Patent Publication Number: US-11648796-B2

Title: Mechanical pencil having lead breakage-prevention mechanism

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a mechanical pencil having a lead breakage-prevention mechanism and, more particularly, to a mechanical pencil having a lead protection pipe provided at a front end of a slider to cover a front end periphery of a pencil lead and prevent lead breakage, wherein the lead protection pipe and the slider move backwardly as the writing proceeds. 
     BACKGROUND INFORMATION 
     Mechanical pencils have been widely known, in which a tapered member is provided at a front end of a main body, a lead protection pipe is retractably inserted through a front end opening formed on the tapered member, and a rear end of the lead protection pipe is fixed to a slider which moves back and forth in the tapered member. In such a mechanical pencil, lead breakage can be prevented by inserting a front end of a lead into the lead protection pipe. The lead protection pipe and the slider move backward as the lead is consumed during use of the mechanical pencil. 
     Knocking-type mechanical pencils usually use a lead-feeding mechanism having a chuck of a collet chuck type, a clamping ring (clutch) which clamps down the chuck, and a chuck spring which urges the chuck rearward. The chuck moves backward by the chuck spring and slips into the inside of the clamping ring. The chuck is thereby closed and grips the lead. When the chuck is knocked, the chuck moves forward while gripping the lead, and during its forward advancement, the clamping ring releases the chuck. By this operation, the lead moves forward and is fed to the position where it protrudes from the lead protection pipe. 
     When the above knocking operation is stopped, the chuck moves backward. At this time, if the lead is not held at an advanced position, the lead moves backward together with the chuck, requiring further knocking a few times in order to feed the lead. Accordingly, in order to keep the lead at the advanced position, a lead-holding portion is provided at the inner face of the slider to temporarily hold the lead, and a slide-resistant portion is provided at the outer face of the slider to prevent backward movement of the slider. Further, when the chuck moves backward, the chuck slips into the clamping ring during its backward movement and therefore the chuck is gradually closed, and concurrently the lead is also retracted slightly. By the retracting force at this stage, once the slider moves backward together with the lead, it becomes difficult to feed the lead again. 
     In a mechanical pencil described in JP-UM-Sho 63-21082A which shows one example of conventional sliders, on an inner face of the slider, a lead-holding portion made of a synthetic resin material is in sliding contact with an outer face of the lead with friction resistance between the two surfaces in sliding contact. On an outer face of the slider, an elastic resistance piece (a piece that increases frictional resistance) made of a synthetic resin is in sliding contact with an inner face of the tapered member. Further, the friction resistance of the elastic resistance piece which is in sliding contact with the inner face of the tapered member is set to be larger than the friction resistance of the lead-holding portion which is in sliding contact with an outer face of the lead. By so setting the friction resistances, when the lead is retracted, the slider does not move backward and the lead is held at the advanced position. 
     On writing, the lead protection pipe abuts on the writing surface, e.g., paper, and retracts backward. The resistance at this time is a resistance obtained by adding a resistance by the elastic resistance piece when the slider is held at a constant position to a resistance by the lead-holding member which temporarily holds the lead. Since this large resistance is always applied to the lead-holding pipe, the resistance at the front end of the writing instrument becomes large, the dragging resistance at the time of writing becomes high, and smooth writing becomes difficult. 
     As mentioned above, the lead breakage can be prevented by the lead protection pipe. Further, a mechanical pencil constituted to prevent lead breakage when an excessively large load is applied at the writing front end of the lead has also been known.  FIGS.  17 A- 17 C  show an example of one such mechanical pencil. Into a tapered member  102  connected to the front end of a main body  101 , a slider  104  having a lead protection pipe  103  is inserted. The slider  104  is constituted in a substantially same structure as described in JP-UM-Sho 63-21082A, and on an inner face of the slider  104 , a lead-holding portion  105  which temporarily holds a lead is provided, and on an outer face of the slider, an elastic resistance piece  106  which has a resistance set to be larger than a resistance of the lead-holding portion  105  is provided. A holding tube  109  holds a lead-feeding mechanism such as a chuck  107 , a clamping ring  108  and a chuck spring  113  and is urged forward by a cushion spring  110  having an elastic force which is larger than a writing pressure. 
     In a writing state, as shown in  FIG.  17 A , the slider  104  and the lead protection pipe  103  move forward and abut on an inward stepped portion  111  of the tapered member  102 . The front end of the lead protection pipe  103  protrudes from the tapered member  102  and then writing can be carried out. As shown in  FIG.  17 B , when an excessively large load is applied to the front end in the direction of the arrow, the lead protection pipe  103  and the slider  104  move backward, and the slider  104  stops at the retracted position. At this time, since the chuck  107  is kept in a state where it clamps a lead  112 , the chuck  107  is pressed by the lead  112  and moves backward together with the holding tube  109  against the biasing force of the cushion spring  110 . The retracted distance at this time is L 1 . 
