EP0726872B1

EP0726872B1 - Outer-boom crane

Info

Publication number: EP0726872B1
Application number: EP93916360A
Authority: EP
Inventors: Kent Larsson
Original assignee: Jonsered Cranes AB
Current assignee: Jonsered Cranes AB
Priority date: 1992-08-04
Filing date: 1993-07-08
Publication date: 2000-02-02
Anticipated expiration: 2013-07-08
Also published as: SE9202279D0; JPH07509682A; EP0726872A1; SE470437B; KR950702935A; FI950494A7; WO1994003389A1; FI108026B; KR0181569B1; JP3074022B2; AU4594293A; FI950494A0; ATE189445T1; DE69327807D1; SE9202279L; ES2142875T3; DE69327807T2

Abstract

An outer-boom crane comprising a first boom section (1) and an outer boom (2) which is pivotable relative to the first boom section by means of a hydraulic cylinder (12) whose one end is connected to the first boom section by a first articulation (13) and whose opposite end is connected to the outer boom (2) by a second articulation (14). Between the two booms (1, 2), there is provided at least one link device (15) which is connected to the first boom section by a third articulation (16) and to the outer boom (2) by a fourth articulation (17) which is spaced from the second articulation (14) and from the third articulation (16), more specifically with the third articulation (16) located at a greater distance from the fourth articulation (17) than the second articulation (14). By means of the link device (15), the outer boom (2) can switch articulation points so as to pivot, within its working range, about a fourth articulation (17) which is situated at a great distance from the main articulation (3) of the first boom section while, when pivoting towards and away from its parking position on the upper side of the first boom section, it is pivoted about the third articulation (16) which is situated considerably closer to the main articulation (3).

Description

Field of the Invention

This invention relates to an outer-boom crane of the type having a first boom section connected to a post by a main articulation, and an outer boom connected to the first boom section and pivotable relative thereto by means of a double-acting hydraulic cylinder which is arranged on the upper side of the first boom section and whose one end, e.g. the cylinder end, is connected to the first boom section by a first articulation and whose opposite end, e.g. the piston-rod end, is connected to the outer boom by a second articulation which is offset a certain distance transversely of an imaginary longitudinal axis through the pivotal articulation of the outer boom to permit pivoting the outer boom between different positions, not only within a working range below an imaginary extension line drawn approximately straight out from or at a small angle to the first boom section, but also in a range of motion situated above this extension line adjacent a parking position.

Background of the Invention

In certain types of outer-boom cranes, it is desirable, within the scope of a given, maximal crane length, to make the first boom section as long as possible in relation to the outer boom. This is especially pronounced in connection with forest cranes used for loading logs on road vehicles, more precisely on a load carrier which is defined by a plurality of vertical stakes mounted along the vehicle sides and having a relatively great height. For reasons of manoeuvrability, it is desirable to be able, with good clearance, to move the tip of the first boom section over the upper ends of the stakes. It is true that the load can always be moved up to a level over the stakes with the aid of the outer boom, but if the first boom section is relatively short, manoeuvring is complicated, thus making the whole loading operation quite time-consuming.
In prior-art crane constructions, however, it has not been possible to make the first boom section as long as desired in relation to the outer boom because of the demands as to geometry that are placed on the crane when to be parked. In connection with parking of at least vehicle-mounted outer-boom cranes, the first boom section is thus lowered to a position in which it is hanging obliquely downwards-outwards from the main articulation of the post while at the same time the outer boom is pivoted upwards to a position near and approximately parallel to the first boom section. The space then available is in practice limited and unconditionally decisive of the maximum length of the first boom section. For this reason, the first boom section and the outer boom are given approximately equal length to provide maximum crane reach within the working range.
In this context, it should also be noted that, in the case of forest cranes, it is of vital importance that the hydraulic cylinder for pivoting the outer boom relative to the first boom section is mounted on the upper side of the latter. In this manner, the hydraulic cylinder is protected from the shocks and impacts that are often produced when the first boom section, by inadvertent manoeuvring, hits a vertical loading stake.

