Patent Publication Number: US-11390109-B2

Title: Mechanical pencil

Description:
FIELD 
     This disclosure relates to a mechanical pencil that includes a chuck for chucking a writing lead and that is capable of feeding out the writing lead by a click operation. 
     BACKGROUND 
     Conventionally, there is known a mechanical pencil including a writing lead tank slidably provided inside a barrel, a chuck fixed to a tip part of the writing lead tank, a chuck ring loosely fitted to the chuck, a sleeve provided between the barrel and the chuck, an elastic body which abuts with the sleeve and which is mounted so that a part thereof is attached with pressure to the writing lead tank, and operating means which compresses the elastic body and makes the writing lead tank movable in an axial direction (for example, refer to PTL 1). 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Patent Application Laid-open No. H 07-290880 (for example, refer to paragraphs 0006, 0007, and 0017) 
       
    
     SUMMARY 
     Disclosure 
     According to the mechanical pencil disclosed in PTL 1, a stroke for pushing out a slider can be sufficiently obtained with a simple structure which integrates a chuck fastening spring with a cushion spring. In addition, excessive writing pressure can be absorbed by the cushion spring, preventing breakage of the writing lead. However, it has conventionally been desired to provide a mechanical pencil that has a cushion spring that not only can prevent breakage of the writing lead thereof but also provides more excellent writing feel than conventional one, such as a softer writing touch. 
     An exemplary object of this invention is to provide a mechanical pencil that has a cushion spring that provides more excellent writing feel than the prior art. 
     In one aspect of the present invention, a mechanical pencil includes a writing lead, a chuck unit for chucking the writing lead, and a cushion spring configured to elastically support the chuck unit so that the chuck unit being retractable with a writing pressure, wherein an output in response to a cushion stroke of the cushion spring is non-linear, and a slope of a spring constant in a region where the cushion stroke is relatively small is smaller than a slope of a spring constant in a region where the cushion stroke is relatively large. 
     In a plurality of exemplary aspects according to this invention, a mechanical pencil that has a cushion spring that provides more excellent writing feel than the prior art can be provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a partial sectional view illustrating a forward-side portion and a rearward-side portion while omitting an intermediate portion of a mechanical pencil according to an embodiment; 
         FIG. 2  is a perspective view illustrating a sleeve having a cushion spring of the mechanical pencil according to the embodiment; 
         FIG. 3  is a diagram illustrating a relationship between a cushion stroke and a writing pressure in the cushion spring of the mechanical pencil according to an embodiment; 
         FIG. 4A  is a partial sectional view of the forward side for explaining operations of the cushion spring performed when writing is continued by the mechanical pencil according to an embodiment, the partial sectional view illustrating an initial state; 
         FIG. 4B  is a partial sectional view of the forward side for explaining the operations of the cushion spring performed when writing is continued by the mechanical pencil according to an embodiment, the partial sectional view illustrating a state in which the cushion spring is bent; 
         FIG. 4C  is a partial sectional view of the forward side for explaining the operations of the cushion spring performed when writing is continued by the mechanical pencil according to an embodiment, the partial sectional view illustrating a state in which the cushion spring is restored; 
         FIG. 5  is a perspective view illustrating modification 1 of the cushion spring of the mechanical pencil according to the embodiments; 
         FIG. 6  is a perspective view illustrating modification 2 of the cushion spring of the mechanical pencil according to the embodiments; and 
         FIG. 7  is a perspective view illustrating modification 3 of the cushion spring of the mechanical pencil according to the embodiments. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments will be described with reference to the drawings. A mechanical pencil  1  according to the present embodiment illustrated in  FIG. 1  is a rear end click-type mechanical pencil in which a writing lead T is fed out and protrudes from a tip of a tip fitting  3  by a click operation on a click button  5 . Moreover, in the following description, a side on which the tip fitting  3  of the mechanical pencil  1  is arranged will be referred to as front and a side on which the click button  5  is arranged will be referred to as rear in a direction of a central axis (i.e., an axial direction) which extends in a longitudinal direction of the mechanical pencil  1 . 
     The mechanical pencil  1  includes a barrel body  2  with an approximately cylindrical shape and the tip fitting  3  having an approximately conical front part and an approximately cylindrical rear part. A barrel is formed so as to include the barrel main body  2  and the tip fitting  3 . The tip fitting  3  is arranged to the front of the barrel body  2 . A cylindrical part  3   a  having an outer diameter smaller than a rear end diameter of a front part of the tip fitting  3  is formed in a rear part of the tip fitting  3 . The tip fitting  3  is fixed to the barrel body  2  by screwing an internal screw part  2   a  formed on an inner circumferential surface of a front end part of the barrel body  2  and an external screw part  3   a   1  formed on an outer circumferential surface of the cylindrical part  3   a  in the rear part of the tip fitting  3 . 
