Abstract:
The disclosure relates to a transmission for a power tool, comprising an input gear driven by a driver, an intermediate shaft carrying first and second intermediate gears, the first intermediate gear being meshed with the input gear, an output shaft carrying an output gear, the output gear being meshed with the second intermediate gear for driving a tool bit, and proximal and distal bearings supporting proximal and distal ends of the intermediate shaft respectively. The first intermediate gear is formed with a receptacle portion which is recessed from a proximal end surface of the first intermediate gear in an axial direction towards a distal side, the proximal bearing being received in the receptacle portion at least in part in the axial direction. The disclosure also relates to a power tool comprising the above transmission. The disclosure provides a compact and robust structure.

Description:
TECHNICAL FIELD 
       [0001]    The disclosure relates to a transmission used in a power tool, and to a power tool, in particular an electric cutting tool, comprising such a transmission. 
       BACKGROUND ART 
       [0002]    A power tool, such as an electric cutting tool, generally comprises an electric motor, a tool bit (for example, a cutting blade) and a transmission for transmitting a force between the electric motor and the tool bit. The transmission generally comprises a gear transmission mechanism for transmitting the output rotational movement of the electric motor to the tool bit at a certain transmission ratio. For achieving a sufficient transmission ratio, the gear transmission mechanism is generally of the type of dual-stage or multi-stage, so that the sizes and positions of the gears can be set properly. However, more stages of the gear transmission mechanism result in a lager size of it. The gears, the shafts and the bearings of the gear transmission mechanism should be arranged in a compact manner to minimize the size of the gear transmission mechanism. 
         [0003]    As an example, an electric cutting tool as shown in  FIG. 1  is disclosed in Chinese patent publication CN101336146A, wherein a start-end side gear  7   a  is coupled with a motor shaft  6   a,  a final-end side gear  7   b  and a saw blade  1  are carried by a saw blade shaft  1   a,  a first gear  7   c  meshing with the start-end side gear  7   a  and a second gear  7   d  meshing with the final-end side gear  7   b  are carried by an intermediate gear shaft  14 , the saw blade shaft  1   a  is supported by bearings  13  and  12  at its proximal and distal sides respectively, and the intermediate gear shaft  14  is supported by bearings  17  and  16  at its proximal and distal sides respectively, wherein the proximal side bearings  13  and  17  are needle bearings which do not function well under axial forces. 
         [0004]    According to the solution disclosed in CN101336146A, the needle bearing  17  on the proximal side of the intermediate gear shaft  14  cannot endure an axial force, which results in deficiencies of the electric cutting tool. For example, such a configuration cannot be used in a reversely operable power tool (a tool having a tool bit that is rotatable in both forward and reverse directions). Further, the strength of the needle bearing itself is low. In addition, high vibration and noise are likely to be generated because the needle bearing has a large radial fitting clearance. These factors will negatively affect cutting precision, operation comfortability and life time of the power tool. 
       SUMMARY OF THE INVENTION 
       [0005]    An object of the disclosure is to provide an improved power tool which has a compact and/or durable structure. 
         [0006]    For this end, the disclosure in one aspect provides a transmission for a power tool, the transmission comprising an input gear driven by a driver (for example, an electric motor) of the power tool; an intermediate shaft carrying first and second intermediate gears, the first intermediate gear being meshed with the input gear; an output shaft carrying an output gear, the output gear being meshed with the second intermediate gear for driving a tool bit of the power tool; and proximal and distal bearings supporting proximal and distal ends of the intermediate shaft respectively; wherein the first intermediate gear is formed with a receptacle portion which is recessed from a proximal end surface of the first intermediate gear in an axial direction towards a distal side, the proximal bearing being received in the receptacle portion at least in part in the axial direction. More specifically, the proximal bearing may be either located in the receptacle portion in part in the axial direction or located in the receptacle portion completely in the axial direction. 
         [0007]    According to a preferred embodiment of the disclosure, each of the proximal bearing and the distal bearing is able to bear both an axial force and a radial force. For example, the proximal bearing and/or the distal bearing is a ball bearing or a conical roller bearing. 
