CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority, under 35 U.S.C. §119(a)-(d), to UK Patent Application No. GB 1216905.8 filed Sep. 21, 2012, the contents of which are incorporated herein by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The present application relates to a hammer drill having a cylinder, in which is located a piston and a ram, the reciprocating movement of the piston reciprocatingly driving the ram via an air spring to impart impacts to a cutting tool. 
       BACKGROUND OF THE INVENTION 
       [0003]    A pavement breaker is a type of hammer drill which operates in a hammer only mode. However, other types of hammer drill operate in two modes, namely a hammer only mode or a hammer and drill mode, or in three modes of operation, namely a hammer only mode, a hammer and drill mode or a drill only mode. 
         [0004]    EP1872913 discloses an example of a pavement breaker which comprises a cylinder in which is mounted a piston which is reciprocatingly driven by a motor via a hammer mechanism. The piston in turn reciprocatingly drives a ram which imparts impacts onto a cutting tool via a beat piece. The cylinder comprises a single bleed hole to refresh the air spring. The characteristics of the performance of the pavement breaker vary depending on the hardness of the material being cut. The problem with this design is that the characteristics of the performance of the hammer can not be adjusted. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    According to a first aspect of the present invention, there is provided a hammer drill according to claim  1 . 
         [0006]    The normal use of the hammer drill is when the hammer drill is running continuously whilst working on a work piece. 
         [0007]    According to a second aspect of the present invention, there is provided a method of altering the performance characteristics of a hammer according to claim  17 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Four embodiments will now be described with reference to the following figures of which: 
           [0009]      FIG. 1  shows a side view of a pavement breaker; 
           [0010]      FIG. 2  shows a vertical cross section of a pavement breaker with a bleed hole in a first position; 
           [0011]      FIG. 3  shows an enlarged view of the middle part of the vertical cross section of the pavement breaker with the bleed hole in the first position as shown in  FIG. 2 ; 
           [0012]      FIG. 4  shows an enlarged view of the tool holder end of the vertical cross section of the pavement breaker with the bleed hole in the first position as shown in  FIG. 2 ; 
           [0013]      FIG. 5A  which shows a diagram of part of the tool holder and beat piece in a second position when the cutting tool is cutting hard material; 
           [0014]      FIG. 5B  which shows a diagram of part of the tool holder and beat piece in a first position when the cutting tool is cutting soft material; 
           [0015]      FIG. 6  shows a graph showing the properties of the pavement breaker of  FIG. 2 ; dependent on the hardness of the material it is working; 
           [0016]      FIG. 7  shows a vertical cross section of a pavement breaker with the bleed hole in a second position; 
           [0017]      FIG. 8  shows a graph showing the properties of the pavement breaker of  FIG. 7 ; 
           [0018]      FIG. 9  shows a first embodiment of the present invention; 
           [0019]      FIG. 10  shows a second embodiment of the present invention; 
           [0020]      FIGS. 11A to 11D  show sketches of a hammer having a single bleed hole with a valve according to a third embodiment; and 
           [0021]      FIG. 12  shows a schematic view of a fourth embodiment with a hollow piston. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Referring to  FIG. 1 , the pavement breaker comprises a body  2  comprising a middle housing  4  connected to an upper housing  6  using bolts  8 . Two handles  10  are moveably mounted on the upper housing via a vibration dampening mechanism  12 . A tool holder  14  is attached to the opposite end of the middle housing to that of the upper housing  6  using bolts  16 . The tool holder  14  comprises a body  90 , a pivotal clamp  16  having a U shaped bracket  18  which holds a cutting tool  22 , such as a chisel, when the pivotal clamp  16  is pivoted to the position shown in  FIG. 1 . The design of such pivotal clamps is well known in the art and therefore will not be described in any further detail. 
         [0023]    Referring to  FIG. 2 , the pavement breaker comprises an electric motor  24  mounted within the upper housing  6 . The motor comprises a rotor  32  rotatably mounted within a stator  36  in well known manner. The motor  24  is powered by a mains electricity supply which is provided via an electric cable  26  which connects to the motor  24  via an electric switch  28 . When the cable is connected to an electricity supply, operation of the electric switch  28  activated the motor causing the rotor  32  together with an output spindle  30  to rotate. 
