Abstract:
A counterweight mechanism for a linearly reciprocating mechanism includes a counterweight configured to balance an inertial force of the reciprocating mechanism, a driving element configured to drive the counterweight to linearly reciprocate, a guiding element configured to guide the counterweight to linearly reciprocate, and an auxiliary driving mechanism configured to auxiliarily drive the counterweight to linearly reciprocate. The auxiliary drive mechanism is configured to exert a force on the counterweight in a direction of the linear reciprocation of the counterweight, which direction alternatingly changes, to promote the linear reciprocation. This configuration enables the reduction or avoidance of vibration and noise of the counterweight mechanism and prolongs a service lifetime thereof. The counterweight mechanism can be included in a power tool.

Description:
[0001]    This application claims priority under 35 U.S.C. § 119 to patent application no. CN 2014 10 284 736.6, filed on Jun. 23, 2014 in China, the disclosure of which is incorporated herein by reference in its entirety. 
         [0002]    The present disclosure relates to a counterweight mechanism and especially to a counterweight mechanism for a linearly reciprocating mechanism and a power tool comprising the counterweight mechanism. 
       BACKGROUND 
       [0003]    In a machine having a linearly reciprocating mechanism, a counterweight mechanism which moves in a direction opposite to the linearly reciprocating mechanism is usually provided to balance a linear reciprocation of the linearly reciprocating mechanism, thereby reducing a vibration of the machine during the linear reciprocation of the linearly reciprocating mechanism. 
         [0004]      FIG. 1  is a simplified schematic view showing a prior linearly reciprocating mechanism and a counterweight of a machine such as a jigsaw, and  FIG. 2  is a side view of  FIG. 1 . As shown in  FIGS. 1 and 2 , the linearly reciprocating mechanism of the jigsaw comprises a gear  3  mounted onto a rotation shaft  1 . An eccentric pin  5  is disposed on one side of the gear  3 , and a free end of the eccentric pin  5  is connected with a working part  9  having a saw blade  7 . A counterweight mechanism of the jigsaw comprises an eccentric cam  11  disposed on the other side of the gear  3  and a counterweight  13  mounted on the eccentric cam  11 , and the eccentric cam  1  I is movably located in an elongated hole  15  in the counterweight  13 . The elongated hole  15  is defined by two generally parallel surfaces and two curved surfaces. Only the two generally parallel surfaces are active surfaces that are in contact with the eccentric cam  11 . When the working part  9  brings the saw blade  7  to move upwards along a direction indicated by an arrow RI as the gear  3  is driven by a driving gear (not shown), the counterweight  13  moves downwards along a direction indicated by an arrow R 2 ; and vice versa. The motion of the counterweight  13  mainly results from a contact force exerted on the counterweight  13  by the eccentric cam  11  disposed on the gear  3 . If the contact force is too big, the eccentric cam  11  itself and the two generally parallel surfaces of the elongated hole  15  in the counterweight  13  will be worn, which consequently increases the vibration and the noise during the operation of the jigsaw and shortens the service lifetime of the eccentric cam and counterweight. 
         [0005]    Thus, there is a need to make improvement on the exiting counterweight mechanism for the linearly reciprocating mechanism. 
       SUMMARY 
       [0006]    An object of the present disclosure is to overcome at least one of the defects in the prior art and to provide an improved counterweight mechanism for the linearly reciprocating mechanism. The counterweight mechanism can reduce the wear on the eccentric cam and the counterweight while the whole machine is light in weight and has a high efficiency, thereby reducing or avoiding the vibration and noise of the eccentric cam and the counterweight and prolonging their service lifetime. 
         [0007]    To this end, according to one aspect of the present disclosure, a counterweight mechanism for linearly reciprocating mechanism is provided, A counterweight mechanism for a linearly reciprocating mechanism comprising:
       a counterweight for balancing an inertial force of the linearly reciprocating mechanism;   a driving element for driving the counterweight to reciprocate linearly; and   a guiding element for guiding the counterweight to reciprocate linearly;   wherein the counterweight mechanism further comprises an auxiliary driving means for driving auxiliarily the counterweight to reciprocate linearly, the auxiliary driving means exerts on the counterweight in a direction of the linear reciprocation of the counterweight a force which promotes the linear reciprocation and whose direction changes alternately.       
