Patent Grant
6978907
Telescopic jibs are used for cranes wherein the jib must be extended for use and retracted for other purposes, such as transport: Thus, such jibs are normally used for vehicular cranes. The sections of such jibs are typically tubular so that the successive sections can nest within each other when retracted and telescope outwardly to extend the jib to a desired length.
Such telescopic jibs execute hoisting operations with the load at their front end. As a result, the jib is exposed to a bending force in two main axes. Viewing the jib in cross section along its longitudinal axis, each jib section, when loaded, is subject to tensile stress on the upper side of the jib while, on the lower side, compressive stresses occur. Due to lateral forces and eccentric loading, horizontal bending and torsion also occur.
Designers of such jibs are principally interested in optimally configuring the cross-section for jib parts loaded in this way. Such a cross-section is easiest to devise when the maximum stresses are the same in every direction and approximate the permissible stress. These requirements are satisfied for instance in the case of thin-walled circular tubes or in the case of a square trussed structure only when uniform forces occur in all directions. If a cross-section is loaded, for instance, more in the vertical direction than in the horizontal, then an optimum rounded cross-section becomes an ellipse and an optimum cornered cross-section becomes a rectangular trussed structure, the cross-sections in both cases being higher than they are wide to account for the imbalanced forces.
A telescopic jib generally as described above is known, for example, from EP 0 499 208 B1. The cross-section of this telescopic jib consists of an upper profile part having a semi-box shaped configuration and a lower profile part, configured as a rounded half shell, welded to the free legs of the former. Although such totally round lower profile parts have good load introduction and stability properties, they do not compete with rectangular trussed structures with respect to stiffness. It is often necessary to install additional members, such as welded stiffeners, to promote stability to counteract buckling or to construct the jib of material that is somewhat thicker which has a negative effect on the weight of the jib overall.
A jib profile for cranes and vehicle cranes is known from EP 0 668 238 A1 in which the two upper leg sections of the lower profile, welded to the lower legs of the upper profile, are configured as straight strips. The remainder of the lower profile part has a curved shell shape. It is also proposed in this document, as an alternative, to employ a straight strip portion at another point of the lower profile part. These straight strip portions produce cross-sectional kinks in the profile at their edges. Due to these kinks the loading properties of such a profile once again approach those of a rectangular trussed structure; i.e., the stiffness can be increased. However, the drawback in such profile designs is that, due to the straight strips employed, the load introduction and stability properties which are particularly advantageous for curved profiles become poorer. Additional stiffeners or thicker material gauges are again needed which disadvantageously increases the overall weight of the jib.
German Utility Model No. 94 02 692 describes a jib profile comprising a substantially semi-box shaped upper section and a rounded lower section connected to the upper section, in which the lower section has at least one planar or flat wall section. This shape is utilized in an attempt to produce both sufficient resistance to buckling and sufficient load resistance against bending. A planar plate segment (wall section) is thus inserted into the cross-section of the lower profile. A disadvantage of this configuration is that planar plate segments or wall sections in such profiles strained by bending and buckling are weak points precisely with respect to buckling resistance. A further disadvantage of the planar segments is that, in the force introduction area between the points of overlap between adjacent jib sections, the planar strips or plates segments are substantially less able than curved shells to absorb transverse forces. Therefore, they have to be strengthened, for example by stiffeners, to counteract buckling.
DE 43 44 795 A1 describes a jib cross-section whose lower profile part consists of nine flat strips with adjacent stripe arranged at an obtuse angle with respect to each other. These strips form the plate segments of the lower profile part. They are all configured as flat plate segments, which again have the disadvantages regarding resistance to buckling.
Furthermore, DE 200 04 016 U1 describes a telescopic jib in which the coupling portion and/or at least one telescopic length consist of profiles, each of which having a lower, round part and an upper, semi-box shaped part, whose facing legs are welded to each other. The upper profile part has the shape of an isosceles trapezium without the longer base part, such that the legs of the upper and lower profile parts abut each other forming an angle which is smaller than 180° on the inner sides of the profiles. The lower profile part is made of material having relatively increased thickness. In this way, it is intended that a better resistance to buckling is achieved. For this purpose, however, the heavier lower profile part has to extend upwards far above the axis of the moment of inertia of the cross section, or the neutral zone, of the jib. Increasing the amount of material in the neutral zone is, however, not advantageous in a jib because it undesirably increases the weight of the jib itself.
