Patent Publication Number: US-8122972-B2

Title: Drive mechanism for a power tool

Description:
FIELD OF THE INVENTION 
     The present invention relates to a drive mechanism for a power tool, and to a power tool incorporating such a mechanism. The invention relates particularly, but not exclusively, to a drive mechanism for a hammer drill, and to a hammer drill incorporating such a mechanism. 
     BACKGROUND OF THE INVENTION 
     Hammer drills are power tools that can generally operate in three modes of operation. The hammer drill will have a tool bit that can be operated in a hammer mode, a rotary mode and a combined hammer and rotary mode. For the hammer and combined hammer and rotary mode, it is necessary to convert the rotary motion of the output shaft of the tool&#39;s motor into a reciprocating motion in order to power the hammering action. 
     A mechanism for converting the rotary motion of the output shaft of the motor into reciprocating motion is described in GB2038986. Referring to  FIG. 1  which shows a partially cut away perspective view of a drive mechanism described in GB2038986, and to  FIG. 2  which shows a cross sectional view of the drive mechanism of  FIG. 1 , a hollow piston  2  is slidably mounted in a sleeve  4  such that the hollow piston  2  can reciprocate relative to the sleeve  4 . A ram (not shown) is slidably disposed in the hollow piston  2  in order to convert the reciprocation of the hollow piston two into a hammering action as will be known to persons skilled in the art. 
     A crank pin  6  connects the hollow piston  2  to a circular crank plate  8  and comprises a cylindrical head  16  which is slidably disposed in a bearing  10  disposed on the rear of a hollow piston  2 . The crank pin  6  also comprises a spherical head  12  which is trapped in a spherical socket  14  disposed in the crank plate  8 . The crank plate  8  is formed from two halves,  8   a  and  8   b , which mate to define a spherical socket  14  for trapping the spherical head  12  therebetween. 
     As the crank plate  8  rotates, the crank pin  6  alternately pushes and pulls the hollow piston  2  forwardly and rearwardly such that the hollow piston  2  reciprocates within the sleeve  4 . During the reciprocating motion of the hollow piston  2 , the spherical head  12  of the crank pin  6  follows a circular path, whilst the cylindrical head  16  of the crank pin  6  slides up and down in bearing  10 , as the bearing  10  and the hollow piston  2  rocks laterally from side to side. As a result of the shape of spherical socket  14 , the spherical head  12  is trapped in the crank plate  8  in order to prevent the crank pin  6  from becoming either disengaged from the bearing  10 , or the crank plate  8 . 
     The above mechanism suffers from the drawback that the spherical head  12  of the crank pin  6  needs to be permanently attached to the crank plate  8 . This means that either the spherical head must be press fitted into the crank plate such that it is in an interference fit or, as in the embodiment shown in  FIGS. 1 and 2 , the crank plate must be formed from a plurality of pieces that come together to form a part-spherical socket. These features both increase the cost and manufacturing complexity of the drive mechanism of GB2038986. 
     Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art. 
     BRIEF SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provided a drive mechanism for a power tool having a housing and a motor disposed in the housing and having an output shaft for actuating a working member of the power tool, the drive mechanism comprising: 
     a reciprocating member adapted to be slidably mounted relative to said housing and adapted to be caused to execute reciprocating movement relative to said housing, wherein said reciprocating member is adapted to slidably receive a first end of a crank pin; 
     a crank plate adapted to be caused to rotate by means of said motor and having a recess adapted to receive a second end of said crank pin such that rotation of said crank plate causes reciprocation of said reciprocating member; and 
     a collar member disposed between said first and second ends of said crank pin, wherein said collar member is adapted to prevent removal of said second end from said recess. 
     By providing a collar member disposed between the first and second ends of a crank pin, wherein said collar member is adapted to prevent removal of said second end from said recess formed in the crank plate, this provides the advantage that the end of the crank pin that engages the crank plate does not need to be permanently held by the crank plate. This reduces the cost of manufacturing the crank plate, and makes the drive mechanism easier to assemble and cheaper to manufacture. 
