EP2709945B1

EP2709945B1 - Composite telescopic crane arm with metal guide blocks and crane comprising said arm

Info

Publication number: EP2709945B1
Application number: EP12731656.0A
Authority: EP
Inventors: Mauro Marco CORTELLINI; Stefano Ferri; Paolo Dario MAINI
Original assignee: CIFA SpA
Current assignee: CIFA SpA
Priority date: 2011-05-17
Filing date: 2012-05-17
Publication date: 2015-07-22
Anticipated expiration: 2032-05-17
Also published as: ITMI20110869A1; EP2709945A1; WO2013050838A1; CN103702921B; CN103702921A

Description

FIELD OF THE INVENTION

The present invention concerns a telescopic arm for cranes and crane comprising said arm.

BACKGROUND OF THE INVENTION

It is known that cranes are work vehicles used to lift heavy loads, in the range of several tons, which generally comprise a truck on which a telescopic arm is mounted, consisting of a plurality of removable segments which can slide reciprocally one inside the other. A hook, or other lifting mean, is provided at an upper end of the arm, to lift the loads.
The arm can be angularly oriented, as well as lengthened telescopically, to carry the hook, or other lifting means, to the desired distance and height.
Generally the telescopic segments are made of metal material, typically steel, and have a tubular shape with a cross section of different shapes, such as rectangular, rectangular with a rounded bottom, hexagonal or other. The end part of each segment has reinforcement elements, of the band type, which also act as stopping means in a non-operative position.
The telescopic segments are guided with respect to each other by means of adjustable blocks made of a material able to absorb the local loads and to reduce the sliding friction.
Moreover, the arm of the crane in question is both able to rotate around a vertical axis, and can also be inclined on a plane perpendicular to the support plane of the crane, being able to pass from a horizontal position to a substantially vertical or sub-vertical position.
Such cranes can be provided with stabilizer support elements, which determine a controlled lifting of the cranes with respect to the support plane, so that they are able to operate in conditions of substantial planarity.
Moreover, ballasts can be provided which allow to balance and stabilize the crane, preventing it from toppling over.
The distances and height which can be reached are variable, as is the lifting capacity.
The distance which can be covered depends on the number of segments that make up the arm, and possibly also their length.
The need to lift loads means the metal segments must be robust, and therefore of a considerable weight, the bigger the loads to be lifted are.
Therefore, one disadvantage of known metal telescopic arms is that the weight of the arm increases as both the distance to be reached and the load to be lifted increase.
Therefore, in this field, there is an urgent need to reduce the weight of such telescopic arms, while still maintaining, and preferably increasing, the load-lifting capacity, as well as the distances and heights that can be reached.
Document GB-A-2.207.109 describes a crane for lifting a light load, such as a cine or video camera. The crane comprises a telescopic arm formed by sections made of composite material based on carbon fiber or aluminum or other composite materials or metal alloys. The sections cooperate with each other in sliding by means of linear guide means that include rolling elements such as balls or cylinders interposed between each other. In particular, GB-A-2.207.109 describes in detail movement means with a belt and pulley for the reciprocal telescopic sliding of the sections. The function of the guide and rolling elements described in GB-A-2.207.109 is to promote the relative sliding of the sections, reducing friction, but the guide and rolling elements do not have any structural function or load supporting function. This is particularly true, considering the presence of the interposed rolling elements which promote the sliding and which, by their very nature, are not suitable for supporting heavy loads, in particular the components transverse to the sections of the type in question. It is quite obvious, in any case, that GB-A-2.207.109 does not deal with the problems typical of cranes for lifting heavy weights, and therefore does not provide any useful teaching for making an arm for cranes which is structurally resistant to heavy loads like those in question.