         [0024]    The output spindle  30  is comprises splines which mesh with the teeth of a first gear  40 . The first gear  40  is rigidly mounted on a rotatable shaft  42 . A second gear  44  is also rigidly mounted on the rotatable shaft  42 . The second gear  44  meshes with a third gear  46  which is rigidly mounted on a rotatable crank shaft  48 . The crank shaft  48  comprises a disk  50  formed at one end on which is rigidly mounted an eccentric pin  52 . Rotation of the spindle  30  of the motor  24  results in rotation of the crank shaft  48  via the gears, which in turn results in rotation of the eccentric pin  52  around the axis of rotation  54  of the crank shaft  48 . 
         [0025]    A tubular cylinder  58  is rigidly mounted within housing  2 . A piston  60  is slideably mounted within the cylinder  58  and is capable of sliding in a direction parallel to longitudinal axis  74  of the cylinder  58 . A con rod  56  is rotationally attached at one end to the eccentric pin  52  via a bearing. The piston  60  is pivotally connected to the other end of the con rod  56 . Rotational movement of the eccentric pin  52  around the axis of rotation  54  of the crank shaft  48 , results in a reciprocating sliding movement of the piston  60  inside the cylinder in well known manner. Each single rotation of the eccentric pin  52  around the longitudinal axis  54  of the crank shaft  48  results in a single back and forth movement of the piston in the cylinder and is referred to as a hammer cycle. As such, rotation of the spindle  30  results in a reciprocating movement of the piston  60  within the cylinder  58 . The piston comprises piston rings  66  which form an air tight seal between the sides of the piston  60  and the inner wall of the cylinder  58 . 
         [0026]    Located inside of the cylinder  58 , forward of the piston  60 , is a ram  64 . The ram  64  can freely slide within the cylinder  58  in a direction parallel to the longitudinal axis  74  of the cylinder  58 . The ram  64  comprises sealing rings  68  which form an air tight seal between the sides of the ram  64  and the inner wall of the cylinder  58 . The ram  64  is connected to the piston  60  via an air spring  62  formed inside of the cylinder  58  between the piston  60  and the ram  64 . As such, the reciprocating movement of the piston  60 , when driven by the motor, is transferred to the ram  64 . 
         [0027]    A bleed hole  94  is formed through the side wall of the cylinder  58  which enables the air spring to be refreshed. The bleed hole is circular in cross section and has a diameter of 2 mm. The maximum amount by which the piston can slide within the cylinder away from the motor is indicated by L3 which shows the position of the front of the piston at this position. The bleed hole is located  151  rearward of this position by 38 mm so that the piston  60  passes over the bleed hole  94  as it is reciprocatingly driven. As such, the piston  60  repeatedly opens and closes the bleed hole  94  when it is to the rear of the bleed hole  94  or when it is covering the bleed hole  94  respectively. The ram  64  comprises a recess  100  formed in its front end. 
         [0028]    Mounted inside of the housing, in front of the cylinder  58 , is a beat piece support structure  70 . Slideably mounted within the beat piece support structure  70  is a beat piece  72 . The beat piece  72  comprises a tubular body  82  with a radially extending flange  84  formed at the front end of the beat piece  72 . The beat piece support structure  70  comprises a tubular section  92  which slidingly engages with the sides of the tubular body  82 . The beat piece  72  can slide in a direction parallel to the longitudinal axis  74  of the cylinder  58 . The rear end of the beat piece projects into the cylinder  58  and is repetitively struck by the base of the recess  100  of the ram  64  when it is reciprocatingly driven by the piston  60  via the air spring  62 . This in turn results in the front end of the beat piece repetitively striking the end of the cutting tool  22  when held in the tool holder  14 . 
         [0029]    A tubular counter mass  76  surrounds the outside of the cylinder  58  and is capable of sliding in a direction parallel to the longitudinal axis  74  of the cylinder  58  along the outside of the cylinder. The tubular counter mass is sandwiched between two helical springs  78 ,  80  which wrap around the cylinder  58  and which are each held in position at one end by the housing. The counter mass  76  oscillates in response to vibrations in the housing. The weight of the counter mass  76  and the strength of the springs  78 ,  80  are set to predetermined values so that oscillation of the counter mass  76  counteracts the vibrations in the housing, thus acting as a vibration dampener. 