     In a preferred embodiment, at least part of said collar member is substantially hollow cylindrical. 
     Said collar member may be a coil spring. This provides the advantage of biasing the second end of the crank pin into engagement with the crank plate. 
     Said reciprocating member may further comprise a bearing disposed adjacent an end thereof, wherein said bearing is adapted to slidably receive said first end of said crank pin. 
     A washer may be disposed between said bearing and said collar member. This provides the advantage of providing a flat abutment between the collar member and the bearing. 
     In a preferred embodiment, at least part of the second end of said crank pin is part-spherical and is adapted to be received in a cup-shaped recess formed in said crank plate, wherein the cup-shaped recess has an upper cylindrical portion and a lower semi-spherical portion. Thus, assembly of the drive mechanism is easier because the second end can be simply inserted into the recess formed in the crank plate. 
     Preferably, the upper cylindrical portion and the lower semi-spherical portion have the same maximum diameter which maximum diameter is slightly greater than that of the corresponding part-spherical second end of said crank pin received therein. As a result, crank pin can pivot, rotate and slide vertically relative to the crank plate whilst the part-spherical second end remains within the confines of the cup-shaped recess. 
     In a preferred embodiment, said collar member is adapted to abut said second end to prevent removal of said second end from the said recess. 
     Said reciprocating member may be a hollow piston having a ram slidably mounted therein, the ram adapted to impart impacts to a working member of the tool as a result of the reciprocating movement of said hollow piston. 
     According to another aspect of the present invention, there is provided a power tool comprising a housing, a motor disposed in the housing and having an output shaft for actuating a working member of the tool, and a drive mechanism as defined above. 
     In a preferred embodiment, the power tool is a hammer drill. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiment of the present invention will now be described by way of example only and not in any limitative sense, with reference to the accompanying drawings in which: 
         FIG. 1  is a partially cut away perspective view of a prior art drive mechanism for a hammer drill; 
         FIG. 2  is a cross-sectional view of the drive mechanism of  FIG. 1 ; 
         FIG. 3  is a perspective view of a hammer drill of a first embodiment of the present invention; 
         FIG. 4  is a side cross-sectional view of the hammer drill of  FIG. 3 ; 
         FIG. 5  is an enlarged side cross-sectional view of part of the hammer drill of  FIG. 4 ; 
         FIG. 6  is a partially cut away perspective view of part of the piston drive mechanism of  FIG. 3  in its rearmost position; 
         FIG. 7  is a partially cut away perspective view of part of the piston drive mechanism of  FIG. 3  advanced through a quarter of a cycle of reciprocation from the position shown in  FIG. 6 ; 
         FIG. 8  is a partially cut away cross section of part of the piston drive mechanism of  FIG. 3  advanced through half a cycle from the position shown in  FIG. 6  to its foremost position; 
         FIG. 9  is a side cross-sectional view of a piston drive mechanism for a hammer drill of a second embodiment of the present invention; 
         FIG. 10  is an enlarged cross-sectional view taken along line A-A of  FIG. 9 ; 
         FIG. 11  is a side cross-sectional view of part of a hammer drill of a third embodiment of the present invention; 
         FIG. 12  is a cross-sectional view taken along line B-B of  FIG. 11 , with parts of the transmission mechanism removed for clarity; 
         FIG. 13  is a cross section taken along line C-C of  FIG. 12 ; 
         FIG. 14  is a side cross-sectional view of a hammer drill of a fourth embodiment of the present invention; 
         FIG. 15   a  is a perspective view from outside of a right clamshell half of a two part transmission housing of a hammer drill of a fifth embodiment of the present invention; 
         FIG. 15   b  is a side view of the outside of the clamshell half of  FIG. 15   a;    
         FIG. 15   c  is a perspective view of the inside of the clamshell half of  FIG. 15   a;    
