Abstract:
A power tool comprising: a housing  2 ; a motor mounted within the housing  2 ; a tool holder  8  rotatably mounted on the housing  2  for holding a cutting tool; at least one striker  612  mounted in a slideable manner within the housing for generating hammering impulses for a cutting tool, which striker is capable of being reciprocatingly driven by the motor along an axis of travel in a reciprocating cycle, when the motor  18  is activated, via a drive mechanism; characterised in that there at least two strikers  612  which are capable of being reciprocatingly driven along their axes of travel by the motor.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to powered rotary hammers, and to power drills having a hammer action.  
       BACKGROUND OF THE INVENTION  
       [0002]     Rotary hammers are known in which a motor drives a spindle supporting a hammer bit, while at the same time causing a piston tightly fitted within the spindle to execute linear reciprocating motion within the spindle. This motion causes repeated compression of an air cushion between the piston and a ram slidably mounted within the spindle, which causes the ram in turn to execute reciprocating linear motion within the spindle and apply impacts to the hammer bit via a beat piece.  
       BRIEF SUMMARY OF THE INVENTION  
       [0003]     Such hammers only have one ram. Therefore, in order to increase the rate at which the number of impacts are generated, the rate at which the ram is reciprocatingly driven has to be increased which is not desirable.  
         [0004]     Accordingly, there is provided a power tool comprising:  
         [0005]     a housing;  
         [0006]     a motor mounted within the housing;  
         [0007]     a tool holder rotatably mounted on the housing for holding a cutting tool;  
         [0008]     at least one striker mounted in a slideable manner within the housing for generating hammering impulses for a cutting tool, which striker is capable of being reciprocatingly driven by the motor along an axis of travel in a reciprocating cycle, when the motor is activated, via a drive mechanism;  
         [0009]     characterised in that there at least two strikers which are capable of being reciprocatingly driven along their axes of travel by the motor.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     Two embodiments of the present invention will now be described with reference to the accompanying drawings of which:  
         [0011]      FIG. 1  shows a perspective view of a hammer; and  
         [0012]      FIG. 2  is an exploded view of a hammer mechanism of a first embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     A hammer drill comprises a housing  2  in which is mounted a motor (not shown). A handle  4  is attached to the rear of the housing which can be activated using a trigger switch  6 . A tool holder is mounted on the front of the housing  2 . The tool holder  8  holds a cutting tool (not shown) such as a drill bit. The motor reciprocatingly drives two bullet shaped impact members  612  which generate hammering impacts for a cutting tool when located within the tool holder in well known manner.  
         [0014]     An embodiment of the hammer mechanism  600  invention is shown in  FIG. 2 , in which axial impacts are imparted to a three-jaw tool holder  602  carrying a drill bit (not shown). The hammer mechanism  600  has a hollow casing  604  (only half of the casing  604  is shown in  FIG. 2 ) fixed relative to the tool housing, the casing  604  having a continuous groove  606  formed around its internal surface which comprises a helical portion  608  and a substantially axial portion  610 . The half of the casing  604  which is not shown contains a helical portion  608  only connecting between the ends  607  of the two helical portions  608  on either side of the axial portion  610  on the half of the casing  604  shown in  FIG. 2 .  
         [0015]     First and second cylinders  614 ,  616  are connected together using screws (not shown) which pass through holes  640  in the first cylinder  614  and screw into threaded holes  642  in the second cylinder. The cylinders  614 ,  616 , when connected together are coaxial and are rotatably, but non axially slidably, mounted within the hollow casing  604 .  
         [0016]     Formed in each of the cylinders  614 ,  616  are a pair of tubular recesses  644  having entrances which, when the cylinders  614 ,  616  are connected together, face towards the other cylinder and which are in alignment with the entrance of a corresponding recess in the other cylinder. The pair of tubular recesses in the first cylinder  614  terminate in apertures  620  formed in the front end of the cylinder  614  which provide access into the recesses from the front of the cylinders  614 ,  616  when the cylinders are connected together. The diameter of the apertures  620  is smaller than the internal diameter of the recesses  644 . Slots  626  are formed in the first and second cylinders  614 ,  616  which pass through the wall of the cylinders  614 ,  616  and engage with the recesses  644  within the cylinders  612 ,  616 .  