Lastly, DE 196 24 312 C2 discloses a telescopic jib for a vehicular crane in which the upper profile part is semi-box shaped and the lower profile part consists of several shell segments adjacent to each other, each having an outwardly curved shape in the form of a circular arc. In this way, it is intended to combine the good load bearing and stability properties of curved profiles with the greater stiffness of a rectangular trussed structure, so that such a telescopic jib can be built particularly lightweight.
Despite the improvements achieved by the various shapes of the upper profile parts and lower profile parts of known jibs, there is still no optimum solution for extreme loads, such as in luffing jib operations, guyed or pre-tensioned systems, or when positioning a jib in an orientation approaching vertical. In such situations the tensile forces in the upper profile portion may be minimized, but large forces act along the main axis of the jib even while the load may be small, resulting in substantial lateral forces. The resulting lateral forces can be very large in these working positions, such that the jib may be in serious danger of buckling.
The invention provides a telescopic jib of the described type in which the disadvantages mentioned above do not occur. In particular, the invention provides a telescopic jib which exhibits increased resistance to buckling and which is, therefore, suitable for carrying extreme loads, such as in luffing jib operations, in guyed systems, or when positioning a jib carrying a substantial load while positioned in a nearly vertical orientation.
The advantages achieved with the invention are based on the fact that the upper profile part of a jib section is formed by several shell segments, each having an outwardly curved shape, with adjacent sections abutting each other at an obtuse angle. In this way, the joints between the individual outwardly curved segments act like idealized stiffeners to counteract buckling. This is of great advantage to luffing jib operations, in pre-tensioned and/or guyed jib systems, and when using a jib to lift a large load while in a nearly vertical orientation since, in a jib according to the invention, both the upper profile part and the lower profile part may be compression loaded. Unlike the telescopic jibs according to the prior art, the cross-section of the upper profile part of the shell is supportive in compression, and stiffness is increased in the telescopic jib profile according to the invention, while simultaneously minimizing the overall weight of the jib. Furthermore, the shape of the upper profile part according to the invention provides a greater capacity to absorb the forces that are transferred from the upper shell of one jib section to the next, larger jib section of a telescopic jib.
As compared to conventional jib profiles, an increase in load bearing is achieved with the configuration in accordance with the invention. This is accomplished with greater material stability, without increasing the amount, thickness or weight of material used. The result is a stronger more stable jib without any corresponding increase in jib weight.
The upper profile part of a telescopic jib according to the present invention consists of at least two curved shell segments. The number of shell segments actually used may vary depending on the desired shape of the jib and on the specific types of loads likely to be encountered. Preferably, three, four or more shell segments may be used. When configuring a “shield” shape, for instance, four shell segments are present in the upper profile part.
According to a preferred feature of the invention, the endmost segments of the upper and the lower profile parts comprise ends formed as straight legs such that the straight leg ends of the upper and lower profile parts can be welded to each other. This results in optimum force transfer from the upper profile part onto the lower profile part and vice versa, depending on the type of load. The welding joint between the lower profile part and the upper profile part is preferably maintained in the area of the neutral zone of the cross section of the jib. This arrangement is facilitated by the structure according to the invention. Since the curved shell segments abutting each other at an obtuse angle in the upper profile part provide a higher level of resistance to bending, the upper profile part can extend further downwardly into the area occupied, in the prior art jibs, by the lower profile part without adversely affecting the load bearing capacity of the jib. As a result it is easily possible to provide the welding joint between the upper profile part and the lower profile part in the area of the neutral zone of the jib cross section.
Rotation of the telescopic parts with respect to each other as a result of torsion is significantly reduced by the cross-sectional shape in accordance with the invention as a result of the multiple joints formed between the shell segments in the upper profile part and the lower profile part.