         [0008]    According to a preferred embodiment of the disclosure, the first intermediate gear is formed integrally with the intermediate shaft; alternatively, the first intermediate gear is formed separately and then fixed onto the intermediate shaft. On the other hand, the second intermediate gear is formed integrally with the intermediate shaft; alternatively, the second intermediate gear is formed separately and then fixed onto the intermediate shaft. The first and second intermediate gears are preferably proximate to each other in the axial direction. 
         [0009]    According to a preferred embodiment of the disclosure, the proximal bearing comprises an inner ring which is supported by the proximal end of the intermediate shaft and an outer ring which is supported by a housing of the power tool. Alternatively, the proximal bearing comprises an inner ring which is supported by a housing of the power tool and an outer ring which is supported in the receptacle portion by the first intermediate gear, with an interference fit being formed between the outer ring and the receptacle portion. 
         [0010]    The disclosure in a further aspect provides a power tool comprising a driver, a tool bit, and a transmission as described above, for transmitting an output movement of the driver to the tool bit. 
         [0011]    The power tool is preferably an electric cutting tool, for example, a circular saw. Further, the power tool is preferably a portable power tool. 
         [0012]    According to the disclosure, the proximal bearing on the intermediate shaft of the gear transmission mechanism of the power tool is accommodated at least partially in the axial direction in the receptacle portion formed in the first proximal intermediate gear, so that the gear transmission mechanism can be arranged in a compact manner. Further, the proximal and distal bearings are of the type that can endure both an axial force and a radial force, thus the supporting ability of the bearings on the intermediate shaft is increased. As a result, the power tool of the disclosure has a compact and/or durable overall structure. In addition, by forming the receptacle portion in the first proximal intermediate gear, the weight of it is reduced. Thus, the total weight of the movable elements and thus the energy loss can be reduced according to the disclosure. 
         [0013]    Other features and benefits of the disclosure will be described later. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a schematic view of a gear transmission mechanism of an electric cutting tool according to prior art. 
           [0015]      FIG. 2  is a schematic sectional view of a gear transmission mechanism of a power tool according a preferred embodiment of the disclosure. 
           [0016]      FIG. 3  is a schematic sectional view of a portion around the intermediate shaft shown in  FIG. 2 . 
           [0017]      FIG. 4  is a schematic sectional view of a portion around an intermediate shaft according to another preferred embodiment of the disclosure. 
           [0018]      FIG. 5  is a schematic sectional view of a portion around an intermediate shaft according to yet another preferred embodiment of the disclosure. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0019]    Now some illustrative preferred embodiments of the disclosure will be described with reference to the drawings. 
         [0020]    The disclosure relates to power tools, in particular portable power tools, such as saws, drills, grinding tools and the like, which may comprise various gear transmission mechanisms. A circular saw is used here as an example for describing the principle of the disclosure though, the disclosure is also applicable in other types of power tools. 
         [0021]      FIG. 2  shows a portion of a circular saw (for example, a portable circular saw) according to a preferred embodiment of the disclosure. The circular saw comprises a housing  20 , an electric motor (not shown) mounted in the housing  20 , a saw blade (tool bit)  22  mounted at least partially outside the housing  20 , and a transmission for transmitting the rotational movement and torque of the electric motor to the saw blade. The electric motor has a motor shaft  24  which is supported in the housing  20  by a bearing  26 . 
         [0022]    The transmission mainly comprises an input gear  28  carried by the motor shaft  24 , a first proximal intermediate gear  32  and a second distal intermediate gear  34  carried by an intermediate shaft  30 , and an output gear  38  carried by the output shaft  36 . 
