SPECIFICATION
TITLE OF THE INVENTION: COUPLING STRUCTURE
TECHNICAL FIELD
[0001] The present invention relates to a coupling structure to couple a member having
an external thread part onto a member having an internal thread part.
BACKGROUND ART
[0002] Usually a casing of a writing implement such as a ballpoint pen is divided into a
plurality of members, those members are coupled by screwing an external thread part
formed in one member to an internal thread part formed in another member (See Patent
document 1, for example).
[00031 When one member in screwed to another member, the relative positions of both
the members are not guaranteed to be always uniform. As one member is rotated
relatively with another member about an axis so that the external thread part is screwed in
to the internal thread part, one member moves toward another member along the axis
direction, then an edge face of one member facing in the axis direction comes into contact
with an edge face of another member opposed to the edge face of one member. However,
one member or another member is able to be further rotated by applying much stronger
operation force (torque) to the members even after the edge faces of both the members are
brought in contact with each other. Therefore the final position of one member fitted
relative to another member can change depending on amount of the operation force
applied to the members.
RELATED ART DOCUMENTS
PATENT DOCUMENTS
[0004]
Patent document 1: Japanese Unexamined Patent Application Publication No.

2001-293996.
SUMMARY OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[0005] It is an object of the present invention to reduce the unevenness of the relative
position of one member with respect to another member when coupling those members
through engaging an external thread part with an internal thread part.
MEANS OF SOLVING THE PROBLEMS
[0006] According to the present invention, a coupling structure is configured to couple a
first member onto a second member by screwing an external thread part formed in the ftrst
member to an internal thread part formed in the second member. The coupling structure
includes a stopper part provided in one of the first member and the second member to
stop the first member rotating relatively with the second member so that the external
thread part is not screwed in to the internal thread part any further when the relative
position of the first member with respect to the second member reaches a predetermined
end position, and a friction increasing part provided in one of the first member and the
second member to increase friction acting on the first member or the second member being
rotated at the stage just before the stopper part stops the first member rotating relatively
with the second member so as to get the friction larger than that acting prior to the stage.
[0007] It is preferable that the stopper part have a contact face that is approximately
orthogonal to a circumferential direction around an axis.
[0008] If an insertion part is provided at the top of the external thread part in the first
member, and besides a tube part whose internal surface is brought into contact with or in
close proximity to the external surface of the insertion part when the insertion part is
inserted into the tube part is provided at the back of the internal thread part in the second
member, screwing the external thread part in to the internal thread part will come to start

after inserting the insertion part into the tube part. Hence, executing operation to rotate
the first member or the second member is facilitated. In addition, stable structure is
fabricated that makes it difficult for one of the members to wobble relatively with the other
while both the members are coupled. In this situation, it is preferable that the stopper
part be provided on the insertion part or the tube part.
[0009] The coupling structure according to the present invention is suitable for the
structure to couple the first member molded of rigid resin onto the second member
molded of rigid resin.
[0010] The first member and the second member may constitute a casing of a writing
implement to contain a refill filled with ink for writing or a lead for writing.
EFFECTS OF THE INVENTION
[0011] The present invention can reduce the unevenness of the relative position of one
member with respect to another member when coupling those members through
engaging the external thread part with the internal thrpad part
BRIEF DESCRIPTION OF THE DRAWINGS
[0012]
FIG. 1 is a front view of a writing implement according to an embodiment of the
present invention.
FIG. 2 is an exploded side view of the writing implement according to the
embodiment.
FIG. 3 is an exploded perspective view showing a front barrel, which is partly
broken, and a rear barrel of the writing implement according to the embodiment.
FIG. 4 is an enlarged cross section in a front view showing an essential part of the
writing implement according to the embodiment.
FIG.5 is an enlarged cross section in a view showing the essential part of the

writing implement according to the embodiment.
FIG. 6 is a cross section in a plan view showing the writing implement according
to the embodiment.
FIG. 7 is an enlarged and exploded perspective view showing an essential part of
a variation of the present invention.
FIG. 8 is an enlarged cross section in a front view showing the essential part of the
variation.
FIG. 9 is a cross section in a plan view showing the variation.
MODE FOR CARRYING OUT THE INVF,NTION
[0013] Described below is an embodiment of the present invention with reference to FIGs.
FIGs. 1 and 2 shuw a writing implement P to which the present invention is applirahln
The writing implement P is mainly composed of a main component 3 that has the function
of writing on papers or the like, a rear barrel 1 and a front barrel 2 that constitute a casing
to contain the main component 3 and be held by a user. The writing implement P
illustrated in FIGs. is a ballpoint pen, the main component 3 includes a refill that has a
vessel filled with ink and a writing mechanism with a ball to transfer the ink onto papers
or the like mounted on the top of the vessel. If the writing implement P is a mechanical
pencil, the main component 3 will include a lead of the mechanical pencil and a
mechanism to let out the lead.
[0014] Each of the rear barrel 1 and the front barrel 2 is formed into an approximately
cylindrical shape extended in its axis direction such that the main component 3 can be
inserted inside the rear barrel 1 and the front barrel 2. In this embodiment, an external
thread part 11 is formed on the rear barrel 1 as a first member, an internal thread part 21 is
formed on the front barrel 2 as a second member, the rear barrel 1 and the front barrel 2 are