Document FR-A-2.759.687 also describes a telescopic arm for a crane or fork lift truck. The arm is formed by sections that slide with respect to each other, made of composite material or metal alloy. FR-A-2.759.687 also provides guide blocks made of plastic material interposed in cooperation between the sections, with the sole function of facilitating the guided sliding of the sections and in any case disposed only at the four outer corners of the terminal ends of the sections, at head and tail. Therefore, in this solution, since the guide blocks are made of plastic, they do not provide any contribution in terms of mechanical resistance to the stresses deriving from the heavy load lifted and, in any case, given the special disposition of the blocks, only at head and tail of the sections, they do not guarantee an optimal distribution and support of the transverse loads acting on the sections.
Document EP-A-1.353.139 describes a system to detect damage to a lifting arm formed by sections that are telescopically mobile. The arm is formed by sections that slide one with respect to the other, of which a first section made of composite material and sections made of metal material, of which a first metal section supports both the weight of the load, both of the second metal section and of the section made of composite material. This solution does not provide, nor suggest, to reduce the weight of the telescopic arm, inasmuch as there is in any case a considerable contribution to said weight given by the metal section of the arm. Furthermore, EP-A-1.353.139 also provides guide elements formed by strips of steel or hard plastic material, such as nylon, disposed along the edges, and sliding blocks, made of plastic material and interposed in cooperation between the sections, with the sole function of facilitating the guided sliding of the sections and in any case disposed only at the four outer corners of the terminal ends of the sections, at head and tail. Therefore, in this solution too, the guide elements and the sliding blocks do not provide any contribution in terms of mechanical resistance to the heavy load lifted and they do not guarantee an optimal distribution and support of the transverse loads acting on the sections.
Finally, document EP-A-0.117.774 describes a crane provided with a telescopic arm consisting of sliding sections. Each section is formed by a multi-layer sandwich structure which includes two surface layers that cover an inner core, to which they are associated by adhesive means. The central core is made of honeycomb plastic material or polyurethane foam, while the surface layers are made of composite material formed by plastic material reinforced with carbon fiber or aluminum alloy. The sections are reciprocally guided by guide blocks and associated sliding elements which allow the sliding of plastic on plastic or plastic on metal, depending on the material they consist of. However, this solution too provides elements and guides only for guiding, not specifically intended to support stresses, in particular stresses transverse to the sections, deriving from the weight of the load lifted.
Purpose of the present invention is therefore to achieve a telescopic arm for cranes which is both lighter than known telescopic arms and which allows, at the same time, to reach great distances, in any case guaranteeing robustness and mechanical resistance, both in the reciprocal coupling between the telescopic segments, particularly in the extended positions, and also in the overall capacity to lift heavy loads.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.