         [0030]    The beat piece support structure  70  abuts against the rear of the tool holder  14 . A circular washer  86  is sandwiched between beat piece support structure  70  and the body  90  of the tool holder  14 . The circular washer  86  has an inner diameter which is greater than that of the tubular body  82  of the beat piece  72  but the same as that of the periphery of the flange  84 , thus forming a inner washer space  87  in which the flange  84  can freely slide inside of the washer  86 . A forward facing chamfer  88  is formed on the forward part of the beat piece support structure  70 . The chamfer  88  tapers from the inner surface, which faces towards the beat piece  72 , of the washer  86  towards the inner wall of the tubular section  92  of the beat piece support structure  70  which slidingly engages the side of the tubular body  82  of the beat piece  72 . The body  90  of the tool holder comprises a tubular recess  96  which extends forward from the rear of the body  90  until a rearward facing chamfer  98  formed inside of the body  90 . An elongate tubular space formed by the tubular recess  96  of the tool holder  14  and the washer space  87 , and which is terminated at one by forward facing chamfer  88  on the beat piece support structure  70  and rearward facing chamfer  98  inside the body  90  of the tool holder  14 . The flange  84  of the beat piece  72  can axially slide within the elongate tubular space  96 ,  87  between a second position where the rear side of the flange  84  abuts the forward facing chamfer  88  on the beat piece support structure  70  and a first position where the forward side of the flange  84  abuts the rearward facing chamfer  98  inside of the body  90  of the tool holder  14 . 
         [0031]    The cutting tool  22  can axially slide in a direction parallel to the longitudinal axis  74  of the cylinder  58 . The cutting tool  22  comprises a rib  102  which limits the range of axial movement of the cutting tool within the tool holder when the pivotal clamp  16  is in the locked position as shown in  FIG. 1 . The cutting tool  22  can slide between a first position (shown in dashed lines  102 ′ in  FIG. 2 ) where the rib  102 ′ abuts against the U shaped bracket  18  and a second position where the rib  102  abuts against the body  90  of the tool holder as shown in  FIG. 2 . 
         [0032]    Referring to  FIG. 4  which shows an enlarged view, during use, the working end (not shown) of the cutting tool  22  is place against a work piece to be cut. The ram  64  strikes the beat piece  72  which in turn strikes the end of the cutting tool  22  which strikes the work piece. When the cutting tool  22  is struck by the beat piece  72 , the cutting tool  22  is pushed forward (left in  FIG. 2 ) out of the tool holder  14  and into the work piece. However, its average position within the tool holder  14  is determined by the hardness of the work piece being cut by the cutting tool. If the work piece is made from hard material, the cutting tool will penetrate the work piece to a lesser extent during each impact of cutting tool and therefore will rebound (to the right in  FIG. 2 ) from the work piece to a greater extent after it has struck it. In this situation, the rib  102  will be located in close proximity to the body  90  of the tool holder  14  as shown in  FIG. 4 . If the work piece is made from soft material, the cutting tool  22  will penetrate the work piece to a greater extent during each impact of cutting tool  22  and therefore the cutting tool  22  will rebound from the work piece to a lesser extent after it has struck it. In this situation, the rib  102 ′ will be located in close proximity to the U shaped bracket  18  of the pivotal clamp  16  (shown in dashed lines  102 ′ as shown in  FIG. 4 ). 
         [0033]    During each impact cycle (i.e. the impact of the cutting tool followed by its rebound) by the cutting tool  22 , whilst the position of the rib  102  will maintain an average position relative to the body  90  of the tool holder  22  (close to the body  90  of the tool holder  14  for hard material; close to the U shaped bracket  18  of the pivotal clamp  16  of the tool holder for soft material), the actual position of the rib  102  will move across a small range of positions whilst it is located at that average position during each impact cycle. 
         [0034]    Referring to  FIG. 5A  which shows the position of the cutting tool  22  and beat piece  72  when the cutting tool  2  is cutting a hard material, the average position of the rib  102  of the cutting tool  22  within the tool holder  14  is in close proximity to the body  90  of the tool holder  14 . During each impact, the rib  102  will move axially during the impact and subsequent rebound (the impact cycle). The rib  102  will move between positions  104  and  106 . The centre point  108  of the rib  102  will travel over the range of movement indicated by Arrow R1 as rib  102  moves between its two end positions  104 ,  106 . However, the rib  102  will remain generally in close proximity to the body  90  of the tool holder  14  and is referred to as the average position  110 . 