Objects and Features of the Invention

The present invention aims at providing a crane construction which meets both of the above-indicated demands, in themselves contradictory, that are placed on a forest crane. A basic object of the invention thus is to provide a crane construction in which the effective length of the first boom section, i.e the distance between the main articulation and the outer boom articulation, is considerably greater than the length of the outer boom when this is moving within the working range while at the same time giving the crane, within the scope of a given parking space, a considerable maximum reach. Another object is, while meeting these demands, to maintain the location of the outer boom cylinder on the upper side of the first boom section. Yet another object of the invention is to provide the aimed-at crane construction without necessitating the use of additional hydraulic cylinders other than the above-mentioned outer boom cylinder.
According to the invention, at least the basic object thereof is achieved by the features recited in claim 1. Preferred embodiments of the invention are recited in claims 2-10.
By arranging, in accordance with the invention, a link with two articulation points between the first boom section and the outer boom, the one, outer articulation point can be used for pivoting the outer boom when this is located within the working range, while the other, inner articulation point is used for pivoting the outer boom within the parking range. In this manner, the crane can be designed with a very great effective length of the first boom section (i.e. a large distance between the main articulation of the post and said outer articulation point) in relation to the length of the outer boom, when the crane is operating within the working range, while the overall length of the link and the outer boom can be as great as the length of the first boom section, which means that the crane can be given maximum reach within the scope of a given parking space.

Brief Description of the Prior Art

The concept of arranging a link between a first boom section and an outer boom is known per se in a number of crane constructions.
In SE-B-315 089 the outer boom is connected to the first boom by means of a pivotal link. The first boom is telescopic and a removable tensile link between fixed pivot axes on the first and outer booms acts together with extension of the first boom to pivot the outer boom about a first axis towards a parking position. A double-acting hydraulic cylinder positioned below the outer boom and connected between the pivotal link and the outer boom acts to cause relative movement of the booms in the working range about a second axis.
US-A-5 016 767 discloses a crane having an outer boom connected by a first pivot to a link which in turn is connected by a second pivot to the first boom. Pivoting of the booms relative to the link is performed by respective double-acting hydraulic cylinders located above the booms and connected pivotally to respective points on the link spaced above the first and second pivots. The cylinders are operable to pivot the outer boom between a working range and a parking position.
SE-B-466 911 discloses a crane having an outer boom pivoted directly to the first boom and with an hydraulic cylinder pivoted at one end to a point above the first boom and at the other end to a pivotal link system which in turn is pivotally connected to the outer boom. Movement of the outer boom from the working range towards the parking position requires external force to move the outer boom through a dead-centre position.

Brief Description of the Drawings

In the drawings,

FIG 1: is a schematic side view showing a crane according to the invention with the first boom section in an appoximately horizontal position and with the outer boom being in a substantially straight extension of the first boom section,
FIG. 2: is a similar side view showing the two booms in a parking position,
FIGS 3-10: are side views on a smaller scale showing different positions of the outer boom when pivoting from a parking position shown in Fig. 3 on the upper side of the first boom section to an extreme swung-back position, shown in Fig. 10, within the lower working range of the outer boom,
FIG. 11: is a cut-away side view on an enlarged scale showing the link device that connects the outer boom to the first boom section,
FIG. 12: is a top plan view of the crane portion in Fig. 11,
FIGS 13-18: are schematic side views showing the function of a locking latch for locking the link relative to the outer boom and the first boom section, respectively, in the different crane positions shown in Figs 3-8, and
FIG. 19: is a plan view showing two hydraulic cylinders for rotating a post being part of the crane.

