BRAKING ARRANGEMENT FOR SECURING A CONVEYING DEVICE, CONVEYING DEVICE AND CRANE SYSTEM

Description

TECHNICAL FIELD

[0001] The present disclosure relates generally and specifically to a brake assembly for securing a conveying device, in particular a crane system. It is intended for an overload situation when, during the conveying of the material, i.e., when the material is in motion, a load exceeding the normal operating load, the overload, occurs.

[0002] Such overloads can occur particularly in crane systems, but also in elevators or other conveyor systems, for example if the material being conveyed becomes caught or jammed during conveyance. TECHNICAL BACKGROUND

[0003] Particularly in crane systems, there is a risk that loads may become caught on objects or projections protruding into the conveyor path during upward movement. In such a case, loads can occur that severely damage the conveyor system or, in the case of freestanding cranes, even cause the conveyor system to tip over. DE 10 2013 200514 discloses a braking arrangement according to the preamble of claim 1.

[0004] In a known overload protection device, such as that specified in DE 202 19 282 U1, a load-dependent clutch disconnects a hoist cable winch from a hoist cable winch drive in the event of an overload. A hoist cable brake, which also acts, allows load-controlled lowering of the hoist load after the clutch is disengaged.

[0005] A particular problem exists with container crane systems, so-called "container bridges," which lift containers from the narrow loading bays of container ships during handling. Containers can become jammed and stuck in these loading bays. The resulting overload can lead to severe overloading of the crane bridge if the handling process continues, and in the worst case, even breaking off and collapsing.

[0006] Due to the sharp increase in handling speeds, such incidents can occur with both a loaded and an unloaded spreader. In addition to the risk of damage to the crane bridge, there is also the risk of damage to the spreader itself or to the loading bays provided in the container ships if a so-called "snag" occurs, in which the load or spreader becomes jammed or stuck during a lifting operation. Various approaches are known for handling such snag incidents.

[0007] Traditional snag-load systems detect overloads and release the haulage ropes via hydraulically controlled unloading slides, allowing the attached containers or spreader to be removed from their jammed position and then, after unloading, to be conveyed properly again. Such hydraulically controlled unloading devices are very complex and maintenance-intensive, requiring complex support rope guidance. This is especially true when, as is common with container gantry cranes, two hoist rope systems are provided for each container spreader that are synchronized during operation. Such systems require a snag-load system for each hoist rope system.

[0008] An improvement to this system is known from EP 1 979 260 B1. The control and regulation arrangement specified therein comprises a braking device acting on the conveyor device, as well as a controller for the braking device. Furthermore, an overload sensor is specified, which detects an overload and emits an overload signal when an overload clutch is triggered or the separation of the overload clutch is detected. In response to the overload signal, the controller acts on the braking device in such a way that it blocks the conveyor device and thus secures the conveyed material.

[0009] In the context of this disclosure, the term "conveyed goods" is intended to encompass both a variable conveyed goods or a load—for example, a container—as well as a device for receiving such a load. Such a device could, for example, be a so-called "spreader," which can engage with corresponding corner fittings of a container at several corner points. The term "conveyed goods" can also include an elevator car or similar device.

[0010] The term "control" shall be used below to refer to both a classic (open) control system, in which one or more input variables influence one or more output variables of a system, and a (closed) control system, in which the control takes place in a control loop and a controlled variable is continuously compared as a dependent variable with a predetermined variable and automatically adjusted to match this so-called "reference variable." The term "control" shall also be used below to refer to a system that performs both open-loop and closed-loop control functions or even simple digital control processes.

[0011] Another approach to detecting a so-called "snag" is known from EP 2 313 336 B1. It is based on the use of a measuring system in which force transducers or load sensors are provided on the lifting cables or locking pins. These sensors incorporate an electrical measuring system, e.g., strain gauges. As soon as the container or the spreader itself is lifted, the measuring areas of the measuring pins deform and generate load-related measurement signals.

[0012] The greater the force acting on the measuring axes, the higher the measurement signal. For example, force signals can be processed or adjusted in such a way that, if a nominal load is exceeded, an emergency shutdown of the crane or an interruption of the lifting process is triggered. Such overload protection devices are also suitable for preventing or detecting a so-called "snag load" condition.

