Telescopic boom

Abstract

 A telescopic boom (1 to 5) of the present invention comprises a base boom (1) and inserted booms (2 to 5) telescopically inserted into this base boom (1). A cross-section of each of the inserted booms (2 to 5) cut in a direction orthogonal to a longitudinal direction thereof is formed by a lower half portion on the side of a ventral surface and an upper half portion on the side of a rear surface. The lower half portion is formed by an arc part along a virtual arc (22) of a semicircle or less having an opening width Wr which is smaller than an outer width Wo of the inserted booms (2 to 5), and a pair of left and right inclined flat plate portions (24) continuous to this arc part. Left and right ends of the upper half portion and left and right ends of the lower half portion are coupled to each other by the inclined flat plate portions (24) formed in the lower half portion.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to a telescopic boom mounted on a rotatable upper rotating body of a wheeled crane or the like so as to be raised and lowered. More specifically, the present invention relates to a telescopic boom having light weight but high buckling strength, and being excellent in the manufacture property of the boom and a pad, in which shim adjustment is easily performed at the time of assembling.

2. Description of Related Art

[0002] A telescopic boom mounted on a rotatable upper rotating body of a wheeled crane or the like so as to be raised and lowered is desirably formed to be as small as possible for weight and size reduction. Preferably, the telescopic boom can be manufactured in a simple manufacturing process.

[0003] As the first conventional technique of such a telescopic boom, there is a technique described in Japanese Patent Laid-Open No. Hei9-501384 for example. In this conventional technique, an upper half portion on the side of a rear surface is formed into a half-box shape and a lower half portion on the side of a ventral surface is formed into a smooth-arc shape in a cross-sectional view cut in the direction orthogonal to the longitudinal direction of the boom. This conventional telescopic boom is excellent in buckling strength in comparison to a telescopic boom formed by building four steel plates into a rectangular shape.

[0004] Next, as the second conventional technique of the telescopic boom, there is a known technique described in European Patent No. 0499208B2 for example. In this conventional technique, an upper half portion on the side of a rear surface is formed into a half-box shape and a lower half portion on the side of a ventral surface is formed into a smooth half-oval shape in a cross-sectional view. In this conventional telescopic boom, in the case where a ratio between the lateral width length and the vertical width length in the cross-section and a ratio of plate thickness are the same, a rate of the width length of parallel side plates in the half-box portion of the upper half portion on the side of the rear surface is smaller than a rate in the telescopic boom according to the first conventional technique. Therefore, there is an advantage that the parallel side plates are not easily buckled.

[0005] The telescopic booms according to the above two conventional techniques are useful to some extent. However, in the case of the telescopic boom according to the first conventional technique, the width length of parallel side plates in the half-box portion of the upper half portion on the side of the rear surface is larger than the width length in the telescopic boom according to the second conventional technique. Therefore, the parallel side plates are easily buckled and there is a problem that the buckling strength for the parallel side plates in the half-box portion of the upper half portion is weaker than the buckling strength in the telescopic boom according to the second conventional technique.

[0006] Meanwhile, the telescopic boom according to the second conventional technique is excellent in the buckling strength for the parallel side plates in the half-box portion of the upper half portion in comparison to the telescopic boom according to the first conventional technique. However, since the curvature radius of a lowest end of the oval is smaller than the curvature radius of the telescopic boom according to the first conventional technique, there is a problem that this part is easily buckled. Since the lower half portion should be formed into an oval shape, there is also a problem that the manufacturing cost for the boom is increased. In addition, since a pad for supporting a telescopic inserted boom is not easily manufactured, there is a problem that the cost for the pad is also increased. Further, there is also a problem that a gap between the booms is not easily adjusted with using a shim at the time of assembling the boom.

SUMMARY OF THE INVENTION

[0007] It is an object of the present invention to provide a telescopic boom having light weight but high buckling strength, and being excellent in the manufacture property of a boom and a pad, in which a gap between the booms is easily adjusted at the time of assembling the boom.

[0008] A telescopic boom of the present invention comprises a base boom, and inserted booms telescopically inserted into this base boom. A cross-section of at least one inserted boom cut in a direction orthogonal to a longitudinal direction thereof is formed by a lower half portion on the side of a ventral surface and an upper half portion on the side of a rear surface. This lower half portion is formed by an arc part along a virtual arc of a semicircle or less having an opening width which is smaller than an outer width of the inserted boom, and a pair of left and right inclined flat plate portions continuous to this arc part. Left and right ends of the upper half portion and left and right ends of the lower half portion are coupled to each other by the inclined flat plate portions formed in the lower half portion.

