shaft, an outer cutting member, and an inner cutting member surrounded by the outer cutting member and rotatable relative to the outer cutting member about the central shaft, the external cutting member has a plurality of hair entry slots, each extending from a radially inner slot end position to a radially outer slot end position along a longitudinal extension of the hair entry slot relative to the central axis; the internal cutting member having a plurality of cutting elements, each cutting element having a cutting edge; a slot angle of each hair entry slot when viewed in an axial direction parallel to the central axis is defined as the acute angle enclosed by a trailing edge of the hair entry slot extending along a longitudinal extension of the hair entry slot relative to the direction of rotation of the internal cutting member and a local radial direction relative to the central axis at said trailing edge; The cutting edge angle of each cutting edge is defined as an acute angle enclosed by the cutting edge and a local radial direction relative to the central axis at the cutting edge when viewed in the axial direction, and the cutting edge angle is in the range of 55° to 85°.

[0002] The invention further relates to a shaving unit comprising at least one hair-cutting unit as previously described, further comprising a support structure arranged to support the at least one hair-cutting unit.

[0003] The present invention further relates to a shaving apparatus having a main housing and the aforementioned shaving units coupled to said main housing, said main housing accommodating a drive system configured to rotate an internal cutting member of each hair-cutting unit of said shaving unit relative to said external cutting member. [Background technology]

[0004] Hair-cutting units having an external cutting member and an internal cutting member covered by the external cutting member and rotatable relative to the external cutting member around a central axis of the hair-cutting unit are well known. Such hair-cutting units may be part of a shaving unit of an electric shaving device. The shaving unit may have two or more such hair-cutting units supported by a support structure of the shaving unit. The shaving device may have a main housing that houses an electric motor. The shaving unit may be coupled to the main housing in a permanent or releasable manner. When the shaving unit is coupled to the main housing, the motor is coupled to the internal cutting member of the hair-cutting unit via a transmission system that allows the motor to rotate the internal cutting member relative to the external cutting member. During rotation of the internal cutting member, hairs can enter the external cutting member through a hair entry slot of the external cutting member and be cut by interaction of a cutting edge of the cutting element of the rotating internal cutting member with a counter-cutting edge formed at a rear edge of the hair entry slot of the external cutting member. It will be appreciated that the trailing edge of the hair entry slot of the external cutting member is defined relative to the direction of rotation of the internal cutting member relative to the external cutting member, i.e., during rotation of the internal cutting member, the cutting edge of the cutting element of the internal cutting member first passes the leading edge of the hair entry slot and subsequently passes the trailing edge of the hair entry slot.

[0005] The hair entry slot of the external cutting member generally extends from a radially inner slot end position to a radially outer slot end position along the longitudinal extension of the hair entry slot relative to the central axis. In known hair-cutting units, the hair entry slot may be straight or curved and may have a main direction extending radially relative to the central axis. Examples of hair-cutting units in which the hair entry slot has a V-shaped portion are also known. In general, when viewed in an axial direction parallel to the central axis, the slot angle of the hair entry slot at a given radial position at the hair entry slot may be defined as the acute angle enclosed by the trailing edge of the hair entry slot extending along the longitudinal extension of the hair entry slot and the local radial direction at said given radial position at the hair entry slot relative to the central axis. In known hair-cutting units, the slot angle may be constant or may vary over the entire extension of the hair entry slot.

[0006] In known hair-cutting units, the cutting elements of the internal cutting member may be arranged on the carrier of the internal cutting member in one or more annular arrays around a central axis, and the cutting edges of the cutting elements may be straight or curved and may have a main direction extending in a radial direction. Examples of hair-cutting units are also known in which the cutting edge of the internal cutting member has a V-shaped portion. In general, when viewed in the axial direction, the cutting edge angle of the cutting edge of each cutting element at a given radial position at the cutting edge may be defined as the acute angle enclosed by the cutting edge and the local radial direction relative to the central axis at said given radial position at the cutting edge. In known hair-cutting devices, the cutting edge angle may be constant or may vary over the radial extension of the cutting edge of the internal cutting member. Summary of the Invention [Problem to be solved by the invention]

[0007] A known problem with hair cutting units of the kind mentioned above is so-called excessive skin doming, i.e. the user's skin may bulge too far into the hair entry slot of the external cutting member. While a certain degree of skin doming is necessary to achieve sufficient tightness of the hair cutting process, excessive skin doming may result in skin contact with the rotating internal cutting member, which may cause skin irritation or skin damage. Excessive skin doming may occur when the user applies the shaving unit to the skin with too much pressure. To reduce skin doming and the resulting skin irritation, WO2019 / 211128 proposes a hair cutting unit of the kind mentioned above, in which the slot angle of the hair entry slot of the external cutting member is in the range of 35°-60°. It has been observed that at such a relatively large slot angle, skin doming is significantly reduced or even avoided. In order to sufficiently prevent the displacement of hair in the hair entry slot (hair manipulation) by the cutting element of the rotating internal cutting member and to obtain sufficient hair cutting performance, WO2019 / 211128 proposes a relatively small shear angle in the range of 0°-15° between the cutting edge of the internal cutting member and the counter-cutting edge of the rear edge of the hair entry slot of the external cutting member in combination with the relatively large slot angle described above. In the embodiment disclosed in WO2019 / 211128, both the cutting edge of the internal cutting member and the counter-cutting edge of the external cutting member are straight, so that the shear angle between the cutting edge and the counter-cutting edge is constant over the entire extension of the cutting edge and the counter-cutting edge. Thus, while the hair-cutting unit of WO2019 / 211128 combines sufficient hair-cutting performance with a relatively low degree of skin irritation and hair manipulation, a drawback of this known hair-cutting unit is that the relatively large slot angle of the hair entry slot reduces the hair-catching performance of the hair-cutting unit compared to hair-cutting units in which the hair entry slot has a smaller slot angle and thus a more radial extension relative to the central axis of the hair-cutting unit. The hair-catching performance relates to the degree to which hair can easily enter the hair entry slot of the external cutting member during use.

[0008] It is an object of the present invention to provide a hair-cutting unit of the kind as previously described in the "field of the invention" section, which provides a degree of skin irritation that is at least similar to or less than the degree of skin irritation provided by the hair-cutting unit of WO2019 / 211128 and a hair-cutting performance that is at least similar to or better than the hair-cutting performance of the hair-cutting unit of WO2019 / 211128, but with hair capture performance that is significantly better than the hair-cutting unit of WO2019 / 211128. [Means for solving the problem]

[0009] In order to achieve the above object, the present invention provides a hair-cutting unit having a central shaft, an outer cutting member, and an inner cutting member surrounded by the outer cutting member and rotatable relative to the outer cutting member about the central shaft, wherein: the external cutting member has a plurality of hair entry slots, each extending from a radially inner slot end position to a radially outer slot end position along a longitudinal extension of the hair entry slot relative to the central axis; the internal cutting member having a plurality of cutting elements, each cutting element having a cutting edge; the slot angle of each hair entry slot, when viewed in an axial direction parallel to the central axis, is defined as the acute angle enclosed by a trailing edge of the hair entry slot extending along a longitudinal extension of the hair entry slot relative to the direction of rotation of the internal cutting member and a local radial direction relative to the central axis at said trailing edge; a cutting edge angle of each cutting edge, when viewed axially, is defined as an acute angle enclosed by the cutting edge and a local radial direction at the cutting edge relative to the central axis, the cutting edge angle being in the range of 55° to 85°; a slot angle of each hair entry slot along the longitudinal extension of the hair entry slot is in the range of 0° to 20° at each radial position of the hair entry slot between a radially inner slot end position and a radially outer slot end position; A trailing edge of each hair entry slot includes a hair retaining structure having an array of hair contacting surfaces disposed along the trailing edge, each hair contacting surface extending transversely to the trailing edge and facing the central axis.

