Spline lubrication for DTH hammers

A down the hole drilling assembly has a top end coupled to a drill string and a bottom cutting end. The drilling assembly includes a piston arranged moveably inside a casing with a piston nose positioned at an axially bottom end and a top working chamber at a top end, a bottom working chamber at the bottom end, a driver sub having axially extending driver sub splines on an internal surface, a drill bit having an axially extending central bore including an elongate shank provided with axially extending shank splines on its outer surface for engagement with the driver sub splines to form a spline area, a guide sleeve forming a seal with the piston nose, the guide sleeve having an inner and outer surface, and at least one air passageway extending through the guide sleeve for fluidly connecting the bottom chamber to the spline area to provide lubrication thereto.

RELATED APPLICATION DATA

This application is a § 371 National Stage Application of PCT International Application No. PCT/EP2021/063275 filed May 19, 2021 with priority to EP 20175331.6 filed May 19, 2020.

TECHNICAL FIELD

The present invention relates to a down-the-hole hammer drill bit assembly arranged to provide improved spline lubrication.

BACKGROUND

Holes can be drilled in rock by means of various rock drilling assemblies. Drilling may be performed with a method of combining percussions and rotation. This type of drilling is called percussive drilling. Percussive drilling may be classified according to whether an impact device is outside the drill hole or in the drill hole during drilling. When the impact device is in the drill hole, the drilling is typically called down the hole (DTH) drilling. Since the impact device in the DTH drilling assembly is located inside the drill hole, the structure of the impact device needs to be compact.

The technique of DTH percussive hammer drilling involves the supply of a pressurised fluid via a drill string to a hammer located at the bottom of a bore hole. The fluid acts to both drive the hammer drilling action and to flush chips and fines resultant from the cutting action, rearwardly through the bore hole so as to optimise forward cutting.

The drilling assembly is provided with a reciprocating percussion piston, which is moved by controlling the feeding and discharging of pressurized fluid into and out of working chambers where the working surfaces of the piston are located. The piston is configured to strike a drill bit being connected directly to the drilling assembly. The most common way to provide rotational driving between the shaft of the drill bit and the driver sub is to use splines both on the exterior of the shaft and on the wall of the bore of the driver sub. It is important that these splines are lubricated, for example by a containing lubricant, in order to prevent galling which would result in damage to and the formation of cracks in the surfaces of the splines.

Traditionally, splines get lubricated via leakage of air from the working chambers. For DTH hammers that have a foot valve, the bottom chamber is sealed off from the foot valve and top diameter of the bit. This creates a buildup of pressure and consequently there will be some leakage of air which will flow into the spline area to ensure lubrication.

Foot valves are typically made of plastic and prone to breaking, therefore it is advantageous to avoid the inclusion of this part to reduce the downtime that would be required to replace broken parts. Therefore, in some DTH hammer designs the foot valve and piston cooperation of earlier designs has been replaced by the nose of the piston creating the sealing with the bore of a bushing. For example this is shown in EP2627850.

However, for DTH hammers that do not have a foot valve, as the bottom chamber sealing is done by the piston nose there is no buildup of high pressure anywhere on the outside surface of the bit. Only during chamber venting will there be air, and even then only for short period, creating some pressure on the outside of the bit, but because the bit center bore offers the path of least resistance this means only very minimal air flow is directed towards the splines, which does not provide sufficient lubrication for the splines. In foot valve less DTH hammers the guide bushing or the top end of the drill bit is provided with scallops to create an air passage for spline lubrication, however the lubrication provided via this means is not sufficient. Therefore, there is need, especially for larger drill bit sizes, to provide a foot valve less drilling bit assembly where there is an increased air flow to the splines in order to provide better lubrication to this region.

SUMMARY

It is an objective of this invention to provide a novel and improved percussive drilling assembly and apparatus for drilling rock whereby there is increased lubricated provided to the splines.