         [0023]    The first intermediate gear  32  is meshed with the input gear  28 , and the second intermediate gear  34  is meshed with the output gear  38 . In this way, a duel-stage gear transmission mechanism is formed by these two pairs of gears, in which the transmission ratio (speed ratio) of each stage of transmission mechanism can be determined properly to obtain a combined total transmission ratio (speed ratio) between the electric motor and the saw blade  22 . In the illustrated embodiment, the two pairs of gears are all cylindrical gears, so that the central axes of the motor shaft  24 , the intermediate shaft  30  and the output shaft  36  are parallel with each other. However, the disclosure does not exclude the conditions that one or both of the two pairs of gears are in the form of bevel gears or other types of gears. Further, the teeth of the two pairs of gears are preferably skewed teeth as illustrated; however the disclosure does not exclude the condition that the teeth of one or both of the two pairs of gears are straight teeth. 
         [0024]    The intermediate shaft  30  is disposed completely in the housing  20 , with its proximal end and distal end respectively being supported by the housing  20  via bearings  40  and  42 . The distal end of the output shaft  36  extends out from the housing  20 , and the remaining portion of the distal end lies in the housing  20 . The output shaft  36  is supported at its proximal end and at a portion between its middle portion and distal end by the housing  20  via bearings  44  and  46  respectively. Here “proximal” refers to a direction towards or a location near the electric motor, while “distal” refers to a direction towards or a location near the saw blade  22 . 
         [0025]    The input gear  28  is mounted to or formed integrally with the motor shaft  24 . In order to provide a sufficient speed ratio, the number of the teeth of the input gear  28  should be small enough (or the diameter of the input gear should be small enough). Thus, it is preferred to form the input gear  28  integrally with the motor shaft  24 , as illustrated. However, the input gear  28  may also be formed separately and then be fixed to the motor shaft  24 . In this case, a defective input gear  28  can be exchanged easily. 
         [0026]    The intermediate shaft  30  is disposed kinetically between the motor shaft  24  and the output shaft  36 . The central axis of the intermediate shaft  30  may be coplanar with the central axis of the motor shaft  24  and the central axis of the output shaft  36 ; however, it may also be not coplanar with them. 
         [0027]    For reasons related with assembling, the housing  20  may comprise at least two the housing portions  48  and  50 , one of the two housing portions (for example,  50 ) being detachable from the other one (for example,  48 ). 
         [0028]    Since the motor shaft  24  is subjected to a relatively large radial force during operation, the bearing  26  which carries the motor shaft  24  is preferably a ball bearing, having an inner ring mounted around the motor shaft  24  and an outer ring fixed in the housing portion  48 , so that the motor shaft  24  is supported stably. 
         [0029]    During operation of the circular saw, the proximal and distal ends of the intermediate shaft  30  is subjected to a relatively large radial force, thus the bearings  40  and  42  which carry the proximal and distal ends of the intermediate shaft  30  may be bearings having relatively large roller elements, for example, standard ball bearings. Further, during operation of the circular saw, the intermediate shaft  30  is also subjected to a certain axial force, thus the bearings  40  and  42  are preferably bearings that can bear an axial pushing force, such as one-direction thrust ball bearings, conical roller bearings or the like. The proximal bearing  40  is carried by a bearing support  48 - 1  of the housing portion  48 , and is able to bear an axial force applied in the proximal direction from the intermediate shaft  30 . The distal bearing  42  is carried by the housing portion  50 , and mainly bears an axial force applied in the distal direction from the intermediate shaft  30 . The bearing  42  has an outer ring which may abut against the housing portion  50  directly or abut against the housing portion  50  indirectly via a washer  68 . The washer  68  is preferably formed of a material having vibration damping property for reducing vibrations generated during operation of the circular saw. 
         [0030]    During operation of the circular saw, the proximal bearing  44  which carries the output shaft  36  is subjected to a relatively small radial force, and is subjected to nearly no axial pushing force. Thus, the bearing  44  may be of any suitable type, such as ball bearing, conical roller bearing, needle bearing or the like. For saving space, the bearing  44  is preferably a needle bearing as illustrated. The bearing  44  having an inner ring mounted around the output shaft  36  and an outer ring fixed in the housing portion  48 . 
