Abstract:
Provided is a jib connection structure capable of connecting a jib base end engagement part and a jib connection shaft even when a boom undergoes natural extension, and capable of restraining a jib from swinging sideways in a state where the jib is projected. This jib connection structure includes: jib connection shafts projecting horizontally toward both sides from a boom tip end part; and jib base end engagement parts respectively provided to bifurcated jib base end parts. Each jib base end engagement part is U-shaped into which the jib connection shaft can be fitted, and is provided with an insertion hole into which a pin for preventing the jib connection shaft from slipping out is inserted. When the jib base end engagement part and jib connection shaft are connected, a gap is formed between the pin and jib connection shaft. When a jib is projected, the gap becomes smaller.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to jib connection structures, and more specifically, to a connection structure between a boom leading end and a jib base end. 
       BACKGROUND ART 
       [0002]    Patent Literature (hereinafter, referred to as “PTL”) 1 discloses a jib connection structure composed of a jib connection shaft projecting in both lateral directions of a boom leading end and jib base end engagement portions provided at a bifurcated jib base end. The jib base end engagement portions are U-shaped and can be engaged with the jib connection shaft. The jib base end engagement portions can be connected to the jib connection shaft by engaging the jib base end engagement portions with the jib connection shaft and inserting pins into through holes in leading ends of the jib base end engagement portions. 
         [0003]    For a jib projection operation, a boom is firstly slightly extended, the jib is then moved to a lower holding position of the boom, and the boom is fully retracted. The jib connection shaft is thereby fitted into the jib base end engagement portions. Subsequently, pins are inserted into through holes in the jib base end engagement portions to connect the jib base end engagement portions to the jib connection shaft. Subsequently, the boom is elevated up, so that the jib is suspended from the boom leading end. Lastly, the jib is projected, by exerting tension on a tension rod. 
         [0004]    In the meantime, a wire telescoping mechanism is known as a telescoping mechanism of a boom (see, for example, PTL 2). When the wire telescoping mechanism is adopted, simultaneous extension or retraction of a multi-section boom is possible by means of telescopic motion of one hydraulic cylinder. In the wire telescoping mechanism, when for some reasons the balance of the tension between each wire for extension and retraction of the boom is disturbed, the so-called natural extension may sometimes occur in which the boom is not fully retracted despite that the hydraulic cylinder is fully retracted, or in which the boom unexpectedly extends after having been retracted. When the boom naturally extends, there is a problem that a pin interferes with a jib connection shaft and cannot be inserted, so that jib base end engagement portions cannot be connected to the jib connection shaft. 
         [0005]    Against this problem, it may be considered to allow clearance between the pin and the jib connection shaft in anticipation of natural extension of the boom. However, an operation in a state where a jib is projected may sometimes cause force acting in a lateral direction on the jib. In this case, there is a problem that the jib swings sideways when the clearance between the pin and the jib connection shaft is too large. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         PTL 1: Japanese Patent Application Laid-Open No. 2006-264956 
         PTL 2: Japanese Patent Application Laid-Open No. 8-127494 
       
     
       SUMMARY OF THE INVENTION 
     Problems to Be Solved by the Invention 
       [0008]    it is therefore an object of the present invention to provide a jib connection structure capable of connecting jib base end engagement portions to a jib connection shaft even when a boom naturally extends, and restraining a jib from swinging sideways in a state where the jib is projected. 
       Solutions to Problems 
       [0009]    A jib connection structure of a first aspect of the present invention is a connection structure between a leading end of a boom and a base end of a jib, the connection structure including a jib connection shaft horizontally projecting in both lateral directions of the leading end of the boom; and jib base end engagement portions provided at respective ends of the bifurcated base end of the jib, in which: the jib base end engagement portions are U-shaped into which the jib connection shaft is allowed to be fitted, and include insertion holes for insertion of pins, the pins being intended for preventing slipping off of the jib connection shaft; the insertion holes in one of the jib base end engagement portions is placed laterally outwardly from a U-shaped bottom portion; and the laterally outwardly placed insertion holes are located at a position at which a gap is formed between the inserted pin and the jib connection shaft in a state where the jib connection shaft is fitted into the jib base end engagement portions and a leading end of the jib is placed laterally from the boom, and at which the gap between the inserted pin and the jib connection shaft becomes narrower in a state where the jib is projected. 
