Patent Publication Number: US-2022219955-A1

Title: Winch brake device

Description:
RELATED APPLICATIONS 
     The content of Japanese Patent Application No. 2021-003628, on the basis of which priority benefits are claimed in an accompanying application data sheet, is in its entirety incorporated herein by reference. 
     BACKGROUND 
     Technical Field 
     Certain embodiments of the present invention relate to a winch brake device used for a work machine. 
     Description of Related Art 
     As a background technique in the present technical field, for example, the related art discloses a technique in which the control characteristics of a brake during the non-freefall of a crane and the braking characteristics of the brake during freefall can be changed by disposing two brake mechanisms (brake cylinders) in series. 
     SUMMARY 
     According to an embodiment of the present invention, there is provided a winch brake device that is used for a work machine and generates a braking force on the basis of a brake manipulation. The winch brake device includes a pressing member that brings braking members into pressure contact with each other to generate the braking force; a first brake cylinder that applies a first pressing force for bringing the braking members into pressure contact with each other to the pressing member; and a second brake cylinder that applies a second pressing force different from the first pressing force to the pressing member. The first brake cylinder and the second brake cylinder are capable of applying the first pressing force or the second pressing force independently of each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an external side view of a crane including a brake device according to one embodiment. 
         FIG. 2  is a hydraulic circuit diagram of a brake device according to the one embodiment. 
         FIG. 3A  is a diagram showing the braking force characteristic of a first piston,  FIG. 3B  is a diagram showing the braking force characteristic of a second piston,  FIG. 3C  is a diagram showing the pressure characteristic of a brake control valve, and  FIG. 3D  is a diagram showing brake control characteristics. 
         FIG. 4  is a flowchart showing a procedure of control processing of a brake device by a controller. 
         FIG. 5  is a hydraulic circuit diagram of a brake device according to Modification Example 1-1. 
         FIG. 6  is a flowchart showing a procedure of control processing of a brake device according to Modification Example 1-2. 
         FIG. 7  is a flowchart showing a procedure of control processing of the brake device according to Modification Example 1-3. 
         FIG. 8  is a hydraulic circuit diagram of a brake device according to another embodiment. 
         FIG. 9A  is a diagram showing control characteristics of an electromagnetic proportional valve,  FIG. 9B  is a diagram showing brake control characteristics in a heavy excavation mode, and  FIG. 9C  is a diagram showing brake control characteristics in a light excavation mode. 
         FIG. 10A  is a diagram showing control characteristics of an electromagnetic proportional valve,  FIG. 10B  is a diagram showing brake control characteristics in a heavy excavation mode, and  FIG. 10C  is a diagram showing brake control characteristics in a light excavation mode. 
     
    
    
     DETAILED DESCRIPTION 
     According to the present invention, the operability of freefall work of the work machine (crane or the like) can be improved. In addition, the problems, configurations, and effects other than those described above will be apparent by the following description of the embodiments. 
     Since the freefall work of the crane occurs in various applications, braking characteristics of the brake according to diversified work (various loads during work) are required. 
     It is desirable to provide a winch brake device capable of improving the operability of freefall work of a work machine (crane or the like). 
     Hereinafter, embodiments in which a brake device according to the present invention is applied to a crane, which is an example of a work machine, will be described with reference to the drawings. 
     One Embodiment 
       FIG. 1  is an external side view of a crane including a brake device according to one embodiment. The crane  100  includes an undercarriage  101 , a turning body  103  turnably provided on the undercarriage  101  via a turning device  102 , a boom  104  pivotably provided on the turning body  103  in a vertical direction, and a cab  108  on which an operator gets. The turning body  103  is mounted with a hoisting winch  105 , which is a winch for hoisting, and a derricking winch  106 , which is a winch for derricking the boom. 
     A hoisting rope  105   a  is wound around the hoisting winch  105 . The hoisting rope  105   a  is wound or unwound by the rotation of the hoisting winch  105 , and a hook  110  is lifted and lowered via a sheave provided at the tip of the boom  104 . A derricking rope  106   a  is wound around the derricking winch  106 . The derricking rope  106   a  is wound or unwound by the rotation of the derricking winch  106 , and the boom  104  is derricked via a sheave provided at the tip of the boom  104 . 
     The cab  108  includes a hoisting operation lever  13  for performing the hoisting and lowering manipulation of the hoisting winch  105 , various operation levers (not shown) for performing the turning manipulation of the turning body  103  and performing the derricking manipulation of the derricking winch  106  of the boom  104 , a controller  18  that performs the control of the crane  100 , a brake mode change-over switch  17  that switches the operation mode of a brake device  4 , a brake control characteristic change-over switch  24  that switches the control characteristics of the brake device  4 , a monitor serving as notification means (not shown), and the like (refer to  FIG. 2 ). 
       FIG. 2  is a hydraulic circuit diagram of the brake device according to the one embodiment of the present invention. In  FIG. 2 , a hydraulic circuit for driving the hoisting winch  105  is shown, and a hydraulic circuit for driving the derricking winch  106  is not shown. 
     The crane  100  includes the controller  18  that controls the operation of each part of the crane  100 , an engine (not shown), and a main pump  12 , a cooling oil pump  19 , and a pilot pump  9  that are driven by the engine (not shown). The main pump  12 , the cooling oil pump  19 , and the pilot pump  9  discharge the hydraulic oil in a tank  10  as pressure oil. 
     Although not shown, the controller  18  includes a CPU that performs various arithmetic operations, a storage device such as a ROM or HDD that stores a program for executing an arithmetic operation by the CPU, a RAM that is a work area when the CPU executes the program, hardware including a communication interface, which is an interface for transmitting and receiving data to and from other devices, and software stored in the storage device and executed by the CPU. Each function of the controller  18  is realized by the CPU that loads various programs stored in the storage device into the RAM to execute the programs. 
     The hoisting winch  105  includes a winding drum  3 , a hydraulic motor  1 , a main pump  12 , a control valve  11 , a planetary reduction mechanism  2 , and the brake device  4 . The main pump  12  supplies the pressure oil to the hydraulic motor  1  and drives the hydraulic motor  1  to drive the hoisting and lowering of the winding drum  3 . The control valve  11  controls the flow of the pressure oil from the main pump  12  to the hydraulic motor  1 . The planetary reduction mechanism  2  transmits the driving force of the hydraulic motor  1  to the winding drum  3 . The brake device  4  prevents the free rotation of the winding drum  3  by braking a carrier shaft  4   g  of the planetary reduction mechanism  2 . 
     The planetary reduction mechanism  2  includes a sun gear  2   a , a planetary gear  2   b , and a ring gear  2   c . An output shaft of the hydraulic motor  1  is connected to the sun gear  2   a  of the planetary reduction mechanism  2 . The planetary gear  2   b  meshes with the sun gear  2   a , and the ring gear  2   c  provided on an inner peripheral side of the winding drum  3  meshes with the planetary gear  2   b . The planetary gear  2   b  is supported by a planetary carrier  2   d . The carrier shaft  4   g  to which the planetary carrier  2   d  is secured is supported by a bearing inside the brake device  4 . An outer peripheral surface of the carrier shaft  4   g  and a through-hole through which the carrier shaft  4   g  is inserted are sealed by an oil seal  4   h.    
