Patent Publication Number: US-9902597-B2

Title: Torque limiter for power winch

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a torque limiter for a power winch, and more particularly to the torque limiter applied to a power winch and capable of timely cutting off the power whenever the reverse torque produced by a carrying load exceeds the bearable torque of the power supply, so as to prevent the power winch from being damaged by overload. 
     Description of the Related Art 
     Power winch is a device designed for hanging or dragging a load. For example, a hoist is a common application of the power winch, and a cable winch installed at the front of a jeep or a cross-country vehicle for trailing another car (to help others) or moving out of danger (to rescue oneself) is another common application of the power winch. The principle of operating the power winch is to output a forward or reverse transmission power by a power supply (such as a power motor) and acted by a reduction mechanism to drive a cable wheel to rotate in a forward or reverse direction to release or retrieve a cable, and a load (such as a heavy object, another car, or another object) is hooked by a heavy-duty hook installed at a front end of the cable, so as to move the load conveniently. 
     Since the winch carries a load (such as goods, objects, or people waiting to be rescued), and the load sometimes exceeds the torque bearable by the power supply (which involves the loading capacity of the power supply), therefore a torque limit mechanism for preventing any torque produced by the load and unbearable by the power supply is generally installed in a transmission mechanism, such that if an overload occurs, the torque limit mechanism will rotate idly and slip and will no longer transmit power, so as to protect the transmission mechanism (such as a power motor, a reduction mechanism, etc) of the power winch, and prevent the components of the power winch from being damaged by the overload. However, the conventional power winch generally uses a conventional circuit breaker as the torque limit mechanism, such that when the torque of the load increases, the current of the power supply current also increases. Therefore, the power of the power supply will be disconnected when there is an overload of current, so as to provide the effect of limiting the torque indirectly. However, the circuit breaker cannot be turned off or on immediately when the circuit breaker is used as a torque limit mechanism, due to the time lapse. Therefore, the power of the power winch cannot be stopped immediately when there is an overload, but the output of power will continue for a short period of time before the power winch stops. Obviously, such application causes tremendous trouble to users and requires further improvements. 
     SUMMARY OF THE INVENTION 
     Therefore, it is a primary objective of the present invention to provide a torque limiter for a power winch, so that when a carrying load produces a reverse load exceeding the load bearable by the power supply, the power will be cut off immediately to prevent the power winch from being damaged by overload. 
     To achieve the aforementioned and other objectives, the present invention provides a torque limiter for a power winch, and the torque limiter comprises: a frame, a power supply installed on a side of the frame and capable of outputting power; a reduction gearbox installed on the other side of the frame and including a reduction mechanism installed therein, and a long shaft being driven to rotate by the power of the power supply to provide a reduction effect, and an end of the long shaft forming a latching end; a cable wheel installed at the middle of the frame and driven to rotate by the power outputted by the reduction gearbox; and a torque limit mechanism being installed in a process of transmitting power from the power supply to the reduction mechanism through the long shaft; characterized in that the torque limit mechanism is comprised of a torsion shaft, a friction plate, a brake pad, a disc-shaped elastic member, an adjustable positioning ring, and a positioning screw ring, wherein the torsion shaft includes an axial through slot formed therein, a latching slot formed at a front end of the torsion shaft, a threaded section formed on an outer periphery of the torsion shaft, and positioning cavity axially and deeply penetrated into the torsion shaft; the friction plate is a ring-shaped plate with friction surfaces; the brake pad is a ring-shaped plate made of a lining material and having a positioning protrusion formed at the inner periphery of the brake pad; the disc-shaped elastic member is substantially in a concave arc shape and has compressive elasticity; the outer periphery of the adjustable positioning ring includes a plurality of positioning plates and selectively bent in a direction towards the positioning screw ring; the positioning screw ring has a shaft hole with a reverse inner thread, and the outer periphery has a plurality of symmetrical positioning grooves formed thereon and provided for deviating, pressing, and remaining the selected positioning screw plate therein. 
