WO2019083353A1

WO2019083353A1 - Method for hoisting objects and related self-correcting lifting-block

Info

Publication number: WO2019083353A1
Application number: PCT/NL2018/000018
Authority: WO
Inventors: Johannes Hendrikus EERTMAN
Original assignee: Ropeblock B.V.
Priority date: 2017-10-24
Filing date: 2018-10-23
Publication date: 2019-05-02
Also published as: NL1042605B1; WO2019083353A4

Abstract

The present invention relates to a method, and a related hoist-block, for correcting deviations in the position of a lifting-block (5) which forms part of a crane (1) which crane further comprises a boom (2) with an upper-block (4), whereby both the upper-block and the lifting-block each comprise at least one sheave (7), which sheave is rotatable over a substantially horizontal rotating-axis and around which sheave a wire rope (3) runs, whereby working with loads can be done almost continuously with the least possible rotating of the ropes and of the lifting-block. The method comprises the steps of detecting of any deviation in position of the lifting-block, in operation, with respect to the calibrated position and/or with respect to the position of the upper-block and correcting the position of the lifting-block when the detected deviation in the position is greater than a preset value by shifting the center of gravity of the lifting-block in such a way to cause the deviation in the position becoming smaller than the preset value.

Description

Method for hoisting objects and related self-correcting lifting-block DESCRIPTION Field of the invention

The present invention relates to a method, and a related hoist-block, for correcting deviations in the position of a lifting-block which forms part of a crane which crane further comprises a boom with an upper-block, whereby both the upper-block and the lifting-block each comprise at least one sheave, which sheave is rotatable over a substantially horizontal rotating-axis and around which sheave a wire rope runs, whereby said lifting-block, in a calibrated position, is substantially parallel to the upper-block, whereby the rotating-axis of the sheaves of both blocks are substantially parallel to one another. Background of the invention

Cranes, mobile as well as off-shore cranes, are used to hoist, lower or move heavy and/or large objects. A crane can be of all sorts and designs, such as a tower crane whether or not rotatable on a vertical axis and on a base, mounted for example on a vehicle, ship, drilling platform etc. Cranes are also often used in harbors to load and unload container ships. Depending on its purpose, a crane has, for instance, a hook, a gripper or a magnet.

Cranes are provided with upper-blocks and lifting-blocks in which blocks wire-sheaves are placed to guide the wire rope. A wire-sheave (in short sheave) comprises a sheave which can be made of metal and/or plastic, with a groove on its periphery to guide the wire rope. The sheaves, mounted on an axis, can freely rotate (around the axis). The axis of the upper- block is connected to the boom and it does not have a free orientation, in contrast to the axis of the lifting-block, which due to the wire rope has a free orientation. Prior to operation, the (upper-block of the) crane is horizontally corrected as much as possible.

A common problem with cranes is that of block rotation, also called cabling, where twisting of a wire rope, built up in the course of time, can lead to a (sudden) rotation of the lifting-block and twisting together of the rope-segments between the lifting-block and the upper-block. In case one timely notices that this is likely to occur, one ought to cease the operation in order to unravel the build-up twist in the wire rope. Unless the problem is timely resolved, the rope-segments can indeed twist together and be damaged, and the crane cannot be used safely anymore. In the worst case, the wire rope can even be cut This problem occurs more severely and more often when an unloaded lifting-block, particularly one with several sheaves, is repeatedly moved up and down.

During the time the problem is being removed, the crane is out of service and the operation is stagnated. This can have serious financial consequences. Besides, the wire ropes and sheaves can be worn out faster.

The severity of the problem can be demonstrated for instance in case of dismantling of a windfarm in the sea. There, for each windmill, the gondola and several blades must be put down from high altitude. The lifting-block is then raised unloaded time after time, as a result of which a constantly increasing twisting of the wire rope builds up. In case a block rotation occurs, or is likely to occur as a consequence of the aforementioned, the crane must be put out of service in order to resolve the problem. It is evident that in this realistic example suspending the hoisting operation entails high costs as well as delay.