 
         [0012]    Preferably, the auxiliary driving means comprises at least one spring which is disposed between the counterweight and a frame of the linearly reciprocating mechanism along the direction of the linear reciprocation of the counterweight such that the spring and the counterweight form a spring-mass system. 
         [0013]    Preferably, the spring is configured such that a component force of a spring force generated by the spring in the direction of the linear reciprocation of the counterweight is greater than that in a direction perpendicular to the direction of the linear reciprocation of the counterweight. 
         [0014]    Preferably, the spring is configured such that an action line of a spring force generated by the spring passes through a center of mass of the counterweight. 
         [0015]    Preferably, the spring is selected such that its spring constant k is given by an equation below: 
         [0000]    
       
      
       k=m×ω 
       2  
      
     
         [0000]    where m is a mass of the counterweight, and ω is an angular frequency of the linear reciprocation of the counterweight. 
         [0016]    Preferably, an inherent angular frequency of the spring-mass system is within a range of an angular frequency of the linear reciprocation of the counterweight. 
         [0017]    Preferably, a spring force generated by the spring is zero when the counterweight is at a mid-position of the linear reciprocation. 
         [0018]    Preferably, the counterweight is configured to reciprocate linearly along a direction opposite to a direction of the motion of the linearly reciprocating mechanism. 
         [0019]    Preferably, the counterweight is configured to reciprocate linearly along a direction perpendicular to a direction of the motion of the linearly reciprocating mechanism. 
         [0020]    Preferably, the at least one spring comprises one spring, one end of the spring is connected to the counterweight, and the other end of the spring is fixed at the frame. 
         [0021]    Preferably, the at least one spring comprises two compression springs, one of the two compression springs is disposed between one end of the counterweight and the frame, and the other one of the two compression springs is disposed between the other end of the counterweight and the frame. 
         [0022]    Preferably, the two compression springs are connected with the frame only at one end while they are not connected to the counterweight at the other end. 
         [0023]    Preferably, the driving element comprises an eccentric cam which is mounted onto a rotation shaft rotatably supported on a frame and rotates with the rotation shaft. 
         [0024]    Preferably, the guiding element comprises a guiding portion fixed at the frame. 
         [0025]    Preferably, an elongated hole defined by two parallel surfaces and two curved surfaces is formed in the counterweight, the eccentric cam is mounted in the elongated hole such that an outer edge of the eccentric cam only contacts the two parallel surfaces of the elongated hole. 
         [0026]    Preferably, the driving element comprises a driving rod which can reciprocate linearly, and the counterweight is mounted on the driving rod. 
         [0027]    Preferably, the guiding element comprises a guiding rod passing through a through hole in the counterweight. 
         [0028]    Preferably, the linearly reciprocating mechanism is a linearly reciprocating mechanism of a power tool. 
         [0029]    Preferably, the power tool is a reciprocating saw or an electric hammer. According to another aspect of the present disclosure, a power tool comprising the linearly reciprocating mechanism is provided. The linearly reciprocating mechanism further comprises a counterweight mechanism as described above. 
         [0030]    Preferably, the power tool is a reciprocating saw or an electric hammer. 
         [0031]    As there is provided at least one spring between the counterweight and the frame along the direction of the linear reciprocation of the counterweight, the counterweight mechanism for the linearly reciprocating mechanism according to the present disclosure may avoid or significantly reduce the friction force and thus the abrasion on the counterweight and components (e.g., driving elements or guiding elements) contacting with the counterweight, thereby reducing or avoiding the vibration and the noise of the counterweight as well as prolonging the service lifetime of the machine. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    In the drawings: 
           [0033]      FIG. 1  is a simplified schematic view showing a linearly reciprocating mechanism and a counterweight of a prior jigsaw; 
           [0034]      FIG. 2  is a side view of  FIG. 1 ; 
           [0035]      FIG. 3  is a schematic view showing a counterweight mechanism for a linearly reciprocating mechanism according to a first preferred embodiment of the present disclosure; and 
           [0036]      FIG. 4  is a schematic view showing a counterweight mechanism for a linearly reciprocating mechanism according to a second preferred embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]    Hereinafter, preferred embodiments of the present disclosure will be described in detail in connection with the examples. It should be understood by a skilled in the art that these exemplary embodiments are not constructed to be any limiting to the present disclosure. 