     Hereinafter, for convenience of explanation, the rotation axis direction of the hoisting winch  105 , that is, the center axis direction of the carrier shaft  4   g  is simply referred to as an axial direction. Additionally, the side of both axial ends of the hoisting winch  105  on which the output shaft of the hydraulic motor  1  is disposed will be described as a front side of the hoisting winch  105 , and an axially opposite side thereof will be described as a rear side of the hoisting winch  105 . 
     Configuration of Brake Device 
     The brake device  4  also has a function as a clutch device for transmitting and cutting off the rotation between the hydraulic motor  1  and the winding drum  3 . That is, the brake device  4  functions as a clutch and a parking brake during power hoisting and stopping, and functions as a service brake (regular brake) during freefall. In addition, in the present specification, the braking force required in a case where the brake device  4  is used as a clutch is defined as a “required clutch force”. 
     The brake device  4  applied to the present embodiment is a negative type and is a wet multi-plate brake having a plurality of friction plates  4   a . The brake device  4  mainly includes the plurality of friction plates (braking member)  4   a , a plurality of mating plates (braking member)  4   b  engaged with the plurality of friction plates  4   a , a first brake cylinder  31 , a second brake cylinder  32 , the carrier shaft  4   g , the oil seal  4   h , and a pressing member  4   j.    
     The plurality of friction plates  4   a  are attached to an outer periphery of the carrier shaft  4   g  by spline coupling so as to be movable in the axial direction and constrained in a rotation direction. On the other hand, the plurality of mating plates  4   b  are alternately disposed with the plurality of friction plates  4   a  in the axial direction and are attached to an inner periphery of a casing by spline coupling so as to be free in the axial direction constrained in the rotation direction. By applying the pressing force to the mating plate  4   b , the friction torque generated between the mating plate  4   b  and the friction plate  4   a  is transmitted to the carrier shaft  4   g  to generate a braking force. 
     The mating plates  4   b  are connected to each other by a spring. For this reason, in a brake-released state, the mating plates  4   b  move so as to be separated from each other, and a gap is formed between each friction plate  4   a  and each mating plate  4   b . By forming the gap between the friction plate  4   a  and the mating plate  4   b , the brake drag torque (rotational resistance caused by the viscosity of the oil between the friction plate  4   a  and the mating plate  4   b  (so-called drag torque)) during freefall is reduced. 
     The first brake cylinder  31  includes a first piston  4   c , a plurality of springs (first springs)  4   e  that urge the first piston  4   c , a first casing  4   f  that accommodates the first piston  4   c , and a first pressure chamber  4   m , which is a space formed in an annular shape between the first casing  4   f  and the first piston  4   c.    
     The second brake cylinder  32  includes a second piston  4   d , a plurality of springs (second springs)  4   k  that urge the second piston  4   d , a second casing  4   i  that accommodates the second piston  4   d , and a second pressure chamber  4   n , which is a space formed in an annular shape between the second casing  4   i  and the second piston  4   d.    
     The first piston  4   c  is urged with a first pressing force to the front side in the axial direction by the plurality of springs  4   e . The second piston  4   d  is disposed coaxially with the first piston  4   c  inside the first piston  4   c . Similar to the first piston  4   c , the second piston  4   d  is urged with a second pressing force to the front side in the axial direction (to the right side in  FIG. 2 ) by the plurality of springs  4   k . Then, the first piston  4   c  and the second piston  4   d  can independently press the pressing member  4   j.    
     Here, the springs  4   e  that urge the first piston  4   c  and the springs  4   k  that urge the second piston  4   d  have the same linear characteristic and the same spring performance (spring constant), and the number of springs  4   e  and the number of springs  4   k  are different from each other. That is, in the present embodiment, the springs  4   e  and the springs  4   k  have the same spring characteristic (the characteristic of being deformed when a force is applied and restored when a force is removed). Of course, the springs  4   e  and  4   k  may have non-linear characteristics or may have different spring constants from each other. Additionally, the number of springs  4   e  and the number of springs  4   k  may be the same. 
     In the present embodiment, the number of springs  4   e  that urges the first piston  4   c  is larger than the number of springs  4   k  that urges the second piston  4   d . More specifically, the ratio of the number of springs  4   e  and the number of springs  4   e  is 6:4. Therefore, the pressing force of the first piston  4   c  to press the mating plates  4   b  is 1.5 times larger than that of the second piston  4   d . In other words, the first piston  4   c  can generate a braking force 1.5 times larger than that of the second piston  4   d . In addition, the total value of braking forces caused by the two brake cylinders  31  and  32  is set to be equal to the required clutch force. Therefore, the required clutch force is proportionally distributed between the first brake cylinder  31  and the second brake cylinder  32  at a ratio of 6:4. In other words, the total of the urging forces of the springs  4   e  and  4   k  is equal to a pressing force required to generate the required clutch force. 
     The pressure oil is supplied to the first pressure chamber  4   m  from a brake cylinder port B 1 . The pressure oil is supplied to the second pressure chamber  4   n  from a brake cylinder port B 2 . When the pressure oil is supplied to the first pressure chamber  4   m , the first piston  4   c  is pressed to the rear side in the axial direction. When the pressure oil is supplied to the second pressure chamber  4   n , the second piston  4   d  is pressed to the rear side in the axial direction. Therefore, the braking force of the brake device  4  becomes the smallest in a state where the first piston  4   c  and the second piston  4   d  are pressed to the rear side in the axial direction, and the braking force caused by the brake device  4  becomes the largest in a state where the first piston  4   c  or the second piston  4   d  is pressed to the front side in the axial direction. 
     In addition, a ratio (A 1 /A 2 ) of a pressure reception area A 1  of the pressure oil acting on the first pressure chamber  4   m  and a pressure oil reception area A 2  acting on the second pressure chamber  4   n  is configured to be equal to the ratio of the pressing force of the first piston  4   c  and the pressing force of the second piston  4   d . As described above, in the present embodiment, the ratio of the number of springs  4   e  that urge the first piston  4   c  to the number of springs  4   k  that urge the second piston  4   d  is 6:4. Thus, the ratio of the pressure reception area A 1  and the pressure reception area A 2  is also configured to be 6:4. Accordingly, a braking force proportional to the stepping amount of the brake pedal  6   a  can be generated. In other words, the ratio of the braking forces proportional to the stepping amount of the brake pedal  6   a  is also configured to be 6:4. 