The aforementioned problem of block rotation may have diverse causes. As such, a wire rope inherently exhibits a tendency to rotate. This occurs because the wire rope is built out of bundles of strands - of steel, fibers (natural or plastic), or a combination thereof (the so- called hybrid rope) - twisted together, as in a helix. Therefore, when loaded and unloaded, these strands tend to rotate. In order to prevent this, rotation-resistant wire ropes are used. In these wire ropes, the strands are twisted partly in another direction, as a result of which the wire rope shows less tendency to twist. A drawback of this is that these wire ropes, when used, wear out faster and must therefore be replaced sooner. Nevertheless, the wire rope twist and block rotation are still not entirely prevented.

Another cause of the aforementioned problem of "cabling" is that a wire rope in operation can make an angle with respect to the sheave. This is called fleet angle. In that case, the wire rope will role from the side of the groove to its bottom during ascending and descending of the crane and that can lead to a twist in the wire rope. This can be reduced by lubrication between the wire rope and sheave, but still, twisting of the wire rope and block rotation is not fully prevented. A still further cause of said problem is the so-called "induced fleet angle". This occurs when more sheaves are used whereby the rope-segments run substantially parallel around the sheaves. Each sheave has, due to friction between the sheave and the wire rope, an efficiency loss. Because of that, the load distribution over those rope- segments will not be equal. Now when the lifting-block is moved with a low load or no load at all, it tends to hang askew (tilted) which causes an earlier mentioned induced fleet angle to come into existence. In case of lifting, the side of the lifting-block with the higher throughout speed of the wire rope (the so-called "faster part" of the wire rope), also called the faster side of the lifting-block, will hang slightly higher than the other side - whereas in case of lowering it will hang slightly lower. The opposite side of the faster side of the lifting-block is then called the slower side. In practice, one may attempt to prevent this askew hanging of the lifting-block, for instance, by means of using more flexible wire ropes and/or by skipping one or more sheaves (the so-called "skip reeving"). Nevertheless, the wire rope twist and block rotation are not entirely prevented.

The aforementioned causes often occur together and can even affect each other (strengthen) and so they can lead to an earlier occurrence of wire rope twist and block rotation.

The above-mentioned solutions are not sufficiently satisfactory and in practice one should frequently cease the operation in order to (timely) resolve the problem of cabling. It is, therefore, a need for a solution that will make it possible to work with loads nearly incessantly, with as little built-up wire rope twist and block rotation as possible. The solution is provided by means of a method according to Claim 1 and a lifting-block according to Claim 8.

Summary of the invention

The invention provides a method, and a related lifting-block, which prevents twisting of the wire rope and block rotation and thereby not only increases the life span of the wire rope in operation but also prevents interlocking of the wire-segments between the lifting- block and the upper-block. This increases the uptime (the time that the crane is in operation), the reliability and safety, and prevents unforeseen costs (because the crane has to be unexpectedly out of service). An object of the invention is to provide a method for correcting deviations correcting deviations in the position of a lifting-block which forms a part of a crane which crane further comprises a boom with an upper-block, whereby each of the upper-block and the lifting-block comprises at least one sheave, which sheave is rotatable around a substantially horizontal rotating-axis and over which sheave a wire rope runs, whereby said lifting-block is, in a calibrated position, substantially parallel to the upper-block, whereby the rotating-axes of the sheaves of both blocks are substantially parallel to one another. To this end, the method is characterized by the following steps: detecting of any deviation in position of the lifting-block, in operation, with respect to the calibrated position and/or with respect to the position of the upper-block;

correcting the position of the lifting-block when the detected deviation in the position is greater than a preset value - by shifting the center of gravity of the lifting-block in such a way to cause the deviation in the position becoming smaller than the preset value.

The deviation in the position, here, refers to the deviation that the lifting-block may have in its position relative to its calibrated position and/or relative to the position of the upper- block. This can be a rotation in the horizontal plane as well as a tilt in the vertical plane. The lifting-block can be first calibrated so that the sheave's rotating-axis of the lifting- block lies substantially in the same vertical plane as that of the upper-block and these rotating-axes are essentially parallel to each other. Ideally, in the calibrated position, both angles - the angle of the rotation as well as the angle of the tilt - are virtually zero.

