Technical field

The present invention relates to a rock drill and, more particularly, to a rock drill for use in percussion rock drilling comprising a drill bit having a first conical surface adapted for connection to a drill rod having a corresponding second conical surface.

Background

In percussive drilling, during drilling, drill bits are connected to a drill rod by means on a threaded connection or conical frictional connection. In the case of the conical frictional connection, otherwise known as a taper joint, the drill bit has a generally conical cavity which is terminated by a cup-shaped cylindrical region, while the end of the drill rod that should be received in the cavity is generally conical.

In known designs the conical section of the cavity connects directly to the cylindrical region. The problem with this sharp connection is that it results in high stresses being located inside the drill bit, which increases the risk of fracture of the drill bit in the region which consequently reduces the lifetime of the drill bit. If the drill bit fractures the percussive shock wave forces may no longer be transmitted to the rock but instead may loosen the firm connection between the drill bit and the drill rod, which could result in the drill bit spinning or even becoming detached and lost in the drill hole.

US20070175671A1, GB-658631A and GB-860768A show traditional conical frictional connections know in the prior art.

Therefore, the problem to be solved is how to reduce stress in the cavity of the drill bit.

Summary

It is an objective of this invention to provide a novel and improved design that reduces the stress in the drill bit in the region of the exit of the conical cavity in a taper joint. This objective is achieved by providing a percussive drill bit comprising a sleeve section and bit section having a longitudinal axis; wherein the sleeve section has an axial end at the opposing end of the drill bit compared to the bit section from which a generally conically shaped internal cavity extends from for receiving a conically shaped drill rod to form a conical frictional connection; wherein the cavity has a conical region nearest to the axial end and a cylindrical region nearest to the bit section; characterised in that: there is connection section joining the conical region to the cylindrical region; the conical region has a straight tapered region having a starting point adjacent to the connection region; wherein the connection section has at a first curved connection region having a first radius, CRi, wherein its tangent, CTi, to a central point along the first curved connection region is positioned at a first distance, CDi, from the taper region starting point.

Advantageously, the inclusion of the curved transition section reduces the stress in the conical cavity of the drill bit. This means the drill bit is less likely to fracture, which reduces the risk of the frictional connection being loosened. Consequently, the lifetime of the drill bit is increased, the risk of the bit spinning with respect to the drill rod or becoming detached from the drill rod is reduced.

Optionally, the connection section additionally has a second curved connection region having a second radius, CR2, wherein its tangent, CT2, to a central point along the second curved connection region is positioned at a second distance, CD2, from the taper region starting point; wherein CD2 < CDi and CR2 > CRi. Advantageously, the inclusion of two different radii provides a reduction in stress whilst maintaining sufficient contact between the conical rod and the conical cavity in the drill bit.

Preferably, CRi is between 1 - 160 mm, more preferably between 5 - 120 mm. Advantageously, this provides an optimal balance between providing a tight connection between the conical rod and the conical cavity of the drill bit and achieving a reduction in stress.

Optionally, the first curved connection region connects directly to the second curved connection region. Advantageously, this design has low levels of the stress in the region of the exit of the conical cavity of the drill bit.

Alternatively, the first curved region connects to the second curved region via an intermediate straight section. Advantageously, this design has low levels of the stress in the region of the exit of the conical cavity of the drill bit.

Optionally, the connection section further comprises a third curved connection region having a having a third radius, CR3, wherein its tangent, CT3, to a central point along the third curved connection section is positioned at a third distance, CD3, from the taper region starting point, wherein CD3 < CD2 and CR3 > CR2. Advantageously, this design has low levels of the stress in the region of the exit of the conical cavity of the drill bit.

Optionally, there is additionally a curved transition section positioned in-between the axial end of the drill bit and the straight tapered region of the conical region. Brief description of the drawing

A specific implementation of the present invention will now be described, by way of example only, and with reference to the accompanying drawings:

Figure 1: Cross section of a drill bit having a conically shaped internal cavity.

Figure 2: Enlargement of the cross section of the connection section according to one embodiment wherein the connection section has a single radius.

Figure 3: Enlargement of the cross section of the connection section according to one embodiment wherein the connection section has two radii wherein the first curved connection region and the second curved connection region are directly connected.

