Patent Publication Number: US-7581896-B2

Title: Safety knock-type writing instrument

Description:
TECHNICAL FIELD 
   The present invention relates, in general, to writing instruments and, more particularly, to a safety knock-type writing instrument configured to automatically retract a cartridge when a user puts the writing instrument, with the cartridge extended, into his/her pocket. 
   BACKGROUND ART 
   Generally, a writing instrument includes a cartridge, a spring, a barrel, and a cartridge control unit. Further, a clip is provided at a predetermined position on the writing instrument so that a user may put the writing instrument in his/her pocket and easily carry the writing instrument. 
   When the user desires to use the writing instrument, the cartridge control unit is manipulated to extend the cartridge out of the barrel. On the other hand, when the writing instrument is not in use, the cartridge control unit is manipulated to retract the cartridge into the barrel and thereby store the writing instrument in his/her pocket. The writing instrument including the cartridge control unit is manipulated to extend or retract the cartridge out of or into the barrel, using a button, a push part, etc. 
   A knock and a gear have been widely used as the cartridge control unit. A knock-type writing instrument is constructed so that the knock and the gear are provided along a central axis in the barrel. Such a construction requires additional space for installing the knock and the gear in the barrel, thus increasing the length and width of the barrel. 
   However, in order to use the conventional writing instrument, the cartridge must be extended using the cartridge control unit that is separately provided. After using the writing instrument, the cartridge must be retracted using the cartridge control unit, thus causing inconvenience to a user. 
   The conventional writing instrument is problematic in that a user&#39;s clothing may be stained by ink dispensed from the cartridge when the writing instrument is put, with the cartridge extended, in his/her pocket. 
   In the writing instrument including a cap as well as the cartridge control unit, the cap must be open to use the writing instrument, and closed after using the writing instrument. 
   The writing instrument with the cap is problematic in that the cap must be removed and replaced whenever a user uses the writing instrument, so that the cap is likely to be lost. 
   Further, the conventional cartridge control unit is disadvantageous in that loud tapping sound may be generated about four times due to the shape of the gear and the elastic force of the spring, when protrusions provided on an inner circumferential surface of the barrel come into contact with the gear rotating about the central axis of the barrel. Thus, a product of high quality is not offered to consumers. 

   
     DESCRIPTION OF DRAWINGS 
       FIG. 1  is a sectional view of a safety knock-type writing instrument, according to the first embodiment of the present invention; 
       FIG. 2  is an exploded view of the writing instrument of  FIG. 1 ; 
       FIG. 3  is a perspective view to illustrate a clip included in the writing instrument of  FIG. 1 ; 
       FIG. 4A  is a front view to illustrate a gear unit included in the writing instrument of  FIG. 1 ; 
       FIG. 4B  is a left side view to illustrate the gear unit included in the writing instrument of  FIG. 1 ; 
       FIG. 4C  is a right side view to illustrate the gear unit included in the writing instrument of  FIG. 1 ; 
       FIGS. 5A through 8F  are views to illustrate operation of a knock unit of the writing instrument of  FIG. 1 ; 
       FIGS. 9A and 9B  are sectional views of a safety knock-type writing instrument, according to the second embodiment of the present invention; 
       FIG. 10  is an exploded perspective view of the safety knock-type writing instrument, according to the second embodiment of the present invention; 
       FIG. 11A  is a front view to illustrate a half gear unit included in the writing instrument of  FIG. 10 ; 
       FIG. 11B  is a left side view to illustrate the half gear unit included in the writing instrument of  FIG. 10 ; 
       FIG. 11C  is a right side view to illustrate the half gear unit included in the writing instrument of  FIG. 10 ; and 
       FIGS. 12A through 15E  are views to illustrate operation of a knock unit of the writing instrument of  FIG. 10 . 
   

   SUMMARY OF THE INVENTION 
   Technical Problem 
   Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a safety knock-type writing instrument, which has a gear unit or a half gear unit at a predetermined position on a clip to automatically retract a cartridge when the writing instrument is put, with the cartridge extended, in a user&#39;s pocket, thus ensuring smooth operation, and preventing his/her clothing from being stained, therefore enhancing operability and convenience. 
   Another object of the present invention is to provide a safety knock-type writing instrument, which is constructed so that a cartridge control unit is provided in a clip outside a barrel, thus allowing a cartridge in the barrel to be lengthened, and reducing thickness of the writing instrument, and reducing thickness of the clip without affecting durability of the clip. 
   A further object of the present invention is to provide a safety knock-type writing instrument of high quality, which is constructed so that friction between a half gear unit of a cartridge control unit and first and second protrusions of a knock unit is small, thus reducing a tapping sound, and having little friction noise generated by the small half gear unit, even if the tapping sound is generated. 
   Technical Solution 
   In order to accomplish the above objects, the present invention provides a safety knock-type writing instrument, including a barrel having a cartridge therein; a gear unit provided on an exterior of the barrel and rotatably seated in a clip; and a knock unit positioned in the barrel, and including first and second protrusions to engage with the gear unit, wherein the cartridge is retracted into the barrel when the first protrusion of the knock unit disengages from the gear unit. 
   Further, in order to accomplish the above objects, the present invention provides a safety knock-type writing instrument, including a barrel having a cartridge therein; a half gear unit provided on an exterior of the barrel and rotatably seated in a clip; and a knock unit positioned in the barrel, and including first and second protrusions to engage with the half gear unit, wherein the cartridge is retracted into the barrel when the first protrusion of the knock unit disengages from the half gear unit, and the half gear unit rotates in a rotating direction and an opposite rotational direction, in response to reciprocating motion of the knock unit in a pushing direction and a releasing direction. 
