The invention refers to a bearing assembly in a mobile hydraulic crane telescopic arm and a mobile hydraulic crane comprising such assembly. According to the International Patent Classification such inventions belong to the field of mobile jib cranes comprising a telescopic arm, namely to the class B 66 C 23/687.

The invention is based on a problem, how to design a bearing assembly in a mobile crane telescopic arm, which should comprise at least two tubular bearing sections, which would have to be inserted within each other and telescopically moveable in the axial direction thereof, wherein on one hand the weight of each of the bearing sections with regard to its maximum allowable carrying capacity while taking into consideration the bending stresses and other stresses during the moving of each load during each regular use of the crane, should be minimized, and at the same time also the shape of each profile, namely the transversal cross-section of each tubular bearing section, should be determined in such manner, that each contact between each mutually overlapping sections should be acceptable both in view of wear as well as in view stresses and deformations even by maximally extracted and maximally loaded telescopic arm, namely in situation where each bending stresses can be transferred from one to another bearing section to another only within relatively narrow local areas, in which said bearing sections remain mutually overlapped. Mobile hydraulic cranes with telescopic arms are widely known from the state of the art, and some of them are also disclosed in patent literature, e.g. in EP 2 789 566 Al, EP 2 683 645 Al, WO 98/17576 A1 and other sources. A telescopic mobile hydraulic crane typically comprises a bearing platform, which can be attached to each vehicle and is generally fitted with at least two telescopically conceived support legs for supporting the crane during the use, by means of which a required stability against leaning and over turning of the crane and/or vehicle is ensured, especially while bearing loads. On said platform, a column is attached with in such manner, that it can be rotated around the vertical axis by means of a suitable driving means, on the free end of which a primary arm of the crane is attached with its first end portion in such manner, that it can be pivoted around the horizontal axis and is simultaneously supported by a hydraulic cylinder, which is pivotally attached to said column. On the opposite free terminal portion of the primary arm a secondary arm is attached, which can also be pivoted around the horizontal geometric axis and is telescopically conceived. Said secondary arm is pivotable relatively to the primary arm around said vertical axis by means of a hydraulic cylinder, which is on one side pivotally connected to the primary arm and is on the other side either directly or indirectly by means of a suitable linking mechanism connected to the secondary arm. Said telescopically conceived second arm consists of at least two tubular bearing sections, which are inserted within each other, wherein the inner bearing section is allowed either to be pushed axially towards the interior of the external bearing section, or to be retracted at certain extent from the external bearing section, which can usually be performed by means of a roller chain, wherein a sufficient area of overlapping must be assured between said bearing sections in order to ensure each required necessary bending strength of such an assembly and consequently the required carrying capacity of the crane. On the free terminal portion of the inner nearing section a mounting point is foreseen, to which either directly or indirectly by means of a suitable hydraulic rotational unit a grabber is attached, which is suitable for manipulating with each load and which is usually powered by hydraulic driving means. In general, such telescopic arm comprises at least two tubular bearing sections, which are inserted within each other, wherein each available bearing section is subsequently by means of a suitable driving means, e.g. by means of a roller chain as installed inside the telescope, telescopically moveable from its initial retracted position, in which all the bearing sections are positioned within each other and are all located together inside of the external bearing section, into its extracted position, in which all bearing sections are as much as possible outwardly extended in their axial direction, so that each inner section is to at certain extend extracted from each external bearing section collaborating there-with, however each pair of mutually collaborating sections are still mutually overlapped at least to such extent that the whole telescopic arm still remains capable of withstanding certain bending stresses, which result on one hand from the weight of the telescope as such, and on the other hand also from each load, which can be attached to the free terminal portion of such telescopic arm.