         [0031]    The bearing  46  which carries the portion between the middle portion and the distal end of the output shaft  36  is supported by the housing portion  50 . Thus, the location of the bearing  46  in the axial direction is distal from the bearing  42 , so that a majority of the radial force from the output shaft  36  is taken by the bearing  46 . For this purpose, the bearing  46  may be a bearing having relatively large roller elements, for example, a standard ball bearing. Further, during operation of the circular saw, the output shaft  36  may also be subjected to a certain axial force, thus the bearing  46  is preferably a bearing that can bear an axial pushing force, such as a one-direction thrust ball bearing, a conical roller bearing or the like. 
         [0032]    The output gear  38  is fixedly mounted to a middle portion of the output shaft  36 , for example, by means of a spring clamper  52 . The bearing  46  has an inner ring mounted around the output shaft  36  and an outer ring fixed in the housing portion  50 . The inner ring of the bearing  46  has a distal side which biases against a shoulder portion  54  on the output shaft  36  and a proximal side which is clamped tightly in the axial direction by the output gear  38  via a separation sleeve  56 . 
         [0033]    The saw blade  22  is clamped onto the distal end of the output shaft  36  by a saw blade clamping device. The saw blade clamping device comprises inner and outer clamping disks  58  and  60 , with the inner clamping disk  58  biasing against a flange portion  62  on the output shaft  36 , and the outer clamping disk  60  being locked tightly by a fastening screw  64  via a washer  66 , so that the saw blade  22  is fixedly clamped between the inner and outer clamping disks  58  and  60 . Of course, other types of saw blade clamping devices can be used alternatively. 
         [0034]    As mentioned above, the proximal bearing  40  which carries the intermediate shaft  30  is a bearing which can bear a relatively large radial force (preferably can also bear a certain axial force), thus it has an inevitably large size. However, since the bearing  40  is located near the bearing  26  which carries the motor shaft  24 , there is likely interference in the radial direction between them. In order to dispose the bearings  40  and  26  is a compact space without interference, the bearing  40  is displaced to the distal side so that it is misaligned from the bearing  26  in the axial direction according to the disclosure. Since the first intermediate gear  32  has a relatively large diameter (substantively larger than that of the input gear  28 , and larger than that of the second intermediate gear  34  to some extent), it is possible to form a receptacle portion  70  in the first intermediate gear  32 . The receptacle portion  70  is a space of a substantially cylindrical shape extending from a proximal side surface of the first intermediate gear  32  towards the distal side in the axial direction. The first intermediate gear  32  has a large size, thus it has a sufficient strength even if the receptacle portion  70  is formed in it. 
         [0035]      FIGS. 3 to 5  show some possible configurations of a portion around the intermediate shaft. 
         [0036]    As shown in  FIG. 3 , according to an embodiment of the disclosure, the second intermediate gear  34  is formed integrally with the intermediate shaft  30 , and the first intermediate gear  32  is formed separately and then fixed to the intermediate shaft  30 . The distal end surface of the first intermediate gear  32  biases against the proximal end surface of the second intermediate gear  34 . The intermediate shaft  30  comprises a proximal end  30 - 1  and a distal end  30 - 2  which have reduced diameters relative to that of the main portion of the intermediate shaft  30  respectively. Mounting shoulders  30 - 3  and  30 - 4  are formed respectively between the proximal end  30 - 1  and the distal end  30 - 2  and the main portion of the intermediate shaft  30 . The inner ring of the bearing  40  is supported by the proximal end  30 - 1  and the mounting shoulder  30 - 3 , and the outer ring of the bearing  40  is supported by the bearing support  48 - 1  of the housing portion  48  (not shown in  FIG. 3 ). The inner diameter of the receptacle portion  70  is larger than the outer diameter of the bearing  40 , so that a ring shaped space is formed therebetween for receiving the bearing support  48 - 1 . The inner ring of the bearing  42  is supported by the distal end  30 - 2  and the mounting shoulder  30 - 4 , and the outer ring of the bearing  42  is supported by a corresponding portion of the housing portion  50  (not shown in  FIG. 3 ). 
