Grab

Abstract: Grab with two grab buckets which are located on both sides of a first plane of symmetry and are each provided with a cutting edge and a grab arm bucket with which they are connected to each other by means of a hinge, comprising further: a grab yoke; closing means for bringing the grab buckets from at least one open excavating position to a closed position, whilst rotating the grab bucket arms about the hinge; means for attaching to the grab yoke a suspension rod or stick, which is provided at the upper end with means for coupling to a crane jib or the like and is suitable for transferring pressure or tensile forces; and rigid connecting rods which extend between the grab yoke and each grab bucket and are each hingedly attached thereto at their respective ends, wherein the cutting edges are located in such a way in relation to the hinge and the ends of the connecting bars, that during their movement from the open to the closed position the cutting edges pass through a substantially horizontal path.

Description: The invention relates to a grab of the kind described in the preamble of claim 1. In addition the invention relates to a method of using such a grab. Grabs of that kind are well known, see for example US-A-2.605.563. In general the suspension yoke hangs on hoisting wires which run parallel to the closing wires, wherein both sorts of wires can be retracted or paid out from the cabin of a crane or the like. During earth moving or ship unloading activities the grab is lowered with opened buckets in order to fall with the cutting edges into the material to be removed. The extent of penetration is hereby determined by the weight and the drop height of the grab. After the initial penetration the closing wires are retracted. These closing wires which run via return pulleys in the grab yoke to return and closing pulleys on the buckets, in particular on the grab bucket arms, ensure that the buckets approach each other until the cutting edges touch each other. Moreover, the buckets rotate about the hinge means, the cutting edges following a curved path and thus pentrating further into the material in a considerable way. Hydraulic grabs are also known, which function without wires but which are connected to a crane jib by means of a hinge coupling. When moving from the open, penetrating position to the closed position, both grab buckets hinge about an own, fixed hinge. The closing motion of the cutting edges hereby has a relatively large vertical component: for a bucket with a volume of 2.5 m 3 the vertical movement of the cutting edge can easily be about 85 cm. For several years now there is an awareness of the presence of layers of sediment, in the main sludge layers which are highly polluted with heavy metals, polycyclic aromatic hydrocarbons etc. and which form a potential danger for the environment. Layers of this kind have, in the meantime, been found in various places, among them harbour basins, estuaries, rivers, streams and lakes. If the water body concerned has to be deepened, efforts will be made to first of all remove the polluted sediment. As the period of deposition of pollutants can have been relatively short in comparison to the period in which the layers of sediment have been deposited, the layer of polluted sediment might generally be relatively thin (for example 35 cm). When removing the polluted layer the risk of also removing unpolluted sediment is then easily incurred. Not only is the time needed for the clearing up activities increased due to this but also the processing of the polluted material will not be as efficient as desired. It is now an object of the invention to provide a grab with which, in a largely controlled way, a layer of material, in particular a layer of polluted sediment can be selectively excavated. For this purpose the grab according to the invention has the characteristics described in claim 1. The rigid connecting rods, which are suitable for transferring pressure and tensile forces, and the connection to a rigid suspension rod ensure that the location of the cutting edges in relation to the grab yoke is now entirely controllable, the grab yoke itself also being accurately positionable. This is already of importance when inserting the grab into ground layer which has to be removed, in which case, when the situation of the ground surface is known, the cutting edges of the grab buckets can be brought to the desired required initial penetration depth, if necessary whilst exercising pressure on the grab and the cutting edges. After initial penetration the cutting edges will, when closing the grab buckets, pass through a substantially horizontal path, so that eventually a substantially horizontal