Abstract:
A method for influencing a cable winch force acting on a cable drive, comprises the method steps providing a cable drive with a drivable winch and with a cable that can be wound on the winch, providing a device for producing a traction sheave cable force on the cable, determining an outer cable force, predetermining a cable drive operating state, providing a control-regulating unit to influence the traction sheave cable force, producing a control-regulating variable by means of the control-regulating unit depending on the outer cable force and the predetermined cable drive operating state, producing the traction sheave cable force by means of the device and influencing the traction sheave cable force by means of the control-regulating unit in such a way that the cable winch force acting on the cable drive can be controlled depending on the respective cable drive operating state and the outer cable force, wherein the device is a traction sheave drive, wherein a four-quadrant operation of the traction sheave drive is reproduced by means of the control-regulating unit, and wherein the four traction sheave drive operating states are no-load lifting, no-load lowering, load lifting and load lowering.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the priority of Patent Application Serial No. DE 10 2013 201 860.6 filed on Feb. 5, 2013, pursuant to 35 U.S.C. 119 (a)-(d), the content of which is incorporated herein as if fully set forth herein. 
     FIELD OF THE INVENTION 
     The invention relates to a method for influencing a cable winch force acting on a cable drive and a device for carrying out a method of this type. 
     BACKGROUND OF THE INVENTION 
     Devices for winding a cable onto a winch of a cable drive are known from DE 10 2004 046 130 A1, from FR 2 843 954 A1, DE 24 51 547 A1, DE 23 01 623 A1, DE 38 19 447 C2, DE 10 2007 031 227 A1, U.S. Pat. No. 4,172,529 and from U.S. Pat. No. 4,204,664. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to improve a method for influencing a cable winch force acting on a cable drive in such a way that the cable winch force acting on the cable drive can be controlled depending on a respective cable drive operating state and an outer cable force. 
     This object is achieved by a method for influencing a cable winch force acting on a cable drive, comprising the method steps of providing a cable drive with a drivable winch and with a cable that can be wound on the winch, providing a device for producing a traction sheave cable force on the cable, determining an outer cable force, predetermining a cable drive operating state, providing a control-regulating unit to influence the traction sheave cable force, producing a control-regulating variable by means of the control-regulating unit depending on the outer cable force and the predetermined cable drive operating state, producing the traction sheave cable force by means of the device and influencing the traction sheave cable force by means of the control-regulating unit in such a way that the cable winch force acting on the cable drive can be controlled depending on the respective cable drive operating state and the outer cable force, wherein the device is a traction sheave drive, wherein a four-quadrant operation of the traction sheave drive is reproduced by means of the control-regulating unit, and wherein the four traction sheave drive operating states are no-load lifting, no-load lowering, load lifting and load lowering. 
     It was recognised according to the invention that a cable winch force can be controlled depending on a respective cable drive operating state and on an outer cable force by exerting a traction sheave cable force on a cable of a cable drive. By combining the cable drive, which basically allows only two operating states, with a device for producing a traction sheave cable force, which is, in particular, configured as a traction sheave drive, four different operating states can be reproduced. As a result, it is possible to influence a cable winch force acting on the cable drive, which can be determined, in particular, by means of a cable force measuring unit, in such a way that the cable winch force can be controlled depending on a cable drive operating state and on an outer cable force. Controlled and low-wear winding is made possible in that the cable is wound on at a cable winch force which is as constant and, in particular, low as possible. An additional drive of the cable in front of the winch can make this possible. This drive principle is based on the cable friction according to the Euler-Eytelwein formula. An acceptable tolerance range of the cable winch force, which is, in particular, +/−20% of a predetermined desired cable winch force, is taken to mean a constant cable winch force. In particular, the acceptable cable winch force range comprises +/−10% of the predetermined desired cable winch force and, in particular, +/−5% of the predetermined desired cable winch force. For example, a value of 2% of a minimum breaking force of the cable or of 10% of the nominal force of the cable drive is used as the desired cable winch force, which is used to prestress the cable for optimal winding or unwinding of the cable. By means