         [0010]    A jib connection structure of a second aspect pf the present invention is a connection structure between a leading end of a boom and a base end of a jib, the connection structure including a jib connection shaft horizontally projecting in both lateral directions of the leading end of the boom; and jib base end engagement portions provided at respective ends of the bifurcated base end of the jib, in which: the jib base end engagement portions are U-shaped into which the jib connection shaft is allowed to be fitted, and include insertion holes for insertion of pins, the pins being intended for preventing slipping off of the jib connection shaft; and the insertion holes in one of the jib base end engagement portions are placed laterally outwardly from a U-shaped bottom portion, and a distance between said insertion holes and said bottom portion in a direction along a central axis of the jib is substantially the same as an outer diameter of the jib connection shaft. 
       Effects of the Invention 
       [0011]    According to the first and/or the second aspect(s) of the invention, when the jib base end engagement portions are connected to the jib connection shaft, the gap is formed between the pin and the jib connection shaft; therefore, even when the boom naturally extends, the jib base end engagement portions can be connected to the jib connection shaft. Further, when the jib is projected, the gap between the pin and the jib connection shaft becomes narrower. Therefore, the jib can be restrained from swinging sideways. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1  is a side view illustrating mobile crane C in which jib  15  is in a stowed condition; 
           [0013]      FIG. 2  is a side view of jib  15  and boom  14 , in which jib  15  is placed in a lower holding position of boom  14 ; 
           [0014]      FIG. 3  is a bottom view of jib  15  and boom  14 , in which jib  15  is placed in the lower holding position of boom  14 ; 
           [0015]      FIG. 4  is an enlarged view of region IV in  FIG. 3 ; 
           [0016]      FIG. 5  is an enlarged view as seen in the direction of arrow V in  FIG. 4 ; 
           [0017]      FIG. 6  is an enlarged view as seen in the direction of arrow VI in  FIG. 4 ; 
           [0018]      FIG. 7  is a bottom view of jib  15  and boom  14 , in which jib  15  is projected; 
           [0019]      FIG. 8  is a side view illustrating a state where jib  15  is suspended from boom leading end  14   a ; 
           [0020]      FIG. 9  is a side view illustrating a state where jib  15  is projected. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0021]    In the following, an embodiment of the present invention will be described with reference to the drawings. 
         [0022]    A jib connection structure according to one embodiment of the invention is applied, for example, to mobile crane C as illustrated in  FIG. 1 . In the meantime, the jib connection structure according to the embodiment can be applied, but not limited, to mobile crane C as illustrated in  FIG. 1  and can be applied to different cranes. 
       Mobile Crane 
       [0023]    To begin with, the basic structure of mobile crane C will be described. 
         [0024]    Reference numeral  11  in  FIG. 1  indicates a traveling chassis provided with wheels for traveling. Slewing base  12  is mounted on chassis  11  in such a manner as to be able to slew 360 degrees in the horizontal plane by a slewing motor. Slewing base  12  is provided with operator&#39;s cab  13 . 
         [0025]    Boom  14  is attached to slewing base  12  such that boom  14  can be freely elevated and lowered. A base end of boom  14  is pivotally supported by a pin on slewing base  12 . A boom elevating cylinder is attached between boom  14  and slewing base  12 . Boom  14  is elevated when the boom elevating cylinder is extended, and lowered when the boom elevating cylinder is retracted. 
         [0026]    Boom  14  is a multi-section boom configured to be telescopic, and extends and retracts by a telescopic cylinder. A telescoping mechanism of boom  14  is a wire telescoping mechanism. The wire telescoping mechanism is a mechanism which is composed of a telescopic cylinder and wires for extension and retraction of a boom, and is configured to transmit telescopic motion of the telescopic cylinder to each tubular section of boom  14  by means of the wires for extension and retraction of the boom. The telescopic motion of the hydraulic cylinder allows simultaneous extension or retraction of the multi-section boom. In the wire telescoping mechanism, when for some reasons the balance of the tension between each wire for extension and retraction of the boom is disturbed, the so-called natural extension may sometimes occur in which the boom is not fully retracted despite that the hydraulic cylinder is fully retracted, or in which the boom unexpectedly extends after having been retracted. 