layer of polluted material will be excavated, along the top of the unpolluted sediment. It is noted that with "horizontal", reference is also made to a plane which is sloping, but is nonetheless perpendicular to the first plane of symmetry and to the (second) plane of symmetry of the grab which itself is perpendicular to the first plane of symmetry. The means for connection with the suspension rod preferably comprise a double cardan coupling, with horizontal hinge axes, and possibly a drivable hinge connection or rotor with a vertical axis of rotation. By means of the rotor the grab can constantly retain the same orientation in the horizontal plane in relation to the pivot of the crane jib, so that dredging can be always be done parallel to a certain working line. In a preferred embodiment of the grab according to the invention the connecting rods, as seen in projection perpendicular to the first plane of symmetry of the grab, are substantially N-shaped or substantially reversed V-shaped, defined by two rods which each run from the central grab yoke to respective bucket sides. In this way the stiffness of the structure is enlarged and thus the relative stability of position of both bucket sides in relation to the grab yoke and thus the controllability of the grab orientation, in particular the cutting edges of the grab buckets, in planes parallel to the first plane of symmetry. According to a further development of the grab according to the invention the closing means comprise at least one pair of cylinder-piston assemblies, which cross each other and are connectable to suitable control means on, for example, a crane by means of control lines and are hingeably connected to the grab yoke with their upper ends and are hingeably connected to the grab buckets with their lower ends. The cylinder-piston assemblies make complicated and vulnerable provisions for closing ropes superfluous, whereby they contribute to the controllability and reliability of the excavating process. Due to an arrangement coinciding and crossing in projection, it is achieved that in a horizontal and vertical direction as little space as possible needs to be taken up by the closing means, while, moreover, forces can be exerted by the cylinder-piston assemblies on the grab buckets with a horizontal and a vertical component. Preferably pairs of cylinder-piston assemblies which cross each other are arranged on both outer sides of the grab, so that the cutting edges, as seen in a horizontal plane, will almost always be parallel to the first plane of symmetry during the beginning of the closing motion. In order to ensure that during the closing motion the hinge means follow a path which is located in the first plane of symmetry, it is provided for that the cylinderpiston assemblies from each pair are connected to each other by means of valves which open and close simultaneously. Alternatively, or in addition, guide means for guiding the hinge means in planes parallel to the first plane of symmetry can be provided, preferably depending and fixedly connected to the grab yoke. Preferably the grab comprises angle gauges for defining the angle between the first and the second plane of symmetry of the grab, respectively, after being placed on the ground to be dredged, and a reference plane or line such as the vertical. This makes it convenient when excavating a layer of a slope in that whilst descending the grab can, in the case of a known slope gradient, be allowed to cant at a corresponding angle perceptible by the angle gauge, so that the plane of the grab opening will be substantially parallel to the gradient of the slope. In addition, in the case of true horizontal bed a warning is given with regard to an unexpected oblique position of the grab. In a further development of the grab according to the invention the grab buckets have a closing off plate on their upper side which is provided with an air and/or water vent, and wherein closing means are attached on the connecting rods, which in the closed position lock the opening and release it in the open position. Thus it is achieved that during the closing motion the vents are automatically and securely closed so that the chance of spill from excavated, polluted material and polluted water during the discharge motion is minimized. The invention will now be explained in more detail on the basis of a number of exemplary embodiments shown in the accompanying drawings. fig. 1 shows a vertical front view of a first embodiment of the grab according to the invention, the grab being in the open position; fig. 2 shows a vertical front view of the grab of figure 1, now, however, in the closed position; Figure 3 shows a vertical side view of