of a cable, the device can be connected at a first cable end to the cable drive and at a second cable end to a load receiving device, such as, for example, a load hook, in particular a hook block. The cable in particular, in each case, loops the two traction sheaves. The cable drive operating state is fixed by an actuating direction of the cable drive, for example by a rotational direction of a winch of the cable drive, in other words a winding or unwinding of the cable. An outer cable force is caused, in particular by a load received by the load receiving device. Various traction sheave drive operating states can be determined depending on the respective cable drive operating state and depending on the outer cable force for a provided device for producing a traction sheave cable force on the cable. Four traction sheave drive operating states are produced, in other words with or without a load suspended on the load receiving device and the winding or unwinding of the cable from the winch of the cable drive. These traction sheave drive operating states are designated no-load lifting, i.e. winding the cable without a load, no-load lowering, i.e. unwinding the cable without a load, load lifting, i.e. winding the cable with a load and load lowering, i.e. unwinding the cable with a load. In particular, it is therefore possible using the method according to the invention to both wind and unwind the cable in a controlled manner, in other words at a constant cable winch force, it being unimportant whether the cable drive is loaded by an outer load, i.e. whether a load is suspended on the load receiving device or not. Wear to the cable, in particular on a winch wound in several layers, is reduced. Since, a monitoring and adaptation of the cable winch force is made possible in particular also when winding the cable onto the winch of the cable drive, winding errors and/or a cable fault as a result of a too loose, unstable cable assembly can be avoided. In particular, it can be avoided that a cable wound on incorrectly in this manner, which is then subject to a strong outer cable force as a result of a high outer load, is drawn in or forced in from an upper layer of a cable winding into deeper layers located therebelow with a looser winding. This form of cable damage is ruled out by the method according to the invention. Additional cable-braking devices, which are called cable baiters, can be dispensed with in the method according to the invention. 
     In particular, a four-quadrant operation of the traction sheave drive can be reproduced by means of a control-regulating unit. 
     Additional operating states are made possible thereby, which are not depicted by means of a two-quadrant operation known from DE 10 2004 046 130 A1 for producing a constant load when winding a cable onto the cable drive. In particular, the method according to the invention allows additional operating states to be depicted. The depiction of the additional operating states takes place by means of an adjustment of the operating states in an incremental control range. This means that the control of the cable force acting on the cable drive is also possible for the additional operating states of load lowering and no-load lowering. The cable guidance and the cable stress are thereby improved. In particular, the situation is ruled out of a so-called hanging cable or slack cable being produced as a result of low stressing of the cable, as, because of the outer cable force, the cable is not stressed by adequate tensile loading. An inadequate tensile loading may be present when, for example, the loading of the cable is only provided by a suspended hook or a reeved load block, and, in particular, an outer load is absent. Furthermore, it can be ruled out that a tearing of the cable will occur as a result of over-stressing. In particular, a method of this type can be advantageously used for mutual control of multiple cable receiving in double cable operation. In multiple cable receiving, the cables can be decelerated very differently because of cable braking forces caused by the sheaves. In this case, a compensation of the cable forces takes place in such a way that a tilting, in other words, a twisting of a double load block is avoided with, in particular, additional devices, which are known from EP 1 924 520 B1 and/or from EP 1 773 706 B1 for avoiding the tilting of a double load block, not being required. These cable force differences can already be dynamically taken into account and avoided, in particular compensated, in the cable run during four-quadrant operation. It is conceivable that a method in the four-quadrant operation mentioned of the traction sheave drive will for the first time allow a double cable drive with extremely long cable lengths of, for example, more than 1000 m. 
     It is advantageous if the traction sheave drive has a drive motor, in particular an electric motor, which, depending on the cable drive operating state, provides a torque of a required size, so that a traction sheave cable force caused by the cable sheave drive leads to a desired cable winch force. In particular, a control algorithm of the traction sheave drive depends directly on the cable drive operating state. 
