Patent Publication Number: US-10316586-B1

Title: Pressurized fluid flow system for a DTH hammer and normal circulation hammer thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable 
     REFERENCE TO A “SEQUENCE LISTING” 
     Not applicable 
     BACKGROUND OF THE INVENTION 
     Field of Application of the Invention 
     The present invention relates generally to pressurized fluid flow systems for percussive mechanisms operating with said fluid, particularly for DTH (Down-The-Hole) hammers and more particularly for normal circulation DTH hammers, and to DTH hammers with said systems. 
     STATE OF THE ART 
     DTH Hammers 
     A numerous variety of percussive drilling mechanisms exist which use a pressurized fluid as the means for transmitting power. Among these are DTH hammers which are widely used in the drilling industry, in mining as well as civil works and the construction of water, oil and geothermal wells. The DTH hammer, of cylindrical shape, is used assembling it on a drill rig located at ground surface. The drill rig also comprises a drill string comprising rods assembled together, the rear end, understood as the end that is farther to the hammer drill bit (element described further along in these specifications), being assembled to a rotation and thrust head and the front end, understood as the end that is closer to the hammer drill bit, coupled to the hammer. Through this drill string the drill rig supplies the necessary pressurized fluid to the hammer for the hammer to operate. 
     Parts of the DTH Hammer 
     The main movable part of the hammer is the piston. This member of the hammer has an overall cylindrical shape and is coaxially and slidably disposed in the inside of a cylindrical outer casing. When the hammer is operative in the mode known as “drilling mode”, the piston effects a reciprocating movement due to the change in pressure of the pressurized fluid contained in two main chambers, a front chamber and a rear chamber, formed inside the hammer and located at opposite ends of the piston. The piston has a front end in contact with the front chamber and a rear end in contact with the rear chamber, and has outer sliding surfaces or sliding sections of the outer surface of the piston (as opposed to sections with recess areas, grooves or bores) and inner sliding surfaces or sliding sections of the inner surface of the piston (again as opposed to sections with recess areas, grooves or bores). The outer sliding surfaces are mainly designed for ensuring guidance and alignment of the piston within the hammer. Besides, in most hammers these surfaces, together with the inner sliding surfaces of the piston, in cooperation with other elements as described further along in these specifications, permit control of the alternate supply and discharge of pressurized fluid into and from the front and rear chambers. 
     The foremost part of the hammer, which performs the drilling function, is known as the drill bit and it is slidably disposed on a driver sub mounted in the front end of the outer casing, the drill bit being in contact with the front chamber and adapted to receive the impact of the front end of the piston. 
     In order to ensure the correct alignment of the drill bit with respect to the outer casing, a component known as drill bit guide is normally used, which is disposed in the inside of the outer casing. The rotating movement provided by the drill rig is transmitted to the drill bit by means of fluted surfaces or splines in both the rearmost part o the drill bit (or shank) and the driver sub. In turn the drill bit head, of larger diameter than the outer casing and than the drill bit shank and driver sub, has mounted therein the cutting elements that fulfill the drilling task and extend forward from the drill bit front face. The movement of the drill bit is limited in its rearward stroke by the driver sub and in its forward stroke by a retaining element especially provided for said purpose. At the rear end of the hammer a rear sub is provided that connects the hammer with the drill string and ultimately to the source of pressurized fluid. 
     In the above description and that one hereinafter provided, the rear end of the hammer is understood to be the end where the rear sub is located and the front end of the hammer, the end where the drill bit is located. 
     Operation of the Hammer 
     When the hammer operates in the so called “drilling mode”, which is explained further along, the front and rear chambers undergo the following states:
         a—supply of pressurized fluid, wherein the fluid coming from the source of pressurized fluid is free to flow into the chamber;   b—expansion or compression, depending on the direction of the piston&#39;s movement, wherein the chamber is tightly sealed and the volume it encloses increases or decreases;   c—discharge of pressurized fluid, wherein the fluid coming from the chamber is free to flow towards the bottom of the hole; this discharge flow enables flushing of the rock cuttings generated by the drill bit, dragged in suspension in the pressurized fluid flow, towards the ground surface (process known as flushing of the hole).       