the grab of figure 2; Figure 4 shows a diagram with the positional relationship between some points of the grab shown in figure 2; figure 5 shows a schematic representation of the path which the cutting edges of the buckets of the grab of figures 1-3 follow when moving from the position of figure 1 to the position of the figure 2 of the grab; and figure 6 shows a detail of the grab according to the invention. Grab 1 is shown in figure 1, and comprises two substantially identical grab bucket members 2 and 3. The grab bucket members 2 and 3 are formed by grab buckets 5 and 6 and grab bucket arms 7 and 8. The grab buckets 5 and 6 have a closed bottom 16 and a closed upper side 17. It is noted that the upper side 17 can be provided with openings, which possibly can be closable for upward escape of air and/or water whilst the grab is being lowered in the position shown in figure 1. On the lower side the bottom 16 of the grab buckets 5 and 6 is provided with a cutting edge 15 which extends perpendicular to the plane of the drawing. At the location of 13 the grab arms 7 and 8 are hingeably connected to each other, the hinge axis extending perpendicular to the plane of the drawing. At the location of the outer end of the upper side 17 brackets 20 are arranged where the rigid connecting "rod" 9, 10 respectively is hingedly attached, the hinge axis extending perpendicular to the plane of the drawing. At its upper side the connecting rod 9, 10 is hingedly attached at the location of 22 to suspension or grab yoke 4. Here, too, the hinge axis is perpendicular to the plane of the drawing. At its upper end the grab yoke 4 is provided with a bracket 14 with a fork 31 on which the fork 28 of the suspension rod or stick 29 is attached in double cardan way. Both the hinge axes extend perpendicular to one another, one of which in the plane of the drawing. Between the bracket 14 and the portion of the grab yoke 4 located below that a rotatable connection or rotor 32 is arranged, the axis of rotation of which runs vertically. The rotation of this connection is preferably remotely controlled. The upper end of the stick 29 is attached to the jib (not shown) of a hydraulic crane placed on a pontoon or an embankment, for example. The lower portion of the grab yoke 4 is formed by a number of fixedly attached vertical plates 23 extending downwards, on which at the location of hingeable connection 24 the upper ends of piston-cylinder assemblies 11, 12 are attached. The hinge axis of these also extends perpendicular to the plane of the drawing. The piston-cylinder assemblies 11, 12 each comprise a cylinder 25 and a cylinder rod 26, the lower end of which is attached at the location of the hinge connection 27 to bracket 19 which is attached in a rotationally fixed way to the transverse strengthening tube 18 of the grab buckets 5, 6. The hinge axis of hinge connection 27 extends perpendicular to the plane of the drawing. As can be seen in the figures 1, 2 and 3 the grab according to the invention has two perpendicular planes of symmetry, these being S 1 and S 2 . It will be understood that where the term "horizontal" is used in the application, a plane is meant which is perpendicular to S 1 and S 2 . In addition in the figure 3 it can be seen that on the front side (a-side) as well as on the rear side (b-side) a pair of piston-cylinder assemblies 11a, 12a and 11b, 12b is arranged which cross each other almost perpendicularly. By this it is achieved that the bucket 5 and with it the cutting edge 15 can be held substantially perpendicular with regard to the plane of symmetry S 2 during the opening and closing motion. The rigidity and thus the controllability is increased further by the N-shaped embodiment of the connecting rods 9 (and also 10), in which connecting rods 9a and 9b are in a "vertical" plane and connect the grab yoke and the grab bucket to each other at the front and back, respectively, and a third rod 9c extends diagonally from the connection grab yoke - rod 9b to the connection rod 9a - grab bucket. Alternatively it is also possible that beneath the hinge connection 22 the connecting rod 9 splits into two connecting rods, which are connected at the location of the brackets 20a and 20 b, respectively, to the front and back side, respectively of the grab bucket 5. It has appeared to be of advantage if the grab with a volume content of 2,5 m 3 has the following dimensional relationships (see figures 2 and 4): a distance between P1 (hinge axis of connection 22) to the plane of symmetry S 1 of 269 mm; a distance between P1 and P3 (axis hinge connection 21) of 2163 mm; a distance between P3 and P6 (axis hinge 13) of 1946 mm; a distance between P3 and P4 (cutting edge 