During operation, it is constantly detected whether the position of the lifting-block differs from the calibrated position and/or the position of the upper-block. Once this deviation is greater than a preset value, the lifting-block will be corrected to make the deviation smaller than the preset value. The preset value will depend, among other things, on dimensions and the largeness of the block, the length of the wire rope and the number of sheaves.

The correction of the position is carried out by means of shifting the center of gravity of the lifting-block in such a way to cause the deviation in the position becoming again smaller than the preset value. The detection of the deviation of the position of the lifting-block is preferably carried out by means of detecting a rotating-angle that sheave's rotating-axis, or the projection thereof, of the lifting-block forms with the sheave's rotating-axis of the upper-block in the same horizontal plane as that of the sheave's rotating-axis of the upper-block. This shows whether the lifting-block is slightly rotated to the right or to the left. If that is the case and this rotation has exceeded the preset value, the position of the lifting-block will be corrected. The horizontal plane can then contain both lines, but it can also contain the sheave's rotating-axis of the lifting-block and the projection of the sheave's rotating-axis of the upper-block. The deviation of the position can also be detected by detecting a rotating-angle that sheave's rotating-axis, or the projection thereof, of the lifting-block forms with the sheave's rotating-axis of the upper-block in the same vertical plane as that of the sheave's rotating-axis of the upper-block. Furthermore, the deviation in the position can preferably be detected, as well, by detecting a rotating-angle (tilting-angle) that the sheave's rotating-axis, or the projection thereof, of the lifting-block forms relative to the sheave's rotating-angle of the upper-block in the same vertical plane as that of the sheave's rotating-axis of the upper-block. This shows whether the block is slightly tilted whereby one side, left or right, hangs higher than the other side. If this is the case and the preset value has been exceeded and the rotating of the lifting-block has exceeded the preset value, the position of the lifting-block will be corrected. The vertical plane can then contain both lines, but it can also contain the sheave's rotating-axis of the lifting-block and the projection of the sheave's rotating-axis of the upper-block.

These angles, one of them or both, can be compared with the preset value(s) and based on that, it can be determined whether, in what direction and to what extent the position of the lifting-block deviates from the calibrated position and/or from the position of the upper- block. If at least one of said angles is greater than the preset value, the position shall be corrected by shifting the center of gravity of the lifting-block toward the faster or slower side of the lifting-block in such a way to cause said rotating-angle becoming smaller than said preset value. In particular, when lifting or lowering a small load or unloaded, the lifting-block tends to hang askew or to tilt Thereby, the side of the lifting-block with the higher throughput of the wire rope, the faster side of the lifting-block, hangs slightly higher, and hangs slightly lower when lowering, than the slower (opposite) side when lifting. This is called here as "tilting in the natural sense". Because of this induced fleet angle, the wire rope, that downwardly around the sheaves runs, will twist in a certain direction (to the left or right, depending on a number of factors). The wire rope, twisted in that direction (to the left or right), tends to rotate back in the opposite direction (to the right or left), which will then cause the block to rotate slightly in mat last-named direction (to the right or left). This rotation of the (lifting-) block is corrected by causing a rotation in the wire rope that is opposite to the direction which causes tilting in the natural sense. This mean: shifting the center of gravity so far that, during the low load or unloaded operation, even a rotating- angle is caused between the sheave's rotating-axis of the lifting-block and that of the upper-block in the same vertical plane. This will result in an opposite tilt of the block, so that the faster side of the block is now lower in case of lifting, and higher in case of lowering. This is, here, referred to as "tilting in the counter-natural sense". Because of this tilting in the counter-natural sense, the downward running wire ripe will now rotate in the direction other than the aforementioned one, thus rotating the block into the direction other than aforementioned direction.

Because of this, a lifting-block that is already slightly rotated, rotates back to its desired position. Since tilting of the block in the natural sense is already known, depending on a number of factors, such as lifting or lowering case, and left or right reeving of the wire rope - the center of gravity can be shifted to the higher side of the block so that the correction can be carried out quickly.