Realisation and exploitation of such telescopic arm in described cranes are connected to several important aspects and consequently also to certain compromises. At the very beginning, it should be bom in mind that in the case of mobile cranes the dimensions of the crane in its collapsed state, in which it is suitable for transporting, are upwardly limited on the basis of strict regulations, which must be taken into account, since otherwise the crane could excessively extend over the width of the vehicle and as such, together with its vehicle, might not be allowed appear in the common road traffic freely and without employing special additional measures. This of course means, that the whole length of the telescopic arm in its initial position, when the bearing sections are completely retracted and are placed within each other, is upwardly limited. On the other hand, the primary goal of installing such telescopic arm on a crane is without doubt to extend the reach of the crane as much as possible, so that it is desired, if the telescopic assembly could be extracted as much as possible. However, the telescope is from a quite practical aspect only extendable to such extent, in which that in the case of completely extracted telescope each remaining areas of mutual overlapping of each mutually collaborating and within teach other inserted bearing sections are still sufficient to ensure withstanding to bending and other stresses, which result from the weight of the load, when carried by the crane, from weight of the bearing sections of the telescopic arm and also from other influences, which unavoidably appear during the use of a crane with or without carrying a load, like e.g. inertia forces and similar impacts. It will be understandable to each person skilled in the art that from the perspective of stresses and deformations, e.g. when lifting and transferring each load with by means of a partially or fully extracted telescopic arm, in particular the areas of mutual overlapping between each mutually collaborating bearing sections are exposed to extreme stresses, wherein in practice a possibility must be always assured, that either a partially or fully extended and maximally loaded telescopic arm must still be functional not only from the perspective of carrying capacity, but also from the perspective of mobility of individual bearing sections relative to each other, which means that even in such case either controlled retraction or controlled extraction of the telescopic arm to its maximal length must be enabled, since otherwise the crane practically could not be controlled by carrying of the load. Critically high loads and elastic deformations in said overlapping areas between mutually collaborating bearing sections of the telescopic arm is therefore a practical reason, why in most currently used mobile telescopic cranes such telescopic arm is still available as a bulky assembly consisting of heavy and thick-walled square or rectangular tubes, and manufactures of cranes precautionary insist in relatively large areas of mutual overlapping of bearing sections in position of maximally extracted telescopic arm, which the results in a relatively small expansion degree of the telescope. This of course means, that on one side the reach of the telescopic arm, and therefore the action radius of the entire crane is significantly smaller than it could be the one achieved by some more thought-through embodiment of the telescopic arm, and on the other hand due to said thick-walled tubular design of said bearing sections also the weight of the telescopic arm as such is relatively high, which during the use of the crane any by using of each disposable driving means for displacing said telescopic arm, results in essential reduction of effective carrying capacity of the crane in position, where the telescopic arm is extracted. The previously mentioned shortcomings and contradictions thus far refer only to the subject of stresses, which may act to the crane during the use thereof. In addition to the higher expenses as a resulting from construction by using heavier telescopic arm and more energy consumption by manufacturing such a crane, a further aspect should also be bom in mind, namely that whenever such crane is mounted onto a motor vehicle, the total mass and carrying capacity of which are during use in the road traffic also limited, which means that each increasing of the weight of the crane itself results in decreasing of each remaining still available carrying capacity, namely effective carrying capacity of the vehicle, which remains available for transporting of other loads. Such enlarged weight of the crane in combination with position of the centre of mass, which is located relatively high above the ground, essentially influences the driving performances of the entire vehicle, including the fuel consumption. The presence of the additional mass on the vehicle during driving furthermore leads to additional inertia forces, which are transferred from the telescopic arm towards the column and the bearing platform and subsequently towards to the bearing construction of the entire vehicle, which means that each superfluous mass of the telescopic arm on the crane results in additional stresses within numerous other bearing elements of the entire mobile crane and each belonging vehicle.

