Abstract:
A hand-held power tool has a torque-limiting unit with which a maximum torque transferred from a motor output shaft to a tool driver is adjustable by an operator. The torque-limiting unit has a spring system with spring elements having substantially similar characteristics (such as length, compression forces, etc.) and yet generating different spring forces.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application derives priority under 35 USC § 119(e) from U.S. Provisional Application No. 61/739,767, filed on Dec. 20, 2012, now pending, which is hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a power tool and particularly to hand- held power tools with torque limiting units. 
       BACKGROUND 
       [0003]    Hand-held power tools, such as cordless screwdrivers, cordless drills or cordless impact drills have a high amount of drive torque. Limiting this torque is desirable for many applications. Adjustable torque limitation makes it possible, e.g., to screw a number of screws into a work piece with the same level of screw-down torque; a torque-limiting unit disengages as soon as the screws apply a certain level of torque resistance to the motor output shaft. The operator can adjust the torque-limiting unit according to the maximum torque required for the task at hand. A hand-held power tool with torque limitation of the type described above is made known in DE 103 09 057 A1. 
         [0004]    Another hand-held power tool with a torque limiting unit is described in U.S. Pat. No. 7,455,123, and is fully incorporated in its entirety by reference. As described therein, the torque limiting unit has springs  26 ,  28  of different spring characteristics, i.e., using different spring rates, widths, lengths and/or stiffness. Requiring such different springs is costly as it is necessary to maintain a full inventory of different springs. It also makes assembly of the power tool difficult, as the assembler has to ensure each spring has been installed correctly in each place. Therefore, the likelihood of manufacturing defects is increased. 
         [0005]    Accordingly, it is an object of the present invention to provide a hand-held power tool with a torque-limiting unit, which is a further improvement of the existing hand-held power tools. 
       SUMMARY 
       [0006]    The present invention is directed to a hand-held power tool with a torque-limiting unit with which a maximum torque transferred from a motor output shaft to a tool driver is adjustable by an operator, the torque-limiting unit including a spring system. The spring system uses a set of similar springs having substantially similar characteristics. 
         [0007]    In this manner a non-linear spring characteristic curve of the spring system can be obtained using simple spring elements without requiring different types of spring elements. As a result, a maximum torque can be easily set in a range of small torques very precisely and over a broad torque range. Typically, an adjustable maximum torque is between 1 Nm and 15 Nm, e.g., to quickly drive screws into wood without damaging the screws or the wood. 
         [0008]    A particularly comfortable adjustment of the maximum torque can be obtained when the maximum torque can be set very precisely in a range of small torques, e.g., up to 5 Nm. To this end, the spring system can have a spring characteristic curve in this range that is flatter than it is in the range of greater torques, in which the maximum torque can be adjusted less precisely. A different action of the spring elements can be achieved when the spring elements are located such that they are staggered in terms of their direction of motion. When the spring system is actuated, for example, only a few spring elements are actuated at first, followed by all spring elements. 
         [0009]    A particularly simple assembly and compact design of the hand-held power tool can be achieved when the spring elements have the same elasticity. By holding the spring elements in different positions, the identical springs would effectively have different spring characteristic curves, e.g, different spring rates or levels of stiffness. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows a front part of a cordless screwdriver with a torque-limiting unit and an overload clutch, in an exploded view, and 
           [0011]      FIG. 2  shows the front part of  FIG. 1  in a sectional view. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]      FIGS. 1 and 2  show a front part of a hand-held power tool designed as a cordless screwdriver, in an exploded view ( FIG. 1 ) and a sectional illustration ( FIG. 2 ). 
         [0013]    The hand-held power tool includes a torque-limiting unit  2 , a motor output shaft  4 , and a tool driver  6 . To drive tool driver  6 , torque from motor output shaft  4  is transferred to three-stage planetary gearing that includes planet gears  8  that therefore rotate on their axes. Planet gears  8  are supported on a planet carrier  10  that, in the normal working mode, is fixedly connected with a housing  12  of the hand-held power tool. Planet gears  8  drive an internal gear  14 , the inner toothing of which encompasses a driving element  16  and drives it. Driving element  16  drives a star wheel  18 , and star wheel  16  drives tool driver  6  via a square socket. 