[0013] "Snag load" refers to an unintentional increase in the load on the crane, e.g., due to the load or spreader becoming caught in a vessel being unloaded or loaded, where the total load can increase symmetrically or asymmetrically if the spreader or container becomes caught or tilted.

[0014] EP 2 313 336 B1 deals with the dynamic detection of such a malfunction (snag load condition), in which the signal from a force sensor is monitored and a shutdown signal is issued when a nominal overload threshold is exceeded. Lifting times with or without additional load are taken into account, averaged weight forces are determined as the base load, and dynamic jump thresholds are defined as overload thresholds that are greater than the base load and less than a nominal overload threshold. This dynamic jump threshold is then used to generate a shutdown signal when the jump threshold is reached.

[0015] However, with all known snag load detection systems, in addition to the problem of resuming normal operation, there is also the problem of braking the various drive elements as quickly as possible in a suitable manner without overloading interacting elements of a drive chain and nevertheless being able to realize the fastest possible, comprehensive braking function.

[0016] In particular, container crane systems have the problem that multi-stage spur gears are connected between relatively fast-rotating drive motors (approx. 2000 revolutions per minute) and the relatively slow-rotating rope drums (approx. 20 revolutions per minute). To achieve the best possible braking effect, brakes act on the drive shafts of the motors (at the gearbox input) and on the drive shafts of the rope drums or the rope drums themselves (at the gearbox output).

[0017] The task is therefore to provide an improved braking arrangement that is capable of implementing the fastest possible emergency braking of the load even in the event of a snag and, at the same time, controlling the braking components that act on different points of a drive chain in such a way that no overloads occur within the load transmission chain.

[0018] A further task can be seen in providing a simple braking arrangement that is suitable for both the snag case and normal service braking operations. SUMMARY

[0019] According to a first aspect, the following disclosure represents a brake arrangement which serves to secure a conveyor device, in particular a crane system. The brake arrangement comprises a first braking device acting on a first drive element, a second braking device acting on a second drive element, a transmission device acting between the first and second drive elements, in particular a gear, and a load sensor which detects a load signal and transmits it to a controller, and the controller is set up such that, in response to a load signal which exceeds an overload threshold, it initiates an emergency braking state and in doing so controls the first and the second braking device such that they act on the first and second drive elements substantially simultaneously within a first braking action time, wherein one of the first and second braking devices is designed such that, in a normal braking state, it acts on the first and / or the second drive element within a second braking action time and the first braking action time is shorter than the second braking action time.

[0020] According to a second aspect, the present disclosure relates to a conveyor device with such a brake arrangement.

[0021] And a third aspect relates to a crane system in which two corresponding conveyor devices are provided which are synchronized for moving a container spreader up and down, wherein the two conveyor devices each have a brake arrangement according to the first aspect, which can be controlled synchronously via a common control system.

[0022] Further aspects and features emerge from the dependent claims, the attached drawings and the following description of embodiments. BRIEF DESCRIPTION OF THE DRAWING

[0023] Embodiments will now be described by way of example and with reference to the accompanying drawings, in which: Fig. 1 is a schematic representation of an embodiment of a crane system with a brake arrangement according to the invention; Fig. 2 is a schematic representation of a control system for controlling the Figure 1 illustrated brake arrangement; Fig. 3 is a schematic representation of the hydraulic functionality of a release device for a first brake device; and Fig. 4 is a schematic representation of the hydraulic functionality of a hydraulic unit for actuating a second brake device. DESCRIPTION OF EMBODIMENTS

[0024] Before a detailed description of the embodiment with reference to Fig. 1 First, general explanations of the designs follow,

[0025] The braking arrangement according to the invention is characterized in that both an emergency braking state and a normal braking state can be realized with the same braking devices. This is made possible by the fact that, in an emergency braking state, at least one of two braking devices acts on a first or a second drive element within a first (short) braking action time, and in a normal braking state, within a second (longer) braking action time.

[0026] The shortened braking time places a significantly increased strain on the braking systems. Especially with lever brakes, the brake levers are subjected to very high loads. This makes it possible to set the desired braking conditions with the same brake both in normal operation (regular operation with extended braking time) and in an emergency (emergency operation with reduced load).

[0027] Typically, in an emergency braking condition, both the first and the second braking device are activated simultaneously, so that the braking effect can be achieved essentially simultaneously on the drive and output sides and a transmission device between the first and second drive element remains largely load-free.