In order to increase the bending strength of the telescopic arm by means of a simple constructional design, a telescopic arm of a mobile crane is proposed in EP 0 583 552 Bl, which also comprises tubular bearing sections, wherein however each of said tubular bearing sections, which are inserted within each other and are telescopically moveable along each other, and is designed with a characteristic transversal cross-section, which is mirror-symmetric with regard to the vertical geometric axis and is on its lower i.e. towards the ground facing side, which is by bending exposed to compression stresses, rounded and is actually shaped as a part of a circle or an ellipse, and is on its upper i.e. away from the ground facing side, which is by bending exposed to tension stresses, designed as a cup with a flat central surface, which, across slightly rounded edge areas, transits into straight sections, which extend towards the sections of said rounded lower part, wherein each converging sections in the area of said transition from said cup-shaped upper side and said rounded lower side are welded with each other in the area of tension stresses above the neutral axis. Such a solution is relatively problematic due to the straight surface in the area, which is exposed to torsional stresses, which is, in the area of mutual overlapping of each internal bearing section and each external bearing section, especially in the final area of each bearing section, locally exposed to significant surface stresses, which results in essential wearing and sometimes even leads to plastic deformations. The wall thickness can in such a case be somewhat smaller than the one employed in classical thick-walled tubular carrying sections, but must nevertheless still maintained relatively significant and sufficient to enable such designed bearing section to retain its shape, carrying capacity and stability when exposed to stresses. Furthermore, such construction of bearing sections with a transversal cross-section, which comprises three rounded areas, is exceptionally complicated in view of manufacturing thereof, starting from achieving each required accuracy, which effects the quality of the welds and the accuracy of each formed bearing section, which should conform to a mirror symmetry and should also be linearly aligned, since on the contrary severe problems could occur in view of mutual collaboration between the bearing sections of the telescopic arm, and could also lead to a pretty unpredictable distribution of stresses due to discrepancies in view of geometry as such. A further problem refers to positioning of the welds between the rounded and cup-shaped side, since welds are positioned on the side, which is exposed to tension stress, and are therefore each per se exposed to combination of tension and shear forces, which can in the dynamically stressed welds especially during intensive long-term exploitation of the crane lead to serious risk of collapsing.

By taking into consideration previously expressed information, a tubular profile of a telescopic arm in a crane is also disclosed in EP 2 185 462 Bl, wherein the lower bearing area thereof, which is by bending exposed to compression stresses, is generally semi-circularly shaped, while both sections attached thereto are straight and are directed towards each other and are in the opposite area, which is by bending of the bearing section exposed to tension stresses, shaped as a widened, downwards and towards said semi-circular area facing letter V. Regarding the previously mentioned approach it is foreseen, that a central area of said substantially semi-circular area, i.e. the vertex area is made out of straight sections, which join together at an obtuse angle, wherein both sheet metals in the area of their mutual connection form an angle, which is suitable for creating a weld without requiring any essential pre-treatment, and said weld is arranged in the compression zone. Such a profile is probably stronger than the one mentioned in EP 0 583 552 Bl, however on both sides of the profile relative large straight surfaces remain available, which might to certain extent contribute to withstanding bending in the vertical plane, however without additional reinforcements have relative poor bearing performance against impacts from other directions. Furthermore, the state of stresses and deformations in the overlapping areas between each pair of within each other insertable profiles of the telescopic arm under bending stresses is in this case relatively unfavourable, especially in each of the end areas of said V-shaped zones in the area exposed to torsional stress of the telescope under bending stress, namely because the outer edge of the internal section protrudes towards the interior of the external bearing section by showing the tendency of pushing away or widening thereof, which is on one hand unfavourable from the perspective of strength and on the other hand from the perspective of surface pressure in the contact areas, which leads to severe wearing which can be enormously problematic during the long-term exploitation of a crane with such a telescopic arm. Excessive deformations and the possibility of widening each external section can in this case be resolved exclusively by increasing the wall thickness of the profile, which however leads to additional weight of the telescope and to already mentioned deficiencies resulting from such increasing of weight.