[0010] Thus, the hair-cutting unit according to the present invention combines a relatively large cutting edge angle of the cutting edge of the internal cutting member, in the range of 55°-85°, with a relatively small slot angle of the hair entry slot of the external cutting member, in the range of 0°-20°. This combination results in a relatively large angle enclosed by the trailing edge of the hair entry slot and the cutting edge of the internal cutting member. The relatively small slot angle of the hair entry slot, in the range of 0°-20°, results in a significantly improved hair capture performance of the hair-cutting unit compared to the hair-cutting unit of WO2019 / 211128.

[0011] The provision of a hair holding structure with an array of hair abutment surfaces disposed along the rear edge of the hair entry slot results in the following technical effects: During operation, a hair entering the hair entry slot is manipulated by the cutting element of the rotating internal cutting member into abutment with one of the hair abutment surfaces of the hair holding structure extending transversely to the rear edge of the hair entry slot. The hair abutment surface prevents further manipulation of the hair in the hair entry slot by the cutting element in a subsequent hair cutting process. As a result of the relatively large cutting edge angle, the cutting edge of the rotating internal cutting member performs a slicing motion through the hair during the hair cutting process, and the hair is held in a stationary position against the rear edge by the hair abutment surface. Said slicing motion results in improved hair cutting performance compared to hair cutting units, where both the hair entry slot of the external cutting member and the cutting edge of the internal cutting member have a main direction extending radially relative to the central axis, resulting in the cutting edge performing a chopping motion through the hair. The slicing motion also results in a lower degree of wear on the cutting edge of the internal cutting member.

[0012] A further insight provided by the present invention is that, as a result of extending transversely to the trailing edge of the hair entry slot, the hair abutment surface and the trailing edge itself do not need to include a counter-cutting edge. The function of the hair abutment surface is only to maintain the hair in a stationary position within the hair entry slot during the hair-cutting process, while the slicing motion of the cutting edge of the internal cutting member through the hair is sufficient to effectively cut the hair. The absence of any counter-cutting edge in the external cutting member simplifies the manufacturing process of the external cutting member.

[0013] The relatively large cutting edge angle of the cutting edge of the internal cutting member, especially in combination with the presence of the hair retention structure at the trailing edge of the hair entry slot, also limits the degree of skin irritation. The limited degree of skin irritation allows for relatively large skin doming into the hair entry slot. As a result, the hair entry slot can have a relatively large width, which further enhances the hair capture performance of the external cutting member.

[0014] It should be noted that although in the description and claims the cutting edge angles and slot angles are defined as having positive values ​​in the ranges of 55° to 85° and 0° to 20°, respectively, the cutting edge angles and slot angles can each have either a positive or negative orientation relative to the local radial direction at the cutting edge and trailing edge, respectively, when viewed axially.

[0015] In an embodiment of the hair-cutting unit according to the invention, when viewed in the axial direction, the shear angle enclosed by each hair abutment surface and the cutting edge of each cutting element at said hair abutment surface is in the range of 0° to 15°. With said shear angle in this range, the hair is optimally maintained in a stationary position in the hair entry slot by the hair abutment surface of the hair holding structure during the hair cutting process. More preferably, said shear angle is in the range of 0° to 5°. In particular, each hair abutment surface may extend substantially parallel to the cutting edge of each cutting element when present at the hair abutment surface.

[0016] In a further embodiment of the hair-cutting unit according to the invention, the cutting edge angle is in the range of 70° to 80°. With the cutting edge angle in this range, a relatively small angle in the range between 10° and 20° is enclosed by the cutting edge and the local direction of movement of the cutting edge in the hair entry slot which is tangential to the central axis. This results in a particularly low degree of skin irritation, while cutting of the hair by a slicing movement of the cutting edge through the hair is very effective.

[0017] In a preferred embodiment of the hair-cutting unit according to the present invention, the slot angle is in the range of 0° to 5°. With the slot angle in this range, the hair-capturing performance of the hair-cutting unit can be further improved.

[0018] In a practical embodiment of the hair-cutting unit according to the invention, the hair holding structure has an arrangement of tooth-like elements arranged along at least one rear edge of the hair entry slot, where the hair abutment surface is formed by a first side edge surface of an individual one of the tooth-like elements facing the central axis. The hair holding structure can thus have a comb-like structure provided at the rear edge. The hair entering the hair entry slot is manipulated by the cutting element of the rotating internal cutting member into one of the spaces present between successive pairs of adjacent tooth-like elements and is cut by the cutting element while abutting against said first side edge surface of one of the tooth-like elements delimiting said space. By providing a relatively large number of tooth-like elements along the rear edge, the hair holding performance of the hair holding structure during the hair-cutting process can be optimized. The second side edge surface of each individual tooth-like element facing away from the central axis can extend substantially parallel to the first side edge surface of the individual tooth-like element. This optimizes the hair capture performance of the hair retention structures provided along the trailing edge.

[0019] In one embodiment of the hair-cutting unit according to the invention, the leading edge of the hair entry slot relative to the direction of rotation of the internal cutting member extends as a straight line from the radially inner slot end position to the radially outer slot end position at a distance from the tooth-like elements. In this embodiment, the hair entry slot has a first region extending as a continuous open slot region along the leading edge from the radially outer slot end position to the radially inner slot end position and a second region along the trailing edge in which the array of tooth-like elements of the hair holding structure is arranged. In this embodiment, the hair is first effectively trapped in said first region of the hair entry slot and can then be efficiently manipulated by the cutting elements of the rotating internal cutting member into said second region of the hair entry slot and held in place by the tooth-like elements of the hair holding structure.

[0020] In another embodiment of the hair-cutting unit according to the invention, the leading edge of the hair entry slot relative to the direction of rotation of the internal cutting member has a further hair holding structure with an arrangement of further tooth-like elements arranged along the leading edge, each further tooth-like element having a further hair abutment surface extending transversely to the leading edge and formed by a side edge surface of the further tooth-like element facing the central axis. In this embodiment, the hair can also be cut while abutting against the further hair abutment surface of the further tooth-like element of the further hair holding structure provided along the leading edge of the hair entry slot. Although the presence of the further tooth-like elements in the hair entry slot may slightly reduce the overall hair capture performance of the hair entry slot, the presence of the further hair holding structure improves the overall hair holding performance of the hair entry slot during the hair cutting process. In one embodiment, the tooth-like elements of the hair holding structure at the rear edge of the hair entry slot and the further tooth-like elements of the further hair holding structure at the leading edge of the hair entry slot can be arranged in a staggered and partially overlapping configuration with respect to each other when viewed in the radial direction. As a result, the open slot area between the hair retention structure at the trailing edge and the further hair retention structure at the leading edge can extend in a zigzag manner from a radially outer slot end position to a radially inner slot end position, which significantly increases the overall hair retention performance of the hair entry slot during the hair-cutting process.