     In the torque limiter for a power winch, the torque limit mechanism has an inner housing installed in front of the reduction mechanism in the reduction gear for entering power and coupled to the reduction mechanism, and a through hole formed at an end of the inner housing, and during assembling, the torsion shaft is passed through the friction plate and the through hole of the inner housing, so that the friction plate is attached to the torsion shaft, and the through slot of the torsion shaft is sheathed on the long shaft, and a latching end at an end of the long shaft is inserted into and closely coupled to the latching slot of the torsion shaft, and the brake pad is mounted onto the torsion shaft outside the inner housing, and the positioning protrusion is entered into the positioning cavity, and the disc-shaped elastic member, the adjustable positioning ring and the positioning screw ring are sheathed on the torsion shaft sequentially, and the shaft hole of the positioning screw ring is screwed and coupled to the threaded section, until the torsion shaft, the friction plate, the inner housing, the brake pad, the disc-shaped elastic member, the adjustable positioning ring and the positioning screw ring are packed tightly, and the positioning plate of the adjustable positioning ring is bent and pressed into the corresponsive positioning groove of the positioning screw ring to limit and prevent the positioning screw ring from being rotated or withdrawn. 
     In the torque limiter for a power winch, the torque limit mechanism is installed in the assembly of the reduction mechanism, and the reduction mechanism has a first planetary gear set comprising three identical small gears with a common center shaft inserted into a base and a cover and arranged into a triangular shape, and the base has a through hole formed at the center of the base, and a large accommodating slot formed on a surface of the base and facing the small gears and connected to plurality of small cavities formed on the inner periphery of the base; and the cover has a through hole corresponsive to the through hole of the base, and the outer periphery of the friction plate further includes a plurality of small flanges, such that during assembling, the torsion shaft is passed through the disc-shaped elastic member and the brake pad, and the positioning protrusion of the brake pad is entered into the positioning cavity of the torsion shaft, and then the torsion shaft is passed through the through hole of the base, so as to attach the brake pad onto the back side of the base, and the friction plate is contained into the large accommodating slot of the base in the manner of corresponding the small flanges one by one to the small cavities and provided for passing the torsion shaft, and then the adjustable positioning ring and the positioning screw ring are sheathed on the torsion shaft sequentially, so that the shaft hole of the positioning screw ring is screwed and engaged with the threaded section until the torsion shaft, the disc-shaped elastic member, the brake pad, the base, the friction plate, the adjustable positioning ring and the positioning screw ring are packed closely, and the positioning plate of the adjustable positioning ring is bent and pressed into the corresponsive positioning groove of the positioning screw ring to limit and prevent the positioning screw ring from being rotated and withdrawn. 
     In the torque limiter for a power winch, when the torque limit mechanism is installed at the power supply that outputs power, a sleeve is coupled to the center shaft of the power supply, and a large accommodating slot is formed at an end of sleeve away the shaft, and the outermost section of the inner periphery of the large accommodating slot has a plurality of small cavities, and the outer periphery of the friction plate has a plurality of small flanges, such that during assembling, the torsion shaft is passed through the friction plate, the brake pad, the disc-shaped elastic member, the adjustable positioning ring and the positioning screw ring sequentially, so that the friction plate is attached onto the torsion shaft, and the positioning protrusion of the brake pad is entered into the positioning cavity of the torsion shaft, and the shaft hole of the positioning screw ring is screwed and engaged with the threaded section of the torsion shaft until the torsion shaft, the friction plate, the brake pad, the disc-shaped elastic member, the adjustable positioning ring and the positioning screw ring are packed closely, and the positioning plate of the adjustable positioning ring is bent and pressed into the corresponsive positioning groove of the positioning screw ring to limit and prevent the positioning screw ring from being rotated and withdrawn, and then the torque limit mechanism is contained in the large accommodating slot of the sleeve, and the small flanges of the friction plate are inserted and positioned into the small cavities of the sleeve in one-to-one correspondence, and the latching end of the long shaft is inserted into the latching slot of the torsion shaft for a connection. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a first preferred embodiment of the present invention; 
         FIG. 2  is an exploded view of the first preferred embodiment of the present invention; 
         FIG. 3  is an exploded view of the first preferred embodiment of the present invention, viewing from another angle; 
         FIG. 4  is an exploded view of a torque limit mechanism of the first preferred embodiment of the present invention; 
         FIG. 5  is a cross-sectional view of the first preferred embodiment of the present invention; 
         FIG. 6  is an exploded view of a second preferred embodiment of the present invention; 
         FIG. 7  is an exploded view of the second preferred embodiment of the present invention, viewing from another angle; 
         FIG. 8  is an exploded view of a torque limit mechanism of the second preferred embodiment of the present invention; 
         FIG. 9  is a cross-sectional view of the second preferred embodiment of the present invention; 
         FIG. 10  is an exploded view of a third preferred embodiment of the present invention; 
         FIG. 11  is an exploded view of the third preferred embodiment of the present invention viewing from another angle; 
         FIG. 12  is an exploded view of a torque limit mechanism of the third preferred embodiment of the present invention; and 
         FIG. 13  is a cross-sectional view of the third preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The technical characteristics, contents, advantages and effects of the present invention will be apparent with the detailed description of a preferred embodiment accompanied with related drawings as follows. 