   Further, according to the present invention, the clip includes a gear seat to rotatably seat the gear unit therein. 
   The gear unit includes a guide groove to axially guide the first protrusion, the guide groove having predetermined depth and length to disengage from the first protrusion when the gear unit rotates; a rotation guide part to be spirally guided by the first protrusion; a first protrusion seat to seat the first protrusion therein; and a second protrusion seat to seat the second protrusion thereon and to be spirally guided by the second protrusion. 
   Further, the barrel is formed so that a barrel body is integrated with a tip holder into a single structure, and includes linear guide slits having a wide opening and a narrow opening, respectively, the linear guide slits having a predetermined length and being opened at a predetermined end so that the first and second protrusions axially slide along the linear guide slits while being projected out of the linear guide slits. 
   Further, according to the present invention, the barrel is coupled to a ring-shaped part of the clip by engagement of a first threaded part of the barrel with a second threaded part of the clip. 
   The barrel has a first tapered contact surface at a position around the first threaded part, and the ring-shaped part of the clip has a second tapered contact surface to be in frictional contact with the first tapered contact surface, the first and second tapered contact surfaces providing a relatively large contact area compared to a flat surface contact manner, thus increasing a coupling force when the first threaded part of the barrel having the guide slits engages with the second threaded part of the clip. 
   Further, the half gear unit has a shape of an eccentric gear which rotates about a central axis thereof, and includes a first rotation guide part providing a spirally inclined slide surface so that the first protrusion of the knock unit slides along the first rotation guide part to rotate the half gear unit within a predetermined angular range; a first inclined groove part provided at a lower end of an inclined surface of the first rotation guide part to form a linearly inclined slide surface and a flat surface in a direction of an axis of rotating shafts, the first inclined groove part serving as a locking step using a height difference; a first protrusion seat provided at an end of the flat surface of the first inclined groove part to have a height different from the first inclined groove part, the first protrusion seat having a spirally inclined slide surface and a sharp corner, thus seating and stopping the first protrusion when the cartridge is extended; a second rotation guide part provided above the sharp corner of the first protrusion seat, and having a toothed shape with a spirally inclined slide surface; a second inclined groove part which is the equal to the first inclined groove part, but has a linear inclined slide surface and a flat surface in an opposite direction to the first inclined groove part; a third rotation guide part having a slide surface so that the second protrusion of the knock unit slides along the third rotation guide part; a second protrusion seat to function as a stopper of the second protrusion; and first and second sidewalls provided outside the first and second inclined groove parts to be perpendicular to the first and second inclined groove parts, the first and second sidewalls guiding and restraining the rotation of the first protrusion within the predetermined angular range. 
   ADVANTAGEOUS EFFECTS 
   A safety knock-type writing instrument according to the present invention is a mechanism constructed so that a gear unit or a half gear unit is provided at a predetermined position on a clip. In this case, the gear unit rotates in a predetermined direction, in response to linear reciprocating motion of a knock unit, or the half gear unit rotates in opposite directions, in response to the linear reciprocating motion of the knock unit, thus allowing a tip of a cartridge coupled to the knock unit to be smoothly retracted or extended without generating loud noise. 
   According to the present invention, although a user puts a writing instrument, with a cartridge extended, in a pocket without first retracting the cartridge, a gear unit or a half gear unit is provided at a predetermined position on a clip, thus allowing the cartridge to be smoothly retracted, and thereby preventing his/her clothing from being stained. 
   In the writing instrument of the present invention, a half gear unit occupying relatively little space is mounted to a clip, thus keeping construction of the clip simple without affecting durability thereof, therefore having competitiveness as a product, promoting sales of the product, and satisfying consumers&#39; demand for unique products. 
   Further, a safe knock-type writing instrument of this invention provides a barrel which is formed so that a barrel body is integrated with a tip holder, thus reducing the number of parts. 
   In a writing instrument of the present invention, a half gear unit is mounted to a clip to be mounted outside an interior of a barrel, thus allowing internal space in the barrel to be more efficiently used, and allowing thickness of the writing instrument to be adjusted as desired. 
   Further, a writing instrument of this invention allows a user to confirm operation of the writing instrument with his/her eyes, thus providing convenience to the user, when a clip equipped with a half gear unit is made of a transparent material. 
   DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
   Reference should now be made to  FIGS. 1 through 8F  showing the first embodiment and  FIGS. 9A through 15E  showing the second embodiment, in which similar reference numerals are used throughout the different drawings to designate the same or similar components. 
   Referring to  FIG. 1 , a safety knock-type writing instrument according to the first embodiment of the present invention includes a barrel  10 , a clip  20  for holding the writing instrument in a user&#39;s pocket or the like, and a gear seat  40  provided in the clip  20 . In this case, a gear unit  30  is rotatably seated in the gear seat  40 . 
   A cartridge  50  is provided in the barrel  10 . Further, a knock unit  60  is provided in the barrel  10  at a position corresponding to an upper portion of the cartridge  50 , and includes two protrusions  61  and  62 . 
   A linear guide slit of a predetermined length is formed along the barrel  10  to be positioned between the gear unit  30  and the knock unit  60 , so that the protrusions  61  and  62  axially reciprocate along the linear guide slit while projecting out of the linear guide slit. 
   Two protrusions  61  and  62 , that is, the first and second protrusions  61  and  62 , project out of the linear guide slit to engage with the gear unit  30 , thus extending or retracting the cartridge of the writing instrument. The first and second protrusions  61  and  62  are integrally provided on an outer circumferential surface of the knock unit  60  to be aligned in a row. For example, it is preferable that the first protrusion  61  have a parallelogram cross-section, and the second protrusion  62  have a wedge-type rectangular cross-section which is inclined at one side. 