     When the excessively large load is released, the holding tube  109  holding the chuck  107 , etc. moves forward by the cushion spring  110 , and therefore, as shown in  FIG.  17 C , the lead  112  in a state where it is clamped by the chuck  107  also advances forward. During advancement, since the chuck  107  does not press the slider  104 , the slider  104  remains stopped at the retracted position where it is retracted by the resistance of the elastic resistance piece. In this state, since the lead protection pipe  103  does not protrude from the front end of the tapered member  102 , only the lead  112  moves forward and protrudes from the front end of the tapered member  102 . The distance of protrusion at this time is a distance L 2  which is equal to the above retracted distance L 1 . Accordingly, the front end portion is exposed outwardly from the tapered member  102  without protection by the lead protection pipe  103  and the lead is therefore likely to break. 
     Usually, in order to secure visibility when writing, i.e., viewing of the writing tip, the lead protection pipe is often used in a state where it protrudes by about 3 to 4 mm from the tapered member. Since mechanical pencils are often used slantwise at an inclined angle, the writing pressure applied to the writing front end of a lead may act in a transverse direction to the lead protection pipe depending on the angle on writing, which means that with respect to the lead protection pipe, the force is exerted in such a direction that the lead protection pipe would be bent on a fulcrum where the lead protection pipe is in contact with the front end of the tapered member. This force becomes a resistance to movement between the outer face of the lead protection pipe and the front end inner face of the tapered member, and also acts on the slider via the lead protection pipe. Accordingly, the slider may sometimes be transversely pressed, bent or moved slantwise, thereby inhibiting straight-line movement of the slider. As mentioned above, when a force in a transverse direction is exerted, the lead protection pipe is bent and straight-line movement of the slider cannot be secured, leading to one of the causes of lead breakage. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a mechanical pencil having a lead breakage-prevention mechanism that prevents or significantly reduces breakage of the pencil lead. 
     Another object of the present invention is to provide a mechanical pencil having a lead breakage-prevention mechanism comprising a slider having on its front end a lead protection pipe for protecting a lead, the slider moving forward when the lead is fed and moving backward as writing proceeds. The lead can be fed by a few knocking operations on the rear end of the mechanical pencil. During writing, the lead protection pipe presents no dragging resistance as it moves on the writing surface, the slider moves backward smoothly, and the lead protection pipe and the slider can move in a stable manner. 
     The above and other objects of the present invention can be implemented by a mechanical pencil having a main body housing a lead-feeding mechanism which includes a chuck and a clamping ring; a tapered member provided at a front end of the main body; a slider mounted to undergo back and forth movement within the tapered member; a lead protection pipe which protrudes from a front end of the tapered member when the slider moves forward; and an outer pipe provided at a front end of the tapered member and surrounding a front end of the lead protection pipe. The slider has, on its outer face, a non-resistant sliding-contact face which is in free sliding contact with an inner face of the tapered member, and has on its inner face, a lead weak-holding portion that weakly holds the lead so that the lead fed by the chuck does not drop by its own weight. 
     In the present disclosure, the direction in which the lead is fed or advanced from the tapered member is referred to as “forward” or “frontward”, and the direction in which the lead is retracted into the main body is referred to as “backward” or “rearward”. 
     The lead weak-holding portion on the slider applies a weak lead-holding force to the lead that is sufficient to hold and prevent the lead from dropping due to its own weight. Additionally, the lead-holding force is selected to ensure that writing can be carried out, without hinderance, while dragging the lead protection pipe on the writing surface. By way of example and not limitation, a gram-force of a few grams has been found to be an acceptable value for the lead-holding force. 
     The outer pipe is fixed to the tapered member or provided on an outer pipe holder inserted into the tapered member. The outer pipe holder is fixed to the inside of the tapered member or disposed to move back and forth in the tapered member. When the outer pipe holder is disposed to move back and forth, a rear end of the outer pipe holder faces a holding pipe which holds the chuck and the clamping ring, and the holding pipe is urged forward by a cushion spring having a spring pressure larger than the writing pressure. 