The objective is achieved by providing a down the hole drilling assembly comprising a down the hole drilling assembly having a top end arranged for coupling to a drill string and bottom cutting end, the drilling assembly comprising: an elongate casing; a fluid powered piston arranged moveably inside the casing which is capable of shuttling axially back and forth having a piston nose positioned at its axially bottom end; a top working chamber at the top end side of the piston and a bottom working chamber at the bottom end side of the piston; a driver sub provided with a set of axially extending driver sub splines on its internal surface; a drill bit having a central bore extending axially therethrough comprising an elongate shank provided with a set of axially extending shank splines on its outer surface for engagement with the driver sub splines to form a spline area; a guide sleeve for forming a first seal with the piston nose wherein the guide sleeve has an inner surface and an outer surface; characterized in that: the guide sleeve forms a second seal with the outer surface of the shank of the drill bit and wherein there is at least one air passageway extending through the guide sleeve and/or the casing for fluidly connecting the bottom chamber to the spline area to provide lubrication thereto.

Advantageously, this means that air is forced to the splines and therefore lubrication of the splines is improved. Consequently, galling is reduced and so the cracking and damage to the surfaces of the splines is minimized. Additionally the increased air flow to the spline area aids in flushing dirt and other debris away, which will improve the lifetime of the components.

Optionally, the first seal and/or second seal are strengthened with an additional sealing medium, such as a piston seal or rod seal. Advantageously, this will improve the strength of the sealing.

Optionally, the air passageway extends exclusively through the guide sleeve. Advantageously, this is easier to manufacture.

Preferably, the guide sleeve has a first section at its axial top end, a second section in axial central region and a third section at its axial bottom end and wherein the air passageway is formed from:at least one top end port located in the first section that extends from a first distal end on an inner surface of the guide sleeve to a second distal end on the outer surface of the guide sleeve and wherein the first distal end is fluidly connected to the bottom chamber;at least one channel positioned in the second section formed between an inner surface of the casing and the outer surface of the guide sleeve that is fluidly connected to the at least one top end port;at least one groove positioned in the outer surface of the third section or at least one bottom end port extending through the third section that is fluidly connected to the channel and the spline area.

Advantageously, this arrangement will provide a good air passageway from the bottom chamber to the spline area without compromising the effectiveness of the guide sleeve to provide alignment between the drill bit and the piston nose.

Preferably there are at least 3 top end ports. Advantageously, this will provide an increased flow of air to the spline area.

Preferably, the top end ports are evenly spaced around the circumference of the guide sleeve. Advantageously, this will provide a well distributed flow of air to the whole of the spline area.

Preferably, the at least one top end port projects at an angle such that the first distal end is nearer the top end of the guide sleeve and compared to the second distal end. Advantageously, this will provide the good fluid pathway for the air to flow from the bottom chamber to the channel in the second section of the guide sleeve.

Preferably, the at least one channel is formed by the outer surface of the guide sleeve being recessed radially inwardly around the entire circumference of the guide sleeve. Advantageously, this structure provides a good air passageway from the top end ports to the grooves or bottom end ports without compromising the strength and effectiveness of the guide sleeve to provide alignment between the drill bit and the piston nose and is easy to manufacture.

Alternatively, the at least one channel is formed by axial sections of the outer surface of the guide sleeve being recessed radially inwardly.

Preferably, there are at least 2 grooves or bottom end ports. Advantageously this will distribute the flow more evenly among the splines.

Alternatively, the air passageway extends through the guide sleeve and the casing. In which case, optionally the air passageway is formed from at least one top end port located in the first section of the guide sleeve and a recess on the inner side of the casing. This alternative could be used for designs where the guide sleeve is thin and therefore there is limited material thickness for the air passageway to extend exclusively through the guide sleeve.

Alternatively, the air passageway extends exclusively through the casing via the recess on the inner side of the casing. This alternative could be used for designs where the guide sleeve is thin and therefore there is limited material thickness for the air passageway to extend through the guide sleeve.

Optionally, the driver sub and/or drill bit has a slot adjacent to a gap between the shank of the drill bit and the driver sub for the air from the spline area to leak through to the outside of the assembly. Advantageously, this will increase the leakage flow and therefore the lubrication.

DETAILED DESCRIPTION

FIGS.1and2show a rock drilling rig1that comprises a movable carrier2provided with a drilling boom3. The boom3is provided with a rock drilling unit4comprising a feed beam5, a feed device6and a rotation unit7. The rotation unit7may comprise a gear system and at least one rotating motor. The rotation unit7may be supported by a carriage8with which it is movably supported to the feed beam5. The rotation unit7may be provided with drill string9which may comprise at least one drilling tube10connected to each other, and a DTH drilling assembly11at an outermost end of the drilling equipment9. The DTH drilling assembly11is located in the drilled bore hole12during the drilling.

