Patent Publication Number: US-2011073341-A1

Title: Power tool for tightening screw joints and release couplling

Description:
FIELD OF THE INVENTION 
     The invention concerns a power tool for tightening screw joints according to the preamble of claim  1 . The invention also concerns a release coupling for such a power tool according to the preamble of claim  6 . 
     BACKGROUND OF THE INVENTION 
     Screw or nut runners for industrial purposes are previously known which comprise means for subjecting an electrically driven motor to a braking torque when a predetermined tightening condition, such as increased torque resistance over a certain level, has been reached. 
     One example of such a device is disclosed in U.S. Pat. No. 4,210,852. In that document, such a condition can be detected by monitoring current supplied to the motor of the device. In order to achieve a rapid tool stop the motor is short-circuited and consequently braked strongly. 
     Even though the previously known device functions well, there is a desire to obtain even higher productivity and thus provide a power tool with high rotational speed that can be used also for screw joints that are characterized by low or virtually no elasticity. 
     Examples of such screw joints are found in applications, where large head screws are used for assembly and mounting purposes, and where there is a clear risk of damaging the screw joint if a high speed tool can not be brought to a stop sufficiently fast. Such problematic screw joints are for example encountered in some parts of the electronics industry, wherein there is also an extremely high productivity requirement. 
     In such cases extremely fast stop is thus desirable in order to obtain screw joints with high quality also when a fast rotating screw or nut driver is used. 
     Aim and Most Important Features of the Invention 
     It is an aim of the present invention to provide a power tool of the kind described above which addresses and at least partially reduces the above problems. 
     This aim is obtained in a power tool according to the above which is characterized by the characterizing features in claim  1 . 
     By providing the tool with a release coupling which disconnects the rotational parts of the motor and transmission elements from the output shaft it is achieved that the major portion of the moment of inertia is disconnected. 
     Hereby the screw joint is not affected by the major part of the kinetic energy stored in rotating masses inside the power tool. Further, the difficulties with detecting fast processes are at least partially avoided. 
     As an example, the inertia factor of the parts that can be disconnected according to the invention may well be as great as 20 a 100 times the inertia factor of the elements that remain connected to the out-put shaft. 
     U.S. Pat. No. 5,155,421 describes a power wrench for tightening screw joints making use of a retardation responsive inertia device. That device is, however, arranged for the activation of a switch as a certain predetermined retardation magnitude in the motor rotor is exceeded. 
     The invention is particularly useful when the rotation motor is an electrically driven motor and when the device for braking the motor is a circuit for initiating an electrical braking torque to the motor, since in such applications there are already examples of very fast rotating and fast stop screw drivers. 
     It is preferred that the first coupling part is subjected to a force in the direction of the first position by at least one from the group: an elastic element, an electric coil, since this allows for an initial toque to be transferred by the coupling and a distinct initial position. The elastic element can be a compression spring or any other suitable elastic means. 
     A coil, which is energized by an electric current, can provide the same function as the elastic element or provide additional holding force. It can further keep the coupling disconnected or even, if for the purpose properly energised, displace the first coupling part to a second position where it is out of engagement with the second coupling part, also when there is no braking action on the tool. It can also provide a stabilizing effect on the first coupling part and prevent it from oscillating. 
     The first coupling part is preferably guided by any one from the group: screw tread, ball and groove, pin and groove, cam and follower. The use of a screw thread is advantageously simple and economic, but allows basically for using the tool in only one rotational direction, if no additional measures are added for allowing reverse direction driving. Ball and groove means or pin and groove means for guiding of the first coupling part makes it possible to use the tool in both rotational directions by shaping and directing the groove accordingly. 
     According to the background art, a tool having a rotational speed of 45 000 rpm can be braked to a resulting rotational speed of 0 rpm in 5 msek. Also such very fast braking action leaves however an unwanted after-rotation of a screw because of the considerable moment of inertia in the rotating parts. 
     The impact of this after-rotation is difficult to calculate and to compensate for in fast processes and is thus generally harmful to the result, particularly in respect of the screw joints discussed above. According to the invention, a great deal of this after-rotation is avoided. 
     The corresponding advantages are obtained in respect of a release coupling device for a screw joint tightening power tool. 
     Further features and advantages of the invention are described in the following description of embodiments. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention will now be described in greater detail by way of embodiments and with reference to the annexed drawings, wherein: 
         FIG. 1  diagrammatically shows a power tool according to the invention, 
         FIG. 2   a  shows, in an axial section, the release coupling of the power tool in  FIG. 1  in a first position, 
         FIG. 2   b  shows, in an axial section, the release coupling of the power tool in  FIG. 1  in a second position, and 
         FIG. 3  shows a detail of the release coupling in a second embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows diagrammatically a power tool  1  for tightening screw joints with only its main functional elements indicated. The power tool  1  comprises a housing  2  wherein is arranged an electrically driven motor  3 . The motor  3  is connected to an intermediate power transmission unit  4  which in turn includes a gear transmission  5  and a release coupling  6 . 
