Patent Publication Number: US-11388857-B2

Title: Cutter blade and lawn mower

Description:
TECHNICAL FIELD 
     The present invention relates to a cutter blade and a lawn mower. 
     BACKGROUND ART 
     Rotary lawn mowers cut lawn grass by rotating a cutter blade along lawn grass. In the lawn mowers, it is known that some rotary lawn mowers include a cutter blade having double blades, i.e., an upper blade and a lower blade (e.g. see the specification of U.S. Patent Application Publication No. 2006/0042216). 
     SUMMARY OF INVENTION 
     Some conventional cutter blades for the lawn mower have structure where ribs, etc. are provided for improvement of rigidity, in order to suppress displacement amount of the blade by the centrifugal force during rotation. However, when outer ends of the blade are displaced upward or downward by the centrifugal force generated during rotation of the cutter blade, the lawn mowing performance (finishing appearance) may be degraded undesirably, and when stress is generated as a result of displacement, the blade strength may be decreased undesirably. 
     The present invention has been made taking such a problem into account, and an object of the present invention is to provide a cutter blade including double blades and a lawn mower in which it is possible to effectively suppress displacement amount during rotation. 
     In order to achieve the above object, the present invention provides a cutter blade for a lawn mower, the cutter blade including an upper blade and a lower blade overlapped with each other in a thickness direction, wherein the upper blade includes an upper blade proximal part having a flat shape in a radial direction perpendicular to a blade rotation axis and an upper outer end positioned radially outside the upper blade proximal part, and above the upper blade proximal part, and the lower blade includes a lower blade proximal part having a flat shape in the radial direction, and a clipping part positioned radially outside the lower blade proximal part and below the lower blade proximal part, and wherein the upper blade proximal part is overlapped on the lower blade proximal part. 
     In the cutter blade of the present invention having the above structure, during rotation, by the influence of the centrifugal force, a force is applied to the upper blade to displace the upper blade downward, and a force is applied to the lower blade to displace the lower blade upward. Thus, the displacement of the upper blade and the displacement of the lower blade are suppressed mutually. Accordingly, it becomes possible to suppress the displacement amount during rotation of the cutter blade effectively. As a result, it become possible to improve the lawn mowing performance (in particular, finishing appearance), and thus suppress generation of the stress as a result of displacement. 
     The upper blade may include a rising part which rises from the upper blade proximal part toward the upper outer end, and the lower blade may include a falling part which falls from the lower blade proximal part to the clipping part, a portion where the rising part starts to rise from the upper blade proximal part is positioned radially outside a portion where the falling part starts to fall from the lower blade proximal part. 
     In the structure, during rotation of the cutter blade, the outermost portion of the lower blade in the radial direction of the lower blade proximal part and the upper blade proximal part push against each other. Therefore, it becomes possible to suppress the displacement amount of the lower blade during rotation of the cutter blade effectively to a greater extent. 
     In a blade rotation direction, one of a radially outer end of the upper outer end and a radially outer end of the clipping part may be positioned on a front side of another of the radially outer end of the upper outer end and the radially outer end of the clipping part. 
     In the structure, it is possible to suppress cancellation of the air flows (wind) generated by the upper outer end and the clipping part during rotation of the cutter blade. Accordingly, it becomes possible to efficiently generate the transportation wind for transporting the clipped lawn grass. 
     The lower blade includes a warping part rising from the clipping part in a direction opposite to the blade rotation direction, in a blade rotation direction, a radially outer end of the upper outer end may be positioned on the front side of the radially outer end of the clipping part. 
     In the structure, during rotation of the cutter blade, it is possible to suppress cancellation of the upward air flow generated by the warping part of the lower blade, by the air flow generated by the clipping part of the upper blade. Therefore, it becomes possible to generate the transportation wind more efficiently. 
     A blade part may be formed in at least part of a front edge in a blade rotation direction of the upper outer end. 
     In the structure, by clipping the lawn grass not only by the lower blade but also by the upper blade, it is possible to finely cut the lawn grass in comparison with the case of clipping the lawn grass only by the lower blade. Therefore, even in the case of transporting long lawn grass and/or heavy lawn grass, the clipped lawn grass rides on the transportation wind easily, and the lawn grass can be transported easily on the transportation wind more efficiently. 
     The lower blade may include a falling part formed between the lower blade proximal part and the clipping part, the falling part being curved downward from the lower blade proximal part toward the radial outside, and a warping part formed to rise from the clipping part in a reverse rotation direction which is a direction opposite to the blade rotation direction, and wherein the falling part and the warping part may include an intersection part where an end of the falling part in the reverse rotation direction and an end of the warping part positioned radially inside intersect with each other. 
     In the structure, it becomes possible to catch long lawn grass and/or heavy lawn grass clipped by the clipping part in the intersection part where the falling part and the warping part intersect with each other. Therefore, after the clipped part is temporarily caught in the intersection part, it becomes possible to transport the clipped lawn grass efficiently. 