     The present invention provides a torque limit mechanism installed during a transmission process of a power winch and provided for timely cutting off the power transmission whenever the torque produced by a carrying load exceeds a torque bearable by the power supply, so that the torque limit mechanism may be installed at any position in the power transmission process. For example, the torque limit mechanism is installed before the power is entered into the reduction gearbox (in other words, it is installed at the front of the reduction mechanism), or it is directly installed in the assembly of reduction mechanism, or it is installed when power is outputted by the power supply, and the preferred embodiments are described in details below. 
     In the first preferred embodiment, the torque limit mechanism is installed in front of the reduction mechanism in the reduction gearbox for entering the power (in other words, it is installed at the front of the reduction mechanism). With reference to  FIG. 1  for a power winch  1  in accordance with the first preferred embodiment of the present invention, the overall appearance of the structure of the power winch  1  comprises: a frame  10 ; a power supply  20 , such as a power motor installed on a side of the frame  10 , and capable of outputting a forward power or a reverse power; a reduction gearbox  30  installed on the other side of the frame  10 , and including a reduction mechanism  32  (as shown in  FIG. 5 ) installed therein, for receiving the power transmission from the power supply  20  for reduction; and a cable wheel  40  installed at the middle of the frame  10  and driven to rotate by the power outputted from the reduction gearbox  30 , so as to release a cable (not shown in the figure) or retrieve the cable. 
     In  FIGS. 2 to 5 , after a rear cover  301  of the reduction gearbox  30  is removed, a long shaft  21  is directly and synchronously driven by a center shaft of the power supply  20  and extended out from a central through hole  311  of an inner circular shell  31 , and an end of the long shaft  21  is formed as a non-circular latching end  211  such as a latching end in a hexagonal shape. An accommodating groove  302  is formed at the center of the innermost-depth end surface of the rear cover  301  (as shown in  FIG. 5 ) and provided for containing and positioning a closely installed bearing  303  therein. A reduction mechanism  32  is installed in the interior of the inner circular shell  31  (as shown in  FIG. 5 ) and driven and reduced by a plurality of layers of planetary gear sets which jointly form the reduction mechanism  32 , and the inner periphery of the outer side of the inner circular shell  31  is an outwardly expanded oblique friction surface  312 . A sectional shaft  33  sheathed on the long shaft  21  without being in contact (in other words, the long shaft  21  is passed through the longitudinal shaft hole of the sectional shaft  33  with a gap from the longitudinal shaft hole). An engaging gear  331 ,  332  and a ring-shaped engaging slot  333 ,  334  are formed on ring walls at both ends respectively, and a positioning hole  335  is formed on a sectional shaft, and an end of the sectional shaft  33  with the engaging gear  331  and the engaging slot  333  is extended into the central through hole  311  of the inner circular shell  31 , and the engaging gear  331  and the reduction gearset  32  at the end are engaged and linked. A C-shaped retainer ring  336  is inserted into the engaging slot  333  (as shown in  FIG. 5 ) and provided for limiting and preventing the sectional shaft  33  from being withdrawn freely. An elastic element  337 , preferably a volute spring, is sheathed on the sectional shaft  33 , and an end of the elastic element  337  is inserted and positioned into the positioning hole  335 . A brake block  34  is made of a rubber lining material and the outer periphery of the brake block  34  is an outwardly expanded oblique friction surface  341 , and a circular groove  342  is formed on the rear side of the brake block  34  (as shown in  FIG. 3 ), and a large through hole  343  is formed at the center of the brake block  34 . A disc-shaped rim  35  is made of a robust material and divided into a disc-shaped portion  351  and a rim portion  352  (as shown in  FIG. 2 ), wherein the disc-shaped portion  351  is directly attached into the circular groove  342  of the brake block  34 , and a plurality of locking elements (such as bolts and screw holes) is provided for securing the brake block  34  with the disc-shaped protrusion  35  to form a jointly rotating body, and the rim portion  352  is entered into the large through hole  343  of the brake block  34 , but there is a significant spacing between the outer peripheral surface of the rim portion  352  and the inner peripheral surface of the large through hole  343 , and the end surface of the outer wall of the rim portion  352  has a plurality of up-and-down bevels  353 , and an end position of the outer wall has a pair of outwardly protruding convex latching bodies  354 , and a rear end surface of the rim portion  352  has a circular abutting slot  355  (as shown in  FIG. 3 ), such that during assembling, the brake block  34  and the disc-shaped protrusion  35  engaged with each other to form the jointly rotating body is sheathed on the sectional shaft  33  as shown in  FIG. 5 , but there is no direct linkage between the two. The other end of the elastic element  34  is abutted into the circular abutting slot  355  formed on the rear end surface of the rim portion  352  for driving the jointly rotating body to move outward. 