[0015] As shown in FIGs. 3-6, the rear barrel 1, which is molded of rigid resin, includes
an insertion part 12 that has an approximately cylindrical shape extended in the axis
direction, the external thread part 11 that is continuous with the rear of the cylindrical part,
an enlarged part 13 as a friction increasing part that is continuous with the rear of the
external thread part 11, and a base part 15 that is continuous with the rear of the enlarged
part 13. Since the insertion part 12 exists, the external thread part 11 is positioned at an
intermediary region of the rear barrel 1 in the axis direction. The outside diameter of the
insertion part 12 is slightly smaller than the major diameter of the external thread part 11
and the minor diameter of the internal thread part 21. The major diameter of the external
thread part 11 means the diameter of an imaginary cylinder or cone circumscribed about
the crest of the external thread, the minor diameter of the intpmal thread part 21 mcuno the
diameter of an imaginary cylinder or cone inscribed in the crest of the internal thread.
The measure of the insertion part 12 in the axis direction is several times longer than the
measure of the external thread part 11 in the axis direction. A protrusion 14 as a stopper
part protrudes forward from the front end face of the insertion part 12 facing in the axis
direction.
[0016] The enlarged part 13 is a region with an outside diameter that is slightly larger
than the major diameter of the external thread. The measure of the enlarged part 13 in
the axis direction is smaller than the lead of the external thread part 11 and the internal
thread part 21. The lead of the external thread part 11 and the internal thread part 21
means the moving distance in the axis direction while going once around the axis along
the helix of the thread, the lead of a single-start thread is equal to the pitch of the thread.
The measure of the enlarged part 13 in the axis direction is about a half of the lead.
[0017] When the rear barrel 1 is fitted to the front barrel 2, the insertion part 12, the

barrel 2 and covered by the front barrel 2. However, the base part 15 of the rear barrel 1 is .
not inserted into the front barrel 2 and exposed to the outside after the rear barrel 1 has
been fitted to the front barrel 2. A clip 16 that is usable to hook the writing implement P
on papers, a plate, a pocket of clothes the user wears or the like is attached to the base part
15 of the rear barrel 1. A decoration 10, for example, a logo may be affixed on the outer
surface of the clip 16 by printing and so on.
[0018] The front barrel 2, which is molded of rigid resin, includes an tube part 22 that has
an approximately cylindrical shape extended in the axis dirprtinn, a reduced part 25 that is
continuous with the front of the tube part 22, the internal thread part 21 that is continuous
with the rear of the tube part 22, and a fitting part 23 as a friction increasing part that is
annular and continuous with the rear of internal thread part 21. The inside diameter of
the tube part 22 into which the insertion part 12 of the rear barrel 1 is inserted is
approximately equal to the outside diameter of the insertion part 12. The internal surface
of the tube part 22 tapers off such that the inside diameter thereof rcducco slightly and
gradually toward the front. The inside diameter of the reduced part 25 is smaller than the
inside diameter of the tube part 22. When the insertion part 12 of the rear barrel 1 is
inserted into the tube part 22 of the front barrel 2, the rear end face of the reduced part 25
facing in the axis direction is opposed to the front end face of the insertion part 12. A
protrusion 24 as a stopper part protrudes rearward from the rear end face. The
protrusion 24 unites with the internal surface of the front end region in the tube part 22.
The measure of the tube part 22 in the axis direction is several times longer than the
measure of the internal thread part 21 in the axis direction. The measure of the tube part
22 in the axis direction is approximately equal to the measure of the insertion part 12 in the
axis direction plus the protrusion length of the protrusion 14 or 24 in the axis direction.