Detailed Description of the Preferred Embodiment

The crane shown in Figs 1 and 2 comprises a first boom section 1 and an outer boom 2. The first boom section 1 is connected at an inner end to a post 4 by a main articulation 3 and can be pivoted relative to the post by means of a hydraulic cylinder 5, in practice termed lift cylinder. The post 4 is mounted on a base 6, more specifically via a live ring device 7 including two hydraulic cylinder units 8, 8' (see also Fig. 19) serving to bring about rotation of the post to different rotational positions relative to the base. In practice, the base 6 may consist or be part of a supporting beam, standing on legs, which is included in a vehicle frame.
The main part of the first boom section 1 suitably is a box girder with an upper side 9 and a lower side 9'. Similarly, the main part of the outer boom 2 also consists of a box girder with an upper side 10 and a lower side 10'. At the free end or tip of the outer boom 2, there is provided a tool carrier 11 which can be mounted either directly on the outer boom or on an extendible telescopic boom incorporated therein. The tool carrier 11 may carry e.g. a rotator, from which is suspended a timber grapple. Naturally, the tool carrier 11 may carry any optional tool.
The outer boom 2 is pivotable relative to the first boom section 1 with the aid of a second hydraulic cylinder 12, in practice termed outer boom cylinder. The hydraulic cylinder 12 is mounted on the upper side of the first boom section and has its one end, in this case the cylinder end 12', connected to the first boom section by a first articulation 13 and its opposite end, in this case the piston rod end 12'', connected to the outer boom 2 by a second articulation 14. The centre of the articulation 13 is spaced a certain distance from the upper side 9 of the first boom section, such that the cylinder end 12' can occupy a position immediately adjacent and parallel to this upper side (see Fig. 3).
According to the invention, there is provided between the first boom section 1 and the outer boom 2 a link device, generally designated 15, in practice comprising two separate links or link plates 15', 15'' (see Fig. 12). This link device is connnected to the first boom section 1 by a third articulation 16 and to the outer boom by a fourth articulation 17 which is spaced from the second articulation 14 and the third articulation 16. More specifically, the third articulation 16 is located at a greater distance from the fourth articulation 17 than the second articulation 14.
As illustrated in Figs 11 and 12, the articulation 14 is made up of a transverse shaft or pin which extends between two wedge-shaped, tapering side flanges 18, 18' which project at an angle from the inner end of the outer boom 2. The pivot pin 14 is disposed at the free ends of the flanges 18, 18', which means that this articulation is offset a certain distance transversely of an imaginary longitudinal axis which is parallel to the outer boom and intersects the articulation 17 thereof. In practice, the articulation 17 also consists of a through shaft or pin, although merely illustrated by a dash-dot line in Fig. 12.
The articulation 16 consists of two spaced-apart pivot pins 16', 16'' which are mounted in flanges 19, 19' fixed on the free, outer end of the first boom section and projecting obliquely upwards from the upper side of the first boom section. In this manner, the piston rod 12'' of the outer boom cylinder 12 can be accommodated in the space between the pivot pins 16, 16'' and the flanges 19, 19', respectively. At its free end, the piston rod 12'' has a connecting ring 20 surrounding the pivot pin 14. The width of the ring 20 is considerably smaller than the distance between the wedge flanges 18, 18', which means that the pivot pin 14 is exposed over a certain distance on both sides of the ring 20.
Each of the two link plates 15', 15'' is generally angular and has, on one side, two edges 21, 22 extending at an angle to each other and joining in a point 23 located at a distance from the fourth articulation 17 which is substantially equal to the distance between the articulations 14 and 17. In practice, the angle between the edges 21 and 22 may be in the range of 150-160°. The point 23 forms an abutment for the second articulation 14. Optionally, a part-circular recess corresponding to the cylindrical shape of the pivot pin 14 may be provided in the area of this point.
At its end spaced from the fourth articulation 17, each link plate 15', 15'' has a terminal edge 24 extending transversely, e.g. at an angle of 75-85°, of the longitudinal extent of the link, the pivot pins 16', 16'' forming the third articulation being arranged in the vicinity of one end of the terminal edge 24 of the two link plates. At the opposite end of the terminal edge 24 (see also Fig. 2), there is provided a suitably semicircular recess or seat 25 adapted to cooperate with a shaft or pin 26 fixedly mounted on the tip of the first boom section and serving as a stop for the link device.
It should now be pointed out that the outer boom 2 can move in two basically distinct ranges of motion, namely one working range and one inactive range or parking range. These two ranges are separated by an intermediate position illustrated in Fig. 6, in which the outer boom projects in the extension of the first boom section, optionally at a certain, small angle in relation thereto. When the outer boom is pivoted downwards from the intermediate position shown in Fig. 6, towards the position shown in Fig. 10, it is located within the working range, while when pivoting upwards from the intermediate position shown in Fig. 6, towards the parking position shown in Fig. 3, it is moving within the parking range.