     The intersection part may include a cup shaped portion on an upper surface of the cutter blade, and the cup shaped portion may be recessed toward a lower surface of the cutter blade. 
     In the structure, it becomes possible to catch long lawn grass and/or wet heavy lawn grass clipped by the blade part more easily in the intersection part. 
     The cup shaped portion may have a spherical surface. 
     In the structure, it becomes possible to catch long lawn grass and/or wet heavy lawn grass clipped by the clipping part in the intersection part. 
     A shape of a marginal portion of the cutter blade in the intersection part may have an arc shape as viewed in a direction of the blade rotation axis. 
     In the structure, it becomes possible to reduce the air resistance during rotation of the blade, and reduce noises such as wind noises. 
     The warping part may have a twisted shape which rises upward in the reverse rotation direction, and which is curved radially inward. 
     In the structure, it is possible to generate the transportation wind more efficiently. 
     Further, the present invention provides a lawn mower including a cutter blade, the cutter blade including an upper blade and a lower blade overlapped with each other in a thickness direction, wherein the upper blade includes an upper blade proximal part having a flat shape in a radial direction perpendicular to a blade rotation axis, and an upper outer end positioned radially outside the upper blade proximal part, and above the upper blade proximal part, and the lower blade includes a lower blade proximal part having a flat shape in the radial direction, and a clipping part positioned radially outside the lower blade proximal part and below the lower blade proximal part, and wherein the blade proximal part is overlapped on the lower blade proximal part. 
     In the lawn mower, the lower blade may include a falling part formed between the lower blade proximal part and the clipping part, the falling part being curved downward from the lower blade proximal part, toward the radially outside, and a warping part formed to rise from the clipping part in a reverse rotation direction which is a direction opposite to a blade rotation direction, and wherein the falling part and the warping part may include an intersection part where an end of the falling part in the reverse rotation direction and an end of the warping part positioned radially inside intersect with each other. 
     In the cutter blade and the lawn mower of the present invention, it becomes possible to effectively suppress displacement amount during rotation of the cutter blade. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a side view showing a lawn mower; 
         FIG. 2  is a plan view showing the lawn mower; 
         FIG. 3  is a perspective view showing a cutter blade mounted in the lawn mower; 
         FIG. 4  is a plan view showing the cutter blade; 
         FIG. 5  is a side view showing the cutter blade as viewed from a blade circumferential direction of the cutter blade; 
         FIG. 6  is a side view showing an outer end of a cutter blade in a radial direction, as viewed inward in a radial direction; 
         FIG. 7  is a side view showing a lawn mower having another structure; 
         FIG. 8  is a plan view showing a cutter blade (having double blades) mounted in the lawn mower in the other structure; 
         FIG. 9  is a perspective view showing the cutter blade (having double blades); and 
         FIG. 10  is a side view showing the cutter blade (double blades) viewed in a circumferential direction of the blade. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A lawn mower  10 A shown in  FIGS. 1 and 2  is a walk-behind, self-propelled working machine for cutting lawn grass. In  FIGS. 1 and 2 , an arrow Fr denotes the front side (same as the front side as viewed from an operator) of the lawn mower  10 A, and an arrow Rr denotes the rear side (same as the rear side as viewed from the operator) of the lawn mower  10 A. (Also in  FIG. 7  showing a lawn mower  10 B having another structure, the arrow Fr denotes the front side of the lawn mower  10 B, and the arrow Rr denotes the rear side of the lawn mower  10 B). 
     The lawn mower  10 A includes a housing  12  as a machine body, left and right front wheels  14 F provided on the front side of the housing  12 , left and right rear wheels  14 R provided on the rear side of the housing  12 , a cutter blade  15  accommodated inside the housing  12  for cutting lawn grass, a prime mover  16  provided above the housing  12 , and a control handle  18  extending backward from the housing  12 . 
     As shown in  FIG. 2 , in a plan view, the lawn mower  10 A rotates the cutter blade  15  clockwise (in the direction indicated by an arrow R) by the prime mover  16  to cut (clip) the lawn grass under the lawn mower  10 A. At this time, the lawn mower  10 A generates flows of the air (swirl air flow or swirl wind) in the housing  12  swirling in the same direction as the cutter blade  15 . By this swirl air flow, the lawn grass cut by the cutter blade  15  can be delivered to, and stored in a grass clippings container  20  through a grass clippings discharge passage  13  formed in the housing  12 . Hereinafter, the lawn grass cut (clipped) by the cutter blade  15  will also be referred to as the “grass clippings”. Further, the “swirl air flow” will also be referred to as the “transportation wind”. 
     In  FIG. 1 , the housing  12  is a casing having an opened bottom where only the surface facing the lawn ground Gr under the housing  12  is opened. The housing  12  has a scroll section having a spiral shape in a plan view, for placing the lawn grass cut by the cutter blade  15  on the transportation wind, and transporting the lawn grass (grass clippings) toward the grass clippings discharge passage  13 . 