     A method according to which the cable winch force acting on the cable drive can be controlled in such a way that it is reduced or increased relative to the outer cable force allows an advantageous control of the cable winch force in relation to the outer cable force. 
     A method according to which the cable winch force acting on the cable drive can be controlled in such a way that the traction sheave cable force follows a predetermined characteristic curve depending on the outer cable force allows a rapid and effective control of the cable winch force. 
     A method according to which the outer cable force is determined indirectly from the load force allows a rapid and uncomplicated determination of the outer cable force. 
     A method according to which the outer cable force is determined directly by means of a cable force measuring device allows a particularly precise determination of the outer cable force. 
     A method according to which the traction sheave cable force is determined that can be transmitted by means of the device from the outer cable force allows the traction sheave cable force to be monitored. 
     A method according to which the rotational direction of the winch, which is predetermined, in particular, by an operator, is considered to produce the control-regulating variable allows improved control of the cable winch force. 
     A method according to which a plurality of input variables, in particular the outer cable force, the load force, the rotational direction and/or the rotational speed of the winch, is used to produce the control-regulating variable allows various influencing variables for producing the control-regulating variable to be taken into account. 
     A method according to which the traction sheave cable force is controlled in such a way that the resulting cable winch force is independent of the rotational speed of the winch allows a control of the traction sheave cable force in such a way that the resulting cable winch force is independent of the rotational speed of the winch. The traction sheave cable force reacts directly to a change in the outer cable force due to the pressure level in the closed control circuit. The method is independent of the speed of the cable and, in particular, of accelerations or decelerations of the cable. 
     A further object of the present invention is to improve a device for influencing a cable winch force acting on a cable drive, in order, in particular, to reduce cable wear and to avoid winding errors when winding the cable. 
     This object is achieved by a device for carrying out a method according to any one of the preceding claims, wherein the device comprises two traction sheaves that can be looped by a cable and at least one drive to drive at least one of the traction sheaves, wherein a traction sheave cable force is produced on the cable by means of the traction sheaves and is influenced by means of a control-regulating unit in such a way that the cable winch force acting on the cable drive can be controlled depending on a respective cable drive operating state and an outer cable force. 
     According to the invention, it was recognised that two traction sheaves are used to exert a traction sheave cable force on a cable of a cable drive, the traction sheaves being drivable independently of one another at least by means of one drive and, in particular, by means of a drive in each case. The device ensures that the cable force acting on the cable drive is monitored independently of the respective operating type of the cable drive. The traction sheave drive can thus be controlled independently of the cable drive. It is thus possible by means of the two traction sheaves to assist the winding and unwinding of the cable from the winch of the cable drive in a targeted manner, i.e. to load the winch of the cable drive or to relieve it. Because of the assisting effect of the traction sheave cable force, the winch of a primary cable drive can be designed to be smaller and, in particular, with a reduced power and brake. As a result, the total weight of the winch arrangement of a work machine can be reduced and the cost outlay reduced. It is also possible to retrofit said device on an already existing work machine. When the device is configured as a refitting kit for an existing work machine, it is, in particular, unnecessary to place increased safety demands on the traction sheaves, as functions relevant to safety such as, for example, a braking function have to be satisfied in any case on the cable drive present on the work machine. In particular, the same safety demands are made of the device as a retrofitting kit as of a primary cable drive. Even a temporary failure of the device, for example the traction sheaves icing over can be tolerated. The device, as a retrofitting kit, can be implemented in an uncomplicated manner, in particular with reduced functions, and economically. It is possible to provided pre-equipped brackets and/or hydraulic lines on an intermediate piece in order to simplify later retrofitting of the device according to the invention. It is advantageous to provide, in the geometric vicinity of a traction sheave drive, a self-sufficient hydraulic unit, which is known, for example, from EP 1 641 703 B1 and to already set up the necessary cables for this beforehand. 