         [0014]    In normal operation, planet carrier  10  is non-rotatably connected with a guide sleeve  34  via two locking discs  22 ,  20 , a spring system  24  preferably composed of six spring elements  26  and two thrust members  30 ,  32 , with guide sleeve  34 , in turn, being non-rotatably fastened to housing  12  of the hand-held power tool. The non-rotatable connection is created by cams  36  on planet carrier  10  that engage with cams on first locking disc  20 , first locking disc  20  with cams  38  being connected with second locking disc  22  via cams  40  on second locking disc  22 . Second locking disc  22  is retained by arms  42  of thrust member  30 , arms  42  extending between raised areas  44  of second locking disc  22 . Both thrust members  30 ,  32  are retained via projections  46  in the inner grooves of guide sleeve  34 . 
         [0015]    A disk spring  48  is located behind planet carrier  10  on the transmission side, disk spring  48  being inserted in a holder  50 . Holder  50  encompasses disk spring  48  and planet carrier  10  via arms  52 , and engages in recesses  54  of first locking disc  20 . Arms  52  are held in recesses  54  via wide sections  56 , holder  50  being held—via a tension with which disk spring  48  is compressed slightly—against locking disk  20  and clamps planet carrier  10  between disk spring  48  and locking disk  20 . A retaining wheel  58  is located behind holder  50 ; it engages via recesses  60  in inner grooves of guide sleeve  34  and is therefore non-rotatably connected with guide sleeve  34  and a wire ring  64  in guide sleeve  34 . 
         [0016]    To adjust a maximum torque to be transferred to tool driver  6 , the spring pressure of spring system  24  applied to second locking disc  20  can be varied with the aid of adjusting element  66 . To this end, adjusting element  66  includes an actuating element  68 , a cam ring  70 , a locking disk  72 , a bolt  74 , and a spring  76 . A recess  78  and a groove  80  non-rotatably hold cam ring  70  and/or locking disk  72  in actuating element  68 . When actuating element  68  is rotated, cam ring  70  also rotates, arms  84  sliding on a cam track  82  of cam ring  70 , which causes second thrust member 32  to move in axial direction  86 . 
         [0017]    Arms  84  extend through recesses  88  in guide sleeve  34  and, loaded by the spring force of coiled springs  24 , are pressed against cam track  82 . When second thrust member  32  moves in axial direction  86 , the spring pressure of spring system  24  with which second locking disc  22  is pressed against first locking disk  20  varies. Locking disk  72 , via its holes in which bolt  74  engages, prevents unintentional displacement of actuating element  68  during operation of the hand-held power tool. 
         [0018]    Spring system  24  preferably includes six spring elements  26  situated in a spring assembly. Spring elements  26  are preferably designed as compression springs in the form of coiled springs. Spring elements  26  may be positioned in a hexagonal pattern. 
         [0019]    As seen in  FIG. 1 , spring elements  26  may be disposed between locking disk  22  and thrust member  30 . Spring elements  26  preferably contact locking disk  22  and thrust member  30 . Locking disk  22  may have different surfaces  22 S,  22 P that contact spring elements  26 . Preferably, the different surfaces  22 S,  22 P will be at different levels for the reasons specified below. For example, surface  22 P is below surface  22 S as seen in  FIG. 1 . 
         [0020]    Similarly, thrust member  30  may have different surfaces  30 S,  30 P that contact spring elements  26 . Preferably, the different surfaces  30 S,  30 P will be at different levels for the reasons specified below. For example, surface  30 S is below surface  30 P as seen in  FIG. 1 . 
