EP1302435A1

EP1302435A1 - A high strength telescopic arm

Info

Publication number: EP1302435A1
Application number: EP01830652A
Authority: EP
Inventors: Guelfo Ganzarolli; Luca Perelli
Original assignee: Effer SpA
Current assignee: Hiab Italia Srl
Priority date: 2001-10-16
Filing date: 2001-10-16
Publication date: 2003-04-16
Anticipated expiration: 2021-10-16
Also published as: ES2670924T3; TR201807546T4; EP1302435B1; DK1302435T3

Abstract

A high-strength telescopic arm, which in cross-section comprises a plurality of straight segments (5) arranged in sequence; each end (6a, 6b) of each straight segment (5) connected to only one end (6a, 6b) of an adjacent straight segment (5), defining a closed section (3). The number of straight segments (5) is between seven and ten.

Description

The present invention relates to a high-strength telescopic arm, in particular for mobile cranes, and a process for making the arm.
As is known, mobile cranes, for example mounted on vehicles, have a telescopic arm which can easily be transported in the fully retracted home position and which, in operating conditions can extend to great heights, to lift and move heavy loads.
In the most common operating condition, the weight is attached to the end of the crane arm and the arm is subjected mainly to a bending load.
The load of the type indicated leads to the formation of tractive stresses in the upper part of the arm and compression stresses in the lower part.
Therefore, the arm section must be of a size which guarantees a safety margin in the presence of the maximum tractive and compression stresses allowed, respectively in the upper and lower zones of the section.
In particular, there are known sections which are hexagonal and have walls with constant thickness, normally obtained from a single metal sheet which is folded and welded.
A hexagonal section made using high-strength materials does not give the best results in terms of weight, dimensions, cost, rigidity, bending strength, shearing strength, resistance to normal stresses and buckling.
A greatly reduced thickness may lead to instability in the shape of the lower panels of the arm, subject to compression, and in the side panels subject to bending and shearing, according to a phenomenon known to experts in the field as buckling of panels with a thin section.
Sections with a small number of straight segments of significant length, including the hexagonal section, do not allow a reduction in the thickness which would bring clear advantages in terms of weight, because if the ratio between the thickness of each compressed segment and the length of the segment is below a given value, its safety in terms of resistance to buckling or shape instability can no longer be guaranteed.
Therefore, even if the use of the most recent special steels, with good structural specifications, would allow a reduction in the thickness of the walls, relative to pure bending strength, and therefore the total weight of the arm.
This is not possible, due to the above-mentioned instability phenomena, unless the size of the section is optimised.
Alongside the solution just introduced, there are known telescopic arm sections formed from curved segments, at least in the compressed zones. As is known, a curvature has good properties regarding shape stability.
The section of a telescopic arm for a crane of the above-mentioned type is described in document US-6,098,824. This section comprises an upper portion in the shape of an inverted U, formed from a flat upper wall and two side walls extending from top to bottom of the end margins of the upper wall, and a lower portion, welded to the upper portion, having a concave - convex wall, with greater thickness and consisting of adjacent arced segments arranged in sequence.
The production of arms with sections of this type is, however, very complex and expensive, given that the entire metal sheet in the zone which will be subject to compression must be processed to give it an arced shape.
Moreover, this process involves lengthy construction times if compared with simple folding along preset lines for the construction of polygonal sections with straight sides, like the hexagonal one described above.
The aim of the present invention is, therefore, to overcome the above-mentioned disadvantages.
In particular, the aim of the present invention is to provide a telescopic arm with good resistance to shape instability and whose weight is contained.
The aim of the present invention is also to provide a telescopic arm which is easy to produce.
Another aim of the present invention is to provide a rapid, economical process for the production of a telescopic arm of the above-mentioned type.
These aims and others, which are more apparent in the description which follows, as achieved by a high-strength telescopic arm as described in the claims herein.
The invention is now described with reference to the accompanying drawings, which illustrate several preferred embodiments of a telescopic arm and a process for its production in accordance with the present invention, without limiting the scope of application, and in which:

Figure 1 is a side view of a truck fitted with a crane with hinged arms with cross-sections in accordance with the present invention;
Figure 2 is a perspective view of one of the tubes which form the trunks of the arm illustrated in Figure 1;
Figure 3 is a cross-section along line III - III of the telescopic arm shown in Figure 1;
Figure 4 is a cross-section of a first embodiment of a trunk of the telescopic arm shown in Figure 1;
Figure 5 is cross-section of a second embodiment of a trunk of the telescopic arm shown in Figure 1;
Figure 6 is a cross-section of a third embodiment of a trunk of the telescopic arm shown in Figure 1;
Figure 7 is a cross-section of a fourth embodiment of a trunk of the telescopic arm shown in Figure 1;
Figure 8 is a cross-section of a fifth embodiment of a trunk of the telescopic arm shown in Figure 1;
Figure 9 is a sheet of metal used to make a trunk of the telescopic arm;
Figure 10 illustrates a folding operation carried out on the sheet of metal shown in Figure 9, to make a trunk of the telescopic arm;
Figure 11 is a side view of a truck fitted with a crane with a telescopic arm, known to experts in the field as a "crane truck", with cross-sections in accordance with the present invention;
Figures 12 to 20 illustrate the folding sequence for obtaining a 10-sided section in accordance with the present invention;
Figure 21 illustrates a six-sided section of an arm for a crane of the known type; and
Figure 22 illustrates a ten-sided section of an arm for a crane in accordance with the present invention.