[0009] The telescopic boom of the present invention has light weight but high buckling strength, and is excellent in the manufacture property of the boom and the pad and capable of closely attaching the inclined flat plate portions to the pad by shim adjustment. Therefore, an assembling work for the boom is easily performed. Since the gap between the booms can be precisely adjusted, it is possible to suppress eccentricity during extending and stowing the boom.

[0010] In a preferable mode of the telescopic boom of the present invention, the arc part may be an arc portion with a circumferential surface in contact with the virtual arc, and an outer width or an inner width of the arc part may correspond to the opening width of the virtual arc. The circumferential surface is in contact with either an inner part of the virtual arc or an outer part of the virtual arc. In the telescopic boom of this mode, a corner portion is not required due to the arc shape, the buckling strength is high.

[0011] In another preferable mode of the telescopic boom of the present invention, the arc part may be a polygonal portion. This polygonal portion is formed by a plurality of flat plate portions coupled by bent portions. Alternatively, in this polygonal portion, the bent portions or inner plane surfaces of the flat plate portions are in contact with the virtual arc, and the outer width or the inner width of the arc part corresponds to the opening width of the virtual arc. The bent portions or inner plane surfaces of the flat plate portions are in contact with either the inner part of the virtual arc or the outer part of the virtual arc. Since the telescopic boom of this mode is formed by the flat plate portions, the processing man-hour can be reduced and manufacture is easily performed in comparison to the telescopic boom formed into an arc shape. Alternatively, in the telescopic boom of this mode, support between the booms can be performed by closely attaching the pad in a planar shape to flat surfaces of the flat plate portions. Therefore, the pad is easily manufactured, and the gap between the booms is easily adjusted, and further, precision of the gap is high in comparison to the telescopic boom formed into an arc shape.

[0012] Further, in a preferable mode of the telescopic boom of the present invention, width lengths of a plurality of the flat plate portions forming the arc part may be equal to each other. Further, the width lengths of the flat plate portions, and width lengths of the inclined flat plate portions may be equal to each other. Furthermore, a plurality of the flat plate portions forming the arc part may be bent at an equal angle to each other. In such a mode, the same die can be used for processing so that the manufacture of the boom is more easily performed.

[0013] In another preferable mode of the telescopic boom of the present invention, outer surfaces of the ends of the inclined flat plate portions of the lower half portion may form outward steps from outer surfaces of the left and right ends of the upper half portion in the coupled condition. Such a mode is excellent in the welding workability so that the strength and the reliability of the boom are high.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] 

Fig. 1 is a side view of a wheeled crane in which a telescopic boom according to the first embodiment of the present invention is installed;

Fig. 2 is an explanatory view of a cross-sectional shape of a first intermediate boom forming a part of the telescopic boom according to the first embodiment of the present invention;

Fig. 3 is an explanatory view of a cross-sectional shape of a first intermediate boom forming a part of a telescopic boom according to the second and third embodiments of the present invention; and

Figs. 4A to 4H are explanatory views of a configuration of an upper half portion on the side of a rear surface in a cross-section cut in the direction orthogonal to the longitudinal direction of the boom.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Hereinafter, telescopic booms according to embodiments of the present invention will be described. Firstly, a telescopic boom according to the first embodiment will be described with reference to Figs. 1 and 2.

[0016] The reference numeral 11 shown in Fig. 1 denotes a wheeled crane in which a telescopic boom B described later is installed. This wheeled crane of the first embodiment is a wheeled crane. This wheeled crane 11 is provided with a traveling platform car 12 in which a front wheel 12a is arranged on the front side in the traveling direction which is a right part of the figure and a rear wheel 12b is arranged on the rear side in the traveling direction which is a left part of the figure. An upper rotating body 13 freely rotated around a vertical axis center through a rotation bearing 12c is installed on this traveling platform car 12. An operator's cab 14 is arranged on the front side of this upper rotating body 13. The telescopic boom B raised and lowered by a telescopic action of a rod of a boom raising and lowering cylinder 15 is arranged on one side of this operator's cab 14.