     According to a feature of the present invention, the slider is disposed within the tapered member provided at the front end of the main body, the slider is disposed to undergo back and forth movement, and the slider has a lead protection pipe which protrudes from the front end of the tapered member when the slider moves forward. At the front end of the tapered member, the outer pipe is provided and surrounds the front end of the lead protection pipe. The slider is provided with, on its outer face, a non-resistant sliding-contact face which slidingly contacts the inner face of the tapered member, and on its inner face, a lead weak-holding portion having a weak holding force such that the lead fed by the lead-feeding mechanism does not drop by its own weight. By this structure, when a chuck of the lead-feeding mechanism is knocked, the outer face of the lead fed by the chuck contacts the lead weak-holding portion in the slider. Since the slider has, on its outer face, a non-resistant sliding-contact face which slidingly contacts the inner face of the tapered member without causing friction resistance, the slider moves forward by the lead slipped into the lead weak-holding portion, and the lead is fed to the advanced position by one knocking whereby writing can be performed in a state where the lead is protected by the lead protection pipe. 
     Taking into account the distance the lead is retracted at the time of backward movement of knocking, in the case of the lead-feeding distance by a knocking operation, the lead is fed a distance larger than the lead-feeding amount for one knocking operation necessary for normal writing by adding the distance when the lead is retracted. And during writing, when the lead protection pipe abuts on the writing surface, since the non-resistant sliding-contact face is provided on the outer face of the slider and the lead-holding force of the lead weak-holding portion is very weak, the lead protection pipe and the slider can move backward readily and the dragging resistance of the lead protection pipe along the writing surface is low, whereby writing can be made smoothly. 
     Further, the outer periphery of the lead protection pipe provided at the front part of the slider is surrounded by an outer pipe provided at the tapered member, and the lead protection pipe protrudes outwardly through the outer pipe, whereby it is protected by the outer pipe. Accordingly, even if a force is exerted during writing by which the lead protection pipe may be bent, the lead protection pipe can be protected by the outer pipe and there is no possibility that the lead protection pipe is bent. It is therefore possible to suppress the inclination of the slider and prevent bending of the lead protection pipe when the mechanical pencil is dropped. As mentioned above, since straight-line movement of the slider can be secured, lead breakage can be further securely prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a cross-sectional view showing one example of a mechanical pencil having a lead-breakage prevention mechanism according to the present invention. 
         FIG.  2    is a partial cross-sectional view and a partial enlarged explanatory view of an example of a lead-breakage prevention mechanism that does not use an outer pipe holder. 
         FIG.  3    shows an example of a slider, wherein  FIG.  3 A  is a plan view,  FIG.  3 B  is a partial cross-sectional view, and  FIG.  3 C  is a right-side elevational view. 
         FIG.  4    shows another example of a slider, wherein FIG.  4 A is a plan view,  FIG.  4 B  is a partial cross-sectional view, and  FIG.  4 C  is a right-side elevational view. 
         FIG.  5    shows an example of an outer pipe holder, wherein  FIG.  5 A  is a plan view,  FIG.  5 B  is a partial cross-sectional view,  FIG.  5 C  is a partial elevational cross-sectional view, and  FIG.  5 D  is a right-side elevational view. 
         FIG.  6    shows another example of an outer pipe holder, wherein  FIG.  6 A  is a plan view, and  FIG.  6 B  is a partial cross-sectional view and a partial enlarged explanatory view. 
         FIG.  7    is an explanatory view showing a state where the slider shown in  FIG.  3    and the outer pipe holder shown in  FIG.  5    are assembled in a tapered member. 
         FIG.  8    is an explanatory view showing a state where the outer pipe holder, etc. shown in  FIG.  7    are retracted. 
         FIG.  9    is an explanatory view showing a state where the slider shown in  FIG.  4    and the outer pipe holder shown in  FIG.  6    are assembled in a tapered member. 
         FIG.  10    is an explanatory view showing a state where the outer pipe holder, etc. shown in  FIG.  9    are retracted. 
         FIG.  11    shows an example of a lead-feeding mechanism, wherein  FIGS.  11 A,  11 B,  11 C and  11 D  are explanatory views, respectively, of a state where a lead protection pipe is housed, a state where knocking is made, a state where a chuck is about to retract, and a writing state. 
         FIG.  12    shows a writing state, wherein  FIG.  12 A  shows an elevational view when the writing starts and  FIG.  12 B  shows an elevational view when a lead protection pipe retracts. 
         FIG.  13    shows a cushion mechanism, wherein  FIG.  13 A  and  FIG.  13 B  show, respectively, a writing state and a state where a cushion spring is compressed. 
         FIG.  14    shows the action of the cushion mechanism, wherein  FIG.  14 A  and  FIG.  14 B  are elevational views showing, respectively, a state where the cushion spring compresses and the lead protection pipe retracts and a state where the cushion spring extends and the lead protection pipe returns to the writing state. 
