TOY BUILDING BLOCK SET

Abstract

A toy building block set includes a socket block piece and at least one of first and second plug block pieces. The socket block piece has a socket wall body and at least one engaging hole of a regular polygonal shape. Each of the first and second plug block pieces has a plug body and at least one polygonal insert plug configured to be inserted into and in frictional engagement with the engaging hole with a first frictional force to permit rotation of the plug block piece relative to the socket block piece by a forcible torque and to keep a predetermined angle of the plug block piece relative to the socket block piece when the forcible torque is removed. The plug body is partially inserted into and in frictional engagement with the engaging hole with a second frictional force larger than the first frictional force.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of Taiwanese Patent Application No. 108102032, filed on Jan. 18, 2019.

FIELD

The disclosure relates to a toy building block set, and more particularly to a toy building block set in which two block pieces are interlockable at a variety of angular positions.

BACKGROUND

In a conventional toy building block set, two identical block pieces are usually interlocked at several angular positions, such as 0, 90 and 180 degrees, which limits the configuration of combination and hence renders use of the toy monotonous.

SUMMARY

Therefore, an object of the disclosure is to provide a toy building block set that can alleviate at least one of the drawbacks of the prior art.

According to the disclosure, the toy building block set includes a socket block piece and at least one of first and second plug block pieces. The socket block piece has a socket wall body and at least one engaging hole which is formed in the socket wall body and bordered by an inner peripheral wall that surrounds a rotating axis normal to the socket wall body. The inner peripheral wall has a cross-section which is of a regular polygonal shape. Each of the first and second plug block pieces has a plug body and at least one polygonal insert plug which extends from the plug body. The polygonal insert plug has an outer peripheral wall with a cross-section which is of a regular polygonal shape to be inserted into the engaging hole in an extending direction parallel to the rotating axis and in frictional engagement with the inner peripheral wall with a first frictional force that is dimensioned to permit rotation of the plug block piece relative to the socket block piece about the rotating axis by a forcible torque and to keep a predetermined angle of the plug block piece relative to the socket block piece when the forcible torque is removed. The plug body is partially inserted into the engaging hole and in frictional engagement with the inner peripheral wall with a second frictional force which is larger than the first frictional force.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the disclosure will become apparent in the following detailed description of the embodiments with reference to the accompanying drawings, of which:

FIG. 1 is a perspective view illustrating a first embodiment of a toy building block set according to the disclosure in an interlocked state;

FIG. 2 is a perspective view illustrating the first embodiment;

FIG. 3 is a fragmentary sectional view illustrating a plug block piece interconnecting two socket block pieces of the first embodiment;

FIG. 4 is a fragmentary sectional view illustrating another plug block piece interconnecting two socket block pieces of the first embodiment;

FIG. 5 is a fragmentary sectional view similar to FIG. 4, but taken from another angle;

FIGS. 6 to 8 are schematic front views illustrating rotation of two socket block pieces relative to each other;

FIG. 9 is a perspective view illustrating a second embodiment of a toy building block set according to the disclosure in an interlocked state;

FIG. 10 is a perspective view similar to FIG. 9, but taken from another angle;

FIG. 11 is a perspective view of the second embodiment;

FIG. 12 is a perspective view similar to FIG. 11, but taken from another angle;

FIG. 13 is a perspective view illustrating a third embodiment of a toy building block set according to the disclosure in an interlocked state;

FIG. 14 is a perspective view of the third embodiment; and

FIGS. 15 and 16 are schematic top views illustrating the third embodiment upon actuation of a control unit to rotate two turning mechanisms.

DETAILED DESCRIPTION

Before the disclosure is described in greater detail, it should be noted that where considered appropriate, reference numerals or terminal portions of reference numerals have been repeated among the figures to indicate corresponding or analogous elements, which may optionally have similar characteristics.

Referring to FIGS. 1 and 2, a first embodiment of a toy building block set 100 according to the disclosure includes three socket block pieces 1, a first plug block piece 2 and a second plug block piece 2′. Each of the socket and plug block pieces 1, 2, 2′ is made from a plastic material.