SUMMARY OF THE INVENTION

The present invention is set forth and characterized in the independent claims, while the dependent claims describe other characteristics of the invention or variants to the main inventive idea.
In accordance with the above purpose, a telescopic arm according to the present invention can be used for a crane able to lift a heavy load.
According to the present invention, the arm comprises a plurality of tubular segments which extend along a common central longitudinal axis. Each of the segments is able to slide in a telescopic manner inside or outside another adjacent segment and consecutive along the central longitudinal axis, between a retracted position and a completely or partly extended position, to selectively determine one or more extended conditions of the arm.
According to the present invention, each of the segments consists of a composite material with a base of reinforcement fibers and thermosetting resin.
Moreover, according to the present invention, the arm comprises longitudinal guide blocks or similar means, consisting of metal with high mechanical characteristics of robustness and disposed between one segment and at least another segment, adjacent and consecutive along the central longitudinal axis, which guide blocks are at least partly drowned in the composite material in corresponding internal surfaces of the at least one segment and external surfaces of the other segment.
According to the present invention, the guide blocks develop continuously along the whole length of the corresponding segment in which they are at least partly drowned, or discretely in correspondence with the usable work part of said length.
Moreover, each of the guide blocks includes a first internal guide element disposed at least partly drowned in the internal surface of the corresponding segment and a second external guide element, at least partly drowned, in a position coordinated with the corresponding first internal guide element, in the external surface of the corresponding segment.
According to the present invention, moreover, each first guide element has coupling means to cooperate directly in contact with mating coupling counter-means of the second guide element, both to guide the reciprocal sliding of the segments and to receive and support, directly on the metal of which the guide blocks are made and substantially along the whole length of the segments, the components of force transverse to the segments, due to the stresses deriving from the heavy load lifted by the arm.
According to the present invention, the reciprocal cooperation between the metal guide elements of the telescopic sections allows to adequately support the transverse loads, which are generally not supported by the composite material as described above, thus achieving, overall, a less heavy structure compared with the state of the art.
The longitudinal guide blocks, at least partly drowned, are advantageously made of metal resistant to wear caused by sliding, and preferably with high mechanical resistance.
By providing, according to the present invention, longitudinal guide blocks at least partly drowned, to allow the reciprocal sliding of the telescopic sections, it is possible to use the arm both in the limit conditions of completely extended segments, or completely retracted, and also in intermediate positions.
Indeed, in the case of contact between composite material and composite material, there would not be adequate structural resistance to the loads applied, since the composite material in question reacts well, in mechanical terms, to tangential stresses, that is, parallel to the fibers, whereas it has little mechanical resistance to perpendicular stresses, that is, orthogonal to the fibers, as described above.
Consequently, the longitudinal guide blocks, at least partly drowned, according to the present invention determine a distribution of the loads, in the working steps, in particular longitudinal loads on the composite material and transverse loads on the metal material of the guide blocks, which is favorable for the composite material, that is, a distribution of the loads that is parallel to the fibers of the material instead of perpendicular. Indeed, the component of stress in a transverse direction, not absorbed by the composite material, is discharged on the metal guide blocks.
The guide blocks, suitably shaped, are such as to allow the mutual sliding of the segments one inside the others telescopically, and also to transfer the loads to the structure in the direction of the fibers of the composite material.
In some forms of embodiment, the segments have a box-like section, defining, with respect to the component of the load raised perpendicular to the arm, upper or lower platbands and lateral flanks, and the guide blocks may be disposed in one and/or the other of the upper or lower platbands and/or in one and/or the other of the lateral flanks.
In some variants, the guide blocks may be provided, also more than one, in the corresponding platbands and/or in the lateral flanks. Moreover, in some specific variant solutions, the guide blocks may be disposed in combination both in the platbands and in the lateral flanks.
In some forms of embodiment of the present invention, the box-like section is rectangular, rectangular with a rounded bottom, rectangular with rounded edges, hexagonal, oval or other.
In variant solutions, the guide blocks may be disposed only in one lateral flank or in both lateral flanks.
Furthermore, in variant solutions, the guide blocks may be disposed in one platband, upper or lower, or in both.
In other variants, the guide blocks may be disposed in both lateral flanks and the upper and lower platbands.
Another variant provides, in one and/or the other of the lateral flanks and/or in one and/or the other of the platbands, a plurality of guide blocks associated in a longitudinal and/or transverse direction.
In one solution according to the present invention, the composite material used to make the segments is based on carbon fibers.