   An upper portion of the knock unit  60  is a push part having a button shape, which is used to extend or retract the cartridge  50 . 
   The push part of the knock unit  60  passes through a center of a ring-shaped part of the clip  20  to be exposed to the outside, so that a user applies finger pressure to the push part, thus rectilinearly reciprocating the knock unit  60  and the cartridge  50 . Further, a spring (not shown) with predetermined elastic force is mounted to a lower portion of the cartridge  50  in the barrel  10 . 
   The spring generates a force for rotating the gear unit  30 , in response to the linear reciprocating motion of the knock unit  60 . 
   Since the writing instrument is constructed so that the gear unit  30  is not provided in the barrel  10 , it is possible to use a cartridge  50  having high capacity, which has a long length corresponding to the length of the barrel  10 . Further, because the cylindrical gear unit  30  is not provided in the barrel  10 , it is possible to reduce the thickness of the barrel  10 . Meanwhile, when the clip  20  is made of a transparent material, a user can confirm the operation of the gear unit  30  in the clip  20 . 
   According to the present invention, the first and second protrusions  61  and  62  of the knock unit  60  have shape corresponding to that of the gear unit  30  to be operated while engaging with the gear unit  30 , so that a loud tapping sound is not generated, but only a little friction noise is generated, thus achieving a writing instrument making little noise. 
   As shown in  FIG. 1 , the safety knock-type writing instrument of this invention is constructed so that the gear unit  30  is not positioned in the barrel  10 , but positioned in the gear seat  40  which is additionally provided in the clip  20 . When the writing instrument is not in use, the gear unit  30  engages with the first protrusion  61 . 
   When the gear unit  30  engages with the first protrusion  61 , the cartridge is retracted into the barrel  10 . In such a state, when a user pushes the push part provided at the upper end of the knock unit  60  to extend the cartridge out of the barrel  10 , the first protrusion  61  disengages from the gear unit  30 , and the gear unit  30  moves while rotating. 
   Thus, the cartridge is extended out of the barrel. After using the writing instrument, the user puts the clip  20  in his/her pocket, without pushing the push part to retract the cartridge  50  into the barrel  10 . In this case, the first protrusion  61  disengages from the gear unit  30  while the cartridge  50  is automatically retracted, thus preventing a user&#39;s clothing from being stained by the extended cartridge  50 . 
     FIG. 2  shows the barrel  10 , the cartridge  50 , and the knock unit  60  provided at the upper portion of the cartridge  50 . The gear unit  30  is disposed to come into contact with either the first protrusion  61  or the second protrusion  62 . 
   On both sides of an inner surface of the gear seat  40  are provided projecting parts  48  and  49  having a hemispherical shape to be coupled to the gear unit  30 . Referring to  FIGS. 4A ,  4 B, and  4 C, recessed parts  38  and  39  are provided on both sides of the gear unit  30  along a central axis thereof to be coupled to the gear seat  40 . The projecting parts  48  and  49  are coupled to the corresponding recessed parts  38  and  39 , so that the coupling functions as a rotating shaft to rotate the gear unit  30  about the projecting parts  48  and  49 . 
   A locking hole  11  is provided at a position around a top opening of the barrel  10 , and a locking projection  21  is provided on an outer circumferential surface of an insert part of the clip  20 . 
   When the insert part of the clip  20  is fitted into the top opening of the barrel  10 , the locking projection  21  is inserted into the locking hole  11 , thus allowing the writing instrument to be rapidly disassembled or assembled as necessary. 
     FIG. 3  is a view to illustrate the clip  20  of the safety knock-type writing instrument according to the present invention. 
   Referring to  FIG. 3 , the gear seat  40  is provided at a predetermined position on the clip  20 , and the gear unit  30  is seated in the gear seat  40 . When the cartridge  50  begins to extend or retract, the gear unit  30  is operated while rotating in the gear seat  40 . 
     FIGS. 4A ,  4 B, and  4 C are enlarged views to show the gear unit  30  seated in the gear seat  40 . 
   Referring to  FIGS. 2 ,  4 A,  4 B, and  4 C, the gear unit  30  has a gear body  31 , with the recessed parts  38  and  39  provided on both sides of the gear body  31  along the central axis thereof to serve as the rotating shaft. 
   Further, a large-diameter part and a small-diameter part are integrally provided on the gear body  31 . The large-diameter part has a plurality of first protrusion seats  34 , and the small-diameter part has a plurality of second protrusion seats  35 . 
   In order to rotate the gear unit  30  within a predetermined angular range, the first and second protrusion seats  34  and  35  have a plurality of inclined slide surfaces in a form similar to a toothed gear, to correspond to the first and second protrusions  61  and  62 . 
   The first protrusion seats  34  have shape to allow the first projection  61  of the knock unit  60  to be slidably seated in the first protrusion seats  34 . Further, rotation guide parts  33  are provided opposite the first protrusion seats  34 . In this case, each of the first protrusion seats  34  corresponds to each of the rotation guide parts  33 . 
   The gear unit  30  has a guide groove  32  to engage with the first protrusion  61 . In this case, it is preferable that the guide groove  32  have a predetermined depth and length and a semi-circular cross-section, so that the guide groove  32  disengages from the first protrusion  61  during the rotation of the gear unit  30 . 
   The second protrusion seats  35  spirally guide the second protrusion  62  of the knock unit  60  so that the second protrusion  62  is slidably seated in the second protrusion seats  35 . Thereby, the second protrusion seats  35  function to rotate the gear unit  30  within a predetermined angular range. 
