Self-hoisting crane and method of mounting and operating such crane

Abstract

Self-hoisting crane (1) and method of mounting and operating such crane The self-hoisting crane is adapted to be hoisted from ground to a nacelle (2) or a tower (46) of a wind turbine (3) by operation of a cable winch (9) arranged at ground (10) and is adapted to be operated in its mounted position on the nacelle or on the tower by operation of the same cable winch. The cable extends from a hook block (6), out through a pedestal (4) of the crane, from the nacelle or from the tower and to the cable winch at the ground. The self-hoisting crane is provided with a cable crawler (11) having at least one motor driven traction sheave (12.sub.2) adapted to, during operation of the self-hoisting crane, transfer a pulling force to the cable (8) through friction. The cable crawler is shiftable between an active state (13) in which the traction sheave may transfer a pulling force to the cable and a passive state in which the traction sheave may transfer no pulling force to the cable.

Claims

1. A self-hoisting crane adapted to be mounted on a nacelle or on a tower of a wind turbine, wherein the self-hoisting crane includes a pedestal, a boom arm, a hook block and at least one cable adapted to lift or lower the hook block in relation to the boom arm for operation of the self-hoisting crane in its mounted position on the nacelle or on the tower, wherein the at least one cable is adapted to extend from the hook block, out through the pedestal, from the nacelle or from the tower and to a cable winch arranged at the ground, wherein the self-hoisting crane is adapted to be hoisted from the ground to the nacelle or to the tower by operation of the cable winch arranged at the ground, and wherein the self-hoisting crane is adapted to be operated in its mounted position on the nacelle or on the tower by operation of the cable winch arranged at the ground, wherein the self-hoisting crane is provided with a cable crawler having at least one motor driven traction sheave adapted to, during operation of the self-hoisting crane in its mounted position on the nacelle or on the tower, transfer a pulling force to the at least one cable through friction, and in that the cable crawler is shiftable between an active state in which the traction sheave may transfer a pulling force or may transfer at least a substantial pulling force to the at least one cable and a passive state in which the traction sheave may transfer no pulling force or at least substantially no pulling force to the at least one cable.

2. A self-hoisting crane according to claim 1, wherein, in the active state of the cable crawler, a first wrap angle represents the angle of contact between the at least one cable and the traction sheave, and wherein, in the passive state of the cable crawler, a second wrap angle represents the angle of contact between the at least one cable and the traction sheave, and in that the first wrap angle is at least substantially greater than the second wrap angle.

3. A self-hoisting crane according to claim 2, wherein the at least one cable, in succession, contacts a first support sheave, the motor driven traction sheave and a second support sheave, in that, seen in a transverse direction of the at least one cable, the motor driven traction sheave runs on a first traction side of the at least one cable and the first and second support sheaves run on a second side of the at least one cable, and in that the motor driven traction sheave is relatively movable in relation to at least one of the first and second support sheaves in the transverse direction of the at least one cable between an active position, in which the cable crawler is in its active state, and a passive position, in which the cable crawler is in its passive state.

4. A self-hoisting crane according to claim 2, wherein a motor drive for the at least one motor driven traction sheave is switchable between an active mode in which an at least substantial rotational moment may be applied to the traction sheave and a free-wheeling mode in which no rotational moment or at least no significant rotational moment may be applied to the traction sheave, and wherein the cable crawler is adapted to switch the motor drive to its free-wheeling mode when the cable crawler is shifted from its active state to its passive state.

5. A self-hoisting crane according to claim 1, wherein the at least one cable, in succession, contacts a first support sheave, the motor driven traction sheave and a second support sheave, in that, seen in a transverse direction of the at least one cable, the motor driven traction sheave runs on a first traction side of the at least one cable and the first and second support sheaves run on a second side of the at least one cable, and in that the motor driven traction sheave is relatively movable in relation to at least one of the first and second support sheaves in the transverse direction of the at least one cable between an active position, in which the cable crawler is in its active state, and a passive position, in which the cable crawler is in its passive state.

6. A self-hoisting crane according to claim 5, wherein the motor driven traction sheave is spring-biased relatively to the at least one of the first and second support sheaves from the passive position of the traction sheave to the active position of the traction sheave.

7. A self-hoisting crane according to claim 6, wherein, in the active position of the motor driven traction sheave, the traction sheave presses the at least one cable against at least one pressure wheel.