         [0038]      FIG. 3  is a schematic view showing a counterweight mechanism for a linearly reciprocating mechanism according to the first preferred embodiment of the present disclosure. For simplicity, a linearly reciprocating mechanism for a reciprocating power tool such as a jigsaw is not shown in  FIG. 3 . As shown in  FIG. 3 , a counterweight mechanism  20  for a linearly reciprocating mechanism according to the first preferred embodiment of the present disclosure comprises a rotation shaft  21  rotatably supported on a frame, an eccentric cam  23  being capable of rotating with the rotation shaft  21 , and a counterweight  25  mounted on the eccentric cam  23 . An elongated hole  27  defined by two generally parallel surfaces  27   a ,  27   b  and two curved surfaces  27   c ,  27   d  is formed in the counterweight  25 . The eccentric cam  23  is mounted in the elongated hole  27  of the counterweight  25  such that the outer edge of the eccentric cam  23  only contacts with the two generally parallel surfaces  27   a ,  27   b  of the elongated hole  27 , i.e., only the two generally parallel surfaces  27   a ,  27   b  of the elongated hole  27  form the active contact surfaces. Further, the counterweight mechanism also comprises the guiding portions  29   a ,  29   d  fixed on the frame to guide the counterweight  25  to reciprocate. 
         [0039]    As known, when the eccentric cam  23  is driven to rotate, it contacts with one of the two generally parallel surfaces  27   a ,  27   b  of the elongated hole  27  in the counterweight  25 , causing the counterweight  25  to move up and down along a direction indicated by an arrow R in the  FIG. 3 , thereby balancing an inertial force of the linearly reciprocating mechanism (not shown) moving in opposite directions. During the up and down movement of the counterweight  25 , the counterweight  25  exerts on the eccentric cam  23  and the guiding portions  29   a ,  29   d  the forces FI, F 2  and F 3  respectively. These forces result in a respective reacting force acting on the counterweight  25 . A resultant force formed by these reacting forces drives the counterweight  25  to reciprocate. The forces F 2  and F 3  always are negligible because they are very small compared to the force F 1 . Thus, in practice, it is mainly the force F 1  exerted on the eccentric cam  23  by the counterweight  25  and its reacting force that result in a friction force and thus an abrasion between the eccentric cam  23  and the counterweight  25  during the up and down movement of the counterweight  25 . 
         [0040]    According to the present disclosure, the counterweight mechanism  20  further comprises a spring  33  which is disposed between the counterweight  25  and the frame  31  alone the direction R of the linear reciprocation of the counterweight  25 . In a preferred embodiment, one end  33   a  of the spring  33  is connected to the counterweight  25 , and the other end  33   b  of the spring  33  is fixed at the frame  31 . In this way, the spring  33  and the counterweight  25  form a spring-mass system which can do a simple harmonic motion. When the counterweight  25  is at its equilibrium position, the spring  33  is free; when the counterweight  25  is at a half cycle above the equilibrium position, the spring  33  is compressed; and when the counterweight  25  is at a half cycle below the equilibrium position, the spring  33  is stretched. Thus, during the linear reciprocation of the counterweight  25  driven by the eccentric cam  23 , the spring  33  exerts on the counterweight  25  in the reciprocating direction of the counterweight  25  a force whose direction changes alternately, thereby assisting to drive the counterweight  25 . 
         [0041]    Theoretically, the closer to the angular frequency of the linear reciprocation of the counterweight the resonant frequency of the spring-mass system is, the smaller the force F 1  exerted on the eccentric cam  23  by the counterweight  25  and its associated reacting force, i.e., the contact force between the eccentric cam  23  and the counterweight  25  will be. Thus, preferably, the spring  33  is selected such that its spring constant k is given by an equation below: 
         [0000]    
       
      
       k=m×ω 
       2  
      
     
         [0000]    where m is the mass of the counterweight  25 , and ω is the angular speed of the linear reciprocation of the counterweight  25 . Accordingly, the resonant frequency of the spring-mass system is approximately equal to the angular frequency of the linear reciprocation of the counterweight  25 . In practice, however, it&#39;s also possible for the resonant frequency of the spring-mass system to be within a range of the angular frequency of the linear reciprocation of the counterweight. 