   The operation of the gear unit and the knock unit will be described in detail with reference to  FIGS. 5   a  through  8   f.    
   First, the operation of extending the cartridge out of the writing instrument will be described below. 
     FIG. 5A  shows the state where the writing instrument of the present invention is normally assembled. For easy description of the present invention, the coupling of the gear unit  30  with the knock unit  60  is shown in  FIG. 5A  in detail. 
   As shown in  FIGS. 4A to 4C , the gear unit  30  includes the guide groove  32  on the gear body  31 . The gear unit  30  also has the rotation guide parts  33  to guide the first protrusion  61 , and the first and second protrusion seats  34  and  35  to seat the corresponding first and second protrusion  61  and  62  therein. 
     FIG. 5A  show the gear unit  30  and the knock unit  60  when the cartridge  50  is retracted. That is, the first protrusion  61  engages with the guide groove  32  provided on the gear body  31 . In such a state, when the knock unit  60  is pushed to extend the cartridge, as shown in  FIG. 5B , the first protrusion  61  disengages from the guide groove  32  while moving downward. Further, when the knock unit  60  is pushed to the maximum, the first protrusion  61  moves further downward, and the second protrusion  62  is seated in the second protrusion seat  35 . This is shown in  FIG. 5C . 
   As shown in  FIG. 5C , when the knock unit  60  is pushed to the maximum, the second protrusion  62  is rotatably seated in the second protrusion seat  35 , so that the knock unit  60  is not pushed any more. In such a state, when a user&#39;s finger is released from the knock unit  60 , as shown in  FIG. 5D , the knock unit  60  moves upward due to the restoring force of the spring, and the first protrusion  61  moves to the first protrusion seat  34 . 
   At this time, the gear unit  30  rotates while the first protrusion  61  is seated in the first protrusion seat  34 . This is shown in  FIG. 5E . As such, when the first protrusion  61  is seated in the first protrusion seat  34 , the cartridge is extended out of the barrel. 
   After using the writing instrument, the process of retracting the cartridge  50  by pushing the knock unit  60  again will be described with reference to  FIGS. 6A to 6E . 
   The process of retracting the cartridge  50  shown in  FIGS. 6A to 6E  is the reverse of the process of extending the cartridge shown in  FIGS. 5A to 5E . 
   That is, as shown in  FIG. 6A , when the first protrusion  61  is seated in the first protrusion seat  34 , the cartridge is extended out of the barrel. In such a state, when the knock unit  50  is pushed to retract the cartridge, the first protrusion  61  is removed from the first protrusion seats  34  while moving downward. This is shown in  FIG. 6B . 
   When the knock unit  60  is pushed to the maximum, as shown in  FIG. 6C , the second protrusion  62  is seated in the second protrusion seat  35 , so that the knock unit  60  is not pushed any more. In this state, when a user&#39;s finger is released from the knock unit  60 , the first protrusion  61  moves upward by the elastic force of the spring while the gear unit  30  rotates. This is shown in  FIG. 6D . As the gear unit  30  rotates, the first protrusion  61  moves upward along the guide groove  32  of the gear unit. This is shown in  FIG. 6E . As such, the first protrusion  61  engages with the guide groove  32  provided on the gear body  31 , so that the cartridge is retracted into the barrel. 
   The safety knock process will be described below with reference to  FIGS. 7A and 7B . Such a process is executed to retract the cartridge  50 , when the writing instrument is put, with the cartridge extended, in a user&#39;s pocket, without pushing the knock unit  60  to retract the cartridge. 
   When the writing instrument is put in his/her pocket without first retracting the extended cartridge of  FIG. 7A , the clip  20  moves away from the barrel  10 . Thus, the first protrusion  61  of the knock unit  60  is removed from the first protrusion seat  34 . 
   Further, as shown in  FIG. 7B , the knock unit  60  moves upward by the restoring force of the spring, while the first protrusion  61  is located at a position on the gear body  31 , so that the cartridge is retracted into the barrel. Thus, although the user puts the writing instrument, with the cartridge extended, in his/her pocket, without first retracting the cartridge, the safety knock process is performed to retract the cartridge into the barrel. The process of extending the retracted cartridge again will be described below with reference to  FIGS. 8A to 8F . 
     FIG. 8A  shows the state where the cartridge is retracted in the safety knock mode. That is, the first protrusion  61  is located at a position on the gear body  31 . In such a state, when the knock unit  60  is pushed to extend the cartridge, the gear unit  30  is rotated (see,  FIG. 8B ), and the first protrusion  61  located at the gear body  31  moves along the rotation guide part  33  of the gear unit  30  to be positioned under the guide groove  32 . This is shown in  FIG. 8C . 
   As shown in  FIG. 8C , the first protrusion  61  disengages from the guide groove  32 , and moves downward. When the knock unit  60  is pushed to the maximum, the first protrusion  61  moves further downward, and thereby the second protrusion  62  is seated in the second protrusion seat  35 . This is shown in  FIG. 8D . 
   As shown in  FIG. 8D , when the knock unit  60  is pushed to the maximum, the second protrusion  62  is seated in the second protrusion seat  35 , so that the knock unit  60  is not pushed any more. In this state, when the user&#39;s finger is released from the knock unit  60 , as shown in  FIG. 8E , the knock unit  60  moves upward, and the first protrusion  61  moves to the first protrusion seat  34 . At this time, the gear unit  30  continuously rotates, while the first protrusion  61  is seated in the first protrusion seat  34 . This is shown in  FIG. 8F . As such, when the first protrusion  61  is seated in the first protrusion seat  34 , the cartridge is extended out of the barrel. 