         FIG.  15    shows a mechanism for protecting the outer pipe, wherein  FIG.  15 A  is an elevational view of a state where the lead protection pipe is housed in the outer pipe,  FIG.  15 B  is an elevational view of a state where the mechanical pencil drops,  FIG.  15 C  is an explanatory view of a state where the lead protection pipe is housed, and  FIG.  15 D  is an explanatory view of a state where the mechanical pencil drops. 
         FIG.  16    is a cross-sectional view of a mechanical pencil not provided with a cushion mechanism. 
         FIG.  17    shows a conventional mechanical pencil, wherein  FIGS.  17 A,  17 B and  17 C  are explanatory views, respectively, of a writing state, a state where the lead retracts when an excessively large load is applied to the front end of the lead protection pipe, and a state where the excessively large load is released. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG.  1    shows one embodiment of a mechanical pencil having a lead breakage-prevention mechanism constructed according to the principles of the present invention. The mechanical pencil comprises a tubular main body  1  having a rear tubular portion  2  on which is provided a clip  3 , and a front tubular portion  4 . The front portion  4  has an inner diameter smaller than that of the rear portion  2 . As conventionally known, a lead-feeding mechanism comprising a chuck  5  and a clamping ring  6  is housed in the tubular front portion  4 . Within the tubular rear portion  2  of the main body is inserted a lead tank  8  provided with an eraser  7  at its rear end, and at the front end of the lead tank  8 , the chuck  5  is fixed. 
     The chuck  5  clamps a lead  9  when the lead is retracted into the clamping ring  6 . The chuck  5  and the clamping ring  6  are inserted into a holding tube  10  which is inserted to undergo back and forth movement in the front portion  4  of the main body  1 . Between an inward stepped portion  11  of the holding tube  10  and the front end of the lead tank  8 , a chuck spring  12  is provided for urging the chuck  5  rearwardly. Further, between the rear end of the holding tube  10  and a shoulder portion  13  formed within the main body  1 , a cushion spring  14  which has a spring pressure larger than normal writing pressure is provided. On an outer periphery of the front portion  4  of the main body, a grip  15  is provided. At the front end of the main body  1 , a tapered member  16  is threadedly mounted, and from the front end of the tapered member  16 , a lead protection pipe  17  made of stainless steel, etc. protrudes. By the above-mentioned structure, when the lead tank  8  is knocked, as conventionally known, the chuck  5  moves forward, and during its forward movement, the chuck is released from clamping by the clamping ring  6 , and the lead  9  is fed forward and protrudes from the lead protection pipe  17 , whereby writing can be made. 
     In the tapered member  16 , an outer pipe holder  18  is provided, and at the front end of the outer pipe holder  18 , an outer pipe  19  is fixedly attached. The lead protection pipe  17  extends slidably through the outer pipe  19  which surrounds the outer periphery of the lead protection pipe. The outer pipe  19  has a strength sufficient to suppress a bending force which acts on the lead protection pipe  17  during writing. Preferably the outer pipe  19  is formed of thin metal pipe material, such as a stainless steel, and has an inner diameter sized such that an outer face of the lead protection pipe  17  is in slidable contact with an inner face of the outer pipe  19  and an outer diameter sized such that visibility of the outer pipe  19  is not lost during writing. Further, the length of the outer pipe  19  is adjusted in size such that when the lead protection pipe  17  protrudes during writing, the front end of the outer pipe  19  is positioned behind the front end of the lead protection pipe  17 . The outer pipe holder  18  may be fixedly provided in the tapered member  16 , but preferably is movable back and forth so that it can move backward when an excessively large load is applied to the outer pipe  19 . 
     The outer pipe  19  may be directly fixed to the front end of the tapered member  16  without employing the outer pipe holder  18  as shown in  FIG.  2   . In the outer pipe holder  18 , a slider  20  is housed to move back and forth, and at the front end of the slider  20 , the lead protection pipe  17  which is inserted into the outer pipe  19  to protect the lead  9  is fixed. In the case of the example as shown in  FIG.  2   , the slider  20  is housed in the tapered member  16 , and a detachment-preventing raised portion  21  to prevent detachment of the slider  20  rearwardly is provided on the inner face at the rear portion of the tapered member  16 . 
       FIGS.  3 A- 3 C  show an example of the slider  20 . The slider  20  has a front side tubular portion  22 , and a rear side tubular portion  23  having a diameter larger than that of the front side tubular portion  22 . The lead protection pipe  17  is fixed to a mounting aperture  24  provided at the front side tubular portion  22 . Inside the rear side tubular portion  23 , is provided a lead weak-holding portion  25  of a tubular shape for temporarily holding the lead  9 . The lead weak-holding portion  25  has one or more slits extending in the longitudinal direction of the tubular portion to increase flexibility of the lead weak-holding portion so that it can elastically hold the lead  9  as in conventional mechanical pencils, but the lead weak-holding portion may be constituted by an elastic material such as a rubber material (not shown). 