     In addition, the ratio of the pressing force of the first piston  4   c  and the pressing force of the second piston  4   d  and the ratio of the pressure reception area A 1  of the first pressure chamber  4   m  to the pressure reception area A 2  of the second pressure chamber  4   n  may not necessarily coincide with each other. For example, a difference between the ratio of the pressing forces and the ratio of the pressure reception area may be in a range of ±10% (preferably in a range of ±5%). This includes a case where effective pressure reception areas are positively adjusted so as to have a required ratio in consideration of the pressure losses caused by the piston seals attached to the brake cylinder  31  and  32  in both pressure chambers  4   m  and  4   n.    
     The pressing member  4   j  is provided so as to be capable of abutting against the first piston  4   c  and the second piston  4   d  and always press the mating plates  4   b  with the urging force of the spring  4   e  acting on the first piston  4   c  and the urging force of the spring  4   k  acting on the second piston  4   d . That is, the brake device  4  always generates a braking force by the pressing force acting on the pressing member  4   j  toward the front side in the axial direction (negative brake). In addition, the shape of the pressing member  4   j  does not matter as long as the pressing member  4   j  is capable of simultaneously abutting against the first piston  4   c  and the second piston  4   d.    
     The brake cylinder port B 1  is connected to the pilot pump  9  via a brake control valve  6 , a brake mode switching valve  5 , and a brake control characteristic switching valve  22  and is also connected to the pilot pump  9  via a brake mode switching valve  21  and a brake control characteristic switching valve  23 . Accordingly, the brake cylinder port B 1  can selectively supply the pressure oil of which the pressure is adjusted by the brake control valve  6  and the pressure oil discharged from the pilot pump  9 . 
     Additionally, similarly, the brake cylinder port B 2  is also connected to the pilot pump  9  via the brake control valve  6 , the brake mode switching valve  5 , and the brake control characteristic switching valve  22  and is connected to the pilot pump  9  via the brake mode switching valve  21  and the brake control characteristic switching valve  23 . Accordingly, the brake cylinder port B 2  can also selectively supply the pressure oil of which the pressure is adjusted by the brake control valve  6  and the pressure oil discharged from the pilot pump  9 . 
     Here, in order to cool the brake device  4 , cooling oil (pressure oil) is forcedly circulated by the cooling oil pump  19 . Specifically, the cooling oil discharged from the cooling oil pump  19  is supplied from the IN port to the inside of the brake device  4 , absorbs the heat of the brake discs (the friction plate  4   a  and the mating plate  4   b ), and is then discharged from an OUT port and returned to the tank  10 . An upper limit of the discharge pressure of the cooling oil pump  19  is limited by a casing internal pressure protection valve  20  that is a relief valve. For that reason, the internal pressure of the brake device  4  is also maintained to be equal to or lower than an upper limit of the casing internal pressure protection valve  20 . In addition, a positional relationship between the IN port and the OUT port may be configured to be in a reverse relationship to that of the present embodiment. 
     Configuration of Hydraulic Circuit 
     The hydraulic circuit that drives the hoisting winch  105  is provided with a motor brake switching valve  7  that is switched and controlled by the manipulation of the hoisting operation lever  13 , a motor brake cylinder  8 , the brake control valve  6  that is manipulated by the brake pedal  6   a , the brake mode switching valves  5  and  21  and the brake control characteristic switching valves  22  and  23 , which are switched and controlled by the controller  18 , and the casing internal pressure protection valve  20 . In addition, the brake control valve  6  may be configured to operate depending on a brake manipulation or may be configured to operate depending on the stepping amount of the brake pedal  6   a . In addition, the brake control valve  6  may be configured to operate depending on the stepping force of the brake pedal  6   a , may be configured to operate by replacing this manipulation with a manual manipulation, or may be configured to operate depending on the rotational speed of the hoisting winch  105 . 
     Additionally, the hydraulic circuit is provided with a plurality of manipulation pressure sensors  15  and one brake circuit pressure sensor  16  in order to switch the brake mode switching valves  5  and  21  to switch the operation mode (neutral brake mode/neutral free mode) of the brake device  4 , and each sensor is connected to the controller  18 . Moreover, the brake mode change-over switch  17  and the brake control characteristic change-over switch  24  are connected to the controller  18 . 
     In addition, the brake mode change-over switch  17  is a switch that selects whether to set the operation mode to the neutral brake mode or the neutral free mode to be described below, and the brake control characteristic change-over switch  24  is a switch that selects whether to set a work mode to a heavy excavation mode and a light excavation mode to be described below. 
     Operation of Hydraulic Circuit 
     Next, the operation of the hydraulic circuit in the neutral brake mode, which is an operation mode for performing power hoisting and lowering, and the operation of the hydraulic circuit in the neutral free mode, which is an operation mode for performing freefall, will be described.  FIG. 2  shows the state of the neutral brake mode. When the brake mode change-over switch  17  is switched to the manipulation position of the neutral brake mode, the brake mode switching valves  5  and  21  and the brake control characteristic switching valves  22  and  23  are brought into a non-excited state as shown in  FIG. 2 . In this state, when the brake mode change-over switch  17  is switched to the manipulation position of the neutral free mode and the other conditions are satisfied, the brake mode switching valves  5  and  21  are excited and the operation mode is switched to the neutral free mode. 
     Operation During Power Hoisting and Lowering (Neutral Brake Mode) 
     When the hoisting operation lever  13  shown in  FIG. 2  is manipulated to the hoisting position or the lowering position, the pilot pressure from the pilot pump  9  acts on the control valve  11  to drive a spool of the control valve  11 , and the pressure oil discharged from the main pump  12  is supplied to the hydraulic motor  1  via the control valve  11 . At the same time, the pilot pressure from the pilot pump  9  also acts on the motor brake switching valve  7  via a high-pressure selection valve  14 . Accordingly, since the motor brake switching valve  7  is switched to a position (A), and the pilot pressure from the pilot pump  9  drives the motor brake cylinder  8  to a brake open side, the hydraulic motor  1  is rotationally driven. In addition, the motor brake switching valve  7  may be an electromagnetic switching valve that performs electrical switching by a detection signal of the manipulation pressure sensor  15 . 
     In the neutral brake mode, the brake mode switching valves  5  and  21  and the brake control characteristic switching valves  22  and  23  are in the non-excited state. Therefore, the brake mode switching valves  5  and  21  are switched to positions (D 1 ) and (D 2 ), respectively, and the brake control characteristic switching valves  22  and  23  are switched to positions (E 1 ) and (E 2 ), respectively. In addition, the brake control characteristic switching valves  22  and  23  may not be switched to the positions (E 1 ) and (E 2 ). That is, the neutral brake mode may be started in a state where the brake control characteristic switching valves  22  and  23  are at positions (F 1 ) and (F 2 ), respectively. 