rear barrel 1 is firmly fitted is approximately equal to the outside diameter of the enlarged
part 13. The measure of the fitting part 23 in the axis direction is smaller than the lead of
the external thread part 11 and the internal thread part 21. The measure of the fitting part
23 in the axis direction is approximately equal to the measure of the enlarged part 13 in the
axis direction.
[0020] The outer periphery of the tube part 22, the internal thread part 21 and the fitting
part 23 in the front barrel 2 is formed into a cylindrical shape that is continuously smooth,
the outside diameter thereof is approximately equal to the outside diameter of the base
part 15 of the rear barrel 1. A decoration 20, for example, a logo may be affixed on this
outer periphery by printing and so on. On the other hand, the outside diameter of the
reduced part 25 in the rear barrel 2 is smaller by one step than the outside diameter of the
tube part 22. The outer periphery of the reduced part 25 is covered with a grip 26, for
example, made of resin softer than the front barrel 2 or the rear barrel 1.
[0021] In assembling the writing implement P, the insertion part 12 of the rear barrel 1 is
inserted into the tube part 22 of the front barrel 2 from the rear end side, that is, the side on
which the internal thread part 21 and the fitting part 23 exist, while the main component 3
is put through the rear barrel 1 and the front barrel 2. The external surface of the insertion
part 12 inserted in the tube part 22 is brought into contact with or in close proximity to the
internal surface of the tube part 22. Then, the rear barrel 1 is rotated relatively with the
front barrel 2 about the axis, the external thread part 11 is screwed in to the internal thread
part 21.
[0022] FIG. 4 shows the situation where the rear barrel 1 can completely coupled onto the
front barrel 2 if the rear barrel 1 is further rotated by about 360 degrees relatively with the
front barrel 2. In this situation, a distance in the axis direction still exists between the

protrusion 24 protruding rearward from the reduced, part 25 into the front end region of
the tube part 22 in the tube part 22. Therefore the protrusions 14 and 24 do not interfere
with each other during operation of further rotating the rear barrel 1 by about 360 degrees
relatively with the front barrel 2. Also, the enlarged part 13 of the rear barrel 1 does not
come into contact with the fitting part 23 of the front barrel 2 yet.
[0023] When the rear barrel 1 is rotated by about 180 degrees relatively with the front
barrel 2 from the situation shown in FIG. 4, the rear barrel 1 moves forward in the axis
direction relatively with the front barrel 2, the enlarged part 13 thereof comes to be fitted
into the fitting part 23 of the front barrel 2. The external surface of the enlarged part 13
fitted in the fitting part 23 is appressed to the internal surface of the fitting part 23 and
generates large friction force, thereby resistant force against the operation force to rotate
the rear barrel 1 or the front barrel 2 about the axis increases suddenly. In other words,
variation of the friction force per unit degree, which is,the variation of the friction force
while the rear barrel 1 is rotated by unit degree relatively with the front barrel 2, at the
stage just before the protrusions 14 and 24 stop the rear barrel 1 rotating relatively with the
front barrel 2 is larger than that in the period prior to the stage.
[0024] Thereafter the rear barrel 1 is further rotated by about 180 degrees relatively with
the front barrel 2 against the friction generated between the enlarged part 13 and the fitting
part 23, and the position of the protrusion 14 of the rear barrel 1 in the axis direction
consequently becomes identical with the position of the protrusion 24 of the front barrel 2
in the axis direction. Accordingly, as shown in FIGs. 5 and 6, a contact face 141 of the
protrusion 14 that is approximately orthogonal to the circumferential direction around the
axis comes into contact with a contact face 241 of the protrusion 24 that is approximately
orthogonal to the circumferential direction around the axis. The protrusions 14 and 24

front barrel 2 so as to screw the external thread part 11 in to the internal thread part 21
more deeply. That is, FIGs. 5 and 6 represent the end position of the rotation about the
axis of the rear barrel 1 or the front barrel 2 as the rear barrel 1 is fitted to the front barrel 2.
[0025] In this embodiment, a coupling structure is configured to couple the first member
(the rear barrel) 1 onto the second member (the front barrel) 2 by screwing the external
thread part 11 formed in the first member 1 to the internal thread part 21 formed in the
second member 2. The coupling structure includes the stopper parts (the protrudes) 14
and 24 provided in at least one of the first member 1 and the second member 2 to come
into contact with the other and stop the first member 1 rotating relatively with the second
member 2 so that the external thread part 11 is not screwed in to the internal thread part 21
any further when the relative position of the first member 1 with respect to the second
member 2 reaches the predetermined end position, and the friction increasing parts (the
enlarged part and the reduced part) 13 and 23 provided in one of the first member 1 and
the second member 2 to increase friction acting on the first member 1 or the second
member 2 being rotated by coming into close contact with the other at the stage just before
the stopper parts 14 and 24 stop the first member 1 rotating relatively with the second
member 2 so as to get the friction larger than that acting prior to the stage.
[0026] According to the embodiment, when coupling the members 1 and 2 through
engaging the external thread part 11 with the internal thread part 21, the operation of
screwing the external thread part 11 in to the internal thread part 21 is always finished in
the position where the stopper part 14 or 24 provided on one of the members 1 and 2
comes into contact with the other member 2 or 1. Hence, the unevenness of the relative
position of one member 1 or 2 wim respect to the other member 2 or 1 can be minimized.
In particular, if the logos 10 and 20 or the like are affixed on the members 1 and 2 each, the