     A wedge block  36  has an engaging gear  361  installed to an inner wall of a center hole of the wedge block  36  engaged and linked with the engaging gear  332  on the ring wall of the sectional shaft  33 . In other words, a direct driving and rotating relation exists between the sectional shaft  33  and the wedge block  36 , a set of C-shaped retainer ring  362  is latched into the engaging slot  334  for limiting the wedge block  36  from displacing beyond the sectional shaft  33 , and the wedge block  36  has a bevel  363  and a convex latching body  364  corresponsive to the bevel  353  and the convex latching body  354  of the disc-shaped rim  35  respectively, but both of the convex latching bodies  354 ,  364  are installed with slightly different angles. An inner housing  37  facing an end of the wedge block  36  is an opening  371  (as shown in  FIG. 3 ), and a through hole  372  is formed at an end of the inner housing away from the wedge block  36  and the interior of the through hole  372  is corresponsive to the convex latching body  354  of the disc-shaped rim  35  and the convex latching body  364  of the wedge block  36 , and an internal ring wall has a corresponsive bump  373  formed thereon. 
     A torque limit mechanism A is comprised of a torsion shaft  38 , a friction plate  391 , a brake pad  392 , a disc-shaped elastic member  393 , an adjustable positioning ring  394  and a positioning screw ring  395 , wherein the torsion shaft  38  has an axial through slot  381  formed therein (as shown in  FIG. 3 ), a non-circular latching slot  382  such as a hexagonal latching slot formed at a front end of the torsion shaft  38  and corresponsive to the latching end  211  of the long shaft  21 , and a small section including a threaded section  383  with a reverse outer thread is formed at the middle of the outer periphery and a positioning recession  384  is formed at a front end and having a positioning cavity  385  formed thereon and penetrated to the inside in an axial direction, and the rear end has a ring wall  386 . The friction plate  391  is a ring-shaped plate having a surface with a friction effect, and the brake pad  392  is a ring-shaped plate made of a lining material and having a positioning protrusion  3921  formed at the inner periphery of the brake pad  391 . The disc-shaped elastic member  393  is in a concave arc shape and has a compressive elasticity. The outer periphery of the adjustable positioning ring  394  has a plurality of positioning plates  3941  selectively bent in a direction towards the positioning screw ring  395 . The positioning screw ring  395  has a shaft hole  3951  with a reverse inner thread, and a plurality of symmetrical positioning grooves  3952  formed on the outer periphery for bending, pressing, and remaining the selected positioning screw plate  3941  therein respectively. 