   The safety knock-type writing instrument of the present invention is capable of retracting the extended cartridge smoothly and automatically, if the writing instrument, with the cartridge extended, is undesirably put in his/her pocket or collar, thus preventing his/her clothing from being stained by the extended cartridge. 
   A safety knock-type writing instrument according to the second embodiment of the present invention will be described hereinafter in detail. 
   Referring to  FIGS. 9A and 9B , a half gear unit  300  is rotatably seated in a gear seat  400  of the clip  20 . 
   The gear seat  400  has a layout which allows rotating shafts of the half gear unit  300 , requiring relatively smaller installation space, to be rotated within a predetermined angular range. In order to force the half gear unit  300  to be fitted into the gear seat  400 , shaft fitting members  490   a  and  490   b  are provided at both sides on an inner surface of the gear seat  400 . 
   The gear seat  400  requires relatively smaller installation space, compared to space for installing the cylindrical gear unit according to the first embodiment, thus allowing the clip  20  to have relatively thin thickness h around the gear seat  400  and have a simple construction, and allowing the thickness of a support part of the clip  20  to be adjusted to provide sufficient durability. 
   A barrel  10  is fabricated by integrating a barrel body  10   a  with a tip holder  12 , through an injection molding process, thus reducing the number of parts. An annular step  12   a  is provided at a predetermined position in the tip holder  12  to support a spring  51 . 
   Further, the barrel  10  has a first step  14  at a position around the tip holder  12 . A second step  15  is provided at a position above the first step  14  to be axially spaced apart from the first step  14  by a predetermined distance. 
   A cartridge  50  is provided in the barrel  10 . Since an end of the spring  51  is supported by a step  52   a  of a tip  52  through which ink is dispensed, and the other end of the spring  51  is supported by the step  12   a  of the tip holder  12 , the cartridge  50  is elastically biased in a direction opposite to a direction of an outside force. A knock unit  60  is fitted over an upper portion of the cartridge  50 . 
   The knock unit  60  has a hollow shape, and includes an upper end which is partially blocked, and a lower end which is completely opened. Two protrusions  61  and  62  are axially positioned on an outer circumferential surface of the knock unit  60  to be aligned in a row. In this case, the protrusions  61  and  62  are projected in a radial direction of the knock unit  60  while being spaced apart from each other by a distance corresponding to the layout of the half gear unit  300 . 
   Further, openings  67  and support ribs  68  are provided at both sides on the circumference of the knock unit  60  to form a bridge structure. This minimizes consumption of material, and more efficiently supports the protrusions  61  and  62 . 
   The first protrusion  61  of the knock unit  60  is relatively lower than the second protrusion  62  in projected height. Thus, the first protrusion  61  engages with a toothed part provided at an outer position of the half gear unit  300 , distant from the rotating shaft, to perform sliding or stopping action, thus being involved in extension or retraction of the cartridge  50 . The first protrusion  61  has a spirally inclined slide surface to correspond to first and second rotation guide parts that will be described later. 
   Further, the second protrusion  62  is higher than the first protrusion  61  in projected height. Thus, the second protrusion  62  engages with an inner portion near the rotating shaft of the half gear unit  300  to perform sliding or stopping action, thus being involved in extension or retraction of the cartridge  50 . The second protrusion  62  has a spirally inclined slide surface to correspond to a third rotation guide part that will be described hereinafter. 
   As shown in  FIG. 10 , linear guide slits  16  and  17  which have predetermined lengths and are opened at a predetermined end are provided on the barrel  10  to be positioned between the half gear unit  300  and the knock unit  60 , so that the first and second protrusions  61  and  62  axially slide along the guide slits  16  and  17  while being projected outside of the guide slit  16  and  17 . 
   In this case, the predetermined end designates an upper end of the guide slits  16  and  17 , and is opened while being coupled to an edge of a top opening of the barrel  10 . The guide slits  16  and  17  comprise a wide opening  16  having a width corresponding to the half gear unit  300  so that the wide opening  16  is not affected by an operating area of the half gear unit  300 , and a narrow opening  17  having a width corresponding to the first and second protrusions  61  and  62  so that the first and second protrusions  61  and  62  smoothly reciprocate. 
   Further, a first threaded part  18 , such as an external thread, is provided on an outer circumferential surface of the upper end of the barrel  10 . A second threaded part  28 , such as an internal thread, is provided on an inner circumferential surface of a ring-shaped part of the clip  20  to engage with the first threaded part  18  (see,  FIG. 9A  or  9 B). 
   A first tapered contact surface  19  is provided at a position around the open upper end of the barrel  10  to be tapered from a circular edge of the open upper end of the barrel  10  to a predetermined position around the first threaded part  18 . 
   A second tapered contact surface  29  contacting the first tapered contact surface  19  is provided at an upper end of the second threaded part  28  on the inner circumferential surface of the ring-shaped part of the clip  20 . 
   The first and second tapered contact surfaces  28  and  29  provide a relatively larger contact area, compared to a flat surface contact manner, thus generating a larger frictional force between the first and second tapered contact surfaces  28  and  29 . Thereby, when the first threaded part  18  of the barrel  10  having the guide slits  16  and  17  engages with the second threaded part  28  of the clip  20 , the first and second tapered contact surfaces  28  and  29  increase a coupling force between the first and second threaded parts  18  and  28 , thus preventing durability of the writing instrument from being reduced due to the guide slits  16  and  17 . 
