Abstract:
A power transmission device that is capable of automatic speed switching according to external load is disclosed, including a frame in which a transmission mechanism and a torque feedback mechanism are received. The torque feedback mechanism includes a torque resistant member so that when the load torque is smaller than its resistant torque, the speed reduction mechanism of the transmission mechanism is retained at a first, high-speed low-torque stage. When the load torque is larger than its resistant torque, a sliding ring of the torque feedback mechanism pushes a shifting gear so that the speed reduction mechanism is shifted to a second, low-speed high-torque stage. The speed reduction mechanism automatically shifts the speed reduction mechanism when the load torque increases or reduces such that the mechanical efficiency of the transmission device can be increased.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a power transmission device that includes a torque feedback mechanism to change the position of a shift gear so as to change the speed of the transmission device. The speed of the transmission device is automatically switched to a proper value when load changes.  
         BACKGROUND OF THE INVENTION  
         [0002]    A conventional power transmission device, especially for electric spinning tools, such as electric drills and electric screwdrivers, includes a multiple-stage power transmission. A speed reduction mechanism is incorporated to provide multiple speeds associated torque change in accordance with the multiplicity of stages. Generally, the speed reduction mechanism is composed of a planetary gear system and clutch or driving members that manually controlled to switch the speed between the multiplicity of stages. Due to the manual control, an operator has to judge the situation of the tool and decide when to activate the speed reduction mechanism in order to obtain desired torque or speed. However, manual operation is apparently not a feasible way to optimize the operation efficiency of the driving motor.  
           [0003]    Therefore, it is desired to have an automatic mechanism for switching the speed of a transmission device based on load torque in order to optimize the operation of the transmission device.  
         SUMMARY OF THE INVENTION  
         [0004]    In accordance with an aspect of the present invention, there is provided an automatic speed switching mechanism for a power transmission device, which comprises a torque feedback mechanism. The torque feedback mechanism includes a pushing wheel and a sliding ring engaging the pushing wheel. A C-shaped clamp and a compression spring which is fit over the frame and retained between ridges of the frame and the clamp. The angular position of the pushing wheel is limited by a torsion spring that is fixed to the frame. The sliding ring is limited to be moved axially in the frame. The transmission mechanism has a shifting gear which has inner teeth engageable with first planet gears and second planet gears. The shifting gear has an annular groove with which a plurality of pins on the clamp engage so as to retain the shifting gear in a first, high-speed low-torque stage while the shifting gear is engaged with the two planet gears, or retain the shifting gear in a second, low-speed high-torque stage and only engaged with the second planet gears. When the load torque on the pushing wheel is smaller than the force of the torsion spring and compression spring, the shifting gear is retained at the first stage and co-rotates with the two planet gears. When the load torque is larger than the force of the torsion spring and compression spring, it rotates and pushes the sliding ring by the inclined faces so that the sliding ring pushes the shifting gear which is in the second stage and cannot rotate due to the engagement of the protrusions of the frame and the notches of the shifting gear. The speed reduction mechanism of the transmission mechanism automatically shifts the speed reduction mechanism when the load torque increases or reduces such that the mechanical efficiency of the transmission device can be increased.  
           [0005]    The present invention will become more obvious from the following description when taken in connection with the accompanying drawings, which show, for purposes of illustration only, a preferred embodiment in accordance with the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is an exploded view of a power transmission device in accordance with the present invention;  
         [0007]    [0007]FIG. 2 is a cross-sectional view of the power transmission device of the present invention in a first stage which is a high-speed low-torque condition;  
         [0008]    [0008]FIG. 3 is a cross-sectional view of the power transmission device of the present invention in a second stage which is a low-speed high-torque condition, and  
         [0009]    [0009]FIG. 4 shows that a pushing wheel of the power transmission device engaging a sliding ring. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0010]    Referring to the drawings and in particular FIGS. 1 and 2, a power transmission device of the present invention comprises a frame  1 , a torque feedback mechanism  2  and a transmission mechanism  3 . The frame  1  comprises a cylindrical case defining a hollow chamber  11  and forming a plurality of protrusions  11   a  extending inward from an inside surface of the chamber  11 . A plurality of slots  12  is defined through the wall of the frame  1  in the longitudinal direction. A plurality of ridges  13  extends from an outer surface of the frame  1 . A slit  14  is defined in the wall of the frame  1  at an open end of the chamber  11 . A plurality of axial grooves  15  is defined through the wall of the frame  1 .  