     The lead-holding force of the lead weak-holding portion  25  in this embodiment is set to be far smaller than that of the lead-holding portion of conventional mechanical pencils. More particularly, when the front end of the main body  1  is directed downward, an extremely weak lead-holding force is applied to the lead  9 ; however, this extremely weak force is sufficient to prevent the lead  9  from falling downward by its own weight. Byway of example, the extremely weak force may be a gram-force of a few grams, which is adequate to prevent falling of the lead  9  due to gravity, and even if writing is conducted while dragging the lead protection pipe  17  on the writing surface, such does not hinder writing. 
     Further, in conventional mechanical pencils, the slider is provided with an elastic piece for resistance which contacts internally the tapered member with a large elastic resistance force so that the slider would not be retracted together when the chuck is moved backward. However, the above rear side tubular portion of the present invention is not provided with such an elastic piece for resistance. Instead, the rear side tubular portion  23  of this embodiment slidingly contacts the inner face of the outer pipe holder  18  and has a non-resistant sliding-contact surface  26  so that no contact resistance would be generated with the outer pipe holder  18 . Here, in the example shown in  FIG.  3   , two detachment-preventing latches  27  are provided at the outer face of the rear side tubular portion  23  so that detachment from the outer pipe holder  18  would be prevented, but the rear end face may be designed to be a detachment-preventing abutting part  28  as shown in an example of  FIGS.  4 A- 4 C . 
       FIGS.  5 A- 5 D  show one embodiment of the outer pipe holder  18 . In this embodiment, the outer pipe holder  18  is formed in a shape of a tubular body configured to be inserted into the tapered member  16 , and at its front end, a taper-shaped protrusion part  29  is formed which protrudes from the front-end opening portion of the tapered member  16 , and inside thereof, amounting aperture  30  for the outer pipe  19  is formed. On the outer face of the outer pipe holder  18 , a shoulder part  31  is formed which acts as detachment prevention by abutting on the inner face of the tapered member  16 . The rear portion of the shoulder part  31  has a small-diameter portion, and rearward of the small-diameter portion is provided a stopper part  32  which abuts on the front end of the holding tube  10 . At the small-diameter portion, two detachment-preventing grooves  33  are provided so that detachment-preventing latches  27  (as shown in  FIG.  3   ) provided on the slider  20  slip thereinto. On the inner face of the outer pipe holder  18 , a stopper part  34  is provided on which the front end of the front side tubular portion  22  abuts when the slider  20  moves forward. Behind the stopper part  34 , a rear side sliding surface  35  is formed on which the non-resistant sliding-contact surface  26  of the slider  20  slides. Further, at the rear end, an inward stepped portion  36  is provided on which the clamping ring  6  abuts so that when the chuck  5  and the clamping ring  6  are moved forward by the knocking operation, the clamping of the chuck  5  by the clamping ring  6  is released during the advance movement. 
       FIGS.  6 A and  6 B  show another example of the outer pipe holder  18 . In this example, instead of the detachment-preventing grooves  33  shown in the outer pipe holder  18  of  FIG.  5   , a raised portion  37  for detachment prevention is provided on which the abutting part  28  for detachment prevention of the slider  20  abuts as shown in  FIG.  4   , and at the rear end thereof, the stopper part  32  which abuts on the front end of the holding tube  10  is provided. 
     To the front side tubular portion  22  of the slider  20  shown in  FIG.  3   , the lead protection pipe  17  is fixed, and to the outer pipe holder  18  as shown in  FIG.  5   , the outer pipe  19  is fixed. In a state where the lead protection pipe  17  is inserted through the outer pipe  19  and the detachment-preventing latches  27  are engaged with the detachment-preventing grooves  33 , the slider  20  and the outer pipe holder  18  are combined and assembled in the tapered member  16  (see  FIG.  7   ). The outer pipe holder  18  is inserted until it stops by abutting on the inner face of the tapered member  16 , and at this time, the lead protection pipe  17  is formed in such a length that the front end of the lead protection pipe  17  protrudes a little forward from the front end of the outer pipe  19 . In this state, if an excessively large load is applied to the front end of the lead protection pipe  17 , the entire lead protection pipe  17  slips into the outer pipe  19  and the rear end of the slider  20  abuts on the stopper part  32 , and concurrently the slider  20  moves backward within the outer pipe holder  18 , and the outer pipe holder  18  also moves backward within the tapered member  16  as shown in  FIG.  8   . 