     A device in which the control-regulating unit has a signal connection to the at least one drive to control or regulate the drive torque and/or drive rotational speed of the drive allows an automatic adaptation and control of the traction sheave cable force by controlling the drive torque and/or drive rotational speed of at least one of the drives. 
     A device in which the at least one drive is a hydraulic motor, an electric motor or a motor-gearing combination allows a simplified and direct activation of the drives. In particular, it is advantageous for a predetermined desired torque to be able to be directly produced and activated. A device with hydraulic drives for the traction sheaves can be can be realised in an uncomplicated and economical manner. In particular, it is possible to provide a supply of the hydraulic drives by means of a hydraulic mechanism which is present in any case on a work apparatus. It is also possible for the hydraulic drives to be activated by a closed, self-sufficient hydraulic circuit. The use, in particular, of frequency-controlled electric motors allows a direct and more precise control of the drive torque. The electric motors can also be more easily integrated into a possible control loop. A control geared at this can take place close to real time. Moreover, electric motors have improved efficiency compared to hydraulic drives. The environmental pollution is reduced due to reduced emissions. The drive can also be configured as a motor-gearing combination. A combination of this type allows a particularly compact implementation of the drive. The drive can thereby be arranged, in particular, advantageously on the device and, overall, allows a compact, weight-reduced configuration of the traction sheave drive. 
     A device in which the at least one drive has an automatic torque control allows a simplified and effective control of the traction sheaves. 
     A device in which each traction sheave has a plurality of grooves for cable guidance allows a targeted and, in particular, robust cable guidance on the traction sheaves. In particular an overlaying of individual cable strands in the device is avoided. A device in which the traction sheaves in each case have a different number of grooves and, in particular, one traction sheave has precisely one groove more than the respective other traction sheave, allows an advantageous cable guidance. 
     A device in which the traction sheaves are arranged in a receiving frame allows an uncomplicated and simultaneously stable arrangement of the device on a work machine, in particular a crane. 
     An embodiment of the invention will be described in more detail below with the aid of the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a perspective view of a lattice boom of a crane with two devices according to the invention, 
         FIG. 2  shows an enlarged detailed view of a device according to  FIG. 1 , 
         FIG. 3  shows a view of a cable guidance on the device, corresponding to  FIG. 2 , 
         FIG. 4  shows a sectional view along the line IV-IV in  FIG. 2 , 
         FIG. 5  shows a schematic diagram of the device with forces acting on a cable of a cable drive in a traction sheave drive operating state no-load lifting, 
         FIG. 6  shows a view corresponding to  FIG. 5  in a traction sheave drive operating state load lifting, 
         FIG. 7  shows a view corresponding to  FIG. 5  in a traction sheave drive operating state load lowering, 
         FIG. 8  shows a view corresponding to  FIG. 5  in a traction sheave drive operating state no-load lowering, 
         FIG. 9  shows a schematic view of a closed hydraulic circuit for the device according to the invention and 
         FIG. 10  shows a view of a characteristic curve for controlling the cable winch force 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows a lattice boom  1  of a crane, which, in particular, may be configured as a boom crane. Two devices  2  according to the invention are fastened to the lattice boom  1 . The devices  2  are arranged along a cable guide of a cable drive, not shown, between a winch, not shown, of the cable drive and a load receiving device, not shown, to receive an outer load. Since two devices  2  are arranged on the lattice boom  1 , the operation of a double hook block as the load receiving device is possible. It is also possible to use precisely one, or more than two devices  2  to operate a hook block on a work machine. 
     The use of the device  2 , which is configured as a traction sheave drive, can be applied to various work machines, in particular a crawler crane. 
     The device  2  will be described in more detail below with the aid of  FIGS. 2 to 4 . The device  2  has a receiving frame  3  in which a first traction sheave  4  and a second traction sheave  5  are arranged. The first traction sheave  4  is driven about its rotational axis  6  by a first drive  7  by means of a first gearing  8 . The first drive  7  is configured as a hydraulic drive. The first gearing  8  is held by means of a first flange  9  on a vertical wall  10  of the receiving frame  3  in the axial direction of the rotational axis  6 . The first gearing  8  furthermore has a second flange  11 , on which the first traction sheave  4  is held in the axial direction along the rotational axis  6 . The first traction sheave  4  is placed with a gearing opening  12  on the first gearing  8 . For simplified assembly of the device  2 , the first gearing  8  is configured to be conically tapering in a portion to be received in the gearing opening  12 . 