         [0011]    The torque feedback mechanism  2  comprises a torsion spring  21 , a pushing wheel  22 , a sliding ring  23 , a C-shaped clamp  24  and a compression spring  25 . The pushing wheel  22  has a plurality of trapezoid blocks  22   a  formed on an outside surface thereof. Inner threads  22   b  are defined in an inner periphery of the pushing wheel  22 . A surface groove  22   c  is defined longitudinally in the outer surface of the pushing wheel  22 . A plurality of trapezoid portions  23   a  is formed on the sliding ring  23 . A plurality of ribs  23   b  is formed on an outside surface of the sliding ring  23 . The sliding ring  23  is received in the chamber  11  of the frame  1  and fit over the pushing wheel  22  with the trapezoid portions  23   a  engaging the trapezoid blocks  22   a  of the pushing wheel  22  and the ribs  23   b  received in the axial grooves  15  of the frame  1  whereby the sliding ring  23  is movable longitudinally in the chamber  11  of the frame  1 . A plurality of lugs  24   a  is formed on an outer surface of the clamp  24  and a plurality of pin holes  24   b  is defined through the clamp  24  and located corresponding to the slots  12  in the frame  1 . Each pin hole  24   b  receives a pin  24   c . The compression spring  25  is fit over the frame  1  and retained between the rides  13  of the frame  1  and the lugs  24   a  of the clamp  24 . The compression spring  25  is deformable by the movement of the clamp  24  in the axial direction so as to provide a longitudinal force. The torsion spring  21  has a first end  21   a  engaging the surface groove  2   c  of the pushing wheel  22 , and a second end  21   b  engaging the slit  14  of the frame  1  so as to resiliently maintain a position relationship between the pushing wheel  22  and the frame  1 .  
         [0012]    The transmission mechanism  3  includes an input gear  31 , a front speed reduction gear  32 , a shifting gear  33  and a rear speed reduction gear  34 . The input gear  31  is connected to an input power source that is not shown. The front speed reduction gear  32  has outer teeth and includes a plurality of planet gears  32   a  rotatably mounted to one surface thereof and a driving gear  32   b  on an opposite surface of the front speed reduction gear  32  for transmitting mechanical power to the rear speed reduction gear  34 . The planet gears  32   a  engage both inner teeth  22   b  of the pushing wheel  22  and the input gear  31  so as to form a planetary speed reduction system. The shifting gear  33  has inner teeth and an annular grooves  33   a  is defined in an outer surface of the shifting gear  33   f . A plurality of notches  33   b  is defined in the outer surface of the shifting gear  33  at an end thereof. The pins  24   c  extend through the slots  12  in the frame  1  and engaging with the annular groove  33   a . The protrusions  11   a  of the frame  1  are engageable with the notches  33   b  for rotatably fixing the shifting gear  33  in the chamber  11  of the frame  1  at a low-speed high-torque condition. The rear speed reduction gear  34  is a circular disk having a surface on which a plurality of planet gears  34   a  is rotatably mounted. An output gear  34   b  is formed on an opposite surface of the circular disk. The planet gears  34   a  engage the drive gear  32   b  and the inner teeth of the shifting gear  33  so as to form a planetary speed reduction mechanism.  
         [0013]    The receiving chamber  11  receives the transmission mechanism  3  and the torque feedback mechanism  2  in sequence. In a first stage which is a high-speed low-torque condition in the illustrated embodiment, the clamp  24  is retained in an initial position by the compression spring  25  where the pins  24   c  are located at an upper section of the slots  12 . Due to the engagement between the pins  24   c  and the annular groove  33   a  of the shifting gear  33 , the shifting gear  33  is located at a topmost position with respect to the frame  1 . Under this circumstance, the trapezoid blocks  22   a  of the pushing wheel  22  and the trapezoid portions  23   a  of the sliding ring  23  completely engage each other. Angular position of the pushing wheel  22  with respect to the frame  1  is retained by the torsion spring  21  while axial position of the sliding ring  23  is retained by the shifting gear  33 , which is retained by the compression spring  25 .  
