Abstract:
An apparatus for providing percussion action in a rotary power tool having a rotary output shaft, the apparatus comprising at least one moveable mass adapted to have a component of movement parallel to the axis of the rotary output shaft to cause impacts to be applied to a working member of the tool; and a device for intermittently converting rotary movement of the rotary output shaft into movement of at least one the moveable mass to cause the impacts to be applied to the working member.

Description:
The present invention relates to an apparatus for providing percussive action in a rotary power tool, and to a rotary power tool incorporating such apparatus. The invention relates particularly, but not exclusively, to hammer action drills. 
   BACKGROUND OF THE INVENTION 
   Repeated hammering action is provided in drills for masonry and other hard materials. In one known type of hammer drill, a drill bit is carried in a chuck fixed to a working shaft which is driven via a gear from another shaft, the working shaft carrying the chuck being free to move axially over a small range of distances. A ratchet ring is fixed to the end of the working shaft opposite to the chuck end, and a corresponding ratchet ring is fixed to the body of the tool. One extreme of the allowable axial movement of the working shaft is set by the contact of the two ratchet rings, and this extreme is a function of the angle of rotation of the working shaft. When a user operates the tool, the working shaft is forced backwards such that the two ratchet plates come into contact with each other, and relative rotation of the ratchet rings causes a series of impulses to occur. 
   Ratchet ring arrangements of this type are relatively inexpensive to construct, but suffer from the drawback that the impulses acting on the working shaft and ultimately passing into the drill bit also have a reaction on the body of the tool, which results in substantial shaking of the tool. A further disadvantage is that friction losses between the two ratchet plates are relatively high. 
   A further known type of hammer drill which benefits from substantially lower tool body vibration, lower loss of torque at the instant of impact, and more effective impact in most cases because the impulses are generated closer to the drill bit, incorporates a flying striker mass. However, hammer drills of this type require direct axial excitation of the flying striker mass, as a result of which they are expensive to construct. 
   BRIEF DESCRIPTION OF THE INVENTION 
   Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art. 
   According to an aspect of the present invention, there is provided an apparatus for providing percussion action in a rotary power tool having a rotary output shaft, the apparatus comprising at least one moveable mass adapted to have a component of movement parallel to the axis of the rotary output shaft to cause impacts to be applied to a working member of the tool; and conversion means for intermittently converting rotary movement of the rotary output shaft into movement of at least one the moveable mass to cause the impacts to be applied to the working member. 
   By providing an apparatus in which rotary movement of a rotary output shaft is intermittently converted into linear movement of a moveable mass which then causes impacts to be applied to an working member such as a drill bit, this provides the advantage that the apparatus is less expensive to manufacture than an apparatus requiring direct axial excitation of a flying striker mass, while reducing wear of moving parts compared with the prior art apparatus using ratchet plates. The invention also has the advantage that because the conversion means intermittently converts rotary movement of the output shaft into movement of at least one moveable mass, under certain circumstances it is possible to arrange the frequency of the percussive impulse applied to the working member of the tool to be substantially independent of the rotational frequency of the output shaft. This is highly advantageous in the field of power tools, since a power tool such as a drill will have an optimum rotational frequency range within which its percussive action operates most efficiently, but the rotational frequency of the drill will reduce when the drill bit encounters resistance. As the rotational frequency of the drill changes, it is difficult to maintain the percussion action within its optimum frequency range if the percussion frequency is dependent upon the rotational frequency of the output shaft. By making the percussion and rotational frequencies substantially independent of each other, this problem can be overcome. 
   The conversion means is preferably adapted to convert said rotary movement of the rotary output shaft into movement of at least one said moveable mass in a direction substantially parallel to the axis of rotation of the rotary output shaft. The conversion means may be adapted to intermittently convert rotary motion of said rotary output shaft into movement of at least one said moveable mass such that times when said conversion means converts said rotary movement of the rotary output shaft into movement of at least one said moveable mass alternate with times when impacts are applied to the working member. This provides the advantage of reducing the extent to which the percussion action transfers impulses to the motor of the tool, which could otherwise cause damage to the tool. 
   The apparatus may further comprise at least one impact member adapted to be impacted by at least one said moveable mass to cause impacts to be applied to the working member, wherein at least one of the mutually impacting surfaces of at least one said impact member and the corresponding movable mass are so shaped that energy associated with said mutual impacts is not dissipated substantially by air damping. This provides the advantage of minimising energy loss through rapid expulsion of air as said moveable mass applies a percussive impulse to the working member of the tool. At least one of said mutually impacting surfaces may be non-planar. 