8. A self-hoisting crane according to claim 6, wherein a motor drive for the at least one motor driven traction sheave is switchable between an active mode in which an at least substantial rotational moment may be applied to the traction sheave and a free-wheeling mode in which no rotational moment or at least no significant rotational moment may be applied to the traction sheave, and wherein the cable crawler is adapted to switch the motor drive to its free-wheeling mode when the cable crawler is shifted from its active state to its passive state.

9. A self-hoisting crane according to claim 5, wherein, in the active position of the motor driven traction sheave, the traction sheave presses the at least one cable against at least one pressure wheel.

10. A self-hoisting crane according to claim 9, wherein the motor driven traction sheave has a diameter of at least 3 times, preferably at least 4 times, the diameter of the at least one pressure wheel.

11. A self-hoisting crane according to claim 10, wherein, in the active position of the motor driven traction sheave, the traction sheave is guided to be relatively displaceable in relation to the at least one pressure wheel in a direction forming an angle of between 30 and 85 degrees, preferably between 45 and 75 degrees, with an axis through a rotation axis of the traction sheave and a rotation axis of the at least one pressure wheel.

12. A self-hoisting crane according to claim 9, wherein, in the active position of the motor driven traction sheave, the traction sheave is guided to be relatively displaceable in relation to the at least one pressure wheel in a direction forming an angle of between 30 and 85 degrees, preferably between 45 and 75 degrees, with an axis through a rotation axis of the traction sheave and a rotation axis of the at least one pressure wheel.

13. A self-hoisting crane according to claim 9, wherein a motor drive for the at least one motor driven traction sheave is switchable between an active mode in which an at least substantial rotational moment may be applied to the traction sheave and a free-wheeling mode in which no rotational moment or at least no significant rotational moment may be applied to the traction sheave, and wherein the cable crawler is adapted to switch the motor drive to its free-wheeling mode when the cable crawler is shifted from its active state to its passive state.

14. A self-hoisting crane according to claim 5, wherein the cable crawler includes a motor housing provided with a motor drive and the at least one motor driven traction sheave, and wherein the motor housing is swingable about a rotation point in relation to a frame carrying the first and second support sheaves between an active position in which the cable crawler is in its active state and a passive position in which the cable crawler is in its passive state.

15. A self-hoisting crane according to claim 14, wherein a spring pack is arranged between the motor housing and the frame so that, in the active position of the motor housing, the spring pack acts on the motor housing with a first moment arm in relation to the rotation point of the motor housing, and, in the passive position of the motor housing, the spring pack acts on the motor housing with a second moment arm in relation to the rotation point of the motor housing, and wherein the first moment arm is greater than the second moment arm.

16. A self-hoisting crane according to claim 5, wherein a motor drive for the at least one motor driven traction sheave is switchable between an active mode in which an at least substantial rotational moment may be applied to the traction sheave and a free-wheeling mode in which no rotational moment or at least no significant rotational moment may be applied to the traction sheave, and wherein the cable crawler is adapted to switch the motor drive to its free-wheeling mode when the cable crawler is shifted from its active state to its passive state.

17. A self-hoisting crane according to claim 1, wherein a motor drive for the at least one motor driven traction sheave is switchable between an active mode in which an at least substantial rotational moment may be applied to the traction sheave and a free-wheeling mode in which no rotational moment or at least no significant rotational moment may be applied to the traction sheave, and wherein the cable crawler is adapted to switch the motor drive to its free-wheeling mode when the cable crawler is shifted from its active state to its passive state.

18. A self-hoisting crane according to claim 17, wherein motor drive for the at least one motor driven traction sheave is provided with a speed control adapted to, at least in the active mode of the motor drive, limit the travel speed of the at least one cable at the position of the self-hoisting crane relatively to the travel speed of the at least one cable at the position of the cable winch arranged at the ground.

19. A self-hoisting crane according to claim 1, wherein the cable crawler is arranged in the self-hoisting crane, preferably in the boom arm, and preferably at an end of the boom arm next to the pedestal.

