Abstract:
A rotary hammer incorporating both a reciprocating drive for generating a first hammering action transmittable to a tool bit and ratcheting mechanism for generating a second hammering action transmittable to the tool bit. The first hammering action and the second hammering actions differing in frequency and force of the impacts transmitted to the tool bit. Furthermore, the reciprocating drive and ratcheting mechanism are selectably engageable so that the first hammering action and the second hammering action may be selectably transmitted to the tool bit individually or in combination depending on the desired mode operation.

Description:
FIELD OF THE INVENTION 
   The present invention relates to powered rotary hammers, and to power drills having a hammer action. 
   BACKGROUND OF THE INVENTION 
   One type of rotary hammers comprise a motor which rotatingly drives a spindle supporting a chuck in which a drill bit can be held, while at the same time causing a piston tightly fitted within the spindle to execute a 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 drill bit via a beat piece. 
   The problem with such designs is that the frequency of impacts per revolution of the spindle is relatively small. 
   In an alternative design of hammer drill, the impacts which are imparted onto the drill bit are generated using two ratchet plates which interact with each other, one of which is fixed, the other rotating with the spindle, the vibrations generated by two ratchets being transferred to the drill bit. EP0613758 discloses such a design. 
   Though this design produces a high frequency of impacts per revolution of the spindle, the size of the impacts is relatively small. 
   BRIEF SUMMARY OF THE INVENTION 
   Accordingly there is provided a rotary hammer comprising: 
   a housing 
   a motor mounted within the housing; 
   a tool holder rotatably mounted on the housing for holding a cutting tool; 
   a striker mounted in a freely slideable manner within the housing for repetitively striking, at a relatively low frequency, an end of a cutting tool when a cutting tool is held by the tool holder, wherein the striker is reciprocatingly driven by the motor, when the motor is activated, via a drive mechanism; 
   wherein the striker strikes an end of a cutting tool via a beat piece slideably mounted within the housing; 
   characterised in that the beat piece is capable of being rotatingly driven by the motor whereby there is further provided a pair of co-operating ratchet plates the first ratchet plate being rigidly connected to the beat piece and the second ratchet plate being connected to the housing  4  and which, when the first and second ratchet plates are engaged and the beat piece is rotatingly driven by the motor, superimpose high frequency impacts onto the low frequency impacts generated by the striker, when the striker is being reciprocatingly driven by the motor. 
   Preferably, the drive mechanism comprises, a support which is capable of being reciprocatingly driven by the motor, ideally by a wobble bearing or a crank mechanism, when the motor is activated; and a spring interconnected between the support and the striker. 
   Tool holders for rotary hammers typically support the end of a drill bit in such a manner that the drill bit can axially slide within the tool holder over a limited range of distance whilst being rotationally fixed within the tool holder. When the tool holder rotates, the drill bit rotates. In such designs, the end of the drill bit is directly struck by the beat piece. As the drill bit can axially slide within the tool holder, no impact force is imparted on to the tool holder itself. The present invention can be utilised with such tool holders, the beat piece striking the end of the tool bit directly. However, the present invention also allows for the end of the tool bit to be struck indirectly, ie via another component such as the tool holder. In such an arrangement, the tool holder can be attached to the beat piece with the impacts being transferred to the cutting tool via the tool holder. 
   Preferably, the ratchet plates are biased apart and are only brought into engagement when a force is applied to the beat piece which over comes the biasing force. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the present invention will now be described with reference to the accompanying drawings of which: 
       FIG. 1  shows a perspective view of a hammer; and 
       FIG. 2  is a schematic cross sectional side view of a hammer mechanism of a rotary hammer of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A rotary hammer drill comprises a housing  2  in which is mounted an electric motor. A chuck support  120  is rotatingly mounted within the housing  2 , one end of which projects from the front of the housing. A chuck  4  is attached to the end of the chuck support  120 . The electric motor is capable of rotatingly driving the chuck support  120  in order for the hammer to perform a drilling function. In addition, the motor can also reciprocatingly drive a ram  142  via a drive mechanism to repetitively strike the chuck support  120 . The impulses on the chuck support  120  are transferred to the rear end of a drill bit located within the chuck  4  in order for the drill bit to perform a chiselling function. A gear mechanism (not shown) is capable of engaging or disengaging the drive to either the chuck support  120  and/or the ram  142  so that the rotary hammer is capable of performing the drilling function and chiselling function separately or in combination. 
     FIG. 2  shows a schematic side cross sectional view of a hammer mechanism of the hammer. 
   A wobble plate  146  is mounted to a rotary shaft  147  by means of bearings  148 . The wobble plate is prevented from rotation around the axis of the shaft  147 . A support  149  is pivotably connected to an arm  153  of the wobble plate  146  remote from the shaft  147  such that rotation of the shaft  147  by means of the motor causes axial reciprocating motion of the upper extremity of the arm  153  of the wobble plate  146  in a direction indicated by Arrow A in  FIG. 2 . This in turn causes reciprocating motion of a ram  142  connected via a spring  144  to the support  149 . As a result, the ram  142  repeatedly strikes a chuck support  120  whose forward (right in  FIG. 2 ) impulses are transferred to the drill bit (not shown) held by the chuck  4  attached to the end of the chuck support  120 . 