         [0021]    By varying the distances between surfaces  22 S,  22 P and  30 S,  30 P, the length of each spring element  26  can be selected to differ from the length of another spring element  26 , without requiring spring elements with differing characteristics. For example, a spring disposed between surfaces  22 P and  30 S will have an effective length that is shorter than a spring disposed between surfaces  22 P and  30 P. Preferably, three spring elements  26  will be disposed between surfaces  22 P and  30 S, while three other spring elements  26  could be disposed between surfaces  22 S and  30 P in an alternating arrangement around the circumference of locking ring  22 . Persons skilled in the art will recognize that the distance between surfaces  22 P and  30 P and/or the distance between surfaces  22 S and  30 P can be selected so that it is substantially equal to the distance between surfaces  22 S and  30 S. 
         [0022]    Such arrangement will effectively create some springs that are shorter and stronger spring action—and others that are longer with weaker spring action, even if all the spring elements  26  the same at-rest characteristics. As a result of this stable arrangement, a single-staged progression of the maximum torque can be attained with uniform displacement of cam ring  70 . 
         [0023]    When the smallest possible maximum torque of 1 Nm is set via cam ring  70 , the longer spring elements  26  are held between locking disk  22  and thrust member  30  with slight preload. When cam ring  70  is rotated toward a larger maximum torque, spring elements  26  are initially compressed, whereas shorter spring elements  26  are still located between locking disk  22  and thrust member  30  with a slight amount of play. Starting at a maximum torque of 4 Nm, when cam ring  70  is rotated further, the shorter spring elements  26  are also compressed, so that the maximum torque now increases more rapidly when cam ring  70  is rotated in a uniform manner, and in fact, up to a value of 15 Nm. 
         [0024]    During normal operation of the hand-held power tool, in which a torque applied to tool driver  6  is below the set maximum torque, planet carrier  10  is stationary relative to housing  12 . If the torque applied to tool driver  6  reaches the maximum torque level that was set, second locking disc  22  is deflected against spring system  24  by beveled flanks of cams  38 ,  40 , and first locking disc  20  can rotate against second locking disc  22  along with planet carrier  10 . Internal gear  14  is stationary, and the transfer of torque from motor output shaft  4  to tool driver  6  is interrupted above the maximum torque. 
         [0025]    To bridge torque-limiting unit  2 , cam ring  70  includes—in addition to a uniformly increasing first segment  90  inside radial cam  82  to realize a drilling mode—a second, more steeply rising segment  92  and a third, flat segment  94  that brings about no change in the spring pressure of spring system  24  when cam ring  70  is rotated. The maximum torque of 1 Nm to 15 Nm is adjusted by moving arms  84  over first segment  90 . 
         [0026]    When the bridging-over setting is set, arms  84  rest on third segment  94  and are deflected away to a maximum extent in the direction of motor output shaft  4  of the hand-held power tool. Spring elements  26  are compressed together so far that pins  96 ,  98  holding spring elements  26  each other. As a result, locking disk  22  is retained between locking disk  20  and thrust member  32  in axial direction  86  such that it cannot be deflected. First locking disk  20  is now unable to slide over second locking disk  22 . In addition, arms  42  extend between recesses  54  of locking disc  20 , by way of which locking disk  20  is non-rotatably connected with guide sleeve  34 . 
         [0027]    In this position, a level of torque that could damage the hand-held power tool and that is dangerous to the operator could be transferred to tool driver 6  by torque-limiting unit  2 . To prevent this much torque from being transferred, an overload clutch that interrupts the flux of force to tool driver  6 when an overload torque is exceeded is located on planet carrier  10 . If a torque level is transferred to tool holder  6  that reaches the level of overload torque specified by the spring force of disk spring  48  in the drilling position, plant carrier  10  is deflected via beveled flanks of cams  36  and the cams on locking disk  20  in the direction toward disk spring  48 , and disk spring  48  is compressed further against its preload. Planet carrier  10  can now rotate against locking disk  20 , by way of which the flux of force from motor output shaft  4  to tool driver  6  between planet carrier  10  and locking disk  20  is interrupted. 
         [0028]    It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of constructions differing from the types described above. 
         [0029]    While the invention has been illustrated and described as embodied in hand-held power tool with a torque-limiting unit, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. 
         [0030]    Without further analysis, the foregoing will reveal fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of the invention.