     The pilot pressure from the pilot pump  9 , which is a brake release pressure, is cut off by the brake mode switching valves  5  and  21 , and the first pressure chamber  4   m  and the second pressure chamber  4   n  communicate with the tank  10 . The first pressure chamber  4   m  and the second pressure chamber  4   n  have tank pressures, the spring  4   e  urges the first piston  4   c , and the spring  4   k  urges the second piston  4   d . The urging forces of the springs  4   e  and  4   k  acting on the first piston  4   c  and the second piston  4   d  are transmitted to the pressing member  4   j , and the pressing member  4   j  presses the mating plates  4   b . As a result, the friction plates  4   a  and the mating plates  4   b  are pressed against each other, a braking force is applied to the planetary carrier  2   d , and the planetary carrier  2   d  is fixed. 
     In the neutral brake mode, when the sun gear  2   a  of the planetary reduction mechanism  2  is pivoted by the rotational driving force of the hydraulic motor  1 , the planetary gear  2   b  rotates on its axis and the winding drum  3  connected to the ring gear  2   c  and the ring gear  2   c  are pivoted, and the hoisting or lowering caused by the power is performed. 
     Operation During Freefall (Neutral Free Mode) 
     When the hoisting operation lever  13  is manipulated to a neutral position, the flow of the pressure oil from the main pump  12  to the hydraulic motor  1  is cut off by the control valve  11 . Since the pilot pressure oil is not supplied to the motor brake switching valve  7  via the high-pressure selection valve  14 , the motor brake switching valve  7  is switched to the position (B). Since the pilot pressure to the motor brake cylinder  8  is cut off by the motor brake switching valve  7 , the motor brake is operated by the urging force of a spring. In this state, when the brake mode switching valves  5  and  21  are excited and the braking force of the brake device  4  is released, the winding drum  3  freely rotates due to the weight of a suspended cargo suspended from the hook  110 . 
     The switching to the neutral free mode is performed as the brake mode switching valves  5  and  21  are operated by the command of the controller  18  on the basis of the respective determination values of the manipulation pressure sensor  15 , the brake circuit pressure sensor  16 , and the brake mode change-over switch  17 . For example, when it is detected that the brake mode change-over switch  17  has been switched to the manipulation position of the neutral free mode in a state where the manipulation pressure detected by the manipulation pressure sensor  15  is smaller than a predetermined threshold (that is, the hoisting operation lever  13  is neutral) and the circuit pressure detected by the brake circuit pressure sensor  16  is smaller than a predetermined threshold (that is, in a state where the brake pedal  6   a  is stepped), the controller  18  switches the operation mode from the neutral brake mode to the neutral free mode. 
     Here, in the present embodiment, the heavy excavation mode (first work mode) and the light excavation mode (second work mode) are prepared in advance as work modes in the neutral free mode. The heavy excavation mode is a mode suitable for a case where a heavy suspended cargo dropped due to its own weight (heavy load work), and the light excavation mode is a mode suitable for a case where a relatively light suspended cargo is dropped due to its own weight (light load work). Specifically, brake control characteristics are different between the heavy excavation mode and the light excavation mode, and the heavy excavation mode is set to have a larger braking force than the light excavation mode. Hereinafter, the operation of the hydraulic circuit in each excavation mode will be described. 
     Heavy Excavation Mode 
     When the brake control characteristic change-over switch  24  switches to the position of the heavy excavation mode, the work mode switches to the heavy excavation mode. In the heavy excavation mode, the brake mode switching valves  5  and  21  are in an excited state, and the brake control characteristic switching valves  22  and  23  are in the non-excited state. That is, in the heavy excavation mode, the brake mode switching valves  5  and  21  are switched to positions (C 1 ) and (C 2 ), respectively, and the brake control characteristic switching valves  22  and  23  are switched to the positions (E 1 ) and (E 2 ), respectively. 
     Then, the pilot pressure adjusted by the brake control valve  6  (the pilot pressure according to the stepping amount of the brake pedal  6   a ) is applied to the brake mode switching valve  5 , and the pilot pressure from the pilot pump  9 , which is the brake release pressure (maximum pressure), is directly applied to the brake mode switching valve  21 . The pilot pressure adjusted by the brake control valve  6  acts on the first pressure chamber  4   m  via the brake cylinder port B 1 . For that reason, the first piston  4   c  presses the pressing member  4   j  with a pressing force corresponding to a difference between the urging force of the spring  4   e  and the pilot pressure acting on the first pressure chamber  4   m.    
     Meanwhile, the pilot pressure (brake release pressure) from the pilot pump  9  directly acts on the second pressure chamber  4   n  via the brake cylinder port B 2 . More specifically, the maximum discharge pressure of the pilot pump  9  acts on the second pressure chamber  4   n . For that reason, the second piston  4   d  is held on the rear side in the axial direction (left side in  FIG. 2 ) against the urging force of the spring  4   k , and the braking caused by the second piston  4   d  is released. 
     In this way, in the heavy excavation mode, the first brake cylinder  31  operates independently of the second brake cylinder  32 , and only a braking force caused by the first brake cylinder  31  is generated. Then, the braking force in the heavy excavation mode follows the spring characteristic of the spring  4   e  that urges the first piston  4   c.    
     Light Excavation Mode 
     When the brake control characteristic change-over switch  24  switches to the position of the light excavation mode, the work mode switches to the light excavation mode. In the light excavation mode, the brake mode switching valves  5  and  21  are in the excited state, and the brake control characteristic switching valves  22  and  23  are in the excited state. That is, in the light excavation mode, the brake mode switching valves  5  and  21  are switched to the positions (C 1 ) and (C 2 ), respectively, and the brake control characteristic switching valves  22  and  23  are switched to the positions (F 1 ) and (F 2 ), respectively. 
     Then, the pilot pressure adjusted by the brake control valve  6  (the pilot pressure according to the stepping amount of the brake pedal  6   a ) is applied to the brake mode switching valve  5 , and the pilot pressure from the pilot pump  9 , which is the brake release pressure, is directly applied to the brake mode switching valve  21 . In the light excavation mode, the positions of the brake control characteristic switching valves  22  and  23  are switched to the positions (F 1 ) and (F 2 ), respectively. Therefore, the pilot pressure adjusted by the brake control valve  6  acts on the second pressure chamber  4   n  via the brake cylinder port B 2 . For that reason, the second piston  4   d  presses the pressing member  4   j  with a pressing force corresponding to a difference between the urging force of the spring  4   k  and the pilot pressure acting on the second pressure chamber  4   n.    
     Meanwhile, the pilot pressure (brake release pressure) from the pilot pump  9  acts directly on the first pressure chamber  4   m  via the brake cylinder port B 1 . More specifically, the maximum discharge pressure of the pilot pump  9  acts on the first pressure chamber  4   m . For that reason, the first piston  4   c  is held on the rear side in the axial direction (left side in  FIG. 2 ) against the urging force of the spring  4   e , and the braking caused by the first piston  4   c  is released. 
     In this way, in the light excavation mode, the second brake cylinder  32  operates independently of the first brake cylinder  31 , and only a braking force caused by the second brake cylinder  32  is generated. Then, the braking force in the light excavation mode follows the spring characteristic of the spring  4   k  that urges the second piston  4   d . By exciting the brake control characteristic switching valves  22  and  23  in this way, the brake cylinders  31  and  32  that generate the braking forces are selectively switched. 