     As shown in  FIG. 2 , a mode switch damper  19  is provided for the grass clippings discharge passage  13 . The mode switch damper  19  can be operated by a control lever (not shown). By switching the open/closed state of the mode switch damper  19 , it is possible to selectively switch the operating mode, between a bagging mode for storing the grass clippings in the grass clippings container  20  and a mulching mode for discharging the grass clippings to a position below the housing  12 . 
     In  FIG. 1 , a prime mover base  22  is attached to an upper surface of the housing  12 . The prime mover  16  is attached to an upper surface of the prime mover base  22 . In the lawn mower  10 A, an engine  16 A is mounted as the prime mover  16 . It should be noted that the prime mover  16  is not limited to the engine  16 A. For example, the prime mover  16  may be an electric prime mover. The prime mover  16  has an output axis  16   a . The output axis  16   a  protrudes downward, and extends into the housing  12 . The output axis  16   a  is substantially perpendicular to the ground Gr. Therefore, in the case where the lawn mower  10 A is present on the horizontal ground Gr, the output axis  16   a  is rotated about the rotation core which is substantially perpendicular to the horizontal plane. 
     The driving power generated by the prime mover  16  is transmitted to the left and right rear wheels  14 R as traveling driving power, through a traveling power transmission mechanism  24  and a transmission  25  coupled to the output axis  16   a . Therefore, the left and right rear wheels  14 R are provided as drive wheels. In the lawn mower  10 A, the traveling power transmission mechanism  24  is in the form of a belt mechanism  24 A. The belt mechanism  24 A includes a drive pulley  26  fixed to the output axis  16   a , a driven pulley  28  coupled to the transmission  25 , and a drive belt  30  wound around the drive pulley  26  and the driven pulley  28 . As the transmission  25 , for example, a hydraulic continuously variable transmission may be used. 
     The cutter blade  15  is coupled to the output axis  16   a  of the prime mover  16 . The rotation driving power is transmitted from the output axis  16   a  to the cutter blade  15  for rotating the cutter blade  15 . Specifically, the cutter blade  15  is coupled to the output axis  16   a  through a blade holder  32 . The blade holder  32  is attached to the center of the cutter blade  15 , and coupled to a lower end of the output axis  16   a  coaxially with the output axis  16   a . Therefore, in the lawn mower  10 A, the rotation axis a 1  (rotation central line) of the cutter blade  15  is in alignment with the rotation core a of the output axis  16   a  (the rotation axis a 1  of the cutter blade  15  is coaxial with the rotation core of the output axis  16   a ). Hereinafter, the rotation axis a 1  of the cutter blade  15  will be referred to as the “blade rotation axis a 1 ”. 
     It should be noted that the blade rotation axis a 1  need not necessarily be in alignment with the rotation core of the output axis  16   a . For example, the blade rotation axis a 1  may be provided in parallel with the rotation core, at a position offset toward the direction perpendicular to the rotation core of the output axis  16   a . Alternatively, the blade rotation axis a 1  may be provided in non-parallel with the rotation core of the output axis  16   a.    
     The cutter blade  15  is a narrow member in the form of a plate (a so-called bar blade). The cutter blade  15  extends in a direction perpendicular to the blade rotation axis a 1 . Hereinafter, the longitudinal direction of the cutter blade  15  will also be referred to as the “radial direction”. The rotation direction of the cutter blade  15  (indicated by the arrow R) may also be referred to as the “blade rotation direction”. The direction opposite to the blade rotation direction may be referred to as the “reverse rotation direction”. The blade rotation direction or the reverse rotation direction may be referred to as the “blade circumferential direction”. 
     The cutter blade  15  includes a pair of blade arms  36  extending in opposite directions about the blade rotation axis a 1 . The pair of blade arms  36  are formed axially symmetrically about the blade rotation axis a 1 . 
     As shown in  FIGS. 3 and 4 , each of the blade arms  36  includes a blade proximal part  38  formed in the radial direction of the cutter blade  15 , a clipping part  40  positioned outside the blade proximal part  38  in the radial direction (radially outside of the blade proximal part  38 ), a falling part  42  formed between the blade proximal part  38  and the clipping part  40 , and a warping part  44  extending from the clipping part  40  in the reverse rotation direction. 
     The blade proximal part  38  is formed in a flat plate shape extending in perpendicular to the blade rotation axis a 1 . The clipping part  40  is continuous from the radially outer end of the falling part  42 , and is a radially outer area of the cutter blade  15 . In  FIG. 5 , the clipping part  40  is positioned below the blade proximal part  38  and the falling part  42 . The clipping part  40  is inclined downward toward the radial outside. Therefore, a radially outer end  40   b  of the clipping part  40  is positioned below a radially inner end  40   a  of the clipping part  40 . 