   The conversion means may include at least one helical spring. The conversion means may further comprise restraining means for resisting expansion of the or each said helical spring in a radial direction. The restraining means may comprise at least one hoop, pin or strut mounted within at least one said spring. 
   The apparatus may further comprise clutch means having a first clutch member adapted to rotate with said rotary output shaft, and a second clutch member connected to said conversion means and adapted to intermittently engage said first clutch member and be rotated thereby to cause movement of at least one said moveable mass. The second clutch member may be adapted to disengage from said first clutch member when the or each said moveable mass applies an impact to said working member. 
   The second clutch member may include a substantially frustoconical outer surface adapted to frictionally engage a corresponding surface of said first clutch member. The cone angle of said substantially frustoconical outer surface is preferably not less than the friction angle between said substantially frustoconical surface and the corresponding surface of said first clutch member. This provides the advantage of minimising the risk of the second clutch member becoming wedged on the first clutch member. 
   The apparatus may further comprise rotation resisting means for causing relative rotation between said rotary output shaft and at least one said moveable mass. This provides the advantage of maximising the extent of actuation of said conversion means. The rotation resisting means may comprise means for resisting rotation of at least one said moveable mass relative to the housing of the tool. The rotation resisting means may be magnetic. 
   The apparatus may further comprise biasing means for biasing at least one said moveable mass in such a direction as to actuate said conversion means. The biasing means may include at least one spring. The biasing means may be magnetic. 
   According to another aspect of the present invention, there is provided a rotary power tool comprising a housing, drive means for causing rotation of a rotary output shaft, a rotary output shaft connected to said drive means, and an apparatus as defined above. The tool may further comprise de-actuating means for de-actuating said apparatus. 
   Limited axial movement of said rotary output shaft relative to the location at which at least one said moveable mass applies a percussive impulse to said working member may be possible. This provides the advantage of minimising transfer of said impulse to the drive means, which could otherwise cause damage to a drive means such as a motor. 
   The tool may further comprise at least one further shaft adapted to be rotated by means of, and move axially relative to, said rotary output shaft. At least one said further shaft may be splined and substantially co-axial with said rotary output shaft. At least one said further shaft may be radially separated from said rotary output shaft. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the 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 side cross-sectional view of part of a first embodiment of the hammer drill of the present invention; 
       FIG. 2  is a perspective view of part of a second embodiment of a hammer drill according to the present invention; 
       FIG. 3  is a further perspective view of the apparatus of  FIG. 2 ; 
       FIG. 4  is a side cross-sectional view of the apparatus of  FIGS. 2 and 3 ; 
       FIG. 5  is a schematic side cross-sectional view of part of a third embodiment of a hammer drill of the present invention; and 
       FIG. 6  is an enlarged view of region A of the drill of  FIG. 5 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , a hammer drill  1  includes a percussive hammer apparatus mounted to a working shaft  2  of the drill. The working shaft  2  is rotated at a generally steady rotational speed by means of a motor (not shown) via a gear reduction mechanism including an integral gear  3  on working shaft  2 . The working shaft  2  is mounted to a housing  4  of the drill by means of bearings  5 ,  6 . 
   The apparatus  1  includes a first mass  7  connected via a helical spring  8  to a second mass  9 , the second mass  9  being larger than the first mass  7 . The first mass  7  and second mass  9  are free to slide and rotate relative to the working shaft  2 , but the second mass  9  is prevented from rotating relative to the housing  4  by means of a pair of parallel bars  10 . 
   The first mass  7  has a generally frustoconical outer surface  11  which mates with a corresponding frustoconical surface  12  on integral gear  3  such that when the frustoconical surfaces  11 ,  12  are fully in contact with each other, the cone angle, which is around 15°, causes a relatively large frictional torque for a relatively small amount of axial force pushing the first mass  7  into contact with the integral gear  3 . The cone angle is not less than the friction angle tan −1 μ, where μ is the coefficient of friction between first mass  7  and integral gear  3 , as a result of which the first mass  7  does not become stuck in engagement with frustoconical surface  12  when the spring  8  exerts any traction force tending to pull the first mass  7  away from the integral gear  3 . 
   At the limiting value of this condition (i.e. when the cone angle is exactly equal to tan −1 μ) the net frictional torque between the integral gear  3  and the first mass  7  has a maximum value of RF S , where R is the mean radius of frustoconical surface  11  and F S  is the compression force in the spring  8 .
 