[0021] In a particular embodiment of the hair-cutting unit according to the invention, the ratio of the first radial distance between the radially inner slot end position and the central axis to the second radial distance between the radially outer slot end position and the central axis is between 0.4 and 0.8. In practice, the radially outer slot end position is at or very close to the outer periphery of the external cutting member. Thus, in this particular embodiment, the hair entry slot can extend over a significant portion of the radius of the external cutting member, i.e. between about 20% and about 60%. This results in a relatively high hair capture performance of the hair entry slot. A preferred range of the above ratio is between 0.5 and 0.6, which in practice may mean that the hair entry slot extends over a radial distance between about 40% and about 50% of the radius of the external cutting member.

[0022] It is to be understood that the practically acceptable minimum value of the ratio of said first radial distance and said second radial distance may depend on the cutting edge angle, in particular in a preferred embodiment of the hair-cutting unit according to the invention, the cutting edge of each cutting element of the internal cutting member extends as a continuous curve from a radially inner slot end position or a radially adjacent position to a radially outer slot end position or a radially adjacent position. In particular in this preferred embodiment, a relatively large cutting edge angle may result in a relatively high value of said ratio, i.e. a relatively short radial extension of the hair entry slot, while a smaller value of the cutting edge angle allows a smaller value of said ratio, i.e. a larger radial extension of the hair entry slot. In this preferred embodiment, the number N of cutting elements of the internal cutting member may be in the range of 2 to 6, preferably 3 or 4, and the cutting edge of each cutting element may extend over an angle of 360° / N around the central axis.

[0023] In a further embodiment of the hair-cutting unit according to the invention, the cutting edges of the cutting elements of the internal cutting member each have a radially inner edge end position that is closer to the central axis than the radially inner slot end position of the respective hair entry slot. As a result, during rotation of the internal cutting member, the user's skin is not exposed via the hair entry slot to the radially inner edge end position of the cutting edge leading the cutting edge in the direction of rotation of the internal cutting member. In particular, the radially inner edge end position of the cutting edge of the internal cutting member is always covered by the central skin contact surface of the external cutting member. Thus, skin irritation due to skin contact with the leading radially inner edge end position is prevented.

[0024] To further improve the hair capturing performance of the hair entry slot, in a further embodiment of the hair-cutting unit according to the invention the hair entry slot is open radially outwardly at a radially outer slot end position.

[0025] A shaving unit according to the present invention comprises at least one hair-cutting unit according to any embodiment as described above, and further comprises a support structure configured to support the at least one hair-cutting unit.

[0026] A shaving apparatus according to the present invention comprises a main housing and a shaving unit according to the present invention as described above, said shaving unit being coupled to said main housing, said main housing housing a drive system configured to rotate and drive an internal cutting member of each hair-cutting unit of said shaving unit relative to said external cutting member.

[0027] The above-mentioned and other aspects of the invention will become apparent from and will be elucidated with reference to the following detailed description of embodiments of a hair-cutting unit, a shaving unit and a shaving apparatus according to the invention. [Brief description of the drawings]

[0028] [Figure 1]1 shows an embodiment of a shaving device according to the present invention; [Diagram 2] 2 shows an embodiment of a shaving unit according to the present invention included in the shaving apparatus of FIG. 1; [Diagram 3] 3 shows an embodiment of a hair-cutting unit according to the invention comprised by the shaving unit of FIG. 2; [Figure 4] 4 shows a drive spindle of the drive system of the shaving apparatus of FIG. 1 in driving engagement with the hair-cutting unit of FIG. 3; [Diagram 5] FIG. 4 shows an external cutting member of the hair-cutting unit of FIG. 3. [Figure 6] FIG. 4 shows an internal cutting member of the hair-cutting unit of FIG. 3. [Figure 7] 7 shows a portion of a hair entry slot of the external cutting member of FIG. 5 and a portion of a cutting element of the internal cutting member of FIG. 6. [Figure 8] 8 is a detailed diagram illustrating the slicing motion of the cutting edge of the cutting element shown in FIG. 7 through a hair held in place by the hair retention structure of the hair entry slot shown in FIG. 7. [Figure 9] FIG. 2 shows an external cutting member of a second embodiment of a hair-cutting unit according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] The invention will now be explained in more detail with reference to the figures, in which equivalent or similar features are designated with the same reference numbers.

[0030] FIG. 1 shows an embodiment of a shaving device 1 according to the invention. The shaving device 1 comprises a main housing 3 designed to be held in the hand of a user during operation. The shaving device 1 further comprises a shaving unit 5 according to the invention coupled to the main housing 3. The shaving unit 5 comprises a support structure 7 and three hair-cutting units 9a, 9b, 9c according to the invention. The support structure 7 is configured to support the three hair-cutting units 9a, 9b, 9c. The support structure 7 may comprise a coupling structure, not shown, of a type well known to a person skilled in the art, for releasably coupling the shaving unit 5 to the main housing 3. Alternatively, the shaving unit 5 may be permanently connected to the main housing 3. It is noted that the shaving unit according to the invention may comprise a different number of hair-cutting units according to the invention, for example one, two or more than two hair-cutting units.

[0031] Fig. 2 shows the shaving unit 5 in a top view. The three hair-cutting units 9a, 9b, 9c are arranged in a triangular shape and each have an external cutting member 11a, 11b, 11c and an internal cutting member 13a, 13b, 13c which is covered by the external cutting members 11a, 11b, 11c and therefore only partially visible in Fig. 2. Furthermore, each hair-cutting unit 9a, 9b, 9c has a support member 15a, 15b, 15c, respectively, which is configured to support the respective external cutting member 11a, 11b, 11c and the respective internal cutting member 13a, 13b, 13c and has a skin contact surface 17a, 17b, 17c surrounding the respective external cutting member 11a, 11b, 11c. The support members 15a, 15b, 15c may be pivotable relative to one another in a manner known to those skilled in the art, so that during use of the shaving apparatus 1 the orientation of the external cutting members 11a, 11b, 11c and the skin contacting surfaces 17a, 17b, 17c of the support members 15a, 15b, 15c may be adjusted to suit the local contours of the user's skin.

[0032] FIG. 3 shows the external cutting member 11a and the internal cutting member 13a of the hair-cutting unit 9a in more detail, where only half of the external cutting member 11a is shown so that the internal cutting member 13a (covered by the external cutting member 11a) is visible to a greater extent. The hair-cutting units 9b and 9c are identical to the hair-cutting unit 9a. The hair-cutting unit 9a has a central axis 19, around which the internal cutting member 13a is rotatable relative to the external cutting member 11a. For this purpose, the external cutting member 11a has a cylindrical bearing pin 21 arranged coaxially with the central axis 19 inside the upper wall 23 of the external cutting member 11a. The internal cutting member 13a has a bearing bush 25 that receives the bearing pin 21 of the external cutting member 11a. The external cutting member 11a further has a cylindrical wall 26 that connects to the outer periphery of the upper wall 23. The top wall 23 and the cylindrical wall 26 together cause the outer cutting member 11a to be cupped and cover the inner cutting member 13a.