     Next, various characteristics of the brake device  4  according to the present embodiment will be described with reference to  FIGS. 3A to 3D .  FIG. 3A  is a diagram showing the braking force characteristic of the first piston  4   c , the horizontal axis represents the pressure acting on the first pressure chamber  4   m , and the vertical axis represents a braking force caused by the first piston  4   c .  FIG. 3B  is a diagram showing the braking force characteristic of the second piston  4   d , the horizontal axis represents the pressure acting on the second pressure chamber  4   n , and the vertical axis represents a braking force by the second piston  4   d.    
     As described above, in the present embodiment, the ratio of the number of springs  4   e  that urge the first piston  4   c  to the number of springs  4   k  that urge the second piston  4   d  is 6:4. Therefore, assuming that the required clutch force is 100% (the maximum value of the braking force), the required clutch force is proportionally distributed such that the maximum braking force of the first piston  4   c  is 60% of the required clutch force (refer to  FIG. 3A ) and the maximum braking force of the second piston  4   d  is 40% of the required clutch force (refer to  FIG. 3B ). Since the spring  4   e  has the linear characteristic, the braking force of the first piston  4   c  decreases linearly as the pressure acting on the first pressure chamber  4   m  increases. Similarly, as the pressure acting on the second pressure chamber  4   n  increases, the braking force of the second piston  4   d  also decreases linearly. 
     In addition, the braking force characteristic of the first piston  4   c  and the braking force characteristic of the second piston  4   d  are adjusted by changing the spring characteristics of the springs  4   e  and  4   k  and the pressure reception areas of the first piston  4   c  and the second piston  4   d.    
       FIG. 3C  is a diagram showing the pressure characteristic of the brake control valve  6 . The horizontal axis represents a pedal stroke. The pedal stroke indicates the proportion of the pedal stroke (stepping amount), assuming that a state where the brake pedal  6   a  is fully stepped is 100% and a non-manipulated state where the brake pedal  6   a  is not stepped is 0%. 
     As shown in  FIG. 3C , the pressure characteristic of the brake control valve  6  is a characteristic that the pressure decreases linearly depending on an increase in the pedal stroke in a range where the pedal stroke is 0% or more and 100% or less. In addition, the pressure characteristic of the brake control valve  6  may be a characteristic that the pressure does not necessarily decrease linearly depending on an increase in the pedal stroke. 
       FIG. 3D  is a diagram showing brake control characteristics, in which the horizontal axis represents the pedal stroke and the vertical axis represents the braking force.  FIG. 3D  shows the proportion of the braking force with the required clutch force as 100%. As shown in  FIG. 3D , in the heavy excavation mode, the braking force increases linearly in a range of 0% to 60% depending on an increase in the pedal stroke. On the other hand, in the light excavation mode, the braking force increases linearly in a range of 0% to 40% depending on an increase in as the pedal stroke. Then, when it is detected that the pedal stroke is 100%, the brake mode switching valves  5  and  21  are immediately brought into the non-excited state and temporarily switched to the neutral brake mode. For that reason, the first brake cylinder  31  and the second brake cylinder  32  cooperate to generate a braking force, and the braking force becomes the required clutch force (100%). 
     In this way, in the present embodiment, the braking force can be generated with different brake control characteristics in the heavy excavation mode and the light excavation mode in a range where the pedal stroke is 0% to 100%. 
     Next, the control of the brake device  4  by the controller  18  will be described.  FIG. 4  is a flowchart showing a procedure of control processing of the brake device  4  by the controller  18 . As shown in  FIG. 4 , first, the controller  18  reads the set value of the current brake control characteristic stored in the RAM (S 101 ). That is, the controller  18  determines whether the current operation mode is the heavy excavation mode or the light excavation mode. 
     Next, the controller  18  determines a brake mode determination as to whether the current operation mode is the neutral free mode or the neutral brake mode (S 102 ). In a case where the result of the brake mode determination is the neutral brake mode, the controller  18  performs a manipulation determination as to whether or not the brake control characteristic change-over switch  24  has been manipulated (S 103 ). In a case where it is determined that the operation to switch to the light excavation mode is performed by an input signal from the brake control characteristic change-over switch  24 , the controller  18  outputs a command to excite the brake control characteristic switching valves  22  and  23  (S 104 ). 
     Here, in the present embodiment, even in a case where the mode is temporarily switched to the neutral brake mode during the hoisting manipulation in S 102 , the mode can be switched to the light excavation mode or the heavy excavation mode by the manipulation of the brake control characteristic change-over switch  24 . However, in a case where the mode is temporarily switched to the neutral brake mode, the switching of the work mode may be prohibited or disabled. In other words, the brake control characteristic change-over switch  24  may be enabled only in a permanent neutral brake mode. 
     Next, the controller  18  writes the set value of the brake control characteristic (in this case, the light excavation mode) to the RAM, and updates the set value (S 105 ). Next, the controller  18  displays the setting of the brake control characteristic on the monitor in the cab  108  (S 106 ). That is, the controller  18  displays, on the monitor, the fact that the mode is in the heavy excavation mode or the light excavation mode. 
     On the other hand, in a case where the controller  18  determined in S 103  that the manipulation to switch to the heavy excavation mode has been performed, a command to non-excite the brake control characteristic switching valves  22  and  23  is output (S 107 ), and the process proceeds to S 105 . 
     Additionally, in S 102 , in a case where the result of the brake mode determination is the neutral free mode, the controller  18  performs a manipulation determination as to whether or not the brake control characteristic change-over switch  24  has been manipulated (S 108 ), and in a case where it is determined that the manipulation has been performed (S 108 /YES), the controller  18  performs, on the monitor, a warning display indicating that the change of the brake control characteristics (the switching between the heavy excavation mode and the light excavation mode) cannot be performed (S 109 ), and disables the manipulation of the brake control characteristic change-over switch  24 . That is, the switching between the heavy excavation mode and the light excavation mode is prohibited during the neutral free mode. 
     On the other hand, in S 108 , when the controller  18  determines that the brake control characteristic change-over switch  24  is not manipulated (S 108 /NO), the process ends. 
     According to the one embodiment configured as described above, the following effects can be obtained. 
     The brake device  4  includes the first brake cylinder  31  and the second brake cylinder  32 , and the first brake cylinder  31  and the second brake cylinder  32  can independently press the pressing member  4   j  to generate a braking force. Also, the braking characteristic of the first brake cylinder  31  and the braking characteristic of the second brake cylinder  32  are different from each other (the numbers of the spring  4   e  and the spring  4   k  are different from each other). For that reason, the operator can manipulate the crane  100  while generating the braking force with the braking characteristic of the first brake cylinder  31  depending on work contents, and can also manipulate the crane  100  while generating the braking force with the braking characteristic of the second brake cylinder  32 . Therefore, the operability of the crane  100  during the freefall work is improved. 