[0017] A rope 16 is rolled up and drawn out along a rear surface of the telescopic boom B with a winch (not shown) housed inside of the upper rotating body 13. A hook 17 raised and lowered by this rope 16 hanging a load is suspended in a top end of the telescopic boom B. It should be noted that the reference numeral 13a denotes a counterweight.

[0018] The telescopic boom B according to the first embodiment of the present invention is a five-step type. That is, the telescopic boom B is formed by a base boom 1 supported by the upper rotating body 13 so as to be raised and lowered, a first intermediate boom 2 telescopically inserted into this base boom 1, a second intermediate boom 3 telescopically inserted into this first intermediate boom 2, a third intermediate boom 4 telescopically inserted into this second intermediate boom 3, and a top boom 5 telescopically inserted into this third intermediate boom 4. These first to third intermediate booms 2 to 4 and the top boom 5 are booms called as inserted booms. This telescopic boom of the first embodiment is the five-step type but may be a four-step type or less or a six-step type or more.

[0019] Fig. 2 shows a cross-sectional shape cut in a direction orthogonal to a longitudinal direction of the booms according to the first embodiment of the present invention. In the case of this first embodiment, the cross-sectional shape is all the same in the booms. Therefore, the first intermediate boom 2 is taken as an example here. That is, a half-box portion 21 opening on the lower side is formed in an upper half portion on the side of a rear surface in a cross-section cut in a direction orthogonal to a longitudinal direction of this first intermediate boom 2. Meanwhile, an arc portion 23 and inclined flat plate portions 24, 24 are continuously provided in a lower half portion on the side of a ventral surface in this cross-section. The arc portion 23 has the curvature radius R which is preliminarily set. An outer periphery surface of the arc portion 23 is in contact with an inner part of a virtual arc 22 of a semicircle or less having an opening width Wr which is smaller than an outer width in the left and right direction in the cross-section orthogonal to the longitudinal direction of the first intermediate boom 2, that is, the outer width Wo of the half-box portion 21. Further, the opening width of this virtual arc 22 corresponds to the outer width Wo of the half-box portion 21. Here, "to correspond" means not only the case of complete correspondence but also the case of substantial correspondence. Meanwhile, the inclined flat plate portions 24, 24 are coupled to ends of parallel side plates 21a of the half-box portion 21 with the opening side of the arc portion 23.

[0020] It should be noted that the curvature radius R of the virtual arc 22 is set so as to be a length (R = 1/2 · Wi + T₁) determined by adding a plate thickness T₁ of the half-box portion 21 of the upper half portion to a half of an inner width Wi of the half-box portion 21, or a length (R = 1/2 · Wi + T₂) determined by adding a plate thickness T₂ of the inclined flat plate portions 24 to a half of the inner width Wi of the half-box portion 21. A height length of the lower half portion including the inclined flat plate portions 24 and the arc portion 23 is set so as not to be longer than a height length of the half-box portion 21 of the upper half portion. In the case where the plate thickness T₂ of the inclined flat plate portions 24 is thicker than the plate thickness T₁ of the half-box portion 21, due to welding work performed between ends of the half-box portion 21 and the inclined flat plate portions 24, the curvature radius R of the virtual arc 22 is set to be (R = 1/2 · Wi + T₂), and it is preferable that outer edges of coupled side ends of the inclined flat plate portions 24 may form outward steps from outer surfaces of the side plates 21a of the half-box portion 21 as shown in Fig. 2.

[0021] Since the arc portion 23 is formed in the lower half portion on the side of the ventral surface, the first intermediate boom 2 of the telescopic boom B according to this first embodiment has light weight as well as the conventional technique. Alternatively, while maintaining high buckling strength, it is possible to reduce the processing man-hour of the boom in comparison to the case of the second conventional technique in which the lower half portion of the boom is formed into a half-oval. Further, since the inclined flat plate portions 24 and the arc portions 23 are only required, pads are easily manufactured. Furthermore, since the inclined flat plate portions 24 can be easily closely attached to the pad by shim adjustment, an assembling work for the boom is easily performed and it is possible to suppress eccentricity during extending and stowing this first intermediate boom 2.

[0022] Next, a telescopic boom according to the second embodiment of the present invention will be described with reference to Fig. 3. It should be noted that the second embodiment of the present invention is different from the first embodiment in terms of a shape of the lower half portion on the side of the ventral surface in the cross-section cut in the direction orthogonal to the longitudinal direction of the boom. Since configuration other than the above is the same, the configuration functioning as the same is given the same reference numerals for a description.