According to a variant, the composite material used to make the segments is based on aramid fibers (Kevlar®), or other similar or comparable fibers, having characteristics substantially similar to carbon in terms of resistance and rigidity.
According to some forms of embodiment of the present invention, the composite material is made with unidirectional fibers, or, according to a variant, is made with interwoven and/or interlaced fibers, or fibers with a random distribution.
In some variants, the composite material is made with a mixture of long fibers and short fibers, with a random or predetermined disposition.
According to a variant of the invention, the composite material used to make the tubular central part is the multi-layer type.
The making of the segments of the arm with a base of composite material makes it much lighter overall than known arms, which are made completely of metal.
Thus, the use of composite material to make the telescopic arm of the crane in question allows to increase the length and/or the number of segments used given the same overall weight; alternatively, the invention allows to reduce the overall weight given the same length and/or number of segments.
Each segment of composite can be made as described in the European patent applications EP-B-1.970.344 and EP-B-2.039.498 and the Italian patent for industrial invention M12011A000273 in the name of the Applican.
Moreover, in some forms of embodiment, the arm according to the present invention could be made by providing terminal ends of the segments at least partly made of metal, as described in the Italian patent application in the name of the Applicant filed on the same day as the present application with application number MI2011A000856 and entitled "BRACCIO TELESCOPICO PER GRU E GRU COMPRENDENTE TALE BRACCIO."
Moreover, in some forms of embodiment, the arm according to the present invention could be made by also providing guide blocks disposed applied on the internal and/or external surfaces of the segments, as described in the Italian patent application in the name of the Applicant filed on the same day as the present application with application number MI2011A000862 and entitled "BRACCIO TELESCOPICO PER GRU E GRU COMPRENDENTE TALE BRACCIO."
The present invention also concerns a method to make a telescopic arm for a crane able to lift a heavy load and comprising a plurality of tubular segments which extend along a common central longitudinal axis, each of the segments being able to slide telescopically inside or outside another adjacent segment and consecutive along a central longitudinal axis, between a retracted position and a completely or partly extended position.
The method provides to make each of the segments consisting of a composite material based on reinforcement fibers and thermosetting resin and to dispose guide blocks between the at least one segment and at least another segment adjacent and consecutive along a central longitudinal axis, the guide blocks being at least partly drowned in the composite material in corresponding internal surfaces of the at least one segment and external surfaces of the other segment, to guide the reciprocal sliding thereof.
The method also provides to dispose the guide blocks continuously along the whole length of the corresponding segment in which they are at least partly drowned, or discretely in correspondence with the usable work part of the length. According to the present invention, each of the guide blocks is obtained from a first internal guide element disposed at least partly drowned in the internal surface of the corresponding segment and a second external guide element disposed at least partly drowned in a position coordinated with the corresponding first internal guide element, in the external surface of the corresponding segment.
Each first guide element is made with coupling means to cooperate directly in contact with mating coupling counter-means of the second guide element, both to guide the reciprocal sliding of the segments and to receive and support, directly on the metal of which the guide blocks are made and substantially along the whole length of the segments, the components of force transverse to the segments, due to the stresses deriving from the heavy load lifted by the arm.
One form of embodiment of the segments of the telescopic arm in question provides a first step in which a suitable quantity of layers of fiber which can already be impregnated or impregnated in a subsequent step, is deposited inside the volume of appropriate female molds.
An alternative form of embodiment provides a first step in which one or more layers of fiber are distributed, together with or without the resinous matrix (that is, pre-impregnated or dry fibers) on the external surface of a mold of the male type.
The method then provides a possible second step in which the composite material is subjected to a heat treatment process, typically in autoclave, and a third step in which the necessary refining mechanical workings are carried out on the segment obtained.
According to this variant, in the course of the first step, or between the first and the second step, the guide blocks are positioned in a region destined to define the internal and/or external surface of the final segment, so that they are at least partly drowned in suitable seatings in the composite material.
The production method of the composite telescopic arm, while not being restrictive, is preferably selected between one or another of the techniques of filament winding, manual deposition of pre-pregs with polymerization in autoclave, poltrusion, hot vacuum, RTM (Resin Transfer Molding), the infusion technique or other similar or comparable method to one or another of the methods indicated above.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other characteristics of the present invention will become apparent from the following description of a preferential form of embodiment, given as a non-restrictive example with reference to the attached drawings wherein:

fig. 1 is a schematic view of an arm according to the present invention;
fig. 2 is an enlarged detail of a section of one part of fig. 1;
fig. 3 is a cross section from III to III in fig. 2;
fig. 4 is an enlarged detail of fig. 3;
fig. 5 is a cross section of a variant of the present invention;
fig. 6 is an enlarged detail of fig. 5;
fig. 7 is a section of another variant of the present invention.

To facilitate comprehension, the same reference numbers have been used, where possible, to identify identical common elements in the drawings.

DETAILED DESCRIPTION OF SOME PREFERENTIAL FORMS OF

EMBODIMENT

With reference to the attached drawings, a telescopic arm according to the present invention is indicated with the number 10, and can be used in a crane for lifting loads, only schematically shown in fig. 1, for example in the building or construction sector.
The telescopic arm 10 comprises a plurality of segments 12, 14, 16 sliding one inside the other, which can be extended linearly to assume an extended operating condition, or retracted one inside the other in a non-operative condition or when the machine is stopped, or to assume intermediate positions between the two limit positions.
The segments 12, 14, 16 have a box-like cross section, defining corresponding axial positioning seatings 25, 27 inside, to accommodate the segments in their retracted position.
The segments 12, 14, 16 have mating reciprocal section shape and size so as to be able to be inserted axially, where possible or required, in the respective axial positioning seatings 25, 27, according to the usual telescopic configuration.
The box-like cross section of the segments 12, 14, 16 also defines upper or lower platbands, that is, the surfaces of the segments that are transverse to the component of the load lifted perpendicular to the arm, and lateral flanks, that is, the surfaces of the segments that are parallel to the perpendicular component.
In this case, a telescopic arm 10 is shown comprising:

a first lower segment 12, angularly constrained in correspondence to a lower end 20 to a support plane by hinging means 42 and associated to a hydraulic cylinder, or other linear actuator 44, which allows the movement of extension/retraction of the corresponding segment of the arm;
a second segment 14 in an intermediate position;
a third upper segment 16, which has an upper end 30 to which a hook 29 is associated to lift the load 31. According to one form of embodiment, the upper terminal end 30 of the third segment 16 may be associated with reinforcement elements, of the band type, which also function as stopping means in the non-operative position.