     To the slider  20  as shown in  FIG.  4   , the lead protection pipe  17  is fixed, and to the outer pipe holder  18  as shown in  FIG.  6   , the outer pipe  19  is fixed. Referring to  FIG.  9   , the lead protection pipe  17  is inserted into the outer pipe  19  so that its front end protrudes from the front end of the outer pipe  19 , and the slider  20  is assembled in the outer pipe holder  18  so that the slider  20  is positioned ahead of the raised part  37  provided within the outer pipe holder  18 , and this state is set in the tapered member  16 . In this state, if an excessively large load is applied to the front end of the lead protection pipe  17 , as shown in  FIG.  10   , the entire lead protection pipe  17  slips into the outer pipe  19  and the rear end of the slider  20  moves backward within the outer pipe holder  18  to the position where the rear end of the slider  20  abuts on the raised part  37 , and further the outer pipe holder  18  also moves backward within the tapered member  16 . 
     The lead-feeding mechanism of the present invention is shown in  FIGS.  11 A- 11 D .  FIG.  11 A  shows a state where the lead protection pipe  17  is hidden in the outer pipe  19  as the writing proceeds. In this state, as shown in  FIG.  8    and  FIG.  10   , the slider  20  is moved backward to a position where it abuts on the stopper part  32  or the raised part  37 . At this time, between the front end of the clamping ring  6  and the inward stepped portion  36 , there is a gap of a distance L 3 , and this distance L 3  is a lead-feeding distance. That is, when the lead tank  8  is knocked as shown in  FIG.  11 B , the chuck  5  and the clamping ring  6  move forward, and during forward movement, the clamping ring  6  stops by abutting on the inward stepped portion  36 , and the chuck  5  is released. As mentioned above, the lead  9  can be fed until the clamping ring  6  abuts on the inward stepped portion  36 , and this distance is a lead-feeding distance. The chuck  5  moves further forward, and moves the slider  20  to the position where it abuts on the stopper part  34 , and causes the front end of the lead protection pipe  17  to protrude from the front end of the outer pipe  19 . At this time, the distance from the front end of the outer pipe  19  to the front end of the lead protection pipe  17  is a maximum, protrusion distance L 4  of the pipe as shown in  FIG.  11 B . 
     When the knocking is stopped as shown in  FIG.  11 C , the chuck  5  is moved backward by the chuck spring  12 . During backward movement, the chuck  5  gradually slips into the clamping ring  6  and moves backward while closing, whereby the lead  9  is retracted backward. At this time, as shown in  FIG.  11 D , since the lead weak-holding portion  25  of the slider  20  holds the lead  9  with an extremely weak holding force, the slider  20  also moves backward, and between the front end of the slider  20  and the stopper part  34 , a gap equivalent to a lead retraction distance L 5  is formed. Although this gap is only a slight amount, the lead  9  is grasped by the chuck  5  and writing can be made without difficulty. Since the slider  20  is not provided with an elastic piece for resistance unlike conventional mechanical pencils, when the chuck  5  moves backward to retract the lead  9  backward, the slider  20  is also retracted. Accordingly, to maintain the writing conditions, the lead-feeding distance L 3  should be set to feed the lead considering the lead retraction distance L 5 . Specifically, in a case where, for example, a lead protrusion distance required for writing is 0.8 mm and the lead retraction distance is 0.7 mm, the lead-feeding distance L 3  is set to be 1.5 mm by adding 0.7 mm to 0.8 mm. By doing so, even if the lead is retracted to move backward by 0.7 mm, the distance at issue is 1.5 mm−0.7 mm=0.8 mm, and it is therefore possible to secure 0.8 mm as the lead-feeding distance L 3  and writing can be made without difficulty. 
     Writing can be conducted under this condition. The lead protection pipe  17  for protecting the lead abuts on a paper face  38  by protruding by a distance L 6  as shown in  FIG.  12 A , and as the lead is worn through use, the lead protection pipe and the slider move backward, and the protrusion distance becomes L 7  as shown in  FIG.  12 B . At the time of backward movement, since the slider  20  has a non-resistant sliding-contact surface  26  on its outer face (see  FIGS.  9  and  10   ), the slider moves without causing resistance with the inner periphery face of the outer pipe holder  18 , and therefore substantially no significant dragging resistance is caused. By continuing the writing, the slider  20  moves backward to a position where its rear end abuts on the stopper part  32  as shown in  FIG.  11 A . When the mechanism is in this condition, by knocking the lead tank  8  as mentioned above, the lead  9  can be fed by one knocking operation as shown in  FIGS.  11 B,  11 C and  11 D . At this time, since the outer periphery of the slider  20  has a non-resistant sliding-contact (low friction) surface  26 , the slider  20  can be moved forward together with the lead  9  at the time of lead-feeding, and there is no possibility that only the lead  9  may protrude from the front end of the lead protection pipe  17 . 