         [0042]    Furthermore, in order to avoid generating an additional torque in the spring-mass system during the simple harmonic motion, an action line of a spring force generated by the spring  33  is configured to be parallel to the direction of the linear reciprocation of the counterweight  25 , more preferably, to pass through a center of mass of the counterweight  25 . It should be understood that, when a plurality of springs are used, the action line of the spring force herein refers to the action line of the resultant force of the spring forces generated by the plurality of springs. With the gravity being ignored, the spring force generated by the spring  33  will be approximate to zero when the counterweight  25  comes to a mid-position of the linear reciprocation. 
         [0043]    According to a structure of a specific machine comprising the linearly reciprocating mechanism, the counterweight mechanism may comprise one or more springs. The one or more springs may be disposed at one or two ends of the counterweight  25  along the direction R of the linear reciprocation of the counterweight  25 . For example, each of the two ends of the counterweight  25  may be provided with one spring  33  along the direction R of the linear reciprocation of the counterweight  25 . In this case, the spring  33  is a compression spring, and the ends of the springs  33  are not necessary to connect to the counterweight  25 . 
         [0044]    Because the spring  33  and the counterweight  25  form the spring-mass system doing the simple harmonic motion when the spring  33  is provided, the force generated by the spring  33  becomes a main force that drives the counterweight  25  to reciprocate linearly. The force generated by the spring  33  will not result in the friction force and thus the abrasion of the counterweight as there is no relative movement between the counterweight  25  and the spring  33 . On the other hand, the force FI exerted on the eccentric cam  23  by the counterweight  25  and its associated reacting force could be significantly reduced as they only serve as a supplement to the energy loss of the spring-mass system. Accordingly, the friction force resulting from the contact force and thus the abrasion of the eccentric cam  23  and counterweight  25  can be avoided or significantly reduced, such that the vibration and noise of the eccentric cam and the counterweight can be reduced or avoided and their service lifetime can be prolonged. 
         [0045]    In order to verify the effect of the present disclosure, on basis of GST90 jigsaw of the BOSCH, two compression springs are disposed at the top of the counterweight along the direction of the linear reciprocation of the counterweight. The upper end of the two compression springs is fixed to the frame (housing) of the jigsaw and the lower end of the two compression springs is not fixed to the counterweight. This means that the springs are active only in the upper half cycle of the counterweight. According to the present disclosure, it could be predicted that the upper surface of the two generally parallel surfaces of the elongated hole of the counterweight contacting with the eccentric cam would be worn, while the lower surface of the two generally parallel surfaces contacting with the eccentric cam would not or hardly be worn. After operation for a long time, it was proved that the upper surface of the two generally parallel surfaces of the elongated hole was worn while the lower surface of the two generally parallel surfaces of the elongated hole was not worn even with the tooling textures remained. 
         [0046]      FIG. 4  is a schematic view showing the counterweight mechanism for the linearly reciprocating mechanism according to a second preferred embodiment of the present disclosure. For simplicity, the linearly reciprocating mechanism for a reciprocating power tool such as a saber saw or an electric hammer is not shown in  FIG. 4 . As shown in  FIG. 4 , the counterweight mechanism  40  for the linearly reciprocating mechanism according to the second preferred embodiment of the present disclosure comprises a driving rod  41  which forms as a part of, for example, a wobble bearing and can reciprocate linearly along a direction indicated by an arrow S, a counterweight  43  which is mounted on the driving rod  41  and can reciprocate linearly together with the driving rod  41  along the direction indicated by the arrow S, and a guiding rod  47  passing through a through hole  45  in the counterweight  43  along a direction parallel to the direction indicated by the arrow S to guide the linear reciprocation of the counterweight  43 . 
         [0047]    As well known, in order to balance the linear reciprocation of the linearly reciprocating mechanism, the center of mass CM of the counterweight  43  is offset to a motion path of the driving rod  41 . For example, during the rightward movement of the counterweight  43  along the direction indicated by the arrow S, the driving rod  41  exerts on the counterweight  43  a force F 4 . As the center of mass eM of the counterweight  43  is offset to the motion path of the driving rod  41 , the counterweight  43  has a leftward inertial force. The leftward inertial force results in a torque which tends to cause the counterweight  43  to rotate anticlockwise; vice versa. Accordingly, as the counterweight  43  is subjected to the torque which tends to cause the counterweight  43  to rotate anticlockwise due to driving force from the driving rod  41 , the portions of the counterweight  43  contacting with the guiding rod  47  are subjected to the forces F 5  and F 6  as shown in  FIG. 4 . The forces F 4 , F 5  and F 6  exert on the driving rod  41  and the guiding rod  47  the respective reacting force respectively. The force F 4  and its associated reacting force can hardly result in any abrasion on the driving rod  41  and the counterweight  43  because there is hardly any relative movement between the driving rod  41  and the counterweight  43 . However, there exists a relative movement between the counterweight  43  and the guiding rod  47  due to the torque tending to cause the counterweight  43  to rotate, thus the forces F 5  and F 6  and their associated reacting force will result in a serious abrasion on the counterweight  43  and the guiding rod  47 . 