         [0035]    Referring to  FIG. 5B  which shows the position of the cutting tool  22  and beat piece  72  when the cutting tool  22  is cutting a soft material, the average position of the rib  102 ′ of the cutting tool  22  within the tool holder is in close proximity to the U shaped bracket  18  of the pivotal clamp  16  of the tool holder. During each impact cycle, the rib  102 ′ will move axially during the impact and subsequent rebound. The rib  102 ′ will move between positions  104 ′ and  106 ′. The centre point  108 ′ of the rib  102 ′ will travel over the range of movement indicated by Arrow R1 as rib  102 ′ moves between its two end positions  104 ′,  106 ′. However, the rib  102 ′ will remain generally in close proximity to the U shaped bracket  18  of the pivotal clamp  16  of the tool holder and is referred to as the average position  110 ′. 
         [0036]    The average position of the cutting tool  22  within tool holder  14  effects the average position of the beat piece  72  within the beat piece support structure  70 . When the cutting tool  22  is cutting hard material, the average position of the rib  102  is close to the body  90  of the tool holder  14  which in turn results in the beat piece  72  being moved to a position where the flange  84  is located in close proximity to the forward facing chamfer  88  formed within the beat piece support structure  70  as shown in  FIG. 5A . When the cutting tool  22  is cutting soft material, the average position of the rib  102 ′ is close to the to the U shaped bracket  18  of the pivotal clamp  16  of the tool holder  14  which in turn results in the beat piece  72  being moved to a position where the flange  84  is located in close proximity to the rearward facing chamfer  98  formed within the body  90  of the tool holder  14  as shown in  FIG. 5B . 
         [0037]    During each impact cycle, whilst the position of the flange  84  of the beat piece  72  will maintain an average position relative to the beat piece support structure  70 , the actual position of the flange  84  will move across a range of positions whilst it is located at that average position during each impact cycle. 
         [0038]    Referring to  FIG. 5A , the average position of the flange  84  is in close proximity to the forward facing chamfer  88  within the beat piece support structure  70 . During each impact cycle, the flange  84  will move axially during the impact and subsequent rebound. The flange  84  will move between positions  112  and  114 . The centre point  116  of the flange  84  will travel over the small range of movement indicated by Arrow R2 as the flange  84  moves between its two end positions  112 ,  114 . However, the flange  84  will remain generally in close proximity to the forward facing chamfer  88  within the beat piece structure  70  and is referred to as the average position  118 . 
         [0039]    Referring to  FIG. 5B , the average position of the flange  84 ′ is in close proximity to the rearward facing chamfer  98  within the body  90  of the tool holder  14 . During each impact cycle, the flange  84 ′ will move axially during the impact and subsequent rebound. The flange  84 ′ will move between positions  112 ′ and  114 ′. The centre point  116 ′ of the flange  84 ′ will travel over the range of movement indicated by Arrow R2 as the flange  84 ′ moves between its two end positions  112 ′,  114 ′. However, the flange  84 ′ will remain generally in close proximity to the rearward facing chamfer  98  within the body  90  of the tool holder  14  and is referred to as the average position  118 ′. 
         [0040]    The average position of the beat piece  72  within the beat piece support structure  70  effects the amount by which the ram  64  can slide within the cylinder  58  away from the piston  60 . When the cutting tool  22  is cutting hard material, the average position of the beat piece  72  within the beat piece support structure  70  is such that the maximum forward position of the front  120  of the ram  64  away from the piston  60  is limited to the position indicated by L1 as shown in  FIG. 3 . When the cutting tool  22  is cutting soft material, the average position of beat piece  72  within the beat piece support structure  70  is such that the maximum forward position of the front  120  of the ram  64  away from the piston  60  is limited to the position as indicated by L2 as shown in  FIG. 3 , which is closer to the tool holder  14 . 
         [0041]    It will be appreciated by the reader that the characteristics of the performance of the pavement breaker will be effected by the type of material that is being work on as the internal average positions of the beat piece  72  and cutting tool  22  will alter together with the maximum amount of travel of the ram  64 . 