     The clipping part  40  is provided in non-parallel with the blade proximal part  38 . Specifically, in  FIG. 5 , the clipping part  40  is inclined with respect to the surface P perpendicular to the blade rotation axis a 1  in a manner that the clipping part  40  is oriented downward toward the radial outside. In  FIG. 6 , the clipping part  40  is inclined downward with respect to the surface P perpendicular to the blade rotation axis a 1  in a manner that the clipping part  40  is oriented in the rotation direction. Therefore, an end  40   c  (front edge) of the clipping part  40  in the rotation direction is positioned below an end  40   d  of the clipping part  40  in the reverse rotation direction. 
     In  FIGS. 3 to 5 , the falling part  42  is a portion which is curved downward from the blade proximal part  38  toward the radial outside. The falling part  42  is continuous with the radially inner end  40   a  of the clipping part  40 . 
     In  FIG. 5 , the falling part  42  is curved in an S-shape between the blade proximal part  38  and the clipping part  40 . That is, a radially inner end  42   a  of the falling part  42  has a curve in an arc shape having the center of curvature on the lower side of the cutter blade  15  as viewed in the blade circumferential direction. A radially outer end  42   b  of the falling part  42  has a curved shape having the center of curvature on the upper side of the cutter blade  15  as viewed in the blade circumferential direction (hereinafter referred to as the “first curved shape”). 
     In  FIGS. 3 and 4 , the warping part  44  protrudes beyond the blade proximal part  38  in the reverse rotation direction. The warping part  44  is formed to rise from the clipping part  40  in the reverse rotation direction. An upper end  44   a  of the warping part  44  extends along (substantially in parallel with) a blade part  54  as viewed in the axial direction of the blade rotation axis a 1 . The warping part  44  has a twisted shape which rises upward in the reverse rotation direction, and which is curved radially inward. 
     As shown in  FIGS. 3 and 6 , the warping part  44  has a shape curved to include an arc (hereinafter referred to the “second curved shape”) oriented upward in the reverse rotation direction. The second curved shape of the warping part  44  may include a plurality of circular arcs having different curvatures. The curvature of the warping part  44  is increased radially outward. Therefore, the curvature in the radially outer portion of the warping part  44  is larger than the curvature in the radially inner portion of the warping part  44 . 
     In  FIG. 5 , the upper end  44   a  (rear edge in the reverse rotation direction) of the warping part  44  is in substantially parallel with the cutter blade  15  in the radial direction (surface P perpendicular to the blade rotation axis a 1 ). The rising height of the warping part  44  from the clipping part  40  is increased toward the radial outside. The upper end  44   a  of the warping part  44  is provided at a position below an upper surface  38   a  of the blade proximal part  38 . The upper end  44   a  of the warping part  44  may be provided at the same height as the upper surface  38   a  of the blade proximal part  38  or above the upper surface  38   a  of the blade proximal part  38 . 
     In  FIGS. 3 to 5 , the falling part  42  and the warping part  44  include an intersection part (overlapping area)  50  where an end of the falling part  42  in the reverse rotation direction and an end of the warping part  44  positioned radially inside intersect with each other. That is, the falling part  42  and the warping part  44  share the intersection part  50 . 
     Specifically, the intersection part  50  includes a cup shaped portion  50   a  on an upper surface of the cutter blade  15 . The cup shaped portion  50   a  is recessed toward the lower surface of the cutter blade  15 . The cup shaped portion  50   a  has a shape formed by combining the first curved shape of the falling part  42  and the second curved shape of the warping part  44 . Therefore, the cup shaped portion  50   a  has a spherical surface. In  FIG. 4 , an area S where the cup shaped portion  50   a  is formed is denoted by hatching. 
     In  FIG. 4 , the shape of a marginal portion  52  of the cutter blade  15  in the intersection part  50  has an arc shape (circular arc shape in  FIG. 4 ) recessed (curved) toward the clipping part  40  as viewed in the axial direction of the blade rotation axis a 1 . The shape of the marginal portion  52  as viewed in the axial direction may include a plurality of circular arcs having different curvatures. 
     In  FIG. 5 , the shape of the marginal portion  52  of the cutter blade  15  at the intersection part  50  has an arc shape recessed (curved) downward as viewed in the blade circumferential direction. It should be noted that the shape of the marginal portion  52  viewed in the blade circumferential direction may be formed to include a plurality of circular arcs having different curvatures. The intersection part  50  is positioned above the clipping part  40 . 
     At each of both ends of the cutter blade  15 , the blade part  54  having a sharp blade edge is formed at the front edge in the rotation direction. As shown in  FIG. 4 , the blade part  54  has a straight shape as viewed in the axial direction of the blade rotation axis a 1 . In the illustrated example, the blade part  54  is formed over the entire front edge of the clipping part  40 , and a radially inner end  54   a  of the blade part  54  is formed at the front edge of the falling part  42 . Therefore, as shown in  FIG. 5 , the blade part  54  is curved at a position connecting the clipping part  40  and the falling part  42 . 