     In particular, in a case where the suspended cargo is heavy, the operator can generate a braking force larger than the light excavation mode by selecting the heavy excavation mode. Therefore, the manipulation of the brake pedal  6   a  according to the operator&#39;s feeling is possible. On the contrary, in a case where the suspended cargo is light, the operator selects the light excavation mode. Accordingly, even when the brake pedal  6   a  is stepped by a predetermined amount, only a braking force smaller than that in the heavy excavation mode is generated. Therefore, the operability is improved even in the light load work. In this way, in the present embodiment, even with one brake device  4 , it is possible to select one of two different braking characteristics depending on the load of the suspended cargo to suitably perform the freefall work. 
     Moreover, in the one embodiment, the braking characteristics of the brake device  4  are physically (mechanically) switched by switching between the first brake cylinder  31  and the second brake cylinder  32 . Therefore, the reliability is high. If the pressure of the pressure oil acting on the first brake cylinder  31  and the second brake cylinder  32  is electrically controlled, there is a possibility that a braking force is not secured when an electronic component fails. Therefore, this is not suitable for brake devices in which high safety is required. However, when the configuration as in the one embodiment is adopted, the braking characteristics are physically (mechanically) switched. Therefore, there is no such concern. 
     Additionally, the selection of the heavy excavation mode and the light excavation mode is controlled so as to be performed only in the neutral brake mode in the brake mode determination (S 102 ). For that reason, work safety is improved. Specifically, since the suspended cargo is dropped by its own weight in the neutral free mode, when the operation mode is switched to the heavy excavation mode or the light excavation mode during this period, the braking characteristics are different. Therefore, the operator needs to adjust the stepping amount of the brake pedal  6   a . Moreover, there is a possibility that the behavior of the hydraulic circuit is temporarily unstable due to the responsiveness during the switching of the operation mode. In this respect, as in the present embodiment, since the heavy excavation mode and the light excavation mode can be selected only in the neutral brake mode, there is an advantage that the work can be safely performed without such a concern. 
     In addition, in the neutral free mode, the switching from the heavy excavation mode to the light excavation mode is prohibited. However, when the switching from the light excavation mode to the heavy excavation mode is enabled, the operability of the operator can be further improved while ensuring safety. 
     Additionally, since the information on the heavy excavation mode, the light excavation mode, various warning information, and the like are displayed on the monitor, the operator can visually recognize the operation mode easily, and the operability of the crane  100  is further improved. In addition, instead of the warning display by the monitor, a warning may be performed by using notification means such as a buzzer or a lamp. 
     Additionally, in the one embodiment, when it is detected that the pedal stroke is 100%, the brake mode switching valves  5  and  21  are immediately brought into the non-excited state and temporarily switched to the neutral brake mode, and a braking force equal to the required clutch force (100%) is generated. For that reason, the crane  100  can be safely manipulated. Of course, a configuration may be adopted in which a braking force lower than the required clutch force is generated when the pedal stroke is 100%. 
     Here, the brake device  4  according to the one embodiment can be variously modified as described below. 
     Modification Example 1-1 
       FIG. 5  is a hydraulic circuit diagram of the brake device  4  according to Modification Example 1-1. Modification Example 1-1 shown in  FIG. 5  has a feature in that a brake control characteristic switching valve  25  in which the functions of the brake control characteristic switching valves  22  and  23  are integrated is provided instead of providing the brake control characteristic switching valves  22  and  23  shown in  FIG. 2 . 
     The brake control characteristic switching valve  25  shown in  FIG. 5  is in the non-excited state, and the brake control characteristic switching valve  25  is switched to a position (G 2 ). When the brake mode switching valves  5  and  21  are excited in this state, the pilot pressure adjusted by the brake control valve  6  acts on the first pressure chamber  4   m , and the pilot pressure (brake release pressure) discharged from the pilot pump  9  acts on the second pressure chamber  4   n . Then, a braking force is generated by the first brake cylinder  31  depending on the stepping amount of the brake pedal  6   a . Therefore, the case where the brake control characteristic switching valve  25  is in the non-excited state is the heavy excavation mode. 
     Meanwhile, in a case where the operation mode is switched from the heavy excavation mode to the light excavation mode, the controller  18  outputs a command to excite the brake control characteristic switching valve  25 . Then, the brake control characteristic switching valve  25  switches from the position (G 2 ) to a position (G 1 ). Then, the pilot pressure adjusted by the brake control valve  6  acts on the second pressure chamber  4   n , and the pilot pressure (brake release pressure) discharged from the pilot pump  9  acts on the first pressure chamber  4   m . Then, a braking force is generated by the second brake cylinder  32  depending on the stepping amount of the brake pedal  6   a.    
     In this way, the brake device  4  according to Modification Example 1-1 also has the same effects as those of the one embodiment. Moreover, in Modification Example 1-1, the number of brake control characteristic switching valves is small. Therefore, there is an advantage that the configuration of the hydraulic circuit can be simplified. 
     Modification Example 1-2 
       FIG. 6  is a flowchart showing a procedure of control processing of the brake device  4  according to Modification Example 1-2. As shown in  FIG. 6 , first, the controller  18  reads the set value (heavy excavation mode or light excavation mode) of the current brake control characteristic stored in the RAM (S 201 ). Next, the controller  18  performs a brake mode determination for determining whether the current operation mode is the neutral free mode or the neutral brake mode (S 202 ). 
     In a case where the result of the brake mode determination is the neutral free mode, the controller  18  performs a control characteristic determination for determining whether the set value of the brake control characteristic read in S 201  is the heavy excavation mode or the light excavation mode. (S 203 ). In a case where the controller  18  determines in S 203  that the current operation mode is the light excavation mode, the controller  18  determines whether or not the hoisting manipulation has been performed (S 204 ). Specifically, the controller  18  determines whether or not the hoisting manipulation by the hoisting operation lever  13  has been performed on the basis of a detection signal input from the manipulation pressure sensor  15 . 
     In a case where the answer is YES in S 204 , the controller  18  detects a load (the load of the suspended cargo) acting on the hoisting rope  105   a  with a load detector (not shown) and performs, on the basis of this detected value, a fall-allowable load determination as to whether or not the freefall of the suspended cargo is permitted (S 205 ). In a case where the detection value of the load detector is less than a threshold, the controller  18  outputs a command to excite the brake control characteristic switching valves  22  and  23  (S 206 ). 
     On the other hand, in a case where the detection value of the load detector is equal to or more than the threshold, the controller  18  performs a warning display to change to the heavy excavation mode on the monitor (S 207 ). Next, the controller  18  outputs a command to non-excite the brake control characteristic switching valves  22  and  23  (S 208 ) and writes the set value of the brake control characteristic to the RAM (S 209 ). In this case, the heavy excavation mode is written in the RAM. Then, the controller  18  displays the current brake control characteristic on the monitor (S 210 ). Specifically, the controller  18  displays, on the monitor, the fact that the current operation mode is the heavy excavation mode or the light excavation mode. 