20. A method of mounting and subsequently operating a self-hoisting crane on a nacelle or on a tower of a wind turbine, whereby the self-hoisting crane includes a pedestal, a boom arm, a hook block and at least one cable adapted to lift or lower the hook block in relation to the boom arm for operation of the self-hoisting crane in its mounted position on the nacelle or on the tower, whereby the at least one cable extends from the hook block, out through the pedestal, from the nacelle or from the tower and to a cable winch arranged at the ground, whereby the self-hoisting crane is hoisted from the ground to the nacelle or to the tower by operating the cable winch arranged at the ground, and whereby the self-hoisting crane is operated in its mounted position on the nacelle or on the tower by operating the cable winch arranged at the ground, wherein the self-hoisting crane is provided with a cable crawler having at least one motor driven traction sheave adapted to transfer a pulling force to the at least one cable through friction, in that the cable crawler is shiftable between an active state in which the traction sheave transfers a pulling force or an at least substantial pulling force to the at least one cable and a passive state in which the traction sheave transfers no or at least substantially no pulling force to the at least one cable, in that, when the self-hoisting crane is operated in its mounted position on the nacelle or on the tower without any or at least substantially without any external load attached to the hook block, the cable crawler is operated in its active state, and in that, when the self-hoisting crane is operated in its mounted position on the nacelle or on the tower with an at least substantial external load attached to the hook block, the cable crawler is operated in its passive state.

Description

(1) The invention will now be explained in more detail below by means of examples of embodiments with reference to the very schematic drawing, in which

(2) FIG. 1 is a side view of a container for use with a self-hoisting crane according to the invention;

(3) FIG. 2 is a top view of the container of FIG. 1;

(4) FIG. 3 is a perspective view of a wind turbine during lifting of a self-hoisting crane according to the invention to the nacelle by means of a cable winch in a container as illustrated in FIGS. 1 and 2;

(5) FIG. 4 is a side view of a self-hoisting crane according to the invention mounted on the nacelle of the wind turbine of FIG. 3;

(6) FIG. 5 is a perspective view of the self-hoisting crane of FIG. 4, whereby the wind turbine is not illustrated;

(7) FIG. 6 is a side view showing a detail on a larger scale of the self-hoisting crane of FIG. 5, whereby the cable crawler is in its active state;

(8) FIG. 7 is a side view corresponding to that of FIG. 6, whereby a motor driven traction sheave of the cable crawler has been slightly moved away from its active position;

(9) FIG. 8 is a side view corresponding to that of FIG. 6, whereby the cable crawler is in its passive state; and

(10) FIG. 9 is an exploded perspective view of part of a boom arm and the cable crawler of the self-hoisting crane of FIG. 5.

(11) In the following, generally, similar elements of different embodiments have been designated by the same reference numerals.

(12) FIG. 4 shows an embodiment of a self-hoisting crane 1 according to the present invention, mounted on a nacelle 2 of a wind turbine 3. The self-hoisting crane includes a pedestal 4, a boom arm 5, a hook block 6 and a first and a second cable 7, 8 or wire rope adapted to lift or lower the hook block 6 in relation to the boom arm 5 for operation of the self-hoisting crane in its mounted position on the nacelle 2. In a well-known manner, the pedestal 4 is mounted in a mounting position on a crane base 32 arranged on the nacelle 2. Although in the illustrated embodiments, the crane base 32 is arranged on the nacelle 2 of the wind turbine 3, in other not shown embodiments, the crane base 32 may be mounted on the tower 46 of the wind turbine 3 rather than on the nacelle 2 of the wind turbine. In such case, the crane base 32 may by means of a not shown clamping mechanism be mounted for instance on a top part of the tower 46 in order for the self-hoisting crane 1 to be used for instance during mounting of the nacelle 2 on the tower 46.

(13) The first and second cables 7, 8 extend from the hook block 6, out through the pedestal 4, from the nacelle 2 and to a cable winch 9 arranged at the ground 10. The cable winch 9 is arranged in a container 33 for transporting the self-hoisting crane 1, as seen in FIGS. 1 and 2.

(14) As seen in FIG. 3, the self-hoisting crane 1 is adapted to be hoisted from the ground 10 to the nacelle 2 or to preferably a top part of the tower 46 by operation of the cable winch 9 in the container 33 arranged at the ground. Furthermore, as illustrated in FIG. 4, and the self-hoisting crane 1 is adapted to be operated in its mounted position on the nacelle 2 or on the tower 46 by operation of the cable winch 9 arranged at the ground 10.