         [0027]    A wire rope provided with a hook (not illustrated) is suspended from leading end  14   a  of boom  14 , and is guided along boom  14  to slewing base  12  and wound around a winch. The winch is driven to rotate forward and reverse by a hoist motor, to wind up and pay out the wire rope, thereby allowing the hook to be moved up and down. 
         [0028]    Combination of slewing of slewing base  12 ; elevating, lowering, extending, and retracting of boom  14 ; and moving up and down of the hook allows loading and unloading in a three-dimensional space. 
         [0029]    Further, mobile crane C is provided with jib  15 . Jib  15  as a whole is an elongated bar member, base end  15   a  of which is bifurcated. Jib  15  is employed for achieving a lifting height and/or operating radius still greater than a lifting height and/or operating radius of the boom length of fully extended boom  14 . When jib  15  is not used, jib  15  is stowed along a side of boom  14  (see  FIG. 1 ). When jib  15  is used, base end  15   a  of jib  15  is connected to leading end  14   a  of boom  14 , and jib  15  is projected forward from boom  14  (see  FIG. 9 ). 
       Jib Connection Structure 
       [0030]    In the following, the jib connection structure of the embodiment will be described. 
         [0031]    The jib connection structure of the embodiment is a structure for connecting leading end  14   a  of boom  14  (hereinafter referred to as “boom leading end  14   a  ”) to base end  15   a  of jib  15  (hereinafter referred to as “jib base end  15   a  ”) in mobile crane C as described above. 
         [0032]      FIGS. 2 and 3  are side and bottom views illustrating a state where jib  15  is placed in a lower holding position in which jib  15  is placed along the bottom side of boom  14 . As will be described below, in a jib projection/stowage operation, boom leading end  14   a  is connected to/disconnected from jib base end  15   a  in the state where jib  15  is placed in the lower holding position. 
         [0033]    As illustrated in  FIG. 3 , in the state where jib  15  is placed in the lower holding position, jib  15  is in an offset placement in which jib base end  15   a  is located at boom leading end  14   a , and a leading end of jib  15  is located laterally from boom  14 . In this respect, the leading end of jib  15  is located on the opposite side of operator&#39;s cab  13  with respect to boom  14 . Hereinafter, in the offset placement, the side on which the leading end of jib  15  is located is referred to as the left side, and the opposite side thereto (the side of operator&#39;s cab  13 ) is referred to as the right side. However, an embodiment in which the right and left sides are reversed is possible. 
         [0034]    As illustrated in  FIG. 4 , boom leading end  14   a  is provided with jib connection shafts  21 ,  22  horizontally projecting in both lateral directions of boom leading end  14   a . Right and left jib connection shafts  21 ,  22  are coaxially placed. Further, respective ends of bifurcated jib base end  15   a  are provided with jib base end engagement portions  31 ,  32 . 
         [0035]    As illustrated in  FIG. 5 , right jib base end engagement portion  31  is formed to have a U-shape composed of a pair of arms  31   a  and bottom portion  31   b  connecting base parts of the arms. The U-shape of jib base end engagement portion  31  has an inner diameter slightly, greater than an outer diameter of right jib connection shaft  21 . As a result, jib connection shaft  21  can be fitted between the pair of arms  31   a . In the meantime, bottom portion  31   b  is a portion in which the inner surface of bottom portion  31   b , in a state where jib  15  is projected, makes contact with jib connection shaft  21  so as to transmit force acting on jib  15  to jib connection shaft  21 . 
         [0036]    Insertion holes  31   h  are formed in leading ends of the pair of arms  31   a . Jib connection shaft  21  is prevented from slipping off by fitting jib connection shaft  21  into jib base end engagement portion  31  and inserting pin  33  into insertion holes  31   h . Jib base end engagement portion  31  can thus be connected to jib connection shaft  21 . 
         [0037]    As illustrated in  FIG. 6 , left jib base end engagement portion  32  is formed to have a U-shape composed of a pair of arms  32   a  and bottom portion  32   b  connecting base parts of the arms. The U-shape of jib base. end engagement portion  32  has an inner diameter slightly greater than an outer diameter of left jib connection shaft  22 . Jib connection shaft  22  can thus be fitted between the pair of arms  32   a . In the meantime, bottom portion  32   b  is a portion in which the inner surface of bottom portion  32   b , in a state where jib  15  is projected, makes contact with jib connection shaft  22  so as to transmit farce acting on jib  15  to jib connection shaft  22 . 