     In addition, in a case where it is determined in S 202  that the mode is in the neutral brake mode and in a case where it is determined in S 203  that the mode is in the heavy excavation mode, and in a case where it is determined in S 204  that the hoisting manipulation is not performed (S 204 /NO), the controller  18  ends the process. 
     In Modification Example 1-2, in a case where the current operation mode is the neutral free mode and the light excavation mode, the manipulation of the hoisting operation lever  13  and the load of the suspended cargo acting on the hoisting rope  105   a  are detected (S 204 , S 205 ), in a case where the load of the suspended cargo is less than the threshold, the light excavation mode is continued as it is and the brake control characteristic switching valves  22  and  23  are excited (S 206 ), and in a case where the load of the suspended cargo is equal to or more than the threshold, the brake control characteristic switching valves  22  and  23  are automatically non-excited and shifted to the heavy excavation mode (S 208 ). Therefore, the usability is excellent for the operator, and the safety is also excellent. 
     In addition, in Modification Example 1-2, the controller  18  may store the state of automatically switching to the heavy excavation mode in S 208  in the RAM and reflect the state in the next hoisting manipulation or may be the process limited to the current hoisting manipulation. Additionally, the load of the suspended cargo may be detected by a device other than the load detector. 
     Modification Example 1-3 
       FIG. 7  is a flowchart showing a procedure of control processing of the brake device  4  according to Modification Example 1-3. As shown in  FIG. 7 , first, the controller  18  reads the current brake control characteristic set value (heavy excavation mode or light excavation mode) stored in the RAM (S 301 ) and performs a control characteristic determination for determining whether the read brake control characteristic set value is the heavy excavation mode or the light excavation mode (S 302 ). In a case where the controller  18  determines in S 302  that the mode is the light excavation mode, the controller  18  outputs a command to excite the brake control characteristic switching valves  22  and  23  (S 303 ). On the other hand, in a case where the controller  18  determines in S 302  that the mode is the heavy excavation mode, the controller  18  outputs a command to non-excite the brake control characteristic switching valves  22  and  23  (S 304 ). Then, the controller  18  displays the current brake control characteristic on the monitor (S 305 ). Specifically, the controller  18  displays, on the monitor, the fact that the current operation mode is the heavy excavation mode or the light excavation mode. 
     Next, the controller  18  performs a key OFF determination for determining whether or not a key switch of an engine is turned off (that is, the operation of the crane  100  is stopped) (S 306 ). In a case where the key switch is not turned off (S 306 /NO), the process returns to S 301 . On the other hand, in a case where the key switch is turned off (S 306 /YES), the controller  18  writes the set value of the brake control characteristic in the RAM (S 307 ). 
     In Modification Example 1-3, by turning off the key switch, the current work mode (heavy excavation mode or light excavation mode) is written in the RAM and the process is ended. Thus, when the next key switch is turned on (that is, the operation of the crane  100  is started), the work mode previously stored in S 301  can be read as an initial value. That is, the previous work mode can be automatically restored. Therefore, by virtue of the control of the brake device  4  according to Modification Example 3, the usability is much more excellent for the operator. 
     Another Embodiment 
       FIG. 8  is a hydraulic circuit diagram of the brake device  4  according to another embodiment. The difference of the other embodiment shown in  FIG. 8  from the one embodiment is that an electromagnetic proportional pressure-reducing valve (hereinafter referred to as an electromagnetic proportional valve)  26  is provided on the upstream side of the brake mode switching valve  21  and a brake control characteristic adjustment dial  27  is connected to the controller  18 . According to this configuration, since the pressure oil of which the pressure is adjusted by the brake control valve  6  and the pressure oil of which the pressure is adjusted by the electromagnetic proportional valve  26  are selectively supplied to the first pressure chamber  4   m  and the second pressure chamber  4   n , the brake control characteristics can be more finely changed. Hereinafter, the features of the brake control characteristics in the other embodiment will be specifically described. 
       FIG. 9A  is a diagram showing control characteristics of the electromagnetic proportional valve,  FIG. 9B  is a diagram showing brake control characteristics in the heavy excavation mode, and  FIG. 9C  is a diagram showing brake control characteristics in the light excavation mode. In addition, in  FIGS. 9A, 9B, and 9C , the control characteristics when the brake control characteristic adjustment dial  27  is set to Level 3 are shown by thick lines, and the control characteristics at Levels 1, 2, 4, and 5 are shown by thin lines. Additionally, the control characteristic for the respective levels of the electromagnetic proportional valve  26  shown in  FIG. 9A  are stored in a look-up table format in the storage device of the controller  18 . 
     In addition,  FIG. 9A  defines the characteristics of the control pressure of the electromagnetic proportional valve  26  with respect to the pressure of the brake control valve  6 . However, instead of the pressure of the brake control valve  6 , the stepping amount (stroke) and the stepping force of the brake pedal  6   a  or the relationship between the rotational speed of the hoisting winch  105  and the characteristics of the control pressure of the electromagnetic proportional valve  26  may be defined. That is, the electromagnetic proportional valve  26  may be configured to be operated by the controller  18  on the basis of a control signal according to the brake manipulation. 
     As shown in  FIG. 9A , the control characteristic of the electromagnetic proportional valve  26  when set to Level 1 is a characteristic in which the control pressure of the electromagnetic proportional valve  26  increases linearly from 0% to 100% depending on an increase in pressure in a range where the pressure of the brake control valve  6  is 0% or more and less than 20% and the control pressure of the electromagnetic proportional valve  26  is maintained at 100% in a range where the pressure of the brake control valve  6  is 20% or more and 100% or less. 
     The control characteristic of the electromagnetic proportional valve  26  when set to Level 2 is a characteristic in which the control pressure of the electromagnetic proportional valve  26  increases linearly from 0% to 100% depending on an increase in pressure in a range where the pressure of the brake control valve  6  is 0% or more and less than 40% and the control pressure of the electromagnetic proportional valve  26  is maintained at 100% in a range where the pressure of the brake control valve  6  is 40% or more and 100% or less. 
     The control characteristic of the electromagnetic proportional valve  26  when set to Level 3 is a characteristic in which the control pressure of the electromagnetic proportional valve  26  linearly increases from 0% to 100% depending on an increase in pressure in a range where the pressure of the brake control valve  6  is 0% or more and less than 60% and the control pressure of the electromagnetic proportional valve  26  is maintained at 100% in a range where the pressure of the brake control valve  6  is 60% or more and 100% or less. 
     The control characteristic of the electromagnetic proportional valve  26  when set to Level 4 is a characteristic in which the control pressure of the electromagnetic proportional valve  26  increases linearly from 0% to 100% depending on an increase in pressure in a range where the pressure of the brake control valve  6  is 0% or more and less than 80% and the control pressure of the electromagnetic proportional valve  26  is maintained at 100% in a range where the pressure of the brake control valve  6  is 80% or more and 100% or less. 