(15) In the embodiment illustrated in FIGS. 5 to 9, the self-hoisting crane 1 according to the invention is provided with a cable crawler 11 having a first and a second motor driven traction sheave 12.sub.1, 12.sub.2 adapted to, during operation of the self-hoisting crane 1 in its mounted position on the nacelle 2 or on the tower 46, transfer a pulling force F.sub.P to the respective first and second cables 7, 8 through friction. The cable crawler 11 is shiftable between an active state 13 illustrated in FIG. 6 in which the first and second traction sheaves 12.sub.1, 12.sub.2 may transfer a pulling force or may transfer at least a substantial pulling force to the respective first and second cables 7, 8 and a passive state 14 illustrated in FIG. 8 in which the first and second traction sheaves 12.sub.1, 12.sub.2 may transfer no pulling force or at least substantially no pulling force to the respective first and second cables 7, 8 through friction.

(16) In the illustrated embodiment, in the active state 13 of the cable crawler 11, as illustrated in FIG. 6, a first wrap angle W.sub.1 represents the angle of contact between each cable 7, 8 and the respective traction sheave 12.sub.1, 12.sub.2. Furthermore, in the passive state 14 of the cable crawler 11, as illustrated in FIG. 8, a second wrap W.sub.2 angle represents the angle of contact between each cable 7, 8 and the respective traction sheave 12.sub.1, 12.sub.2. As seen, the first wrap angle W.sub.1 is substantially greater than the second wrap angle W.sub.2. The first wrap angle W.sub.1 may for instance be at least 1.5 times and preferably at least 2 times greater than the second wrap angle W.sub.2.

(17) As illustrated in FIG. 4, due to the relatively larger first wrap angle W.sub.1 in the active state 13 of the cable crawler 11, the first and second motor driven traction sheaves 12.sub.1, 12.sub.2 may transfer a total pulling force F.sub.P to the respective first and second cables 7, 8 through friction. Said total pulling force, together with a force F.sub.2 corresponding to weight of the hook block 6, is large enough to counteract a force F.sub.1 corresponding to the weight of the part of the first and second cables 7, 8 extending from hook block 6, i.e. from the top of the boom arm 5, to the cable winch 9 arranged at the ground 10. In other words: F.sub.2+F.sub.P>F.sub.1.

(18) On the other hand, due to the relatively smaller second wrap W.sub.2 angle in the passive state 14 of the cable crawler 11, the first and second motor driven traction sheaves 12.sub.1, 12.sub.2 may transfer only a relatively very small or almost negligible total pulling force to the respective first and second cables 7, 8 through friction. Therefore, in the passive state 14 of the cable crawler 11, the cable crawler may practically be prevented from providing any substantial unwanted braking force to the cables during the lifting operation.

(19) FIGS. 6 to 8 illustrate a longitudinal section through the boom arm 5 of the self-hoisting crane 1 as seen in FIG. 4. As seen, the cable 8, in succession, contacts a first support sheave 15.sub.2, the motor driven traction sheave 12.sub.2 and a second support sheave 16.sub.2. Seen in a transverse direction T of the cable 8, the motor driven traction sheave 12.sub.2 runs on a first traction side 17 of the cable 8 and the first and second support sheaves 15.sub.2, 16.sub.2 run on a second side 18 of the cable 8. Comparing FIGS. 6 and 8, it is seen that the motor driven traction sheave 12.sub.2 is relatively movable in relation to the first and second support sheaves 15.sub.2, 16.sub.2 in the transverse direction T of the cable 8 between an active position 19 seen in FIG. 6, in which the cable crawler 11 is in its active state 13, and a passive position 20 seen in FIG. 8, in which the cable crawler 11 is in its passive state 14. Likewise and parallel arranged, the cable 7, in succession, contacts a first support sheave 15.sub.1, the motor driven traction sheave 12.sub.1 and a second support sheave 16.sub.1. Seen in a transverse direction T of the cable 7, the motor driven traction sheave 12.sub.1 runs on a first traction side 17 of the cable 7 and the first and second support sheaves 15.sub.1, 16.sub.1 run on a second side 18 of the cable 7. As understood from FIG. 9, and as it will be explained in further detail below, the first and second motor driven traction sheaves 12.sub.1, 12.sub.2 are driven on a common axle by a common motor drive 27. The common motor drive 27 may include a gear box and/or a brake.