With reference to FIGS. 3 to 5, each socket block piece 1 has a socket wall body 11 in the form of an elongated plate, and three engaging holes 12 which are formed in the socket wall body 11 and spaced apart from each other in a lengthwise direction of the socket wall body 11. Each of the engaging holes 12 is bordered by an inner peripheral wall that surrounds a rotating axis normal to the socket wall body 11, i.e., transverse to the lengthwise direction. The number of the engaging holes 12 of each socket block piece 1 may be varied, such as one, two or at least four engaging holes 12. In this embodiment, each of the engaging holes 12 penetrates through the socket wall body 11. Alternatively, the engaging holes 12 may be blind holes. The inner peripheral wall of each engaging hole 12 has a cross-section which is of a regular polygonal shape, such as an octagonal shape in this embodiment.

Each of the first and second plug block pieces 2, 2′ is disposed to interconnect two adjacent ones of the socket block pieces 1. The first plug block piece 2 has a plug body 21 and two polygonal insert plugs 22 which extend from two ends of the plug body 21 and opposite to each other. The polygonal insert plugs 22 are respectively inserted into the engaging holes 12 of two adjacent ones of the socket block pieces 1. The second plug block piece 2′ has a plug body 21′, a polygonal insert plug 22 which extends from an end of the plug body 21′, and a circular insert plug 23 which extends from the other end of the plug body 21′ and away from the polygonal insert plug 22. The polygonal and circular insert plugs 22, 23 are respectively inserted into the engaging holes 12 of two adjacent ones of the socket block pieces 1. Specifically, the polygonal insert plug 22 has an outer peripheral wall with a cross-section which is of a regular polygonal shape, such as an octagonal shape, to be inserted into the engaging hole 12 in an extending direction parallel to the rotating axis and in frictional engagement with the inner peripheral wall with a first frictional force that is dimensioned to permit rotation of the plug block piece 2, 2′ relative to the socket block piece 1 about the rotating axis by a forcible torque and to keep a predetermined angle of the plug block piece 2, 2′ relative to the socket block piece 1 when the forcible torque is removed. In this embodiment, the predetermined angle may be 0, 45, 90, 135 or 180 degrees. Alternatively, the cross-section of the polygonal insert plug 22 and the engaging hole 12 may be of another polygonal shape, such as a regular triangular shape, a regular pentagonal shape, a regular heptagonal shape, etc. The circular insert plug 23 has an outer peripheral wall with a cross-section which is of a circular shape to be inserted into the engaging hole 12 in the extending direction and in frictional engagement with the inner peripheral wall with a third frictional force that is dimensioned to permit rotation of the second plug block piece 2′ relative to the socket block piece 1 about the rotating axis by a forcible torque and to keep any angle of the plug block piece 2′ relative to the socket block piece 1 when the forcible torque is removed. The plug body 21, 21′ of each of the first and second plug block pieces 2, 2′ is partially inserted into the engaging hole 12 when the polygonal or circular insert plug 22, 23 is inserted into the engaging hole 12, and is in frictional engagement with the inner peripheral wall with a second frictional force which is larger than the first frictional force or the second frictional force.

In this embodiment, each of the first and second plug block pieces 2, 2′ has two insert plugs 22 or 22, 23 to interconnect two of the socket block pieces 1. Alternatively, each plug block piece 2, 2′ may have only one insert plug 22 or 23. In this embodiment, the insert plugs 22 of the first plug block piece 2 are configured to be aligned with each other in the extending direction such that the two socket block pieces 1 interlocked by the first plug block piece 2 are kept relative to each other at an angular position by an included angle of 0, 45, 90, 135 or 180 degrees. The two socket block pieces 1 interlocked by the second plug block piece 2′ are kept relative to each other at an angular position by any included angle of 0 to 180 degrees. Each the first and second plug block pieces 2, 2′ also has a tubular hole 24 which extends through the plug body 21, 21′ and the polygonal insert plugs 22 or the polygonal and circular insert plugs 22, 23 in the extending direction. Each of the first and second plug block pieces 2, 2′ also has a plurality of splits 25 which are formed in and extend through the outer peripheral wall of the polygonal or circular insert plug 22, 23 to be in communication with the tubular hole 24 and angularly displaced from each other so as to permit resilient engagement of the outer peripheral wall with the inner peripheral wall of the socket block piece 1 and so as to facilitate insertion of the polygonal or circular insert plug 22, 23 into the engaging hole 12. With the splits 25, the polygonal or circular insert plug 22, 23 is rotatably inserted into the engaging hole 12 to permit rotation of the first or second plug block piece 2, 2′ relative to the socket block piece 1 to adjust the included angle therebetween, as shown in FIGS. 6 to 8. Specifically, the two socket block pieces 1 interlocked by the polygonal insert plug 22 of the first plug block piece 2 can be rotated relative to each other about the rotating axis to have an included angle of 0, 45, 90, 135 or 180 degrees. The two socket block pieces 1 interlocked by the circular insert plug 23 of the second plug block piece 2′ can be rotated relative to each other about the rotating axis to have any included angle of 0 to 180 degrees. For example, the socket block pieces 1 may be formed as limb parts of a robot (not shown), and the first and second plug block pieces 2, 2′ may be formed as joints of the limb parts. Moreover, in this embodiment, the tubular hole 24 has a cross-section of a dodecagonal shape for a stem (not shown) having a regular hexagonal cross-section to be inserted thereinto by two different angles.