   The first and second protrusions  61  and  62  are integrally provided on the outer circumferential surface of the knock unit  60  to be arranged in a row. The upper end of the knock unit  60  which is partially blocked has a push part in a button shape to extend and retract the cartridge  50 , and an air hole  69  is provided at a predetermined position on the push part. 
   The coupling of the safety knock-type writing instrument and the shape of the half gear unit  300  according to the present invention are as follows. 
   A finger holder  13  is fitted over the barrel  10  to be positioned between the first and second steps  14  and  15 . 
   The cartridge  50  having the tip  52  equipped with the spring  51  is inserted into the barrel  10  through the open upper end of the barrel  10 . 
   The open lower end of the knock unit  60  is fitted over the cartridge  50  so that the knock unit  60  is positioned between the outer circumference of the cartridge  50  and the inner circumference of the barrel  10 . 
   In this case, the knock unit  60  is inserted into the barrel  10  by a predetermined distance such that the first and second protrusions  61  and  62  are located at the guide slits  16  and  17 . 
   Rotating shafts  390   a  and  390   b  of the half gear unit  300  are fitted into the corresponding shaft fitting members  490   a  and  490   b  of the gear seat  400  so that the gear unit  300  is rotatable within a predetermined angular range. 
   In the ring-shaped part of the clip  20 , the first threaded part  18  of the barrel  10  engages with the second threaded part  28  of the clip  20 . In this case, the push part provided at the upper end of the knock unit  60  passes through the upper end of the barrel  10  and the interior of the ring-shaped part of the clip  20  to be exposed to the outside. 
   The upper end of the cartridge  50  is in contact with an inner surface of the upper end of the knock unit  60 , and the first and second protrusions  61  and  62  are located at initial positions which will be described below in detail. Further, whenever the user pushes the push part of the knock unit  60  having the air hole  69 , the spring  51  generates restoring force acting against the user&#39;s force, and the restoring force is transmitted through the cartridge  50  to the knock unit  60  and the first and second protrusions  61  and  62 . 
   That is, when the user applies finger&#39;s pressure to the push part, the knock unit  60  and the cartridge  50  rectilinearly reciprocate. The spring  51  is supported in the tip holder  12  and generates the restoring force to return the knock unit  60  and the cartridge  50  to original positions thereof. Further, the spring  51  functions to generate a force for rotating the half gear unit  30  within a predetermined angular range, in response to the linear reciprocating motion of the cartridge  50  or the knock unit  60 . 
   In the safety-knock type writing instrument of the present invention, the half gear unit  300  requires a small installation space, thus allowing the construction of the clip  20  to be simple. Further, the first and second protrusions  61  and  62  of the knock unit  60  move while engaging with corresponding parts of the half gear unit  300 , so that there is no loud tapping sound, but only a small friction noise is generated, thus achieving a writing instrument making little noise. 
   The above-mentioned features are accomplished by the half gear unit  300  described below in detail. 
   As shown in  FIGS. 11A ,  11 B, and  11 C, the half gear unit  300  includes the rotating shafts  390   a  and  390   b  which are projected in opposite directions, and a gear body having a shape of an eccentric gear which is constructed to be eccentric from a central axis of the rotating shaft. 
   The gear body having the shape of the eccentric gear includes a first rotation guide part  310 , a first inclined groove part  320 , a first protrusion seat  330 , a second rotation guide part  340 , a second inclined groove part  350 , a third rotation guide part  360 , a second protrusion seat  370 , and first and second sidewalls  380   a  and  380   b.    
   The first rotation guide part  310  provides a spirally inclined slide surface, and the first protrusion of the knock unit slides along the first rotation guide part to rotate the half gear unit within a predetermined angular range. 
   The first inclined groove part  320  is provided at a lower end of the inclined surface of the first rotation guide part  310  to form a linearly inclined slide surface and a flat surface in a direction of an axis of the rotating shafts  390   a  and  390   b . The first inclined groove part  320  serves as a locking step using a height difference. 
   The first protrusion seat  330  is provided at an end of the flat surface of the first inclined groove part  320  to have a height different from the first inclined groove part  320 . Further, the first protrusion seat  330  has a spirally inclined slide surface and a sharp corner, thus seating and stopping the first protrusion of the knock unit when the cartridge is extended. 
   The second rotation guide part  340  is provided above the sharp corner of the first protrusion seat  330 , and has a toothed shape with a spirally inclined slide surface. 
   The second inclined groove part  350  is equal to the first inclined groove part  320 , but has a linear inclined slide surface and a flat surface in an opposite direction to the first inclined groove part  320 . 
   The third rotation guide part  360  provides a slide surface so that the second protrusion of the knock unit slides along the third rotation guide part, and is used to rotate the half gear unit within a predetermined angular range in an opposite direction to the above-mentioned rotating direction. 
   The second protrusion seat  370  functions as a stopper of the second protrusion of the knock unit. 
   The first and second sidewalls  380   a  and  380   b  have a shape of a hexagonal wing, and are provided outside the first and second inclined groove parts  320  and  350  to be perpendicular to the first and second inclined groove parts  320  and  350 . In this case, the first and second sidewalls  380   a  and  380   b  function to guide and restrain the rotation of the first protrusion within a predetermined angular range. 
   The operation and use of the safety knock-type writing instrument of the present invention will be described in brief with reference to  FIGS. 9A and 9B . 
   When the half gear unit  300  engages with the first protrusion  61 , the cartridge is retracted into the barrel. In such a state, when a user pushes the push part of the knock unit  60  to extend the tip used to write, the first protrusion  61  disengages from the half gear unit  300 , and simultaneously, the half gear unit  300  rotates by a predetermined angle in a predetermined direction. Thereafter, the first protrusion  61  comes into contact with the half gear unit  300  by the restoring force of the spring  51 . At this time, the writing end of the tip  52  is extended out of the tip holder  12 . 