(20) As further seen in FIGS. 6 to 8, the first and second support sheaves 15.sub.1, 15.sub.2, 16.sub.1, 16.sub.2 are fixed rotatably on a frame 30 in the form of the boom arm 5 of the self-hoisting crane 1. The motor driven traction sheaves 12.sub.1, 12.sub.2 are arranged displaceably in relation to said frame 30. Alternatively, the motor driven traction sheaves could be fixed rotatably on such frame, and the first and second support sheaves could be arranged displaceably in relation to said frame. In any case, a relative displacement, seen in the transverse direction T of the cables 7, 8, between the motor driven traction sheave and the first and second support sheaves, may change said wrap angle between a relatively large first wrap angle W.sub.1 in the active state 13 of the cable crawler 11 and a relatively very small second wrap angle W.sub.2 in the passive state 14 of the cable crawler.

(21) In the illustrated embodiment, the cable crawler 11 includes a motor housing 28 provided with the motor drive 27 and the first and second motor driven traction sheaves 12.sub.1, 12.sub.2. The motor housing 28 is swingable about a rotation point 29 in relation to the frame 30 carrying the first and second support sheaves 15.sub.1, 15.sub.2, 16.sub.1, 16.sub.2 between an active position in which the cable crawler 11 is in its active state 13 and a passive position in which the cable crawler 11 is in its passive state 14.

(22) Furthermore, in the illustrated embodiment, the motor driven traction sheaves 12.sub.1, 12.sub.2 are spring-biased relatively to the first and second support sheaves 15.sub.1, 15.sub.2, 16.sub.1, 16.sub.2 from the passive position 20 of the traction sheaves 12.sub.1, 12.sub.2 to the active position 19 of the traction sheaves. Thereby, the cable crawler 11 may automatically shift between its active state 13 and its passive state 14 depending on whether the hook block 6 is carrying an external load or not. Instead of being spring-biased by means of mechanical springs, the traction sheaves may also be biased by means of hydraulic means or any other suitable means, such as a gas spring. When the hook block 6 is carrying an external load, the cables 7, 8 may tend to follow an at least relatively straight path and thereby displace the motor driven traction sheaves relatively to the first and second support sheaves, against the spring-bias, so that the traction sheaves 12.sub.1, 12.sub.2 reach their passive position 20, as illustrated in FIG. 8. Thereby, the motor driven traction sheaves 12.sub.1, 12.sub.2 may be prevented from providing any substantial unwanted braking force to the cables 7, 8 during the lifting operation. On the other hand, when the hook block 6 is not carrying any external load, the cables 7, 8 may follow the path formed by the motor driven traction sheaves 12.sub.1, 12.sub.2 and the first and second support sheaves 15.sub.1, 15.sub.2, 16.sub.1, 16.sub.2, in the active position 19 of the traction sheaves 12.sub.1, 12.sub.2, as seen in FIG. 6, without pressing the motor driven traction sheaves 12.sub.1, 12.sub.2 relatively to the first and second support sheaves 15.sub.1, 15.sub.2, 16.sub.1, 16.sub.2, against the spring-bias. Thereby, the traction sheaves 12.sub.1, 12.sub.2 may remain in their active position 19 and may therefore transfer a suitable pulling force to the cables 7, 8 through friction, as discussed above. Due to this automatic shift between the active state 13 and the passive state 14 of the cable crawler 11, the operator of the self-hoisting crane 1 may not need to control the cable crawler manually.

(23) Moreover, in the illustrated embodiment, the motor driven traction sheaves 12.sub.1, 12.sub.2 are spring-biased from their passive position 20 illustrated in FIG. 8 to their active position 19 illustrated in FIG. 6 by means of a spring pack 31 arranged between the motor housing 28 and the frame 30 of the boom arm 5. The spring pack 31 is loaded with a number of compression springs 34, so that, in the active position of the motor housing 28, the spring pack 31 acts on the motor housing with a first moment arm M.sub.1 in relation to the rotation point 29 of the motor housing, as seen in FIG. 6, and, in the passive position of the motor housing 28, the spring pack 31 acts on the motor housing with a second moment arm M.sub.2 in relation to the rotation point 29 of the motor housing 28, as seen in FIG. 8, and the first moment arm M.sub.1 is greater than the second moment arm M.sub.2. Thereby, in the passive state of the cable crawler 11 as seen in FIG. 8, the motor driven traction sheaves may be biased by means of a suitable relatively low force against the cables 7, 8, although the spring force of the spring pack may be larger than in the active state of the cable crawler, due to relatively more compression of the springs 34. Consequently, the cable crawler 11 may not interfere with the operation of the self-hoisting crane 1 when an external load is lifted by the hook block.