     In  FIG. 5 , during assembling, the torsion shaft  38  is passed through the friction plate  391  and the through hole  372  of the inner housing  37 , so that the friction plate  391  is attached to the ring wall  386 , and then the through slot  381  of the torsion shaft  38  is sheathed on the long shaft  21 , and the latching end  211  at an end of the long shaft  21  is inserted into the latching slot  382  of the torsion shaft  38  of a close connection. During the process, the end of the opening  371  of the inner housing  37  is inserted precisely into the gap between the rim portion  352  and the large through hole  343  for a non-closely connecting insertion, while the convex latching bodies  364 ,  354  of the wedge block  36  and the disc-shaped rim  35  are disposed adjacent to the bump  373  of the inner housing  37 , and then the brake pad  392  is sheathed on the torsion shaft  38  outside the inner housing  37 , and the positioning protrusion  3921  is entered into the positioning cavity  385 , and then the disc-shaped elastic member  393 , the adjustable positioning ring  394  and the positioning screw ring  395  are sheathed on the torsion shaft  38  sequentially, and the shaft hole  3951  of the positioning screw ring  395  is connected to the threaded section  383  through a tight screwing engagement, until the torsion shaft  38 , the friction plate  391 , the inner housing  37 , the brake pad  392 , the disc-shaped elastic member  393 , the adjustable positioning ring  394  and the positioning screw ring  395  are packed tightly (in other words, the torque limit mechanism A and the inner housing  37  are packed), and the level of packing (or the number of screws used for connecting the shaft hole  3951  and the threaded section  383 ) can be adjusted according to the torque bearable by the power supply  20  (it is noteworthy that the torque bearable by the power supply  20  is indirectly proportional to the level of packing). After the level of packing is selected, the positioning plate  3941  of the adjustable positioning ring  394  is bent towards and pressed into the corresponsive (adjacent) positioning groove  3952  of the positioning screw ring  395  to limit and prevent the positioning screw ring  395  from being rotated and withdraw. The positioning plate  3941  not corresponsive (adjacent) to the positioning groove  3952  will not be bent. Finally, the rear cover  301  is covered, so that the positioning recession  384  of the torsion shaft  38  is entered and positioned into the inner periphery of the bearing  303  contained in the accommodating groove  302  of the rear cover  301 . 
     When the power supply  20  is turned on (regardless of outputting a forward power or a reverse power), the long shaft  21  is driven to rotate by the power supply  20 , and the torque limit mechanism A and the inner housing  37  are synchronously rotated by the close connection between the latching end  211  and the latching slot  382 , and then the bump  373  inside the inner housing  37  pushes the convex latching bodies  354 ,  364  to rotate synchronously. Since the engaging gear  361  of the wedge block  36  and the engaging gear  332  on the ring wall of the sectional shaft  33  are engaged and linked with each other, the sectional shaft  33  is driven to rotate, so as to provide a predetermined deceleration effect of the reduction mechanism  32  and drive the cable wheel  40  to rotate and release a cable (not shown in the figure) or retrieve the cable. 
     When the power supply  20  is turned off (through a manual control/operation or a power failure), the long shaft  21 , the torque limit mechanism A and the inner housing  37  will be stopped immediately, and then the cable of the cable wheel  40  carrying a load will produce a reverse torque, and the linkage of the cable and the reduction mechanism  32  pushes the sectional shaft  33  and the wedge block  36  to produce a reverse rotation, so that the convex latching body  364  of the wedge block  36  is separated with respect to the bump  373  inside the inner housing  37 , and the bevel  363  of the wedge block  36  momentarily press the bevel  353  of the disc-shaped rim  35 , so that the disc-shaped rim  35  is moved quickly towards the inner circular shell  31 , and the oblique friction surface  341  of the brake block  34  is attached quickly to the oblique friction surface  312  of the inner circular shell  31  to produce a braking effect for braking the brake block  34  and stopping the wedge block  36 , the sectional shaft  33 , the reduction mechanism  32  and the cable wheel  40  with the cable accordingly. 
     The power supply  20  regardless of being turned on or off will produce a reverse torque as long as the cable is loaded. When the reverse torque produces the brake effect and/or the power supply  20  is capable of bearing the torque, then there will be no problem for the application. However, if the reverse torque produces the brake effect and/or reaches a level almost not bearable by the power supply  20 , then the reverse torque will force the wedge block  36  to push the inner housing  37  to rotate in a reverse direction. Now, the torsion shaft  38  is stopped together with the power supply  20 , so that rotation of the inner housing  37  will force the friction plate  391  to rotate altogether, but the torsion shaft  38  and the brake pad  392  will be released from the compressed status with the inner housing  37  to remain still, so as to protect the power supply  20  from being damaged. 