         [0042]      FIG. 6  shows a graph showing the properties of the pavement breaker shown in  FIG. 2  dependent on the hardness of the material it is working on. The piston is being reciprocatingly driven at 15.2 Hz by the motor. 
         [0043]    The horizontal axis (X axis)  130  is the Restitution Coefficient and is an indicator of the harness of the material being work on. The Restitution coefficient is the return speed of the ram  64  (after it has impacted the material) divided by the impact speed of the ram (Restitution coefficient (RC)=return speed ram (V re)/speed ram (V) [m/s/m/s]). The harder the material, the faster the ram  64  will bounce back. For example, for a soft material such as lime stone, the Restitution Coefficient, Vre/V, is 2/20=0.1 (when the impact speed is 20 ms −1 ). For a hard material, such as granite, the Restitution Coefficient, Vre/V is 10/20=0.5 (when the impact speed is 20 ms −1 ). The higher the value of the Restitution Coefficient, the harder the material being worked on. 
         [0044]    Four graphs are shown in  FIG. 6 , each having a different Y axis. 
         [0045]    The first Y axis  132  is the ETA which ranges from 0 to 1.0. The ETA is the number of Watts of energy delivered by the ram to the cutting tool divided by the amount of energy in the connecting rod driving the piston. As such, it is a measure of the efficiency of the hammer mechanism. This varies depending on the hardness of the material being worked on and produces the graph  134  when the ETA is compared with the Restitution Coefficient. 
         [0046]    The second Y axis  136  is power delivered by the hammer in Watts. This varies depending on the hardness of the material being worked on and produces the graph  138  when the power is compared with the Restitution Coefficient. 
         [0047]    The third Y axis  140  is the impact speed of the ram in metres per second. This varies depending on the hardness of the material being worked on and produces the graph  142  when the impact speed is compared with the Restitution Coefficient. 
         [0048]    The fourth Y axis  144  is the amount of compression of the air spring  62  in cylinder  58 . The amount of compression is determined by the maximum air pressure of the air spring  62  divided by the pressure of the atmosphere. This varies depending on the hardness of the material being worked on and produces the graph  146  when the amount of compression is compared with the Restitution Coefficient. 
         [0049]    The characteristics of the performance of the pavement breaker are effected by the size and axial location of the bleed hole  94  in the cylinder  58  relative to the piston  60 .  FIG. 7  shows a second design of pavement breaker which is identical to that shown in  FIG. 2  except that the size and axial position of the bleed hole  150  has been altered. Where the same features are present in the second design shown in  FIG. 7  are present in the first design as shown in  FIG. 2 , the same reference numbers have been used. The bleed hole  150  is a circular in cross section and 4 mm in diameter and has been located  152  further forward (80 mm) of the bleed hole  150  shown in  FIG. 2  and forward of the maximum amount L3 by which the piston  60  can slide within the cylinder  58  away from the motor. The larger diameter allows more air to pass through it. The ram  64  passes over the bleed hole  150  as it is reciprocatingly driven by the piston  60 . As such, the ram  64  repeatedly opens and closes the bleed hole  150  when it forward of the bleed hole  150  or when it is covering the bleed hole respectively. This results in the timing of when the bleed hole  150  is open and closed within a hammer cycle being altered when compared to that disclosed in  FIG. 2 . 
         [0050]    Again, it will be appreciated by the reader that the characteristics of the performance of this hammer will be effect by the type of material that is being work on.  FIG. 8  shows a graph showing the properties of the pavement breaker shown in  FIG. 7  dependent on the hardness of the material it is working on. The piston  60  is being reciprocatingly driven at 15.2 Hz by the motor. The same reference numbers for the Restitution Coefficient, ETA, impact speed, power and compression used in  FIG. 6  have been used for the same features in  FIG. 8 . 
         [0051]    As can be seen when comparing  FIG. 6  with  FIG. 8 , when the bleed hole  150  is of the size and is located in the position shown in  FIG. 7 , the performance of the pavement breaker on hard material is greatly improved when compared to a bleed hole  94  of the size and position shown in  FIG. 2 . However, when the bleed hole  150  is of the size and is located in the position shown in  FIG. 7 , the performance of the hammer on soft material is reduced when compared to a bleed hole  94  of the size and position shown in  FIG. 2 . 