     The click button  5  formed in a bottomed cylindrical shape is attachably and detachably mounted to a rear end of a writing lead tube  11  arranged inside the barrel body  2 . An inner circumferential surface of a front end opening of the click button  5  is attachably and detachably fitted to an outer circumferential surface of the rear end of the writing lead tube  11 . An outer circumferential surface of a front part of an eraser ferrule  4 , formed in an approximately cylindrical shape, is attachably and detachably fitted and assembled to an inner circumferential surface of the rear end of the writing lead tube  11 . The eraser ferrule  4  has a forward-side small diameter part  4   a  and a rearward-side large diameter part  4   b . An outer circumferential surface of an eraser  6  is attachably and detachably fitted and assembled to an inner circumferential surface of the large diameter part  4   b  of the eraser ferrule  4 . 
     The writing lead tube  11 , internally housing the writing lead T, is formed in an approximately cylindrical shape and arranged inside the barrel body  2 . A chuck  12  is assembled to a front part of the writing lead tube  11 . The chuck  12  is formed so as to be capable of chucking the writing lead T by chucking/clamping in a radial direction when each chuck piece formed by dividing a tip of the chuck  12  into three parts in a circumferential direction elastically deforms toward a central axis. The chuck  12  has a rear end base part  12   a  fixed by being inserted into the writing lead tube  11 , a beam-like part  12   b  extending forward from the base part  12   a , and a bulging part  12   c  formed at a front end of the beam-like part  12   b . A chuck ring  13  is fittably and detachably mounted to an outer circumference of the bulging part  12   c . An approximately cylindrical cushion member  7  is arranged on the rear side of the chuck ring  13  so as to cover the range from the beam-like part  12   b  of the chuck  12  to the front part of the writing lead tube  11 , from an outer diameter direction. 
     As illustrated in  FIGS. 1 and 2 , a cylindrical sleeve part  7   a  that is inserted from the cylindrical part  3   a  of the tip fitting  3  into the tip fitting  3  along an inner circumferential surface of the tip fitting  3  is formed in a front part of the cushion member  7 . An approximately cylindrical spring part  7   b , arranged behind the cylindrical part  3   a  of the tip fitting  3  and having a diameter larger than that of the sleeve part  7   a , is formed in the rear part of the cushion member  7 . A flange part  7   c , arranged behind the cylindrical part  3   a  of the tip fitting  3  and having a diameter larger than that of the spring part  7   b , is formed in the connecting portion between the sleeve part  7   a  and the spring part  7   b . An annular wall  7   a   1  is formed so as to protrude inward in the radial direction on an inner circumferential surface of a front end part of the sleeve part  7   a.    
     The cushion member  7  is integrally formed of a resin. As illustrated in  FIG. 2 , the spring part  7   b  of the cushion member  7  is formed by forming a pair of elongated holes  7   b   1  so as face each other with an axis therebetween, the elongated holes  7   b   1  opening along a circumferential direction of the cylindrical outer wall, and by arranging the pairs of plurality of elongated holes  7   b   1  along the axial direction. The pairs of elongated holes  7   b   1 , adjacent to each other along the axial direction, are arranged in the circumferential direction so as to be in a relative positional relationship in which the longitudinal holes  7   b   1  rotate 90 degrees about the axis. In this arrangement, the length of the spring part  7   b  in the axial direction can be reduced. In the present embodiment, five pairs of elongated holes  7   b   1  are formed along the axial direction. An elastic beam-like part  7   b   2  is formed between the pairs of elongated holes  7   b   1  adjacent to each other in the axial direction. Furthermore, inter-hole struts (i.e., axially extending parts)  7   b   3  are formed between the pairs of elongated holes  7   b   1  arranged in the circumferential direction. The elastic beam-like part  7   b   2  is formed in such a manner that the thickness thereof in the axial direction gradually decreases from the front-side inter-hole strut  7   b   3  toward the rear-side inter-hole strut  7   b   3  that is adjacent thereto in the axial direction. When applied a force in the axial direction, the spring part  7   b  generates an elastic force as the elastic beam-like part  7   b   2  bends in a direction in which the opening of each elongated hole  7   b   1  shrinks. 
     As illustrated in  FIG. 3 , the spring part  7   b  of the cushion member  7  is configured in such a manner that an output in response to a stroke (i.e., a cushion stroke) of the spring part  7   b  is non-linear. Details will be described below. 
     Returning to  FIG. 1 , an outer circumference of the sleeve part  7   a  of the cushion member  7  is guided in proximity to an inner circumferential surface of the cylindrical part  3   a  of the tip fitting  3 . The inner circumferential surface of the sleeve part  7   a  is guided in proximity to the outer circumferential surface of the writing lead tube  11 . A plurality of ribs  2   b , extending in the axial direction, are formed on the inner circumferential surface of the barrel body  2  corresponding to the outer circumference of the spring part  7   b  of the cushion member  7 . Each of the plurality of ribs  2   b  forms a stepped part  2   b   1  between a front end of each rib  2   b  and the inner circumferential surface of the barrel body  2 . The stepped part  2   b   1  is formed so as to face a rear surface of the flange part  7   c  of the cushion member  7 . A protruding part  2   c  that annularly protrudes inward in the radial direction is formed on the inner circumferential surface of the barrel body  2  at the rear of the spring part  7   b  of the cushion member  7 . A rear end surface of the spring part  7   b  of the cushion member  7  abuts with a front surface of the protruding part  2   c.    