[0028] This is especially necessary in cases where a relatively fast-rotating electric drive motor is provided on the drive side, which drives a cable drum (second drive element) via a drive shaft (first drive element) and multi-stage reduction gear (transmission device) on the output side with reduced speed but increased torque, for example.

[0029] In the emergency braking condition, the drive shaft and the output shaft of the cable drum or the cable drum itself are brought to a standstill almost simultaneously, so that the transmission device connected in between remains largely load-free.

[0030] In a normal braking condition, the second braking device is generally not used at all and the braking action time on the first drive element is increased (the brake applies more slowly) and thus the braking process is overall softer and less stressful for the individual elements.

[0031] There are designs in which the first braking device (e.g., acting on the drive shaft of a drive motor) comprises an electrohydraulic brake release device, in which a fluid pressure in a piston that counteracts the braking force can be reduced by means of two electromagnetically actuated valves. The first braking action time can be achieved by de-energizing both valves, and the second braking action time can be adjusted by de-energizing one of the valves. The resulting variation in the flow cross-section in the valve also determines the speed of the pressure reduction.

[0032] There are also designs where one valve is provided for the second braking action time and two additional valves are provided for the first braking action time – a total of three valves. With such a design, increased functional redundancy can be achieved, which increases the safety of a corresponding braking arrangement.

[0033] There are designs in which the second braking device can be controlled via a hydraulic unit and / or an electro-hydraulic brake release device, which comprises two redundant electrically controllable solenoid valves and by de-energizing one of the valves, a fluid pressure counteracting the braking force can be reduced and the braking effect on the second drive element can thus be triggered.

[0034] There are also designs in which two controllable solenoid valves are provided in a single hydraulic unit for two second braking devices, which reduce the pressure in both second braking elements by de-energizing them.

[0035] In another embodiment, with two second brake assemblies, a separate electrohydraulic brake release device is provided for each of the braking devices. Each comprises two redundant electrically controlled solenoid valves, each of which, by de-energizing, reduces the fluid pressure counteracting the braking force, thus triggering a rapid braking effect on the second drive element(s).

[0036] There are brake arrangements in which the control of both valves results in a braking action time of between 25 and 40 ms and by controlling only one valve a second braking action time of 180 and 250 ms can be achieved.

[0037] It is also possible that the first braking action time on the first braking device is slightly different from the first braking action time on the second braking device. In particular, a first braking action time on the first braking device can be shorter than the first braking action time on the second braking device.

[0038] In a conveyor system designed as a rope hoist, a crane system, or a component of a container crane system, the first braking device is usually located on the high-speed drive shaft of the drive motor, and the second braking device is located on the drive shaft of a rope drum, which has a higher force but slower speed, or on the rope drum itself. Different initial braking times or emergency braking times on the components are advantageous in terms of load-bearing capacity. Thus, the high-speed motor drive shaft is quickly braked and brought to a standstill first, and the rope drum is braked with a certain delay, with simultaneous or slightly delayed braking onset.

[0039] This also makes the emergency braking process gentle, especially for an intermediate reduction gear, since the input-side gear is already almost at a standstill when the slow-running output-side gear is brought to a standstill. This prevents overloading of the gear pairs within the gear unit. This is particularly important in a so-called "snag load" situation, where emergency braking is required when lifting a load.

[0040] There are designs in which the overload sensor is mounted on a load-handling device that is coupled to the conveyor. This could, for example, be a hook or a so-called "twistlock" on a container spreader, which is used to attach the loads to be suspended.

[0041] For container spreaders, the load sensor can be mounted on a load-carrying element (twistlock) itself and / or optionally additionally on a suspension cable connection. This allows for reliable detection of overloads and, in particular, asymmetrical snag load conditions on both a loaded container spreader (with a container attached) and an unloaded container spreader. Different overload thresholds can be taken into account, which are typically higher for a loaded spreader than for an unloaded spreader.

[0042] The invention is particularly applicable to conveyor systems configured as rope hoist systems, crane systems, or components of a container crane system. It is also particularly applicable to crane systems with two conveyor systems that are synchronized for moving a container spreader up and down. The two conveyor systems each have a braking arrangement according to the invention, and these two braking arrangements can also be controlled synchronously via a common control system.

