Patent Publication Number: US-2006000645-A1

Title: Driver

Description:
The present invention relates to a driver or dogging element for transferring torsion forces between a rotating outer tube and the drill bit of a down-the-hole hammer.  
      In the case of countersink hammers, rotational forces, feed, and percussion energy are transferred from a drilling unit to a down-the-hole hammer that is lowered into the hole to be drilled. The rotational forces are transferred with the aid of a rotating outer tube such as to cause the drill bit to rotate, and the percussion forces are transferred with the aid of a piston or ram that moves reciprocatingly in the tube in its longitudinal direction. The rotational force is transferred from the outer tube by means of a chuck or dogging element screwed firmly into the outer tube. Arranged inside the dogging element or chuck is an array of splines which co-act with splines arranged on the outside of the drill bit shaft. The drill bit, the outer tube and the dogging element or chuck are all made of steel.  
      This technique has been used with satisfaction for many years. Because of the greater powers to which a drilling unit is subjected, resulting in higher drilling speeds, the stresses and strains to which component materials are subjected have also increased. In the present case, this has resulted in greater wear on the spline transmission which, in turn, has led to the formation of cracks and fissures which, in turn, have led to breakdowns caused by fracturing of the drill bit shaft. This can result in the loss of the drill bit, with the drill bit remaining in the drill hole. It may lead to that the lost drill bit cannot be recovered and that a new hole must be drilled from the beginning.  
      Accordingly, an object of the present invention is to provide for the transfer of torsion forces between a rotating outer tube and a drill bit of a down-the-hole hammer a novel driver or dogging element that overcomes the aforesaid problems.  
      This object is achieved with a driver or dogging element according to the present invention. The invention is characterised in that the dogging element is made from a material that is softer than the material from which those parts of the drill bit that co-act with the dogging element are made. 
    
    
      The invention will now be described in more detail with reference to an exemplifying embodiment and also with reference to the accompanying drawings, in which  
       FIG. 1  is a longitudinally sectioned view of a drill bit for a down-the-hole hammer and shows those parts that are in closest connection with the drill bit; and  
       FIG. 2  is a cross-sectional view of an inventive dogging element included in the  FIG. 1  embodiment. 
    
    
       FIG. 1  thus shows a drill bit  1  that has rock drilling buttons  2  mounted on its front end in a known manner. Typically, the drill bit  1  also includes a shaft  3  which is provided with longitudinally extending splines  4  along a part of its length. The drill bit also has a rear end  5  against which a percussion piston is intended to strike. The drill bit  1  typically includes a longitudinally extending channel  6  through which compressed air is blown out from the front part of the drill to blow the drill cuttings away from the drill bit and out of the drill hole.  
      The shaft  3  carries a driver or dogging element  7  which is provided internally with longitudinally extending splines  8  that co-act with the splines  4 . The dogging element  7  is generally in the form of a sleeve and is provided along at least part of its length with an external screw thread  9  by means of which the dogging element  7  can be screwed securely to the front end of an outer tube  10  that surrounds at least a part of both the dogging element  7  and the shaft  3  of the drill bit  1 . The outer tube  10  also surrounds the percussion piston  11 . The down-the-hole hammer can be spliced or joined to allow the drill run to be extended to a desired length. The drilling unit causes the outer tube  10  to rotate, wherewith the rotational movement of the tube is transmitted to the dogging element  7  which, in turn, transfers the rotational movement to the drill bit  1  through the medium of the spline connection  8 ,  4 .  
      According to the invention, the dogging element  7  is made of a material that is significantly softer than those parts of the drill bit  1  that co-act with the dogging element, i.e. than the shaft  3 , therewith avoiding those problems associated with the formation of cracks, fissures, and with breakage of the shaft  3 . It has been found that a bronze alloy is a suitable material for the dogging element. This alloy may be a tin bronze alloy or a chromium bronze alloy. A tin bronze alloy produced by Applicants Atlas Copco Secoroc AB and designated 11 9351 93 has been found to be excellent in this respect.  
      When the dogging element  7  is made of this softer material, it is this material that suffers the wear and tear rather than the steel in the shaft  3  of the drill bit co-acting with the dogging element. As the dogging element  7  rotates in response to the rotation of the outer tube  10 , force is transmitted by the splines  8  on the dogging element to the splines  4  on the shaft  3 , wherewith the splines  8  on the dogging element, which are softer than the splines  4 , will become worn instead of the splines  4  on shaft of the drill bit  1 . There is thus obtained a controlled and observable wear and the dogging element can be replaced when it is noticed that the degree of wear on the dogging element makes this necessary. The splines  8  on the dogging element may be allowed to wear down to about half their original width before needing to replace the dogging element. Breakage or fracture of the dogging element  7  will not cause any serious damage, since it is only the dogging element that breaks while the drill bit will still be held firmly by the outer tube  10 .  
      The preferred material is not only softer than steel, but also has higher thermal conductivity and lower friction than steel, meaning that the stresses and strains in the spline connection will be much lower than those experienced with known techniques using steel on steel contact.