     In  FIG. 4 , a radially inner end  50   b  of the intersection part  50  is positioned radially inside a radially inner end position  54 P of the effective blade part length L of the blade part  54 . That is, the position of the radially inner end  50   b  of the intersection part  50  is offset radially inward from the radially inner end position  54 P of the effective blade part length L of the blade part  54 . The blade width W of the blade part  54  in the range of the effective blade part length L (see  FIG. 6 ) is substantially constant. In the blade part  54 , the blade width of the radially inside part compared to the range of the effective blade length L is decreased toward the radial inside ( FIG. 4 ). 
     In  FIG. 4 , the cutter blade  15  has a shape with a sweepback angle inclined in a reverse rotation direction of the cutter blade  15  (anti-clockwise in a plan view) with respect to the radiation direction about the blade rotation axis a 1 . That is, the cutter blade  15  (pair of blade arms  36 ) is curved or bent in the reverse rotation direction of the cutter blade  15  (opposite to the direction indicated by an arrow R), at an intermediate position between the blade rotation axis a 1  and an outer end  36   a  of the cutter blade  15 . It should be noted that the cutter blade  15  may have a shape which does not include any sweepback angle (shape extending straight in the radial direction). 
     Next, operation of the lawn mower  10 A having the above structure will be described. 
     In  FIG. 1 , when the cutter blade  15  is rotated under operation of the prime mover  16 , the cutter blade  15  cuts (clips) lawn grass grown on the lawn ground right under the lawn mower  10 A. The clipped lawn grass (grass clippings) is swirled by the transportation wind (swirl wind) produced by rotation of the cutter blade  15 , and transported to the grass clippings container  20  through the grass clippings discharge passage  13  formed in the housing  12 . 
     During rotation of the cutter blade  15 , the cutter blade  15  generates upward air flow by the warping part  44 . Therefore, by generating the upward air flow to orient the lawn grass growing on the lawn ground to stand upright, it is possible to cut (clip) the lawn grass by the cutter blade  15  efficiently. Further, after the lawn grass (grass clippings) cut by the cutter blade  15  is lifted upward, and swirled in the housing  12  by the transportation wind, i.e., upward air flow and the swirl air flow generated by the warping part  44 , the lawn grass can be transported into the grass clippings container  20  efficiently. 
     In this case, the lawn mower  10 A including the cutter blade  15  offers the following advantages. 
     In the cutter blade  15  of the lawn mower  10 A, as shown in  FIGS. 3 to 5 , the falling part  42  and the warping part  44  include the intersection part  50  where the end of the falling part  42  in the reverse rotation direction and the end of the warping part  44  positioned radially inside intersect with each other. In the structure, it becomes possible to catch long lawn grass and/or wet heavy lawn grass clipped by the clipping part  40  in the intersection part  50  where the falling part  42  and the warping part  44  intersect with each other. Therefore, after the clipped lawn grass is caught in the intersection part  50  temporarily, it becomes possible to transport the lawn grass by the transportation wind efficiently. 
     In particular, the intersection part  50  has the cup shaped portion  50   a  on the upper surface of the cutter blade  15 . The cup shaped portion  50   a  is recessed toward the lower surface of the cutter blade  15 . In the structure, it becomes possible to catch long lawn grass and/or wet heavy lawn grass clipped by the blade part  54  in the intersection part  50 . Further, since the cup shaped portion  50   a  has the spherical surface, it becomes easier to catch long lawn and/or wet heavy lawn clipped by the blade part  54  in the intersection part  50  more suitably. 
     The marginal portion  52  of the cutter blade  15  in the intersection part  50  has an arc shape as viewed in the direction of the blade rotation axis a 1 . Further, the shape of the marginal portion  52  of the cutter blade  15  in the intersection part  50  has an arc shape recessed downward, as viewed in the blade circumferential direction. In the structure, it becomes possible to reduce the air resistance during rotation of the blade, and reduce noises such as wind noises. 
     The warping part  44  has a shape which rises upward in the reverse rotation direction, and which is curved radially inward. In the structure, it is possible to generate the transportation wind more efficiently. 
     The clipping part  40  is inclined downward toward the radial outside, and the upper end  44   a  of the warping part  44  is formed substantially in parallel with the radial direction. In the structure, it becomes possible to generate the transportation wind more efficiently. 
     The rising height of the warping part  44  from the clipping part  40  is increased toward the radial outside. In the structure, it is possible to generate the transportation wind more efficiently. 
     The radially inner end  50   b  of the intersection part  50  is positioned radially inside with respect to the radially inner end position  54 P of the effective blade part length L of the blade part  54 . In the structure, it is possible to catch the lawn grass clipped by the radially inner end of the blade part  54  in the intersection part  50  suitably. 