         FIG. 15   d  is a side view of the inside of the clamshell half of  FIG. 15   a;    
         FIG. 15   e  is a front view of the clamshell half of  FIG. 15   a;    
         FIG. 15   f  is a cross-sectional view taken along line A-A of  FIG. 15   d;    
         FIG. 15   g  is a cross-sectional view taken along line B-B of  FIG. 15   d;    
         FIG. 15   h  is a cross-sectional view along line F-F of  FIG. 15   b;    
         FIG. 16   a  is a perspective view from the outside of a left clamshell half corresponding to the right clamshell half of  FIGS. 15   a  to  15   h;    
         FIG. 16   b  is a side view of the outside of the clamshell half of  FIG. 16   a;    
         FIG. 16   c  is a perspective view of the inside of the clamshell half of  FIG. 16   a;    
         FIG. 16   d  is a side view of the inside of the clamshell half of  FIG. 16   a;    
         FIG. 16   e  is a front view of the clamshell half of  FIG. 16   a;    
         FIG. 16   f  is a cross-sectional view along line A-A of  FIG. 16   d;    
         FIG. 16   g  is a cross-sectional view taken along line B-B of  FIG. 16   d;    
         FIG. 16   h  is a cross-sectional view taken along line F-F of  FIG. 16   d;    
         FIG. 17  is an enlarged perspective view of the inside of the clamshell half of  FIG. 16 ; 
         FIG. 18  is a partially cut away top view of part of a hammer drill incorporating the clamshell halves of  FIGS. 15 and 16 ; 
         FIG. 19  is a partially cut away perspective view of part of the hammer drill of  FIG. 18 ; 
         FIG. 20  is another side cross-sectional view of the piston drive mechanism; 
         FIG. 21  is a cross-sectional view of a prior art piston drive mechanism; 
         FIG. 22  is an enlarged partial cross-sectional view of the piston drive mechanism of  FIG. 21 ; 
         FIG. 23  is a cross-sectional view along line V-V of  FIG. 22 ; 
         FIG. 24   a  is a cross-sectional view of a hollow piston of a hammer drill of a sixth embodiment of the present invention; 
         FIG. 24   b  is a perspective view from the side of the hollow piston of  FIG. 24   a;    
         FIG. 24   c  is a top view of the hollow piston of  FIG. 24   a;    
         FIG. 24   d  is a view from the front of the hollow piston of  FIG. 24   a;    
         FIG. 25  is a rear view of a piston drive mechanism incorporating the hollow piston of  FIGS. 24   a  to  24   d  mounted in a spindle; 
         FIG. 26  is a perspective view from the rear of the piston drive mechanism of  FIG. 25 ; 
         FIG. 27  is a side view of a hammer drill of a seventh embodiment of the present invention; and 
         FIG. 28  is a side cross-sectional view of the hammer drill of  FIG. 26 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 3 , a battery-powered hammer drill comprises a tool housing  22  and a chuck  24  for holding a drill bit (not shown). The tool housing  22  forms a handle  26  having a trigger  28  for activating the hammer drill  20 . A battery pack  30  is releasably attached to the bottom of the tool housing  22 . A mode selector knob  32  is provided for selecting between a hammer only mode, a rotary only mode and a combined hammer and rotary mode of operation of the drill bit. 
     Referring to  FIG. 4 , an electric motor  34  is provided in the tool housing  22  and has a rotary output shaft  36 . A pinion  38  is formed on the end of output shaft  36 , the pinion  38  meshing with a first drive gear  40  of a rotary drive mechanism and a second drive gear  42  of a hammer drive mechanism. 
     The rotary drive mechanism shall be described as follows. A first bevel gear  44  is driven by the first drive gear  40 . The first bevel gear  44  meshes with a second bevel gear  46 . The second bevel gear  46  is mounted on a spindle  48 . Rotation of the second bevel gear  46  is transmitted to the spindle  48  via a clutch mechanism including an overload spring  88 . The spindle  48  is mounted for rotation about its longitudinal axis by a spherical ball bearing race  49 . A drill bit (not shown) can be inserted into the chuck  24  and connected to the forward end  50  of spindle  48 . The spindle  48  and the drill bit rotate when the hammer drill  20  is in a rotary mode or in a combined hammer and rotary mode. The clutch mechanism prevents excessive torques being transmitted from the drill bit and the spindle  48  to the motor  34 . 
