Abstract:
A hammer drill with rolling contact at the contact surfaces for transmission of axial force between a drive shaft and hammer shaft. In the case of ball bearings, point contact is obtained. In the case of roller bearings, line contact is obtained. The area of contact is thus close to zero as opposed to a relatively large area in engagement systems using toothed surfaces. Use of point or line contact reduces heat generation and reduces energy loss due to friction.

Description:
BACKGROUND OF THE INVENTION 
     Hammer drills are known in which rotation of toothed surfaces against each other causes a hammering action. Also, in U.S. Pat. Nos. 3,149,681 and 3,133,602, rotary impact hammers with a ball on tooth engagement provide for a hammering action only in one direction of rotation. A ball on tooth engagement also tends to wear a groove in the tooth, which tends to create a wide contact area between ball and tooth. Together with the immobility of the tooth surface, the wide contact area increases friction losses and heating of the tool. 
     SUMMARY OF THE INVENTION 
     The present invention provides a hammer drill with rolling contact at the contact surfaces for transmission of axial force between a drive shaft and hammer shaft. In the case of ball bearings, point contact is obtained. In the case of roller bearings, line contact is obtained. The area of contact is thus close to zero as opposed to a relatively large area in engagement systems using toothed surfaces. Use of point or line contact reduces heat generation and reduces energy loss due to friction. 
     In some prior art products, a release clutch is used to release torque when pressure is critically increased and to prevent engagement parts from shear. In the case of a hammer drill with rolling contact, relatively low torque generators may be used where the torque does not exceed shearing stresses. The hammer drill of the present invention does not require the release clutch because it provides its function by rolling friction. When torque increases, the rotating bearing elements in the drive assembly are pushing the rotating bearing elements in the hammer assembly, thus separating the hammer assembly from the drive assembly and releasing the torque. This repetitive action also generates a hammering effect. The contact points between the rotating bearing elements are between 0 and 90 degrees to the tool axis. This offset makes the shearing component of the reaction force to rotate the rotating bearing elements inside the cavities and its axial component makes rotating bearing elements climb on each other. 
     To provide easier assembly and better interaction control between driver half and hammer half of the hammer drill, the bearing holders are provided by a plate with cavities backed up with a back plate. Rotating bearing elements, preferably balls, inserted into the cavities are exposed above the front surface of the cavity plate. The rotating bearing elements are prevented from axial motion in relation to the bearing holder, but are allowed to rotate freely within the cavities of the bearing holder. 
     The balls of one bearing holder may be fewer in number than the balls of the other bearing holder, and the balls of one bearing holder may be radially offset in relation to the balls of the other bearing holder. An on-off switch is also provided to turn the hammering action on and off. 
     These and other aspects of the invention are described in the detailed description of the invention and claimed in the claims that follow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     There will now be described preferred embodiments of the invention, with reference to the drawings, by way of illustration only and not with the intention of limiting the scope of the invention, in which like numerals denote like elements and in which: 
     FIG. 1 is a section through a hammer drill according to the invention; 
     FIGS. 2A and 2B are schematics showing relative ball positions of balls used in the hammer drill of FIG. 1; 
     FIG. 3 is a graph showing relative ball movement in the hammer drill adapter of FIG. 1, for one revolution; and 
     FIG. 4 is a section through a second embodiment of a hammer drill according to the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In this patent document, the word comprising” is used in its non-limiting sense to mean that items following the word in the sentence are included and that items not specifically mentioned are not excluded. The use of the indefinite article “a” in the claims before an element means that one of the elements is specified, but does not specifically exclude others of the elements being present, unless the context clearly requires that there be one and only one of the elements. 
     Referring to FIG. 1, there is shown an adapter  10  for a hammer drill, which includes two subassemblies mounted within a housing  12 . A driver assembly  14  is directly connected to the chuck of a drill or power tool (not shown) and transfers torque from drill to a hammer assembly  16 . The hammer assembly  16  converts received torque into torque and axial stroke motion. The drive assembly  14  may be formed as an integral part of a power tool. 