[0023] In the second embodiment of the present invention, the half-box portion 21 opening on the lower side is formed in the upper half portion on the side of the rear surface in the cross-section cut in the direction orthogonal to the longitudinal direction of the first intermediate boom 2. Meanwhile, a polygonal portion 25 having a plurality of flat plate portions 26 with width lengths which are preliminarily set is formed in the lower half portion on the side of the ventral surface in this cross-section. This polygonal portion 25 also has the curvature radius R which is preliminarily set. Convex parts of bent portions 27 of this polygonal portion 25 are in contact with the inner part of the virtual arc 22 of a semicircle or less having the opening width Wr which is smaller than the outer width Wo of the half-box portion 21 of this first intermediate boom 2. Further, the outer width of the arc portion 23 corresponds to the opening width Wr of the virtual arc 22. Here, "to correspond" means not only the case of the complete correspondence but also the case of the substantial correspondence. The inclined flat plate portions 24, 24 are coupled to the ends of the parallel side plates 21a of the half-box portion 21 of this first intermediate boom 2 with the opening side of the polygonal portion 25. The width of the flat plate portions 26 of the polygonal portion 25 is set to be smaller than the width of the inclined flat plate portions 24.

[0024] It should be noted that the curvature radius R of the virtual arc 22 is set so as to be the length (R = 1/2 · Wi + T₁) determined by adding the plate thickness T₁ of the half-box portion 21 of the upper half portion to a half of the inner width Wi of the half-box portion 21, or the length (R = 1/2 · Wi + T₂) determined by adding the plate thickness T₂ of the inclined flat plate portions 24 to a half of the inner width Wi of the half-box portion 21 as well as the case of the first embodiment. The height length of the lower half portion including the inclined flat plate portions 24 and the polygonal portion 25 is set so as not to be longer than the height length of the half-box portion 21 of the upper half portion.

[0025] Since the polygonal portion 25 having the flat plate portions 26 with the width lengths which are preliminarily set is formed in the lower half portion on the side of the ventral surface, the first intermediate boom 2 according to this second embodiment has light weight and it is possible to maintain high buckling strength. Since the polygonal portion 25 is formed by bending at the width lengths which are preliminarily set, all the polygonal portions 25 in the lower half portions of the booms can be processed with the same die. It should be noted that the width lengths of the flat plate portions 26 is not necessarily the same length, although the workability with the die is reduced.

[0026] Therefore, in the case of the second embodiment, it is possible to manufacture the boom with less processing man-hour in comparison to the case of the first embodiment. Further, since the flat plate portions 26 are only flat surfaces, the pads are more easily manufactured. Furthermore, the inclined flat plate portions 24 and the flat plate portions 26 can be more easily closely attached to the pad by shim adjustment. Therefore, the assembling work for the boom is easily performed and it is possible to suppress the eccentricity during extending and stowing this first intermediate boom 2.

[0027] A telescopic boom according to the third embodiment of the present invention will be described with reference to Fig. 3 which is used for the description of the second embodiment. It should be noted that the third embodiment of the present invention is different from the second embodiment in terms of the width of the flat plate portions of the polygonal portion and a bending angle of the polygonal portion, and configuration other than the above is the same.

[0028] The bent portions 27 of the polygonal portion 25 according to the third embodiment of the present invention are bent at an equal angle so as to be in contact with the inner part of the virtual arc 22. According to this third embodiment, since the polygonal portion 25 is formed by bending at the width length which is preliminarily set at an equal angle, all the polygonal portions 25 can be processed with the same die, the same pressing force and the same forced amount. Therefore, it is possible to more easily manufacture the boom in comparison to the case of the second embodiment of the present invention.

[0029] Next, a telescopic boom according to the fourth embodiment of the present invention will be described with regard to the cross-sectional shape cut in the direction orthogonal to the longitudinal direction of the boom. In the case of the first intermediate boom forming a part of the telescopic boom of the third embodiment, the width lengths of the flat plate portions of the polygonal portion are set to be smaller than the width length of the inclined flat plate portions. Meanwhile, in the case of the telescopic boom according to the fourth embodiment of the present invention, the width length of the inclined flat plate portions is also set to be the same as the width length of the flat plate portions of the polygonal portion having the equal width length bent at an equal angle.