FIG.2indicates a top end42or axially rearward end of the drilling assembly11and bottom end44or axially forward end of the drilling assembly. The DTH drilling assembly11comprises an impact device (not shown). The impact device is at the opposite end of the drill string9in relation to the rotation unit7. During drilling, a drill bit14is connected directly to the impact device, whereby percussions P generated by the impact device are transmitted to the drill bit14. The drill string9is rotating around its longitudinal axis in direction R by means of the rotation unit7shown inFIG.1and, at the same, the rotation unit7and the drill string9connected to it are fed with feed force F in the drilling direction A by means of the feed device6. Then, the drill bit14breaks rock due to the effect of the rotation R, the feed force F and the percussion P. Pressurized fluid is fed from a pressure source PS to the drilling assembly11through the drilling tubes10. The pressurized fluid may be compressed air and the pressure source PS may be a compressor. The pressurized fluid is directed to influence to working surfaces of a percussion piston19(shown onFIG.3) of the drilling assembly and to cause the piston19to move in a reciprocating manner and to strike against impact surface of the drill bit. After being utilized in working cycle of the drilling assembly11pressurized air is allowed to discharge form the drilling assembly11and to thereby provide flushing for the drill bit14. Further, the discharged air pushes drilled rock material out of the drill hole in an annular space between the drill hole and the drill string9. Alternatively, the drilling cuttings are removed from a drilling face inside a central inner tube passing through the impact device. This method is called reverse circulation drilling.

FIG.3shows a cross section of a DTH drilling assembly11. The drilling assembly11comprises an elongate casing15, which may be a relatively simple sleeve-like frame piece in the form of a substantially hollow cylinder. The drill bit14is at least partially accommodated within the bottom end44of the casing15. At a top end42of the casing15a top sub (or connection piece)80is mounted providing means for the drilling assembly11to be connected to a drill tube (not shown). The top sub80is at least partially accommodated within the top end42of the casing15. In connection with the top sub80is an inlet port18for feeding pressurized fluid to the impact device13. The inlet port18may comprise a valve means18a,which allows feeding of fluid towards the impact device but prevents flow in an opposite direction. The piston19, which is substantially an elongated cylinder extends axially within the casing15and is capable of shuffling back and forth longitudinally through the DTH drilling assembly11. The bottom end44of the piston19is arranged adjacent to the drill bit14. The drill bit14is provided with a central, axially extending, bore20forming a passageway for flushing medium to flow through. The central bore20has a centre line61.

At the top end42side of the piston19is a top working chamber21and at the opposite end, towards the bottom end44, is a bottom working chamber22. Movement of the piston19is configured to open and close fluid passages for feeding and discharging the working chambers21,22and to thereby cause the piston19to move towards an impact direction A and return direction B. At the bottom end44of the piston19is the piston nose24.

The drill bit14is provided with a plurality of tungsten carbide inserts66. The drill bit14is formed with an axially extending shank29. The shank29is provided with a set of axially extending shank splines31on its outer surface. Rotational force is applied to the drill bit14through a hollow, cylindrical driver sub34(otherwise known as the chuck), which is also provided with a set of axially extending driver sub splines30on its inner surface which engage with the shank splines31to transmit rotational drive from the driver sub34to the drill bit14. The region where the driver sub splines30and the shank splines31engage is referred to as the spline area32. Air needs to be delivered to the spline area32to provide lubrication thereto.

The assembly further comprises a bit retaining ring36, which is typically formed in two half annular parts for ease of assembly which functions to prevent the drill bit14from disengaging with the remaining components of the drilling assembly11, such as the casing15.

A guide sleeve23(otherwise known as a bushing or guide bushing), which is used in place of a foot valve, is arranged to co-operate with the piston nose24. The guide sleeve23is positioned radially inward and adjacent to the casing15. The piston nose24is able to pulse in and out of the guide sleeve23at its top end42and the shank29of the drill bit14is partially enclosed inside the guide sleeve23at its bottom end44. The purpose of the guide sleeve23is to align the drill bit14with the piston nose24to help stabilise, guide and provide a timing event for the piston19.