     A power winch  1  in accordance with the second preferred embodiment of the present invention has the same structural look and effects as the embodiment as shown in  FIG. 1 , and thus they will not be repeated, and the same element of both first and second preferred embodiments adopts the same name and numeral for simplicity. The difference between these two preferred embodiments resides on that the torque limit mechanism of the second preferred embodiment is installed in the assembly of the reduction mechanism. With reference to  FIGS. 6 to 9  for the power winch  1  of the second preferred embodiment of the present invention, after the rear cover  301  and the inner circular shell  31  of the reduction gearbox  30  are removed, a second inner circular shell  43  and a first planetary gear set  321  are exposed, and the inner periphery of the second inner circular shell  43  is an inner gear  431 , and the first planetary gear set  321  is the first layer of the reduction mechanism  32  and comprised of three identical small gears, and the center shaft is jointly inserted into a base  41  and a cover  42  and they are arranged into a triangular shape, wherein the base  41  has a through hole  411  formed at the center of the base  41 , a large accommodating slot  412  formed on a side of the base  41  facing the small gears, and a plurality of small cavities  413  formed at the inner periphery of the base  41 , and three insert holes  414  are formed around. The cover  42  has a through hole  421  formed at the center of the cover  42  and three insert holes  424  formed around the periphery of the cover  42 , such that during assembling, the insert holes  414 ,  424  are provided for inserting and positioning the center shaft at both ends of the three small gears, and the through holes  411 ,  421  are configured to be corresponsive to the insert holes  414 ,  424  respectively. 
     A torque limit mechanism B, is comprised of a torsion shaft  48 , a friction plate  491 , a brake pad  492 , a disc-shaped elastic member  493 , an adjustable positioning ring  494  and a positioning screw ring  495 , wherein the torsion shaft  48  has an axial through slot  481  formed therein and provided for passing the through the long shaft  21  with spacing, and a small section of the outer periphery has a threaded section  483  with a reverse outer thread, and a positioning cavity  485  is formed at a selected position of the outer periphery and penetrated to the inside in an axial direction, and its rear end has an engaging gear  486 . The friction plate  491  is a ring-shaped plate having a surface with a friction effect, and the outer periphery further includes a plurality of small flanges  4911 , and the friction plate  491  and the small flanges  4911  have sizes precisely corresponsive to those of the large accommodating slot  412  and the small cavity  413  of the base  41 . The brake pad  492  is a ring-shaped plate made of a lining material, and its inner periphery has a positioning protrusion  4921 . The disc-shaped elastic member  493  is substantially in a concave arc shape and has a compressive elasticity. The outer periphery of the adjustable positioning ring  494  has a plurality of positioning plates  4941  which can be selectively bent towards the positioning screw ring  495 . The positioning screw ring  495  has a shaft hole  4951  with a reverse inner thread, and a plurality of symmetrical positioning grooves  4952  formed at the outer periphery and provided for bending, pressing and remaining the selected positioning screw plate  4941 . 
     In  FIG. 9 , during assembling, the torsion shaft  48  is passed through the disc-shaped elastic member  493  and the brake pad  492 , and the positioning protrusion  4921  of the brake pad  492  is entered into the positioning cavity  485  of the torsion shaft  48 , and then the torsion shaft  48  is passed through the through hole  411  of the base  41 , so that the brake pad  492  is attached onto the back side of the base  41 , and then the friction plate  491  is contained into the large accommodating slot  412  of the base  41  by corresponding the small flanges  4911  with the small cavities  413  in a one-to-one correspondence, so as to allow the torsion shaft  48  to pass through, and then the adjustable positioning ring  494  and the positioning screw ring  495  are sheathed on the torsion shaft  48  sequentially, and the shaft hole  4951  of the positioning screw ring  495  and the threaded section  483  are connected with a tight screwing engagement, until the torsion shaft  48 , the disc-shaped elastic member  493 , the brake pad  492 , the base  41 , the friction plate  491 , the adjustable positioning ring  494  and the positioning screw ring  495  are packed (in order words, the torque limit mechanism B and the base  41  are packed). Of course, the level of packing (or the quantity of screws used for connecting the shaft hole  4951  and the threaded section  483 ) may be adjusted according to the torque bearable by the power supply  20  (it is noteworthy that the torque bearable by the power supply  20  is directly proportional to the level of packing). After the level of packing is selected, the positioning plate  4941  of the adjustable positioning ring  494  is bent towards the corresponsive (adjacent) positioning groove  4952  of the positioning screw ring  495  to limit and prevent the positioning screw ring  495  from being rotated and withdrawn. The positioning plate  4941  not corresponsive (adjacent) to the positioning groove  4952  will not be bent. Finally, the base  41  and the three small gears and the cover  42  are assembled into the first planetary gear set  321 , and then the first planetary gear set  321  including the torque limit mechanism B is put into the second inner circular shell  43 , so that the outermost teeth of the three small gears are engaged with the inner teeth  431  of the second inner circular shell  43 , while the engaging gear  486  of the torsion shaft  48  is being extended into the second inner circular shell  43  and coupled with any other layer of the planetary gear set of the reduction mechanism  32  installed further inside the second inner circular shell  43 . After an end of the aforementioned sectional shaft  33  having the engaging gear  331  and the engaging slot  333  is extended into the central through hole  311  of the inner circular shell  31 , the end of the sectional shaft  33  is supported by a bearing  432 , and the C-shaped retainer ring  336  is inserted into the engaging slot  333  (as shown in  FIGS. 5 and 10 ), for limiting the sectional shaft  33  from being withdrawn freely. In the process, the engaging gear  331  of the sectional shaft  33  is inserted into a space enclosed by the three small gears, and the engaging gear  331  is engaged with the innermost teeth of the three small gears. Finally, the inner circular shell  31  and the rear cover  301  are used for the covering purpose. 