         [0037]    The proximal end  30 - 1  extends through the receptacle portion  70  in the first intermediate gear  32  in the axial direction towards the proximal side, and the proximal end surface of the proximal end  30 - 1  extends preferably beyond the proximal end surface of the first intermediate gear  32  in the axial direction towards the proximal side. Further, for mounting the bearing  40 , the proximal end surface of the shoulder  30 - 3  lies beyond a bottom surface  70 - 1  of the receptacle portion  70  which faces towards the proximal side in the axial direction towards the proximal side. 
         [0038]    A locating member (not shown) can be used for preventing relative rotation between the first intermediate gear  32  and the intermediate shaft  30 , so that the first intermediate gear  32  is able to drive the intermediate shaft  30  to rotate with it. Preferably, this locating member or an additional locating member may prevent the first intermediate gear  32  from moving relative to the intermediate shaft  30  in the axial direction towards the proximal side. 
         [0039]      FIG. 4  shows another embodiment of the intermediate shaft portion, wherein the first intermediate gear  32  is formed integrally with the intermediate shaft  30 , and the second intermediate gear  34  is formed separately and then fixed to the intermediate shaft  30 . The proximal end surface of the second intermediate gear  34  biases against the distal end surface of the first intermediate gear  32 . The proximal end  30 - 1  and the distal end  30 - 2  of the intermediate shaft  30  have reduced diameter relative to that of the main portion of the intermediate shaft  30  respectively, so that mounting shoulders  30 - 3  and  30 - 4  are formed between the proximal and distal ends  30 - 1  and  30 - 2  on one hand and the main portion of the intermediate shaft  30  on the other hand respectively. The proximal end  30 - 1  extends in the receptacle portion  70  in the axial direction towards the proximal side, and the proximal end surface of the proximal end  30 - 1  extends preferably beyond the proximal end surface of the first intermediate gear  32  in the axial direction towards the proximal side. Further, for mounting the bearing  40 , the shoulder  30 - 3  protrudes in the axial direction towards the proximal side from the bottom surface  70 - 1  of the receptacle portion  70  which faces towards the proximal side. 
         [0040]    The inner ring of the bearing  40  is supported by the proximal end  30 - 1  and the mounting shoulder  30 - 3 , and the outer ring of the bearing  40  is supported by the bearing support  48 - 1  of the housing portion  48  (not shown in  FIG. 4 ). The inner diameter of the receptacle portion  70  is larger than the outer diameter of the bearing  40 , so that a ring shaped space for receiving the bearing support  48 - 1  is formed therebetween. The inner ring of the bearing  42  is supported by the distal end  30 - 2  and the mounting shoulder  30 - 4 , and the outer ring of the bearing  42  is supported by a corresponding portion of the housing portion  50  (not shown in  FIG. 4 ). 
         [0041]    Other aspects of the embodiment shown in  FIG. 4  are similar to that of the embodiment shown in  FIG. 3  and are not described again. 
         [0042]      FIG. 5  shows yet another embodiment of the intermediate shaft portion, wherein the first intermediate gear  3  is formed integrally with the intermediate shaft  30 , and the second intermediate gear  34  is formed separately and then fixed to the intermediate shaft  30 . The proximal end surface of the second intermediate gear  34  biases against the distal end surface of the first intermediate gear  32 . The distal end  30 - 2  of the intermediate shaft  30  has a reduced diameter relative to that of the main portion of the intermediate shaft  30 , so that a mounting shoulder  30 - 4  is formed between the distal end  30 - 2  and the main portion of the intermediate shaft  30 . 
         [0043]    The proximal end of the intermediate shaft  30  is terminated at a bottom surface  70 - 1  of the receptacle portion  70  which faces towards the proximal side, rather than protruding from the bottom surface  70 - 1 . 