[0033] The shaving apparatus 1 comprises a drive system arranged to rotationally drive the internal cutting members 13a, 13b, 13c of each hair-cutting unit 9a, 9b, 9c of the shaving unit 5 relative to the respective external cutting members 11a, 11b, 11c. The drive system is not visible in Fig. 1 but may be of a type known to those skilled in the art. In particular, the drive system may comprise an electric motor arranged in the main housing 3, three rotatable drive spindles arranged in the support structure 7 of the shaving unit 5, each spindle being coupled to a respective one of the internal cutting members 13a, 13b, 13c, and a transmission system arranged in the main housing 3 and in the support structure 7 of the shaving unit 5, via which the electric motor can rotationally drive the three drive spindles. Fig. 4 shows one of the drive spindles 27 of the drive system, which is in driving engagement with the internal cutting member 13a of the hair-cutting unit 9a. The drive spindle 27 has a coupling head 29 which engages in a coupling cavity 31 of the internal cutting member 13a. The coupling cavity 31 is provided on a carrier 33 of the internal cutting member 13a, which is located in the center of the internal cutting member 13a. The carrier 33 also carries the bearing bush 25 of the internal cutting member 13a described above.

[0034] Figure 5 shows the external cutting member 11a of the hair-cutting unit 9a in a top view, i.e. viewed in an axial direction parallel to the central axis 19. The top wall 23 of the external cutting member 11a has a plurality of hair entry slots 35, each extending along the longitudinal extension of the hair entry slot relative to the central axis 19 from a radially inner slot end position 37 to a radially outer slot end position 39. Viewed in the axial direction, the radially inner slot end position is the radially innermost point of the hair entry slot 35 relative to the central axis 19 and the radially outer slot end position is the radially outermost point of the hair entry slot 35 relative to the central axis 19. In the embodiment shown in Figure 5, the external cutting member 11a has 32 hair entry slots 35 equally spaced from one another around the central axis 19. Alternatively, the external cutting member can have a different number of hair entry slots, and the hair entry slots may be irregularly distributed around the central axis 19. In the embodiment shown in Figure 5, the inner slot end positions 37 of the hair entry slots 35 are equal distances from the central axis 19, but alternatively the inner slot end positions 37 may be different distances from the central axis 19. Similarly, in the embodiment shown in Figure 5, the outer slot end positions 39 of the hair entry slots 35 are equal distances from the central axis 19, but alternatively the outer slot end positions 39 may be different distances from the central axis 19.

[0035] As shown in Figure 5, the leading edge 41 and the trailing edge 43 of each hair entry slot 35 both extend along the longitudinal extension of the hair entry slot 35 and are both defined relative to the direction of rotation of the internal cutting member 13a indicated by the arrow R. That is, during rotation of the internal cutting member 13a in the direction of rotation R, when a particular portion of the internal cutting member 13a, in particular a cutting edge 45a of the internal cutting member 13a described in more detail below, passes through the hair entry slot 35, said particular portion first passes through the leading edge 41 of the hair entry slot 35 and then passes through the trailing edge 43 of the hair entry slot 35. When viewed axially as shown in Figure 5, the slot angle α of each hair entry slot 35 is defined as the acute angle enclosed by the trailing edge 43 of the hair entry slot 35 and the local radial direction relative to the central axis 19 at the trailing edge 43. In the embodiment of FIG. 5, the slot angle α of the hair entry slot 35 is 0° because the trailing edge 43 extends radially relative to the central axis 19. In accordance with the present invention, the slot angle α of each hair entry slot 35 along the longitudinal extension of the hair entry slot 35 ranges from 0° to 20° at each radial position of the hair entry slot 35 between the radially inner slot end position 37 and the radially outer slot end position 39. The slot angle α may be the same at each radial position of the hair entry slot 35. Alternatively, the slot angle α is different at different radial positions of the hair entry slot 35. As in the embodiment of FIG. 5, the trailing edge 43 of the hair entry slot 35 may be straight, but alternatively the trailing edge 43 may be slightly curved.

[0036] As further shown in Fig. 5, the rear edge 43 of each hair entry slot 35 of the external cutting member 11a is provided with a hair retaining structure 47. In the embodiment shown in Fig. 5, the hair retaining structure 47 has an array of tooth-like elements 49 arranged along the rear edge 43 of each hair entry slot 35. A first side edge surface 51 of each tooth-like element 49 facing the central axis 19 extends transversely to the rear edge 43 and forms a hair abutment surface 53 of the hair retaining structure 47 that functions in a manner described in detail below. Thus, the hair retaining structure 47 has an array of hair abutment surfaces 53 arranged along the rear edge 43, each hair abutment surface 53 extending transversely to the rear edge 43 and facing the central axis 19. A second side edge surface 55 of each tooth-like element 49 of the hair retaining structure 47 faces away from the central axis 19 and extends substantially parallel to the first side edge surface 51. The leading edge 41 of each hair entry slot 35 extends in a straight line from a radially inner slot end location 37 to a radially outer slot end location 39, spaced from the tooth elements 49 of the hair retention structure 47 at the trailing edge 43. Additionally, as best seen in Figure 3, the hair entry slots 35 open radially outwardly at the radially outer slot end location 39.

[0037] Figure 6 shows the internal cutting member 13a of the hair-cutting unit 9a in a top view, i.e. in an axial direction parallel to the central axis 19. As shown in Figures 3 and 6, the internal cutting member 13a has a cutter body 57 with three cutting elements 59a, 59b, 59c. The cutter body 57 is fixed to the carrier 33 of the internal cutting member 13a described above, which is located in a central opening 61 of the cutter body 57. The cutting elements 59a, 59b, 59c are formed as ridges 63a, 63b, 63c, respectively, which project relative to an upper surface 65 of the cutter body 57. The upper periphery of each ridge 63a, 63b, 63c forms a cutting edge 45a, 45b, 45c of a respective cutting element 59a, 59b, 59c. Each cutting edge 45a, 45b, 45c extends as a continuous curve from a radially inner edge end location 67 to a radially outer edge end location 69 over an angle of 120° relative to the central axis 19. The radially inner edge end location 67 of the cutting edges 45a, 45b, 45c has a radial location relative to the central axis 19 that is at or near the radial location of the radially inner slot end location 37 of the hair entry slot 35. The radially outer edge end location 69 of the cutting edges 45a, 45b, 45c has a radial location relative to the central axis 19 that is near the radial location of the radially outer slot end location 39 of the hair entry slot 35. Viewed axially as shown in Figure 6, the cutting edge angle β of each cutting edge 45a, 45b, 45c is defined as the acute angle enclosed by the cutting edge 45a, 45b, 45c and the local radial direction 44 at said cutting edge 45a, 45b, 45c relative to the central axis 19. According to the invention, the cutting edge angle β ranges from 55° to 85° at any location of the cutting edge 45a, 45b, 45c.