     The hammer drive mechanism shall now be described as follows. The pinion  38  of motor output shaft  36  meshes with a second drive gear  42  such that rotation of the second drive gear  42  causes rotation of a crank plate  52 . A crank pin  54  is driven by the crank plate  52  and slidably engages a cylindrical bearing  56  disposed on the end of a hollow piston  58 . The hollow piston  58  is slidably mounted in the spindle  48  such that rotation of the crank plate  52  causes reciprocation of hollow piston  58  in the spindle  48 . A ram  60  is slidably disposed inside hollow piston  58 . Reciprocation of the hollow piston  58  causes the ram  60  to reciprocate with the hollow piston  58  as a result of expansion and contraction of an air cushion  93 , as will be familiar to persons skilled in the art. Reciprocation of the ram  60  causes the ram  60  to impact a beat piece  62  which in turn transfers impacts to the drill bit (not shown) in the chuck  24  when the hammer drill operating in a hammer mode or a in combined hammer and rotary mode. 
     A mode change mechanism includes a first and a second drive sleeves  64 ,  66  which selectively couple the first and second drive gears  40 ,  42  respectively, to the first bevel gear  44  and the crank plate  52 , respectively, in order to allow a user to select between either the hammer only mode, the rotary only mode or the combined hammer and rotary mode. The mode change mechanism is the subject of UK patent application no. 0428215.8. 
     A transmission mechanism comprises the rotary drive mechanism, the hammer drive mechanism and the mode change mechanism. The transmission mechanism is disposed inside a transmission housing  80 . The transmission housing  80  also supports the electric motor  34 . The transmission housing is formed from two clamshell halves of durable plastics material or cast metal, the two clamshell halves compressing an o-ring  82  therebetween. The o-ring  82  seals the transmission housing  80  to prevent dust and dirt from entering the transmission housing and damaging the moving parts of the transmission mechanism. 
     The transmission housing  80  is slidably mounted inside the tool housing  22  on parallel rails (not shown) and is supported against to the tool housing  22  by first and second damping springs  84  and  86  disposed at its rearward end. The transmission housing  80  can therefore move by a small amount relative to tool housing  22  in order to reduce transmission of vibration to the user during operation of the hammer drill  20 . The spring co-efficients of the first and second damping springs  84  and  86  are chosen so that the transmission housing  80  slides to a point generally mid-way between its limits of forward and rearward travel when the hammer drill  20  is used in normal operating conditions. This is a point of equilibrium where the forward bias of the damping springs  84  and  86  equals the rearward force on the transmission housing  80  caused by the user placing the hammer drill  20  against a workpiece and leaning against the tool housing  22 . 
     Referring to  FIG. 5 , the hammer drive mechanism will be described in more detail. The crank pin  54  comprises a cylindrical link member  68  rigidly connected to a part-spherical bearing  70 . The part-spherical bearing  70  is slidably and rotatably disposed in a cup-shaped recess  72  formed in the crank plate  52 . The cup-shaped recess  72  has an upper cylindrical portion  72   a  and a lower generally semi-spherical portion  72   b . The upper cylindrical portion  72   a  and a lower semi-spherical portion  72   b  have the same maximum diameter which is slightly greater than that of the part-spherical bearing  70 . As a result, the part-spherical bearing  70  can be easily inserted into the cup-shaped recess. The crank pin  4  can pivot, rotate and slide vertically relative to the crank plate whilst the part-spherical bearing remains within the confines of the cup-shaped recess  72 . 