(24) As illustrated in FIG. 9, the spring pack 31 is composed by four parallel sets each comprising two compression springs arranged in series, i.e. aligned with each other on a pin 35. By using parallel sets of compression springs, it may be possible to use relatively softer springs and thereby obtain a longer service life. Furthermore, this arrangement does not take up too much space. The pins 35 with compression springs 34 are arranged between a set of two cross-pins 36, 37 of which one 36 is mounted rotatably in a bearing 38 of the boom arm 5 and of which another 37 is mounted rotatably in a bearing 39 of the motor housing 28. The spring pack 31 is further provided with two parallel locking rods arranged between the set of two cross-pins 36, 37 and parallel with and between the pins 35. By means of the locking rods 40 and corresponding locking nuts 41 threaded thereon, the spring pack 31 may be locked in the passive position of the motor housing 28 as seen in FIG. 8, during mounting of the self-hoisting crane 1 or for maintenance purposes.

(25) In order to transfer an even larger pulling force from the motor driven traction sheaves 12.sub.1, 12.sub.2 to the respective cables 7, 8 through friction, in the active position 19 of the motor driven traction sheaves 12 illustrated in FIG. 6, the traction sheaves 12.sub.1, 12.sub.2 press the cables 7, 8 against respective pressure wheels 22.sub.1, 22.sub.2 arranged in bearings 42 in the boom arm 5. Preferably, the motor driven traction sheaves have a diameter of at least 3 times, preferably at least 4 times, the diameter of the pressure wheels. Consequently, the motor driven traction sheave of the cable crawler 11 may transfer an even larger pulling force to the at least one cable through friction.

(26) As illustrated in FIG. 6, in the active position 19 of the motor driven traction sheaves 12.sub.1, 12.sub.2, the traction sheaves 12.sub.1, 12.sub.2 are guided to be relatively displaceable in relation to the pressure wheels 22.sub.1, 22.sub.2 in a direction D forming an angle a of between 30 and 85 degrees, preferably between 45 and 75 degrees, with an axis A through a rotation axis 23 of the traction sheaves and a rotation axis 24 of the pressure wheels 22.sub.1, 22.sub.2. Thereby, a wedge-like effect may be obtained when the motor driven traction sheaves are pressed against the respective pressure wheels with the respective cables in between, thereby resulting in that the motor driven traction sheaves may transfer an even larger pulling force to the cables through friction, as explained above.

(27) The motor drive 27 for the motor driven traction sheaves 12.sub.1, 12.sub.2 may be switchable between an active mode in which an at least substantial rotational moment may be applied to the traction sheaves 12.sub.1, 12.sub.2 and a free-wheeling mode in which no rotational moment or at least no significant rotational moment may be applied to the traction sheaves 12.sub.1, 12.sub.2. The cable crawler 11 may be adapted to switch the motor drive 27 to its free-wheeling mode when the cable crawler 11 is shifted from its active state 13 to its passive state 14.

(28) In the illustrated embodiment, switching between the active mode and the passive mode is performed by controlling software in that a signal is received from an inductive switch 43 arranged on the swingable motor housing 28, as seen for instance in FIG. 8, when the inductive switch 43 is near a metal disc 44 arranged on the boom arm 5, as it is the case in FIG. 6. FIG. 7 illustrates an intermediate position of the motor housing 28, in which the motor housing has just left its active position of FIG. 6 and the traction sheaves 12.sub.1, 12.sub.2 have just left their active position 19. In this intermediate position, the inductive switch 43 has already switched from the active mode to the passive mode of the motor drive 27. Thereby it may be ensured that in the active mode of the motor drive 27, the cable crawler 11 may transfer sufficient pulling force to the cables 7, 8, whereas in the passive mode of the motor drive, the cable crawler may practically not interfere with the operation of the self-hoisting crane 1 when an external load is lifted by the hook block. In the passive mode of the motor drive 27, the controlling software may simply disconnect the electric connection to an electric motor of the motor drive 27. Furthermore, in the passive mode of the motor drive 27, the controlling software may release a brake for the motor drive. Alternatively, in the passive mode of the motor drive 27, a frequency converter used to control the electric motor may just be set to no moment. It is preferred that the electric motor is an alternating current motor having coils in both the rotor and in the stator so that it is possible to achieve a very low moment in the passive mode of the motor drive. Otherwise, in an alternative embodiment, switching between the active mode and the passive mode may be performed by a mechanical clutch or any other suitable mechanical system.