In a further advantageous embodiment of the method according to the present invention, shifting the center of gravity of the lifting-block is carried out by means of a steerable counterweight which can freely move toward the faster or slower side of the lifting-block. Another object of the present invention is to provide a lifting-block, adapted to be used in a crane with at least a boom with an upper-block, whereby each of the upper-block and the lifting-block comprises at least one sheave which sheave is rotatable around a substantially horizontal rotating-axis and over which sheave a wire rope runs. To this end, the lifting-block comprises:

- detection means for detecting any deviation in position of the lifting-block, in operation, with respect to the calibrated position and/or with respect to the position of the upper-block;

- correction means for correcting the position of the lifting-block when said deviation in the position is greater than a preset value, which correction means are adapted to shift the center of gravity of the lifting-block in such a way to cause the deviation in the position becoming smaller than the preset value.

The detection means preferably comprise one or more cameras, compasses, and/or spirit levels which are adapted to detect at least one of the angles - that sheave's rotating-axis, or the projection thereof, of the lifting-block forms with the sheave's rotating-axis of the upper-block in the same horizontal plane as that of the sheave's rotating-axis of the upper- block - or that the sheave's rotating-axis, or the projection thereof, of the lifting-block forms with the sheave's rotating-axis of the upper-block in the same vertical plane as that of the sheave's rotating-axis of the upper-block. The detection means can also comprise one or more compasses or spirit levels, for example digital, which detect at least one of the said angles.

These detected angles, one of them or both, can, for instance, be calculated by means of a processor and they can be compared to the preset value(s). In the event that at least one of the above-mentioned angles is greater than the preset value, the processor generates a signal that can be sent directly to the correction means. Thereupon, the correction means, based on the signal from the processor, can shift the center of gravity of the lifting-block to such a side to cause the aforementioned angle becoming smaller than the said preset value.

Preferably, the correction means shift the center of gravity so far that even a rotating-angle will arise, as a result of this shift, between the sheave's rotating-axis, or the projection thereof, of the lifting-block and that of the upper-block in the same vertical plane, whereby the said rotating-angle has a value greater than a preset value. By so doing, a detected rotation of the block will be reversed until this detected rotating-angle becomes smaller than a preset value. Another advantageous embodiment of the lifting-block according to the present invention is characterized in that the correction means comprise a cylinder and a guide rail, whereby the cylinder can controllably move on the rail from one side to the other side and thus causing the center of gravity of the lifting-block to be shifted and whereby the cylinder has a certain weight as such that when it is moved to the end of either side of the lifting-block, it causes the lifting-block to tilt so that a rotating-angle arises between the sheave's rotating-axis, or the projection thereof, of the lifting-block and the sheave's rotating-axis of the upper-block in the same vertical plane.

Alternatively, the correction means may comprise a propulsive pendulum, whereby the pendulum has an arm with a certain length and weight. The pendulum can freely swing from one side to the other side and thus quickly shift the center of gravity of the lifting- block. Thereby, the aforementioned weight and arm length of the pendulum must be chosen in such a way that when the pendulum is moved to an extreme end of a side of the lifting-block, it causes the lifting-block to tilt so that a rotating-angle arises between the sheave's rotating-axis, or the projection thereof, of the lifting-block with the sheave's rotating-axis of the upper-block in the same vertical plane.

The weight of the cylinder or pendulum, but also the arm length of the pendulum, are decisive for shifting the center of gravity and realizing of the desired tilt, and they depend, among other things, on the size, weight and dimensions of the lifting-block.

Another object of the present invention is to provide a crane for lifting and lowering of objects comprising a boom, an upper-block and a lifting-block, whereby each of the upper- block and the lifting-block comprises at least one sheave, which sheave is rotatable around a substantially horizontal rotating-axis and over which sheave a wire rope runs, whereby said lifting-block, in a calibrated position, is substantially parallel to the upper-block, whereby the rotating-axes of the sheaves of both blocks are substantially parallel to one another, whereby the crane further comprises detection means for detecting deviation in position of the lifting-block, in operation, with respect to the calibrated position and/or with respect to the position of the upper-block - whereby said lifting-block further comprises correction means for correcting the position of the lifting-block, which correction means cause the center of gravity of the lifting-block to be shifted in such a way to cause the deviation in the position becoming smaller than a preset value. Thereby, the detection means can be placed, for instance, on the upper-block whereby the deviation in the position of the lifting-block is detected from the upper-block.