     The cylindrical link member  68  is slidably disposed in a cylindrical bearing  56  formed in the end of the hollow piston  58 . Sliding friction in the cup-shaped recess  72  is slightly greater than in the cylindrical bearing  56 . The cylindrical link member  68  therefore slides up and down in the cylindrical bearing  56  while the part-spherical bearing rocks back and forth in the cup-shaped recess. A cylindrical collar member  74  surrounds the cylindrical link member  68  of the crank pin  54  and can slide between a lower position in which it abuts the upper surface of the part-spherical bearing  70  and an upper position in which it abuts and the underside of the cylindrical bearing  56 . The collar member  74  is precautionary feature that limits movement of the part-spherical bearing  70  towards the cylindrical bearing  56  so that it is impossible for the crank pin  54  and its the part-spherical bearing  70  to move totally out of engagement with the cup-shaped recess  72 . The cylindrical collar member  74  can be mounted to the crank pin  54  after construction of the crank plate  52  and crank pin  54  assembly. 
     Referring to  FIGS. 6 to 8 , as the crank plate  52  rotates in the anti-clockwise direction from the upright position shown in  FIG. 6 , to the position shown in  FIG. 7 , it can be seen that the crank pin  54  pushes the hollow piston  58  forwardly and also tilts to one side. As the crank pin  54  tilts, the cylindrical link member  68  slides downwardly in the cylindrical bearing  56 . As the crank plate  52  rotates from the position of  FIG. 7  to the position of  FIG. 8  to push the hollow piston  58  to its foremost position, the crank pin  54  re-adopts an upright position and the cylindrical link member  68  of the crank pin  54  slides upwardly inside cylindrical bearing  56 . It can be seen that by engagement of the collar member  74  with the underside of the cylindrical bearing  56  and the top of the part-spherical bearing  70 , the crank pin  54  is prevented from moving too far inside the cylindrical bearing and out of engagement with the crank plate  52 . There is therefore no need for an interference fit to trap the crank pin into engagement with the crank plate, which significantly simplifies assembly of the drive mechanism. 
     A hammer drill of a second embodiment of the invention is shown in  FIGS. 9 and 10 , with parts common to the embodiment of  FIGS. 3 to 8  denoted by like reference numerals but increased by 100. 
     Crank pin  154  is of the same construction as the embodiment of  FIGS. 3 to 8 . However, in the embodiment of  FIGS. 9 and 10  the collar member  176  is a coil spring. A washer  178  is provided between the collar coil spring  176  and the cylindrical bearing  156 . The collar coil spring  176  has the further advantage of biasing the part-spherical bearing  170  of the crank pin  154  into engagement with the cup-shaped recess  172  of the crank plate  152  so that the part-spherical bearing is prevented from even partially moving out of engagement with the crank plate  152 . 
     A hammer drill of a third embodiment of the invention is shown in  FIGS. 11 to 13 , with parts common to the embodiment of  FIGS. 3 to 8  denoted by like reference numerals but increased by 200. 
     The transmission housing  280  is formed from two clamshell halves of durable plastics or cast metal material. The two clamshell halves trap and compress an O-ring  282  therebetween. The transmission housing  280  is supported by first and second damping springs  284  and  286  at its rearward end. The transmission housing  280  is also mounted on parallel rails (not shown) disposed within the tool housing  222  such that the transmission housing  280  can slide a small distance relative to the tool housing  222  backwards and forwards in the direction of the longitudinal axis of the spindle  248 . 
     The spring coefficients of damping springs  284  and  286  are chosen so that the transmission housing  280  slides to a point generally mid-way between its limits of forward and backward travel when the hammer drill is used in normal operating conditions. This is a point of equilibrium where the forward bias of the damping springs  284  and  286  equals the rearward force on the transmission housing  280  caused by the user placing the hammer drill  220  against a workpiece and leaning against the tool housing  222 . 