With reference to the accompanying drawings, the numeral 1 denotes a telescopic arm for mobile cranes.
The telescopic arm 1 comprises a plurality of coaxial tubes 2 which form the trunks of the arm 1.
They have sections 3 with steadily decreasing sizes and are inserted inside one another. The tubes 2 can slide relative to one another and, thanks to suitable drive means, which are not illustrated, can move the arm 1 from the home position to the operating position and vice versa.
Each tube 2 consists of a plurality of panels 4 which form the side wall of the tube 2.
A generic cross-section 3 of each tube 2 is, therefore, formed by a plurality of straight segments 5 arranged in sequence.
Each straight segment 5 has a length b and thickness t1, t2 and is connected to the next segment 5 at one end 6a and to the previous segment 5 at the opposite end 6b, so as to form a closed polygonal section 3.
Moreover, two adjacent segments 5 form an angle α facing the inside of the section 3 which is less than one hundred and eighty degrees, therefore, the internal edge 7 of the section 3 consists of a concave line.
In all of the embodiments illustrated, the number of straight segments 5, and therefore the number of panels 4 which form each tube 2 is between seven and ten.
The section 3 preferably has an axis of symmetry Y, therefore the segments 5 of which it consists are distributed symmetrically relative to the axis Y.
In some embodiments, illustrated in Figures 4, 5, 6, 7 and 22, the segments 5 of each section all have the same thickness t1.
The ratio between the thickness tl and length b of the sides of the section is preferably such that it allows a similar safety margin relative to the strength of the zones subject to normal, shearing and bending stresses (that is to say, the sides in particular) and resistance to shape instability (that is to say, buckling).
In the embodiments illustrated in Figures 4, 5, 6, 7 and 22, each tube 2 which is part of the telescopic arm 1 is made from a single rectangular sheet 8 of metal with constant thickness t1, also making construction of the sections particularly economical.
The sheet of metal 8 is deformed along fold lines 9 parallel with the longitudinal edges 10 of the sheet 8, which are substantially the same length as the tube 2 to be made.
The folds are made in such a way as to bring the two longitudinal edges 10 together, until they are touching.
Each fold line 9 on the metal sheet 8 delimits two adjacent panels 4 forming dihedral angles α less than one hundred and eighty degrees or, similarly in a section of metal sheet 8 at a right angle to the longitudinal edges 10, two straight segments 5 of the section 3.
To obtain a number of panels 4, forming the side wall of the tube 2, or segments 5 of the section 3, numbering between seven and ten, the process comprises between seven and ten folding steps.
Finally, the edges 10, which are touching, are joined by welding.
In a first preferred embodiment, illustrated in Figure 4, the section 3 consists of seven straight segments 5, arranged symmetrically relative to an axis of symmetry Y of the section 3 which coincides with the main direction of application of the loads.
This seven-sided section 3 is obtained by folding the metal sheet 8 along seven parallel lines 9 to obtain six complete segments 5 and two half- segments 5a, 5b.
The two half- segments 5a, 5b are brought together and welded along the edges, keeping them in the same plane, to form a single segment 5, with a weld seam 11 on the axis of symmetry Y of the section 3.
A second embodiment, illustrated in Figure 5, comprises a section 3 consisting of eight straight segments 5, again arranged symmetrically relative to an axis of symmetry Y of the section 3.
The eight-sided section 3 is obtained by folding the metal sheet 8 along eight parallel lines 9 to obtain seven complete segments 5 and two half- segments 5a, 5b.
As already indicated, the two half- segments 5a, 5b are brought together and welded along the edges, keeping them in the same plane, so as to form a single segment 5, with a weld seam 11 on the axis of symmetry Y of the section 3.
A third embodiment, illustrated in Figure 6, comprises a section 3 consisting of nine straight segments 5, again arranged symmetrically and made using the same procedure as was used for the first two.
Finally, a fourth embodiment, illustrated in Figure 7, consists of ten segments.
The section 3 is obtained by folding the metal sheet 8 along nine parallel lines 9 to obtain ten complete segments 5. The end segments 5 are brought together at a given angle α and welded along the edges 10a, 10b, leaving the weld seam 11 on the edge.
The folding process, which is common to all of the embodiments mentioned, is carried out by placing the metal sheet 8 on a die 12 and pressing it along the fold lines 9 with a punch or knife 13.
In order to use the knife 13 on all of the fold lines 9, a first set of folds is made in succession, starting from one longitudinal edge 10 towards the inside of the metal sheet, then moving on to a second set, starting from the opposite longitudinal edge 10 and continuing as far as the final fold of the first set.
In this way, as illustrated in Figure 10, the knife 13 can be removed after the last fold, from the gap left by the metal sheet 8 between the two longitudinal edges 10 which are almost touching.
Figures 12 to 20 illustrate the folding sequence used to obtain a 10-sided section.
The sides are labelled with capital letters from "A" to "J" and are obtained by folding first the right-hand part of the section (Figures 12 - 15), then the left-hand part of the section (Figures 16 - 19), then making a central fold on the side opposite the gap left by the metal sheet.
Again, the knife 13 can be removed after the final fold has been made, from the gap left by the metal sheet between the two longitudinal edges which are almost touching. Moreover, since it is straight, the knife 13 is more economical than curved knives and can be used for all sections, whether large or small, because special knife curved shapes are not required for its removal from the profile being folded.
Advantageously, as a further guarantee of shape stability, or resistance to buckling, the part 14b of the section 3 which supports mainly compression loads can be made with a segment 5 thickness t2 greater than the segment 5 thickness t1 for the remaining part 14a, designed to resist only tractive and shearing loads.