[0038] FIG. 7 shows a portion of the hair entry slot 35 of the external cutting member 11a and a portion of the cutting element 59a of the internal cutting member 13a as the cutting edge 45a of the cutting element 59a passes through the hair entry slot 35 during operation of the shaving device 1. The shear angle γ enclosed by the hair abutment surface 53 of the toothed element 49 of the hair holding structure 47 and the cutting edge 45a when present at said hair abutment surface 53, as viewed in an axial direction parallel to the central axis 19 as in FIG. 7, is 0° in the embodiment shown in FIG. 7. In other words, at the position of the toothed element 49, the hair abutment surface 53 extends parallel to the cutting edge 45a, as shown in FIG. 7. According to the present invention, said shear angle γ is in the range of 0° to 15°.

[0039] During operation of the shaving device 1, the internal cutting members 13a, 13b, 13c rotate relative to the external cutting members 11a, 11b, 11c. In the following, the operation of the shaving device 1 is described with reference to the hair-cutting unit 9a with the external cutting member 11a and the internal cutting member 13a. The operation of the hair-cutting units 9b and 9c is similar. During operation, the cutting edges 45a, 45b, 45c perform a sliding movement along the inside of the angular portion of the upper wall 23 of the external cutting member 11a, in which the hair entry slot 35 is provided. As the cutting edges 45a, 45b, 45c are provided on narrow ridges 63a, 63b, 63c protruding relative to the upper surface 65 of the cutter body 57, an optimal sliding contact is achieved between the cutting edges 45a, 45b, 45c and the inside of said angular portion of the upper wall 23 of the external cutting member 11a.

[0040] During operation, hair on the user's skin enters the hair entry slot 35 of the external cutting member 11a. A relatively small slot angle α of the hair entry slot 35 in the range of 0°-20° results in a high hair-catching performance of the hair-cutting unit 9a, i.e. a high hair-accepting capacity of the hair entry slot 35. With regard to said hair-catching performance, a preferred range of the slot angle α of the hair entry slot 35 is 0°-5°. As a result of the fact that the hair entry slot 35 opens radially outward at the radially outer slot end position 39, as shown in FIG. 3, the hair-catching performance is further increased.

[0041] A hair that has entered one of the hair entry slots 35 is manipulated by one of the cutting elements 59a, 59b, 59c of the rotating internal cutting member 13a into one of the spaces 71 that exist between a pair of adjacent tooth elements 49 of the hair holding structure 47 at the rear edge 43 of the hair entry slot 35. This manipulation of the hair is the result of the fact that the cutting elements 59a, 59b, 59c move from the leading edge 41 towards the trailing edge 43 of the hair entry slot 35 in combination with the fact that the cutting edge angle β of the cutting edges 45a, 45b, 45c is less than 85°. After being manipulated into one of the spaces 71 of the hair holding structure 47, the hair is manipulated into position against the hair abutment surfaces 53 of the tooth elements 49 that surround the space 71. The hair abutment surface 53 prevents further manipulation of the hair in the hair entry slot 35 by the cutting elements 59a, 59b, 59c during the subsequent hair cutting process, particularly as a result of friction between the hair and the hair abutment surface 53.

[0042] 5 and 7, the hair entry slot 35 has a first region 73 extending as a continuous open slot region along the leading edge 41 from the radially outer slot end position 39 to the radially inner slot end position 37, and a second region 75 along the trailing edge 43 in which the array of tooth elements 49 of the hair retaining structure 47 are disposed. In this embodiment, the hair is first effectively trapped in said first region 73 of the hair entry slot 35 and then efficiently manipulated by the cutting elements 59a, 59b, 59c of the rotating internal cutting member 13a into said second region 75 of the hair entry slot 35 and into contact with the hair abutment surface 53 of the hair retaining structure 47.

[0043] As a result of the relatively large cutting edge angle β of the cutting edges 45a, 45b, 45c, the cutting edges 45a, 45b, 45c of the rotating internal cutting member 13a perform a slicing movement through the hair during the hair cutting process, which is held in a stationary position in abutment against the hair abutment surface 53 of the hair holding structure 47. This slicing movement is shown diagrammatically in FIG. 7, which shows the cutting edge 45a and a hair 77 held in place by the hair holding structure 47 and partially cut by the cutting edge 45a. The slicing movement of the cutting edge 45a through the hair 77 is shown in detail in FIG. 8. The local direction of movement of the cutting edge 45a through the hair 77 is indicated by the arrow M in FIGS. 7 and 8, which is tangential to the central axis 19. The acute angle δ enclosed by the cutting edge 45a and the arrow M at the location of the hair 77 is equal to 90°-β. Because the angle δ is relatively small, the local velocity of the cutting edge 45a passing through the hair 77 in a direction parallel to the cutting edge 45a (i.e., the slicing motion velocity V SM ) is much greater than the local velocity of the cutting edge 45a through the hair 77 in a direction perpendicular to the cutting edge 45a (i.e. the chopping motion velocity VCM shown in FIG. 8). Said relatively high slicing motion velocity VSM of the cutting edge 45a through the hair 77 results in an improved hair cutting performance compared to hair-cutting units in which the hair entry slot of the outer cutting member and the cutting edge of the inner cutting member both have a main direction extending radially relative to the central axis, and as a result the cutting edges perform substantially only a chopping motion through the hair. Furthermore, as a result of said slicing motion of the cutting edges 45a, 45b, 45c through the hair, the cutting process of a single hair is performed by a relatively large portion of the cutting edges 45a, 45b, 45c. This results in a relatively lower degree of wear of the cutting edges 45a, 45b, 45c.

[0044] As mentioned above, the only function of the hair contacting surface 53 of the hair holding structure 47 is to maintain the hair in a stationary position in the hair entry slot 35 during the hair cutting process. To this end, in the above-described embodiment, the shear angle γ enclosed by each hair contacting surface 53 and the cutting edges 45a, 45b, 45c when present at said hair contacting surface 53 is 0°. That is, each hair contacting surface 53 extends substantially parallel to the cutting edges 45a, 45b, 45c when present at the hair contacting surface 53. In general, to optimally maintain the hair in a stationary position in the hair entry slot 35 by the hair contacting surface 53 of the hair holding structure 47 during the hair cutting process, the shear angle γ should be in the range of 0° to 15°. More preferably, the shear angle γ is in the range of 0° to 5°. As previously mentioned, the trailing edge 43, the toothed elements 49 and especially the hair abutting surface 53 of the hair holding structure 47 do not need to be provided with a counter-cutting edge since the hair is effectively cut solely by the slicing motion of the cutting edges 45a, 45b, 45c through the hair. The absence of any counter-cutting edge in the external cutting members 11a, 11b, 11c simplifies the manufacturing process of the hair-cutting units 9a, 9b, 9c.

[0045] The relatively large cutting edge angle β of the cutting edges 45a, 45b, 45c of the internal cutting members 13a, 13b, 13c, particularly in combination with the presence of the hair retention structure 47 at the rear edge 43 of the hair entry slot 35, limits the degree of skin irritation. The limited degree of skin irritation allows for relatively large skin doming into the hair entry slot 35. As a result, the hair entry slot 35 can have a relatively large width. In particular, the first region 73 of the hair entry slot 35 can have a relatively large width in a direction perpendicular to the radial direction. This further improves the hair capture performance of the external cutting members 11a, 11b, 11c.