     The control characteristic of the electromagnetic proportional valve  26  when set to Level 5 is a characteristic in which the control pressure of the electromagnetic proportional valve  26  increases linearly from 0% to 100% depending on an increase in pressure in a range where the pressure of the brake control valve  6  is 0% or more and 100% or less. 
     In this way, since the pressure of the electromagnetic proportional valve  26  is controlled in accordance with the control characteristics shown in  FIG. 9A , the brake control characteristics shown in  FIG. 9B  are obtained in the heavy excavation mode, and the brake control characteristics shown in  FIG. 9C  are obtained in the light excavation mode. 
     For example, in a case where the set value of the brake control characteristic adjustment dial  27  is Level 3, in the heavy excavation mode, as shown in  FIG. 9B , the braking force increases linearly with the same inclination as the characteristic of the first piston  4   c  alone with an increase in the pedal stroke in a range where the pedal stroke is in a range of 0% or more and less than 40%, and the assist characteristic of the second piston  4   d  is added in a range where the pedal stroke is 40% or more and 100 or less, so that a brake control characteristic is obtained in which the braking force increases linearly with a steeper inclination than the characteristic of the first piston  4   c  alone. 
     Additionally, as shown in  FIG. 9C , in the light excavation mode, even in a case where the set value is the same Level 3, the braking force increases linearly with the same inclination as the characteristic of the second piston  4   d  alone with an increase in the pedal stroke in a range where the pedal stroke is in a range of 0% or more and less than 40%, and the assist characteristic of the first piston  4   c  is added in a range where the pedal stroke is 40% or more and 100 or less, so that a brake control characteristic is obtained in which the braking force increases linearly with a steeper inclination than the characteristic of the second piston  4   d  alone. 
     Also, even at the same Level 3, the heavy excavation mode has a gentler inclination of a straight line than the light excavation mode in a range where the pedal stroke is 0% or more and less than 40% and has a steeper inclination of a straight line in a range where the pedal stroke is 40% or more and 100 or less. 
     In this way, in the other embodiment, more detailed brake control characteristics can be realized. Therefore, the usability is much more excellent for the operator. In particular, the other embodiment is suitable for the work of switching between the heavy excavation mode and the light excavation mode. 
     Specifically, one crane  100  is usually mounted with two or three winch devices. For this reason, there is a case where the load form of each winch device is different depending on the work contents. For example, in clamshell work, a winch device that supports a bucket and controls the lifting and lowering of the bucket and a winch device that controls the opening and closing of the bucket are used. 
     During excavation, in the manipulation of landing by freefall manipulation in a state where the bucket is open (released state), the falling speed is adjusted by the winch device for supporting the bucket, which is a heavy load, and the winch device for opening and closing the bucket, which is a light load, executes the manipulation of closing the bucket by following the falling speed of the supporting winch device. Here, there is a concern that when the follow-up is delayed, the bucket closes and then lands, while when the follow-up precedes, the wire rope of the opening and closing winch device may loosen and cause random winding. For this reason, in the clamshell work, high operability is required particularly for the winch device for opening and closing the bucket. 
     In this way, in a case where the load is different for each winch device and specialization is desired according to each load form, the other embodiment is effective. 
     In addition, the brake control characteristic adjustment dial  27  is set to five steps of Levels 1 to 5, but this setting is optional. Additionally, a continuously variable dial having an intermediate value of 1 to 5 Steps may be used. 
     Modification Example 2-1 
     Additionally, the control characteristics of the electromagnetic proportional valve  26  according to the other embodiment may be modified so as to be curvedly controlled. 
       FIG. 10A  is a diagram showing control characteristics of the electromagnetic proportional valve,  FIG. 10B  is a diagram showing brake control characteristics in the heavy excavation mode, and  FIG. 10C  is a diagram showing brake control characteristics in the light excavation mode. As shown in  FIG. 10A , the control characteristic of the electromagnetic proportional valve  26  when set to Level 1 is a characteristic in which the control pressure of the electromagnetic proportional valve  26  increases curvedly from 0% to 100% depending on an increase in pressure in a range where the pressure of the brake control valve  6  is 0% or more and less than 20% and the control pressure of the electromagnetic proportional valve  26  is maintained at 100% in a range where the pressure of the brake control valve  6  is 20% or more and 100% or less. As shown in the figure, Levels 2 to 5 also have a control characteristic in which the control pressure of the electromagnetic proportional valve  26  increased curvedly with an increase in the pressure of the brake control valve  6 . 
     In this way, since the pressure of the electromagnetic proportional valve  26  is controlled in accordance with the control characteristics shown in  FIG. 10A , the brake control characteristics shown in  FIG. 10B  are obtained in the heavy excavation mode, and the brake control characteristics shown in  FIG. 10C  are obtained in the light excavation mode. 
     For example, in a case where the set value of the brake control characteristic adjustment dial  27  is Level 3, in the heavy excavation mode, as shown in  FIG. 10B , the braking force increases linearly with the same inclination as the characteristic of the first piston  4   c  alone with an increase in the pedal stroke in a range where the pedal stroke is in a range of 0% or more and less than 40%, and the assist characteristic of the second piston  4   d  is added in a range where the pedal stroke is 40% or more and 100 or less, so that a brake control characteristic is obtained in which the braking force increased curvedly with an increase in the pedal stroke. Additionally, even in the light excavation mode, the brake control characteristics as shown in  FIG. 10C  can be obtained. 
     Modification Example 2-1 is suitable because it is possible to realize the operability that matches the operator&#39;s feeling in a case where the braking energy accompanied by the freefall changes in a quadratic curve due to gravitational acceleration. Moreover, when the other embodiment and Modification Example 2-1 are configured to be selectable by the operator, higher functionality is achieved. 
     In addition, the present invention is not limited to the aforementioned embodiment, and various modifications can be made without departing from the concept of the present invention, and all technical matters included in the technical idea described in the claims are included are the subject of the present invention. Although the above-mentioned embodiment shows a preferred example, those skilled in the art can realize various alternatives, alternations, modifications, or improvements from the contents disclosed in the present specification. These are included in the technical concept described in the appended claims. 
     In each of the above-described embodiments, the negative type brake device  4  is exemplified, but a positive type brake device may be used. That is, in the present invention, the brake device is not limited to the negative type. 
     Additionally, a crawler crane has been exemplified as an example of the crane. However, the present invention is not limited to this and can be applied to all kinds of cranes such as tower cranes, ceiling cranes, jib cranes, retractable cranes, stacker cranes, portal cranes, unloaders, and other basic machines such as earth drills, in addition to other mobile cranes such as wheel cranes, truck cranes, rough terrain cranes, and all terrain cranes. Additionally, the present invention can be applied to work machines other than cranes, for example, road machines, hydraulic excavators, wheel loaders, and the like. Additionally, the brake device of the present invention can be applied to other than winches. 
     It should be understood that the invention is not limited to the above-described embodiment, but may be modified into various forms on the basis of the spirit of the invention. Additionally, the modifications are included in the scope of the invention.