     A bearing journal  14 , which is rotatably mounted in a floating bearing  15 , which is arranged in a bearing opening  16  of the first traction sheave  4 , is provided on a bearing vertical wall  13  arranged opposite the drive vertical wall  10 . The first drive  7 , the first gearing  8 , the bearing journal  14  and the floating bearing  15  are oriented concentrically with respect to one another along the rotational axis  6 . 
     The first traction sheave  4 , at its outer cylinder casing face, has four grooves  17 , which are used to guide the cable during the winding and unwinding of a cable from the first traction sheave  4 . The grooves  17  are in each case separated by groove rims arranged in between. Furthermore, the first traction sheave  4  has flanks  18  directed obliquely outwardly from the grooves  17 . 
     The second traction sheave  5  is held in an identical manner on the receiving frame  3 . The second traction sheave  5  can be driven about its rotational axis  19  by means of a second drive  20  by means of a second gearing  21 . The only difference is that the second traction sheave  5  has three instead of four grooves  17 . As a result, a guidance of a cable  22  shown by a dash-dot line in  FIG. 3  is made possible. According to  FIG. 3 , the cable  22  runs from the top left, coming from the load receiving device, into the first traction sheave  4 . The cable  22  is wound along the grooves  17  of the traction sheaves  4 ,  5 . The number of loops  23  of the cable  22  is produced from the number of grooves  17  of the two traction sheaves  4 ,  5 . Since the first traction sheave  4  has one groove  17  more than the second traction sheave  5 , the cable  22  is deflected on the traction sheaves  4 ,  5  of the device  2  in such a way that an inlet  24  of the cable  22 , coming from the load receiving device, and an outlet  25  of the cable  22  toward a cable drive, are arranged in the same plane.  FIG. 3  shows the inlet  24  at the top left and the outlet  25  at the bottom right. The inlet  24  and the outlet  25  are parallel to one another. 
     The device  2  furthermore has a control unit, not shown in  FIGS. 1 to 4  and described below, which has a signal connection to the drives  7 ,  20 . The control unit is used to control the drive torque and/or the drive rotational speed of the drives  7 ,  20 . In addition or as an alternative, each drive  7 ,  20  may have an automatic torque control, not shown, which is called a mooring control. The pump pressure is used as the control variable for the mooring control. The predetermined pressure is kept constant by the pump, from which a constant torque follows at the hydraulic motors of the traction sheave drive, regardless of the rotational direction of the drives 
     The mode of functioning of the device  2 , in other words a method for influencing a cable winch force acting on a cable drive, will be described in more detail below with the aid of  FIGS. 5 to 8 . 
     The device  2  is connected at a first cable end of the cable  22 , shown on the left in  FIG. 5 , by means of a cable pulley  26  of the upper and lower load block to the load receiving device in the form of a hook block  27  shown symbolically. With multiple cable receiving, a plurality of cable pulleys  26  may also be provided. Furthermore, the device  2  at a second cable end of the cable  22 , shown on the right in  FIG. 5 , is connected to a winch  28 . The winch  28  is part of the cable drive designated  29  as a whole. The cable drive  29  has a drive, not shown, for driving the winch  28  about the rotational axis  30  of the winch. According to the view in  FIG. 5 , no load is suspended on the hook block  27 . A load force  31  introduced by the hook block  27  is small and is substantially based on the inherent weight of the hook block  27  and the cable  22 . In  FIG. 5 , the cable  22  is wound by the cable drive  29  onto the winch  28 . For this purpose, the winch  28  is rotated according to  FIG. 5  in the clockwise direction about the rotational axis  30  of the winch along the rotational direction  32  of winding. A cable drive operating state is thereby fixed, in other words, in particular, by the predetermining of the rotational direction  32  of winding of the winch  28 .  FIG. 5  shows the traction sheave drive operating state no-load lifting. 