FIG.4shows an enlargement of the cross section of the drilling assembly in the region of the guide sleeve23. A first seal25is formed between the guide sleeve23and the piston nose24and a second seal28is formed between the guide sleeve23and the outer surface of the shank29and the drill bit14. Therefore a seal is created between the central bore20and the outer surface of the shank29. This means that the main air flushing path (through the central bore20) is separated from the spline area32. Typically, the first and second seals25,28are created by having a tight clearance in these regions. Optionally, the first and/or second seals25,28can be strengthened by introducing an additional sealing medium, such as a polymer, a piston seal or rod seal or other suitable material. The guide sleeve23has an inner surface38which is adjacent to the piston nose24and an outer surface39, which is adjacent to the casing15. The guide sleeve23has been specially adapted to have an air passage55which allows air to flow directly from the bottom chamber22to the spline area32. The flow of air along the air passageway55from the bottom chamber22to the spline area32is indicated onFIG.4by arrows27.

The guide sleeve23comprises at least one air passageway55that fluidly connected the bottom chamber22to the spline area32to provide lubrication thereto. Preferably, the at least one guide sleeve23can be considered to be made up of three sections. In a first section56, at the top end42of the guide sleeve23, there is at least one top end port37that projects from a first distal end50on an inner surface38of the guide sleeve23, to a second distal end51on the outer surface39of the guide sleeve. Preferably, the at least one top end port37projects at an angle such that the first distal end50is nearer the top end42of the guide sleeve23and compared to the second distal end51. Preferably, there is more than one port37, such as 3 or more, or such as 4 or more, or such as 5 or more. The number and size of the top end port(s)37can be varied to facilitate the required volume of air being delivered to the spline area32. Preferably, the top end ports37are evenly spaced around the circumference of the guide sleeve23. In a second section57, at a central portion of the guide sleeve23, the outer surface39is scalloped or recessed radially inwardly so that at least one channel52is formed between an inner surface63of the casing15and the outer surface39of the guide sleeve23around either the entire circumference or in axial sections of the guide sleeve23, such that grooves are formed. The channel52is fluidly connected to the at least one top end port37. In a third section58, at the bottom end44of the guide sleeve23, there is at least one groove59in the outer surface39of the guide sleeve23. The at least one groove59extends axially along the outer surface39of the guide sleeve in the third section58to fluidly connect the channel52to the spline area32. Preferably, there is more than one groove59, such as at least 2 grooves, more preferably at least 3 grooves. The number and dimensions of the groove59can be varied to facilitate the required volume of air being delivered to the spline area35. In one embodiment the air passageway55is formed from the at least one top end port37, the at least one channel52and the at least one groove59.

FIG.5shows the guide sleeve23of the present invention more detail.

Alternatively, the at least one top end port37in the first section56could be replaced by a passageway between the casing15and the guide sleeve23.

Alternatively, the at least one channel52in the second section57could be replaced by at least one axial hole projecting through the guide sleeve23.

FIG.6shows that alternatively, the at least one groove59in the third section58could be replaced by at least one bottom end port62.

The number of top end ports37in the first section56could be the same or different to the number of grooves59or bottom end ports62in the third section58.

As the piston nose24moves out of the guide sleeve23the bottom chamber22is vented and so all air passes through the central bore20. As the piston nose24moves into the guide sleeve23, just before the striking point, pressurized air in the bottom chamber22becomes fluidly connected to the air passageway55via the at least one top end port37. The design of the piston nose24can also be used to control the injection of air to the spline are32.

Once the air has passed through the spline area32it will leak to the outside of the assembly11through a gap64between the shank29on the drill bit14and the driver sub34. Additional flow area can be provided either by adding a slot65adjacent to the gap64on the driver sub34, as shown inFIG.7or on the drill bit14as shown inFIG.8or a combination of both to further increase the leakage and therefore further increase the lubrication.

FIG.9shows that alternatively the air passageway55may extend partially through the guide sleeve23and partially through the casing15. For example, the air passageway55may be formed from at least one top end port37located in the first section56of the guide sleeve23and a recess60on the inner side of the casing15.

FIG.10shows that alternatively the air passageway55may extend entirely and exclusively through the casing15to fluidly connect the bottom working chamber22to the spline area32via the recess60on the inner side of the casing15.