 T   S   =RF   S 
 
   The characteristics of the helical spring  8  are such that it causes a coupling between twist and axial compression/extension deformation. For some limited range of deformation, the torque and compression force in the spring are generally linearly related to the axial compression deformation and twist deformation of the spring through three spring constants k FF , k FT , k TT  as follows: 
   
     
       
         
           
             [ 
             
               
                 F 
                 S 
               
               
                 T 
                 S 
               
             
             ] 
           
           = 
           
             
               [ 
               
                 
                   
                     k 
                     FF 
                   
                   ⁢ 
                   
                     k 
                     FT 
                   
                 
                 
                   
                     k 
                     FT 
                   
                   ⁢ 
                   
                     k 
                     TT 
                   
                 
               
               ] 
             
             ⁡ 
             
               [ 
               
                 δ 
                 α 
               
               ] 
             
           
         
       
     
   
   In which F S  and T S  are the compression force and torque in the spring, and δ and α are the compression deformation and twist deformation of the spring respectively. Spring constants k FF , k TT , and k FT  are the spring constants corresponding to compression, twist, and combined compression and twist respectively. The torque is defined such that positive T S  corresponds to a torque tending to accelerate the second mass  9  in the same direction as the rotation of the working shaft  2 . 
   The general increment in stored energy ΔSE in the spring for a change in deformation Δδ, Δα is as follows:
 
Δ SE=F   S.   Δδ+T   S.   Δα=k   FF   δ.Δδ+k   FT   α.Δδ+k   FT   δ.Δα+k   TT α.Δα
 