In order to achieve the purpose of creating each bearing section of the telescopically designed section in a mobile crane from a single piece of sheet metal, the thickness of which should be smaller than that in the previously known solutions from the state of the art, in EP 2 185 461 Bl a profile cross-section is suggested, which is on its lower side, which is by bending in the vertical direction exposed to compression stresses, semi-circularly shaped, wherein in the top two thirds it is narrowed in the area of straight and towards the remaining side converging walls, namely on that side, which is by bending in the vertical direction exposed to tension stress, so that said straight walls on one side pass into said semi-circular area in the compression zone, and on the other hand pass into the narrowed area in the tension zone, which is conceived as a V-shaped area with distinctly opened and in the direction towards said walls extending sections. When starting from such shape of cross-section, which actually matches the one from the previously mentioned document EP 2 185 462 Bl, the problems related to critical stresses, surface pressure and wearing in substantially stressed areas in mutually overlapping areas between each two collaborating bearing sections of each completely extended telescopic arm are in this case remain substantially similar. Despite to initially emphasised goal like in EP 2 185 461 Bl, that the section would have to be made out of a single piece of sheet metal, in said source some different possibilities are foreseen from the perspective of assembling a profile by using two symmetrical cups, which would be welded in the area of both vertex points in the vertical plane, namely exactly in that points, which are exposed to extreme compression or tensional stresses. Additionally, in said document a possibility of using a material of different thicknesses along the same contour of the profile is foreseen, wherein the material would be thickened towards the inside. All these measures generally confirm that local stresses and deformations in the case of thinning the wall thickness, especially within each overlapping areas between each mutually collaborating bearing sections in the state, when the telescopic arm is under stress, may no doubt reach or even exceed critical values, so that any reasonable thinning of the walls in these areas is practically not feasible. It is also worth to express that introduction of different wall thicknesses and subsequent welding makes the process substantially more expensive while from quite technical point of view it is also much more difficult for carrying it out, because in addition to increasing the weight, by welding also some additional internal tensions may occur in combination with hardly predictable deformations, which can even subsequently have essential impact to geometries of each bearing section during use of the crane and might therefore influence the actual state of stresses in one or more bearing sections of the telescopic arm, which may jeopardize accuracy of collaboration between said bearing sections.

The present invention is also dealing with such bearing assembly in a mobile hydraulic crane telescopic arm, which generally comprises at least two tubular bearing sections, which are inserted within each the other and are by means of a suitable driving means in a controlled manner telescopically moveable along each other in the axial direction thereof, namely an outer tubular bearing section having a pre-determined length and an inner tubular bearing section having a pre determined length, so that thanks to their similarity and complementary shape of their transversal cross-sections, insertion of one into another is enabled by simultaneously assuring of at least minimal overlapping length Lg and also at least approximately uniform spacing between said sections along the complete circumference thereof. Suitable sliding pads are inserted within each gap between each adjacent bearing sections, which are correspondingly spaced apart from each other in the axial direction of the arm and are also properly arranged along the circumference of said sections in order to enable suitable matching between said mutually abutting bearing sections by simultaneously allowing each required movements relatively to each other in the axial direction in order to maintain the friction as low as possible. Each of said tubular bearing sections, when observed in its cross-section, is designed with a substantially uniform wall thickness along the complete circumference thereof, while at the same time said cross-section is also mirror-symmetric with regard to the vertical geometric axis, in the direction of which the load weight force extends in situation, when said assembly is during the use of the crane exposed exclusively to static loads and is exposed to stresses resulting from a bending moment around the horizontal geometrical axis as a neutral axis of each effective bearing cross-section of each bearing section, below which a compression zone is located, while a tension zone is located above said neutral geometrical axis.