         [0038]    Sides of arms  32   a  are each provided with extension plates  32   c , with extension plates  32   c  projecting outwardly. These extension plates  32   c  are provided with insertion holes  32   h . Jib connection shaft  22  is prevented from slipping off by fitting jib connection shaft  22  into jib base end engagement portion  32  and inserting pin  34  into insertion holes  32   h . Jib base end engagement portion  32  can thus be connected to jib connection shaft  22 . 
         [0039]    As illustrated in  FIG. 4 , left jib base end engagement portion  32  is provided with extension plates  32   c , and extension plates  32   c  are provided with insertion holes  32   h . Insertion holes  32   h  are thus placed laterally outwardly from bottom portion  32   b  on which a load is applied in a state where jib  15  is projected. In a state where jib connection shafts  21 ,  22  are fitted into jib base end engagement portions  31 ,  32  and the leading end of jib  15  is placed laterally from boom  14 , gap d 1  is formed between pin  34  inserted into insertion holes  32   h  and jib connection shaft  22 . 
         [0040]    In the meantime, jib  15  is supported by a base end boom and jib connection shafts  21 ,  22  are fixed to a leading end boom. Accordingly, when boom  14  naturally extends, the position of jib  15  remains unchanged and jib connection shafts  21 ,  22  move in the direction of the boom leading end. As a result, jib connection shaft  22  moves toward pin  34 . Normally, when boom  14  naturally extends in a case where there is no gap between pin  34  and jib connection shaft  22 , pin  34  interferes with jib connection shaft  22  and cannot be inserted into insertion holes  32   h . Jib base end engagement portion  32  thus cannot be connected to jib connection shaft  22 . 
         [0041]    In contrast, in the embodiment, when jib base end engagement portion  32  is connected to jib connection shaft  22 , gap d 1  is formed between pin  34  and jib connection shaft  22 . Accordingly, when it is within the range of gap d 1  that boom  14  naturally extends, pin  34  can be inserted into insertion holes  32   h  without interfering with jib connection shaft  22  and jib base end engagement portion  32  can be connected to jib connection shaft  22 . 
         [0042]      FIG. 7  is a bottom view illustrating a state where jib  15  is projected. When jib  15  is projected, jib  15  that has been in the offset placement rotationally moves in a lateral direction to be in substantially straight alignment with boom  14 . That is, central axis O of jib  15  becomes parallel to a central axis of boom  14  and vertical with respect to jib connection shafts  21 ,  22   
         [0043]    As illustrated in  FIG. 7 , in left jib base end engagement portion  32 , distance L between insertion holes  32   h  and bottom portion  32   b  in a direction along central axis O of jib  15  is substantially the same as the outer diameter of jib connection shaft  22 . The phrase “substantially the same” here means not only that distance L is the same as the outer diameter of jib connection shaft  22 , but also that distance L is slightly greater than the outer diameter of jib connection shaft  22 . 
         [0044]    In a state where jib  15  is projected, gap d 2  between pin  34  inserted into insertion holes  32   h  and jib connection shaft  22  becomes narrower than in the case of the offset placement (d 1 ). This is because, when jib  15  is projected, jib  15  that has been in the offset placement rotationally moves in the counterclockwise direction in  FIG. 7 , so that pin  34  approaches jib connection shaft  22 . 
         [0045]    An operation in a state where jib  15  is projected, for example slewing with a load being suspended causes force acting on jib  15  in a lateral direction. In this case, when clearances between pins  33 ,  34  and jib connection shafts  21 ,  22  are too large, jib  15  swings sideways. 
         [0046]    In contrast, in the embodiment, insertion holes  32   h  are placed in the position as described above; therefore, pin  34  is in contact with jib connection shaft  22  or there is a small clearance between pin  34  and jib connection shaft  22 . That is, jib connection shaft  22  is brought into a state where jib connection shaft  22  is held between bottom portion  32   b  of jib base end engagement portion  32  and pin  34  and is supported from the front and rear. As a result, rotational movement of jib  15  in the counterclockwise direction in  FIG. 7  is restrained. 
         [0047]    As described above, when jib  15  is projected, the gap between pin  34  and jib connection shaft  22  becomes narrower; therefore, the jib can be restrained from swinging sideways. In the meantime, rotational movement of jib  15  in the clockwise direction may be restrained by means of other mechanisms provided on jib connection shaft  21  and jib base end engagement portion  31  on the right side. 