Specifically, referring to FIGS. 2 and 3, the inner peripheral wall of each engaging hole 12 has a middle wall portion 121 extending in a direction of the rotating axis, two larger-diameter wall portions 122 respectively extending from two ends of the middle wall portion 121, and two shoulder portions 123 each interposed between the middle wall portion 121 and a respective one of the larger-diameter wall portions 122. In this embodiment, each of the middle and larger-diameter wall portions 121, 122 has a cross-section of a regular octagonal shape. Each of the polygonal and circular insert plugs 22, 23 has a plurality of flange sections 221, 231 formed at a terminal end thereof. When the polygonal or circular insert plug 22, 23 is inserted into the engaging hole 12, the flange sections 221, 231 are received within one larger-diameter wall portion 122 and abut against the corresponding shoulder portion 123, and the plug body 21 is partially received within the other larger-diameter wall portion 122 and abuts against the corresponding shoulder portion 123. Thus, the polygonal or circular insert plug 22, 23 can be firmly engaged in the engaging hole 12. In this embodiment, the plug body 21 has a regular octagonal cross-section to match with the larger-diameter wall portion 122 so as to prevent rotation of the first plug block piece 2 relative to the socket block piece 1 when the plug body 21 is partially inserted into the engaging hole 12. The plug body 21′ is circular in cross-section so as to render the second frictional force larger to prevent rotation of the second plug block piece 2′ relative to the socket block piece 1. Moreover, each of the socket block piece 1 has two surrounding flange walls 13 formed at two sides of the socket wall body 11 and surrounding the engaging holes 12 for concealing the plug bodies 21, 21′ of the first and second plug block pieces 2, 2′ when the first and second plug block pieces 2, 2′ are disposed to interconnect two socket block pieces 1. Furthermore, each polygonal insert plug 22 has a plurality of ribs 222 which are in frictional engagement with the inner peripheral wall of the engaging hole 12 to increase the friction therebetween.

Referring to FIGS. 9 to 12, in a second embodiment, the toy building block set 100 includes two socket block pieces 1′, a first plug block piece 2″, a second plug block piece 2′, and a retaining piece 3. Each of the socket block pieces 1′ has five of the engaging holes 12. Each larger-diameter wall portion 122 of the inner peripheral wall of each engaging hole 12 has a plurality of slots 124 angularly spaced apart from each other and each extending in the direction of the rotating axis. The plug body 21″ of the first plug block piece 2″ has a plurality of keys 211 formed thereon and angularly spaced apart from each other to be respectively engaged in the slots 124 so as to prevent rotation of the first plug block piece 2″ relative to the socket block piece 1′. In this embodiment, eight of the slots 124 are formed in the larger-diameter wall portion 122. Eight of the keys 211 are formed on the plug body 21″ of the first plug block piece 2. The retaining piece 3 has a retaining shank 32 which extends to be inserted into the tubular hole 24 of the first plug block piece 2″ in the extending direction from one polygonal insert plug 22 when the polygonal insert plug 22 is inserted into the engaging hole 12 so as to decrease resilient deformation of the polygonal insert plug 22, and a round-disc shaped head 31 which is disposed on and extends radially and outwardly from an end of the retaining shank 32 to abut against the polygonal insert plug 22 and be received in the larger-diameter wall portion 122 of the engaging hole 12. The retaining piece 3 further has a plurality of filling ribs 321 which are formed on the retaining shank 32 to be respectively filled in the splits 25 when the retaining shank 32 is inserted into the tubular hole 24. With the retaining shank 32 and the filling ribs 321 disposed to decrease the resilient deformation of the polygonal insert plug 22, the first plug block piece 2″ is retained firmly to the socket block piece 1′ for preventing rotation relative to and removal from the engaging hole 12. In this embodiment, the second plug block piece 2′ has the polygonal insert plug 22 retained to one socket block piece 1′, and the circular insert plug 23 rotatably engaged with the other socket block piece 1′. The retaining piece 3 can be also disposed to be inserted into the tubular hole 24 of the second plug block piece 2′ or the first plug block piece 2 in the first embodiment.