15) of 1403 mm; a distance between P4 and P6 of 2352 mm; and a distance between P2 (axis hinge connection 24) and PS (axis hinge connection 27) of, in retracted state of the piston-cylinder assembly 11, 1141 mm. For larger or smaller bucket volumes multiples thereof can be taken. The imaginary line sections P3, P4 and P3, P6 are preferably perpendicular to each other, and they are preferably, with the imaginary line section P4 P6, in a ratio of 3:4:5 respectively to each other. It is further preferred, as in this exemplary embodiment, that the length of the imaginary line section P4 P6 is larger than the length of the imaginary line section P1 P3 . It is furthermore advantageous that, as can be seen in figure 1, in the opened position the imaginary line sections P4 P3 and P3 P1 together include an obtuse angle α, so that in a closing motion the hinge connection 21 does not have to be moved through a dead centre. By placing the connection 20 near the bottom side 16 and by the large length of the imaginary line section P4 P6 , in other words the large distance between the cutting edge 15 and the hinge 13, a span which is as large as possible has in that way been provided after all. By exerting the correct amount of force with the crane via the fork of stick 29 to the rest of the grab 1, it can be ensured that the cutting edges 15 can be forced to the desired depth in the sediment layer which is to be removed. Because of the existence of the rigid connecting rods 9 and 10 the position of the cutting edge 15 in relation to the grab yoke 4 and via the fork of stick 29 in relation to the crane jib is, after all, substantially fixed. With the help of an angle gauge not shown the angle between plane S 1 and the vertical is defined. The angle between S 2 and the vertical is defined by comparable means. As a result of the measures discussed above and/or said preferred dimensional relationships, the cutting edges 15 will move in a substantially horizontal direction C when retracting the piston rods 26 in the direction A 1 and A 2 , respectively, whilst holding the grab yoke 4 at substantially the same vertical height by means of the crane arm and fork of stick 29. Then the hinge 13 will move in the plane of symmetry S 1 upwards in the direction B until the situation shown in figure 2 has been reached. The piston rods 26 are then almost entirely retracted in the cylinders 25. The hinge 13 is hereby situated somewhat above the upper ends of the piston-cylinder assemblies 11, 12, and, in actual fact, within the yoke portion, as a result of which, when opening completely, a large span is obtained, which is much larger than with existing buckets, and consequently the penetration depth for a certain volume of earth to be excavated can be limited and the structure height of the grab is also kept limited despite the large span, . During the closing motion the point Q, which is located at the centre of the imaginary line section P4 P6 , will describe an almost circular path in the direction D about the point M, which is located in the middle between the cutting edges 15. The crossed arrangement of the piston-cylinder assemblies 11, 12 is advantageous compared to, for example, a horizontal arrangement, because it would soon be necessary that, during the opening motion of the grab buckets the piston rod thereof would again have to be retracted somewhat when the hinge moves in a downward direction through the dead centre between both lower ends of the connecting rods. A vertical arrangement of the piston-cylinder assemblies would cause difficulties as far as the size of the attainable stroke is concerned. With the crossed arrangement in the opening and closing motion respectively, the movement of the piston rod can remain single while, in the mutually perpendicular position as shown of the piston-cylinder assemblies 11 and 12, a maximum stroke length is attained with a minimal occupation of space. It will be understood that suitable control means for the piston-cylinder assemblies 11, 12 will be available, but for the sake of the clarity of the drawinge these have not been shown. For an expert it will be clear which means can be used for that purpose. In order to ensure that during closing the hinge 13 will move exactly in the plane of symmetry S 1 , vertical guide beams 33, 34 are attached to the yoke 4, which extend downward from the yoke 4 and define between them a vertical passage 35, in which the hinge 13, which is externally roller-shaped, is fittingly guided so as to move according to a line which coincides with the first plane of symmetry S 1 . By the guide buckets 33, 34 in effect a roller support for hinge 13 is realized on the yoke, as a result of which