         [0014]    Referring to FIGS. 3 and 4, when the a large torque is required, the input gear  31  of the transmission mechanism  3  increases the torque transmitted to the front speed reduction gear  32 , which in turn causes the planet gears  32   a  of the front speed reduction gear  32  to impart a reaction torque in reverse direction to the inner teeth  22   b  of the pushing wheel  22  so as to rotate the pushing wheel  22 . Nevertheless, the rotation of the pushing wheel  22  is restrained by the force of the torsion spring  21  and the compressing spring  25 . Inclined side faces of the trapezoid blocks  22   a  of the pushing wheel  22  induce a camming action on inclined side faces of the trapezoid portions  23   a  of the sliding ring  23  whereby, when the reaction torque of the pushing wheel  22  is large enough, the sliding ring  23  is forced to move axially by the camming action of the inclined side faces of the trapezoids  22   a ,  23   a  and guided by the engagement between the ribs  23   b  and the axial grooves  15 . The sliding ring  23  pushes the shifting gear  33 , which, due to the engagement between the pins  24   c  and the annular groove  33   a , drives the clamp  24  to axially move in unison therewith. The clamp  24  is resiliently biased by the compression spring  25  and a reaction force against the movement of the clamp  24  is induced. Under this circumstance, when the torque applied to the pushing wheel  22  by the front speed reduction gear  32  reaches a predetermined threshold value, the trapezoid portion  23   a  of the sliding ring  23  moves along the inclined sides of the trapezoid blocks  22   a  of the pushing wheel  22 , bringing the sliding ring  23  away from the pushing wheel  22 . This disengages the shifting gear  33  from the front speed reduction gear  32  and the shifting gear  33  is now only engaging the planet gears  34   a . A further speed reduction is obtained and a maximum torque is induced on the output gear  34   b . The shifting gear  33  now reaches the bottom position to allow the notches  33   b  to engage the protrusions  11   a  in the chamber  11  of the frame  1  thereby fixing the shifting gear  33 .  
         [0015]    The torque of the shifting speed reduction mechanism is determined by the torsion spring  21  and the compression spring  25 . This can be changed by replacing the springs  21 ,  25  with new ones having different spring constants.  
         [0016]    The automatic shifting device can be used as a power transmission device in electric drills. When drilling, if a small amount of torque is required, the torque applied on the pushing wheel  22  from the front speed reduction gear  32  cannot overcome the resistant force from the torsion spring  21  and the compression spring  25 , so that the pushing wheel  22  does not rotate. The sliding ring  23  and the shifting gear  33  are retained in their first stage of speed. The shifting gear  33  is engaged with the planet gears  34   a  of the rear speed reduction gear  34  and the front speed reduction gear  32 . The shifting gear  33  co-rotates with the front speed reduction gear  32  and the rear speed reduction gear  34 . The result is located in the maximum value of the curve of the torque vs. revolution. If a large torque is required, the input gear  31  increases the torque gradually and the torque applied onto the pushing wheel  22  from the front speed reduction gear  32  overcomes the resistant force from the torsion spring  21  and the compression spring  25 . The pushing wheel  22  rotates when the torque increases and the sliding ring  23  and the shifting gear  33  are in its lower most position. The shifting gear  33  is disengaged from the front speed reduction gear  32  and engaged with the planet gears  34   a  of the rear speed reduction gear  34 . The shifting gear  33  is not rotated due to the engagement of the notches  33   b  and the protrusions  11   a his provides the first stage of speed and the result is located in the maximum value of the curve of the torque vs. revolution.  
         [0017]    While we have shown and described the embodiment in accordance with the present invention, it should be clear to those skilled in the art that further embodiments may be made without departing from the scope of the present invention.