Brief description of the drawings

Figure 1 displays a side view of a simple mobile crane (1).

Figure 2 displays a front view of an unloaded lifting-block (5) in operation.

Figure 3 displays schematically a top view of the upper-block (4) and an underlying rotated lifting-block (5).

Figure 4a displays a front view of the lifting-block equipped with pendulum, without tilting. Figure 4b displays a front view of the lifting-block equipped with pendulum and with tilting in the natural sense.

Figure 4c displays a front view of the lifting-block equipped with a pendulum in an extreme position, with tilting in the counter-natural sense.

Detailed description of the drawings

Figure 1 shows a simple crane (1) which comprises an upper-block (4), a lifting-bock (5) and a boom (2) and which is placed on a vehicle. A wire rope runs through the (sheaves of the) blocks and is used to hoist, move or put down objects. The blocks can comprise more sheaves. These sheaves rotate around a rotating-axis. The rotating-axis of the sheaves of the lifting-block will be, in an ideal case, parallel to the sheave's rotating-axis of the upper- bock. However, in practice, the rotating-axis of the lifting-block is, during operation, often not parallel (anymore) to that of the upper-block and the lifting-block will thus be slightly rotated and/or tilted, for example as shown in Figure 2 that shows a slightly tilted lifting- block (5) that is equipped with four sheaves (7) and a hook (6).

Tiling occurs, particularly, in multi-sheaves blocks and when lifting or lowering unloaded or with a low load. This is caused by the loss of efficiency over the sheaves. As a result, the load distribution over the rope-segments will not be equal. When the lifting-block is moved with a low or without load, the lifting-block is inclined to hang askew (tilted); the side of the lifting-block with the higher throughput of the cable, the faster side of the lifting-block, will then hang slightly higher when lifting and hang slightly lower when lowering. This tilting causes an induced fleet angle. In Figure 2, a schematic front view of a lifting-block (5) is displayed where the lifting- block is moved up and down unloaded. The faster side of the lifting-block hangs, in case of lifting, slightly higher than its opposite side and, in case of lowering, slightly lower.

Which side of the lifting-block is the faster side, depends on rope reeving of the lifting- block. In Figure 1, seen from the perspective of the cabin (8), a left-handed reeved lifting- block has its faster side at the left side, during lifting, while a right-handed reeved lifting block has its faster side at its right side. The upper-block and the lifting-block are usually reeved in the same way.

When the lifting-block of Figure 2 is lifted, the faster side is going to hang higher and the lifting-block will tilt. This is a so-called "tilting in the natural sense". This will cause an angle, in the vertical plane, between the sheave's rotating-axes of both blocks (i.e. between the sheave's rotating-axis of the lifting-block and a horizontal line in the same vertical plane). This rotating-angle is, here for clarity's sake, called "β". As a result of this tilting, the wire rope - that from the upper-block runs downward into and around sheaves of the lifting-block - will rotate to the left A wire rope, rotated to the left, tends to rotate back, to the right, and with it also the lifting-block (S) tends to rotate to the right, and so in a horizontal plane a rotating-angle, here called "a", will be caused between the sheave's rotating-axes of both blocks as shown in Figure 3. In practice, both angles will exist at the same time so that the sheave's rotating-axes of the blocks will not lie perfectly in a plane (horizontally or vertically). In that case the angles are then formed between the sheave's rotating-axis of the upper-block and the projection of the sheave's rotating-angle of the lifting-block on the related plane (horizontal or vertical).

Figure 3 displays a top view of the lifting-block (5) that is rotated to the right relative to the upper-block (4) - and the rotating-angle "a". If, as a result of a rotation in the wire rope, this rotating-angle becomes too large, this may lead to sudden distortion of the lifting- block and thus to twisting together of the parallel rope-segments between lifting-block and upper-block.