the yoke is statically determined with regard to the rest of the grab. It is further noted that, for reasons of clarity, this provision has been omitted from figure 3. The piston-cylinder assemblies can alternatively be provided with valves which open and close simultaneously, so that the piston rods thereof will be moved simultaneously. The cutting edges 15 will follow a substantially horizontal path during the closing motion, as is schematically shown in figure 5. In the example described above with a digging opening of approximately 4.35 m, the vertical drift will be only approximately 6 cm, i.e. ± 3 cm compared to the average horizontal line of movement, thus amounting to a deviation of only 2.8% of the digging opening, so that a horizontal "slice" of earth is excavated, of which the volume will correspond substantially to the volume that was intended to be excavated within one movement of the grab. Preferably penetration depth measurements will have been carried out, with the help of usual means for that purpose. Dredging can thus be done very effectively, with the grab buckets being filled to an optimum, whilst as little non-polluted earth as possible is removed. The deviance from a true horizontal digging line will be very small, even less than 3.5%, with regard to the digging opening. Use of rigid connecting rods in the grab according to the invention offers yet another advantage. With reference to figure 6, in which the grab 1 is shown in detail, box beams 130 are attached between the connecting rods 9, 10 in a direction perpendicular to the plane of the drawing, to which brackets 131 are fixedly attached. These brackets 131 are, in turn, attached to valve plates 132 extending perpendicularly to the plane of the drawing, which are provided on the lower side with elongated rubber closing coverings 133. These rubber coverings 133 fit precisely in a sealing manner onto openings 134, which openings are elongated and extend perpendicular to the plane of the drawing and are arranged in the upper side 17 of the grab buckets 5 and 6. If the grab 1 is in a position shown in figure 1, the rubber covering 133 will be relatively swivelled away from the opening 134, and water and/or air will be able to escape upwards from the inner space of the bucket 5. If the grab 1 is closed, however, and is moving to the state shown in figure 5, relative rotation in the direction T will have taken place at the location of the hinge connection 21 between the connecting rod 9 and the upper side 17 of the bucket 5. In the closed end position shown in figure 5 the rubber covering 133 will connect to the opening 134 and seal the latter, so that the earth material present in the grab buckets 5 and 6 cannot escape.

Claims: Grab (1) comprising two grab members (2, 3) substantially corresponding to each other, which are located on both sides of a first plane of symmetry (S 1 ) and are each provided with a grab bucket (5, 6) having a cutting edge (15) and with a grab bucket arm (7, 8), with which they are rotatably about a horizontal axis connected to each other by means of hinge means (13), said grab having a second plane of symmetry (S 2 ) perpendicular to said first plane of symmetry (S 1 ), said grab further comprising: a grab yoke (4); closing means (11, 12) for bringing the grab buckets from at least one open excavating position to a closed position, in which the cutting edges abut one another, whilst rotating the grab bucket arms about the hinge means; means (31) for attaching the grab yoke (4) to a crane jib or the like; and rigid connecting rods (9, 10) which extend between the grab yoke (4) and each grab bucket (5, 6) and are each hingedly attached thereto at their respective ends, characterized in that the cutting edges (15) are located in such a way in relation to the hinge means (13) and the ends of the connecting rods (9, 10), that during their movement from the open to the closed position the cutting edges (15) pass through a substantially horizontal path, that is to say a path which is in a plane perpendicular to the first plane of symmetry as well as to the second plane of symmetry, the grab further comprising a suspension rod or stick (29), which is attached to the grab yoke by the attaching means and is provided at its upper end with means for coupling to a crane jib or the like and is suitable for transferring pressure or tensile forces. Grab (1) according to claim 1, wherein during the closing motion the points located on the middle of the imaginary line sections between the hinge means (13) and the cutting edges (15) follow a substantially circular path. Grab according to claim 1 or 2, wherein in the open position in which the mutual distance of the