Coupling Between a Drill Bit and a Drill Shaft

The invention relates to a device for obtaining in two end positions located in angular distance from each other a rigid torque connection in one rotation direction between a drill shaft and a drill bit being in opposite direction freely rotatable on the drill shaft between said end positions, whereby the drill shaft comprises a first abutting means in the first end position being engaged in a first stop means arranged within the drill bit, and a second abutting means in the second end position being engaged in a second stop means arranged in the drill bit.

Couplings of this kind are used between a drill bit and the means transferring energy to the drill bit, when drilling a drill hole, whereby simultaneously the drill hole is lined with a lining tube. The drill bit can by the coupling be turned between a retracted position, whereby it is totally included in the lining tube, and an extended working position, whereby it in part is radially located outside the lining tube. The drill force is via the coupling transferred to the drill bit. As a result of the heterogeneity of the material, the coupling is exposed to considerable varying stresses in the direction of rotation as well as in the direction of the shaft. In percussion drilling the stresses in shaft direction occurring on impact and the arising recoil become particularly large. Admittedly, earlier known couplings allow that the drill bit can be freely turned from the torque transmitting working position to the retracted position, but, in order to meet these requirements, it could not be avoided that there exists a lash between the force transferring coupling parts. This lash has--besides the consequence of an increased impact stress and a wear of material--also led to an overheating of the coupling details with still heavier wear of material and a final collapse as the result. It has been tried to avoid this negative effect by increasing the dimensions of the coupling and hereby to reduce the surface stresses. This measure did not lead to the desired result.

The purpose of the present invention is to eliminate the drawbacks mentioned above and to achieve, at least in one rotation direction, a reliable and durable coupling--free from play--of the drill bit to the drill shaft, whereby simultaneously the drill bit in opposite rotation direction can be freely turned in relation to the drilling rod to a second end position on the drill shaft.

This is solved thereby that the first abutting means of the drill shaft comprises a helical first abutting surface arranged at the circumference, and that the first stop means of the drill bit comprises a helical first stop surface arranged at the interior surface and parallel to the first abutting surface of the shaft, which stop surface in the first end position is engaged in the first abutting surface of the drill shaft, and that limiting means are provided on the drill bit and the drill shaft in order to define in both shaft directions the axial range of movement of the drill bit along the drill shaft.

Further properties and advantages of the invention can be seen from the example of the embodiment, which in the following is described with reference to attached drawings, which in

FIG. 1 show a section of a drill bit in extended working position on the end of a drill shaft

FIG. 2 in a section show the drill bit in retracted position on the drill shaft.

The drill hole 1, which is drilled by the drill bit 7, is during the drilling lined with a lining tube 2, which follows the drill bit 7 down into the hole 1 and prevents a collapse of the drill hole wall. A guide 4 having a contact surface 5 that rests against the inside of the lining tube 2, connects the drill bit 7 with an energy transferring means 3, for instance a drill rod or a sinker bore hammer. The guide 4 continues in a shaft end 6 on which the drill bit 7 is mounted in the form of a hub enclosing the shaft 6. The drill bit 7 is fixed on the shaft 6 by aid of a pin 8 firmly mounted in the drill bit 7, which pin laterally protrudes into a notch 9 extending around the shaft 6. The pin and notch coupling allows the drill bit 7 to be freely turned on the shaft 6, whereby simultaneously the drill bit 7 is axially secured on the shaft 6. When there exists axial play between the pin 8 and the notch 9, the drill bit 7 has a limited axial range of movement on the drill shaft 6.

The drill shaft 6 is provided with a helical spline 10 having two parallel flanks 10a, 10b, located opposite to each other. The spline 10, which has an axial width b' and a pitch s, co-operates with a helical spline 11 shaped within the drill bit 7 and with the same pitch s as the spline 10 and with an axial width b". The spline 11 of the drill bit 7 comprises also two parallel flanks 11a, 11b, located opposite to each other. The pitch s is greater than the sum of the axial widths b'+b" of the splines 10, 11--in the example shown in the drawing about twice as great.

When the shaft 6 is rotated in the rotation direction 12, the flank 10b will softly bevel against the flank 11b on the spline 11 arranged in the drill bit 7 and turn the drill bit 7 being eccentrically retracted in the lining tube 5 to the extended working position shown in FIG. 1. Simultaneously, the axial component of the engagement force between the flanks 10b and 11b will actuate the drill bit 7 in a direction towards the drill rod 3. The shaft 6 is provided with one or several stop planes 14, 16, against which corresponding pressure planes 15, 17, arranged on the drill bit 7 will be pressed. The stop plane of the shaft 6 can consist of a flange plane 16, or of an end plane 14, or of both, whereby the drill bit 7 is provided with corresponding reaction surfaces, i.e. the annular end plane 17 or the bottom plane 15, respectively of the drill bit 7. On torque transmission the drill bit 7 will thus, free from play, be secured between the force transmission plane and a stop plane provided along the shaft 6, so that axial forces--drill percussions as well as recoils--can be carried without any occurrence of overheating phenomena and strong wear of material in the force transmission surfaces. The stop surfaces and the force transmission surfaces can moreover be made rather large, which further reduces the surface strain with a diminished material degradation as the result.

When the shaft 6 is turned in opposite direction 13, the spline flank 11b of the drill bit 7 is disengaged from the spline flank 10b of the drill shaft 6. The pitch s is of course so great that the joint does not become self-locking. Due to the fact that the pitch s is greater than the sum of the spline widths b'+b", the shaft 6 can be turned an angle before the spline flank 10a will get in contact with the spline flank 11a of the drill bit. The torsion restriction backwards need not necessarily be obtained by aid of a helical spline coupling, but can be performed by aid of co-operating projections on the shaft 6 and in the drill bit 7, or for instance thereby, that the notch 9 contains a stop shoulder, against which shoulder the pin 8 abuts, because merely an insignificant torque and no axial force need be transferred in this rotation direction 13.

If the spline flanks 10a, 11a and 10b, 11b, respectively constitute the sole torsional restriction and the drill bit 7 is axially fixed on the shaft 6, the free torsion angle

is obtained.

The drill bit 7 can thus be turned approximately 180.degree., when--as in the drawing figure--the sum of the spline widths amounts to half the pitch s. The free torsion angle can be increased by allowing the drill bit 7 a limited axial range of movement on the drill shaft 6, for instance thereby, that the pin 8 with rather large axial play is contained in the notch 9.

In order to increase the operating reliability under difficult conditions and to prevent an infiltration of external impurities, the mechanism is protected on the one hand by a shaft packing 19, and on the other hand by a non-return valve arranged in the scavenging air channel 18.

The present invention is of course not restricted to the shown example. For instance, several splines can be provided in the drill bit and on the shaft, i.e. there exist several entrances. The capacity of transferring the torque increases thereby, but this occurs at the cost of the free torsion angle.