     As in the case of the lawn mower  10 A ( FIG. 1 , etc.), a lawn mower  10 B having another structure in  FIG. 7  is a walk-behind, self-propelled working machine for cutting lawn grass. The lawn mower  10 B is different from the lawn mower  10 A in respect of structure of a cutter blade  60 . The cutter blade  60  includes an upper blade  60 A and a lower blade  60 B that are overlapped with each other in the thickness direction. That is, the cutter blade  60  is in the form of double blades. The upper blade  60 A and the lower blade  60 B are coupled together by a blade holder  32  in a manner that the upper blade  60 A and the lower blade  60 B are not rotatable relative to each other. Therefore, during rotation of the output axis  16   a , the upper blade  60 A and the lower blade  60 B rotate together. 
     As shown in  FIG. 8 , the cutter blade  60  has an axially symmetrical shape with respect to the blade rotation axis a 1 . The phase of the upper blade  60 A is shifted from the phase of the lower blade  60 B in the blade rotation direction (indicated by an arrow R). Therefore, as viewed in the axial direction of the blade rotation axis a 1 , the lower blade  60 B and the upper blade  60 A include an area where the lower blade  60 B and the upper blade  60 A are overlapped with each other, and an area where the lower blade  60 B and the upper blade  60 A are not overlapped with each other. 
     The upper blade  60 A includes a pair of upper blade arms  62  extending in opposite directions from the blade rotation axis a 1  at the center. In  FIGS. 8 and 9 , each of the upper blade arms  62  includes an upper blade proximal part  64 , an upper outer end  66 , and a rising part  68 . The upper blade proximal part  64  has a flat shape in the radial direction perpendicular to the blade rotation axis a 1 . The upper outer end  66  is positioned radially outside the upper blade proximal part  64 , and above the upper blade proximal part  64 . The rising part  68  rises upward from the upper blade proximal part  64  toward the upper outer end  66 . 
     The upper blade arm  62  has a shape where the blade width (width in the circumferential direction) is decreased toward the radial outside. Therefore, the blade width of the rising part  68  is smaller than the blade width of the upper blade proximal part  64 . The blade width of the upper outer end  66  is smaller than the blade width of the rising part  68 . Specifically, the rear edge of the upper blade arm (edge in the reverse rotation direction) is inclined in the blade rotation direction toward the radial outside. It should be noted that the blade width of the upper blade arm  62  may have a substantially constant shape toward the radial outside. 
     The upper blade proximal part  64  has a flat shape extending perpendicular to the blade rotation axis a 1 . The upper blade proximal part  64  is overlapped with a lower blade proximal part  76  of the lower blade  60 B described later. That is, a lower surface  64   b  of the upper blade proximal part  64  contacts an upper surface  76   a  of the lower blade proximal part  76 . 
     The upper outer end  66  is a portion extending from the radially outer end of the rising part  68  to the radial outside of the rising part  68 . A blade part  70  having a sharp blade edge is provided at the front edge of the upper outer end  66  in the blade rotation direction. Therefore, the upper outer end  66  also functions as a clipping part for clipping lawn grass. As viewed in the axial direction of the blade rotation axis a 1 , the blade part  70  has a straight shape in parallel with the axis of the longitudinal direction of the upper blade  60 A. 
     In the blade rotation direction, a radially outer end  66   a  of the upper outer end  66  is positioned on the front side of a radially outer end  78   a  of a clipping part  78  described later, of the lower blade  60 B. In the blade rotation direction, the entire radially outer end  66   a  of the upper outer end  66  is not required to be positioned on the front side of the radially outer end  78   a  of the clipping part  78 . It is sufficient that at least part of the radially outer end  66   a  of the upper outer end  66  is positioned on the front side of the radially outer end  78   a  of the clipping part  78 . In this case, in the blade rotation direction, the upper outer end  66  should be positioned on the front side of a warping part  82  of the lower blade  60 B described later. 
     Unlike the above described structure, in the case of adopting structure where the phase of the upper blade  60 A is shifted from the phase of the lower blade  60 B in the reverse rotation direction (opposite to the direction indicated by the arrow R), in the blade rotation direction, the radially outer end  78   a  of the clipping part  78  may be positioned closer to the front end, from the radially outer end  66   a  of the upper outer end  66 . 
     In  FIG. 10 , the upper outer end  66  is provided in parallel with the upper blade proximal part  64 . The upper outer end  66  may be provided in non-parallel with the upper blade proximal part  64 . Therefore, the upper outer end  66  may be inclined with respect to the surface P in perpendicular to the blade rotation axis a 1  in a manner that the upper outer end  66  is inclined downward or upward toward the radial outside. Alternatively, the upper outer end  66  may be inclined with respect to the surface P in perpendicular to the blade rotation axis a 1  in a manner that the upper outer end  66  is inclined downward or upward in the blade rotation direction. 