In the present invention, the deviation in the position of the lifting-block is detected during operation, relative to the calibrated position and/or relative to the position of the upper- block. Based on that, it can be determined whether, in which direction and to what extent the position of the lifting-block deviates from the calibrated position. If this deviation in the position is greater than a preset value - which then depends on a number of factors among which the largeness, sizes and weight of the lifting-block - the position of the lifting-block shall be corrected by shifting the center of gravity of the lifting-block, in such a way that the deviation in the position becomes smaller than the said preset value.

The deviation in the position, here, is the deviation that shows the position of the lifting- block in relation to its calibrated position. This can be both a rotation in the horizontal plane, the aforementioned rotating-angle "a", as a rotation in the vertical plane, the rotating-angle "β". The lifting-block can be calibrated first, so that the sheave's rotating- axis of the lifting-block is substantially in the same vertical plane as that of the upper-block and these sheave's rotating-axes are essentially substantially to each other. Ideally, in the calibrated position, both angle of rotation and angle of tilt are virtually zero. According to the invention, it is constantly detected whether the position of the lifting- block differs from the calibrated position. Once this deviation is greater than a preset value, the lifting-block will be corrected to make the deviation smaller than the preset value, which value will then depend, among other things, on dimensions and largeness of the lifting-block, the rope's length, the number of sheaves and so on.

The detection of the deviation in the position of the lifting-block is preferably carried out by detecting the angle "a", as show in Figure 3. This angle can be detected by means of a camera (not shown) which, for instance, is attached to the upper-block or the lifting-block. Once an angle "a" has been detected which is greater than a preset value, the lifting-block will have to be corrected. Instead of a camera, this angle can also be determined with compasses, placed o the blocks. In this way, it can be determined whether the lifting-block is slightly rotated to the right or left side, in the horizontal plane.

The deviation in the position can also be detected by detecting the angle "β". That means, it is detected whether the block is slightly tilted whereby the faster or slower side is going to hang higher than its opposite side. This angle can be seen in Figure 2. The angle can be detected by means of a camera (not shown), which, for example, is attached to the upper- block or lifting-block. Once an angle "β" has been detected which is greater than a preset value and the angle "a" is smaller than a preset value, the lifting-block will have to be corrected. Instead of using a camera, this angle can also be detected with a (digital) spirit level placed on the lifting-block. By so doing, it can be determined whether the lifting- block is slightly tilted clockwise or counterclockwise in the vertical plane. Figure 4a displays a front view of the lifting-block provided with a pendulum (9) in zero- position, which means there is neither tilt, nor rotation.

Figure 4b shows the lifting-block of Figure 4a which is now tilted with an angle "β". The lifting-block, here, has a tilt in the natural sense. This rotating-angle is detected by detection means, for example a camera (not show here) or a (digital) spirit level - as soon as the rotating-angle exceeds the permissible value and the angle "a" is smaller than a preset value, it will be corrected for this by means of shifting the center of gravity. A processor (not shown) can serve to calculate and determine the angles, decide for correction and the extent of the correction.

In Figure 4c, the center of gravity can be shifted by means of a pendulum (9) and it can thus be corrected for tilting in the natural sense. The pendulum, here, shows its maximum swing. In this figure, the center of gravity is moved in such a way that not only the original tilt of the lifting-block (tilting in the natural sense, as shown in Figure 4b,) has been corrected, but also that the lifting-block is still further tilted, but now in the opposite direction than that in the natural (sense) tilt; this is thus "tilting in the counter-natural sense". By causing, a tilt in the counter-natural sense, will be corrected for the rotating of the lifting-block, the angle "a".

As explained above, a tilt in the natural sense of the lifting-block will cause the wire rope, that runs around the sheaves, to have a rotation, in the horizontal plane, in a certain direction (left or right). As a consequence, the wire rope tends to rotate back again but now in the opposite direction (right or left). As a result, the lifting-block will also tend to rotate in the same direction. By causing a tilt in the counter-natural sense, the wire rope as well as the lifting-block, both, will rotate back again and this will lead to smaller rotating-angle "a". This tilting in the counter-natural sense will cause a rotating-angle "β" in the opposite direction.