     The outer cable force  33  acting on the cable drive  29  is determined by means of a cable force measuring device, not shown, which may be configured, in particular, as a load torque limiter that is present in any case on a crane. The outer cable force  33  provides the prestressing, with which the cable  22  is wound onto the winch  28 . The cable force  33  determined can, in particular, be used as an input signal for the control unit  34  of the device  2 . As an alternative to the cable force measuring device, which allows a direct determination of the outer cable force  33 , it is also possible to indirectly determine the outer cable force  33  from the load force  31 . The indirect determination of the outer cable force  33  is possible in an uncomplicated manner. In particular, the apparatus outlay for this is small. 
     The cable  22  is wound using a cable winch force  35  onto the winch  28 . In order to ensure that in the operating state no-load lifting, the cable  22  is wound with adequate prestressing, in other words not too loosely, onto the winch  28 , the traction sheaves  4 ,  5  of the device  2  are activated and, in particular controlled, in such a way that a traction sheave cable force  36  on the cable  22  counteracts the cable winch force  35 . The cable winch force  35  is controlled by the traction sheave cable force  36 . The cable winch force  35  is the resultant of the traction sheave cable force  36  and outer cable force  33 . The outer cable force  33  is produced from the load force  31  depending on the loading condition from the system, comprising the cable  22 , the load block, or a simple load receiving device. The outer cable force  33  counteracts the cable winch force  35 . This means that the outer cable force  33  and the cable winch force  35  compensate one another. The outer cable force  33  and the cable winch force  35  are the same in terms of amount, in particular during conventional operation of the device, and mutually cancel one another. A resulting force formed from these two forces  33 ,  35  is 0. In order to be able to change the cable winch force  35  with a predetermined outer cable force  33 , in particular to increase or reduce it, the traction sheave drive  2  is inserted. Depending on the load condition and operating type, the cable winch force  35  can be increased or reduced by the traction sheave cable force  36 . In particular, the direction of action of the traction sheave cable force  36  can be adjusted by the drive direction of the traction sheaves  4 ,  5 . The traction sheave cable force  36  can thus be adjusted to be in the same or opposite direction to the cable winch force  35 . 
     The interrelation of the cable forces  33 ,  35  and  36  is graphically shown in the characteristic curve graph according to  FIG. 10 , in that the cable winch force  35  and the traction sheave cable force  36  are in each case shown as a function of the outer cable force  33 . By way of example, the respective forces are given in N in  FIG. 10 . The characteristic curve graph shows purely qualitatively the interrelations between the forces  33 ,  35  and  36 . The cable winch force  35  is shown as a dashed line in the characteristic curve graph according to  FIG. 10 . The cable winch force  35  is identical to the outer cable force  33  if no traction sheave cable force  36  is provided. Accordingly, the cable winch force  35  is a line through the origin with the slope  1 . The traction sheave cable force  36  is shown in  FIG. 10  by means of a continuous line. The continuous line corresponds to a possible predetermined characteristic curve for the traction sheave cable force  36 . The traction sheave cable force  36  has a linearly rising region, the slope of the characteristic curve being smaller than that of the cable winch force  35 . On reaching or exceeding a critical outer cable force  33  F crit , which, according to the characteristic curve graph shown, is 180 N, the traction sheave cable force  36  follows a plateau, i.e. the traction sheave cable force  36  is constant for an outer cable force  33 , which is greater than F crit . The characteristic curve  36  can also have a falling portion. The characteristic curve can, at least in portions, also be non-linear and, in particular, have a square, cubic, exponential, logarithmic or otherwise curved functional course. Furthermore, a dash-dot line  42  is shown in  FIG. 10 . The dash-dot line  42  shows the course of the cable winch force  35 , which is reduced by the traction sheave cable force following the characteristic curve  36 . This applies to the operating states no-load lifting ( FIG. 5 ) and no-load lowering ( FIG. 8 ). It is also possible for the characteristic line  36  to be added to the cable winch force  35 . This applies to the operating states load lifting ( FIG. 6 ) and load lowering ( FIG. 7 ). 