According to the present invention, at least one, advantageously each, of the segments 12, 14, 16 is made of composite material, in this case based on carbon fibers, which makes them much lighter than state-of-the-art segments.
According to the present invention, the arm 10 comprises guide blocks or other equivalent means disposed between the segments 12, 14, 16, at least partly drowned in the composite material in surfaces of direct cooperation of the segments 12, 14, 16, to facilitate the reciprocal sliding thereof (figs. 2, 3 and 5). The guide blocks 45 of the present invention determine a distribution of the loads, in the working steps, which is favorable for the composite material, that is, a distribution of the loads parallel, as indicated by the arrows D in figs. 4 and 6, to the fibers of the material instead of perpendicular.
The guide blocks are made of metal material, such as for example a material based on steel or titanium or alloys thereof, or other equivalent material resistant to sliding wear.
The guide blocks may be provided along the whole length of the segments 12, 14, 16, or only on one part, advantageously the usable work part that affects the segments 12, 14, 16 in the various operating conditions.
Hereafter we shall refer, for convenience of explanation and simply as an example, to the second 14 and third 16 segment slidingly coupled by means of guide blocks 45 applied for cooperation of the external surface 36 of the third segment 16 with the internal surface 37 of the second segment 14 (figs. 4 and 6). The guide blocks 45 allow the segments 14, 16 to slide in the typical telescopic movement of the arm 10 of the crane in question.
It is clear that this description also applies for guide blocks, for convenience not shown in the attached drawings but substantially identical to the guide blocks 45, which allow the reciprocal guided sliding of the first 12 and second 14 segment, part of which is shown in figs. 4 and 6, indicated in dashes by the reference number 47, at least partly drowned in the internal surface 38 of the second segment 14.
Furthermore, the guide blocks 45 could be at least partly drowned, also more than one, in only one or both of the lateral flanks of the segments 12, 14, 16, in correspondence with the central tract as in figs. 3 and 4, or in correspondence with the shoulders connecting with the platbands as in figs. 5 and 6, and/or in only one or both the lower and upper platbands. One variant, not shown, may also provide a combination of guide blocks 45 disposed as in figs. 3, 4 and 5, 6, that is, on the lateral flanks and also on the connecting shoulders between the lateral flanks and the platbands.
Each guide block 45 is formed in this case by a first internal guide element 49 at least partly drowned in the composite material of the internal surface 37 of the second segment 14 and a second external guide element 51 at least partly drowned in the composite material of the external surface 36 of the third segment 16, coupled slidingly in a position coordinated with the corresponding first guide element 49.
As can be seen in figs. 4 and 6, the first guide element 49 is at least partly drowned in a seating 57 made in the composite material of the internal surface 37 of the second segment 14, while the second guide element 51 is at least partly drowned in a seating 55 made in the composite material of the external surface 36 of the third segment 16.
The part of the block 47 at least partly drowned in the composite material of the internal surface 38 of the second segment 14, which is totally identical to the second guide element 51, is at least partly drowned in a seating 65 made in the external surface 38 of the second segment 14. Moreover, the part of the block 47 in turn will be slidingly coupled with a mating part of a block applied on the internal surface of the first segment 12, not shown here, which is totally identical to the first guide element 49.
In this case, the first guide element 49 has coupling means 61 configured to cooperate in guided sliding with mating coupling counter-means 59 of the second guide element 51.
The other blocks have a similar configuration, as can be seen for the part of a block 47 which has coupling counter-means 63 to cooperate with coupling means of another part of a block, analogous to the first guide element 49, provided in the first segment 12.
The coupling means 61 of the first guide element 49 can be the female type, comprising a hollow, a groove or other longitudinal seating and coordinately the coupling counter-means 59, 63 of the second guide element 51, 47 may be the male type, comprising a tooth, a rib or other longitudinal protruding portion of mating sizes. Consequently, in the variant shown as an example in fig. 4, the first guide element 49 is completely drowned in the internal surface 37 of the second segment 14, with the coupling means 61 included in the bulk of the latter, while the second guide element 51 is partly drowned in the external surface 36 of the third segment 16, with the coupling counter-means 59 protruding from the latter.
Or, in a mechanically equivalent manner, the coupling means 61 can be the male type and the coupling counter-means 59, 63 of the second guide element 51, 47 can be the female type. Consequently, in the variant not shown, the first guide element 49 is partly drowned in the internal surface 37 of the second segment 14, with the coupling means 61 protruding from it, while the second guide element 51, 47 is completely drowned in the external surface 36 of the third segment 16, with the coupling counter-means 59, 63 included in the bulk of the latter.
Fig. 7 shows a variant embodiment of the present invention, which provides to make the first guide element 49 in a single body, indicated for convenience by the reference number 67, with the coordinated part of block 47, disposed in a mating position in the same second segment 14, of the guide block that is directly adjacent, not completely shown in the drawings, which allows sliding of the first segment 12 and second segment 14. in this solution, the block in the single body 67 therefore has both the coupling means 61, whether they are male or female, and also the coupling counter-means 63, whether they are male or female. By single body we mean both the solution where the first guide element 49 and the coordinated part of the block 47 do not have a break in continuity of material with each other, since they are a single piece, and also the solution where the first guide element 49 and the coordinated part of block 47 are made starting from two distinct components that are put together and solidly constrained with each other.