       Jib Projection Operation 
       [0048]    In the following, a jib projection operation be described. 
         [0049]    (1) As illustrated in  FIG. 1 , in a state where jib  15  is in a stowed position, jib  15  is stowed along a side of boom  14 . 
         [0050]    (2) Firstly, boom  14  is slightly extended. Subsequently, jib  15  is moved to the lower holding position in which jib  15  is placed along the bottom of boom  14 . Jib  15  is supported by first jib supporting member  16  and second jib supporting member  17 . First jib supporting member  16  is provided with hydraulic cylinder  16   a . Jib  15  can be turned to the lower holding position by extending hydraulic cylinder  16   a . Boom  14  is then fully retracted. Accordingly, the state as illustrated in  FIGS. 2 and 3  is achieved. In this operation, jib connection shafts  21 ,  22  are fitted into jib base end engagement portions  31 ,  32 , as illustrated in  FIG. 4 . 
         [0051]    (3) Pins  33 ,  34  are then inserted into insertion holes  31   h ,  32   h . Jib base end engagement portions  31 ,  32  are thus connected to jib connection shafts  21 ,  22 . Tension rod  15   b  provided on jib  15  is also connected. 
         [0052]    At this stage, jib  15  is in the offset placement in which jib base end  15   a  is located at boom leading end  14   a , and the leading end of jib  15  is located laterally from boom  14 . As described above, gap d 1  is formed between pin  34  and jib connection shaft  22 . Accordingly, even when boom  14  naturally extends and is not fully retracted by the aforementioned operation of fully retracting the boom, pin  34  can be inserted into insertion holes  32   h  without interfering with jib connection shaft  22 . That is, jib base end engagement portion  32  can be connected to jib connection shaft  22 . 
         [0053]    (4) Boom  14  is then elevated. Subsequently, when boom  14  is slightly extended, first jib supporting member  16  and second jib supporting member  17  are disconnected. Then, as illustrated in  FIG. 8 , jib  15  rotates about jib connection shafts  21 ,  22  to become suspended from boom leading end  14   a.    
         [0054]    (5) Tilt cylinder  15   c  is mounted on jib  15 . A rod of tilt cylinder  15   c  is connected to tension rod  15   b . When tilt cylinder  15   c  is extended, tension can be generated on tension rod  15   b , and jib  15  can be projected forward from jib connection shafts  21 ,  22 .  FIG. 9  illustrates a state where tilt cylinder  15   c  is fully extended, in which jib  15  is in substantially straight alignment with boom  14 . 
         [0055]    When jib  15  is projected, bottom portions  31   b ,  32   b  of jib base end engagement portions  31 ,  32  are brought into contact with jib connection shafts  21 ,  22 . Accordingly, force acting on jib  15  is transmitted from bottom portions  31   b ,  32   b  of jib base end engagement portions  31 ,  32  to jib connection shafts  21 ,  22 . 
         [0056]    As illustrated in  FIG. 7 , in a state where jib  15  is projected, gap d 2  between pin  34  inserted into insertion holes  32   h  and jib connection shaft  22  becomes narrower than in the case of the offset placement (d 1 ). Jib connection shaft  22  is brought into a state where jib connection shaft  22  is held between bottom portion  32   b  of jib base end engagement portion  32  and pin  34  and is supported from the front and rear. Therefore, jib  15  can be restrained from swinging sideways. 
       Jib Stowage Operation 
       [0057]    A jib stowage operation is carried out in a procedure reverse to the jib projection operation. In the jib stowage operation, pins  33 ,  34  are extracted from insertion holes  31   h ,  32   h  in a state where jib  15  is in the lower holding position and in the offset placement. Also in this case, because there is gap d 1  formed between pin  34  and jib connection shaft  22 , no frictional force acts between pin  34  and jib connection shaft  22 , and pin  34  can be easily extracted. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           14  Boom 
           14   a  Boom leading end 
           15  Jib 
           15   a  Jib base end 
           21 ,  22  Jib connection shaft 
           31 ,  32  Jib base end engagement portion 
           32   c  Extension plate 
           31   h ,  32   h  Insertion hole 
           33 ,  34  Pin