cutting edges (15) of the grab buckets (5, 6) is at a maximum, the line between the lower end of the connecting rod (9, 10) and the cutting edge makes an internal obtuse angle with the connecting rod, as seen in projection in a vertical plane perpendicular to the first plane of symmetry (S 1 ). Grab according to claim 1, 2 or 3, wherein the connecting rods (9, 10) , as seen in projection perpendicular to the first plane of symmetry (S 1 ) of the grab, are substantially reversed V-shaped, defined by two rods which each run from the central grab yoke (4) to respective bucket sides, or wherein the connecting rods as seen in projection perpendicular to the first plane of symmetry of the grab, are substantially N-shaped, defined by three rods (9a, 9b, 9c) which form a lattice with the grab yoke (4) and the grab bucket (5). Grab according to any one of the preceding claims, wherein the means for connection of the grab yoke (4) and the suspension rod form a double cardan connection with mutually perpendicular horizontal hinge axes and wherein preferably an operable hinge connection, such as a rotor (32), with a vertical hinge axis is present in these connecting means or in the grab yoke. Grab according to any one of the preceding claims, wherein the hinge means (13) have one common hinge axis, which is located in the first plane of symmetry (S 1 ) of the grab. Grab according to any one of the preceding claims, wherein the distance between the cutting edge (15) and the hinge axis (13), as seen in projection in a vertical plane perpendicular to the first plane of symmetry (S 1 ), is larger than the length of the connecting rod (9, 10). Grab according to any one of the preceding claims, wherein the lower end of each connecting rod (9, 10) is located near the bottom (16) of the grab bucket concerned, on the end thereof distant from the cutting edge (15), wherein preferably, as seen in projection in a vertical plane perpendicular to the first plane of symmetry (S 1 ), the imaginary vertical connecting line between the cutting edge and the lower end of the connecting rod is substantially perpendicular to the imaginary connecting line between that lower end and the hinge means (13). Grab (1) according to any one of the preceding claims, wherein the closing means comprise at least one pair of cylinder-piston assemblies (11, 12), which cross each other and are connectable to suitable control means on, for example, a crane by means of control lines and are hingeably connected to the grab yoke (4) with their upper ends and are hingeably connected to the grab buckets (5, 6) with their lower ends, wherein preferably in the closed position of the grab the upper ends of the cylinder-piston assemblies are located on both sides of the hinge means (13), as seen in projection in a vertical plane perpendicular to the first plane of symmetry (S 1 ), wherein preferably on both grab sides a pair of cylinder-piston assemblies (11, 12) which cross each other is arranged, which cylinder-piston assemblies are preferably provided with valves which open and close simultaneously. Grab according to any one of the preceding claims, comprising guiding means (33, 34), such as guide beams depending from the grab yoke (4), for guiding the hinge means (13) in a sidewardly confining way along a plane parallel to the first plane of symmetry (S 1 ) during the movement thereof from the closed to the open position and vice versa. Grab according to any one of the preceding claims, comprising angle gauges for defining the angle between the first and second plane of symmetry (S 1 , S 2 ), respectively, of the grab (1), after being placed on the ground to be dredged, and a reference plane or line such as the vertical. Grab according to any one of the preceding claims, wherein the grab buckets (5, 6) have a closing off plate (17) on their upper side, which is provided with an air and/or water vent, and wherein closing means (132) are attached on the connecting rods (9, 10), which in the closed position lock the opening and release it in the open position. Assembly of a crane and a grab (1) according to any one of the preceding claims attached to said crane. Method for selectively removing a layer of polluted sediment under water, wherein use is made of a grab according to any one of the preceding claims, wherein in the open position the grab is forced in an active way with the cutting edges in the layer of sediment up to a predetermined depth, and wherein during the closing of the grab buckets the grab yoke is held at the same level.

Inventors: KORF DIRK, KORF, DIRK
Applicants: BOS KALIS BV, BAGGERMAATSCHAPPIJ BOSKALIS BV
Application year: 1995
Main classification: B66C3
IPC codes: B66C3, E02F3