     The driver assembly  14  includes a drive shaft  18  with one end having hexagonal shape in cross-section for connection into a chuck (not shown) of a conventional power tool, and another end oblong shape in cross-section for connection with the hammer assembly  16 . The middle section of the drive shaft  18  is round in section and has a step  20  for fitting a roller bearing  22  that supports the drive shaft  18  within the housing  12  for rotation relative to the housing  12 . A cone shaped extension  24  covers roller bearing  22 . The housing  12  is formed of a cylindrical outer case  26 , a bearing housing  28  and end cup  30 . Bearing housing  28  is a cylinder shaped part, and has an opening for fitting roller bearing  22  and has a round opening, partially flattened with a flat portion to create a D-shape, for positioning a bearing holder or ball holder cassette  32 . A snap ring  34  engages a groove  36  on the drive shaft  18  to secure the bearing holder  32  in place and fixed axially in relation to the drive shaft  18 , while the bearing holder  32  is fixed rotationally in relation to the housing  12 . 
     The bearing holder  32  fits in the D-shaped opening of bearing housing and has  12  circular distributed cavities for positioning  12  balls  38 . A back plate  40  is inserted on the drive shaft  18  between bearing housing  28  and bearing holder  32 , and the back plate may be secured by a snap ring  41 . Back plate  40  is made from hardened steel to protect the bearing housing  28  from impact wearing due to action of the balls  38 . 
     The hammer assembly  16  includes a hammer shaft  42 , which is cylindrically shaped. The hammer shaft has an oblong profile cavity for connection with the drive shaft  18 . The matching sections of the drive shaft  18  and hammer shaft  42  permit the shafts to rotate together while allowing relative axial movement between them. Hammer shaft  42  also has a D-shape opening for inserting a bearing holder or ball cassette  44 . A snap ring  46  is received in a snap ring groove  48  for securing the ball holder  44  on the hammer shaft  42 , so that the bearing holder is held axially and rotationally stationary in relation to the hammer shaft  42 . The working end  50  of the hammer shaft  42  is hexagonal shaped for receiving a drill bit. 
     Bearing holder  44  has for  12  circular distributed cavities for positioning  12  balls  52 , with the balls  52  backed up by back plate  45 . The back plate  45  may be secured by snap ring  47 . End cup  30  of the housing  12  is cylindrically shaped for locating a bushing  54  that permits relative rotational movement of housing  12  in relation to hammer shaft  42 . Both the drill assembly  14  and the hammer assembly  16  are secured within the housing  12  formed by shell  26 , bearing housing  28  and end cup  30  by suitable means such as threads, snap lock or glue. 
     Drive shaft  18  receives torque from a source (portable drill or electric motor), and transfers torque to hammer shaft  42 . Bearing holder  32  remains fixed in motion relative to the housing  12  by virtue of the D shape of the bearing holder  32  within the D shaped opening in bearing housing  28 . Bearing housing  28  stays steady in relation to the housing  12  due to threaded connection of the bearing housing  28  to the outer casing  26 . Balls  38  are free to rotate in the cavities in the bearing holder  32 . Bearing holder  32  is held against axial movement on the drive shaft  18  by snap ring  34 . 
     Bearing holder  44 , inserted in hammer shaft  42  is secured by snap ring  46 , and stays steady relative to hammer shaft  42 . When hammer shaft  42  rotates, balls  52  in the bearing holder  44  rotate with the hammer shaft  42  about the central longitudinal axis of the hammer shaft  42 . With axial compression on the drive shaft  18  and hammer shaft  42 , the balls  38  are initially located in gaps between balls  52 . The balls  38  should not contact the surface of the bearing holder  44  between the balls  52 , and the balls  52  should not contact the surface of the bearing holder  32  between the balls  38 . Rather, at the point of minimum separation between the bearing holder  38  and bearing holder  52 , the balls  38  should rest on balls  52  with point contact, each ball of one bearing holder resting on two balls of the other bearing holder. As the hammer shaft  42  rotates, pulling the bearing holder  44  with it, the balls  38  climb over the balls  52 , pushing the hammer shaft  42  away, and then sink down between the balls  52  under axial compression. The axial displacement is a function of the ball size and ball separation. If there are twelve balls  38  on bearing holder  32 , and eight balls  52  on bearing holder  44 , the stroke of the hammer shaft  42  is repeated  12  times per revolution to generate a hammer action. 