Claims

Telescopic arm for cranes able to lift a heavy load, comprising a plurality of tubular segments (12, 14, 16) which extend along a common central longitudinal axis, each of said segments (12, 14, 16) being able to slide telescopically inside or outside another segment (12, 14, 16) adjacent and consecutive along said central longitudinal axis, between a retracted position and a completely or partly extended position, characterized in that each of said segments (12, 14, 16) is made of a composite material with a base of reinforcement fibers and thermosetting resin and in that it comprises longitudinal guide blocks (45) made of metal with high mechanical characteristics of strength and disposed between one segment (12, 14, 16) and at least another segment (12, 14, 16) adjacent and consecutive along said central longitudinal axis, which guide blocks (45) are at least partly drowned in the composite material in corresponding internal surfaces (37) of said at least one segment (12, 14, 16) and external surfaces (36, 38) of said other segment (12, 14, 16), said guide blocks (45) developing continuously along the whole length of the corresponding segment (12, 14, 16) in which they are partly drowned, or discretely in correspondence to the usable work part of said length, each of said guide blocks (45) comprising a first internal guide element (49) at least partly drowned in the internal surface (37) of the corresponding segment (12, 14, 16), and a second external guide element (51, 47), at least partly drowned, in a position coordinated with the corresponding first internal guide element (49), in the external surface (36, 38) of the corresponding segment (12, 14, 16) each first guide element (49) having coupling means (61) to cooperate directly in contact with mating coupling counter-means (59, 63) of the second guide element (51, 47), both to guide the reciprocal sliding of the segments (12, 14, 16) and also to receive and support, directly on the metal of which the guide blocks (45) are made and substantially along the whole length of the segments (12, 14, 16), the components of force transverse to said segments (12, 14, 16), due to the stresses deriving from the heavy load lifted by the arm.
Arm as in claim 1, characterized in that said longitudinal guide blocks (45) are made of material resistant to wear caused by sliding.
Arm as in any claim hereinbefore, characterized in that said segments (12, 14, 16) have a box-like section, defining with respect to the component of the load raised perpendicular to the arm, upper and lower platbands and lateral flanks, and in that said longitudinal guide blocks (45), at least partly drowned or incorporated, are provided on one and/or the other of said upper and lower platbands and/or on one and/or the other of said lateral flanks.
Arm as in any claim hereinbefore, characterized in that the first guide element (49) of one guide block (45) is made in a single piece with a coordinated second guide element (47), disposed in a mating position in the same segment (12, 14, 16) of another directly adjacent guide block which allows the guided sliding of the same segment (12, 14, 16) and another segment (12, 14, 16) so as to define a block in a single body (67) which cooperates on one side with a second guide element (51) and on the other side with another first guide element.
Crane comprising a telescopic arm as in any claim hereinbefore.
Method to achieve a telescopic arm for cranes suitable to lift a heavy load, comprising a plurality of tubular segments (12, 14, 16) which extend along a central common axis, each of said segments (12, 14, 16) being able to slide telescopically inside and outside of another segment (12, 14, 16) adjacent and consecutive along said central longitudinal axis, between a retracted position and a completely extended position, characterized in that it provides to achieve at least one of said segments (12, 14, 16) made of composite material with a base of reinforced fibers and thermosetting resin and to dispose guide blocks (45) between one segment (12, 14, 16) and at least another segment (12, 14, 16) adjacent and consecutive along said central longitudinal axis, which guide blocks (45) are at least partly drowned or incorporated in the composite material in corresponding internal surfaces (37) of said at least one segment (12, 14, 16) and external surfaces (36, 38) of said other segment (12, 14, 16), said method providing to achieve said guide blocks (45) in a continuous way along the whole length of the corresponding segment (12, 14, 16) in which they are partly drowned, or in a discrete way in correspondence to the useful working part of said length, each of said guide blocks (45) comprising a first internal guide element (49) at least partly drowned in the internal surface (37) of the corresponding segment (12, 14, 16), and a second external guide element (51, 47), at least partly drowned, in a position coordinated with the corresponding first internal guide element (49), in the external surface (36, 38) of the corresponding segment (12, 14, 16) each first guide element (49) having coupling means (61) to cooperate in sliding with mating coupling counter-means (59, 63) of the second guide element (51, 47), both to guide the reciprocal sliding between the segments (12, 14, 16) and to receive and support, directly on the metal of which the guide blocks (45) are made and substantially along the whole length of the segments (12, 14, 16), the strength components to said segments (12, 14, 16) due to the stresses deriving from heavy loads lifted by the arm.