         [0017]     A bullet shaped impact member  612  is located within each of the two sets of recesses together with a compression spring  618  such that each impact member  612  is urged forwardly by their respective compression spring  618  so that its forward portion protrudes through the corresponding aperture  620  in the first cylinder  614 . The bullet shaped impact members together with the compression springs are inserted into the cylinders  614 ,  616  prior to the two cylinders being screwed together to secure them to each other.  
         [0018]     Each impact member  612  has a part-spherical recess  622  for receiving a corresponding ball bearing  624  which protrudes through the slot  626  formed in the first and second cylinders  614 ,  616 . As such the bullet shaped impact members can slide within the recesses  644  within the cylinders  614 ,  616 . The ball bearings  624  engage the groove  606  in the casing  604  when the assembled cylinders are located within the casing  604 . As a result, rotation of the cylinders  614 ,  616  about its longitudinal axis  660  relative to the casing  604  causes rearward movement of the impact members  612  relative to the cylinders  614 ,  616  against the action of the corresponding compression springs  618  due to the ball bearings  624  travelling along the helical portion  608  of the groove  606  until the ball bearings reach an axial part  610  of the groove  606 , after which the springs  618  urge the impact members  612  forward along an axis of travel  664  so that its forward end protrudes through the corresponding apertures  620  in the first cylinder  614  to impart an impact on the end of a shaft  662  which supports the tool holder  602 . This is achieved due to the location of the axial portion  610  of continuous groove  606  relative to the axis  668  of the shaft  662  which ensures that the axis of travel  664  of the bullet shaped impact member is aligned and co-axial with the axis  668  of rotation of the shaft when the bullet shaped member protrudes through the corresponding aperture  620  in the first cylinder. In addition, as the cylinders rotate, the bullet shaped impact members rotate about the longitudinal axis  660  of the cylinders  614 ,  616 , the longitudinal axis  660  of the cylinders  614 ,  616  being parallel to the axes of travel of the bullet shaped impact members  612 .  
         [0019]     The cylinders  614 ,  616  are rotated relative to the casing  604  by means of a conical clutch  628  engaging a gear  630  which is in turn driven by a gear  632  on a shaft  634  rotated by means of the motor (not shown). The shaft  634  also causes rotation of the tool holder  602  by means of engagement with a gear  636  on shaft  634  with teeth on the external periphery of the gear  638  connect to the tool holder  602 .  
         [0020]     It will be appreciated by a person skilled in the art that the path of the groove  606  around the internal surface of the casing  604  can be varied in order to generate different types of hammering action. By way of example, the groove  606  may contain two axial parts  610  located directly opposite each other on the internal surface of the casing  604 . This would result in the two bullet shaped impact members  612  striking simultaneously, twice every time the first and second cylinders  614 ,  616  make one complete revolution. The position of the axis of rotation  660  could then be aligned with axis  668  of rotation of the shaft  662  so that the two bullet shaped impact members  612  strike the side of the gear  638  simultaneously, the motion being transferred to the tool holder  602  via the shaft  662 .  
         [0021]     Though the first and second cylinders  614 ,  616  can be continually rotated, it will be further appreciated that the first and second cylinders  614 ,  616  could be held stationary whilst one or both of the bullet shaped members  612  travel along the axial part  610  of the groove  606  due to the biasing force of their respective spring  618 , the first and second cylinders  614  then being rotated after the impact, to move the bullet shaped impact members  612  away from the shaft  662  and gear  638  against the biasing force of their respective spring  618  in preparation for the next impact.  
         [0022]     A second embodiment of the hammer mechanism will now be described. The construction of the second embodiment is very similar to that of the first embodiment. However, in the second embodiment, the axis  660  of rotation of the first and second cylinders  614 ,  616  are aligned and co-axial with the axis  668  of rotation of the shaft  662 . The first and second cylinders  614 ,  616  which are rotated in the first embodiment, are held stationary, whilst the hollow casing  604 , which is held stationary in the first embodiment relative to the tool housing, is rotated about its longitudinal axis inside the tool housing. This results in the groove  606  rotating around the first and second cylinders  614 ,  616  causing the bullet shaped impact members to repetitively strike the gear  638  in a manner similar to that described in the first embodiment.