   A person skilled in the art will appreciate that the wobble bearing can be replaced by a crank mechanism. 
   The chuck support  120  is mounted within a mount  124 . The mount  124  is rigidly attached to the housing  4 . The chuck support  120  is capable of freely rotating as well as axially sliding within the mount  124 . A compression spring  150  biases the chuck support forward (right in  FIG. 2 ) in a direction parallel to its axis. The amount of forward axial movement is limited by the mount  124 . The chuck support  120  is capable of being rotatingly driven within the mount  124  by the motor of the hammer. Details of the rotary drive mechanism have omitted for clarity. Any type of drive which will be well known to a person skilled in the art can be used. 
   An annular first ratchet plate  154  is integrally formed on rear end of the chuck support  120 . An annular second ratchet plate  151  is integrally formed on the mount  124 . The compression spring biases the two ratchet plates  154 ,  151  apart. 
   The hammer mechanism can be operated in at least two different manners of operation. In the first manner, the motor drives both the chuck support  120  in a rotating manner and the ram  142  in a reciprocating manner. 
   In use in the first manner, a drill bit is held by the chuck  4 . When the end of the drill bit is placed against the surface of a work piece in order to drill a hole, the chuck support is biased forward to its furthest extent. The drill bit is rotatingly driven by the chuck support  120 . When the shaft  147  is rotatingly driven by the motor, the ram  142  is reciprocatingly driven by the wobble plate  146  via the support  149  and the spring  144 . However, the hammer mechanism is arranged so that the ram  142  either very narrowly misses the chuck support  120  when it is in its forward most position or just touches the chuck support when it is in its forward (right in  FIG. 2 ) most position. As such, no forward impulses are generated which would be passed forward to the drill bit. 
   However, when an operator applies further pressure onto the drill bit through the hammer in order to push the tip of the drill bit into the work piece to be drilled, the chuck support  120  slides backwards against the biasing force of the spring  150 . This results in the reciprocatingly driven ram  142  to strike the chuck support  120 . The impulses generated by this striking action are then passed onto the chuck  4 . As such, a hammer action is superimposed onto the drill bit as it is pressed against a work piece. It should be noted that a chuck  4  needs to be constructed in a sufficiently robust manner in order to withstand the impacts imparted by the reciprocating ram  142  over a long period of time. 
   If sufficient force is applied onto the drill bit, the chuck support slides rearwardly (left in  FIG. 2 ) to its maximum extent until the two ratchet plates  151 ,  154  engage. As the chuck support  120  is being rotatingly driven, the ratchet plates  151 ,  154  will ride over each other causing the chuck support  120  to oscillate. The oscillating motion will be superimposed onto the hammer motion caused by the striking of the ram  142 . The frequency of the oscillation due to the ratchet plates  151 ,  154  will be much higher than that caused by the striking action of the ram  142  but will be much smaller in amplitude. 
   In the second manner of operation, the motor initially drives the chuck support  120  in a rotating manner, no drive being applied to the ram  142 . 
   In use in the second manner, a drill bit is held by the chuck  4 . When the end of the drill bit is placed against the surface of a work piece in order to drill a hole, the chuck support is biased forward to its furthest extent. The drill bit is rotatingly driven by the chuck support  120 . No forward impulses are generated which would be passed forward to the drill bit. 
   If sufficient force is applied onto the drill bit, the chuck support slides rearwardly (left in  FIG. 2 ) to its maximum extent until the two ratchet plates  151 ,  154  engage. As the chuck support  120  is being rotatingly driven, the ratchet plates  151 ,  154  will ride over each other causing the chuck support  120  to oscillate. The oscillating motion will be superimposed onto the rotating motion of the bit. 
   However, if for a short period of time the operator wishes to increase the hammering action, the operator can engage the drive to the ram  142  to cause it to reciprocate. This results in the reciprocatingly driven ram  142  striking the chuck support  120 . The impulses generated by this striking action are then passed onto the chuck  4 . As such, a larger hammer action is superimposed onto the drill bit as it is pressed against a work piece. 
   The second manner of operation allows the action of the reciprocating ram  142  to be used sparingly, thus enabling a standard design of chuck to be utilised without damage due to the large vibration caused by the ram  142 . If the hammer mechanism is intended to be used predominantly in the second manner of operation, the strength of the spring can be reduced to allow easy engagement of the ratchet plates  151 ,  154 . 
   The present invention has been described in relation to a chuck mounted on the chuck support. However, a person skilled in the art will realise that it can also be utilised with typical tool holders for rotary hammers which support the end of a drill bit in such a manner that the drill bit can axially slide within the tool holder over a limited range of distance whilst being rotationally fixed within the tool holder. When the tool holder rotates, the drill bit rotates. When such designs of tool holders are used, the end of the drill bit is directly struck by the chuck support  120 . As the drill bit can axially slide within the tool holder, no impact force is imparted on to the tool holder itself. The chuck support and the tool holder will be separately rotatingly driven. The tool holder will be rotatingly driven to rotate the bit. The chuck support  120  will rotatingly driven to generate the hammering effect due to the ratchet plates  151 ,  154 .