     After the power supply  20  is turned on (regardless of outputting a forward power or a reverse power), and the power is supplied to the sectional shaft  33 , the three small gears of the first planetary gear set  321  are driven to rotate around the inner teeth  431  of the second inner circular shell  43 , so that the whole first planetary gear set  321  is rotated. Now, the torque limit mechanism B and the base  41  are situated at a compressed status, so that the torque limit mechanism B is rotated synchronously with the first planetary gear set  321 . When the power supply  20  is turned off (by manual control or operation, or by power failure), the first planetary gear set  321  and the torque limit mechanism B will be stopped immediately, and the reverse torque produced by the carrying load pushes the torque limit mechanism B and the first planetary gear set  321  to produce a reverse rotation through the linkage with the cable and another layer planetary gear set on the reduction mechanism  32 . If the reverse torque is not bearable by the power supply  20 , then the reverse torque will force the torsion shaft  48  to rotate in a reverse direction. Now, the sectional shaft  33  and the first planetary gear set  321  are stopped with the power supply  20 , therefore the reverse rotation of the torsion shaft  48  will force the brake pad  492  to rotate altogether but the base  41  and the friction plate  491  will be separated from the compressed status with the brake pad  492  and will remain still, so as to protect the power supply  20  from being damaged. 
     A power winch  1  in accordance with the third preferred embodiment of the present invention has the same structural look and effects as the embodiment as shown in  FIG. 1 , and thus they will not be repeated, and the element of the third preferred embodiment same as that of the first and second preferred embodiments adopts the same name and numeral for simplicity. The difference between the third preferred embodiment and the first and second preferred embodiments resides on that the torque limit mechanism of the third preferred embodiment is installed when the power supply outputs power. 
     With reference to  FIGS. 10 to 13  for the power winch  1  in accordance with the third preferred embodiment of the present invention, an engaging gear  221  is installed at a front end of the center shaft  22  of the power supply  20 ; a sleeve  23  is formed at an end of an ring-shaped inner engaging tooth  231  corresponsive to the center shaft  22 , and the engaging gear  221  of the center shaft  22  is engaged with the inner engaging tooth  231  of the sleeve  23 , so that the sleeve  23  and the center shaft  22  are connected, and a large accommodating slot  232  is formed at an end of the sleeve  23  away from the center shaft  22 , and the outermost section of the inner periphery further has a plurality of small cavities  233 ; 
     A torque limit mechanism C is comprised of a torsion shaft  58 , a friction plate  591 , a brake pad  592 , a disc-shaped elastic member  593 , an adjustable positioning ring  594  and a positioning screw ring  595 , wherein the torsion shaft  58  contains a latching slot  581  substantially in a non-circular shape (such as a hexagonal shape) in the axial direction and provided for engaging an end of the non-circular (or hexagonal) latching end  212  corresponsive to the long shaft  21 , and a small section of the outer periphery of the torsion shaft  58  has a threaded section  583  with a reverse outer thread, and a positioning cavity  585  formed at a selected position of the outer periphery and penetrated into the inside in an axial direction; the friction plate  591  is a ring-shaped plate having a surface with a friction effect, and the outer periphery further has a plurality of small flanges  5911 , and the friction plate  591  and the small flanges  5911  have sizes precisely corresponsive to those of the large accommodating slot  232  and the small cavity  233  of the sleeve  23 ; the brake pad  592  is a ring-shaped plate made of a lining material, and has a positioning protrusion  5921  formed at the inner periphery; the disc-shaped elastic member  593  is substantially in a concave arc shape and has a compressive elasticity; the outer periphery of the adjustable positioning ring  594  has a plurality of positioning plates  5941  which can be selectively bent towards the positioning screw ring  595 ; the positioning screw ring  595  has a shaft hole  5951  with a reverse inner thread, and a plurality of symmetrical positioning grooves  5952  formed at the outer periphery for bending, pressing and remaining the selected positioning screw plate  5941 . 