According to a preferred embodiment of the invention, the crane comprises means for locking the link device relative to the outer boom 2 when this is moving in the range between the parking position according to Fig. 3 and the intermediate position according to Fig. 6, i.e. within the parking range.
According to another preferred embodiment, the crane also comprises means for locking the link relative to the first boom section 1 when the outer boom 2 is moving within the above-mentioned working range.
According to a particularly preferred embodiment of the invention, one and the same locking means is adapted, on the one hand, to lock the link relative to the outer boom when this is moving within the parking range and, on the other hand, to lock it relative to the first boom section when the outer boom is moving within the working range. Figs 11 and 12 show, in combination with Figs 13-18, such a particularly preferred locking means which is able to lock the link device with respect to both the outer boom and the first boom section. This locking means comprises two latches 27, 27' which, via an articulation 28, are pivotally connected to the associated link plates 15, 15''. As appears, e.g. from Fig. 14, the individual latch 27 has a first, suitably fork-shaped female member 29 serving, in locking, to receive the pivot pin 14. The latch also comprises a second female member in the form of a semicircular recess 30 which can receive or at least partially embrace the pin 26 serving as a stop. Each locking latch cooperates with a guide member 31, 31' which is fixed to the upper side of the first boom section in the immediate vicinity of the boom tip. The guide member is essentially hook-shaped and has an outer, arcuately rounded hook portion 32. Each guide member 31 is located in the same vertical plane as the associated locking latch 27, such that the latch can ride on the guide member.
The function of the locking latches appears from Figs 13-18. Before going into the details of these Figures, reference is however made to Figs 3-10 which schematically illustrate different crane positions from the parking position to a lower working position, according to Fig. 10, situated close to an end position. Thus, Fig. 3 shows how the outer boom 2 in its parking position is located adjacent and substantially parallel to the first boom section 1. In this position, the outer boom cylinder 12 has been shortened to minimal length, i.e. the piston rod 12'' is maximally retracted into the cylinder 12'. Moreover, the link device 13 is locked relative to the outer boom, such that the link device and the outer boom together form a rigid unit. When the outer boom cylinder is lengthened or extended, the outer boom, together with the link device, will be pivoted outwards and upwards from the first boom section beyond the positions shown in Figs 4 and 5, up to the intermediate position illustrated in Fig. 6. In this position, further pivotal movement of the link device 15 is prevented by the recess 25 engaging the associated stop pin 26. A continued extension of the outer boom cylinder 12 will therefore entail switching of the articulation point of the outer boom from the previous articulation point 16 to the articulation point 17, such that the outer boom, upon further extension of the cylinder 12 to the different positions shown in Figs 7-10, will operate within its working range with the pivot pin 17 as articulation point. When the cylinder 12 is shortened, the crane will operate in the reverse manner. As long as the outer boom is still within the working range below the intermediate position shown in Fig. 6, the pivot pin 17 will still serve as articulation point. As soon as the outer boom reaches the intermediate position according to Fig. 6, the articulation point is however switched, so that a continued pivotal movement of the outer boom upwards towards the parking position according to Fig. 3 will take place about the articulation 16.
Reference is now again made to Figs 13-18 which illustrate the function of the locking latches 27, 27' (only the function of the latch 27 will be described hereinafter, since it is obvious that the latch 27' acts in a similar way). In the parking position of the outer boom according to Fig. 13 (corresponding to Fig. 3), the fork-shaped female member 29 engages the pivot pin 14, more precisely by engaging the exposed portion of the pin 14 located between the connecting ring 20 and the flange 18. In this position, the outer edge portion of the latch 27, or here its lower edge portion, engages the upper side of the guide member 31. When pivoting the outer boom up to and beyond the position shown in Fig. 14, the fork-shaped or female member 29 of the latch will be maintained in continuous engagement with the pivot pin 14 by the latch continuously engaging and riding on the guide member 31, which means that the link and the outer boom are locked relative to each other so as to form a rigid unit. Also in the position according to Fig. 15, the locking engagement of the latch with respect to the pivot pin 14 is maintained by the guide member 31 still preventing the latch from pivoting anticlockwise about the articulation 28. In Fig. 16, on the other hand, the latch has been disengaged from the guide member 