     The forward end of the transmission housing  280  has a generally part-conical portion  290 , which abuts a corresponding part-conical portion  292  formed on the tool housing  222 . The part conical portions  290  and  292  form an angle of approximately 15° with the longitudinal axis of the spindle  248 . The interface defined by the part-conical portions  290  and  292  defines a stop at which the transmission housing  280  rests against the tool housing  222  when the hammer drill  220  is in its inoperative condition. When the hammer drill  220  is being used in normal operating conditions, a gap opens up between the surfaces of the part-conical portions  290  and  292  which helps to damp axial and lateral vibrations that would otherwise be directly transmitted from the tool bit (not shown) to the user holding the hammer drill  220 . Naturally, this gap slightly increases as the transmission housing moves backwards against the bias of the damping springs  282 ,  286 . This helps to damp the increased axial and lateral vibrations which may arise when the user applies greater forward pressure to the hammer drill  220 . However, the gap is sufficiently small that the hammer drill  220  and the transmission housing  280  can always be adequately controlled by the user via the interface between the part-conical portions  290 ,  292  which maintains alignment of the transmission housing  280  with the tool housing  222 . 
     A hammer drill of a fourth embodiment of the invention is shown in  FIG. 14 , with parts common to the embodiment of  FIGS. 3 to 8  denoted by like reference numerals but increased by 300. 
     The hammer drill  320  has a tool housing  322 . In this embodiment, the transmission housing  380  is formed from three housing portions. A generally L-shaped first housing portion  380   a  accommodates the transmission mechanism except for the first and second gears  340 ,  342  and the front end  348   a  of the spindle  348 . The bottom end of the first housing portion  380   a  is mounted upon a second housing portion  380   b  such that a first O-ring  382   a  is trapped between the two portions to prevent the ingress of dust and dirt. The second housing portion  380   b  holds the lower parts of the transmission mechanism inside the first housing portion  380   a  and accommodates the first and second gears  340 ,  342 . The second housing portion  380   b  has a motor output aperture  390  to allow the motor output shaft  336  access to the inside of the transmission housing and to enable the pinion  338  to drive the first and second gears  340 ,  342  of the transmission mechanism. A third housing portion  380   c  is mounted to the front end of the first housing portion  380   a  such that a second O-ring  382   b  is trapped between the two portions to prevent the ingress of dust and dirt. The third housing portion  380   c  holds the front parts of the transmission mechanism inside the first housing portion  380   a  and accommodates the front end  348   a  of the spindle. 
     The generally L-shaped first transmission housing portion  380   a  allows the transmission mechanism to be fully assembled inside the first transmission housing portion  380   a  from both its ends. For example, the hollow piston and spindle assemblies can be inserted into the front end of the first transmission housing portion  380   a , and the first transmission housing portion  380   a  can then be turned through 90° and the various gears and mode change mechanism can be inserted through the bottom end and dropped into place to engage the spindle  348  and hollow piston  358 . The second and third transmission housing portions  380   b  and  380   c  can then be mounted to the first transmission housing portion  380   a  in order to cap off the open ends of the first transmission housing portion  380   a.    
     The first transmission housing portion  380   a  can be used as a standard platform (including standard hammer drive, rotary drive and mode change mechanisms) for several power tools, and the second and third transmission housing portions  380   b  and  380   c  changed to accommodate motors and spindles of differing sizes. 
     A hammer drill of a fifth embodiment of the invention has a transmission housing shown in  FIGS. 15 to 20 , with parts common to the embodiment of  FIGS. 3 to 8  denoted by like reference numerals but increased by 400. 
     Referring to  FIGS. 15 and 16 , a transmission housing is formed from a right clamshell half  421   a  and a left clamshell half  421   b  formed from injection moulded high-grade strong plastics material. The clamshell halves  421   a ,  421   b  each have a plurality of threaded holes  423   a ,  423   b  respectively adapted to receive screws (not shown) such that the clamshell halves  421   a ,  421   b  can be joined together to form the transmission housing which encapsulates the transmission mechanism. 
     The two-part transmission housing is adapted to hold all the components of the transmission mechanism. Various indentations are moulded in the clamshell halves to provide support for these components. For example, first drive gear indentations  427   a  and  427   b  are shaped to support the first drive gear  40 . A motor support portion  425   a  and  425   b  is adapted to support and partially encapsulate the top part of the electric motor  34 . 