In other words, at least two of the straight segments 5 have a thickness t2 which is different to the thickness tl of the remaining segments 5.
Therefore, around half of the section 3, considered along the axis of symmetry Y, in an embodiment illustrated in Figure 8, consists of sheet metal which is thicker than in the other half.
The precise calculation is done with reference to the neutral axis of the section, which also depends on the thickness, according to known processes which are not described here.
Given the presence of two different thicknesses t1, t2, the section 3, in the above-mentioned embodiment, is obtained from a pair of metal sheets 8a, 8b.
Each metal sheet in the pair 8a, 8b is folded along a plurality of fold lines 9, in accordance with the above-mentioned process, without bringing the longitudinal edges 10a, 10b of each sheet 8a, 8b together, but leaving the distance between the two edges 10a of the first sheet 8a equal to the distance between the two edges 10b of the second sheet 8b.
Advantageously, to obtain a closed section 3 consisting of between seven and ten segments 5, the sum of the fold lines 9 on the first and second sheets 8a, 8b must be between five and ten.
Therefore, the two sheets 8a, 8b are brought together so that the longitudinal edges 10a of the first sheet and those 10b of the second sheet are touching, forming the tube 2 with polygonal cross-section.
The edges 10a, 10b of the two sheets 8a, 8b are then welded together.
Advantageously, the weld seam 11 is on the lower part of the section 3, subject to compression, because it enhances its stress resistance characteristics.
Figures 21 and 22 illustrate two embodiments of crane arm sections, one a six-sided section 15 of the known type, and the other a ten-sided section 16 in accordance with the present invention.
These sections 15, 16 are shown by way of example to help provide a clear explanation of the inventive concept, by without limiting its scope, which extends to all embodiments covered by the inventive concept expressed in the description and in the claims.
Both sections 15, 16 have maximum dimensions within a rectangle labelled 27 which has a width of 305 mm and height of 450 mm. Both sections have the same vertical plane bending inertia coefficient (W - 759 cm³) and the same resistance to buckling.
The overall dimensions of section 15 are height 477.5 mm and width 235 mm.
It consists of six sides with 120 degree reciprocal angles. The sides 15a are 288 mm long and the upper and lower sides 15b are 103 mm long. The radius of the sides is 21 mm.
The overall dimensions of section 16 are height 409.4 mm and width 304 mm.
It consists of ten sides. The sides 16a are 273 mm long and are at a 115 degree angle to the upper and lower external sides 16b, which are 70 mm long. The external sides 16b in turn are at an angle of 162° 30' to the upper and lower internal sides 16c. The latter sides 16c are at an angle of 165 degrees to one another at the centre line of the section 16.
The length of the sides 16c is such that it allows the obtainment of a section 16 with an overall width of 304 mm and is substantially 53 mm.
In both cases the sections 15 and 16 are made using a high-strength material, marketed as Weldox 1100, a registered trade-mark of the company SSAB Oxelosund AB, which has a yield strength of 1100 MPA (N/mm²).
In the first case, section 15 is optimised in such a way that a thickness of at least 7 mm is required to obtain sufficient resistance to yielding and buckling.
In the second case, section 16 is optimised in such a way that a thickness of just 6 mm is required to obtain sufficient resistance to yielding and buckling similar to those of section 15.
Thanks to the new ten-sided shape, the 6 mm thickness is sufficient to obtain a similar safety coefficient against buckling of the section sides subjected to normal, shearing and bending stresses.
The example also shows how the ten-sided section is around 7% lighter, the area of the ten-sided section being 73.9 cm², compared with 79.6 cm² for the area of the six-sided section.
Moreover, in comparison with the six-sided section in the example, the ten-sided section is also 17% more resistant to bending on the horizontal plane, because the ten-sided section is wider, despite remaining within the dimensions of the design in the example.
The greatest advantages with the sections disclosed in the present invention are obtained with materials which have a yield strength of more than 800 MPA (N/mm²). Since the materials currently available reach yield strength values of 1100 MPA (N/mm²), this is the upper limit value. However, said advantages will also apply with future materials whose yield strength values are even higher.
Another possible use for the sections disclosed relates to materials with lower yield strength values, of around 300 - 400 MPA (N/mm²).
With these materials, with a lower yield strength, it is still possible to obtain advantages in terms economic savings. Relatively large sections can be produced with thin metal sheets, to obtain a reduction in the weight of the material used and, therefore, a less expensive construction.
The present invention brings important advantages.
Firstly, the entire section of the telescopic arm in accordance with the present invention allows a reduction in the thicknesses and, therefore, the weight of the arm.
Thanks to its shape, the section disclosed guarantees a good safety margin relative to possible instability in the zones which are compressed and those subjected to bending and shearing, because it allows a similar safety coefficient relative both to strength and to the tendency to buckle.
It should also be noticed that every trunk of the arm, with a section as described in the claims, is easily made, since it does not involve the production of arced profiles or shapes which are not straight.
Moreover, the entire process is fast because the time required for the moulding step, during which the metal sheet is folded, is less than the time necessary to obtain semi-finished products with a curved section and two or more weld seams.
The process in accordance with the present invention, therefore, allows production to be speeded up, with the consequent limitation of costs.
The invention described can be subject to numerous modifications and variations without thereby departing from the scope of the inventive concept.
Moreover, all the details of the invention may be substituted by technically equivalent elements.