     A chuck spring  15 , which includes a coil spring, is assembled between the outer circumferential surface of the beam-like part  12   b  of the chuck  12  and the inner circumferential surface of the sleeve part  7   a  of the cushion member  7 . A front end of the chuck spring  15  abuts with a rear surface of the annular wall  7   a   1  of the sleeve part  7   a  and a rear end of the chuck spring  15  abuts with a front end surface of the writing lead tube  11 . The chuck spring  15  is assembled in a state of being compressed in the axial direction between the cushion member  7  and the writing lead tube  11 . Since the writing lead tube  11  and the chuck  12  are biased rearward with respect to the sleeve part  7   a  of the cushion member  7  by a biasing force of the chuck spring  15 , the chuck ring  13  fitted to the chuck  12  is also biased rearward with respect to the sleeve part  7   a . Consequently, a rear end surface of the chuck ring  13  abuts with a front surface of the annular wall  7   a   1  of the sleeve part  7   a.    
     An annular stepped part  3   b  is formed on the inner circumferential surface of the tip fitting  3  in such a manner as to face the front end surface of the sleeve part  7   a  of the cushion member  7 . The front end surface of the sleeve part  7   a  of the cushion member  7  abuts with the stepped part  3   b  while being biased forward by the spring part  7   b  of the cushion member  7 . 
     A stepped part  3   c  is formed on the inner circumferential surface of the tip fitting  3 , to the front of the stepped part  3   b , in such a manner that the front end surface of the chuck ring  13  is abuttable with the stepped part  3   c  when moving forward. When the front end surface of the chuck ring  13  abuts with the stepped part  3   c  of the tip fitting  3 , the chuck ring  13  separates rearward from the chuck  12  and the writing lead T is released from the chuck of the chuck  12 . A series of feed-out operations by the mechanical pencil  1  will be described below in detail. 
     At a position to the front in the vicinity of the front end surface of the chuck  12 , a guide tube  31  is arranged so as to be movable in the axial direction. The guide tube  31  has a disk-shaped base end part  31   b , a central hole into which the writing lead T is inserted in the axial direction, and a plurality of forward protruding parts  31   a  formed around the central hole in such a manner as to protrude forward from the base end part  31   b . A tip of each forward protruding part  31   a  is formed in a hook shape as illustrated and, when engaged inside a slit  32   a  formed in a writing lead holder  32  to be described in detail below, the forward protruding part  31   a  is locked so as to be relatively movable in a front-rear direction with respect to the writing lead holder  32 . 
     A stepped part is formed at an outer circumferential edge of the base end part  31   b  of the guide tube  31 . In addition, a stepped part of an inner circumference of the tip fitting  3  is formed on the inner circumferential surface of the tip fitting  3  that faces the stepped part of the base end part  31   b  of the guide tube  31  in the axial direction. A return spring  36  that is a compression coil spring for biasing the guide tube  31  rearward in the axial direction with respect to the tip fitting  3  is arranged between the base end part  31   b  of the guide tube  31  and the stepped part of the inner circumference of the tip fitting  3 . In a state in which the guide tube  31  is biased rearward by the return spring  36 , a rear end surface of the base end part  31   b  of the guide tube  31  approaches the front end surface of the chuck  12  from the front. In addition, when the chuck  12  moves forward, a rear end surface of the guide tube  31  abuts with a tip surface of the chuck  12  from the front. In this state, the rear end surface of the guide tube  31  approaches or abuts with the tip surface of the chuck  12  from the front. Therefore, the base end part  31   b  of the guide tube  31  supports the writing lead T protruding from the tip surface of the chuck  12 , in a direction perpendicular to the axial direction. Therefore, a bending moment of the writing lead T chucked by the chuck  12 , which acts on a position on the tip surface of the chuck  12  in the axial direction, can be reduced, preventing writing lead breakage of the writing lead T at the position on the tip surface of the chuck  12 . 
     At a position to the front of the guide tube  31 , the writing lead holder  32  is arranged so as to be movable in the axial direction. A plurality of slits  32   a  extending in the front-rear direction are formed on the writing lead holder  32 . The hook-shaped tips of the forward protruding parts  31   a  of the guide tube  31  are brought into engagement with the slits  32   a  in a slidable manner. Accordingly, the guide tube  31  is locked so as to be relatively movable in the front-rear direction with respect to the writing lead holder  32 . The central hole into which the writing lead T is inserted in the axial direction is formed in the writing lead holder  32 . A holding part  32   b  that sandwiches and holds the writing lead T inward in the radial direction is formed at a tip portion of the central hole of the writing lead holder  32 . To the front of the writing lead holder  32  is arranged a tip tube  30  having an approximately tapered cylindrical shape which has an outer circumferential surface sliding against an inner circumferential surface of the opening part  3   d  of the tip fitting  3  and is configured so as to be movable in the axial direction while supporting the writing lead T from the outer diameter direction. A contact part  30   a  obtained when the tip tube  30  comes into contact with a paper surface (i.e., a tip outer circumference of the tip tube  30 ) is rounded to form a roundedly chamfered edge/corner. Therefore, even in a case where the contact part  30   a  of the tip tube  30  moves while in contact with the paper surface, writing can be performed while favorably causing the tip tube  30  to retract, without having the tip tube  30  caught on the paper surface. The writing lead holder  32  is inserted into the tip tube  30  from the rear and assembled to the tip tube  30 . 