[0043] There are also versions where the synchronization of the conveyor systems can be canceled in the event of an overload, and the control system acts optionally on the braking system of one or both conveyor systems in response to a relief signal. This also allows for a controlled lowering or raising of one side of the container spreader.

[0044] Returning to Fig. 1This shows a crane system 100 with two brake assemblies 1 according to the invention. The crane system 100 comprises two drive motors 2, each of which acts on a second drive element embodied as a cable pulley 5 via a first drive element embodied as a drive shaft 3 and a transmission device embodied as a reduction gear 4. The cable pulley 5 is coupled via an output shaft 6 to the reduction gear 4 and thus also to the drive shaft 3 and the drive motor 2.

[0045] In an alternative embodiment, the output shaft 6 itself can also form the second drive element.

[0046] A first braking device 7 (service brake) acts on the drive shaft 3, which can exert a braking force on the drive shaft 3, for example via a brake disc or brake drum connected to the drive shaft 3 in a rotationally fixed manner.

[0047] A second braking device 8 (safety brake) acts on the cable pulley 5 or on the output shaft 6, which can exert a braking force either via a brake disc formed on the brake drum 5 or, optionally, via a brake drum or brake disc non-rotatably coupled to the output shaft 6. Both braking devices 7, 8 can be controlled via a controller 9. Both brakes are so-called industrial brakes, in which the braking force is applied via a preloaded brake spring, and the braking effect can be released via an electro-hydraulic actuator (brake release device). A hydraulic cylinder overcomes the spring force when pressure builds up and releases the brake either via a brake lever linkage or directly.

[0048] A so-called "container spreader" 11, which can accommodate a container 12, is attached to the cable 10 of the cable drum. Load sensors 13 are provided to detect load signals. These sensors detect either the cable force or cable load and transmit a corresponding signal to a controller 9. Alternatively, or optionally, load sensors 13' can also be provided. These sensors are located on a load-carrying element (e.g., a so-called "twistlock"), which serves to pick up a load (e.g., a container). These load sensors 13' also detect the corresponding load and transmit a corresponding load signal to the controller 9.

[0049] The control system 9 comprises a crane control system 9a via which the normal operation of the crane system is controlled and via which, for example, the synchronization of the two drive motors 2 and the various braking devices 7 and 8 takes place.

[0050] The control system 9 also includes a safety control system 9b, which is designed to control an emergency braking condition. During normal operation, the crane system is controlled via the drive motors 2 and the first braking devices 7, which are designed as service brakes. For this purpose, the drive motors 2 are accelerated and decelerated accordingly, and if necessary, an additional braking effect is applied via the service brakes 7.

[0051] The second braking devices 8 or safety brakes are generally not used in normal operation.

[0052] At least the braking devices 7 are designed such that they can act on the drive shaft 3 within a first braking action time and / or within a second braking action time. The first braking action time is shorter than the second braking action time. The first braking action time serves for an emergency braking condition, while the (longer) second braking action time applies to a normal braking condition. The second braking action time causes a significantly lower load on the braking components of the first braking device 7 and the drive shaft 3.

[0053] The first braking action time is significantly shorter and serves to very quickly stop the drive shaft 3 in an emergency braking condition. Typical first braking action times are between 25 and 40 ms, while the second braking action times are between 180 and 250 ms. In the normal braking condition, the second braking device 8 is normally not used. In an emergency braking condition, however, the second braking device or safety brake is also used to additionally stop the cable drum 5 itself and thus end the movement of the container spreader 11 as quickly as possible. For this purpose, the second braking device, i.e. the safety brake 8, is also activated within the shorter first braking action time, so that both brakes 7 and 8 come to a standstill at approximately the same time.

[0054] However, it is possible that the first braking action time on the second braking device is slightly longer than the first braking action time on the first braking device 7. Typical first braking action times on the second braking device 8 (safety brake) are between 70 and 100 ms.

[0055] The control 9 is in the Figure 2 The crane control system 9a controls the motors 2, the first braking devices 7, and the second braking devices 8 via corresponding signal lines 9c. The crane control system 9a can close any braking device 7, 8 at any time. During normal operation, however, it only controls the first braking devices 7 (service brakes).

[0056] The emergency brake control 9b (also referred to as the BOSS control) is connected to the force sensors 13 and 13', respectively, and monitors and compares the two output channels of the load sensors 13 and / or 13'. If the signals from the two output channels of the load sensors 13 and 13' do not match, a load sensor defect is detected and an emergency brake condition is triggered.