     The rising part  68  forms a step (bent portion) between the upper blade proximal part  64  and the upper outer end  66 . The rising part  68  is inclined upward, toward the radial outside. Therefore, a lower surface  66   b  of the upper outer end  66  is positioned above an upper surface  64   a  of the upper blade proximal part  64 . Further, the rising part  68  may be a vertical part which extends from the upper blade proximal part  64  to the blade rotation axis a 1 , i.e., which stands upright in perpendicular to the upper blade proximal part  64 . Alternatively, the rising part  68  may have an S-shaped curve as viewed in the blade circumferential direction. 
     The portion where the rising part  68  starts to rise from the upper blade proximal part  64  (portion connecting the upper blade proximal part  64  and the rising part  68 ), is positioned radially outside the portion where a falling part  80  described later starts to fall from the lower blade proximal part  76  (portion connecting the lower blade proximal part  76  and the falling part  80 ). That is, the radially inner end of the rising part  68  is positioned radially outside the radially inner end of the falling part  80 . 
     The lower blade  60 B includes a pair of lower blade arms  74  extending in opposite directions to each other on both sides of the blade rotation axis a 1  at the center. Each of the lower blade arms  74  includes a lower blade proximal part  76 , the clipping part  78 , the falling part  80 , and the warping part  82 . The lower blade proximal part  76  has a flat shape in the radial direction. The clipping part  78  is positioned radially outside the lower blade proximal part  76 , and positioned below the lower blade proximal part  76 . The falling part  80  falls from the lower blade proximal part  76  toward the clipping part  78 . The warping part  82  extends from the clipping part  78  in the reverse rotation direction. 
     The lower blade proximal part  76  has a flat plate shape extending perpendicular to the blade rotation axis a 1 . The clipping part  78  is continuous with the radially outer end of the falling part  80 , and is a portion forming a radially outer area of the cutter blade  60 . The front edge of the clipping part  78  in the blade rotation direction includes a blade part  79  having a sharp blade edge. In  FIG. 10 , the clipping part  78  is inclined downward, toward the radial outside. 
     The falling part  80  is a portion which is curved downward from the lower blade proximal part  76  toward the radial outside, and which forms a step (bent portion) between the lower blade proximal part  76  and the clipping part  78 . The falling part  80  is continuous with the radially inner end of the clipping part  78 . In  FIGS. 8 and 9 , a plurality of ribs  84  are provided on the lower surface of the falling part  80  for improving rigidity. The ribs  84  protrude downward, and are arranged at intervals in the blade circumferential direction. Recesses  84   a  as back surfaces of the ribs are provided on the upper surface of the falling part  80 , at positions corresponding to the ribs. 
     In  FIG. 9 , the warping part  82  protrudes in the reverse rotation direction beyond the lower blade proximal part  76 , and rises from the clipping part  78 . The warping part  82  has an arc shape curved upward in the reverse rotation direction. 
     In  FIG. 10 , an upper end  82   a  (rear edge in the reverse rotation direction) of the warping part  82  is substantially in parallel with the radial direction of the cutter blade  60  (surface P perpendicular to the blade rotation axis a 1 ). The upper end  82   a  of the warping part  82  is positioned above the upper surface  76   a  of the lower blade proximal part  76 . It should be noted that the upper end  82   a  of the warping part  82  may be provided at the same height as the upper surface  76   a  of the lower blade proximal part  76  or below the upper surface  76   a  of the lower blade proximal part  76 . 
     The upper end  82   a  of the warping part  82  is provided below the lower surface  66   b  of the upper outer end  66  of the upper blade  60 A. It should be noted that the upper end  82   a  of the warping part  82  may be provided at the same height as the lower surface  66   b  of the upper outer end  66  of the upper blade  60 A, or above the lower surface  66   b  of the upper outer end  66  of the upper blade  60 A. 
     Next, operation of the lawn mower  10 B having the above structure will be described. 
     In  FIG. 7 , when the cutter blade  60  is rotated under driving operation of the prime mover  16 , the cutter blade  60  cuts (clips) lawn grass grown on the lawn ground right under the lawn mower  10 B. The clipped lawn grass (grass clippings) is transported by the transportation wind (swirl wind) produced by rotation of the cutter blade  60  to the grass clippings container  20  through the grass clippings discharge passage  13  formed in the housing  12 . 
     During rotation of the cutter blade  60 , the cutter blade  60  generates an upward air flow by the warping part  82  of the lower blade  60 B. In this manner, the upward air flow is generated to orient the lawn grass growing on the lawn ground to stand upright. Therefore, it is possible to cut (clip) the lawn grass by the cutter blade  60  efficiently. Further, after the lawn grass (grass clippings) cut by the cutter blade  60  is lifted upward, and swirled in the housing  12  by the transportation wind, i.e., upward air flow and the swirl air flow generated by the warping part  82 , the lawn grass can be transported into the grass clippings container  20  efficiently. 