     A flange part is formed at a rear end of the tip tube  30 . An O-ring  38  that elastically supports the tip tube  30  and the writing lead holder  32  in the direction perpendicular to the axial direction is assembled to an outer circumference of the flange part. The O-ring  38  is configured so as to impart a prescribed sliding resistance with respect to movements of the tip tube  30  and the writing lead holder  32  in the front-rear direction. In the present embodiment, the prescribed sliding resistance is configured to be a sliding resistance capable of holding the tip tube  30  and the writing lead holder  32  so that the writing lead T having been fed out can be held in the axial direction. The prescribed sliding resistance is configured to be a sliding resistance which, when a larger pressing force in the axial direction is applied to the tip tube  30  or the writing lead holder  32 , allows movements of the tip tube  30  and the writing lead holder  32  so that the tip tube  30  protrudes from the tip fitting  3  or the tip tube  30  is housed inside the tip fitting  3 . Forward movements of the tip tube  30  and the writing lead holder  32  are restricted as the flange part of the tip tube  30  abuts with a stepped part  3   d   1  formed on the inner circumferential surface of the opening part  3   d  of the tip fitting  3 . 
     A chuck unit capable of chucking and feeding out the writing lead T includes the chuck  12 , the chuck ring  13 , the sleeve part  7   a  of the cushion member  7 , and the chuck spring  15 , and is housed inside the barrel. The chuck unit is elastically supported by the spring part  7   b  of the cushion member  7  (i.e., the cushion spring) so as to be retractable by a load (so-called “writing pressure”) generated in the axial direction by writing. 
     Retracting of the chuck unit is restricted by the rear surface of the flange part  7   c  of the cushion member  7  coming into abutment with the stepped part  2   b   1  of the front end of the rib  2   b  formed on the inner circumferential surface of the barrel body  2 . The space (i.e., cushion stroke) between the flange part  7   c  and the stepped part  2   b   1  of the rib  2   b  in the set state of the cushion member  7  is set within a range of a prescribed stroke length in which the spring part  7   b  is not damaged when the elongated holes  7   b   1  of the spring part  7   b  of the cushion member  7  shrink and thereby the elastic beam-like part  7   b   2  is repeatedly brought into close/solid contact with the spring part  7   b.    
     Moreover, it is desired that the cushion stroke be limited within the range of the prescribed stroke length in which the writing lead T is not bent/broken by the writing pressure when the writing lead T protrudes from the tip of the tip tube  30  by the same length as the cushion stroke. For example, when the cushion stroke is set at 0.8 mm, the writing lead T protrudes by at least 0.8 mm, which is the same as the cushion stroke, from the tip of the tip tube  30  that is brought into contact with the paper surface and retracts as a result of a cushion operation, after the writing lead T is released from the writing pressure. Even if writing applies the writing pressure to the writing lead T, then bending/breaking of the writing lead T by the writing pressure can be prevented by the configuration in which the protruded writing lead T is housed substantially entirely in the tip tube  30  again due to bending of the cushion spring (e.g., the spring part  7   b ). In order to achieve this configuration, it is preferred that the cushion stroke (i.e., full stroke) falls substantially in the range of, for example, 0.8 mm±0.4 mm. 
     Furthermore, the cushion member  7  can be assembled so as to have an arbitrary prescribed set load in a state in which the spring part  7   b  of the cushion member  7  is compressed by the stepped part  3   b  of the inner circumferential surface of the tip fitting  3  and the protruding part  2   c  of the inner circumferential surface of the barrel body  2 . Actions and effects of the spring part  7   b , functioning as the cushion spring, are described below in detail. 
     In the present embodiment, the set load of the spring part  7   b  of the cushion member  7  is set to be equal to or lower than a sliding resistance obtained by an O-ring  38  of the tip tube  30  (i.e., the resistance force resulting from axial retracting of the tip tube  30 ). According to this configuration, the cushioning operation of the spring part  7   b  of the cushion member  7  can be executed so that the tip of the tip tube  30  easily comes into contact with the paper surface. In addition, in the present embodiment, the holding force of the writing lead holder  32  holding the writing lead T (the sliding resistance in the axial direction thereof), too, is set to be smaller than the sliding resistance caused by the O-ring  38  of the tip tube  30 . According to this configuration, the cushioning operation of the spring part  7   b  can be executed so that the tip of the tip tube  30  comes into contact with the paper surface more easily. Moreover, the set load of the spring part  7   b  of the cushion member  7  in the present embodiment is set to be smaller than the holding force of the writing lead holder  32  holding the writing lead T. According to this configuration, the cushioning operation of the spring part  7   b  can be executed so that the tip of the tip tube  30  comes into contact with the paper surface more easily. With the mechanical pencil  1  of the present embodiment, writing by the writing lead T can be executed because the tip tube  30  in contact with the paper surface can further retract by the writing pressure, even when the spring part  7   b  of the cushion member  7  performs the cushioning operation so that the tip of the tip tube  30  comes into contact with the paper surface and when the writing lead T is housed in the tip tube  30 . 
