Hair-cutting unit for a shaving device

A hair-cutting unit for use in a shaving device, said hair-cutting unit comprising an external cutting member and an internal cutting member which is rotatable relative to the external cutting member in a rotational direction about an axis of rotation, wherein:

• the internal cutting member comprises a plurality of cutting elements, each having a cutting edge with a respective main directional component of extension in a radial direction relative to the axis of rotation and located on a side of said cutting element leading in said rotational direction;
• the external cutting member comprises an annular wall having an outer surface facing away from the internal cutting member and a plurality of hair-entry slits which are mutually separated by hair-guiding strip portions of the annular wall, each hair-entry slit and each hair-guiding strip portion being elongated and having a respective main directional component of longitudinal extension in the radial direction relative to the axis of rotation, and each hair-guiding strip portion having a counter-cutting edge for co-operation with the cutting edges of the internal cutting member during rotation of the internal cutting member in said rotational direction;
• each hair-guiding strip portion has an inward surface facing the internal cutting member, an outward surface being part of said outer surface, a first side surface facing in the rotational direction, and a second side surface facing in a direction opposite to the rotational direction, wherein said inward surface and said second side surface mutually connect at the counter-cutting edge of the respective hair-guiding strip portion;
• and wherein, as seen in a cross-section taken perpendicularly to the radial direction at least within a range of radial positions relative to the axis of rotation:
• a maximum depth D of a respective hair-entry slit is defined as a length of a maximum axial extension of a hair-guiding strip portion adjacent to the respective hair-entry slit in an axial direction parallel to the axis of rotation;
• a depth y is defined as a depth within the respective hair-entry slit as measured from said outer surface and along the axial direction, wherein 0≤y≤D;
• a first normal vector at a respective first side surface and a second normal vector at a respective second side surface are each defined in a direction facing away from the corresponding hair-guiding strip portion;
• said first normal vector having a non-zero component in said axial direction facing away from the internal cutting member, a first angle α1 between the first normal vector and said axial direction is defined in the acute angular range 0°<α1<90°, wherein α1(y) is a value of the first angle α1 in a position on the first side surface at the depth y;
• said second normal vector having a non-zero component in said axial direction facing away from the internal cutting member, a second angle α2 between the second normal vector and said axial direction is defined in the acute angular range 0°<α2<90°, wherein α2(y) is a value of the second angle α2 in a position on the second side surface at the depth y;
• said first side surface has an upper segment extending from the outer surface to a depth y1 and a lower segment extending from a depth y2 in a direction away from the upper segment, wherein y2≥y1, y1≤0.2*D and y2≤0.6*D;
• said second side surface has an upper segment extending from the outer surface to a depth y3, an intermediate segment extending from the depth y3 to a depth y4>y3, and a lower segment extending from the depth y4 in a direction away from the intermediate segment, wherein y3≤0.2*D, y4≤0.6*D and y4−y3≥0.2*D;
• an average angle of normal vectors of said intermediate segment of said second side surface α2AV=0.5*{α2(y3)+α2(y4)};
• α2(y3)=α2AV+/−α2δ and α2(y4)=α2AV+/−α2δ;
• α2AV is in a range from 20° to 50°;
• α2δ is in a range from 0° to 10°;
• in each position on the upper segment of said first side surface α1≤α1(y1) and dα1(y)/dy≥0;
• in each position on the lower segment of said first side surface 70°≤α1≤110°;
• in each position on the upper segment of said second side surface α2≤α2(y3) and dα(y)/dy≥0;
• and
• in each position on the lower segment of said second side surface 70°≤α2≤110°.