         [0052]    A first embodiment of the present invention will now be described with reference to  FIG. 9 . The design of the embodiment is the same as the hammer described previously with reference to  FIG. 2  except for the provision of two bleed holes  200 ,  202  and a switching mechanism for opening and closing the bleed holes  200 ,  202  depending on the average position of the beat piece  72  within the beat piece support structure  70 . Where the same features are present in the first embodiment are present in the pavement breaker described with reference to  FIG. 2 , the same reference numbers have used. Please note the vibration dampener is not shown in  FIG. 9  to aid clarity. 
         [0053]    Referring to  FIG. 9 , the cylinder comprises two bleed holes  200 ,  202  formed through the side of the cylinder  58 . The position and size of the first bleed hole  200  is the same as the bleed hole shown in  FIG. 2 . The position and size of the second bleed hole  202  is the same as the bleed hole shown in  FIG. 7 . Surrounding the cylinder is a sleeve  204  having two apertures  206 ,  208  formed through it. The sleeve  204  is cable of axially sliding along the cylinder  58  in a direction (Arrow A) parallel to the longitudinal axis  74  of the cylinder  58  but is prevented from rotating around the longitudinal axis  74 . Each aperture  206 ,  208  is capable of aligning with a corresponding bleed hole  200 ,  202  on the cylinder  58 . The length of each of the apertures  206 ,  208  (in a direction parallel to the longitudinal axis  74  of the cylinder  58 ) is greater then the diameter of its corresponding bleed hole  200 ,  202  enabling the each aperture  206 ,  208  to align with its corresponding bleed hole  200 ,  202  whilst the sleeve  204  is in a range of axial positions. The width (in a direction perpendicular to the longitudinal axis  74  of the cylinder  58 ) of each of the apertures  206 ,  208  is slightly greater than the diameter of the corresponding bleed hole  2002 ,  202 . A lubricating grease is sandwiched between the cylinder  58  and the sleeve  204  to form an air tight seal between the two. 
         [0054]    The positions of the apertures  206 ,  208  in a direction parallel to the longitudinal axis  74  of the cylinder  58  is greater than the distance between the bleed holes  200 ,  202  and is such that when one first aperture  206  is aligned with the first bleed hole  200 , the second aperture  208  is located away form the second bleed hole  202 , the sleeve  204  sealing the second bleed hole  202 . As the sleeve  204  slides along the cylinder  58  away from the beat piece support structure  70 , the first aperture  206  ceases to be aligned with the first bleed hole  200 , the second aperture  208  becoming aligned with the second bleed hole  202 . In this location, the sleeve  204  seals the first bleed hole  200 . During the transition, the positions of the apertures  206 ,  208  on the sleeve  204  are such that both bleed holes  200 ,  202  can not be open at the same time. As such, only one bleed hole is open at any one time depending on the axial position of the sleeve  204  on the cylinder  58 . 
         [0055]    The amount of sliding movement of the sleeve  204  is limited so that the sleeve  204  can slide between two positions, a first position where the first aperture  206  is aligned with the first bleed hole  200 , with the second bleed hole  202  being sealed by the sleeve  204 , and a second position where the second aperture  208  is aligned with the second bleed hole  202 , with the first bleed hole  200  being sealed by the sleeve  200 . 
         [0056]    A spring  210  is sandwiched between the housing  4  and a bar  212  attached to the sleeve  204  which urges the sleeve  204  forward towards its first position where it is closest to the beat piece support structure  70 . Movement of the sleeve  204  from its first position to its second position, away from the beat piece support structure  70 , is against the biasing force of the spring  210 . 
         [0057]    A rod having three sections  214 ,  216 ,  218  is attached to the sleeve  204 . The third section  218  is located inside and capable of sliding within a passage  220  formed through the beat piece support structure  70 . The end  222  of the rod projects in to the inner washer space  87  in which the flange  84  of the beat piece  72  can slide. The maximum amount by which the end  222  can project into the space  87  is limited by the middle section  216  of the rod abutting against the rear of the beat piece support structure  70  under the biasing force of the spring  210 . When the end  22  of the rod extends by its maximum amount into the inner washer space  87 , the sleeve  204  is in its first position. 