. always uniform, and the product will become superior and .beautiful asa result
[0027] Since the friction increasing parts 13 and 23 increase the friction force generated
between the member 1 and the member 2 rapidly (or stepwise) just before the stopper
parts 14 and 24 stop the member 1 and the member 2 rotating relatively, both the members
1 and 2 are firmly coupled though the stopper parts 14 and 24 is brought in contact with
each other, the engagement of the external thread part 11 with the internal thread part 21
does not loosen easily.
[0028] Also, at the stage just before the stopper parts 14 and 24 stop both the membprs 1
and 2 rotating relatively, in other words, at the stage just before both the members 1 and 2
reach the end position shown in FIGs. 5 and 6, the resistance against the operation to rotate
the members 1 and 2 about the axis becomes large. Thereby an operator or automatic
machinery for assembling the writing implement P can mechanically sense that both the
members 1 and 2 are just before the end position of the rotating, fracturing the member 1
or 2 (or the stopper part 14 or 24 and so on) by continuing to apply an pxrpssivp nppration
force to the members 1 and 2 is avoided.
[0029] Since the stopper parts 14 and 24 each have the contact faces 141 and 241
approximately orthogonal to the circumferential direction around the axis, the relative
rotation of both the members 1 and 2 about the axis can be stopped surely by hitting the
contact faces 141 and 241 against the other members 2 and 1.
[0030] Since the insertion part 12 is provided at the top of the external thread part 11 in
the first member 1, and besides the tube part 22 is provided at the back of the internal
thread part 21 in the second member 2, the internal surface of the tube part 22 being
brought into contact with or in close proximity to the external surface of the insertion part
12 when the insertion part 12 is inserted into the tube part 22, screwing the external thread

tube part .22. Hence, executing the operation to rotate the first member 1 or the second
member 2 about the axis is facilitated. In addition, stable structure is fabricated that
makes it difficult for one of the members 1 and 2 to wobble relatively with the other 2 or 1
while both the members 1 and 2 are coupled.
[0031] If external force to bend the writing implement P acts on it while the rear barrel 1
as the first member is fitted to the front barrel 2 as the second member, the rear barrel 1
and the front barrel 2 will not be shaky mutually owing to an overlap of the shaft part 12
and the tube part 22, the barrel of the writing implement P is stable. Loosening or
wobbling the threads by stress concentration on the thread parts 11 and 12 can be avoided
when the user grips the writing implement P tightly. That is, strength and durability of
the barrel of the writing implement P are enhanced.
[0032] Since the stopper parts are each provided on the insertion part 12 and the tube
part 22, the relative rotation of both the members 1 and 2 can be restrained at the point
apart from the region where the external thread part 11 is engaged with the internal thread
part 21.
[0033] The present invention is not limited to the above-described embodiment. For
example, the stopper part may be provided on a region other than the insertion part 12 and
the tube part 22. According to an example shown in FIGs. 7-9, a protrusion 17 is
provided such that the protrusion 17 protrudes outward in the radial direction from the
external surface of the enlarged part 13 in the rear barrel 1 as the first member, and besides
a recess 27 that the protrusion 17 can enter is provided by notching a part of the inner
periphery of the fitting part 23 in the front barrel 2 as the second member. As shown in
FIG. 9, when the rear barrel 1 is completely coupled onto the front barrel 2 after inserting
the insertion part 12 in the tube part 22 and screwing the external thread part 11 to the