(29) The motor drive 27 for the motor driven traction sheaves 12.sub.1, 12.sub.2 may be provided with a speed control adapted to, at least in the active mode of the motor drive 27, limit the travel speed of the cables 7, 8 at the position of the self-hoisting crane 1 relatively to the travel speed of the cables 7, 8 at the position of the cable winch 9 arranged at the ground 10. For instance, as an example, the travel speed of the cables 7, 8 at the position of the self-hoisting crane 1 may be controlled to be within 15 to 20 percent of the travel speed of the cables 7, 8 at the position of the cable winch 9. Thereby, it may ensured that the travel speed of the cable does not differ substantially between the position of the self-hoisting crane and the position of the cable winch, as explained above.

(30) The motor drive 27 for the motor driven traction sheaves 12.sub.1, 12.sub.2 is preferably provided with a moment control system adapted to ensure that a suitable rotational moment may be applied to the traction sheaves 12.sub.1, 12.sub.2 in the active mode of the motor drive 27. In order to adjust and input the desired rotational moment to the moment control system, in the mounted position of the self-hoisting crane 1 on the nacelle 2 or on the tower 46, the self-hoisting crane may simply be set to lower the hook block 6 from the top of the boom arm 5 without any external load attached to the hook block 6, whereby the hook block 6 is lowered by means of the motor drive 27 rotating the motor driven traction sheaves 12.sub.1, 12.sub.2. The moment control system is set to raise the rotational moment until a pair of ballast weights for the cables 7, 8, arranged in the container 33, are lifted by the cables 7, 8. When this happens, the rotational moment with which the traction sheaves 12.sub.1, 12.sub.2 are rotated, is large enough to outbalance the weight of the part of the cables 7, 8 extending from the nacelle 2 or from the tower 46 to the container 33. This rotational moment may subsequently be set as the desired rotational moment for said moment control system. The system of ballast weights for the cables 7, 8 is well-known in the art and is described in WO 2020221716 A1 mentioned above.

LIST OF REFERENCE NUMBERS

(31) a angle A axis through rotation axis of traction sheave and rotation axis of pressure wheel D guide direction for traction sheave F.sub.1 force corresponding to weight of cables extending from hook block to cable winch F.sub.2 force corresponding to weight of hook block F.sub.P pulling force M.sub.1 first moment arm M.sub.2 second moment arm T transverse direction of cable W.sub.1 first wrap angle W.sub.2 second wrap angle 1 self-hoisting crane 2 nacelle of wind turbine 3 wind turbine 4 pedestal 5 boom arm 6 hook block 7, 8 cable 9 cable winch arranged at ground 10 ground 11 cable crawler 12.sub.1, 12.sub.2 motor driven traction sheave 13 active state of cable crawler 14 passive state of cable crawler 15.sub.1, 15.sub.2 first support sheave 16.sub.1, 16.sub.2 second support sheave 17 first traction side of cable 18 second side of cable 19 active position of traction sheave 20 passive position of traction sheave 21 intermediate position of traction sheave 22.sub.1, 22.sub.2 pressure wheel 23 rotation axis of traction sheave 24 rotation axis of pressure wheel 25 rotation axis of first support sheave 26 rotation axis of second support sheave 27 motor drive for motor driven traction sheave 28 motor housing of cable crawler 29 rotation point of housing 30 frame carrying first and second support sheaves 31 spring pack 32 crane base 33 container for self-hoisting crane 34 compression spring 35 pin for compression spring 36, 37 cross-pin 38 bearing of boom arm 39 bearing of motor housing 40 locking rod of spring pack 41 locking nut 42 bearing in boom arm 43 inductive switch 44 metal disc for inductive switch 45 ballast weight for cable in container 46 tower of wind turbine