         [0058]    When the pavement breaker is working on a soft material, the beat piece  72  is located in its forward average position. The flange  84 ′ (indicated by dashed lines in  FIG. 9 ) of the beat piece  72  is in front of the end  222  of the rod and makes no contact with the rod. As such, the end  22  of the rod is allowed to extend by its maximum amount into the space  87 . When the rod is in this position, the sleeve  204  is located in its first position. In this position, the first aperture  206  is in alignment with the first bleed hole  200  allowing the first bleed hole  200  to be functional. The second aperture  208  is located forward of the second bleed hole  202  and as such, the second bleed hole  202  is sealed closed by sleeve  204 . As such, only the first bleed hole  200  is operational. This results in an improved performance of the pavement breaker for soft material as the pavement breaker will have the performance characteristics shown in  FIG. 6 . 
         [0059]    When the hammer is working on a hard material, the beat piece  72  is located in its rearward average position (indicated by solid lines in  FIG. 9 ). In this position, the flange  84  of the beat piece  72  is located adjacent the forward facing chamfer  88  formed in the beat piece support structure  70  and engaged with the end  222  of the rod which is pushed rearward by the flange  84 . When the rod is in this position, the sleeve  204  is pushed to its second rearward position by the rod. In this position, the second aperture  208  is in alignment with the second bleed hole  202  allowing the second bleed hole  202  to be functional. The first aperture  206  is located rearward of the first bleed hole  200  and as such, the first bleed hole  200  is sealed closed by the sleeve  204 . As such, only the second bleed hole  202  is operational. This results in an improved performance of the pavement breaker for hard material as the pavement breaker will have the performance characteristics shown in  FIG. 8 . 
         [0060]    During each impact cycle, the flange  84  moves axially over a small range of movement during the impact and subsequent rebound. When the flange  84  is in its rearward position in engagement with the end  222  of the rod, this small range of movement will be transferred to the rod which in turn will be transferred to the sleeve  204 . This movement is accommodated by the fact that the length of the first aperture  206  (in a direction parallel to the longitudinal axis  74  of the cylinder  58 ) is not only greater then the diameter of the first bleed hole  200 , but is sufficiently greater than small range of axial movement of the sleeve to enable the aperture  206  to remain aligned with the first bleed hole  200  whilst the sleeve  204  moves over the small range of axial positions. 
         [0061]    It will be appreciated by the reader that a dampener could be added to limit the movement of the sleeve  2004  caused by the limited movement of flange  84  over the impact cycle, the sleeve  204  only moving in response to the movement of the average position of the flange  84 . 
         [0062]    A second embodiment of the present invention will now be described with reference to  FIG. 10 . The design of the second embodiment is the same as the first embodiment except that the mechanism comprising the rod  214 ,  216 ,  218  for moving the sleeve  204  in response to the position of the beat piece  72  within the beat piece support structure  70  has been replaced by a manual switching mechanism. Where the same features are present in the second embodiment are present in the first embodiment, the same reference numbers have used. Please note the vibration dampener is not shown in  FIG. 10  to aid clarity. 
         [0063]    Referring to  FIG. 10 , the slideable sleeve  204  with the apertures  206 ,  208  function in the same manner as in the first embodiment to open and close the two bleed holes  200 ,  202 . However, the use of the rod  214 ,  216 ,  218  has been removed and replaced with a manual switch. The manual switch comprises a rigid arm  300  attached to the sleeve  204  and which extends from the sleeve  204  in a direction perpendicular to the longitudinal axis  74  of the cylinder  58  from the sleeve  204  and through an aperture  302  formed through the wall of the middle housing  4 . Attached to the end of the arm  300  is a finger pad  304  which can be engaged by an operator. A catch comprising a rib  306  mounted on the end of a leaf spring  308  which is attached to and extends side ways from the arm  300  is biased towards a slide pad  312  which comprises two notches  314 ,  316 . An operator can engage the finger pad  304  and slide it (Arrow A) between a first position (shown in dashed lines) where the rib  306  engages the first notch  316  to a second position (shown in solid lines) where it engages the second notch  314 , or vice versa. The sliding movement of the finger pad results in a corresponding sliding movement of the sleeve  204 . In the first position, the first aperture  206  of the sleeve  204  is in alignment with the first bleed hole  200 , with the second bleed hole  202  sealed by the sleeve  204 . In the second position, the second aperture  208  of the sleeve  204  is in alignment with the second bleed hole  202 , with the first bleed hole  200  sealed by the sleeve  204 . 