orthogonal, to .the circumferential direction around the axis comes into contact with a
contact face 271 of the protrusion 27 that is approximately orthogonal to the
circumferential direction around the axis. The protrusions 17 and 27 butting with each
other prohibit the rear barrel 1 from being rotated relatively with the front barrel 2 so as to
screw the external thread part 11 in to the internal thread part 21 more deeply. That is,
FIG. 9 represents the end position of the rotation about the axis of the rear barrel 1 or the
front barrel 2 as the rear barrel 1 is fitted to the front barrel 2.
[0034] With respect to the friction increasing part, a mode different from the above
embodiment may be employed. According to an example shown in FIG. 8, the inside
diameter of a front end region 221 of the tube part 22 in the front barrel 2 is slightly smaller
than that of the other regions of the tube part 22. At the stage just before the stopper parts
17 and 27 stop the first member 1 rotating relatively with the second member 2, in other
words, at the stage just before both the members 1 and 2 reach the end position shown in
FIG. 9, a front end region 121 of the insertion part 12 in the rear barrel 1 is fitted into this
region 221 of the tube part 22 in the front barrel 2. At this time, the external surface of the
front end region 121 of the insertion part 12 is appressed to the internal surface of the front
end region 221 of the tube part 22 and generates large friction force, thereby the resistant
force against the operation force to rotate the rear barrel 1 or the front barrel 2 about ihe
axis increases suddenly.
[0035] The friction increasing part may be provided on the external thread part 11 and/or
the internal thread part 21. More specifically, the friction increasing part can be
configured by enlarging the major diameter or the minor diameter (the diameter of an
imaginary cylinder or cone circumscribed about the root of the external thread) of a part of
the external thread in comparison with the other part thereof, and/or, by reducing the

inscribed in the root of the internal thread) of a part of the internal thread in comparison. ..
with the other part thereof.
[0036] The friction increasing parts 12, 23,121 and 221 are not always continuous along
the circumferential direction around the axis.
[0037] Though the outside diameter of the insertion part 12 is approximately equal to the
inside diameter of the tube part 22, and besides the external surface of the insertion part 12
is brought into contact with or in close proximity to the internal surface of the tube part 22
in the above embodiment, the outside diameter of the insertion part 12 may be smaller
than the inside diameter of the tube part 22, and the external surface of the insertion part 12
may be apart from the internal surface of the tube part 22 accordingly.
[0038] The measure of the insertion part 12 in the axis direction and the measure of the
tube part 22 in the axis direction can be arbitrarily set, it is not necessary that those be
several times longer than the measure of the external thread part 11 in the axis direction
and the measure of the internal thread part 21 in the axis direction, those may be shorter.
[0039] The measure of the enlarged part 13 in the axis direction and the measure of the
fitting part 23 in the axis direction can also be arbitrarily set, it is not necessary that those be
about a half of the lead of the threads.
[0040] Of course, the present invention is applicable to objects other than a casing of a
writing implement.
[0041] Regarding to the concrete structures of the respective components, various
modifications are possible without departing from the scope and spirit of the present
invention.
INDUSTRIAL APPLICABILITY
[0042] The present invention is applicable to a coupling structure to couple a member
having an external thread part onto a member having an internal thread part.

DESCRIPTION OF THE.REFERENCE NUMERAL
[0043]
P writing implement
1 first member (rear barrel)
11 external thread part
12 insertion part
13 friction increasing part (enlarged part)
14 stopper part (protrusion)
141 contact face
2 second member (front barrel)
21 internal thread part
22 tube part
23 friction increasing part (fitting part)
24 stopper part (protrusion)
241 contact face
3 main component

CLAIMS
1. A coupling structure to couple a first member onto a second member by screwing
an external thread part formed in the first member to an internal thread part formed in the
second member, the coupling structure comprising:
a stopper part provided in one of the first member and the second member to stop
the first member rotating relatively with the second member so that the external thread
part is not screwed in to rhp internal thread part any further when the relative position of
the first member with respect to the second member reaches a predetermined end position;
and
a friction increacing part provided in one of the first member and the second
member to increase friction acting on the first member or the second member being rotated
at the stage just before the stopper part stops the first member rotating relatively with the
second member so as to get the friction larger than that acting prior to the stage.
2. The coupling structure according to claim 1,
wherein the stopper part has a contact face that is approximately orthogonal to a
circumferential direction around an axis.
3. The coupling structure according to claim 1 or 2,
wherein an insertion part is provided at the top of the external thread part in the
first member, and besides a tube part is provided at the back of the internal thread part in
the second member, the internal surface of the tube part being brought into contact with or
in close proximity to the external surface of the insertion part when the insertion part is
inserted into the tube part

screwing the external thread part in to the internal thread part starts after inserting
the insertion part into the tube part.
4. The coupling structure according to claim 3,
wherein the stopper part is provided on the insertion part or the tube part.
5. The coupling structure according to claim 1,2,3 or 4,
wherein the first member and the second member constitute a casing of a writing
implement to contain a refill filled with ink for writing or a lead for writing.