In the context of resolving of the initially presented technical problem, the invention proposes that each bearing section is in its cross-section designed in such manner, that in the area of said tension zone above said neutral axis and at a suitable distance apart from said neutral axis a smaller substantially semi-circular area with a smaller radius is arranged, while in the area within the compression zone below said neutral axis a larger substantially semi-circular area with a larger radius is arranged, which ends in its terminal points below the neutral axis and at a suitable distance apart from it, and is on its both sides symmetrically with regard to the vertical axis tangentially extended by straight sections, which extend across said neutral axis, so that each of them at a pre-determined distance apart from said neutral axis and at an obtuse angle and symmetrically with regard to the vertical axis, coincides with a complementary straight section, by which said substantially semi-circular area with a smaller radius is extended in each of its terminal points.

In one of the embodiments of the invention, said straight sections, which extend tangentially with respect to said larger substantially semi-circular area with a larger radius and protrude above said neutral axis, are arranged parallel with each other.

In a further embodiment of the invention, said straight sections, which extend tangentially with regard to said larger substantially semi-circular area with a larger radius and protrude above said neutral axis, are inclined relatively to each other and symmetrically with regard to said vertical axis converge towards the said smaller substantially semi-circular area with a smaller radius.

Therein the radius R _t of said smaller substantially semi-circular area within the tension zone above the neutral axis and the radius R2 of the larger substantially semi-circular area within the compression zone below the neutral zone are determined in such a manner that the condition 1/4 < (R/R ₂ ) £ 3/4 is fulfilled.

The height h of each tubular bearing section, namely the distance between the vertex points of said substantially semi-circular areas, relative to the width b of the tubular bearing section, which corresponds to diameter of the larger substantially semi-circular area within the compression zone below the neutral axis, is determined in such manner that the condition 1/2 < (b/h) < 4/5 is fulfilled, wherein the preferred ratio between the width b and the height h is estimated to approximately 3/4.

The length d, which presents the distance between the neutral axis and each intersection between each tangentially from the smaller substantially semi-circular area within the tension zone above the neutral axis extending straight section and each tangentially from the larger substantially semi-circular area within the compression zone below the neutral axis extending straight section in the direction towards the smaller substantially semi-circular area is with regard to the total height h of said tubular bearing section determined in such a manner that the condition hi 5 £ d£ hi A is fulfilled.

Additionally, the angle g between each tangentially from the smaller substantially semi-circular area within the tension zone above the neutral axis extending straight section and each belonging tangentially from the larger substantially semi-circular area within the compression zone below the neutral axis extending straight section is selected within the range 140° < g£ 170°.

In the embodiment of the invention, wherein the straight sections, which extend tangentially with respect to said larger substantially semi-circular area with a larger radius and protrude above said neutral axis, are not arranged parallel with each other, the angle b between the normal line of each tangentially from the larger substantially semi-circular area within the compression zone below the neutral axis extending straight section and the neutral bending axis is selected within the range 0 < b < 25°. Taking this into account, the wall thickness t of each tubular bearing section is selected within the range 3 mm < t < b! 20, furthermore the shortest length L ₉ of the mutually overlapping area in the state, in which each internal tubular bearing section consisting of steel is in its axial direction maximally extracted from each external tubular bearing section consisting of steel, with regard to the height h of said internal tubular bearing section fulfils the condition 1.5 h < Lg £ 3 h.

In the cases where each bearing section in the telescopic bearing assembly consists of a cold formed steel plate, it can be shaped as a continuous shell and subsequently welded in the area of the vertex point on the larger substantially semi-circular area within the compression zone below the neutral axis, or in the areas of transition from the larger substantially semi-circular area into each belonging tangential straight sections within the compression zone below the neutral axis. Whenever each of said substantially semi-circular areas above and below the neutral axis is, either for the purpose of simplifying manufacturing or in order to employ existing technological equipment realized by bending of a metallic sheet having a pre-determined thickness, and is suitably approximated by means of a regular equilateral polygon, which is an at least 16 sided polygon, namely a regular equilateral polygon, wherein the number of vertices exceeds 16, and is preferably an at least 24 sided polygon.