[0046] In the embodiment described above, the cutting edge angle β is 75°. It will be clear to those skilled in the art that the extent to which the movement of the cutting edges 45a, 45b, 45c through the hair is a slicing movement increases with increasing cutting edge angle β. However, at cutting edge angles close to 90°, the component of movement of the cutting edges 45a, 45b, 45c perpendicular to the cutting edges 45a, 45b, 45c is too small for an effective hair cutting process. Therefore, the upper limit of the range of cutting edge angles β according to the present invention is 85°. The lower limit of the range of cutting edge angles β according to the present invention is 55°. With a cutting edge angle β of 55°, the component of slicing movement of the cutting edges 45a, 45b, 45c through the hair (i.e. the component of movement parallel to the cutting edges 45a, 45b, 45c) is still large enough to achieve an effective hair cutting process without the need for a counter cutting edge at the external cutting members 11a, 11b, 11c. The preferred range of cutting edge angles β is 70° to 80°. With the cutting edge angle β in this range, a particularly low degree of skin irritation is achieved, whilst the slicing motion of the cutting edge through the hair makes cutting of the hair very effective.

[0047] Due to the presence of the tooth elements 49, the hair retention structure 47 provided along the rear edge 43 of the hair inlet slot 35 in the previously described embodiment has a comb-like structure, which allows for effective manipulation of the hair by the cutting elements 59a, 59b, 59c of the rotating internal cutting member 13a, 13b, 13c into one of the spaces 71 present between successive pairs of adjacent tooth elements 49. By providing a relatively large number of tooth elements 49 along the rear edge 43, the hair retention performance of the hair retention structure 47 during the hair cutting process can be improved. The second side edge surface 55 of each tooth element 49 facing away from the central axis 19 may extend substantially parallel to the first side edge surface 51 as in the previously described embodiment, or may extend non-parallel to the first side edge surface 51.

[0048] Figure 9 shows, in a top view similar to Figure 5, an external cutting member 111 of a second embodiment of a hair-cutting unit according to the invention. In this second embodiment, the hair-cutting unit has an internal cutting member similar to the internal cutting member 13a of the hair-cutting unit 9a according to the previous embodiment. The internal cutting member, not visible in Figure 9, is covered by the external cutting member 111 and is rotatable relative to the external cutting member 111 about the central axis 119 of the hair-cutting unit. The external cutting member 111 has a number of hair entry slots 135. Each slot extends along the longitudinal extension of the hair entry slot 135 relative to the central axis 119 from a radially inner slot end position 137 to a radially outer slot end position 139.

[0049] Similar to the embodiment of the external cutting member 11a in Fig. 5, the leading edge 141 and the trailing edge 143 of each hair entry slot 135 of the external cutting member 111 of the second embodiment of the hair-cutting unit each extend as a straight line along the longitudinal extension of the hair entry slot 135 with a main extension direction radial to the central axis 119. However, unlike the embodiment of the external cutting member 11a, the leading edge 141 and the trailing edge 143 of the external cutting member 111 do not extend exactly in the radial direction. As a result, as shown in Fig. 9, each hair entry slot 135 has a non-zero slot angle α bounded by the trailing edge 143 of the hair entry slot 135 and the local radial direction 144 at the trailing edge 143 with respect to the central axis 119. Since the trailing edge 143 extends as a non-radial straight line, the slot angle α is different at different positions along the trailing edge 143, i.e. along the longitudinal extension of the hair entry slot 135. In particular, the slot angle α increases from a minimum at the radially outer slot end position 139 to a maximum at the radially inner slot end position 137. Here, in accordance with the present invention, the slot angle α along the longitudinal extension of the hair entry slot 135 is a maximum of 20° at each radial position of the hair entry slot 135 between the radially inner slot end position 137 and the radially outer slot end position 139.

[0050] Similar to the embodiment of the external cutting member 11a of Fig. 5, the rear edges 143 of the hair entry slots 135 of the external cutting member 111 of the second embodiment of the hair-cutting unit each comprise a hair retaining structure 147 having an array of tooth-like elements 149 arranged along the rear edge 143. A first side edge surface 151 of each tooth-like element 149 facing the central axis 119 extends transversely to the rear edge 143 and forms a hair abutment surface 153 of the hair retaining structure 147. Thus, the hair retaining structure 147 at the rear edge 143 has an array of hair abutment surfaces 153 arranged along the rear edge 143, where each hair abutment surface 153 extends transversely to the rear edge 143 and faces the central axis 119. A second side edge surface 155 of each tooth element 149 of the hair retention structure 147 at the trailing edge 143 faces away from the central axis 119 and extends obliquely relative to the first side edge surface 151 .

[0051] Different from the embodiment of the external cutting member 11a of Fig. 5, in the external cutting member 111 of the second embodiment of the hair-cutting unit the front edges 141 of the hair entry slots 135 are each provided with a further hair retaining structure 181 with an arrangement of further tooth-like elements 183 arranged along the front edge 141. Each further tooth-like element 183 has a first side edge surface 185, which faces the central axis 119 and extends transversely to the front edge 141 and forms a further hair abutment surface 187 of the further hair retaining structure 181. Thus, the further hair retaining structure 181 provided on the front edge 141 has an arrangement of further hair abutment surfaces 187 arranged along the front edge 141, where each further hair abutment surface 187 extends transversely to the front edge 141 and faces the central axis 119. A second side edge surface 189 of each further tooth-like element 183 of the further hair retention structure 181 at the leading edge 141 faces away from the central axis 119 and extends obliquely relative to the first side edge surface 185 .

[0052] As shown in FIG. 9, the tooth elements 149 of the hair retention structure 147 arranged along the rear edge 143 of the hair entry slot 135 and the further tooth elements 183 of the further hair retention structure 181 arranged along the front edge 141 of the hair entry slot 135 are staggered relative to each other and arranged in a partially overlapping configuration when viewed radially.

[0053] In the external cutting member 111 of the second embodiment of the hair-cutting unit, hair may also be cut while abutting with the further hair abutment surface 187 of the further tooth-like element 183 of the further hair retaining structure 181 provided along the leading edge 141 of the hair entry slot 135. Although the presence of the further tooth-like element 183 in the hair entry slot 135 may slightly reduce the overall hair capturing performance of the hair entry slot 135 compared to the hair entry slot 35 of the external cutting member 11a described above, the overall hair retention performance of the hair entry slot 135 during the hair cutting process is improved due to the presence of the further hair retaining structure 181 compared to the hair entry slot 35 of the external cutting member 11a. As a result of the staggered arrangement of the tooth elements 149 of the hair retention structure 147 at the trailing edge 143 and the further tooth elements 183 of the further hair retention structure 181 at the leading edge 141 and their mutual arrangement in a partially overlapping configuration, viewed in the radial direction, the open slot area between the hair retention structure 147 at the trailing edge and the further hair retention structure 181 at the leading edge 141 extends in a zigzag manner from the radially outer slot end position 139 to the radially inner slot end position 137. This zigzag extension of said open slot area significantly increases the overall hair retention performance of the hair entry slot 135 during the hair-cutting process. To achieve optimal hair retention performance during the hair-cutting process, the shear angle enclosed by each hair abutment surface 153 of the hair retention structure 147 and the cutting edge of each cutting element of the internal cutting member should be in the range of 0°-15°, preferably in the range of 0°-5°. Similarly, the further shear angle enclosed by each further hair abutment surface 187 of the further hair holding structure 181 and the cutting edge of each cutting element of the internal cutting member should be in the range of 0° to 15°, preferably in the range of 0° to 5°. In the embodiment shown in Figure 11, both said shear angle and said further shear angle are 0°.