31, whereby it becomes free to pivot anticlockwise about the articulation 28, whereupon the latch becomes free, with its female recess 30, to engage the stop pin 26 which has then already engaged the recess 25 in the terminal edge 24 of the link. When, thereafter, the outer boom upon continued extension of the outer boom cylinder 12 has been pivoted to the position shown in Fig. 17, the pivot pin 14 is completely disengaged from the fork-shaped member 29, whereby the continued pivotal movement of the outer boom will take place about the articulation 17. When the outer boom is moving within the working range, the link device 15 will always be affected by a torque tending to hold the link device applied against the stop 26. For this reason, the link device need not be locked relative to the first boom section. In certain types of displacements, the outer boom may however tend to pivot upwards and occasion jerky movements in the link, should this be freely pivotable relative to the first boom section. By the provision of locking latches 27, 27', such jerky movements are however prevented since the engagement of the female recess 30 with the associated stop pin 26 prevents the link device from pivoting anti-clockwise about the articulation 16.
Although the two locking latches 27, 27' are shown in the drawings without any particular aids for moving them into engagement with the associated stop pins 26, it is of course possible to combine them, for example with a spring, which constantly tends to pivot the latch anti-clockwise about the articulation 28, or optionally also a particular hydraulic cylinder which controls the movements of the locking latches.
According to the invention, the overall length of the outer boom (calculated as the distance between the articulation 17 and the tool carrier 11) and the link (calculated as the distance between the articulations 16 and 17) may be substantially equal to or optionally only slightly smaller than the length of the first boom section 1 (calculated as the distance between the main articulation 3 and the link articulation 16). The length of the outer boom 2 in relation to the length of the link device 15 may vary quite considerably. In the Example shown in the drawings, the outer boom is about 3.5 times longer than the link device. It is however also possible to design the crane with, relatively speaking, considerably longer link devices. However, the outer boom should always be at least equally long as the link device. Suitably, the length of the outer boom should be in the range of 1-5 or preferably 2-4 times the length of the link device.
The advantages of the invention are obvious. By means of the link device provided between the outer boom and the first boom section, the crane will, when the outer boom passes the intermediate position described above, automatically switch articulation points in order, when operating within the working range, to pivot about an articulation point comparatively remote from the main articulation of the first boom section, while when moving within the parking range it will pivot about an articulation point situated closer to the main articulation of the first boom section. In other words, the effective length of the first boom section, during the pivotal movement of the outer boom within the working range, is considerably greater than the length of the first boom section proper, which means that the inventive crane can be parked using a relatively small space while at the same time it becomes much easier to operate the crane in the operative state since the outer boom can be pivoted about an articulation point located at a great distance from the main articulation of the first boom section.
Reference is now made to Fig. 19 in combination with Fig. 2. As earlier mentioned, the post 4 is mounted on a rotary device 7 comprising two hydraulic cylinder units 8, 8' serving to bring about rotation of the post about its vertical centre axis. In practice, each unit consists of two cooperating, single-acting hydraulic cylinders, to which is connected a reciprocating straight rack which meshes with a gear rim integrated in the rotary device 7. Using two cylinder units confers the advantage that an even torque can be imparted to the post, which is twice the size of the torque achievable with only one cylinder. One drawback of using double-cylinder units of the type which in prior-art crane constructions are parallel to each other, is however that the ends of the cylinder units which project from the rotary device prevent the first boom section from being pivoted downwards, like the outer boom, to a parking position situated close to the post.
In the construction illustrated in Fig. 19, the two hydraulic cylinder units 8, 8' are inclined relative to each other, such that two first, free ends of cylinders 32, 32' are located considerably closer to each other than the opposite ends of cylinders 33, 33'. As a result, there is formed between the ends 33, 33' a space 34 which is so large that the first boom section 1 and the associated outer boom 2 can be accommodated therebetween.
Although the inventive solution shown in Fig. 19 is especially valuable in connection with the crane construction described above, in which the outer boom in its parking position is located on the upper side of the first boom section, it is also applicable to other types of cranes in which it is desirable to park a first boom section close to the post, and also to be able to rotate the post with a great force.