A mobile telescopic hydraulic crane is furthermore foreseen according to the invention, which comprises a bearing platform, which is adapted for mounting of said crane on each motor vehicle and is optionally furnished with at least a pair of telescopic supporting legs, which are suitable for supporting said crane on each ground during transporting of each load in order to ensure its carrying capacity and stability. A column is in the area of its first terminal portion pivotally around the vertical geometric axis attached to said platform, wherein a primary bearing arm of the crane is in the area of its first terminal portion pivotally around the horizontal geometric axis attached to the second terminal portion of said column and is on said column supported and pivoted around said horizontal geometrical axis by means of a hydraulic cylinder, which is pivotally connected on one side with said column and on the other side with said primary bearing arm. A telescopically conceived secondary bearing arm of the crane is with its first end portion pivotally around the horizontal geometric axis attached to the second terminal portion of said primary arm and is on its second free terminal portion equipped with an attachment point, which is suitable for mounting a grabber or any other suitable assembly for manipulating with each load. Furthermore, said telescopic secondary arm is on said primary arm supported and pivoted around said horizontal geometric axis by means of a hydraulic cylinder, which is on the one hand either directly or indirectly via a suitable linking mechanism pivotally connected to said primary arm and on the other hand either directly or indirectly via a suitable linking mechanism attached to said secondary arm. Furthermore, said secondary arm comprises a telescopically in the longitudinal direction extendable bearing assembly according to any of the previously described features.

The invention will be further explained by embodiments and in connection with attached drawings, wherein

Fig. 1 presents a mobile hydraulic crane, which is suitable for mounting on a not- shown motor vehicle and comprises a telescopic arm with a bearing assembly according to the invention;

Fig. 2 shows a schematic presentation of a telescopic bearing assembly consisting of two tubular bearing sections according to the invention, which are inserted within each other;

Fig. 3 shows the first embodiment of one of the tubular bearing sections a cross- section along the plane II - II according to Fig. 2; and Fig. 4 shows the second embodiment of one of the tubular bearing sections a cross-section along the plane II - II according to Fig. 2.

A bearing assembly 40 in a mobile hydraulic crane telescopic arm 4 according to Fig. 1 is as such presented of Fig. 2 and comprises at least two tubular bearing sections 44, 45, which are inserted within each the other and are by means of a suitable driving means in a controlled manner telescopically moveable along each other in the axial direction thereof. In the shown example and exclusively for the purposes of simplification, only one external tubular bearing section 44 having a pre-determined length L ₀ and one inner tubular bearing section 45 having a pre determined length L are shown, wherein thanks to their similarity and complementary shape of their transversal cross-sections, insertion of the internal tubular bearing section 45 into the external bearing section 44 is enabled by simultaneously assuring of at least minimal overlapping length L ₉ and also of at least approximately uniform spacing between said sections 44, 45 along the complete circumference thereof. Suitable sliding pads 46’, 46” are inserted within each gap between each adjacent bearing sections 44, 45, which are correspondingly spaced apart from each other in the axial direction of the arm 4 and are also properly arranged along the circumference of said sections 44, 45 in order to enable suitable matching between said mutually abutting bearing sections 44, 45 by simultaneously allowing each required movements relatively to each other in the axial direction X in order to maintain the friction as low as possible. Each of said tubular bearing sections 44, 45, when observed in its cross-section, is designed with a substantially uniform wall thickness t along the complete circumference thereof, while at the same time said cross-section is also mirror- symmetric with regard to the vertical geometric axis Z, in the direction of which the load FQ weight force extends in situation, where said assembly 40 is during the use of the crane exposed exclusively to static loads and is exposed to stresses resulting from a bending moment around the horizontal geometrical axis F as a neutral axis of each effective bearing cross-section of each bearing section 45, below which a compression zone is located, while a tension zone is located above said neutral geometrical axis Y.