     With respect to the length of the lead protection pipe  17  that protrudes from the tapered member  16  during writing, in a case of conventional mechanical pencils having no outer pipe  19 , the outer periphery of the lead protection pipe  17  is not covered by an outer pipe  19  and substantially the entire length of the lead protection pipe  17  protruding from the tapered member  16  is exposed. On the other hand, in the present invention, since the outer periphery of the lead protection pipe  17  is covered by the outer pipe  19  provided at the tapered member  16 , the length of the exposed portion of the lead protection pipe  17  is the same as the length extending from the outer pipe  19 , and the length of the exposed portion of the lead protection pipe  17  is shorter than that of conventional mechanical pencils. 
     When writing, as shown in  FIG.  12 A , since the main body  1  is gripped slantwise for writing, a slantwise load created by writing pressure is applied to the front end of the lead protection pipe  17 , and this load is a bending force tending to bend the lead protection pipe  17 . The bending force is equal to “a load applied to the front end of the lead protection pipe  17 ”×“a length of extended portion of the lead protection pipe”. Since the front end of the lead protection pipe of the present invention protrudes from the front end of the outer pipe as mentioned above, the protruded length can be made shorter than that of conventional ones where no outer pipe is provided and the lead protection pipe is directly exposed from the tapered member. The bending force applied to the lead protection pipe  17  can thereby be made small. Accordingly, the friction resistance applied when the lead protection pipe  17  and the slider  20  move backward can be reduced, the straight movement of the slider  20  is secured, and lead breakage can be satisfactorily prevented. 
     Further, movement of the lead protection pipe  17  is guided by the outer pipe  19  and therefore its guided length is longer than that of conventional mechanical pencils having no outer pipe  19 . Therefore, the deviation of the lead protection pipe  17  and the slider  20  becomes small, and backward movement can be conducted under stable conditions, whereby prevention of lead breakage can be further improved. When the mechanical pencil is dropped, the lead protection pipe  17  moves backward into the outer pipe  19  and the front end of the lead protection pipe  17  can be protected. 
     In the example shown in  FIG.  1   , a cushion spring  14  is disposed behind the holding tube  10  and provides an appropriate cushion function when an excessively large writing pressure is applied. With reference to  FIG.  13 A , a lead  9  is gripped by the chuck  5  and the clamping ring  6  (see  FIG.  1   ). Under this condition, when an excessively large load is applied to the front end of the lead  9 , as shown in  FIGS.  13 B and  14 A , the lead protection pipe  17  and the slider  20  move backward via the lead  9  (see  FIG.  14 A ), and concurrently the holding tube  10  housing the chuck  5  and the clamping ring  6  moves backward while compressing the cushion spring  14  as shown in  FIG.  13 B . At this time, the length of the cushion spring  14  is compressed to a distance L 8 , and a gap L 9  is formed between the outer pipe holder  18  and the holding tube  10 . 
     When the excessively large load disappears, the holding tube  10  moves forward to close the gap L 9  by the action of the cushion spring  14 , the slider  20  and the lead protection pipe  17  move forward the same distance, and the mechanism returns to the writing condition shown in  FIGS.  13 A and  14 B . At this time, since no elastic piece for resistance is provided at the outer periphery of the slider  20 , the slider  20  moves forward together with the lead  9 . Accordingly, unlike the conventional mechanical pencil shown in  FIG.  17 C , there is no possibility that the slider  20  stops part way during movement in the tapered member whereby only the front end of the lead unwantedly protrudes, and therefore reliable protection can be secured up to the front end of the lead by the lead protection pipe. 
       FIG.  15    shows a protection mechanism for the outer pipe  19 . In this example, an outer pipe holder  18  is inserted to undergo back and forth movement in a tapered member  16 . As shown in  FIG.  15 A  and  FIG.  15 C , a lead protection pipe  17  is housed in the outer pipe  19 . In this state, when the main body is dropped on a floor  39 , as shown in  FIG.  15 B  and  FIG.  15 D , the outer pipe holder  18  to which the outer pipe  19  is fixed moves backward by a distance L 10  within the tapered member  16 , abuts on a holding tube  10 , and moves the holding tube  10  backward against a cushion spring  14 . The cushion spring  14  compresses to a length L 11 . By the buffering effect of the cushion spring  14 , the outer pipe  19  is protected from breaking, and when the pencil is raised from the floor, by the action of the cushion spring  14 , the holding tube  10  and the outer pipe holder  18  move forward and the mechanism returns to the original state. 