Claims

A high-strength telescopic arm, comprising in cross-section:

a plurality of straight segments (5) arranged in sequence; each straight segment (5) having a length (b), a thickness (t1, t2) and two ends (6a, 6b); each end (6a, 6b) of each straight segment (5) being connected to one end (6a, 6b) of an adjacent straight section (5) to form a closed section (3);

characterised in that the number of straight segments (5) is between seven and ten.
The telescopic arm according to claim 1, characterised in that two adjacent straight segments (5) form an angle (α) opposite the inside of the closed section (3) which is less than or equal to one hundred and eighty degrees.
The telescopic arm according to claim 1 or 2, characterised in that the straight segments (5) are arranged symmetrically relative to an axis of symmetry (Y) of the section (3) in its plane.
The telescopic arm according to any of the foregoing claims, characterised in that all of the straight segments (5) have the same thickness (t1).
The telescopic arm according to any of the claims from 1 to 3, characterised in that at least two of the straight segments (5) have a thickness (t2) greater than the thickness (t1) of the remaining segments (5).
The telescopic arm according to any of the foregoing claims, characterised in that it also comprises at least one weld seam (11) joining two adjacent straight segments (5).
The telescopic arm according to any of the foregoing claims, characterised in that it is at least partly made from a material with a yield strength of more than 800 MPA (N/mm²).
The telescopic arm according to any of the claims from 1 to 6, characterised in that it is completely made entirely from a material with a yield strength of more than 800 MPA (N/mm²).
A process for making a high-strength telescopic arm, comprising the following steps:

obtaining a rectangular metal sheet (8) with two parallel edges (10) opposite one another;

deforming the metal sheet (8) along a plurality of fold lines (9) parallel with the edges (10), to bring the edges (10) together until they are touching; each fold line (9) on the metal sheet (8) delimiting two panels (4) forming dihedral angles (α) of less than one hundred and eighty degrees; the panels (4) forming, in a section (3) at a right angle to the fold lines (9), respective straight segments (5) arranged in sequence;

joining the edges (10) of the metal sheet (8) to form a tube (2) with a polygonal cross-section; characterised in that the step of deforming the metal sheet (8) comprises between six and ten folding steps, each along a fold line (9).
The process according to claim 9, characterised in that the step of joining the edges (10) of the metal sheet (8) consists in welding the edges (10).
The process according to claim 9 or 10, characterised in that each fold along a single fold line (9) is made by placing the metal sheet (8) on a die (12) and pressing the metal sheet (8) with a punch (13) along the line (9).
A process for making a high-strength telescopic arm, comprising the following steps:

obtaining a pair of rectangular metal sheets (8a, 8b), each having two parallel edges (10a, 10b) opposite one another;

deforming the first of the pair of sheets (8a) along a plurality of fold lines (9) parallel with the edges (10a); each fold line (9) on the first sheet of the pair (8a) delimiting two panels (4) forming dihedral angles (α) of less than one hundred and eighty degrees; the panels (4) defining, in a section (3) at a right angle to the edges (10a) respective straight segments (5) arranged in sequence;

deforming the second of the pair of sheets (8b) along a plurality of fold lines (9) parallel with the edges (10b); each fold line (9) on the second sheet of the pair (8b) delimiting two panels (4) forming dihedral angles (α) of less than one hundred and eighty degrees; the panels (4) defining, in a section (3) at a right angle to the edges (10b) respective straight segments (5) arranged in sequence;

joining the edges (10a) of the first sheet of the pair (8a) to the edges (10b) of the second sheet of the pair (8b) to form a tube (2) with polygonal cross-section;

characterised in that the two steps of deforming the metal sheets (8a, 8b) comprise a total of five to ten folding steps, each along a fold line (9).
The process according to claim 12, characterised in that the thickness (t2) of the second sheet of the pair (8b) is greater than the thickness (t1) of the first sheet of the pair (8a).
The process according to claim 12 or 13, characterised in that the step of joining the edges (10a) of the first sheet of the pair (8a) to the edges (10b) of the second sheet of the pair (8b) consists of making at least two weld seams.
The process according to any of the claims from 12 to 14, characterised in that each fold along a single fold line (9) on each sheet of the pair (8a, 8b) is made by placing the metal sheet (8) on a die (12) and pressing the metal sheet (8) with a punch (13) along the line (9).