[0054] In the embodiment of the external cutting member 11a shown in Figure 5, the hair entry slots 35 have the same hair retention structure 47 located at their rear edge 43, whereas in the embodiment of the external cutting member 111 shown in Figure 9, the hair entry slots 135 have the same hair retention structure 147 located at their rear edge 143 and the same further hair retention structure 181 located at their front edge 141. It should be noted that the present invention also covers hair-cutting units, in which the hair entry slots of the external cutting member have mutually different hair retention structures located at different rear edges and, if present, mutually different further hair retention structures located at different front edges. An example of such an alternative external cutting member has both a hair entry slot of a similar type to the hair entry slot 35 of the external cutting member 11a shown in Figure 5 and a hair entry slot of a similar type to the hair entry slot 135 of the external cutting member 111 shown in Figure 9. In any embodiment, the hair holding structure at the trailing edge and the further hair holding structure at the leading edge (if present) have an arrangement of hair abutment surfaces arranged along the trailing and leading edges, respectively, where each hair abutment surface extends transversely to the trailing and leading edges, respectively, and faces the central axis. Instead of being formed as side edge surfaces provided on the tooth-like elements of the comb-like structure, as is the case in the embodiments shown in Figures 5 and 9, the hair abutment surfaces can also be provided in a different manner on the hair holding structure.

[0055] In Figure 5, D1 denotes a first radial distance between a radially inner slot end position 37 of the hair entry slot 35 and the central axis 19, and D2 denotes a second radial distance between a radially outer slot end position 39 of the hair entry slot 35 and the central axis 19. In the embodiment of Figure 5, the ratio D1 / D2 is 0.5. The radially outer slot end position 39 of the hair entry slot 35 is very close to the outer circumference of the external cutting member 11a, so that in this embodiment the hair entry slot 35 extends over a significant portion of the radius of the external cutting member 11a, i.e. about 50%. This results in a relatively high hair capture performance of the hair entry slot 35.

[0056] In the embodiment of the hair-cutting unit 9a, the cutting edges 45a, 45b, 45c of the internal cutting member 13a each extend as a continuous curve from a radially inner edge end position 67 close to the radially inner slot end position 37 to a radially outer edge end position 69 close to the radially outer slot end position 39, and the substantially acceptable minimum value of the ratio D1 / D2 depends on the value of the cutting edge angle β of the cutting edges 45a, 45b, 45c. In particular, a relatively large cutting edge angle β results in a relatively short radial extension of the cutting edge of the internal cutting member, resulting in a relatively large first radial distance D1 and a relatively high value of the ratio D1 / D2. A relatively small cutting edge angle β allows a relatively long radial extension of the cutting edge of the internal cutting member, resulting in a relatively small first radial distance D1 and a relatively low value of the ratio D1 / D2. Generally, in embodiments of the hair-cutting unit in which the cutting edge of the internal cutting member extends as a continuous line as described above for the internal cutting member 13a, the ratio D1 / D2 may be between 0.4 and 0.8, preferably between 0.5 and 0.6. This ratio means that the hair entry slot 35 extends over a radial distance of between about 20% and about 60%, preferably between about 40% and about 50% of the radius of the external cutting member 13a.

[0057] The radially inner edge end positions 67 of the cutting edges 45a, 45b, 45c have a radial position closer to the central axis 19 than the radially inner slot end position 37 of the hair entry slot 35 of the outer cutting member 11a. As a result, the radially inner edge end positions 67 of the cutting edges 45a, 45b, 45c of the inner cutting member 13a are always covered by the central closed portion of the top wall 23 of the outer cutting member 11a. Thus, during rotation of the inner cutting member 13a, the user's skin is not exposed to the radially inner edge end positions 67 of the cutting edges 45a, 45b, 45c at the radially inner slot end position 37 of the hair entry slot 35. These radially inner edge end locations 67 lead the cutting edges 45a, 45b, 45c in the direction of rotation R of the internal cutting member 13a, and therefore may injure the skin if the skin were exposed to these radially inner edge end locations 67. As previously discussed and as shown in FIG. 6, each cutting edge 45a, 45b, 45c of the internal cutting member 13a extends as a continuous curve from the radially inner edge end location 67 to the radially outer edge end location 69, so that the skin is not exposed to the leading edge-end points of any of the cutting edges 45a, 45b, 45c. Thus, skin irritation or skin injury due to contact between the rotating cutting edges 45a, 45b, 45c and the skin is minimized, particularly as a result of the relatively large cutting edge angle β.

[0058] As described above and as shown in FIG. 6, each of the three cutting edges 45a, 45b, 45c of the internal cutting member 13a extends over an angle of 120° relative to the central axis 19. It should be noted that the internal cutting member can have a different number of cutting elements. In an embodiment in which the cutting edges each extend as a continuous curve from a radially inner edge end position to a radially outer edge end position, as in FIG. 6, the number of cutting elements N may depend on the cutting edge angle β of the cutting edges, especially when the cutting edges do not overlap each other in the tangential direction around the central axis of the hair-cutting unit. In particular, when the cutting edge angle β is relatively small, the number of cutting elements N may be relatively large. In this embodiment, in general, the number of cutting elements N of the internal cutting member may be in the range of 2 to 6, preferably 3 or 4. The cutting edge of each cutting element may extend over an angle of 360° / N relative to the central axis, as in the embodiment of FIG. 6. Alternatively, the cutting edges may extend around the central axis over different angles from each other, and consequently, if the cutting edges have similar cutting edge angles β, the cutting edges may have different radial extensions from each other. In embodiments where the cutting edges overlap each other tangentially around the central axis of the hair-cutting unit, the number N of cutting edges may be increased compared to embodiments where the cutting edges do not overlap each other tangentially around the central axis.

[0059] It should be noted that in the embodiment of the internal cutting member 13a as shown in FIG. 6, the cutting edge angle β of each cutting edge 45a, 45b, 45c is equal at all positions on the cutting edge 45a, 45b, 45c between the radially inner edge end position 67 and the radially outer edge end position 69. This requires a specific curvature shape of the cutting edge 45a, 45b, 45c between the radially inner end position 67 and the radially outer end position 69, which can be straightforwardly designed by a person skilled in the art. Alternatively, the cutting edge angle β of the cutting edge 45a, 45b, 45c may vary along the cutting edge 45a, 45b, 45c. In that case, according to the present invention, the cutting edge angle β should have a value in the range of 55° to 85° at each position on the cutting edge between the radially inner edge end position and the radially outer edge end position.