   When the user finishes using the writing instrument, the user puts the clip  20  in his/her pocket without pushing the push part of the knock unit  60  to retract the cartridge  50 . In this case, the first protrusion  61  disengages from the half gear unit  300 , so that the cartridge  50  returns to an original position thereof by the restoring force of the spring  51 , and thereby, the cartridge  50  is retracted into the barrel, thus preventing the user&#39;s clothing from being stained by the extended cartridge  50 . 
   The operation of the half gear unit  30  and the knock unit  60  according to normal extension, normal retraction, safety retraction and normal extension of the tip of the cartridge will be described in detail with reference to  FIGS. 12A to 15E . 
   First, the normal extension of the tip of the cartridge will be described with reference to  FIGS. 12A to 12E . 
   As shown in  FIG. 12A , in an initial state in which the cartridge is retracted, the first protrusion  61  of the knock unit  60  is relatively low, so that the first protrusion  61  is slidably in contact with the spirally inclined slide surface, and located at a corner between the first rotation guide part  310  and the first sidewall  380   a  of the half gear unit  300 . 
   At this time, the second protrusion  62  is provided above the first protrusion  61  while being aligned in a row. The half gear unit  300  may rotate about the rotating shafts  390   a  and  390   b  within a predetermined angular range. 
   In order to normally extend the cartridge in such a state, the user pushes the push part of the knock unit  60 . 
   As shown in  FIG. 12B , when the push part of the knock unit  60  is pushed to move the knock unit  60  in a pushing direction F, the first protrusion  61  of the knock unit  60  slidably comes into contact with the spirally inclined slide surface of the first rotation guide part  310  of the half gear unit  30  while moving in the same direction as the pushing direction F. The half gear unit  300  rotatably coupled to the first protrusion starts rotating within the predetermined angular range in a predetermined rotating direction R by the first rotation guide part  310 . 
   When the first sidewall  380   a  of the half gear unit  300  comes into contact with the inner surface of the gear seat, the rotation of the half gear unit  300  stops. At this time, a corner between the second sidewall  380   b  of the half gear unit  300  and the first inclined groove part  320  comes into contact with the first protrusion  61  of the knock unit  60 . 
   Since the user keeps pushing the knock unit  60  to a maximum moving position in the pushing direction F, the first protrusion  61  is guided by the second sidewall  380   b  to rectilinearly move along the slide surface and the flat surface of the first inclined groove part  320 . At this time, the knock unit  60  horizontally supports the first protrusion  61  with respect to the cartridge, by the openings and the support ribs that form the bridge structure, as described above. Thus, the first protrusion  61  may move linearly while thrusting the first inclined groove part  320  in a projecting direction of the first protrusion  61 . 
   In this case, a vertical force corresponding to a support force of the knock unit  60  is transmitted to the clip equipped with the half gear unit  300 , so that the clip is slightly deformed within its elastic limits. Thereby, the clip slightly moves and then returns to an original position thereof, thus preventing the return of the first protrusion  61  to the first inclined groove part  320 , and making the first protrusion  61  move along the slide surface of the first protrusion seat  330 . 
   That is, as shown in  FIG. 12C , when the first protrusion  61  of the knock unit  60  moves to the maximum moving position, the second protrusion  62  is relatively higher and comes into contact with the second protrusion seat  37 . Therefore, the movement of the second protrusion  62  and the first protrusion  61  in the pushing direction F is stopped, and thereby, the knock unit  60  is not pushed any more. 
   As shown in  FIG. 12D , when the user&#39;s finger is released from the pushed knock unit  60 , the knock unit  60  moves in a releasing direction S opposite to the pushing direction F, by the restoring force of the spring. 
   In this case, the slide surface of the first protrusion  61  comes into contact with the slide surface of the first protrusion seat  33 , so that the half gear unit  300  starts rotating in an opposite rotational direction Q. Therefore, the first protrusion  61  is seated in the corner between the first protrusion seat  330  and the second rotation guide part  340 . Thereby, the movement of the knock unit  60  in the releasing direction S and the rotation of the half gear unit  300  in the opposite rotational direction Q are stopped. 
   At this time, the slide surface of the second protrusion  62  is partially aligned with the slide surface of the third rotation guide part  360  in an imaginary straight line while being spaced apart from it. This is shown in  FIG. 12E . 
   As shown in  FIG. 12E , when the first protrusion  61  is seated in the first protrusion seat  330 , the tip of the cartridge is extended, and the cartridge does not move upward by the engagement of the first protrusion  61  with the first protrusion seat  330 , thus allowing the user to smoothly write using the writing instrument. 
   After using the writing instrument, the process of normally retracting the tip of the cartridge by pushing the knock unit  60  will be described with reference to  FIGS. 13A to 13E . 
   As shown in  FIG. 13A , the first protrusion  61  is seated in the first protrusion seat  330 , and the slide surface of the second protrusion  62  is partially aligned with the slide surface of the third rotation guide part  360  in an imaginary straight line while being spaced apart from it. Further, a part of the tip is extended out of the tip holder. 
   As shown in  FIG. 13B , in order to normally retract the cartridge in the above-mentioned state, the push part of the knock unit  60  is pushed, and simultaneously, the knock unit  60  moves in the pushing direction F. At this time, the first protrusion  61  of the knock unit  60  disengages from the first protrusion seat  330 , so that rotation of the half gear unit  300  is possible. 