Possible Modifications of the Invention

It goes without saying that the invention is not restricted only to the embodiment described above and shown in the drawings. Thus, it is conceivable to replace the single locking means in the form of the pair of latches 27, 27' which are able to lock the link device not only relative to the outer boom but also relative to the first boom section, with separate locking means, either only for locking the link device relative to the outer boom or only for locking the link device relative to the first boom section. It is even conceivable to dispense with each particular locking means for locking the link device. Instead of pins 26, it is also possible to use other optional means for arresting the link in the described intermediate position in order to switch the articulation point of the outer boom. Although the link device described comprises two separated, parallel link plates, it is also conceivable to design the device with only one link or link plate.

Claims

An outer-boom crane comprising a first boom section (1) connected to a post (4) by a main articulation (3), and an outer boom (2) connected to the first boom section and pivotable relative thereto by means of a double-acting hydraulic cylinder (12) which is arranged on the upper side of the first boom section and whose one end, e.g. the cylinder end, is connected to the first boom section by a first articulation (13) and whose opposite end, e.g. the piston-rod end, is connected to the outer boom (2) by a second articulation (14) which is offset a certain distance transversely of an imaginary longitudinal axis through the pivotal articulation of the outer boom to permit pivoting the outer boom between different positions, not only within a working range below an imaginary extension line drawn appoximately straight out from or at a small angle to the first boom section, but also in a range situated above this extension line adjacent a parking position, wherein there is provided between the two booms (1, 2) at least one link or link device (15) connected to the first boom section by a third articulation (16) and to the outer boom (2) by a fourth articulation (17) which is spaced from both said second articulation (14) and said third articulation (16), more specifically with the third articulation (16) disposed at a greater distance from the fourth articulation (17) than the second articulation (14), the outer boom (2), on the one hand, being pivotable together with the link device, by extension of the hydraulic cylinder (12), from a parking position close to the upper side (9) of the first boom section (1) to an intermediate position in which the link device is applied against and arrested by a stop (26) and from which continued extension of the hydraulic cylinder (12) causes only the outer boom (2) to pivot, now about the fourth articulation (17), and, on the other hand, being pivotable, by shortening of the hydraulic cylinder (12), in the opposite direction about the fourth articulation (17) from arbitrary positions within the working range to said intermediate position in which continued shortening of the hydraulic cylinder causes the outer boom to again pivot together with the link device (15) about the third articulation (16) back to said parking position.
An outer-boom crane as claimed in claim 1, characterised in that the link device (15) comprises two separate link plates (15', 15''), each of which is generally angular and has, on a first side, two edges (21, 22) making an angle of e.g. 150-160° with each other and joining in a point (23) located at a distance from said fourth articulation (17) that is substantially equal to the distance between said second and fourth articulations (14, 17), said point (23) forming an abutment for said second articulation (14) as long as the outer boom (2) is located between said parking position and said intermediate position.
An outer-boom crane as claimed in claim 2, characterised in that the link plate, at its end remote from said fourth articulation (17), has a terminal edge (24) extending transversely e.g. at an angle of 75-85°, of the longitudinal extent of the link plate, said third articulation (16) being arranged in the vicinity of one end of said terminal edge, while a means (25) for engaging said stop (26) is arranged at the opposite end of the terminal edge.
An outer-boom crane as claimed in claim 3, characterised in that the stop consists of a shaft or pin (26), and that said engagement means consists of a suitably semicircular recess (25) in which said pin (26) can engage when the link device is in said intermediate position.
An outer-boom crane as claimed in any one of the preceding claims, characterised in that it comprises means (27, 29) for locking the link device (15) relative to the outer boom (2) when this is moving in the range between said parking position and said intermediate position.
An outer-boom crane as claimed in any one of the preceding claims, characterised in that it comprises means (27, 30) for locking the link device (15) relative to the first boom section when the outer boom (2) is moving within said working range.
An outer-boom crane as claimed in claims 5 and 6, characterised in that one and the same locking means (27) is adapted, on the one hand, to lock the link device relative to the outer boom (2) when this is moving in the range between said parking position and said intermediate position, and, on the other hand, to lock the link device relative to the first boom section (1) when the outer boom (2) is moving within said working range.
An outer-boom crane as claimed in claim 7, characterised in that the locking means consists of at least one latch (27, 27') connected to the link device by an articulation (28) and having a first, suitably fork-shaped female member (29) for receiving a shaft or pin (14) forming said second articulation, and a second female member, e.g. a semicircular recess (30), for receiving said stop pin (26), said latch cooperating with a guide member (31) positively holding said first female member (29) in engagement with said second pivot pin (14) over at least the major part of the range of pivotal movement of the outer boom between said parking position and said intermediate position.
An outer-boom crane as claimed in any one of the preceding claims, characterised in that the overall length of the outer boom (2) and the link device (15) is substantially equal to the length of the first boom section (1).
An outer-boom crane as claimed in any one of the preceding claims, characterised in that the length of the outer boom (2) is 1-5, suitably 2-4 times the length of the link device (15).