     Furthermore, in the tip tube  30  of the present embodiment, the resistance force is generated by the frictional resistance between the O-ring  38  and the inner circumferential surface of the tip fitting  3 , but in another embodiment, a prescribed resistance force may be generated by a set load of any spring configured to elastically support the tip tube  30  in the axial direction. 
     How the writing lead T is fed out by the mechanical pencil  1  is described next. In a state in which the tip tube  30  is housed inside the tip fitting  3 , the base end part  31   b  of the guide tube  31  and the rear end of the writing lead holder  32  come into abutment with each other. From this state, by performing the click operation in which the click button  5  is pressed, the guide tube  31  moves forward along with the chuck  12  moving forward, and thereby the writing lead holder  32  and the tip tube  30  protrude from the tip fitting  3 , obtaining the state illustrated in  FIG. 1 . The click operation in the state illustrated in  FIG. 1  is described hereinafter. 
     As a result of the click operation of clicking the click button  5 , the writing lead tube  11 , the chuck  12  to which the chuck ring  13  is fitted, and the writing lead T chucked by the chuck  12  move forward against a biasing force of the chuck spring  15 . The guide tube  31  abutting with the chuck  12  having moved forward is also pressed forward and moves forward against a biasing force of the return spring  36 . As a result of the guide tube  31  having moved forward, a front surface of the base end part  31   b  of the guide tube  31  comes into abutment with the rear end of the writing lead holder  32 . In this manner, the writing lead holder  32  that is pressed by the base end part  31   b  of the guide tube  31  moves forward until the flange part of the tip tube  30  comes into abutment with the stepped part  3   d   1  on the inner circumferential surface of the tip fitting  3  along with the tip tube  30  assembled to the writing lead holder  32 , and the writing lead holder  32  then protrudes forward from the tip fitting  3 . Once the chuck  12  and the chuck ring  13  have moved by a prescribed interval, the front end surface of the chuck ring  13  abuts with the abutting surface of the stepped part  3   c  formed on the inner circumferential surface of the tip fitting  3 , and the chuck ring  13  disengages rearward from the bulging part  12   c  of the chuck  12 . Once the chuck ring  13  disengages, each chuck piece of the chuck  12  opens outward in the radial direction due to elasticity and the writing lead T is released. The writing lead T is fed out by a prescribed feed-out amount per click operation in the mechanical pencil  1  and is then released, and in this position the writing lead T is sandwiched by the writing lead holder  32 . When the click button  5  is released from being clicked and the chuck unit is released from the click operation, the chuck  12  and the chuck ring  13  retract, leaving the writing lead T at the position where the writing lead T was fed out and released, and once again chuck the writing lead T at a position farther rear than the position obtained prior to the click operation. Furthermore, when the chuck unit is released from the click operation and the chuck  12  and the chuck ring  13  retract, the guide tube  31  is biased by the return spring  36  and retracts to the position where the rear end of the guide tube  31  approaches or abuts with the chuck  12 . 
     The mechanical pencil  1  is used when writing in the state in which a prescribed amount of the writing lead T protrudes from the tip tube  30  or in the state in which the writing lead T is housed in the tip tube  30 . The writing pressure during writing is applied to the writing lead T, the chuck  12  and the chuck ring  13  that chuck the writing lead T, and the sleeve part  7   a  of the cushion member  7 . The spring part  7   b  functioning as the cushion spring elastically deforms in accordance with this writing pressure, whereby the cushioning operation occurs in which the writing lead T, the chuck  12 , the chuck ring  13 , and the sleeve part  7   a  of the cushion member  7  retract rearward with respect to the barrel body  2 . In a case where the tip of the tip tube  30  comes into contact with the paper surface, the tip tube  30  retracts rearward in the axial direction by the writing pressure, against the sliding resistance with the tip fitting  3 . 