According to another preferred embodiment of the invention, the outwardly curved shell segments in the upper profile part may have positioned between them one, two or several straight or flat segments. This achieves a more even distribution of stresses and reduces ovaling of the cross-section due to strains from bending in the vertical plane. This is advantageous when substantial tensile forces are imposed on the upper profile part of the jib. In particular when such a straight or flat shell segment is situated in the upper horizontal area of the upper profile part, the advantages of conventional semi-box shaped upper profile parts can be utilized, as is sensible for certain applications. Introducing such a straight shell segment in a jib according to the invention reduces the overall height of the jib cross-section. This, in turn, reduces the jib height in the lowered position of the jib and, thus, the overall height of the crane with the jib stowed. This is of value when transporting a nested crane.
Furthermore, it is noted that the lowermost part of a jib section, which is received and supported in the distal end of the next larger section of the telescopic jib, must be supported overall by sliders. Such sliders are situated in the area of the cross section where the curved shell segments abut each other at an obtuse angle. By introducing a straight or flat shell segment, the sliders can be omitted at this point. The lengths of the sliders in the direction of the jib main axis can be optimized by varying the width of the straight segment.
Lastly, a straight or flat shell segment in the upper profile part is helpful for transport, production and assembly, since assembling devices for supporting and positioning such a jib section are not necessary.
The invention will be more fully understood in light of the following description considered in conjunction with the appended drawings, in which:
The drawing figures show a cross-section through a section of a telescopic jib. It is to be understood that the invention applies to either or both of a jib base section that may be supported on a vehicle or other base part of a crane, or to an extensible telescopic jib section that nests within the base section or within a further telescopic section. Typically, a jib comprises a base jib section and several telescopic sections of identical or substantially identical cross-sectional shape. This allows the telescopic parts to be nested within each other and within the base part with very small clearances from each other. Nesting of jib sections in compact fashion is facilitated by the invention since, for the reasons explained above, stiffening means such as additional welded-on stiffeners to counteract buckling may be dispensed with, and a thin wall structure may be employed. This results in a stable, more lightweight and compact telescopic jib.
A first embodiment of a telescopic jib section is shown in
The jib section of
The lower profile part is formed by three outwardly curved shell segments 14b, 14c and 14d. Each section 14b, 14c and 14d has the shape of a circular arc, though with respectively different radii of curvature. Segments 14b and 14d each include one of the straight leg portions 14a that are welded to the upper profile part.
In a similar way, the upper profile part 12 consists of three outwardly curved shell segments 12b, 12c and 12d, each of which likewise has the shape of a circular arc, with respectively different radii of curvature. The two shell segments 12b and 12d include the straight parts 12a which are welded to the straight parts 14a of the lower profile part 14.
As can be seen, the shell segments 12b, 12c, and 12d form obtuse angles with each other at their respective meeting points and at the points where they meet with the connecting straight parts 12a, respectively. This also applies to the lower shell segments 14a, 14b, 14c and 14d.
The embodiment according to
The number of curved shell segments in the upper profile part 12, which is shown as three in the embodiment of
The outwardly curved segments of the jib provide excellent resistance to compressive forces. The relatively sharp “creases” formed at the joints where the curved segments meet at obtuse angles provide enhanced stiffness. This avoids any need for additional stiffeners, thus maintaining a desirably clean profile, desirably low overall weight and a compact nested jib structure. All of this is achieved also without unnecessarily increasing the thickness of the material from which the jib is fabricated and, thus, avoids undesirable increase in the dead weight of the jib. This enhanced strength and rigidity is especially important in the upper profile part of the jib, as discussed above.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3516553 | Reske | Jun 1970 | A |
| 5884791 | Vohdin et al. | Mar 1999 | A |
| 6108985 | Paschke et al. | Aug 2000 | A |
| Number | Date | Country |
|---|---|---|
| 94 02 692 | May 1994 | DE |
| 43 44 795 | Jun 1995 | DE |
| 196 24 312 | Jan 1998 | DE |
| 196 24 312 | Jan 1998 | DE |
| 200 04 016 | May 2001 | DE |
| 0 499 208 | Aug 1992 | EP |
| 0 668 238 | Aug 1995 | EP |
| 203878 | Dec 1967 | SU |
| Number | Date | Country | |
|---|---|---|---|
| 20030106871 A1 | Jun 2003 | US |