Such telescopic assembly 40 is characterized by that each bearing section 44, 45 is in its cross-section designed in such manner, that in the area of said tension zone above said neutral axis Y and at a suitable distance apart from said neutral axis F a smaller substantially semi-circular area nti with a smaller radius Rj is arranged, while in the area within the compression zone below said neutral axis Y a larger substantially semi-circular area m ₂ with a larger radius R ₂ is arranged, which ends in its terminal points C, C below the neutral axis F and at a suitable distance apart from it, and is on its both sides symmetrically with regard to the vertical axis Z tangentially extended by straight sections n ₂ , n ₂ which extend across said neutral axis Y, so that each of them at a pre-determined distance d apart from said neutral axis F and at an obtuse angle y and symmetrically with regard to the vertical axis Z, coincides with a complementary straight section n «/, by which said substantially semi-circular area nt _j with a smaller radius R _j is extended in each of its terminal points A, A

According to one of the embodiments of each of the tubular bearing sections 44, 45 of the bearing assembly 40 according to the invention (Fig. 3), the straight sections n ₂ , n ₂ , which extend tangentially with respect to said larger substantially semi-circular area m ₂ with a larger radius R ₂ and protrude above said neutral axis Y, are arranged parallel with each other.

According to a further embodiment of each of the tubular bearing sections 44, 45 of the bearing assembly 40 according to the invention (Fig. 4), the straight sections n ₂ , n ₂ ', which extend tangentially with regard to said larger substantially semi circular area m ₂ with a larger radius R ₂ and protrude above said neutral axis F, are inclined relatively to each other and symmetrically with regard to said vertical axis Z converge towards the said smaller substantially semi-circular area nt with a smaller radius R .

The radius R _j of the smaller substantially semi-circular area m _t within the tension zone above the neutral axis F and the radius R ₂ of the larger substantially semi- circular area m ₂ within the compression zone below the neutral zone Y are determined in such a manner that the condition 1/4 < (R _J /R ₂ ) < 3/4 is fulfilled.

The height h of each tubular bearing section 44, 45, namely the distance between the vertex points E, F (Figs. 3 and 4) of said substantially semi-circular areas m ₂ , m ₂ , relative to the width b of the tubular bearing section 44, 45, which corresponds to diameter of the larger substantially semi-circular area m ₂ within the compression zone below the neutral axis Y, is determined in such manner that the condition 1/2 < (b/h) £ 4/5 is fulfilled, however the preferred ratio between the width b and the height h is estimated to approximately 3 : 4.

The length d, which presents the distance between the neutral axis Y and each intersection B, B' between each tangentially from the smaller substantially semi circular area nt within the tension zone above the neutral axis Y extending straight section n ₂ , n ₂ ’ and each tangentially from the larger substantially semi-circular area m ₂ within the compression zone below the neutral axis Y extending straight section n ₂ , n ₂ ' in the direction towards the smaller substantially semi-circular area nt is with regard to the total height h of said tubular bearing section 44, 45 determined in such a manner that the condition h/5 £ d£ A/4 is fulfilled.

The angle ^between each tangentially from the smaller substantially semi-circular area nt _j within the tension zone above the neutral axis Y extending straight section n _{h tii} and each belonging tangentially from the larger substantially semi-circular area m ₂ within the compression zone below the neutral axis Y extending straight section n ₂ , n ₂ is selected within the range 140° < g£ 170°. In the cases where the straight sections n ₂ , n ' are not parallel with each other (Fig. 4), the angle b between the normal line of each tangentially from the larger substantially semi-circular area m ₂ within the compression zone below the neutral axis Y extending straight section n ₂ , n ₂ ' and the neutral bending axis Y is selected within the range 0 < b £ 25°. In the case where the straight sections n ₂ , n ₂ are parallel with each other (Fig. 3), the condition b- 0 is fulfilled.

It would be clear to a person skilled in the art from what was described so far, that each tubular bearing section 44, 45 is in the area above the neutral axis equipped with an additional longitudinal crease, which is consists of two straight sections nl, n2; nV, n2, which converge at an obtuse angle g and each individually tangentially belong to the larger substantially semi-circular areas m _h m ₂ .