[0057] An emergency braking condition is also triggered if a load signal exceeding an overload threshold is detected in one or more limit switches 14. In this case, the control system 9, 9b sends corresponding signals to switching relays 15, which control the first and second braking devices (service and safety brakes) 7, 8 so that they each engage within the first (shorter) braking action time (emergency braking condition). Optionally, a further switching relay can be provided, via which the crane control system 9a can also trigger an emergency braking condition under certain circumstances.

[0058] To trigger the different braking action times, special electro-hydraulic release devices 20, 30 are provided, the function of which is determined by the Figures 3 and 4 be explained.

[0059] For the drive shaft 3, which is subjected to higher speeds but lower forces, a lever brake 7 is provided, which is connected to the Figure 3The brake is provided with the brake release device 20 shown. The release device 20 comprises an actuating cylinder 21, which is adjustable via an electrically driven hydraulic pump 22. A check valve 23 is provided between the hydraulic pump 22 and the actuating cylinder 21. The valve 24 and / or the valves 25, which are each controlled by the control unit 9, as is the motor of the hydraulic pump 22, serve to reduce the pressure. The actuating cylinder 21 releases or releases the brake against a spring force and remains in its release position when the valves 24 and 25 are closed.

[0060] For the normal braking condition, pressure reduction occurs via a valve 24. The return speed of the actuating cylinder 21 depends on the fluid cross-section of the valve 24. This second braking action time for a normal braking condition is typically between 180 and 250 ms, during which the actuating cylinder 21 is retracted by the spring or braking force acting on the brake lever linkage. The smaller the flow cross-section, the longer the braking action time.

[0061] In an emergency braking condition, both valves 25 are actuated, also via the control 9. They provide an increased flow cross-section, which shortens the retraction of the actuating cylinder 21 and thus the braking action time, which is usually between 25 ms and 40 ms.

[0062] Additional safety considerations are taken into account if, during normal operation, it is determined that the actuating cylinder does not reset when valve 24 is actuated (de-energized, de-energized) . The two valves 25 are then automatically activated (de-energized, de-energized) to trigger the braking effect. An optional pressure relief valve 26 is provided, which limits the pressure buildup by the hydraulic pump 22. The required hydraulic fluid is provided in a reservoir 27.

[0063] Figure 4shows the release device 30 for the second braking device 8, the safety brake. The release device 30 is constructed similarly to the release device 20, but does not include its own actuating cylinder. Instead, it acts via two output lines 31 on the brake cylinder 8a integrated into the second braking device 8 (safety brake). Pressurization is also carried out via a hydraulic pump 32, which is connected to the output connections 31 via a check valve 33. In the electrically controllable valves 34, which are designed to be open when de-energized, the pressure built up by the hydraulic pump 32 is held in the closed state (energized) and released in the open state (de-energized). The valves 34 are also controlled via the controller 9, 9b. Here, too, an optional pressure relief valve 35 is provided, which limits the pressure built up by the hydraulic pump 32. The hydraulic fluid is also provided in a reservoir 36.Optionally, a measuring connection 37 and a pressure switch 38 can be provided, which can also be coupled to the control 9.

[0064] To release the brake, the valves 34 are closed and the desired actuating pressure is built up via the hydraulic pump 32, and the brake cylinder is moved to its released (ventilated) position. To trigger or apply the brake, the valves 34 are de-energized and the hydraulic fluid flows back into the reservoir 36. The brake spring causes the actuating pistons to extend, which then act on a brake disc of the cable drums 5 or output shafts 6 via corresponding brake shoes. The valves 34 are designed redundantly, so that even if one of the valves 34 fails or jams, the braking process is always carried out via the second valve - albeit with a slightly increased braking effect time.

[0065] Optionally, the release device 30 can be equipped with a manual release device, in which pressure can be built up using a hand pump 40, which is coupled to the output connections 31 via a check valve 41. For this purpose, a manually operable valve 42 must be closed, and two pressure relief valves prevent excessive pressure buildup upstream and downstream of the check valve 41. With this release device 30, a braking action time of 70 to 100 ms can be achieved in conjunction with a suitable brake.

[0066] The interaction and targeted control of brake release devices 20 and 30, or braking devices 7 and 8, makes it possible to reliably implement emergency braking conditions without the need for additional components such as separating clutches or auxiliary brakes. The system is also suitable for retrofitting existing crane systems.