     The diagram of  FIG. 3  illustrating the relationship between the cushion stroke and the writing pressure is used to specifically describe embodiments. In  FIG. 3 , in a state in which the cushion member  7  is assembled in the barrel at a prescribed set load, a cushion stroke x (mm) is defined as 0. In the present embodiment, the prescribed set load mentioned above is substantially 0. At point A on the diagram, since the elasticity of the cushion member  7  is configured in such a manner that the writing pressure is approximately 48 g and the cushion stroke is approximately 0.3 mm, the chuck unit retracts approximately 0.3 mm when the writing pressure is approximately 48 g. Similarly, as indicated by point B, the elasticity of the cushion member  7  is configured so that the chuck unit retracts approximately 0.6 mm when the writing pressure is approximately 180 g. Here, the slope of the spring constant is defined as the slope of the tangent line of an arbitrary point on the relationship diagram illustrated in  FIG. 3 . In the present embodiment, the slope of the spring constant of the spring part  7   b  of the cushion member  7  is configured in such a manner that the slope of the spring constant in a region where the cushion stroke x (mm) is relatively small (for example, 0 to less than 0.4 mm), is smaller than the slope of the spring constant in a region where the cushion stroke x (mm) is relatively large (for example, 0.4 mm or more). Specifically, for example, the slope of the spring constant at point A is θ1, and the slope of the spring constant at point B is θ2, wherein θ1&lt;θ2. Therefore, the spring part  7   b , which is the cushion spring, can be cushion-operated to generate the prescribed cushion stroke x (mm) even when a load extremely smaller than the normal writing pressure (approximately 300 g is considered standard) is applied. On the other hand, with a load close to the normal writing pressure, a cushion stroke is less likely to occur (i.e., the writing feel is harder) compared to a region with a load smaller than the normal writing pressure. The present embodiment is configured in such a manner that the slope of the spring constant increases together with (according to) the cushion stroke. With this configuration, while the cushion operation can be caused with a relatively small writing pressure, the cushion stroke can be prevented from becoming excessively large. In this case, an excellent writing feel with a soft writing touch and a firm feel can be obtained. Another embodiment may be configured in such a manner that, for example, the slope of the spring constant monotonically increases at a constant rate as the cushion stroke increases. In this case, the output of the cushion spring at the time of a cushion stroke is performed can be easily predictable. 
     Continuously writing with the mechanical pencil  1  consumes and wears out the writing lead T protruding from the tip of the tip tube  30 , as illustrated in  FIG. 4A . At this moment, the contact part  30   a  at the tip of the tip tube  30  comes into contact with the paper surface. As a result, the writing lead T receives a force directed axially rearward (i.e., writing pressure) from the paper surface, along with the tip tube  30  and the writing lead holder  32 . Once the tip tube  30  and the writing lead holder  32  receive the writing pressure, the tip tube  30  and the writing lead holder  32  retract against the resistance force that occurs between the O-ring  38  of the tip tube  30  and the inner circumferential surface of the tip fitting  3 . At this moment, since the guide tube  31  is biased rearward by the return spring  36 , the guide tube  31  moves rearward together with the tip tube  30  and the writing lead holder  32 . At the same time, once the writing lead T receives the writing pressure, the spring part  7   b  of the cushion member  7  bends and the chuck unit moves rearward. In other words, when the writing lead T receives the writing pressure, the chuck  12 , the chuck ring  13 , the sleeve part  7   a  of the cushion member  7 , the chuck spring  15 , and the writing lead tube  11  move rearward.  FIG. 4B  illustrates a state in which the writing lead T, together with the tip tube  30  and the writing lead holder  32 , receives the writing pressure and the tip tube  30 , the writing lead holder  32 , and the chuck unit move rearward. 
     Once the contact part  30   a  of the tip at the tip tube  30  separates from the paper surface, the spring part  7   b  of the cushion member  7  is restored by the elasticity thereof, moving the chuck unit forward and returns the chuck unit to the original position thereof. Specifically, as illustrated in  FIG. 4C , the tip of the sleeve part  7   a  of the cushion member  7  once again comes into abutment with the stepped part  3   b  on the inner circumferential surface of the tip fitting  3 , whereby the chuck  12 , the chuck ring  13 , the sleeve part  7   a  of the cushion member  7 , the chuck spring  15 , and the writing lead tube  11  return to the positions same as those illustrated in  FIG. 4A . The tip tube  30  and the writing lead holder  32 , on the other hand, remain at the position illustrated in  FIG. 4B  to which the tip tube  30  and the writing lead holder  32  have retracted in response to the writing pressure illustrated in  FIG. 4B . When the spring part  7   b  of the cushion member  7  is restored as described above, the output of the spring part  7   b  of the cushion member  7  (i.e., a restoring force due to the elasticity) is greater than the force of the writing lead holder  32  holding the writing lead T, and the resistance force of the tip tube  30  against the tip fitting  3  is greater than the force of the writing lead holder  32  holding the writing lead T. According to this configuration, the writing lead T can be caused to protrude from the tip of the tip tube  30  without performing the click operation of the mechanical pencil  1 . Therefore, the mechanical pencil  1  in which the writable distance obtained by a single click operation is longer than that of the prior art can be obtained. In order to cause the writing lead T to protrude from the tip of the tip tube  30  within a short cushion stroke without performing the click operation as described above, it is desirable to have the configuration in which the slope of the spring constant increases together with (according to) the cushion stroke, as in the spring part  7   b  of the cushion member  7  of the present embodiment. 