     The transmission housing is slidably mounted on a pair of guide rails (not shown) in the tool housing  22 . As the transmission housing is disposed inside of the tool housing  22  and out of sight of the user, high-grade strong plastics material can be used in the construction of the transmission housing. This type of material is normally not suitable for external use on a power tool due to its unattractive colour and texture. High-grade strong plastics material also generally has better vibration and noise damping properties than metal. Strengthening ribs (not shown) can also be moulded into the plastics material to increase the strength of the transmission housing. 
     Referring to  FIGS. 15 to 20 , each of the clamshell halves  421   a  and  421   b  includes integrally formed overflow channels  429   a  and  429   b . The clamshell halves also include respective ball bearing race support recesses  431   a  and  431   b  which are adapted to hold the ball bearing race  49  to support the spindle  48 . 
     Referring in particular to  FIGS. 18 to 20 , the clam shell halves  421   a  and  421   b  mate to define a first transmission housing chamber  433  and a second transmission housing chamber  435  disposed on either side of the ball bearing race  449 . The first and second transmission housing chambers  433  and  435  are interconnected by channels  429   a  and  429   b . The rear end of the hollow piston  458 , cylindrical bearing  456 , the crank pin  454  and crank plate  452  are disposed in the first transmission housing chamber  433 . The majority of the spindle  448  and the over-load spring  458  are disposed in the second transmission housing chamber  435 . Part of the spindle  448  in the second transmission housing chamber has a circumferential array of vent holes  448   a . The vent holes  448   a  allow communication between the second transmission housing chamber  435  and a spindle chamber  448   b  located inside the spindle  448  in front of the hollow piston  458  and the ram  460 . 
     In hammer mode, the hollow piston  458  is caused to reciprocate by the crank plate  452 . When the hollow piston  458  moves into the first transmission housing chamber  433  air pressure in the first transmission housing chamber  433  increases due to the reduction in the volume of first transmission housing chamber caused by the arrival of the hollow piston. At the same time, the hollow piston  458  and the ram  460  move out of the spindle  448 . This causes a decrease in air pressure in the spindle chamber  448   b  due to the increase in volume in the spindle chamber caused by the departure of the hollow piston and the ram. The second transmission housing chamber  435  is in communication with the spindle chamber  448   b , via the vent holes  448   b , and so the air pressure in the second transmission housing chamber  435  decreases too. The air pressure difference is equalised by air flowing from the first transmission housing chamber  433  through the overflow channels  429   a  and  429   b  and into the second transmission housing chamber  435  and the spindle chamber  448   b.    
     Conversely, when the hollow piston  458  goes into the spindle  448 , air pressure in the first transmission housing chamber  433  decreases due to the increase in the volume of first transmission housing chamber caused by the departure of the hollow piston. At the same time, this causes an increase in air pressure in the spindle chamber  448   b  due to the decrease in volume in the spindle chamber caused by the arrival of the hollow piston and the ram. As mentioned above, the second transmission housing chamber  435  is in communication with the spindle chamber  448   b , via the vent holes  448   b , and so the air pressure in the second transmission housing chamber  435  increases too. The air pressure difference is equalised by air flowing back from the second transmission housing chamber  435  and the spindle chamber  448   b  through the overflow channels  429   a  and  429   b  and into the first transmission housing chamber  433 . 
     As a result of this cyclic back and forth movement of air in the overflow channels  429   a ,  429   b , compression of the air is eliminated, or significantly reduced, during reciprocation of the hollow piston  58 . As such, the hammer drive mechanism does less work and loses less energy through inadvertently compressing trapped air. This increases the efficiency of the motor and the battery life of the hammer drill. 
     A hammer drill of a sixth embodiment of the invention has a hammer drive mechanism shown in  FIGS. 24 to 26 , with parts common to the embodiment of  FIGS. 3 to 8  as denoted by like reference numerals but increased by 500. 