     In the above example, the cushion mechanism is provided, but the cushion mechanism may be omitted.  FIG.  16    shows this example, and since this example basically has the same structure as the example shown in  FIG.  1   , common parts are indicated by the same reference numerals for explanation. A main body  1  is formed in a tubular shape, its rear portion  2  has a clip  3  at the outside, and a grip  15  is provided at the outer periphery of a front portion  4  which has an inner diameter smaller than that of the rear portion  2 . At the front end of the front portion  4 , a chuck  5  and a clamping ring  6  are housed to undergo back and forth movement, and the rear part of the chuck  5  is connected to a lead tank  8  housed in the main body  1 . Between the front portion of the lead tank  8  and an inward stepped portion  11  formed within the front portion of the main body, a chuck spring  12  is provided to urge the chuck  5  rearwardly. At the rear end of the lead tank  8 , a conventional eraser-feeding mechanism  40  is provided and removably holds an eraser  7 . Alternatively, the eraser may be mounted at the rear end of the lead tank as shown in  FIG.  1   . At the front portion  4  of the main body  1 , a tapered member  16  is threadedly mounted and an outer pipe  19  is inserted into and fixed to the tapered member  16 . In the outer pipe holder  18 , a slider  20  is movably inserted to undergo back and forth movement, and at the front end of the slider  20 , a lead protection pipe  17  is fixed. The lead protection pipe  17  is movably inserted to undergo back and forth movement in the outer pipe  19  and protrudes from the front end thereof. 
     Inside the slider  20 , a lead weak-holding portion  25  for temporarily holding the lead  9  is provided. The lead weak-holding portion  25  is tubular-shaped and provided with one or more longitudinal slits to add flexibility to the lead weak-holding portion  25  so that it can elastically hold the lead  9 . Alternatively, the lead weak-holding portion  25  may be formed by an elastic material such as a rubber material (not shown). The lead-holding force exerted by the lead weak-holding portion  25  is set to be far weaker as compared to conventional mechanical pencils. Specifically, when the front end of the main body  1  is directed downward, the lead-holding force is slightly greater than the force necessary to prevent the lead from dropping by its own weight. By way of example, a gram-force of a few grams has been found effective to provide a holding force sufficient to ensure that the lead does not drop by its own weight. That is, the load is set to be such a level that even if writing is conducted while dragging the lead protection pipe  17  on the writing surface, no hindrance to writing occurs. Further, on the outer face of the slider  20  is formed a non-resistant sliding-contact surface  26  which slidingly contacts the inner face of the outer pipe holder  18  with substantially no resistance. On the inner face of the outer pipe holder  18 , a raised part  37  is provided to prevent detachment of the slider  20 . 
     In the above structure, when the lead tank  8  is knocked, the chuck  5  moves forward and the lead  9  is fed. When the knocking is stopped and the chuck  5  moves backward, the chuck  5  slips into the clamping ring  6 , and concurrently the lead  9  is retracted. Therefore, the distance of feeding the lead by the knocking operation is determined taking into account the retraction distance. That is, the lead feeding distance is a distance obtained by adding the lead-retracting distance to the protrusion amount of the lead necessary for writing. By doing so, when the chuck moves backward, even if the lead  9  is retracted rearwardly, a length necessary for writing can be secured and writing can be conducted without difficulty. 
     By employing the above structure, the mechanical pencil shown in the example of  FIG.  16    can move the slider  20  and the lead protection pipe  17  forward to feed the lead  9  by one knocking operation. As the writing proceeds, since the slider  20  and the lead protection pipe  17  move backward without generating resistance, even if writing is conducted while dragging the lead protection pipe  17 , no uncomfortable feeling is given to the user and writing is not hindered. Further, since the outer periphery of the lead protection pipe  17  is covered by the outer pipe  19 , the lead protection pipe  17  can endure the bending force exerted on its front end on writing, and is hardly bent, whereby lead breakage can be prevented. 
     It will be appreciated by those of ordinary skill in the art that obvious changes, alterations and modifications can be made to the examples and embodiments described in the foregoing description without departing from the broad inventive concept thereof. It is understood, therefore, that this disclosure is not limited to the particular examples and embodiments disclosed, but is intended to cover all obvious changes, alterations and modifications thereof which are within the scope and spirit of the disclosure as defined by the appended claims.