     One of both of the sets of balls  38 ,  52  may be replaced by rollers, for example conical rollers, with line contact, roller to roller or point contact, ball to roller. Although it is possible for one set of balls to be replaced by rollers, it is preferable to use either balls in both bearing holders or rollers in both bearing holders to reduce manufacturing costs. The term rotating bearing elements includes both rollers and balls. As shown in FIG. 2A, bearing holder  44  may have  8  circular cavities  54  for receiving the balls  52 . As shown in FIG. 2B, bearing holder  32  may have  12  circular cavities  56  for receiving balls  38 . The balls  38 ,  52  may be offset radially relative to each other, for example as shown in FIGS. 2A and 2B so that for example the centers of the cavities  54  may be closer to the center of the bearing holder  44  than are the cavities  56  in relation to the center of the bearing holder  32 , and vice versa. The resulting pattern of movement of the balls  38 ,  52  is shown in FIG.  3 . 
     To allow separate operation of the hammer drill adapter in both a rotary drilling action and a hammer action, an on-off device is provided as shown FIG.  4 . In FIG. 4, bearing  60  is mounted with loose fit on hammer shaft  43  inside bearing housing  61  and is secured by snap ring  62 . On/off collar  64  fits over housing casing  65 , and has four threaded holes  66  distributed equally around its periphery. Pins  68  thread into the holes  66  and fit through angular slots  69  at 45 degrees when viewed sideways in the housing casing  65  and into holes  67  in the bearing housing  61 . End cap  70  is secured to the bearing housing  61  by screws  72 , and together with the bearing housing  61 , housing casing  65  and bearing housing  29 , forms a housing for retaining drive assembly  19  and hammer shaft  43 . Bearing  60  press fits inside bearing housing  61  and is secured by snap ring  74 . 
     To switch off hammering action, collar  64  is rotated at a 45 degree angle in relation to the housing casing  65 , pulling bearing housing  61  and hammer shaft  43  away from the drive assembly  19 . As a result, the balls of respective bearing holders  76  and  78  disengage, thus terminating the hammering action, but permitting drilling since drive assembly  19  remains engaged with hammer shaft  43  for the transfer of torque. 
     Lubrication between hammer shaft  42  and drive shaft  18  in FIG. 1, and between hammer shaft  43  and drive assembly  19  in FIG. 4, is provided by respective cavities  80 ,  81  at the end of hammer shafts  42 ,  43 , communicating with holes  82 ,  83  drilled in the hammer shafts  42 ,  43  perpendicularly to the center axis of the hammer shafts, which holes  82 ,  83  lead out to oil reservoirs  84 ,  85 . Two small grooves  87  (FIG.  4 ), not shown in FIG. 1, are added along hammer shafts  42 ,  43 . When hammer shafts  42 ,  43  move forward, they create a vacuum effect that sucks grease from reservoirs  84 ,  85  and transfers grease through grooves on frictional surface. 
     The use of ball bearing or roller bearing engagement (BBE) is to reduce friction, which generates heat and results in loss of energy. Here is a formula to calculate energy generated by friction: 
     E=K×F×A 
     Where F—is the acting force 
     A—is the area of contact 
     K—is the friction coefficient 
     As we can see from the given equation, we need to minimize any of the given components to achieve the minimum energy (E). Acting Force is a result of pressure applied by operator through the tool on the drilling surface and it cannot be minimized. Friction Coefficient is a function of materials, surface grade and action character (dragging or rolling). In case BBE we are minimizing K because: 
     a) The balls have a smoother surface than the teeth in Tooth &amp; Tooth Engagement (TTE); 
     b) BBE provides rolling action as opposed to dragging in TTE. 
     As we can see, K in BBE is significantly smaller than in TTE. 
     The design shown is suited to the commercial market. For a consumer hammer drill adapter, it is preferred to use a sleeve bearing for the ball bearing  22 , and the cone shaped cover  24  may be smaller. In addition, the balls may be installed directly on the bearing housing, drive shaft or hammer shaft, without use bearing cassettes. In this case, the material of the bearing housing, drive shaft or hammer shaft supporting the balls is the bearing holder referred to in the claims. In addition, instead of an oblong shaped connection between the drive shaft and hammer shaft, one or the other may be keyed and the other slotted to effect a non-rotating connection between hammer shaft and drive shaft. 
     A person skilled in the art could make immaterial modifications to the invention described in this patent document without departing from the essence of the invention.