     Referring to  FIGS. 24 to 26 , a hollow piston  558  comprises a cylindrical bearing  556  that is adapted to receive a crank pin  554  in order to cause the hollow piston  558  to reciprocate inside the spindle  548 . A ram (not shown) is slidably disposed inside the hollow piston  558  such that the ram is caused to execute a hammering action due to the air spring effect created inside hollow piston  558 . A plurality of longitudinal ridges  559  are formed on the outer circumferential surface of the generally cylindrically-shaped hollow piston  558  to reduce the surface area of contact between the hollow piston  558  and the generally cylindrically-shaped spindle  548 . A plurality of convex curvilinear shaped grooves  561  are formed in the gaps between the ridges. The grooves  561  circumscribe a cylinder of slightly reduced diameter than that of the outer circumferential surface of the hollow piston  558 . As such, the grooves  561  are shallow enough to retain lubricant of normal viscosity throughout normal operation of the hammer drive mechanism. 
     The hollow piston  558  is slidably disposed inside the spindle  548 . Rotation of crank plate  552  causes the crank pin  554  to act on cylindrical bearing  556  such that the hollow piston  558  reciprocates inside of the spindle  548 . The spindle  548  may also rotate about the hollow piston  558 . The longitudinal ridges  559  formed on the outer surface of the hollow piston  558  slidingly engage the inner surface of the spindle  548 . It can be seen that the area of contact between the hollow piston  558  and the spindle  548  is reduced due to the engagement of only the ridges  559  with the inner surface of the spindle  548 . The lubricant  563  contained in the grooves  561  reduces friction between the spindle  548  and the hollow piston  558 . Air may also pass between the hollow piston  558  and the spindle, via the space created by the grooves  561 , thereby improving cooling of the transmission mechanism. This air passage through the grooves may also assist in the equalisation of air pressure in the first and second transmission housing chambers  433 ,  435  already discussed under the heading of the fifth embodiment. 
     A hammer drill of a seventh embodiment of the invention having a motor cooling system is shown in  FIGS. 27 and 28 , with parts common to the embodiment of  FIGS. 3 to 8  denoted by like reference numerals but increased by 600. 
     A hammer drill  620  comprises a tool housing  622  in which a plurality of air vents  669  is formed. The air vents are adapted to either receive cool air from outside of the hammer drill or expel warm air from the inside of the hammer drill. 
     Referring to  FIG. 28 , a motor cooling fan (not shown) is disposed on the axis of the motor  634  in a position that is between the upper field coil (not shown) and the lower commutator (not shown) of the motor  634 . A transmission housing  680 , which may be of the two-part type or the three-part type described above, substantially encapsulates the transmission mechanism. 
     During operation of the power tool the cooling fan is driven by the motor. The cooling fan draws air axially through the motor and expels the air radially outwardly through holes  675  formed in the outer housing  677  of the motor  634 . The cooling fan is vertically aligned with the holes  675  to make the radial expulsion of air easier. This causes air to be drawn in through the air vents  669  formed on the top of the housing  622 , in the side of the housing  622  and between the housing  622  and the battery pack  630 . The cool air follows a path through the tool housing  622  shown by cool air arrows  671 . The cool air flows around the outside of the transmission housing  680  but inside the tool housing  622  such that air does not pass through the transmission mechanism which is sealed to prevent ingress of dirt. 
     A plurality of motor openings  635  are formed in the outer housing  677  of the motor  634  to enable cool air to pass into the motor to cool the motor. As a result of the position of the cooling fan, cool air is drawn across both the field coils of the motor and the motor commutator such that each of these components is individually cooled by air flowing downwards over the field coils and upwards over the commutator. Warm air is expelled through a front vent  669  in the front of the housing following a path shown by warm air arrows  673 . The front vent  699  is vertically aligned with the holes  675  in the outer housing  677  of the motor  634 . Warm air may also be expelled through a rear vent  699  that is disposed between the tool housing  622  and the releasable battery pack  630 . 
     It will be appreciated by persons skilled in the art that the above embodiment has been described by way of example only and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims.