Abstract:
An automatic speed shifting device includes a frame having a transmission device and a torque feedback device received therein. The torque feedback device has a torque resistant member so that when the load torque is smaller than its resistant torque, the speed reduction mechanism of the transmission device is remained at the first stage speed status. When the load torque is larger than its resistant torque, a pushing wheel devices a clamp to compress a shifting gear so that the speed reduction mechanism is shifted to another stage of speed. The speed reduction mechanism of the transmission device 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 
     The present invention relates to an automatic speed shifting device which employs a torque feedback device to change the position of a shift gear so as to engage corresponding shifting member in the transmission device. The speed reduction device automatically shifts the transmission device when the load torque increases or reduces. 
     BACKGROUND OF THE INVENTION 
     A conventional power transmission device, especially for electric spinning tools, such as electric drills and electric screwdrivers, includes a multiple-stage power transmission device and a speed reduction device is used to provide multiple speeds or torque. Generally, the speed reduction device is composed of a planetary gear system and clutch or driving members in the speed reduction device are manually controlled such that some parts are fixed or moved in the planetary gear system and the purposes of speed reduction of output or input can be achieved. An operator has to judge the situation of the tool and then decide to operate the manual device to activate proper speed reduction device to obtain desired torque or revolutions of speed. It is difficult to make the efficiency of the driving motor optimized by the manual operation. Therefore, a feedback device for the load torque is needed so as to shift proper steps of the speed reduction device. 
     SUMMARY OF THE INVENTION 
     In accordance with an aspect of the present invention, there is provided an automatic speed shifting device that comprises a frame having a transmission device and a torque feedback device received therein. The frame has a plurality of triangle shaped slots defined through a wall thereof. A radial groove is defined through the wall of the frame. A plurality of protrusions extending from an inside of the frame and ridges are defined on an outer surface of the frame. The transmission device has a shifting gear that has inner teeth that are engaged with first planet gears and second planet gears. The shifting gear has an annular groove with which a plurality of pins on a clamp are engaged so as to retain the shifting gear in a first stage speed position and to engage the shifting gear with the two planet gears, or retain the shifting gear in a second stage speed position and only engage the shifting gear with the second planet gears. The torque feedback device has a pushing wheel, a C-shaped clamp and a compression spring. The pushing wheel has a lever extending from an outer surface thereof and the clamp has a plurality of pin extending through the slots of the frame and engaged with the annular groove of the shifting gear. An elongate hole is defined through the clamp and located corresponding to the radial groove of the frame and the lever of the pushing wheel. The compression spring is mounted to the frame and retained between rides and the ridges on the frame. When the pushing wheel is rotated, the lever is moved in the radial groove of the frame and drives the clamp via the elongate hole in the clamp. The clamp compresses the compression spring by a movement along an inclined surface of the radial groove of the frame so as to generate resistant force. 
     When the torque applied on the pushing wheel from the front speed reduction gear cannot overcome the resistant force from the torsion spring and the compression spring, the shifting gear is in its first stage of speed status and is engaged with the first planet gears and the second planet gears. When the torque applied onto the pushing wheel overcomes the resistant force from the torsion spring and the compression spring, the pushing wheel rotates and drives the clamp to rotate the shifting gear, and the shifting gear is not rotated due to the engagement of the notches and the protrusions. 
     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 
     FIG. 1 is an exploded view of an automatic speed shifting device constructed in accordance with the present invention; 
     FIG. 2 is a cross-sectional view of the automatic speed shifting device in a first stage; 
     FIG. 3 is a cross-sectional view of the automatic speed shifting device in a second stage; 
     FIG. 4A shows a front view of the speed shifting device of the present invention at the first stage of speed, 
     FIG. 4B shows a right side view of the speed shifting device of the present invention in the first stage; and 
     FIG. 4C shows a left side view of the speed shifting device of the present invention in the second stage. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 2, an automatic speed shifting device of the present invention comprises a frame  1 , a torque feedback device  2  and the transmission device  3 . The frame  1  is a cylindrical case and has a hollow chamber  11 . A plurality of protrusions  11   a  extends from an inside of the chamber  11 . A plurality of triangle shaped slots  12  is defined through the wall of the frame  1  in the longitudinal direction and each slot  12  includes a peak portion and a base from which two recesses  12   a  are defined in communication with two ends of the base. A plurality of ridges  13  extends from an outer surface of the frame  1  and is located close to the base of the slots  12 . A slit  14  is defined in the wall of the open end of the chamber  11  and a radial groove  15  defined through the wall of the frame  1 . 