[0030] In the telescopic boom according to the fourth embodiment of the present invention, the width length of not only the flat plate portions of the polygonal portion but also the inclined flat plate portions is set to be the same length. Therefore, all the inclined flat plate portions and the flat plate portions of the polygonal portion can be processed with the same die, the same pressing force and the same forced amount. Consequently, in the telescopic boom according to the fourth embodiment of the present invention, it is possible to process the boom with further less processing man-hour and to more inexpensively manufacture the telescopic boom in comparison to the case of the telescopic boom of the third embodiment of the present invention.

[0031] In the telescopic booms according to the above first to fourth embodiments, the description is given as an example to the case where the half-box portion 21 including the left and right parallel side plates 21a, and an upper flat plate portion continuous to the upper side of the left and right side plates 21a through left and right R surfaces having the predetermined bending radius is formed in the upper half portion on the side of the rear surface in the cross-section cut in the direction orthogonal to the longitudinal direction.

[0032] However, the configuration of the upper half portion of the telescopic boom is not limited to the shape of the half-box portion 21 mentioned above, but various configurations of the upper half portion described later can be adopted. Hereinafter, various specific examples of the configurations of this upper half portion will be described with reference to Figs. 4A to 4H as follows. The configurations of the upper half portion shown in Figs. 4A to 4H are variation examples of the half-box portion 21 according to the above first to fourth embodiments.

[0033] The upper half portion shown in Fig. 4A is provided with left and right upward-spreading side plates in which a gap is gradually enlarged toward the side of the rear surface of this first intermediate boom 2. The upper half portion is provided with an upper flat plate portion continuous to upper ends of the left and right side plates through the left and right R surfaces having the predetermined bending radius. The upper half portion shown in Fig. 4A is formed into an inverted trapezoidal shape as a whole.

[0034] The upper half portion shown in Fig. 4B is provided with left and right upward-narrowing side plates in which the gap is gradually narrowed toward the side of the rear surface of this first intermediate boom 2. The upper half portion shown in Fig. 4B is formed into a trapezoidal shape as a whole.

[0035] The upper half portion shown in Fig. 4C is provided with left and right parallel side plates and an upper flat plate portion continuous to upper ends of the left and right parallel side plates at a right angle.

[0036] The upper half portion shown in Fig. 4D is provided with left and right parallel side plates. Left and right inclined plate portions inclined at a predetermined inclination angle are coupled to upper ends of the left and right parallel side plates, and ends of these left and right inclined plate portions are coupled to each other. The upper half portion shown in Fig. 4D is formed into a pentagonal shape as a whole.

[0037] The upper half portion shown in Fig. 4E is provided with left and right parallel side plates. Left and right inclined plate portions inclined at a predetermined inclination angle are coupled to upper ends of the left and right parallel side plates, and an upper flat plate portion is coupled to upper ends of these inclined plate portions.

[0038] The upper half portion shown in Fig. 4F is provided with left and right curved side plates opening on the lower side, and an upper curved plate continuous to these left and right side plates and protruding upward. More specifically, the upper half portion is formed into a half-oval shape having a short diameter in the left and right width direction and a long diameter in the up and down direction.

[0039] The upper half portion shown in Fig. 4G is provided with left and right parallel side plates, and an upper half-arc plate with lower opening ends continuous to upper parts of these left and right parallel side plates. The configuration of the upper half portion on the side of a rear surface of this first intermediate boom 2 may be the one shown in Fig. 4H. The upper half portion shown in Fig. 4H is provided with left and right parallel side plates, and a half-oval plate continuous to these left and right parallel side plates. This half-oval plate has a long diameter in the left and right width direction and also has a short diameter in the up and down direction.

[0040] In the above, the various configurations of the upper half portion of the telescopic boom are described. However, the configurations of the upper half portion mentioned above are only mere specific examples, and the present invention is not limited to these configurations of the upper half portion.