   At the same time, the slide surface of the second protrusion  62  slidably comes into contact with the slide surface of the third rotation guide part  360 , and thereby, the half gear unit  300  rotates in the opposite rotational direction Q by a contact distance between the slide surface of the second protrusion and the slide surface of the third rotation guide part. This is shown in  FIG. 13C . 
   As shown in  FIG. 13C , the slide surface of the first protrusion  61  of the knock unit  60  is positioned to be spaced apart from the slide surface of the second rotation guide part  340 , and the second protrusion  62  moves along the slide surface of the third rotation guide part  360  to be in surface contact therewith. 
   As shown in  FIG. 13D , when the user&#39;s finger is released from the pushed knock unit  60 , the knock unit  60  moves in the releasing direction S due to the restoring force of the spring. 
   In this case, when the slide surface of the first protrusion  61  comes into contact with the slide surface of the second rotation guide part  340 , the half gear unit  300  keeps rotating in the opposite rotational direction Q, corresponding to the slant direction of the slide surfaces. 
   Therefore, as shown in  FIG. 13E , the first sidewall  380   a  supported by the inner surface of the gear seat comes into contact with the first protrusion  61 , thus stopping the rotation of the half gear unit  300  in the opposite rotational direction Q. 
   Due to the elastic force of the spring, the first protrusion  61  passes over the linearly inclined slide surface and the flat surface of the second inclined groove part  350 , thus returning to the original position thereof. 
   At this time, as described above, the clip equipped with the half gear unit  300  moves slightly due to the vertical force corresponding to the support force of the knock unit  60 , and then returns to the original position. 
   Thus, the return of the first protrusion  61  to the second inclined groove part  350  is restrained, so that the first protrusion  61  may move along only the first rotation guide part  310 . 
   Therefore, the half gear unit  300  does not rotate in only a predetermined rotating direction different from a prior art, but rotates in the rotating direction R and the opposite rotational direction Q to correspond to the reciprocating motion of the knock unit  60  in the pushing direction F and the releasing direction R. Thus, the extension of the tip of the cartridge coupled to the knock unit  60  is performed and stopped stepwise, so that the sound is very quiet. 
   The safety retraction of the cartridge will be described with reference to  FIGS. 14A and 14B . Such a process is executed to retract the cartridge, when the writing instrument is put, with the cartridge extended, in a user&#39;s pocket, without pushing the knock unit  60  to retract the cartridge. 
   When the user puts the writing instrument, with the cartridge extended as shown in  FIG. 14A , in his/her pocket, a predetermined portion of the clip having the gear seat moves away from the barrel. 
   Thus, the first protrusion seat  330  of the half gear unit  300  is removed from the first protrusion  61  of the knock unit  60 , or the first protrusion  61  of the knock unit  60  disengages from the first protrusion seat  330  of the half gear unit  300 , so that axial movement of the cartridge and the knock unit  60  is possible. 
   In this case, as shown in  FIG. 14B , the cartridge is retracted by the restoring force of the spring, and the knock unit  60  coupled to the cartridge immediately moves in the releasing direction S. 
   At this time, rotation of the half gear unit  300  does not occur, but the second protrusion  62  moves upward to be positioned above either the third rotation guide part  360  or the second protrusion seat  370 . Simultaneously, the first protrusion  61  is located at a position on a surface between the third rotation guide part  360  and the first rotation guide part  310  of the half gear unit  300 . 
   Thus, although the user puts the writing instrument, with the cartridge extended, in his/her pocket, the cartridge is automatically retracted by the action of the half gear unit  300  and the knock unit  60 . 
   The process of extending the retracted cartridge will be described below with reference to  FIGS. 15A to 15E . 
   The normal extension of  FIGS. 15A to 15E  which is executed after the safety retraction is very similar to the normal extension of the tip of the cartridge which is shown in  FIGS. 12A to 12E . 
   That is, when the cartridge is retracted as shown in  FIG. 15A , the first protrusion  61  of the knock unit  60  is stopped at a predetermined position on the surface of the half gear unit  300  between the third rotation guide part  360  and the first rotation guide part  310 . In order to normally extend the cartridge in such a state, the user pushes the push part of the knock unit  60 . 
   In this case, as shown in  FIG. 15B , the first protrusion  61  of the knock unit  60  moves in the pushing direction F, so that the half gear unit  300  rotates within the predetermined angular range in the rotating direction R. Thus, the corner between the first rotation guide part  310  of the half gear unit  300  and the first sidewall  380   b  comes into contact with the first protrusion  61  of the knock unit  60 . 
   As shown in  FIG. 15C , when the user pushes the knock unit  60  to the maximum moving position in the pushing direction F, the first protrusion  61  is guided by the second sidewall  380   b  to pass over the first inclined groove part  320 , but is stopped together with the knock unit  60  and the second protrusion  62  by the second protrusion  62  coming into contact with the second protrusion seat  370 . 
   As shown in  FIG. 15D , when the user&#39;s finger is released from the pushed knock unit  60 , the knock unit  60  moves in the releasing direction S by the restoring force of the spring, so that the first protrusion  61  of the knock unit  60  comes into contact with the slide surface of the first protrusion seat  330 . 
   As shown in  FIG. 15E , the half gear unit  300  rotates in the opposite rotational direction Q, in response to the movement of the first protrusion  61  of the knock unit  60 , so that the sharp corner provided between the first protrusion seat  330  and the second rotation guide part  340  restrains the linear movement of the first protrusion  61 . In this case, the tip of the cartridge is extended out of the tip holder, thus allowing the user to write using the writing instrument. 
   Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.