Referring to FIGS. 13 to 16, in a third embodiment, the toy building block set 100 is assembled as a toy vehicle, and includes a main vehicle frame 10, two turning mechanisms 20, a control unit 30, two rear wheels 40 and two motors 50.

The turning mechanisms 20 are disposed at two sides of a front end of the main vehicle frame 10. Each turning mechanism. 20 includes a fixed block piece 1″, two adaptor block pieces 4, two socket block pieces 1, four second plug block pieces 2′, a front wheel 201 and a rotating shaft 5.

The fixed block piece 1″ is fixed on the front end of the main vehicle frame 10, and has a wall body 11′ of a C-shaped configuration, and two engaging holes 12 formed in two terminal ends of the wall body 11′. Each adaptor block piece 4 has a curved wall body 41, an adaptor engaging hole 42 and an adaptor insert plug 43. The curved wall body 41 has first and second wall segments 411, 412 extending transverse to each other. The adaptor engaging hole 42 is formed in the first wall segment 411 and configured to be engaged with a polygonal insert plug 22 of the second plug block piece 2′ (or a polygonal insert plug 22 of the first plug block piece 2 in the previous embodiment). The adaptor insert plug 43 projects from the second wall segment 412 and is configured to be engaged in an engaging hole 12 of the socket block piece 1. Specifically, the adaptor engaging hole 42 has a cross-section of a regular polygonal shape to allow insertion and frictional engagement of the polygonal insert plug 22 of the second plug block piece 2′ (or the first plug block piece 2). The extending direction of the adaptor insert plug 43 is transverse to the inserting direction of the adaptor engaging hole 42. The adaptor insert plug 43 has a cross-section of a regular polygonal shape to be inserted into and frictionally engaged with the engaging hole 12 of the socket block piece 1. The adaptor insert plug 43 has a configuration similar to that of the polygonal insert plug 22. Alternatively, the extending direction of the adaptor insert plug 43 may be not transverse to the inserting direction of the adaptor engaging hole 42. Still alternatively, the adaptor insert plug 43 may have a cross-section of a circular shape and may be similar to the circular insert plug 23 in configuration. The adaptor engaging holes 42 of the adaptor block pieces 4 are disposed inboard of the C-shaped wall body 11′ of the fixed block piece 1″ and respectively aligned with the engaging holes 12 of the fixed block piece 1″. Two of the second plug block pieces 2′ are disposed to interconnect each adaptor block piece and the fixed block piece 1″. Specifically, the polygonal and circular insert plugs 22, 23 of the second plug block piece 2′ are respectively engaged in the aligned adaptor engaging hole 42 and the engaging hole 12 so as to allow rotation of the adaptor block piece 4 relative to the fixed block piece 1″. One of the socket block pieces 1 erects and has upper and lower engaging holes 12 respectively engaged with the adaptor insert plugs 43 of the adaptor block pieces 4, and a middle engaging hole 12 aligned with an axial hole (201a) in the front wheel 201. One of the second plug block pieces 2′ is disposed to interconnect the socket block piece 1 and the front wheel 201 to have the polygonal insert plug 22 engaged with the middle engaging hole 12 of the socket block piece 1, and the circular insert plug 23 engaged with the axial hole (201a) of the front wheel 201 to allow rotation of the front wheel 201 relative to the socket block piece 1 for rolling on a support surface (not shown). The other one of the socket block pieces 1 is placed horizontally above the fixed block piece 1″, and has an engaging hole 12 aligned with the engaging hole 12 of the fixed block piece 1″. The polygonal insert plug 22 of one second plug block piece 2′ is engaged in the engaging hole 12 of the socket block piece 1. The rotating shaft 5 has a cross-section of a polygonal shape, such as a regular hexagonal shape, and is made from a copper material. The rotating shaft 5 is configured to be inserted into the tubular holes 24 of the three aligned second plug block pieces 2′ so as to operatively drive rotation of the second plug block pieces 2′ with the rotating shaft 5 about the rotating axis. Thus, when the horizontal socket block piece 1 above the fixed block piece 1″ is rotated relative to the rotating shaft 5, the adaptor block pieces 4 are driven to rotate about and relative to the rotating shaft 5 so as to turn the front wheel 201 and the upright socket block piece 1 about the rotating shaft 5 to perform turning of the front wheel 201.