[0060] In the embodiment of Fig. 5, the multiple hair entry slots 35 arranged in an annular fashion around the central axis 19 can be considered to form a single so-called shaving track of the hair-cutting unit 9a. Also, the embodiment of the hair-cutting unit with the external cutting member 111 as shown in Fig. 9 has a single shaving track. It is noted that the present invention also covers embodiments in which the hair-cutting unit has more than one shaving track. In such embodiments, the external cutting member has two or more annular arrangements of hair entry slots arranged concentrically around the central axis, and the internal cutting member has two or more annular arrangements of cutting elements with cutting edges arranged concentrically around the central axis. In such embodiments, the number of cutting elements in each individual shaving track may be greater than the number of cutting elements in the embodiment with a single shaving track, especially if the cutting edge angle β is similar in all embodiments.

[0061] The cutting edges 45a, 45b, 45c of the internal cutting member 13a described above have a wedge angle θ of approximately 90°. As shown in FIG. 3, the wedge angle θ is the angle enclosed by the upper surface 60 and the front surface 62 of the cutting elements 59a, 59b, 59c at the location of the cutting edges 45a, 45b, 45c. By reducing the wedge angle θ to a value smaller than 90°, the cutting force required to cut the hair can be reduced. This can be achieved by locally providing an oblique portion of the front surface 62 at the location of the cutting edges 45a, 45b, 45c. Alternatively, the entire front surface 62 can be arranged in an oblique direction relative to the upper surface 60.

[0062] The scope of the present invention is not limited to the above examples, and it will be apparent to those skilled in the art that some modifications and changes can be made thereto without departing from the scope of the present invention defined in the appended claims. It is intended that the present invention be construed as including all such modifications and changes insofar as they are within the scope of the claims or their equivalents. Although the present invention has been illustrated and described in detail in the drawings and description, such illustrations and descriptions are merely explanatory or exemplary and not restrictive. The present invention is not limited to the disclosed embodiments. The drawings are schematic, in which details that are not necessary for understanding the present invention may be omitted and are not necessarily drawn to scale.

[0063] 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 figures, the description, and the appended claims. In the claims, the word "comprising" does not exclude other steps or elements, and the indefinite article "a" or "an" does not exclude a plurality. Any reference signs in the claims should not be construed as limiting the scope of the invention.

[0064] Elements and aspects discussed for or in connection with a particular embodiment can be combined with elements and aspects of other embodiments as appropriate, unless otherwise expressly stated. Thus, 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.

[0065] The terms "having" and "including" as used herein will be understood by those skilled in the art to cover the term "consisting of." Thus, the terms "having" or "including" may mean "consisting of" in one embodiment, but in another embodiment may mean "including / having / comprising at least the specified species and, optionally, one or more other species."

Claims

1. A hair-cutting unit having a central axis, an outer cutting member, and an inner cutting member surrounded by the outer cutting member and rotatable relative to the outer cutting member about the central axis; the external cutting member having a plurality of hair entry slots, each extending along a longitudinal extension of the hair entry slot from a radially inner slot end position to a radially outer slot end position relative to the central axis; the internal cutting member having a plurality of cutting elements, each cutting element having a cutting edge; a slot angle of each hair entry slot, when viewed in an axial direction parallel to the central axis, is defined as an acute angle enclosed by a trailing edge of the hair entry slot extending along a longitudinal extension of the hair entry slot relative to a direction of rotation of the internal cutting member and a local radial direction relative to the central axis at the trailing edge; a cutting edge angle of each cutting edge is defined as an acute angle enclosed by the cutting edge and a local radial direction at the cutting edge relative to the central axis when viewed in an axial direction, the cutting edge angle being in the range of 55° to 85°; a slot angle of each hair entry slot along the longitudinal extension of the hair entry slot is in the range of 0° to 20° at each radial position of the hair entry slot between the radially inner slot end position and the radially outer slot end position; a hair-cutting unit, wherein the rear edge of each hair entry slot comprises a hair retaining structure having an array of hair abutment surfaces disposed along the rear edge, each hair abutment surface extending transversely to the rear edge and facing the central axis.

2. 2. The hair-cutting unit according to claim 1, wherein a shear angle enclosed by each hair abutting surface and a cutting edge of each cutting element at said hair abutting surface, when viewed in the axial direction, is in the range of 0° to 15°.

3. A hair-cutting unit according to claim 1 or 2, wherein the cutting edge angle is in the range of 70° to 80°.

4. A hair-cutting unit according to any of the preceding claims, wherein the slot angle is in the range of 0° to 5°.

5. 5. A hair-cutting unit according to claim 1, wherein the hair holding structure comprises an array of tooth-like elements arranged along the rear edge of at least one of the hair entry slots, and each of the hair abutment surfaces is formed by a first side edge surface of a respective one of the tooth-like elements facing the central axis.

6. 6. A hair-cutting unit according to claim 5, wherein a second side edge surface of each individual tooth element facing away from the central axis extends substantially parallel to the first side edge surface of the individual tooth element.

7. 7. A hair-cutting unit as claimed in claim 6, wherein a leading edge of the hair entry slot relative to the direction of rotation of the internal cutting member extends linearly from the radially inner slot end position to the radially outer slot end position at a distance from the tooth elements.

8. 6. A hair-cutting unit as claimed in claim 5, wherein a leading edge of the hair entry slot relative to the direction of rotation of the internal cutting member has a further hair retaining structure with an array of further tooth-like elements arranged along said leading edge, each further tooth-like element having a further hair abutment surface extending transversely to said leading edge and formed by a side edge surface of the further tooth-like element facing the central axis.

9. 9. A hair-cutting unit as claimed in claim 8, wherein the tooth-like elements of the hair holding structure at the rear edge of the hair entry slot and the further tooth-like elements of the further hair holding structure at the front edge of the hair entry slot are mutually arranged in a staggered configuration and in a partially overlapping configuration when viewed in the radial direction.

10. 10. A hair-cutting unit according to any one of the preceding claims, wherein a ratio of a first radial distance between the radially inner slot end position and the central axis to a second radial distance between the radially outer slot end position and the central axis is between 0.4 and 0.

8.

11. 11. A hair-cutting unit as claimed in any preceding claim, wherein the cutting edge of each cutting element of the internal cutting member extends as a continuous curve from a radial position at or near the radially inner slot end position of each hair entry slot to a radial position at or near the radially outer slot end position of each hair entry slot, and the number of cutting elements of the internal cutting member is in the range of 2 to 6.

12. 12. A hair-cutting unit according to claim 11, wherein the cutting edge of each cutting element of the internal cutting member extends over an angle of 360° / N about the central axis, where N is the number of cutting elements of the internal cutting member.

13. 13. A hair-cutting unit as claimed in any preceding claim, wherein the cutting edges of the cutting elements of the internal cutting member each have a radially inner edge end position, relative to the central axis, closer to the central axis than the radially inner slot end position of each hair entry slot.

14. A hair-cutting unit according to any preceding claim, wherein the hair entry slot opens radially outwardly at the radially outer slot end.

15. A shaving unit comprising at least one hair-cutting unit according to any one of the preceding claims and a support structure for supporting said at least one hair-cutting unit.

16. A shaving apparatus having a main housing and a shaving unit as described in claim 15, said shaving unit being coupled to said main housing, said main housing housing accommodating a drive system for rotationally driving said internal cutting member of each hair-cutting unit of said shaving unit relative to said external cutting member.

Images