Abstract:
A speed changing mechanism includes two planetary gear sets and a movable gear that is controlled to be movably engaged with one of the two planetary gear sets to generate two different speeds. A third speed can be output when the movable gear is moved to a position where the movable gear is not engaged with either of the two planetary gear sets.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a three-speed speed changing mechanism that includes two planetary gear sets and a movable gear is controlled to be engaged with one or the two or both of the two planetary gear sets so as to obtain three different speeds. 
     BACKGROUND OF THE INVENTION 
     A conventional electric tools, especially for those tools have a rotating output shaft such as electric drills, are equipped with a speed reduction mechanism so as to provide different speeds and torque. In general, the speed reduction mechanism uses a planetary gear set and an input shaft is engaged with a chosen gear such as the sun gear, the inner gear, or the planet gears so that different speeds can be obtained. Nevertheless, due to the limited space in the casing of the tools, only two speeds can be obtained by the mechanism, and the speeds and torque are not satisfied by the users. 
     The present invention intends to provide a speed changing mechanism that outputs three different speeds by movably engaging a movable gear between two planetary gear sets. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, there is provided a speed changing mechanism which comprises a base in which a first planetary gear set and a second planetary gear set are respectively received. An input device and an output device are also received in the base. A movable gear is received in the base and has bosses which are movably engaged with protrusions in the base and a ring member of the input device. The planet gears of the two planetary gear sets are respectively engaged with the input gear of the input device. The movable gear is movably engaged between the two planetary gear sets. A push button controls the movable gear so as to engage the desired gears to output two different speed. When the movable gear is positioned in dependent from the two planetary gear sets, a third speed can be obtained. 
    
    
     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 to show the speed changing mechanism of the present invention; 
     FIG. 2 is an end cross sectional view to show the base with the mechanism received therein; 
     FIG. 3A shows the push button is positioned one of three positions of the body in which the speed changing mechanism is received; 
     FIG. 3B shows an end cross sectional view of the push button in the body; 
     FIG. 4 shows the movable gear is positioned at the first position; 
     FIG. 5 shows the movable gear is positioned at the second position; and 
     FIG. 6 shows the movable gear is positioned at the third position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 4, the speed changing mechanism of the present invention comprises a base  1  which is a tubular case and has a receiving chamber  11  defined in an end thereof and a plurality of first protrusions  14  as shown in FIG. 2, extend from an inner periphery of the chamber  11 . A plurality of recesses  13  are defined in the inner periphery of the receiving chamber  11  and communicate with the open end of the base  1 . 
     A first planetary gear set includes a first disk  6  which has a plurality of first planet gears  62  rotatably mounted onto three rods  63  extending from a first side of the first disk  6 . An output shaft  61  extends from a second side of the first disk  6 . 
     A second planetary gear set includes a second disk  5  which has first outer teeth  51  and a plurality of second planet gears  52  are rotatably mounted to three rods  53  extending from a convex portion  54  on a first side of the second disk  5 . First inner teeth  55  are defined in a second side of the second disk  5 . 
     A ring-shaped inner gear  4  has second inner teeth  41  and second outer teeth  42 . The convex portion  54  of the second disk  5  is engaged in a neck portion  43  extending from a side of the inner gear  4 . 
     A ring-shaped movable gear  3  has third inner teeth  31  and a plurality of bosses  32  extend from two respective edges of the movable gear  3 . A groove  33  is defined in an outer periphery of the movable gear  3 . 
     A third planetary gear set includes a third disk  20  which has a plurality of third planet gears  202  rotatably mounted to three rods  201  extending from a first side of the third disk  20  and an input shaft  21  extends from a second side of the third disk  20 . 
     The first planet gears  62  and the second planet gears  52  are respectively engaged with the input shaft  21 . 
     A ring member  22  has fourth inner teeth  221  and a plurality of second protrusions  23  extending radially outward from an outer periphery of the ring member  22 . The second protrusions  23  of the ring member  22  are engaged with the recesses  13  of the base  1 . The third planet gears  202  are engaged with the fourth inner teeth  221  of the ring member  22  and a driving shaft member  100  as shown in FIG.  4 . The ring member  22  and the input shaft  21  are called as an input device  2  which transfers input power into the speed changing device. 
     The first planet gears  62  engaged with the first inner teeth  55  f the second disk  5  and the second planet gears  52  are engaged with the second inner teeth  41  of the inner gear  4 . 
     Further referring to FIGS. 3A and 3B, an engaging member  34  has two distal ends  342  which are inserted into holes  12  defined through the wall of the base  1  and engaged with the groove  33 . A push button  7  is connected to a top section  341  of the engaging member  34 . The push button  7  includes two flanges  71  which are slidably engaged with two sides of a rail  81  defined in a body  8 . Three notches  82  are defined in each of the two sides of the rail  81  and each flange  71  has a positioning protrusion  72  which is engaged with one of the three notches  82  so as to position the push button  7 . By moving the push button  7 , the movable gear  3  can be shifted. 
     The movable gear  3  can be moved to be engaged with the first outer teeth  51  of the second disk  5  as shown in FIG. 4, the bosses  32  are engaged with the first protrusions  14  in the chamber  11  and the third inner teeth  31  are engaged with the first outer teeth  51 . The input force drives the second planet gears  52  of the second disk  5  and the planet gears  62  of the first disk  6 . The inner gear  4  is rotated by the second planet gears  52  and the first disk  6  is rotated by the first inner teeth  55  which drive the first planet gears  62 . The output shaft  63  outputs a force. 
     As shown in FIG. 5, when the push button  7  is shifted to another position where the movable gear  3  is shifted such that the bosses  32  are engaged with the second protrusions  23  of the ring member  22 . The third inner teeth  31  are engaged with the second outer teeth  42  such that the input force drives the second planet gear  52  and the first planet gears  62 . The second disk  5  is driven by the engagement of the second inner teeth  41  and the second planet gears  52 , and the first disk  6  is rotated by the engagement of the first planet gears  62  and the first inner teeth  55 . The output shaft  61  outputs a force. 
     As shown in FIG. 6, when the push button  7  is pushed to a mediate position where the bosses  32  of the movable gear  3  are not engaged with any of the second protrusions  23  and the first protrusions  14 . The third inner teeth  31  are engaged with the first outer teeth  51  and the second outer teeth  42  simultaneously. The inner gear  4  and the second disk  5  are co-rotated with the movable gear  3 . The input force drives the second planet gears  52  and the first planet gears  62  directly. The first disk  6  is rotated by the engagement of the first planet gears  62  and the first inner teeth  55 . The output shaft  61  outputs a force. The three output forces are different in speed and torque. 
     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.