[0067] Further variants and embodiments of the present invention will become apparent to the person skilled in the art within the scope of the claims. LIST OF REFERENCE SYMBOLS

[0068] 100 Crane system 1 Brake assembly 2 Drive motor 3 Drive shaft (first drive element) 4 Reduction gear (transmission device) 5 Rope pulley (second drive element) δ Output shaft (second drive element) 7 First braking device (service brake) 8 Second braking device (safety brake) 8a Brake cylinder 9 Control system 9a Crane control system 9b Emergency brake control system 9c Signal line 10 Rope 11 Container spreader 12 Container 13 Load sensor 13' Load sensor 14 Limit switch 15 Switching relay 20 Lifting device 21 Actuating cylinder 22 Hydraulic pump 23 Check valve 24 Valve 25 Valve 26 Pressure relief valve 27 Reservoir 30 Lifting device 31 Output connection 32 Hydraulic pump 33 Check valve 34 Valve 35 Pressure relief valve 36 Reservoir 37 Measuring connection 38 Pressure switch 40 Hand pump 41 Check valve 42 Hand valve 43 Pressure relief valve

Claims

1. Brake arrangement (1) for securing a hoisting device, in particular a crane system (100), with: a first brake device (7) which acts on a first drive element (3), a second brake device (8) which acts on a second drive element (5), a transmission device (4), in particular a gear mechanism, which acts between the first and the second drive element, a load sensor (13, 13') which detects a load signal and outputs it to a controller (9), and the controller (9) is configured in such a way that, upon a load signal which exceeds an overload threshold, it initiates an emergency brake state and in the process actuates the first and the second brake device (7, 8) in such a way that they act on the first and second drive element (3, 5) at the same time within a first brake active time, characterized in that one of the first and second brake devices is configured in such a way that it acts on the first and / or the second drive element within a second brake active time in a normal brake state and the first brake active time is shorter than the second brake active time.

2. Brake arrangement (1) according to Claim 1, in which the first drive element (3) is a drive shaft, coupled to the transmission device (4), of a drive motor.

3. Brake arrangement (1) according to Claim 1 or 2, in which the second drive element (5) is a cable drum which is coupled to the transmission device (4) via an output shaft (6).

4. Brake arrangement (1) according to Claim 1, 2 or 3, in which the first brake device (7) comprises an electrohydraulic brake thrust unit (20), in which a fluid pressure which counteracts the brake force can be dissipated in a piston by means of two electromagnetically actuable valves (24, 25), and the first brake active time can be realised by deenergizing the two valves, and the second brake active time can be set by deenergizing one of the valves.

5. Brake arrangement (1) according to one of the preceding claims, in which the second brake device (8) can be actuated via a hydraulic assembly and / or an electrohydraulic brake thrust unit (20) which comprises two redundant electrically actuable solenoid valves, and a fluid pressure which counteracts the brake force can be dissipated by deenergizing one of the valves, and the brake action on the second drive element can thus be triggered.

6. Brake arrangement (1) according to Claim 4, in which, when the valve (24) is switched off, the first brake active time is 25 and 40 ms and, when the valves (25) are switched off, the second brake active time is 180 and 250 ms.

7. Brake arrangement (1) according to one of the preceding claims, in which the overload sensor is arranged on a load suspension arrangement which is coupled to the hoisting device.

8. Brake arrangement (1) according to Claim 7, in which the load suspension arrangement is configured as a container spreader (11), and the load sensor (13, 13') is arranged on a load suspension element and / or on a suspension cable connector.

9. Hoisting device with a brake arrangement (1) according to one of the preceding claims, wherein the hoisting device is configured as a cable lifting system, as a crane system (100) or as a component of a container crane system.

10. Crane system (100) with two hoisting devices according to Claim 9 which are synchronized in order to move a container spreader (11) up and down, wherein the two hoisting devices each have a brake arrangement (1) according to one of Claims 1 to 8, which brake arrangements can be actuated in a synchronized manner via a common controller (9).

11. Crane system (100) according to Claim 10, in which, in the case of an overload, the synchronization of the hoisting devices can be cancelled and, upon a relief signal, the controller (9) acts selectively on the brake device of one of the two hoisting devices or of the two hoisting devices.