   the total stored energy SE therefore being
 
 SE= ½ k   FF δ 2   +k   FT δα+ 1 / 2    k   TT α 2 
 
   this is positive for all values of δ and α if
 
(k FF   k   TT )−( k   FT ) 2 ≧0
 
   Provided that k FT  is positive (i.e. the handedness of the helical spring is such that turning the end nearest the integral gear  3  in the direction of rotation of the working shaft  2  tends to elongate the spring  8 ) then the presence of any torque at the interface between the first mass  7  and integral gear  3  will tend to increase the axial force reacted at the contact between frustoconical surfaces  11 ,  12  and therefore increase the maximum possible interface torque. 
   The characteristics of the spring of the apparatus of the present invention are therefore chosen such that the existence of any positive torque at the interface between frustoconical surfaces  11 ,  12  rapidly leads to the elimination of any rotational slip. It follows that the spring characteristic should be such that any increase in T S,  ΔT S , which takes place without extension of the spring should result in an increase in F S,  ΔF S,  greater than ΔT S /R. This condition is satisfied if k FF R is greater than k FT . 
   The rotation of the first mass  7  causes axial movement of the second mass  9 , which delivers percussive impulses to an impulse face  13  mounted on the working shaft  2  near to a chuck  14  to which a drill bit (not shown) is mounted. The second mass  9  has a recess  15  adjacent the working shaft  2  to minimise energy loss caused by rapid expulsion of air from between two parallel surfaces. The second mass  9  is biased by means of a pair of springs  16  towards the integral gear  3 . 
   The operation of the hammer drill  1  shown in  FIG. 1  will now be described. 
   If the working shaft  2  is rotating at a steady rotational speed and the first mass  7 , second mass  9  and spring  8  are initially stationary and the first mass  7  is not in contact with the integral gear  3 , the small pre-load force of springs  16  urges the first mass  7  into contact with the integral gear  3 . At the moment of contact, a torque at the interface between frustoconical surfaces  11 ,  12  rotates first mass  7  and increases the compressive force in helical spring  8 . 
   The increase in compressive force increases the frictional torque between integral gear  3  and first mass  7 , which rapidly causes the interface to lock so that the first mass  7  has the same angular velocity as the working shaft  2 . Because the second mass  9  is prevented by parallel bars  10  from rotating with the first mass  3 , the helical spring  8  then begins to acquire twist, as a result of which the axial compression force in the helical spring  8  increases significantly. 
   As a result, the second mass  9  is urged towards impulse face  13  while the spring has a compressive force. The compressive force of spring  8  then decreases, causing the first mass  7  to separate from integral gear  3 , and the second mass  9  then strikes impulse face  13 . The second mass  9  is then urged by springs  16  back towards integral gear  3  to bring first mass  7  into contact with the integral gear, and the process then repeats itself. After a small number of cycles, the system develops a steady state behaviour in which there is a regular impulse, and the frequency of this impulse is set largely by the mass of the second mass  9  and the characteristics of helical spring  8 . It is therefore found that this frequency is generally insensitive to the rotational speed of the working shaft  2 . 
   Referring now to  FIGS. 2 to 4 , in which parts common to the embodiment of  FIG. 1  are denoted by like reference numerals but increased by  100 , a hammer drill  101  of a second embodiment of the invention has a first mass  107  connected to a second mass  109  by means of a helical spring  108  including three individual helices  120 ,  121 ,  122  which are connected together by means of a series of rings  123 . The spring  108  of the embodiment of  FIGS. 2 to 4  has the advantage of minimising radial expansion of the spring  108  as it acquires twist, which may otherwise reduce the extent to which the spring  108  converts rotary movement of first mass  107  into axial movement of second mass  109 . 
   Referring to  FIGS. 5 and 6 , in which parts common to the embodiment of  FIG. 1  are denoted by like reference numerals but increased by  200 , a hammer drill  201  of a third embodiment of the invention has a working shaft  202  comprising a rear part  202   a  fixed relative to integral gear  203  and motor (not shown), and a front part  202   b  which is axially slidable to a limited extent relative to the rear part  202   a.  As shown in greater detail in  FIG. 6 , which is an enlarged schematic view of region A in  FIG. 5 , the rear part  202   a  and front part  202   b  are connected to each other by means of a generally frustoconical projection  230  on rear part  202   a  (shown in dotted lines in  FIG. 6 ), which is received in a correspondingly shaped recess in the front part  202   b.  The front and rear parts  202   a,    202   b  are splined, i.e. provided with ridges and grooves  231  so that when the front part  202   b  is in mating contact with the rear part  202   a,  rotation of the rear part causes corresponding rotation of the front part. 
   In the third embodiment of  FIGS. 5 and 6 , when the second mass  209  strikes impulse face  213 , part of the impulse delivered to the housing (acting towards the right in  FIG. 5 ) is transferred to the front part  202   b  of working shaft  202 . This causes front part  202   b  to move to a limited extent to the right in  FIGS. 5 and 6 , which minimises the extent to which the impulse is transmitted via rear part  202   a  to the motor. This in turn minimises the extent to which the impulse transmitted to the tool housing is transferred via working shaft  202  to the motor, which could otherwise damage the motor. 
   It will be appreciated by persons skilled in the art that the above embodiments have 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. For example, instead of providing a working shaft  202  which consists of two parts  202   a,    202   b  which can move axially relative to each other, it is possible to minimise the extent to which the impulse delivered to the tool housing is transferred back to the working shaft  202  by rotating the drill bit by means of a further shaft parallel to the working shaft  202 , so that the working shaft does not need to be in direct engagement with the motor. Also, it is possible to provide means to selectively disengage the hammer action of the present invention, for example by providing means for permanently disengaging the first mass  7  from the integral gear  3  and/or clamping the second mass  9  to the impulse face  13  when not in hammer mode (i.e. when in conventional drilling mode).