For the embodiment displayed in Figures 4, there is a propulsive pendulum (9) shown which then shifts the center of gravity of the lifting-block toward the faster or the slower side and thus adjusts the deviation in the position. Shifting of the center of gravity can also be done in other ways, for example by means of a cylinder that can move on a rail. The cylinder as well as the pendulum are provided with a certain weight (10) that must be chosen in such a way that the lifting-block is tilted when they move to the extreme. This weight (10) shall be determined on the basis of, among other things, the weight and size of the lifting-block and the length of the wire rope. For practical values, it can be referred to the following type, which demonstrates a non-restrictive example from practice using a lifting-block of brand RopeBlock ® (type no. FRB 575.28.7.250E). The center of gravity for this lifting-block - equipped with 7 sheaves with sheave's diameter 575 mm, with the own weight of 2,840 kg - is located at 641 mm below the heart of the sheaves' axis.

In an unloaded lifting-block with an efficiency loss of 2% for each sheave, a tilt will occur with an angle of 1.6°. For this lifting-block a pendulum is needed - which has an arm length of around 500 mm in its extreme swing position and a weight of approximately 100 kg - in order to get the aforementioned lifting-block in an almost horizontal position.

When a rotation of the lifting-block is detected that is greater than a preset value, then also a tilt in the counter-natural sense is desired, for example with an angle of 1.6°. In such a case, another 100 kg is needed.

Besides the pendulum's weight (10), it is also possible to adjust the (extent of the) swing of the pendulum (9), and/or to select a (longer) pendulum arm with a sliding weight There are also alternatives in case a sliding cylinder is chosen.

As mentioned earlier, a lifting-block (5), provided with sheaves (7), tends to tilt, especially when lifted or lowered unloaded or with a low load, due to efficiency loss over the sheaves. A lifting-block that is left-handed reeved, seen from the perspective of the cabin (8), according to Figure 1, will slightly tilt clockwise when lifting and counter-clockwise when lowering. The correction of the position is then carried out by causing the lifting-block to tilt slightly in an opposite direction than tilting in the natural sense, and as such for a left- handed reeved block, when lifting, causing to tilt slightly counterclockwise and when lowering to tilt slightly clockwise. When changing between lifting and lowering, the counterweight will move to the opposite side until the rotating-angle "a" is reduced and, thus, less than the preset value, and the block has almost reached its calibrated position. The principle of the "induced fleet angle" is thus applied to offset the rotating of the wire rope and of the lifting-block, and after this, tilting of the block is prevented in order not to introduce any new rotation of the wire rope. It is possible to apply the present invention to the existing hoisting systems. In an existing hoisting system, where the wire ripe has already been rotated, correction means such as a pendulum, for instance, can be mounted on the lifting-block. Next, the lifting-block is calibrated and the method according to the present invention can then be applied. 0 The shifting of the center of the gravity is hard to realize in case the lifting-block is moved up or down when loaded. This is due to the fact that in the case of a loaded lifting-block, shifting of the counterweight will not have much effect on the location of the center of gravity. In this way, the counterweight - when shifting between lifting and lowering - can remain in place, move to the middle (zero position), or move to the other side.

It should be clear to a person skilled in the art of the invention that variations of the invention are conceivable without departing from the principle of the invention. For example, it is possible that a cylinder as well as a pendulum at the same time, two cylinders, or two pendulums are used. The weight can adjustable move on the pendulum's arm. Also, the pendulum's swing can be adjustable. As detection means, only one or more cameras, only one or more spirit level, only one or more compasses, or the combination of these means can be used.

Claims

CLAIMS 1. A method for correcting deviations in the position of a lifting-block which forms a part of a crane which crane further comprises a boom with an upper-block, whereby each of the upper-block and the lifting-block comprises at least one sheave, which sheave is rotatable around a substantially horizontal rotating-axis and over which sheave a wire rope runs, whereby said lifting-block is, in a calibrated position, substantially parallel to the upper-block, whereby the rotating-axes of the sheaves of both blocks are substantially parallel to one another, which method comprises the steps of: a. detecting of any deviation in position of the lifting-block, in operation, with respect to the calibrated position and/or with respect to the position of the upper-block; b. correcting the position of the lifting-block when the detected deviation in the position is greater than a preset value - by shifting the center of gravity of the lifting-block in such a way to cause the deviation in the position becoming smaller than the preset value.