     In order to ensure the traction sheave cable force  36  according to  FIG. 5 , the traction sheaves  4 ,  5 , which rotate in the same direction as the winding rotational direction  32  along a traction sheave rotational direction  37  about the respective rotational axis  6 ,  19 , are driven with a drive torque in the same direction. The two traction sheaves  4 ,  5  are driven by a hydraulic drive  7 , which is supplied by a hydraulic pump  38  with hydraulic medium, controlled by the control unit  34 . 
     The traction sheave drive operating state load lifting according to  FIG. 6  differs from that according to  FIG. 5  in that an outer load  39  is attached to the hook block  27 . The load force  31  is comparatively high. Because of the high load force  31 , the drives  7 ,  20  are activated by the control unit  34  with a drive torque in such a way that the traction sheaves  4 ,  5  are driven, assisting the winch  28 , along the traction sheave rotational direction  37 . As a result, a traction sheave cable force  36  is brought about on the cable  22  and acts in the same direction as the cable winch force  35 . This means that the traction sheave cable force  36  caused by the traction sheaves  4 ,  5  relieves the winch  28 . The cable  22  is wound onto the winch  28  at the cable winch force  35 , the cable winch force  35  being less than the outer cable force  33 . This can avoid the cable  22  being wound too tightly as a result of a high load  39 . Unacceptably high elongations of the cable  22  are avoided. 
     The traction sheave drive operating state load lowering according to  FIG. 7  differs from the operating state according to  FIG. 6  in that the load  39  is lowered, i.e. the winch  28  is rotated about the rotational axis  30  along the unwinding rotational direction  40 . The cable  22  is unwound from the winch  28 . In order to reduce the high load force  31  as a result of the load  39 , the traction sheaves  4 ,  5  of the device  2  are activated by the control unit  34  in such a way that the traction sheaves  4 ,  5  are loaded with a drive torque directed counter to the unwinding rotational direction  40 . The traction sheave cable force  36  acts in a relieving manner in the same direction as the cable winch force  35  in order to avoid too high a traction load of the load  39  during unwinding. The cable  22  is decelerated by the device  2  during unwinding. The device  2  relieves the winch  28 . 
     The traction sheave drive operating state no-load lowering shown in  FIG. 8  differs according to the state shown in  FIG. 7  in that the cable  22  is unwound without a load. Accordingly, the winch  28  is rotated about the rotational axis  30  along the unwinding rotational direction  40 . Since no load is suspended on the hook block  27 , the load force  31  is small. So that the cable winch force  35  is large enough when unwinding the cable  22  from the winch  28 , a traction sheave cable force  36  is provided by the device  2 , which counteracts the winch force  35 . The traction sheave cable force  36  thus brings about an unwinding with a predetermined cable winch force  35 , in that the cable  22  is drawn off from the winch  28  by means of the device  2 . In this case, the cable winch force  35  is greater than the outer cable force  33 . 
     The mode of functioning of the hydraulic control will be described in more detail below with the aid of  FIG. 9  and the traction sheave drive operating state load lowering according to  FIG. 7 . The hydraulic drives  7 ,  20  are controlled in such a way that, depending on the outer cable force  33  and the traction sheave rotational direction  37 , the drive torques of the traction sheaves  4 ,  5  bring about the traction sheave cable force  36 . In the embodiment shown according to  FIG. 9 , the drive torques of the hydraulic drives  7 ,  20  are realised by means of an automatic torque control. The drives  7 ,  20  are activated by means of a hydraulic pump  38  working in a closed hydraulic circuit. The amount and the direction of the drive torque are controlled by means of a pressure unit  41  at the pump.