         [0044]    The inner ring of the bearing  42  is supported by the distal end  30 - 2  and the mounting shoulder  30 - 4 , and the outer ring of the bearing  42  is supported by a corresponding portion of the housing portion  50  (not shown in  FIG. 5 ). The inner ring of the bearing  40  is supported by the bearing support  48 - 2  of the housing portion  48 , and the outer ring of the bearing  40  is supported by the first intermediate gear  32 . More specifically, the inner diameter of an inner cylindrical wall defined in the receptacle portion  70  corresponds to the outer diameter of the bearing  40 , with interference fit formed therebetween, so that the outer ring of the bearing  40  is kept and supported in the receptacle portion  70 . 
         [0045]    Other aspects of the embodiment shown in  FIG. 5  are similar to that of the embodiments shown in  FIGS. 3 and 4  and are not described again. 
         [0046]    As an alternative to the embodiment shown in  FIG. 5 , the second intermediate gear  34  may be formed integrally with the intermediate shaft  30 , and the first intermediate gear  32  may be formed separately and then fixed to the intermediate shaft  30 . 
         [0047]    It is appreciated that, in a possible embodiment which is not shown, the first intermediate gear  32  and the second intermediate gear  34  may both be formed integrally with the intermediate shaft  30 ; alternatively, the first intermediate gear  32  and the second intermediate gear  34  may both be formed separately and then fixed to the intermediate shaft  30 . 
         [0048]    Further, in the embodiments described above, the axial length of the receptacle portion  70  may be smaller than the axial length or the width of the bearing  40 , in order to avoid significant reducing of the strength of the first intermediate gear  32  caused by forming the receptacle portion  70 . In this case, the bearing  40  is located in part in the axial direction in the receptacle portion  70 . However, the disclosure does not exclude the condition that the axial length of the receptacle portion  70  equals to or is larger than the axial length of the bearing  40 . In other words, the disclosure covers both the conditions that the bearing  40  is located partly and completely in the receptacle portion  70 . 
         [0049]    According to the disclosure, the proximal bearing  40  of the intermediate shaft  30  is located at least partially in the axial direction in the receptacle portion  70  formed in the first proximal intermediate gear  32 , so that the bearing  40  is misaligned in the axial direction with the bearing  26  on the motor shaft  24 . In this way, sufficient accommodating spaces in radial direction are provided for both the bearings  40  and  26 , so that the inner space in the housing  20  can be used efficiently, while the whole gear transmission mechanism can be disposed compactly. 
         [0050]    Further, the proximal and distal ends of the intermediate shaft  30  are each supported by a bearing that can bear both a radial force and an axial force, thus the bearing on the intermediate shaft  30  can bear various loads that may be generated during operation of the circular saw. For example, even when the circular saw operates with its saw blade rotating in a reverse direction, the bearing on the intermediate shaft  30  can provide a sufficient support. As a result, the bearing on the intermediate shaft  30  is more durable, so that the life time of the whole the circular saw can be increased. 
         [0051]    Furthermore, it is appreciated that, by using a dual-stage (or even multi-stage) gear transmission mechanism in the circular saw of the disclosure, and by disposing the gears of the gear transmission mechanism in a compact manner, the diameter of the output gear  38  can be reduced, and the cutting depth of the saw blade can be increased. 
         [0052]    Furthermore, it is appreciated that, the basic principle of the disclosure, i.e., the proximal bearing on the intermediate shaft of the gear transmission mechanism is located at least partially in the axial direction in the receptacle portion formed in the first proximal intermediate gear, is also applicable in other types of power tools in which multi-stage gear transmission mechanisms are used, such as electric cutting tools, in particular portable electric cutting tools. In these cases, the technical effect of compactly disposing the gear transmission mechanism can be obtained similarly. Further, by forming the receptacle portion in the first proximal intermediate gear, the total weight of the movable elements and thus the energy loss can be reduced. 
         [0053]    While certain embodiments of the disclosure have been described here, they are presented by way of explanation only and are not intended to limit the scope of the disclosure. Various modifications, substitutions and changes can be made by those skilled in the art within the scope and spirit of the disclosure as defined in the attached claims and their equivalents.