     In writing characters and the like using the mechanical pencil  1 , the tip of the mechanical pencil  1  (e.g., the writing lead T or the tip tube  30 ) repeatedly comes into contact with and separates from the paper surface. In the present embodiment, the tip tube  30  and the chuck unit retract under a load sufficiently smaller than the writing pressure, and the chuck unit holding the writing lead T returns forward while leaving the tip tube  30  at the retracted position thereof when the writing lead T and the tip tube  30  separate from the paper surface. Therefore, writing can be continued using a prescribed length of the writing lead T without performing the click operation of feeding out the writing lead T of the mechanical pencil  1 . The prescribed length of the writing lead T available for writing continuously is the length of the writing lead T corresponding to the length from a protrusion limit of the tip tube  30  (i.e., the position where the flange portion of the tip tube  30  abuts with the stepped part  3   d   1  of the tip fitting  3 ) to an indentation limit of the tip tube  30  (i.e., the position in the tip fitting  3  where the tip of the tip tube  30  is approximately flush with the tip of the tip fitting  3 , into which the tip tube  30  is indented). 
     It is preferred that the set load of the spring part  7   b  of the cushion member  7  be set to be equal to or lower than the resistance force generated by the tip tube  30  retracting in the axial direction, so that the writing lead T and the chuck unit holding the writing lead T can retract together with the tip tube  30  retracting. As described above, the spring part  7   b  is configured in such a manner that an output thereof in response to a cushion stroke is non-linear as illustrated in  FIG. 3 , wherein the slope of the spring constant is small in a region where the cushion stroke is relatively small. In the present embodiment, favorable operations are performed as illustrated in  FIGS. 4A to 4C  in which, without being aware of the operation of the spring part  7   b  of the cushion member  7 , the writing lead T is caused to naturally retract together with the tip tube  30  during writing so that the writing lead T is exposed from the tip tube  30 . Furthermore, since the slope of the spring constant is large in the region where the cushion stroke is relatively large, the cushioning effect obtained due to the writing pressure of the spring part  7   b  of the cushion member  7  can be experienced within a relatively short cushion stroke range. In this case, the writing pressure can be reduced appropriately before the writing lead T is bent/broken. 
     In the present embodiment, the region where the cushion stroke is relatively small may be defined as, as described above, the first half of the diagram illustrated in  FIG. 3  where the cushion stroke x (mm) is 0 to less than 0.4 mm but is more preferably the front portion of the diagram where the cushion stroke x (mm) is 0 to less than 0.2 mm, as long as the abovementioned effect is exerted. Most preferably, the region may indicate the front end portion in the diagram where the cushion stroke is 0 mm or the initial movement portion of the cushion spring (e.g., the spring part  7   b ). Similarly, in the present embodiment, the region where the cushion stroke is relatively large may be defined as, as described above, the latter half of the diagram illustrated in  FIG. 3  where the cushion stroke x (mm) is equal to or greater than 0.4 mm, but is more preferably the latter portion of the diagram where the cushion stroke is 0.6 mm or more, as long as the abovementioned effect is exerted. Most preferably, the region may indicate the rear end portion of the diagram where the cushion stroke is 0.8 mm or the final movement portion of the cushion spring (e.g., the spring part  7   b ). For example, the slope of the spring constant of the cushion spring (e.g., the spring part  7   b ) at the initial movement portion can be set to be smaller than the slope of the spring constant at the final movement portion. Alternatively, in another embodiment, the cushion spring may be configured in such a manner that, in accordance with the average value of the slope of the spring constant in any region defined as above, the average value of the slope of the spring constant in the region where the cushion stroke is relatively small and is smaller than the average value of the slope of the spring constant in the region where the cushion stroke is relatively large. In this case, a preferable mechanical pencil can be configured using various cushion springs, including the use of a cushion spring that includes a region portion where the slope of the spring constant decreases arbitrarily as the cushion stroke increases. 
     Modifications of the spring part  7   b  (e.g., the cushion member  7 ) that are the cushion springs according to the other embodiments are described next. 
     (Modification 1) 
       FIG. 5  illustrates a cushion member  71  of modification 1. As with the cushion member  7  according to the embodiment illustrated in  FIG. 2 , the cushion member  71  of modification 1 has a sleeve part  71   a , a spring part  71   b  functioning as the cushion spring, and a flange part  71   c . A pair of elongated holes  71   b   1 , a beam-like elastic part  71   b   2 , and an inter-hole strut  71   b   3  are formed in the spring part  71   b . In this modification 1 as well, the beam-like elastic part  71   b   2 , which is in the shape of a tapered cantilever beam on a cylindrical surface, is formed in such a manner that an output of the spring part  71   b  functioning as the cushion spring is a non-linear output. The spring part  71   b  of the cushion member  71  of modification 1 further includes a plurality of restricting protrusions  71   b   4  protruding from an inter-hole strut  71   b   3  toward the inter-hole strut  71   b   3  that is adjacent thereto via the elongated hole  71   b   1  located forward in the axial direction. The restricting protrusions  71   b   4  are arranged in such a manner that the restricting protrusions  71   b   4  adjacent to each other along the axial direction are in a relative position rotated 90 degrees circumferentially about the axis. 