Such a concept of creating of such cross-section results in substantial reinforcement of each tubular bearing section 44, 45 from the perspective of strength and deformations and at the same time also leads to a substantially more effective collaboration between sections 44, 45 even in the case of the smallest length Lg of their mutual overlapping.

Consequently, the wall thickness t of each tubular bearing section 44, 45 can be selected within the range 3 mm < t £ (b/ 20). Furthermore, the length L of the mutually overlapping area of the sections 44, 45 in the state, in which each internal tubular bearing section 45, having the length L _} and consisting of steel is in its axial direction maximally extracted from each external tubular bearing section 44 having the length L ₀ and consisting of steel, with regard to the height h of said internal tubular bearing section 45 fulfils the condition 1.5 h < L ₉ < 3 h. In one embodiment of the invention, each of said tubular bearing sections 44, 45 in the telescopic bearing assembly 40 consists of a cold formed steel plate and is shaped as a continuous shell and welded in the area of the vertex point F on the larger substantially semi-circular area m ₂ within the compression zone below the neutral axis Y. In an alternative embodiment of the invention it is foreseen, that each of said tubular bearing sections 44, 45 in the telescopic bearing assembly 40 consists of a cold formed steel plate shaped as continuous shell and is welded in the areas C, C of transition from the larger substantially semi-circular area m ₂ into each belonging tangential straight sections n ₂ , n ₂ within the compression zone below the neutral axis Y.

However, if manufacturing of perfectly rounded or at least exactly enough rounded areas m _} , m ₂ by using each required technical equipment for such a purpose might either be unavailable or impossible or too complicated, each of said substantially semi-circular areas m _lf m ₂ above and below the neutral axis Y could also be approximated by bending of a metallic sheet having a pre-determined thickness t, and is therefore suitably approximated by a regular equilateral polygon, which is an at least 16 sided polygon, preferably an at least 24 sided polygon. Even in this case, namely by approximating of a circular arc with an inscribed or circumscribed polygon with a sufficiently large number of vertices when solving the initially presented problem, sufficiently similar conditions to those present in the case of rounded areas m ₂ , m ₂ can be achieved, which may at least in view of deformations and reliable collaboration between sections 44, 45, in practice lead to assuring of advantages of the present invention.

The scope of the invention also includes a mobile telescopic hydraulic crane with a bearing platform 1, which is adapted for mounting of said crane on each motor vehicle and is optionally furnished with at least a pair of telescopic supporting legs 11, which are suitable for supporting said crane on each ground during transporting of each load in order to ensure its carrying capacity and stability. A column 2 is in the area of its first terminal portion 21 pivotally around the vertical geometric axis attached to said platform 1, furthermore, a primary bearing arm 3 of the crane is in the area of its first terminal portion 31 pivotally around the horizontal geometric axis attached to the second terminal portion 22 of said column 2 and is on said column 2 supported and pivoted around said horizontal geometrical axis by means of a hydraulic cylinder 21 , which is pivotally connected on one side with said column 2 and on the other side with said primary bearing arm 3. A telescopically conceived secondary bearing arm 4 of the crane is by its first end portion 41 pivotally around the horizontal geometric axis attached to the second terminal portion 32 of said primary arm 3 and is on its second free terminal portion 42 equipped with an attachment point 5, which is suitable for mounting a grabber 6 or any other suitable assembly for manipulating with each load. Said telescopic secondary arm 4 is on said primary arm 3 supported and pivoted around said horizontal geometric axis by means of a hydraulic cylinder 34, which is on the one hand either directly or indirectly via a suitable linking mechanism pivotally connected to said primary arm 3 and on the other hand either directly or indirectly via a suitable linking mechanism attached to said secondary arm 4.

For such a type of mobile crane it is typical, that said secondary arm 4 comprises a telescopically in the longitudinal direction X extendable bearing assembly 40 according to any of the previously described features.