     In  FIG. 13 , during assembling, the torsion shaft  58  is passed through the friction plate  591 , the brake pad  592 , the disc-shaped elastic member  593 , the adjustable positioning ring  594  and the positioning screw ring  595 , so that the friction plate  591  is attached onto the back side of a disc-shaped area of the torsion shaft  58 , and the positioning protrusion  5921  of the brake pad  592  is entered into the positioning cavity  585  of the torsion shaft  58 , and the shaft hole  5951  of the positioning screw ring  595  and the threaded section  583  of the torsion shaft  58  are connected with a tight screwing engagement, until the torsion shaft  58 , the friction plate  591 , the brake pad  592 , the disc-shaped elastic member  593 , the adjustable positioning ring  594  and the positioning screw ring  595  are packed (in other words, the torque limit mechanism C is packed closely), and its level of packing (or the quantity of screws used for connecting the shaft hole  5951  and the threaded section  583 ) may be adjusted according to the torque bearable by the power supply  20  (it is noteworthy that the torque bearable by the power supply  20  is indirectly proportional to the level of packing). After the level of packing is selected, the positioning plate  5941  of the adjustable positioning ring  594  is bent and pressed into the corresponsive (adjacent) positioning groove  5952  of the positioning screw ring  595  to limit and prevent the positioning screw ring  595  from being rotated and withdrawn. The positioning plate  5941  not corresponsive (adjacent) to the positioning groove  5952  will not be bent. Finally, the torque limit mechanism C is contained in the large accommodating slot  232  of the sleeve  23 , and the plurality of small flanges  5911  of the friction plate  591  are inserted and positioned into the small cavities  233  of the sleeve  23  in a one-to-one correspondence, and provided for inserting the latching end  212  of the long shaft  21  into the latching slot  581  of the torsion shaft  58  to achieve a connection, and the latching end  211  at the other end of the long shaft  21  is inserted at a corresponsive connecting position, therefore both ends of the long shaft  2  are limited from being separated freely. 
     When the power supply  20  is turned on (regardless of outputting a forward power or a reverse power), and the power is supplied from the center shaft  22  to the sleeve  23 , the torque limit mechanism C is packed and the plurality of small flanges  5911  of the friction plate  591  are inserted and packed into the small cavities  233  of the sleeve  23  in a one-to-one correspondence, and the latching end  212  of the long shaft  21  is inserted into the latching slot  581  of the torsion shaft  58  to achieve a connection, and the torque limit mechanism C and the long shaft  21  are driven by the sleeve  23  to rotate synchronously. When the power supply  20  is turned off (by manual control or operation, or by power failure), the sleeve  23  and the long shaft  21  will be stopped immediately, but the reverse torque produced by a carrying load will push the long shaft  21  but the linkage of the cable and the reduction mechanism  32 , so that the torque limit mechanism C and the sleeve  23  produce a reverse rotation. If the reverse torque is not bearable by the power supply  20 , then the reverse torque will force the torsion shaft  58  to rotate in a reverse direction. Now, the sleeve  23  is stopped together with the power supply  20 , so that the reverse rotation of the torsion shaft  58  will force the brake pad  592  to rotate accordingly, but the sleeve  23  and the friction plate  591  are released from the compressed status with the brake pad  592  and remain still, so as to protect the power supply  20  from being damaged. 
     In view of the aforementioned three preferred embodiments of the present invention, a torque limit mechanism is selectively installed in a power winch during a power transmission process for outputting power from the power supply  20  and transmitting the power through the long shaft  21  to the reduction mechanism  32 , and the torque limit mechanism is installed before the power is supplied to the reduction mechanism inside the reduction gear (in order word, it is installed at the front end of the reduction mechanism), installed directly in the assembly of the reduction mechanism, or installed when the power supply outputs power, so that if the reverse torque produced by the carrying load of the power winch exceeds the reverse torque bearable by the power supply, the power transmission will be cut off immediately to assure that the use of the power supply  20  will not exceed the loading capacity, so as to prevent the power winch from being damaged.