2. The method according to claim 1, characterized in that step a. is carried out by means of detecting a rotating-angle that sheave's rotating-axis, or the projection thereof, of the lifting-block forms with the sheave's rotating-axis of the upper-block in the same horizontal plane.

3. The method according to claim 1, characterized in that step a. is carried out by means of detecting a rotating-angle that sheave's rotating-axis, or the projection thereof, of the lifting-block forms with the sheave's rotating-axis of the upper-block in the same vertical plane.

4. The method according to claim 1, characterized in that step a. is carried out by means of detecting a rotating-angle that sheave's rotating-axis, or the projection thereof, of the lifting-block forms with the sheave's rotating-axis of the upper-block in the same horizontal plane - and detecting a rotating-angle that sheave's rotating- axis, or the projection thereof, of the lifting-block forms with the sheave's rotating- axis of the upper-block in the same vertical plane.

5. The method according to any of the claims 2 - 4, characterized in that correcting the position of the lifting-block in step b. is carried out by means of shifting the center of gravity of the lifting-block in such a way to cause the said rotating-angle becoming smaller than the said preset value.

6. The method according to claim 3 or 4, characterized in that correcting the position of the lifting-block in step b. is carried out by means of shifting the center of gravity of the lifting-block in such a way to cause the said rotating-angle becoming smaller than the preset value.

7. The method according to claim 5 or 6, characterized in that shifting the center of gravity of the lifting-block is carried out by means of a steerable counterweight toward one side of the lifting-block.

8. A lifting-block, adapted to be used in a crane which further comprises a boom with an upper-block, whereby each of the upper-block and the lifting-block comprises at least one sheave which sheave is rotatable around a substantially horizontal rotating-axis and over which sheave a wire rope runs, whereby said lifting-block, in a calibrated position, whereby the lifting-block further comprises:

9. The lifting-block according to claim 8, characterized in that the detection means comprise one or more cameras, compasses, and/or spirit levels which are adapted to detect at least one of the following angles: a rotating-angle that sheave's rotating- axis, or the projection thereof, of the lifting-block forms with the sheave's rotating- axis of the upper-block in the same horizontal plane - and a rotating-angle that sheave's rotating-axis, or the projection thereof, of the lifting-block forms with the sheave's rotating-axis of the upper-block in the same vertical plane.

10. The lifting-block according to claim 8 or 9, characterized in that the correction means comprise a cylinder and a guide rail, whereby the cylinder can controllably move on the rail in the direction of the slower or the faster side of the lifting-block and thus causing the center of gravity of the lifting-block to be shifted.

11. The lifting-block according to claim 8 or 9, characterized in that the correction means comprise a propulsive pendulum, whereby the pendulum has an arm and a certain weight and can freely swing toward the slower or the faster side of the lifting-block and thus causing the center of gravity of the lifting-block to be shifted.

12. The lifting-block for lifting and lowering of objects comprising a boom, an uppcr- block and a lifting-block according to any of the claims 8 - 11.

13. A crane for lifting and lowering of objects comprising a boom, an upper-block and a lifting-block, whereby each of the upper-block and the lifting-block comprises at least one sheave, which sheave is rotatable around a substantially horizontal rotating-axis and over which sheave a wire rope runs, whereby said lifting-block, in a calibrated position, is substantially parallel to the upper-block, whereby the rotating-axes of the sheaves of both blocks are substantially parallel to one another, whereby the crane further comprises detection means for detecting deviation in positon of the lifting-block, in operation, with respect to the calibrated position and/or with respect to the position of the upper-block - whereby said lifting-block further comprises correction means for correcting the position of the lifting-block, which correction means cause the center of gravity of the lifting-block to be shifted in such a way to cause the deviation in the position becoming smaller than a preset value.