         [0064]    The range of movement of the finger pad  304  is limited by the end stops  320  limiting the range of movement of the rib  306 . 
         [0065]    When an operator knows that he is going to use the pavement breaker on a soft material such as limestone, he slides the finger pad  304  to its first position so that only the first bleed hole  200  is operative. When an operator knows that he is going to use the pavement breaker on a hard material such as limestone, he slides the finger pad  304  to its second position so that only the second bleed hole  200  is operative. 
         [0066]    The spring  210  biases the finger pad  304  to its first position where the performance characteristics of the pavement breaker are more uniform when used on materials with a range of hardness. However, the leaf spring  308  has sufficient strength to hold the rib  306  within the second notch  314  against the biasing force of the spring  210  when it is moved to this position. 
         [0067]    Whilst the embodiments described above relate to a pavement breaker, it will be appreciated by the reader that the invention can be utilized on any type of hammer drill having a cylinder, inside of which is a piston and ram, where the reciprocating movement of the piston reciprocatingly drives the ram via an air spring. 
         [0068]    A third embodiment will now be described with reference to  FIGS. 11A to 11D . The third embodiment is similar to the previous embodiments except that the two bleed holes in the previous embodiments have been replaced with a single bleed hole and a valve.  FIGS. 11A to 11D  show a schematic diagram of a hammer comprising a cylinder  504 , a piston  502  slidingly mounted within the cylinder  504  which is reciprocatingly driven by a con rod  506  within the cylinder. A ram  508  is mounted within the cylinder and is reciprocatingly driven by the piston  502  via an air spring  510 . The ram  508  repetitively strikes a beat piece  512  which in turn strikes a cutting tool held in the tool holder. A single bleed hole  524  is formed through the wall of the cylinder  504  for proving air to replenish the air spring  510 . A valve  526  controls the timing and volume of the air flow through the bleed hole.  FIGS. 11A to 11D  show the positions of the component parts of the hammer mechanism over the course of a hammer cycle. 
         [0069]    The valve  526  is opened and closed electronically. The timing of the opening and closing of the valve  526  is related to the position of the piston which is measured using a sensor  528  which produces a signal for use by the valve which is indicative of the position of the piston. By controlling when the valve is opened and closed versus the position of the piston  502 , it is possible to mimic the position of the bleed holes shown in the previous embodiments. Furthermore, by controlling the volume of the air which passes through the bleed hole  524 , it can also mimic the sizes of the bleed holes in the previous embodiments. The determination of the timing of the opening and closing of the valve relative to the piston position and volume can be preset by an operator dependent on the hardness of the material the hammer is intended to be used upon, or by sensing the position of the beat piece  512 , which is dependent on the position of the cutting tool, which in turn is dependent on the hardness of the material the hammer is working on, in a similar manner as described in the first embodiment. 
         [0070]    A fourth embodiment is shown in  FIG. 12 . The fourth embodiment is similar to the third except for the fact that the piston  502  is a hollow piston, the ram  508  being slidingly mounted within the piston, the air spring  510  being located between the ram  508  and the piston  502 . 
         [0071]    A bleed hole  600  is formed through the end of the piston  502  to connect between the air spring  510  and the surrounding atmosphere. A valve  602  is attached to the bleed hole  600 . A cable  604  attaches between the valve  602  and the sensor  528 . The timing of the air flow and the amount of air allowed to pass through the bleed hole  600  can be controlled by the valve  602  in the same manner as the third embodiment.
A hammer drill having a motor mounted within a body and an output spindle; a tool holder mounted on the body capable of holding a cutting tool; a hammer mechanism having a piston; a reciprocating drive for converting rotary movement of the motor into reciprocating movement of the piston; a ram reciprocatingly driven by the piston via an air spring to strike a cutting tool held in the tool holder, the hammer mechanism performing one hammer cycle each time the ram strikes a cutting tool during normal use; an air replenishment mechanism capable of refreshing the air spring during certain time periods during normal use; the air replenishment mechanism capable of being adjusted to refresh the air spring during time periods within the hammer cycle and/or the system allows different volumes of air into or out of the air spring during the refreshment time periods.