Abstract:
A writing instrument has a writing refill slidably disposed in a tubular casing. A rotary cam mechanism extends and retracts the writing refill in response to knocking of a knocking member at a rear end of the casing. The rotary cam mechanism includes a cam cylinder inserted axially into the rear end of the casing, and projections on the cam cylinder engage with apertures in the casing to prevent axial movement of the cam cylinder inside the casing. Rotational movement of the knocking member rotates the cam cylinder and disengages the projections from the apertures, thereby permitting withdrawal of the cam cylinder from the casing and replenishment of the writing refill.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to writing instruments and, more specifically, to a writing instrument, such as a ballpoint pen, having a rotary cam mechanism for feeding a writing refill of the writing instrument. 
     2. Background Information 
     Various types of conventional writing instruments having a structure in which a writing refill is fed by a knocking operation using a rotary cam mechanism have been known as described in U.S. Pat. Nos. 5,413,428, 5,263,786 and 5,004,364. The rotary cam mechanism has a stationary cam formed on an inner face of a barrel, a rotary cam abutting a rear end of a writing refill and engaged with the stationary cam, and a knocking cam inserted in the stationary cam and which moves the rotary cam in an axial direction. The rotary cam mechanism is constructed so that by knocking the knocking cam, the rotary cam is rotated and its engagement position with the stationary cam is changed, by which the writing refill is protruded to a writing position or retracted to an accommodated position within the barrel. More specifically, the stationary cam has a guide groove extending in the axial direction and a slant cam face. The rotary cam has a guide projection which enters the guide groove of the stationary cam and a rotary cam face which engages with the slant cam face of the stationary cam. The knocking cam has an engagement projection which enters the guide groove of the stationary cam and a knocking cam face which engages with the rotary cam face of the rotary cam. The writing refill is urged backward by a spring and its rear end abuts on the rotary cam. Accordingly, the rotary cam and the knocking cam are urged rearwardly by the spring. 
     When the knocking cam is knocked, the rotary cam advances, the guide projection comes out of the guide groove of the stationary cam, and the rotary cam rotates by the actions of the rotary cam face and the knocking cam face. By this construction and operation, the rotary cam face of the rotary cam engages with the slant cam face of the stationary cam, and the rotary cam is maintained at the advanced position. Since the writing refill is advanced by the rotary cam, the front end of the writing refill protrudes from the front end of the barrel and can be used for writing. 
     When the knocking cam is knocked again in the writing state, the rotary cam is pressed and advanced by the knocking cam, the slant cam face and the rotary cam face are disengaged, and the rotary cam is further rotated. Then, the guide projection of the rotary cam enters the guide groove of the stationary cam, whereby the rotary cam and the writing refill urged backward by the spring are retracted, and the front end of the writing refill is accommodated in the barrel. 
     Since the entire body of the rotary cam mechanism is pressed backward by the spring which urges the writing refill backward as described above, the stationary cam forming part of the rotary cam mechanism is integrally formed on an inner wall of the barrel as described in U.S. Pat. Nos. 5,004,364 and 5,263,786, or a cam body including the stationary cam is formed and the cam body is fixed under a detachment-preventing state on the inner face of the barrel as described in U.S. Pat. No. 5,413,428. 
     Accordingly, the conventional writing instruments require the rotary cam and the knocking cam to be inserted in the stationary cam which is fixed as described above, from the front portion of the stationary cam, whereby the conventional writing instruments having the rotary cam mechanism are constructed so that the front portion of the barrel can be opened. As the structure for opening the front portion of the barrel, a nose cone (tip) is detachably disposed at the front end of the barrel by screwing, or the barrel is constructed so as to be separable into upper and lower parts. Accordingly, when the writing refill is changed, in many cases, the barrel constructed of separable parts must be disassembled, or the nose cone screwed at the front end of the barrel must be detached to open the front end of the barrel. Since such a screwed part is necessarily provided, the nose cone portion becomes large in diameter to some extent, or when the barrel is transparent, the screwed part or the like is seen from the exterior thereof which presents an unsightly appearance. Accordingly, the conventional writing instruments are associated with restrictions in both design and aesthetic appearance. 
     SUMMARY OF INVENTION 
     It is an object of the present invention to provide a writing instrument having a rotary cam mechanism for feeding a writing refill, wherein the writing refill can be changed from the rear end side of a casing or barrel without detaching a nose cone or disassembling the barrel of the writing instrument. 
     It is another object of the present invention to provide a writing instrument in which a writing refill can be changed by rotating a knocking part of the rotary cam mechanism without deterioration of the external appearance of the writing instrument. 
     It is still a further object of the present invention to provide a writing instrument which has a simple construction and which may be made economically and used in a convenient manner. 
     The foregoing and other objects of the present invention are carried out by a writing instrument comprising a tubular casing having a longitudinal axis, a through-hole extending in a longitudinal direction of the longitudinal axis, a wall surface, and an aperture formed in the wall surface. A writing refill is inserted in the through-hole of the tubular casing for extension out of the casing to a writing position and retraction into the casing to a retracted position. A rotary cam mechanism is accommodated in the through-hole of the casing for extending and retracting the writing refill between the writing position and the retracted position, respectively. The rotary cam mechanism comprises a cam cylinder having a projection engaging the aperture of the casing and a stationary cam, a rotary cam engageable with the stationary cam of the cam cylinder, and a knocking cam engageable with the stationary cam of the cam cylinder and the rotary cam. A knocking member is integrally connected to the knocking cam and is connected to the cam cylinder in a non-rotatable state while permitting movement of the knocking member relative to the cam cylinder in the longitudinal direction. 
     The stationary cam has a guide groove and a slant cam surface. The rotary cam has a guide projection inserted in the guide groove of the stationary cam and a rotary cam surface engageable with the slant cam surface of the stationary cam. The knocking cam has a locking projection inserted in the guide groove of the stationary cam and a knocking cam surface engageable with the rotary cam surface of the rotary cam. 
     In another embodiment, the writing instrument has a stopper integrally connected to an inner surface of the casing. Instead of being formed in the wall surface of the casing, the aperture is formed in the stopper for engagement with the projection of the cam cylinder. 
     Preferably, in the foregoing embodiments of the writing instruments, the cam cylinder has a tubular construction, and the rotary cam and the knocking cam are inserted in the cam cylinder. The stationary cam is formed in an inner surface of the cam cylinder. The projection of the cam cylinder engages the aperture of the casing so that the cam cylinder is prevented from undergoing movement in the longitudinal direction relative to the casing while permitting rotational movement of the cam cylinder relative to the casing in a direction of rotation. The projection of the cam cylinder has a slant surface slanting towards the direction of rotation of the cam cylinder. 
     The writing instruments according to the present invention preferably further comprise means for permitting the projection of the cam cylinder to undergo elastic movement in a radial direction of the cam cylinder during rotation of the cam cylinder in the direction of rotation. In one embodiment, the means for permitting the projection of the cam cylinder to undergo elastic movement comprises an aperture formed in the cam cylinder adjacent to the projection. In another embodiment, the means for permitting the projection of the cam cylinder to undergo elastic movement comprises a pair of slits formed in the cam cylinder with the projection disposed between the slits. 
     Thus the writing instrument according to the present invention has an aperture formed in the casing or in a stopper integrally connected to the casing. The cam cylinder is inserted in the stopper or casing and is provided with the stationary cam, and the knocking element is connected to the cam cylinder so that it does not rotate but moves in the axial direction relative to the cam cylinder. By this construction, the projection disposed on the cam cylinder can be detachably engaged with the aperture of the casing or the stopper. 
     Thus, it is possible to easily assemble the foregoing components in the casing by firstly connecting the knocking element to the cam cylinder, accommodating the rotary cam and the knocking cam into the cam cylinder, and then inserting the cam cylinder from a rear end of the casing directly or doing so under the condition that the cam cylinder is inserted in the stopper. Thereafter, when the writing refill is changed by rotating the knocking element, the cam cylinder rotates and the projection of the cam cylinder disengages from the aperture of the casing or the stopper, whereby the cam cylinder can be separated from the stopper or casing, and the cam cylinder and the knocking element can be taken out of the rear portion of the casing. Accordingly, unlike with conventional writing instruments, the writing refill can be changed without opening a front portion of the casing. 
     After the writing refill is replaced with a new one, the cam cylinder is inserted in the stopper or casing. In this instance, if a guide edge is provided at the rear end of the stopper or on the inner face of the casing so as to guide the projection to the aperture of the casing or stopper, the projection can be engaged with the aperture without positioning and only by pressing the cam cylinder in the axial direction, whereby the assembling operation is simplified. Furthermore, it is also possible to omit the screw and the connecting structure of the connecting portion which are required for writing instruments of which the casing is separated into a front part and a rear part and in which these parts are connected by screwing, whereby a nose cone portion can be made small in diameter. In addition, by using the stopper, the aperture engageable with the projection of the cam cylinder is not seen from the exterior of the casing, whereby the aesthetic appearance of the writing instrument is improved and the casing can be formed in various shapes and designs and from various materials. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The foregoing summary, as well as the following detailed description of a preferred embodiments of the invention, will be better understood when read in conjunction with the accompanying drawings. For the purpose of illustrating the invention, there is shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangement and instrumentalities shown. In the drawings: 
         FIG. 1  is a partial cross-sectional view showing a first embodiment of a writing instrument according to the present invention; 
         FIG. 2  is a partial exploded view of the writing instrument according to the first embodiment with the cam cylinder shown in phantom; 
         FIG. 3  is a partial cross-sectional view showing the rotary cam mechanism of the writing instrument shown in  FIG. 1  with the cam cylinder shown in phantom; 
         FIG. 4  shows is a front view of the stationary cam of the writing instrument shown in  FIG. 1 ; 
         FIG. 5  is a perspective view of the rotary cam of the writing instrument shown in  FIG. 1 ; 
         FIG. 6  is a perspective view of the knocking cam of the writing instrument shown in  FIG. 1 ; 
         FIGS. 7(A)-7(B)  show a stopper of the writing instrument shown in  FIG. 1 , where  FIG. 7(A)  is a partial cross-sectional view and  FIG. 7(B)  is a plane view; 
         FIG. 8  is a partial cross-sectional view showing an example of the structure of the stopper and the barrel for preventing relative rotation between the stopper and the barrel; 
         FIGS. 9(A)-9(D)  show a cam cylinder of the writing instrument shown in  FIG. 1 , where  FIG. 9(A)  is a partial cross-sectional view,  FIG. 9(B)  is a plane view,  FIG. 9(C)  is an enlarged cross-sectional view taken along line  9 (C)- 9 (C) in  FIG. 9(B) , and  FIG. 9(D)  is a view showing an engagement projection; 
         FIG. 10  is a partial cross-sectional view showing a modified version of the hole edge of the stopper and the edge face of the projection of the cam cylinder; 
         FIG. 11  is a partial cross-sectional view showing another modified version of the hole edge of the stopper and the edge face of the projection of the cam cylinder; 
         FIG. 12  is a plane view showing another modified version of the cam cylinder shown in  FIGS. 9(A)-9(D) ; 
         FIG. 13  showing another modified version of the cam cylinder shown in  FIGS. 9(A)-9(D) ; 
         FIGS. 14(A)-14(D)  show a knocking element of the writing instrument shown in  FIG. 1 , where  FIG. 14(A)  is a plane view,  FIG. 14(B)  is a partial cross-sectional view,  FIG. 14(C)  is an enlarged cross-sectional view taken along line  14 (C)- 14 (C) in  FIG. 14(B) , and  FIG. 14(D)  is an enlarged cross-sectional view taken along line  14 (D)- 14 (D) in  FIG. 14(B) ; 
         FIG. 15  is a view illustrating the state in which the cam cylinder and the knocking element of the writing instrument shown in  FIG. 1  are connected together; 
         FIGS. 16(A)-16(B)  show the state in which the stopper is connected to the connected cam cylinder and knocking element shown in  FIG. 15 , where  FIG. 16(A)  and  FIG. 16(B)  illustrate the state before and after the connection, respectively; and 
         FIG. 17  is a partial cross-sectional view showing another embodiment of the writing instrument according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While this invention is susceptible of embodiments in many different forms, this specification and the accompanying drawings disclose only examples of the use of the invention. The invention is not intended to be limited to the embodiments so described, and the scope of the invention will be pointed out in the appended claims. 
       FIGS. 1-9D  show an embodiment of the writing instrument, generally designated at  100 , according to the present invention. As shown in  FIGS. 1-2 , the writing instrument  100  has a barrel  1  in the form of a tubular member or casing having a through-hole extending longitudinally therethrough along a longitudinal axis A. A writing refill  2 , such as a ballpoint pen refill, is accommodated in the through-hole of the barrel  1  in the direction of the longitudinal axis A (“longitudinal direction”). Disposed in a rear portion of the barrel  1  are a tubular stopper  3  integrally connected to the barrel  1 , a tubular cam cylinder  4  extending through the stopper  3 , and a knocking member or element  5  connected to the cam cylinder  4  so as to allow relative linear movement between the cam cylinder  4  and the knocking element  5  in the longitudinal direction while preventing relative rotation between the cam cylinder  4  and the knocking element  5 . 
     The writing instrument  100  has a rotary cam mechanism for feeding or moving the writing refill  2  in the longitudinal direction between a first position, in which a writing tip  2   a  of the writing refill  2  is accommodated in the barrel  1  (retracted position), and a second position, in which the writing tip  2   a  of the writing refill  2  protrudes from the front end of the barrel  1  (writing position). With reference to  FIG. 3 , the rotary cam mechanism comprises a stationary cam  6  formed in an inner surface of the cam cylinder  4 , a rotary cam  7  disposed in the cam cylinder  4 , and a knocking cam  8  extending into the cam cylinder  4 . As show in  FIGS. 3-4 , the stationary cam  6  has a guide groove  6   a  extending in the longitudinal direction and a slant cam face  6   b . As show in  FIGS. 3 and 5 , the rotary cam  7  has a guide projection  7   a  that enters the guide groove  6   a  of the stationary cam  6  and a rotary cam face  7   b  for engagement with the slant cam face  6   b  of the stationary cam  6 . As shown in  FIGS. 3 and 6 , the knocking cam  8  has a guide projection  8   a  that enters the guide groove  6   a  of the stationary cam  6 , a knocking cam face  8   b  for engagement with the rotary cam face  7   b  of the rotary cam  7 , and a rod  8   c  detachably connected to the knocking element  5 . The writing refill  2  is urged backward (i.e., toward the rear portion of the barrel  1 ) by a spring  50  and a rear end of the writing refill  2  abuts on the rotary cam  7  ( FIG. 2 ). Accordingly, the rotary cam  7  and the knocking cam  8  are also urged backward by the spring  50  as well. 
     By the foregoing construction, while the writing tip  2   a  of the writing refill  2  is in the retracted position, the knocking element  5  can be knocked to advance the writing tip  2   a  of the writing refill  2  in the longitudinal direction so as to extend or protrude from the front end of the barrel  1  to place the writing tip  2   a  in the writing position. Likewise, while the writing tip  2   a  of the writing refill  2  is in the writing position, the knocking element  5  can be knocked to retract the writing refill  2  in the longitudinal direction and position the writing tip  2   a  in the barrel  1  in the retracted position. 
     Referring now to  FIGS. 1 ,  7 (A)- 7 (B) and  8 , the stopper  3  is formed in a substantially cylindrical shape and has a front end  10  abutting on an inner stepped portion  9  formed in the barrel  1 . A rotation-preventing portion  11  of the stopper  3  engages with a rotation-preventing member  1   a  formed in the barrel  1  so as to prevent relative rotation between the stopper  3  and the barrel  1  about the longitudinal axis A. An engagement portion  13  is formed on an outer wall surface of the stopper  3  for engagement with an inner engagement portion  12  formed in the inner surface of the barrel  1 . As best shown in  FIG. 7(B) , a pair of apertures or holes  14  are formed in the outer wall surface of the stopper  3  and are spaced approximately 180 degrees from one another around the circumference of the outer wall surface. A rear portion of the stopper  3  is provided with an inner slant surface  16  formed in the axial direction of the holes  14  for guiding the respective projections  15  of the cam cylinder  4  to engage the holes  14  as further described below. A guide edge  17  of the stopper  3  has rear edge portions terminating in hill-shaped protruding parts  17   a . The inner slant surface  16  forms a valley part  17   b  at the rear portion of the stopper  3 . The valley part  17   b  and the protruding parts  17   a  are connected together by arcuate curved surfaces  17   c.    
     In the embodiment shown in  FIG. 8 , the rotation-preventing portion  11  of the stopper  3  and the rotation-preventing member  1   a  formed in the barrel  1  are shown in the form of plane surfaces disposed in contact with one another to prevent relative rotation between the stopper  3  and the barrel  1 . However, it is understood by those skilled in the art that other forms of rotation preventing means, such as surfaces provided with engaging rib portions, may be employed for preventing relative rotation between the stopper  3  and the barrel  1 . 
     Referring now to  FIGS. 9(A)-9(D) , the cam cylinder  4  has an inner wall surface  4   a  and an outer wall surface  4   b . The stationary cam  6  is formed on the inner wall surface  4   a  and the projections  15  project from the outer wall surface  4   b . The projections  15  are spaced approximately 180 degrees from one another around the circumference of the outer wall surface  4   b  of the cam cylinder  4 . A flange  18  is formed at a rear portion and extends from the outer wall surface  4   b  of the cam cylinder  4 . A window hole or aperture  19  is formed in the outer wall surface  4   b  of the cam cylinder  4  and extends in a radial direction at a position between the projections  15 . A guide groove  20  contiguous with the aperture  19  is formed in an inner surface of the flange  18  and opens toward the rear portion of the cam cylinder  4 . The guide groove  20  has a width smaller than that of the aperture  19 . The aperture  19  constitutes means for allowing the projections  15  to undergo elastic movement in the radial direction during rotation of the cam cylinder  4  to facilitate engagement between the projections  15  and the respective holes  14  of the stopper  3 . 
     As shown in  FIGS. 9(B)-9(D) , each of the projections  15  has a vertical edge surface  21  and a slant or inclined edge surface  22 . The vertical edge surface  21  (i.e., the surface  21  is generally perpendicular to the longitudinal axis A) is configured for engagement with an edge  14   a  ( FIG. 7(B) ) of the corresponding stopper hole  14  and extends in an axial direction of the corresponding stopper hole  14 . By this construction, the projections  15  can be securely engaged with the vertical edge surface  21  of the respective holes  14  while preventing the projections  15  from slipping off in the axial direction. Stated otherwise, when engaged with the holes  14 , the projections  15  are prevented from detaching in the axial direction (i.e., in the direction of the longitudinal axis A). The inclined edge surface  22  (i.e., the surface  22  is not generally perpendicular to the longitudinal axis A) of each projection  15  is configured for detachable engagement with an edge  14   b  ( FIG. 7(B) ) of the corresponding stopper hole  14  and extends in a radial or circumferential direction of the corresponding stopper hole  14 . By this construction, when the cam cylinder  4  is rotated, the projections  15  will slip off the respective stopper holes  14  and readily enter the inner surface of the stopper  3 . 
     In the embodiment shown in  FIGS. 9(A)-9(D) , the projections  15  of the cam cylinder  4  are provided with the inclined edge surface  22 . Alternatively, as shown in  FIG. 10 , the edge  14   b  of the stopper holes  14  may be in the form of an inclined edge surface and the edge surface  22  of the cam cylinder  4  may be in the form of a vertical edge surface. In another embodiment, as shown in  FIG. 11 , both the edge  14   b  of the stopper holes  14  and the edge surface  22  of the cam cylinder  4  may be formed as conforming inclined edge surfaces. In yet another embodiment, as shown in  FIG. 12 , inclined edge surfaces  22  may be formed on both side surfaces of the projections  15  rather than on only one side surface as shown in  FIG. 11 . By these alternative forms of construction, the projections  15  detachably engage with the respective stopper holes  14  when the cam cylinder  4  is rotated. In this case, the aperture  19  of the cam cylinder  4  permits the projections  15  to undergo elastic movement in the radial direction during rotation of the cam cylinder  4  so that the projections  15  can readily engage the respective holes  14  of the stopper  3 . 
       FIG. 13  shows a modified form of the cam cylinder  4 . In  FIG. 13 , for each projection  15  of the cam cylinder  4 , a pair of slits  4   c  are formed in the outer wall surface  4   b  of the cam cylinder in directions generally perpendicular to the longitudinal axis A so that the projection  15  is disposed between the slits  4   c . The slits  4   c  further facilitate elastic movement of the projections  15  in the axial direction during rotation of the cam cylinder  4 . It is understood that elastic movement of the projections  15  in the axial direction during rotation of the cam cylinder  4  may be facilitated by means other than the slits  4   c , such as by making the outer wall of the cam cylinder  4  thin. Accordingly, it will be appreciated that elastic movement of the projections  15  in the axial direction during rotation of the cam cylinder  4  may be facilitated by structural means other than those described above without departing from the spirit and scope of the invention. 
     Referring now to  FIGS. 14(A)-14(C) , the knocking element  5  has a flange  23  and a tubular connecting portion  24  integral with and extending from one side of the flange  23 . The connecting portion  24  is configured to be inserted into the cam cylinder  4  and has a substantially rectangular cross-sectional shape with generally planar or flat side surfaces for engagement with the inner wall surface  4   a  of the cam cylinder  4  to prevent relative rotation between the cam cylinder  4  and the knocking element  5 . A knocking head  25  is integral with and extends from an opposite side of the flange  23  so that the knocking head  25  protrudes exteriorly from the rear end portion of the barrel  1 . The connecting portion  24  has connecting projections  26  and rib portions  27  integral with the connecting projections  26 . 
     As shown in  FIG. 14(C) , each of the connecting projections  26  extends in the circumferential direction of the knocking element  5  so that they slidably fit into the aperture  19  of the cam cylinder  4 . As shown in  FIGS. 14(A) and 14(D) , each of the rib portions  27  extend in the direction of the longitudinal axis A and are inserted in the guide groove  20  of the cam cylinder  4 . The rib portions  27  of the knocking element  5  and the guide groove  20  of the cam cylinder  4  constitute means for guiding the relative sliding movement between the cam cylinder  4  and the knocking element  5  in the longitudinal direction. An axial slit  28  is formed in the connecting portion  24  to facilitate insertion of the connecting portion  24  into the cam cylinder  4 . As shown in  FIG. 1 , the rod  8   c  of the knocking cam  8  is inserted into the connecting portion  24  of the knocking element  5  in the assembled state of the writing instrument  100 . 
     FIGS.  15  and  16 (A)- 16 (B) show states for connecting the stopper  3 , cam cylinder  4  and knocking element  5  together into an integral unit prior to assembling the integral unit into the barrel  1 . As shown in  FIG. 15 , the connecting portion  24  of the knocking element  5  is inserted into the cam cylinder  4  by inserting the rib portions  27  in the guide groove  20  of the cam cylinder  4 . In this state, relative rotation between the cam cylinder  4  and the knocking element  5  is prevented while the knocking element  5  is permitted to slide in the longitudinal direction relative to the cam cylinder  4 . Thereafter, as shown in  FIG. 16(A) , the cam cylinder  4  is inserted into the stopper  3  until the projections  15  of the cam cylinder  4  engage the respective holes  14  of the stopper  3 . At this point, a front end portion of the cam cylinder  4  projects from a front end  10  of the stopper  3 .  FIG. 16(B)  shows the assembled state of the stopper  3 , cam cylinder  4  and knocking element  5  into the integral unit  150 . 
     Prior to connecting the integral unit  150  to the barrel  1 , the rotary cam  7  and the knocking cam  8  are assembled relative to the stopper  3 , the cam cylinder  4 , and the knocking element  5 . More specifically, the rod  8   c  of the knocking cam  8  is inserted into the cam cylinder  4  and into the connecting portion  24  of the knocking element  5  and connected thereto by friction fit. The rotary cam  7  is then inserted into the cam cylinder  4  and the rotary cam face  7   b  of the rotary cam  7  is engaged with the knocking cam face  8   b  of the knocking cam  8 . 
     Thereafter, in the state in which the rotary cam  7  and the knocking cam  8  have been assembled relative to the integral unit  150  and the writing refill  2  has been preliminarily inserted in the barrel  1 , the front end portion of the cam cylinder  4  is inserted into a rear end  1   b  of the barrel  1  until the front end  10  of the stopper  3  abuts on the inner stepped portion  9  of the barrel  1 . At this point, the engagement portion  13  of the stopper  3  engages with the inner engagement portion  12  of the barrel  1  and, at the same time, the rotation-preventing portion  11  of the stopper  3  is locked with the rotation-stopping member  1   a  of the barrel  1 , whereby the stopper  3  and the barrel  1  are connected together in a non-rotatable state. Furthermore, the guide projection  7   a  of the rotary cam  7  and the guide projection  8   a  of the knocking cam  8  enter the guide groove  6   a  of the stationary cam  6  in the barrel  1 , and the rear end of the writing refill  2  abuts on the front end of the rotary cam  7 . In this assembled state of the writing instrument  100 , the writing refill  2  can be advanced and retracted in the longitudinal direction by knocking the knocking element  5  to position the writing tip  2   a  of the writing refill between the writing position and the retracted position. 
     Since the knocking element  5  is connected in a non-rotation state relative to the cam cylinder  4 , replacement of the writing refill  2  can be accomplished by rotating the knocking element  5 . Upon rotation of the knocking element  5 , the projections  15  of the cam cylinder  4  are detached from the respective holes  14  of the stopper  3 . At this point, by pulling the knocking element  5  rearwardly in the longitudinal direction (i.e., by pulling the cam cylinder  4  to the right in  FIG. 1 ), the cam cylinder  4  can be separated from the stopper  3 . Thereafter, by pulling the cam cylinder  4  and the knocking element  5  (still in a connected state) out of the rear end  1   b  of the barrel  1 , the writing refill  2  can be taken out from the rear end  1   b  of the barrel  1 . After a new writing refill  2  is inserted into the barrel, the connected cam cylinder  4  and knocking element  5  are inserted into the barrel  1  from the rear end  1   b  thereof and the cam cylinder  4  is engaged with the stopper  3  as described above to place the writing instrument in the assembled state for a writing operation. 
       FIG. 17  shows another embodiment of a writing instrument  200  according to the present invention. The structure and function of the writing instrument  200 , including the barrel  1 , cam cylinder  4  and knocking element  5 , are generally the same as described above for the writing instrument  100 , except as further described below. For ease of understanding, the same numerals used with reference to the writing instrument  100  will be used to describe the corresponding components of the writing instrument  200 . 
     The writing instrument  200  does not have a stopper as described above for the writing instrument  100 . In the writing instrument  200 , a pair of apertures or holes  30  are formed in an outer side wall surface of the barrel  1  and are spaced approximately 180 degrees from one another around the circumference of the outer side wall surface. The projections  15  of the cam cylinder  4  are prevented from detaching in the axial direction (i.e., in the direction of the longitudinal axis A) by detachable engagement with the respective holes  30  of the barrel  1 . The side surface of the projection  15  is formed as an inclined edge surface  22  as shown in  FIG. 9(D) . A guide edge  31  and an inner inclined edge  32  are formed in the inner surface of the barrel  1  for guiding the projections  15  of the cam cylinder  4  to the respective holes  30  when the cam cylinder  4  is inserted into the barrel from the rear end  1   b  of the barrel  1 . 
     By this construction, the writing instrument  200  can simply be assembled by inserting the cam cylinder  4  in the barrel  1  from the rear end  1   b  of the barrel  1 . Furthermore, by rotating the cam cylinder  4 , the projections  15  can be disengaged from the holes  30  of the barrel  1  and the cam cylinder  4  can be taken out of the barrel  1  to replace the writing refill as described above for the writing instrument  100 . The writing instrument  200  has a simpler structure and is more economical to manufacture than the writing instrument  100  because it does not require the use of the stopper. 
     The writing instrument according to the present invention has a simpler construction and is more economical to manufacture as compared to conventional writing instruments, such as disclosed in U.S. Pat. Nos. 5,413,428, 5,263,786 and 5,004,364. The writing instrument of the present invention provides a rotary cam mechanism which is capable of efficiently positioning a writing refill between a writing position and a retracted position while permitting the writing refill to be replaced by rotating a knocking part of the rotary cam mechanism. Furthermore, the writing refill can be replaced from the rear end side of a barrel by removing parts of the rotary cam mechanism without detaching a nose cone or disassembling the barrel of the writing instrument. Thus, the present invention obviates the need for barrels having screwed portions which must be disengaged in order to replace a writing refill, thereby removing restrictions in the aesthetic appearance of the writing instrument, such as the provision of a transparent barrel. 
     From the foregoing description, it can be seen that the present invention comprises an improved writing instrument. It will be appreciated by those skilled in the art that obvious changes can be made to the embodiments described in the foregoing description without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but is intended to cover all obvious modifications thereof which are within the scope and the spirit of the invention as defined by the appended claims.