[0041] In the telescopic booms according to the above first to fourth embodiments, the description is given as an example to the case where an outer periphery surface of the arc portion is in contact with an inner part of the virtual arc of a semicircle or less having the opening width which is smaller than the outer width of the boom as the boom in which the arc portion is formed in the lower half portion on the side of the ventral surface in the cross-section cut in the direction orthogonal to the longitudinal direction. However, an inner periphery surface of the arc portion may be in contact with an outer part of the virtual arc in the present invention. Alternatively, as the boom in which the polygonal portion having the flat plate portions is formed in the lower half portion on the side of the ventral surface in this cross-section, the description is given as an example to the case where the bent portions of the polygonal portion are in contact with an inner part of the virtual arc of a semicircle or less having the opening width which is smaller than the outer width of the boom. However, inner flat surfaces of the flat plate portions of the polygonal portion may be in contact with an outer part of the virtual arc in this case as well.

[0042] It should be noted that although the first intermediate boom is described as an example in the telescopic booms according to the above first to fourth embodiments, any other booms are applicable like the same manner. Further, although the description is given as an example to the case where the telescopic booms according to the above first to fourth embodiments are used for a type of wheeled crane of commonly using an operator's cab for operating a crane and an operator's cab for traveling, the present invention is not particularly limited to use for this type of wheeled crane. For example, the present invention can also be applied to a telescopic boom used for a truck crane separately provided with the operator's cab for operating the crane and the operator's cab for traveling, or a telescopic boom used for a crawler traveling type crane provided with a crawler type traveling platform car.

[0043] Although the invention has been described with reference to the preferred embodiments in the attached figures, it is noted that equivalents may be employed and substitutions made herein without departing from the scope of the invention as recited in the claims.

[0044] A telescopic boom of the present invention comprises a base boom 1 and inserted booms 2 to 5 telescopically inserted into this base boom 1. A cross-section of each of the inserted booms 2 to 5 cut in a direction orthogonal to a longitudinal direction thereof is formed by a lower half portion on the side of a ventral surface and an upper half portion on the side of a rear surface. The lower half portion is formed by an arc part along a virtual arc 22 of a semicircle or less having an opening width Wr which is smaller than an outer width Wo of the inserted booms 2 to 5, and a pair of left and right inclined flat plate portions 24 continuous to this arc part. Left and right ends of the upper half portion and left and right ends of the lower half portion are coupled to each other by the inclined flat plate portions 24 formed in the lower half portion. The telescopic boom of the present invention has light weight but high buckling strength, and is excellent in the manufacture property of the boom and a pad, in which a gap between the booms is easily adjusted at the time of assembling of the boom.

Claims

1. A telescopic boom comprising a base boom (1), and at least one inserted boom (2, 3, 4 or 5) telescopically inserted into said base boom (1), a cross-section of said inserted boom (2, 3, 4 or 5) cut in a direction orthogonal to a longitudinal direction thereof being formed by a lower half portion on a side of a ventral surface and an upper half portion on a side of a rear surface,
characterized in that:

said lower half portion is formed by an arc part along a virtual arc (22) of a semicircle or less having an opening width (Wr) which is smaller than an outer width (Wo) of said inserted boom (2, 3, 4 or 5), and a pair of left and right inclined flat plate portions (24) continuous to said arc part, and

left and right ends of said upper half portion and left and right ends of said lower half portion are coupled to each other by said inclined flat plate portions (24) formed in said lower half portion.

2. A telescopic boom according to claim 1, wherein
said arc part is an arc portion (23) with a circumferential surface in contact with said virtual arc (22), and an outer width or an inner width of said arc part corresponds to said opening width (Wr) of said virtual arc (22).

3. A telescopic boom according to claim 1, wherein
said arc part is a polygonal portion (25) formed by a plurality of flat plate portions (26) coupled by bent portions (27) in which said bent portions (27) or inner plane surfaces of said flat plate portions (26) are in contact with said virtual arc (22), and an outer width or an inner width of said arc part corresponds to said opening width (Wr) of said virtual arc (22).

4. A telescopic boom according to claim 3, wherein
width lengths of a plurality of said flat plate portions (26) forming said arc part are equal to each other.

5. A telescopic boom according to claim 3, wherein
width lengths of a plurality of said flat plate portions (26) forming said arc part, and width lengths of said inclined flat plate portions (24) are equal to each other.

6. A telescopic boom according to claim 3 or 4, wherein
a plurality of said flat plate portions (26) forming said arc part are bent at an equal angle to each other.

7. A telescopic boom according to claim 1, wherein
outer surfaces of the ends of said inclined flat plate portions (24) of said lower half portion form outward steps from outer surfaces of the left and right ends of said upper half portion in a coupled condition.

Drawing