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 of 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 while the end of the drill rod that should be received in the cavity is generally conical.

In known designs the exit of conical cavity of the drill bit has the form of a straight line with a chamfer. The problem with this is that high stresses are formed at the exit of the cavity in 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 drill bit in the region of the exit of the conical cavity in order to increase the performance and lifetime 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; characterised in that: the internal cavity has a transition section extending between the axial end of the drill bit and the a straight tapered region; wherein the transition section has at a first curved region having a first radius, TRi, wherein its tangent, TTi, to a central point along the first curved transition section is positioned at a first distance, TDi, from the axial end.

Advantageously, the inclusion of the curved transition section reduces the stress in the region of the exit of 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.

Preferably, the transition section additionally has a second curved section having a second radius, TR2, wherein its tangent, TT2, to a central point along the second curved transition section is positioned at a second distance, TD2, from the axial end; wherein TD2 > TDi and TR2 > TRi. 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, TRi is between 0.1 - 1.5. Advantageously, this provides an optimal balance between providing a tight connection between the conical rod and the conical cavity of the drill bit and also achieving a reduction in stress.

Preferably, TD2 is between 0.5 - 5.0 mm. Advantageously, this reduces the stress in the exit region of the cavity.

Preferably, TRi is between 0.1 - 1.0 mm. Advantageously, this provides optimal stress reduction.

Optionally, the first curved region connects directly to the second curved 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 a first 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 transition section further comprises a third curved region having a third radius, TR3, wherein its tangent, TT3, to a central point along the third curved transition section is positioned at a third distance, TD3, from the axial end, wherein TD3 > TD2 and TR3 > TR2. Advantageously, this design has low levels of the stress in the region of the exit of the conical cavity of the drill bit. Optionally, additionally there is a curved connection section between the straight tapered region and the cylindrical region. Advantageously, this provides additional stress reduction in the cavity of the drill bit.

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 transition section according to one embodiment wherein the transition section has a single radius.

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

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

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

Figure 6: Transition section according to one embodiment wherein there is additionally a curved connection 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 to a cup-shaped cylindrical region 16 that terminates the cavity 8 and is positioned adjacent to the bit section 6. The conical region 14 comprises a straight tapered region 26 positioned adjacent to the cylindrical region 16 and a transition section 32 that is positioned in-between the axial end 18 of the drill bit 2 and the straight tapered section 26.

Figure 2 shows an enlargement of the drill bit 2 in the area of the transition section 32 according to one embodiment of the present invention. In this embodiment the internal surface 10 of the transition section 32 has a first curved transition section 20 having a first radius, TRi, wherein its tangent TTi to a central point along the first curved transition section 20 is positioned at a first distance, TDi, from the axial end 18 of the drill bit 2.

Figure 3 shows an alternative embodiment, enlarged in the same way as in figure 2, wherein the internal surface 10 of the transition section 32 has a first curved transition region 20 having a first radius, TRi, wherein its tangent TTi to a central point along the first curved transition section 20 is positioned at a first distance, TDi, from the axial end 18 of the drill bit 2 and a second curved transition region 22 having a second radius, TR2, wherein its tangent TT2 to a central point along the second curved transition section 22 is positioned a second distance, TD2, from the axial end 18 of the drill bit 2. In the present invention TD2>TDI and TR2 > TRi. TRi is between 0.1-1.5 mm, preferably between 0.1-1.0 mm. TR2 is between 20-300 mm, preferably between 35-200 mm. TD2 is between 0.5-5.0 mm, preferably between 1.5-2.5 mm. In this embodiment the first curved transition region 20 is directly connected to the second curved transition region 22 and the second transition region 22 is directly connected to the straight tapered region 26.

Figure 4 shows an alternative embodiment, enlarged in the same way as in figure 2, wherein the first curved transition region 20 is connected to the second curved transition region 22 via an intermediate straight section 34.

Figure 5 shows an alternative embodiment, enlarged in the same way as in figure 2, wherein the transition section 32 additionally comprises a third curved transition region 24. The third transition region 24 has a third radius, TR3, wherein its tangent, TT3, to a central point along the third curved transition section 24 is positioned at a third distance, TD3, from the axial end 18 of the drill bit 2, wherein TD3 > TD2. TR3 will be different to TRi and TR2, preferably TR3 > TR2. Optionally, the transition section 32 could also comprise more than three curved transition sections, each having a different radius. Each of the curved transition 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 connection section 40 between the straight tapered region 26 and the cylindrical region 16.

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 transition region. Figures 8 shows the von Mises equivalent stress taken of the conical region 14 for an inventive drill bit, wherein there is transition section having a first and second curved transition regions. 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 exit region of the drill exit region of the cavity 8 is reduced for the inventive drill bit compared to the prior art drill bit.