The control unit 30 has an elongated socket block piece 1′ and two second plug block pieces 2′. The socket wall body 11 of the elongated socket block piece 1′ is in the form of an elongated plate which extends in an elongated direction and is longer than the socket block piece 1 of the first embodiment, and has a plurality of the engaging holes 12 which are spaced apart from each other along the elongated direction. Two of the engaging holes 12 at two ends thereof are respectively aligned with the rear engaging holes 12 of the horizontal socket block pieces 1 of the turning mechanisms 20. The polygonal insert plugs 22 of the two second plug block pieces 2′ are respectively engaged in the rear engaging holes 12 of the horizontal socket block pieces 1 of the turning mechanisms 20, and the circular insert plugs 23 thereof are respectively engaged in the end engaging holes 12 of the elongated socket block piece 1′ so as to allow rotation of the elongated socket block piece 1′″ relative to the horizontal socket block pieces 1 of the turning mechanisms 20. Thus, as shown in FIGS. 15 and 16, when the elongated socket block piece 1″ is actuated to move transversely, the horizontal socket block pieces 1 of the turning mechanisms 20 are rotated relative to the rotating shafts 5 so as to turn the front wheels 201. For example, the control unit 30 may have an actuating unit (not shown) to actuate the movement of the elongated socket block piece 1 so as to control the turning of the front wheels 201.

The rear wheels 40 are disposed at two sides of a rear end of the main vehicle frame 10. The motors 50 are disposed on the main vehicle frame 10 to respectively drive the rear wheels 40 to move forwards the toy vehicle.

As illustrated, with the engaging holes 12 of a regular polygonal shape and the polygonal insert plugs 22 configured to be inserted into the engaging holes 12, rotation of the plug block pieces relative to the socket block pieces is allowed and can keep a predetermined angle of the plug block pieces relative to the socket block pieces for playing and using the toy building block set with more variety.

In the description above, for the purposes of explanation, numerous specific details have been set forth in order to provide a thorough understanding of the embodiments. It will be apparent, however, to one skilled in the art, that one or more other embodiments may be practiced without some of these specific details. It should also be appreciated that reference throughout this specification to “one embodiment,” “an embodiment,” an embodiment with an indication of an ordinal number and so forth means that a particular feature, structure, or characteristic may be included in the practice of the disclosure. It should be further appreciated that in the description, various features are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of various inventive aspects.

While the disclosure has been described in connection with what are considered the exemplary embodiments, it is understood that this disclosure is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.

Claims

1. A toy building block set comprising: a socket block piece having a socket wall body and at least one engaging hole which is formed in said socket wall body and bordered by an inner peripheral wall that surrounds a rotating axis normal to said socket wall body, said inner peripheral wall having a cross-section which is of a regular polygonal shape; andat least one of first and second plug block pieces having a plug body and at least one polygonal insert plug which extends from said plug body, said polygonal insert plug having an outer peripheral wall with a cross-section which is of a regular polygonal shape to be inserted into said engaging hole in an extending direction parallel to the rotating axis and in frictional engagement with said inner peripheral wall with a first frictional force that is dimensioned to permit rotation of said plug block piece relative to said socket block piece about the rotating axis by a forcible torque and to keep a predetermined angle of said plug block piece relative to said socket block piece when the forcible torque is removed, said plug body being partially inserted into said engaging hole and in frictional engagement with said inner peripheral wall with a second frictional force which is larger than the first frictional force.