     In this case, the lawn mower  10 B having the cutter blade  60  offers the following advantages. 
     As shown in  FIGS. 8 to 10 , the cutter blade  60  includes the upper blade  60 A and the lower blade  60 B overlapped with each other in the thickness direction. The upper blade  60 A includes the upper blade proximal part  64  and the upper outer end  66 . The upper blade proximal part  64  has a flat shape in the radial direction perpendicular to the blade rotation axis a 1 . The upper outer end  66  is positioned radially outside the upper blade proximal part  64 , and above the upper blade proximal part  64 . The lower blade  60 B includes the lower blade proximal part  76  having a flat shape in the radial direction, and the clipping part  78  positioned radially outside the lower blade proximal part  76 , and below the lower blade proximal part  76 . Then, the upper blade proximal part  64  is overlapped on the lower blade proximal part  76 . 
     With the structure, during rotation of the cutter blade  60 , by the influence of the centrifugal force, a force is applied to the upper blade  60 A to displace the upper blade  60 A downward, and a force is applied to the lower blade  60 B to displace the lower blade  60 B upward to suppress displacement of the upper blade  60 A and displacement of the lower blade  60 B mutually. Therefore, it becomes possible to suppress displacement amount during rotation of the cutter blade  60  effectively. Accordingly, it becomes possible to improve, in particular, the finishing appearance (flatness of the lawn after clipping) as one of the features of the lawn mowing performance. Further, since stress generation by displacement is suppressed, it becomes possible to reduce requirements of rigidity and/or the strength of the upper blade  60 A and the lower blade  60 B. 
     The upper blade  60 A includes the rising part  68  rising from the upper blade proximal part  64  to the upper outer end  66 , and the lower blade  60 B includes the falling part  80  falling from the lower blade proximal part  76  to the clipping part  78 . Further, the position where the rising part  68  starts to rise from the upper blade proximal part  64  is positioned radially outside the position where the falling part  80  starts to fall from the lower blade proximal part  76  ( FIG. 10 ). In the structure, during rotation of the cutter blade  60 , the outermost portion of the lower blade proximal part  76  in the radial direction and the upper blade proximal part  64  push against each other. Therefore, it becomes possible to suppress the displacement amount of the lower blade  60 B during rotation of the cutter blade  60  effectively to a greater extent. Since the displacement amount of the lower blade  60 B is suppressed, it becomes possible to improve the finishing appearance to a greater extent. 
     In the blade rotation direction, one of the upper radially outer end  66   a  of the upper outer end  66  and the radially outer end  78   a  of the clipping part  78  is positioned on the front side of another of the upper radially outer end  66   a  of the upper outer end  66  and the radially outer end  78   a  of the clipping part  78  ( FIG. 8 ). In the structure, it is possible to suppress cancellation of the air flows (wind) generated by the upper outer end  66  and the clipping part  78  during rotation of the cutter blade  60 . Accordingly, it becomes possible to efficiently generate the transportation wind for transporting the clipped lawn grass. 
     In particular, in the case of the lawn mower  10 B, the lower blade  60 B includes the warping part  82  rising from the clipping part  78  in a direction opposite to the blade rotation direction. In the blade rotation direction, the radially outer end  66   a  of the upper outer end  66  is positioned on the front side of the radially outer end  78   a  of the clipping part  78  ( FIG. 8 ). In the structure, during rotation of the cutter blade  60 , it is possible to suppress cancellation of the upward air flow generated by the warping part  82  of the lower blade  60 B, by the air flow generated by the upper outer end  66  of the upper blade  60 A. Therefore, it becomes possible to generate the transportation wind more efficiently. 
     The blade part  70  is formed in at least part of the front edge of the upper outer end  66  in the blade rotation direction. In the structure, by clipping the lawn grass not only by the lower blade  60 B but also by the upper blade  60 A, it is possible to finely cut the lawn grass in comparison with the case of clipping the lawn grass only by the lower blade  60 B. Therefore, even in the case of transporting long lawn grass and/or wet heavy lawn grass, the clipped lawn grass rides on the transportation wind easily, and the clipped lawn grass can be transported easily on the transportation wind more efficiently. 
     In the lawn mower  10 B, it should be noted that the cutter blade  15  shown in  FIG. 3 , etc. may be adopted as the lower blade of the cutter blade  60 , instead of the above described lower blade  60 B. In this manner, in addition to the above advantages offered by the lawn mower  10 B, the same advantages as in the case of the lawn mower  10 A can be obtained. For example, after the long lawn grass and/or wet heave lawn grass clipped by the clipping part  78  is caught in the intersection part  50  ( FIG. 3 ), it is possible to transport the lawn grass on the transportation wind efficiently. 
     The present invention is not limited to the above described embodiments. Various modifications can be made without deviating from the gist of the present invention.