Figure 4: Cross section of the connection section according to one embodiment wherein the connection section has two radii wherein the first curved connection region and the second curved connection region of the connection section are connected via an intermediate straight section.

Figure 5: Enlargement of the cross section of the connection section according to one embodiment wherein there is additionally a third curved connection region in the connection section.

Figure 6: Cross section of the connection section according to one embodiment wherein there is additionally a curved transition section between the straight tapered region and the cylindrical region.

Figure 7: Stress image of the conical region of the prior art drill bit.

Figure 8: Stress image of the conical region of an inventive drill bit.

Detailed description

Figure 1 shows a cross section of a drill bit 2 comprising a sleeve section 4 and a bit section 6 which are integrated together. The sleeve section 4 has an internal cavity 8, having a generally conical shape, for receiving a conically shaped drill rod (not shown). The drill bit 2 has a longitudinal axis 30. The cavity 8 opens from an axial end 18 of the drill bit 2, furthest from the bit section 6. The cavity 8 has the largest cross-sectional diameter at the axial end 18 of the drill bit 2. The cavity 8 comprises a conical region 14 that tapers inwardly from the open axial end 18 along a straight tapered region 26 to a cup-shaped cylindrical region 16 that terminates the cavity 8 and is positioned adjacent to the bit section 6. A connection section 40 joins the conical region 14 to the cylindrical region 16. The tapered region 26 has a taper region starting point 48 adjacent to the connection section 40.

Figure 2 shows an enlargement of the drill bit 2 in the area of the connection section 40 according to one embodiment of the present invention. In this embodiment the internal surface 10 of the connection section 40 has a first curved connection region 42 having a first radius, CRi, wherein its tangent CTi to a central point along the first curved connection region 42 is positioned at a first distance, CDi, from the taper region starting point 48.

Figure 3 shows an alternative embodiment wherein the internal surface 10 of the connection section 40 has a first curved connection region 42 having a first radius, CRi, wherein its tangent CTi to a central point along the first curved connection region 42 is positioned at a first distance, CDi, from the taper region starting point 48 and a second curved connection region 44 having a second radius, CR2, wherein its tangent CT2 to a central point along the second curved connection section 44 is positioned a second distance, CD2, from the taper region starting point 48. In the present invention CD2< CDi and CR2 > CRi, in other words the curved connection section having the smaller radius is adjacent to the cylindrical region 16. CRi is between 1-160 mm, preferably between 5-120 mm. CR2 is between >1 - 160 mm, preferably between 5 - 120 mm. In this embodiment the first curved connection region 42 is directly connected to the second curved connection region 44 and the second connection region 44 is directly connected to the straight tapered region 26.

Figure 4 shows an alternative embodiment, wherein the first curved connection region 42 is connected to the second curved connection region 44 via an intermediate straight section 34.

Figure 5 shows an alternative embodiment, wherein the connection section 40 additionally comprises a third curved connection region 46. The third curved connection region 46 has a third radius, CR3, wherein its tangent, CT3, to a central point along the first curved connection region 46 is positioned at a third distance, CD3, from taper region start point 48, wherein CD3 < CD2. CR3 will be different to CRi and CR2, preferably CR3 > CR2. Optionally, the connection section 40 could also comprise more than three curved connection sections, each having a different radius. Each of the curved connection sections could either be directly connected to each other could be connected to each other via an intermediate straight section 34.

Figure 6 shows an alternative embodiment wherein there is additionally a curved transition section 32 that is positioned in-between the axial end 18 of the drill bit 2 and the straight tapered region 26. Figure 7 shows the von Mises equivalent stress taken of the conical region 14 of a prior art drill bit, wherein there is no curved connection region. Figure 8 shows the von Mises equivalent stress taken of the conical region 14 for an inventive drill bit, wherein there is a first curved connection region. In the models one rod and one bit are included and the models are axi-symmetry. An axial load of 190 kN is applied on the end of the rod and the bit face is fully supported. An elastic material with Young's modulus of 206 GPa, Poisson's ratio of 0.3 and a volumetric mass density of 7800 kg/m ³ were used. The implicit solver in LS-Dyna R10 were used to solve the models and the images of the von Mises equivalent stress were taken at the maximum load and HyperView 2019 were used as post-processor. It can be seen that the stress in cavity 8 of the drill bit 2 is reduced for the inventive drill bits compared to the prior art drill bit.