     The torque feedback device  2  includes a torsion spring  21 , a pushing wheel  22 , a C-shaped clamp  24  and a compression spring  25 . The pushing wheel  22  has a lever  22   a  extending from an outer surface thereof, and 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 lugs  24   a  extends from 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 . An elongate hole  24   d  is defined through the clamp  24  and located corresponding to the radial groove  15  and the lever  22   a . The compression spring  25  is mounted to the frame and retained between the ridges  13  of the frame  1  and the lugs  24   a  of the clamp  24 . The compression spring  25  is deformable by movement of the clamp  24  in the axial direction. The torsion spring  21  has a first end  21   a  that is engaged with the surface groove  2   c  of the pushing wheel  22 , and a second end  21   b  that is engaged with the slit  14  of the frame  1  so as to maintain the pushing wheel  22  to face the frame  1 . 
     The transmission device  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  includes a plurality of planet gears  32   a  on one side thereof and a driving gear  32   b  on the other side of the front speed reduction gear  32  so as to transmit power to the rear speed reduction gear  34 . The planet gears  32   a  are engaged with inner teeth  22   b  of the pushing wheel  22  and the input gear  31  so as to form a planetary speed reduction mechanism. The shifting gear  33  has an annular groove  33   a  in an outer surface thereof and a plurality of notches  33   b  are defined in the outer surface of an end of the shifting gear  233 . The pins  24   c  extend through the slots  12  in the frame  1  and are engaged with the annular grooves  33   a . The protrusions  11   a  of the frame  1  are engaged with the notches  33   b . The rear speed reduction gear  34  is a disk and is connected to a plurality of planet gears  34   a  on one end and the other end of the disk is connected to an output gear  34   b  so as to transmit power to an output mechanism that is not shown. The planet gears  34   a  are engaged with the drive gear  32   b  and the inner teeth of the shifting gear  33 . 
     The receiving chamber  11  receives the transmission device  3  and the torque feedback device  2  in sequence. In a first stage of speed, the clamp  24  is retained by the torsion spring  25  and the pins  24   c  are located at the peak portion of the slots  12 , and the pins  24   c  are engaged with the annular groove  33   a  of the shifting gear  33 , so that the shifting gear  33  is located at a top most position. As shown in FIGS. 4A and 4B, the lever  22   a  of the pushing wheel  22  is engaged with the radial groove  15  of the frame  1  so that the rotation angle is limited. The angle of the lever  22   a  is also limited by the retaining of the torsion spring  21  and the clamp  24 . 
     Referring to FIGS. 3,  4 A,  4 B, and  4 C, when the a large resistant torque is applied, the input gear  31  of the transmission mechanism  3  increases the torque of the front speed reduction gear  32  gradually so that the planet gears  32   a  of the front speed reduction gear  32  apply an reaction force in reverse direction to the inner threads  22   b  of the pushing wheel  22  so as to rotate the pushing wheel  22 . Nevertheless, the pushing wheel  22  rotates only when the force of the torsion spring  21  and the force of the compressing spring  25  are overcome. When the pushing wheel  22  rotates, the lever  22   a  drives the clamp  24  via the elongate hole  24   d . When the clamp  24  rotates, the pins  24   c  are lowered along the inclined surface of the radial groove  15  in the frame  1  so that the clamp  24  depresses the compression spring  25 . Under the circumstance, the pins  24   c  drive the shifting gear  33  downward till the torque that the front speed reduction gear  32  applies onto the pushing wheel  22  reaches a pre-set value, the pushing wheel  22  rotates a largest angle. The shifting gear  33  reaches the lower most position and the notches  33   b  are engaged with the protrusions  11   a  of the frame  1 . 
     The torque of the shifting speed reduction mechanism can be decided by choosing proper torsion spring  21  and the compression spring  25 . 
     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 shifting gear  33  is in its first stage and 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  provides a first stage when the front speed reduction gear  32  co-rotates with 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 number of the revolution of the pushing wheel  22  increases when the torque increases and the shifting gear  33  is in its lower most position and disengages from the front speed reduction gear  32 . In the meanwhile, the shifting gear  33  is still 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 . This provides the first stage of speed and the result is located in the maximum value of the curve of the torque vs. revolution. 
     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.