         [0048]    According to this embodiment, the counterweight mechanism  40  of the present disclosure also comprises a spring  51  disposed between the counterweight  43  and the frame  49  along a direction S of the linear reciprocation of the counterweight  43 . In this preferred embodiment, it is shown that each of the two ends of the counterweight  43  is provided with one spring  51  along the direction S of the linear reciprocation of the counterweight  43 . In this case, the spring  51  is a compression spring, and the ends of the spring  51  could be not connected to the counterweight  43 . Of course, the ends of the spring  51  could be connected to the counterweight  43  if necessary. It should be understood that, however, it is possible to provide only one spring  51  between the counterweight  43  and the frame  49  along the direction S of the linear reciprocation of the counterweight  43 , as shown in the figure of the first preferred embodiment. One end of the spring  51  is connected to the counterweight  43 , and the other end of the spring is fixed at the frame  49 . Of course, it is possible to provide a plurality springs  51 . Therefore, when the counterweight  43  is driven by the driving rod  41  to reciprocate linearly, the spring (s)  51  exert (s) on the counterweight  43  in the reciprocating direction of the counterweight  43  a force whose direction changes alternately, thereby assisting to drive the counterweight  43 . 
         [0049]    The spring  51  is selected such that its spring constant k is given by an equation below: 
         [0000]    
       
      
       k=m×ω 
       2  
      
     
         [0000]    where m is the mass of the counterweight  43 , and ω is the angular frequency of the linear reciprocation of the counterweight  43 . Accordingly, the inherent angular frequency of the spring-mass system is approximately equal to the angular frequency of the linear reciprocation of the counterweight  43 . In practice, however, it is possible for the inherent angular frequency of the spring-mass system to be within a range of the angular frequency of the linear reciprocation of the counterweight. 
         [0050]    In order to avoid generating an additional torque in the spring-mass system during the simple harmonic motion, an action line of a spring force generated by the spring  51  is configured to be parallel to the direction of the linear reciprocation of the counterweight  43 , more preferably, to pass through a center of mass of the counterweight  43 . It should be understood that, when a plurality of springs are used, the action line of the spring force herein refers to the action line of the resultant force of the spring forces generated by the plurality of springs. With the gravity being ignored, the spring force generated by the spring  51  will be approximate to zero when the counterweight  43  comes to a mid-position of the linear reciprocation. 
         [0051]    It should be understood that an electromagnetic device, a hydraulic device or a pneumatic device could be provided as an alternative to the spring in the foregoing preferred embodiments to achieve a similar function as the spring. 
         [0052]    The present disclosure has been described in detail in connection with the particular embodiments. Obviously, it should be understood that embodiments described above and shown in the figures are illustrative rather than limiting. For example, in the foregoing preferred embodiments, the counterweight is configured to reciprocate linearly along a direction opposite to the direction of the linear reciprocation of the linearly reciprocating mechanism, however, it should be understood that in the case that the inertial force in the direction of the linear reciprocation of the linearly reciprocating mechanism is translated into an inertial force in a direction perpendicular to the direction of the linear reciprocation by the eccentric cam, the counterweight can also be configured to reciprocate linearly in the direction perpendicular to the direction of the linear reciprocation of the linearly reciprocating mechanism. Further, it should be contemplated by the skilled in the art that the direction of the linear reciprocation of the counterweight can also be configured to be oblique to the moving direction of the linearly reciprocating mechanism at an angle. In this case, a force exerted on the counterweight by an auxiliary driving means refers to a component force in the direction of the linear reciprocation of the counterweight, and preferably the auxiliary driving means is configured such that its component force in the direction of the linear reciprocation of the counterweight is greater than its component force in the direction perpendicular to the direction of the linear reciprocation of the counterweight. Various modifications and changes within the scope of the present disclosure can be made by the skilled in the art without departing the spirits of the present disclosure.