     The release coupling  6  is on an output side connected to an output shaft  7  which at its distal end carries a screw engagement member  8 . 
     With  9  is indicated a central processing unit, which monitors the operation of the power tool. A circuit  10  for initiating a braking torque is functionally connected to the processing unit and to the electrically driven motor  3 . 
     The unit  3  can be arranged to detect a particular condition representative that a predetermined torque has been reached by for example monitoring current supplied to the motor of the device. In order to achieve a rapid tool stop the motor can be short-circuited and consequently braked strongly. Other per se known braking actions to the motor can also be used. The motor can be battery driven or be connected to a power supply in net over an electrical cord  22 . 
     In  FIG. 2   a , the release coupling  6  is shown in more detail. In particular the release coupling  6  includes a first coupling part  11  which is supported by a support shaft  14 . The support shaft  14  is in turn rotationally connected to the electrically driven motor. Ball bearings are indicated with  20 . 
     The first part  11  is pressed by a helical compression spring  13 , between a shoulder  15  on the support shaft  14  and a shoulder surface  16  on a sleeve portion  17  of the first part  11 , to the shown position which is a first position of the first coupling part. In that position the first coupling part is in engagement with a second coupling part  12  of the release coupling  6 . 
     Furthermore, the first coupling part  11  is guided in respect of the support shaft  14  over a screw thread  18  in such a way that relative rotation of the first coupling part  11  in respect of the support shaft  14  causes a relative axial displacement of the first coupling part  11  in respect of the support shaft  14  and also of the second coupling part  12 . 
     In  FIG. 2   b  the release coupling  6  is shown in a second position of the first coupling part, wherein the latter has been axially displaced such that torque transmission between the first and the second coupling parts  11  and  12  has been interrupted. 
     According to the invention, this second position of the first coupling part  11  is reached when the electrically driven motor  3  is subjected to a braking torque which in turn also induces a strong retardation of the support shaft  14 . Because of the kinetic energy stored in the first coupling part during rotation and the braking of the support shaft  14 , a relative rotational inertia movement between the first coupling part  11  and the support shaft  14  will be the result. 
     Hereby the first coupling part  11 , through the guidance of the thread  18 , will leave its first position and reach the second position which is shown in  FIG. 2   a . It should be noted that  10  the “first position” is clearly defined whereas “second position or positions” can be defined as all positions where the first coupling part is out of engagement with the second coupling part. 
     When the first coupling part is in the second position or positions the only remaining rotational parts that are still connected to the screw engagement member will therefore be the output shaft  8  and the second coupling part  12 , the kinetic energy of which having a level such that it will not harmfully affect the resulting screw joint. It should, however, be noted that one or more gear steps (not shown) could be positioned between the release coupling and the output shaft. Such elements do not, however, harmfully affect the effect of the invention if dimensioned properly. 
     The invention can be modified within the scope of the following claims. The release coupling may be constructed otherwise than with conical contact surfaces which are shown in  FIGS. 2   a  and  2   b , for example with plane contact surfaces. 
     The helical compression spring  13  may be replaced with another actuating means such as for example an electrical coil, indicated with  19 , which can have the combined features of 1) providing holding force for the coupling, 2) providing a continued release position of the first coupling part, and 3) providing a means for stabilizing the first coupling part in order to avoid unwanted oscillations. The power supply to the coil  19  can be controlled by the processing unit  9 . The coil can also be a complementary element to an elastic element such as the compression spring  13 . 
     In  FIG. 3  is shown an alternative to the thread  18  in  FIGS. 2   a  and  2   b . A support shaft  14 ′ is provided with a guide groove  21  having an angular displacement of α° to a plane which is transverse to the axis of the shaft  14 ′. The first coupling part can be guided by the groove  21  by the intermediate of a ball a pin or the like. Further solutions are also possible such as a cam and follower. 
     The motor of the tool can also be pneumatically driven, even if such tools are not likely to provide the high rotational speed where the invention is most useful. 
     There is a desire to arrange the release coupling as close to the screw joint as possible, since this leaves correspondingly little remaining moment of inertia after release. It is, however, because of dimensional issues, sometimes motivated to have at least one transmission step between the release coupling and the output shaft. 
     The invention is most useful in small, low torque machines but can be applied also to larger devices.