     According to the spring part  71   b  of the cushion member  7  of modification 1, even if the spring part  71   b  is compressed and thereby the elongated holes  71   b   1  shrink in the axial direction, each restricting protrusion  71   b   4  abuts with a portion facing a tip of said restricting protrusion  71   b   4  (e.g., the portion being each inter-hole strut  71   b   3  in modification 1), preventing the contraction of the elongated holes  71   b   1  from tightly attaching/solidly closing together the beam-like elastic portions  71   b   2  axially adjacent to each other. In this manner, by forming the restricting protrusions  71   b   4  as protrusions extending in the axial direction of the spring part  71   b  functioning as the cushion spring, a deterioration in performance of the spring part  71   b  caused by an excessive cushion operation such as the tight attachment/full-close of the spring (i.e., attachment/solidly close between the beam-like elastic parts  71   b   2 ) can be prevented. 
     Using the cushion member  71  of modification 1 can omit regulation of the compression of the spring part  7   b  caused by the abutment between the stepped part  2   b   1  on the inner circumferential surface of the barrel body  2  of the present embodiment and the flange part  7   c  of the cushion member  7  illustrated in  FIG. 1 , thereby simplifying the mechanism. 
     (Modification 2) 
       FIG. 6  illustrates a cushion member  72  of modification 2. The cushion member  72  of modification 2 is a modification of the spring part  7   b  of the cushion member  7  of the present embodiment illustrated in  FIG. 2 . The cushion member  72  of modification 2 includes an approximately cylindrical sleeve part  72   a , a spring part  72   b  functioning as the cushion spring, and a flange part  72   c . The spring part  72   b  of modification 2 is a resin spring formed in a spiral shape and generates an axial elastic force. A cushion member having an excellent design, such as the cushion member  72  of modification 2, may be used as long as the output of the spring part functioning as the cushion spring is a non-linear output, as described above. In this case as well, any shape of the spring part that is thick in the radial direction may be changed arbitrarily so that the output of the spring part is a non-linear output. 
     (Modification 3) 
       FIG. 7  illustrates a cushion member  73  of modification 3. The cushion member  73  of modification 3 is a modification of the spring part  7   b  of the cushion member  7  of the present embodiment illustrated in  FIG. 2 . The cushion member  73  of modification 3 includes an approximately cylindrical sleeve part  73   a , a spring part  73   b  functioning as the cushion spring, and a flange part  73   c.    
     The spring part  73   b  has a front-side cylindrical part  73   b   1  and a rear-side cylindrical part  73   b   2 . Two torsion springs  73   b   3  connecting the front-side cylindrical part  73   b   1  and the rear-side cylindrical part  73   b   2  are formed between the front-side cylindrical part  73   b   1  and the rear-side cylindrical part  73   b   2  in such a manner as to extend in a direction oblique to the axial direction. When a compressive force is applied to the cushion member  73  in the axial direction, the front-side cylindrical part  73   b   1  and the rear-side cylindrical part  73   b   2  rotate relatively in opposite directions in the circumferential direction around the axis, and the torsion springs  73   b   3  are compressed along the axial direction so as to be twisted, thereby generating an elastic force. 
     Embodiments and modifications of the present invention have been described above. However, the present invention can be implemented in various modes without being limited to the embodiments and modifications described above. For example, in the embodiments, although the sleeve and the cushion spring are formed integrally of a resin material, the sleeve part  7   a  and the spring part  7   b  may be formed separately. In so doing, the cushion spring can be formed into various elastic members such as a metal coil spring. 
     REFERENCE SIGNS LIST 
     
         
           1  Mechanical pencil 
           2  Barrel body 
           2   a  Internal screw part 
           2   b  Rib 
           2   b   1  Stepped part 
           2   c  Protruding part 
           3  Tip fitting 
           3   a  Cylindrical part 
           3   a   1  External screw part 
           3   b  Stepped part 
           3   c  Stepped part 
           3   d  Opening part 
           3   d   1  Stepped part 
           4  Eraser ferrule 
           4   a  Small diameter part 
           4   b  Large diameter part 
           5  Click button 
           6  Eraser 
           7  Cushion member 
           7   a  Sleeve part 
           7   a   1  Annular wall 
           7   b  Spring part 
           7   b   1  Elongated hole 
           7   b   2  Elastic beam-like part 
           7   b   3  Inter-hole strut 
           7   c  Flange part 
           11  Writing lead tube 
           12  Chuck 
           12   a  Base part 
           12   b  Beam-like part 
           12   c  Bulging part 
           13  Chuck ring 
           15  Chuck spring 
           30  Tip 
           30   a  Contact part 
           31  Guide tube 
           31   a  Forward protruding part 
           31   b  Base end part 
           32  Writing lead holder 
           32   a  Slit 
           32   b  Holding part 
           36  Return spring 
           38  O-ring 
           71  Cushion member 
           71   a  Sleeve part 
           71   b  Spring part 
           71   b   1  Elongated hole 
           71   b   2  Beam-like elastic part 
           71   b   3  Inter-hole strut 
           71   b   4  Restricting protrusion 
           71   c  Flange part 
           72  Cushion member 
           72   a  Sleeve part 
           72   b  Spring part 
           72   c  Flange part 
           73  Cushion member 
           73   a  Sleeve part 
           73   b  Spring part 
           73   b   1  Front-side cylindrical part 
           73   b   2  Rear-side cylindrical part 
           73   b   3  Spring 
           73   c  Flange part