2. The toy building block set as claimed in claim 1, wherein said engaging hole penetrates through said socket wall body.

3. The toy building block set as claimed in claim 2, wherein said first plug block piece has two of said polygonal insert plugs which extend from said plug body and away from each other.

4. The toy building block set as claimed in claim 2, wherein said second plug block piece has a circular insert plug which extends from said plug body and away from said polygonal insert plug, said circular insert plug having an outer peripheral wall with a cross-section which is of a circular shape to be inserted into said engaging hole in the extending direction and in frictional engagement with said inner peripheral wall with a third frictional force that is dimensioned to permit rotation of said second plug block piece relative to said socket block piece about the rotating axis by a forcible torque and to keep any angle of said second plug block piece relative to said socket block piece when the forcible torque is removed, and that is smaller than the second frictional force.

5. The toy building block set as claimed in claim 2, wherein said at least one of said first and second plug block pieces has a tubular hole which extends through said plug body and said polygonal insert plug in the extending direction.

6. The toy building block set as claimed in claim 5, wherein said at least one of said first and second plug block pieces has a plurality of splits which are formed in and extend through said outer peripheral wall to be in communication with said tubular hole and are angularly displaced from each other so as to permit resilient engagement of said outer peripheral wall with said inner peripheral wall of said socket block piece.

7. The toy building block set as claimed in claim 6, further comprising a retaining piece which has a retaining shank that extends to be inserted into said tubular hole in the extending direction to decrease resilient deformation of said polygonal insert plug, and a head that is disposed on and extends radially and outwardly from an end of said retaining shank to abut against said polygonal insert plug.

8. The toy building block set as claimed in claim 7, wherein said retaining piece has a plurality of filling ribs which are formed on said retaining shank to be respectively filled in said splits when said retaining shank is inserted into said tubular hole.

9. The toy building block set as claimed in claim 5, further comprising a rotating shaft which is configured to be inserted into said tubular hole of said at least one of said first and second plug block pieces so as to operatively drive rotation of said at least one of said first and second plug block pieces relative to said socket block piece about the rotating axis.

10. The toy building block set as claimed in claim 9, wherein each of said tubular hole and said rotating shaft has a cross-section of a polygonal shape.

11. The toy building block set as claimed in claim 1, further comprising an adaptor block piece, said adaptor block piece having a curved wall body, at least one adaptor engaging hole which is formed in said curved wall body and which has a cross-section of a regular polygonal shape to allow insertion and frictional engagement of said polygonal insert plug of said at least one of said first and second plug block pieces, and at least one adaptor insert plug which projects from said curved wall body and which has a cross-section of a regular polygonal or circular shape to be inserted into and frictionally engaged with said engaging hole of said socket block piece.

12. The toy building block set as claimed in claim 11, wherein extending directions of said adaptor insert plug and said adaptor engaging hole are transverse to each other.

13. The toy building block set as claimed in claim 1, wherein said socket wall body of said socket block piece is in the form of an elongated plate which extends in an elongated direction, and has a plurality of said engaging holes which are spaced apart from each other along the elongated direction.

14. The toy building block set as claimed in claim 1, wherein said cross-section of said polygonal insert plug is octagonal.

15. The toy building block set as claimed in claim 2, wherein said inner peripheral wall of said engaging hole has a middle wall portion extending in a direction of the rotating axis, two larger-diameter wall portions respectively extending from two ends of said middle wall portion, and two shoulder portions each interposed between said middle wall portion and a respective one of said larger-diameter wall portions, said polygonal insert plug of said at least one of said first and second plug block pieces having a plurality of flange sections formed at a terminal end thereof such that, when said polygonal insert plug is inserted into said engaging hole, said flange sections are received within one of said larger-diameter wall portions and abut against a corresponding one of said shoulder portions, and said plug body is partially received within the other one of said larger-diameter wall portions and abuts against a corresponding one of said shoulder portions.

16. The toy building block set as claimed in claim 15, wherein said inner peripheral wall of said engaging hole has a plurality of slots formed in each of said larger-diameter wall portions and each extending in the direction of the rotating axis, said plug body of said at least one of said first and second plug block pieces having a plurality of keys which are formed thereon and angularly displaced from each other so as to be engaged with said slots, respectively.