     In accordance with the piston&#39;s reciprocating movement, starting from the position in which the piston is in contact with the drill bit and the latter is disposed at the rearmost point of its stroke (position known as impact position), and ending in the same position (with the impact of the piston over the drill bit), the respective sequence for the states of the front and rear chambers are the following: [a−b(expansion)−c−b(compression)−a] and [c−b(compression)−a−b(expansion)−c]. The transition from one state to the other is independent for each chamber and is controlled by the position of the piston with respect to other parts of the hammer in such a way that the piston acts in itself as a valve, as well as an impact element. 
     In a first operative mode or “drilling mode”, when pressurized fluid is supplied to the hammer and the hammer is in the impact position, the piston immediately begins the reciprocating movement and the drill bit is impacted in each cycle by the piston, the front end of the drill bit thereby performing the function of drilling the rock at each impact. The rock cuttings are exhausted to the ground surface by the pressurized fluid discharged from the front and rear chambers to the bottom of the hole. As the depth of the hole increases, the magnitude of the pressurized fluid column with rock cuttings also increases, producing a greater resistance to the pressurized fluid discharge from the chambers. This phenomenon negatively affects the drilling process. In some applications the leakage of water or any other fluid into the hole increases even more this resistance, and the operation of the hammer may cease. 
     In some hammers, this operative mode of the hammer can be complemented with an assisted flushing system which allows discharge of part of the flow of pressurized fluid available from the source of pressurized fluid directly to the bottom of the hole without passing through the hammer cycle. The assisted flushing system allows the hole to be cleaned thoroughly while it is being drilled. 
     In a second operative mode of the hammer or “flushing mode”, the drill string and the hammer are lifted by the drill rig in such a way that the drill bit loses contact with the rock and all the pressurized fluid is discharged through the hammer directly to the bottom of the hole for cleaning purposes without going through the hammer cycle, thus ceasing the reciprocating movement of the piston. 
     The pressurized fluid coming from the assisted flushing system has an energy level substantially similar to that of the pressurized fluid coming out from the source of pressurized fluid, as opposed to what happens with the pressurized fluid exhausted from the chambers, which is at a pressure substantially lower due to the exchange of energy with the piston. 
     Industrial Applications 
     These drilling tools are used in two fields of industrial application: 
     1) Production, where a kind of hammer known as “normal circulation hammer” is used, wherein the rock cuttings produced during the drilling operation are flushed to the ground surface through the annular space defined by the wall of the hole and the outer surface of the hammer and the drill string, producing wear on the outer surfaces of the hammer and the drill string by the action of said cuttings. The pressurized fluid coming from the chambers and from the assisted flushing system is discharged through a central passage inside the drill bit which extends from its rear end to its front end. This passage may be divided into two or more passages ending in the front face of the drill bit in such a way that the discharge of the pressurized fluid is mainly generated from the center and across the front face of the drill bit towards the peripheral region of the same and towards the wall of the hole, and then towards the ground surface along the annular space between the hammer and the wall of the hole and between the drill string and the wall of the hole. The rock cuttings are exhausted by drag and are suspended in the pressurized fluid discharged to the bottom of the hole. 
     Normal circulation hammers are used in mining in underground and surface developments. Due to their ability to drill medium to hard rock, the use of this type of hammers has also extended to the construction of oil, water and geothermal wells. In general the soil or rock removed is not used as it is not of interest and suffers from contamination on its path to the surface. 
     2) Exploration, where a kind of hammer known as “reverse circulation hammer” is used, which allows the rock cuttings from the bottom of the hole to be recovered at the ground surface by means of the pressurized fluid discharged to the bottom of the hole. The pressurized fluid coming from the chambers is discharged along the peripheral region of the front end of the drill bit, therefore producing a pressurized fluid flow across the front face of the drill bit towards the inside of a continuous central passage formed along the center of the hammer, typically through an inner tube known as sampling tube extending from the drill bit to the rear sub, and through the double walled rods that conform the drill string. This central passage begins in the inside of the drill bit at a point where two or more flushing passageways originated in the front face of the drill bit converge. The rock cuttings are dragged towards the central passage by the action of the pressurized fluid, said rock cuttings being recovered at the ground surface. The pressurized fluid flow with suspended rock cuttings produce wear on the inner surfaces of all the elements that form said central passage. 
     Either, the drill bit or a cylindrical sealing element of the hammer which has a diameter substantially similar to the diameter of the drill bit head and larger than the external diameter of the outer casing, performs the function of preventing the leakage of pressurized fluid and rock cuttings into the annular space between the hammer and the wall of the hole and between the drill string and the wall of the hole when the hole is being drilled (as happens with a normal circulation hammer), forcing these cuttings to travel through the sampling tube and drill string to the ground surface by the action of the pressurized fluid. If it is the drill bit that performs this sealing function, it has a peripherial region that isolates the front face of the drill bit from said annular space. 
     The use of this type of drilling tool allows for the recovery of more than 90% of the rock cuttings, which do not suffer from contamination during their travel to the ground surface and are stored for further analysis. 
     Performance Variables 
     From the user&#39;s point of view, the variables used to evaluate the performance and usefulness of the hammer are the following: 
     1) rate of penetration, which is given by the power generated in the pressurized fluid cycle in the hammer and which value depends on two variables: the pressurized fluid consumption and the cycle&#39;s energy conversion efficiency, this being defined as the power generated per unit of pressurized fluid mass consumed;
 
2) durability of the hammer related to wear induced by the pressurized fluid flow dragging rock cuttings toward the ground surface and the interaction between moving parts, the durability being strongly dependent on the characteristics of the rock cuttings, the materials used to manufacture the hammer parts and the thickness of the parts in contact with the pressurized fluid flow;
 
3) consumption of pressurized fluid, which is strongly dependent on the passive volume of the front chamber, the passive volume of the rear chamber and the design of the pressurized fluid cycle of the hammer;
 
4) deep drilling capacity, which depends on the ability of the hammer to deliver pressurized fluid with a high level of energy to the bottom of the hole;
 
5) manufacturing costs, which depend on manufacturing complexity, the amount of components of the hammer and the amount of raw material used;
 
6) reliability of the hammer, which depends on the quality of the manufacture process and the sturdiness of the design of the tool; and
 
7) rock cuttings recovery efficiency (only for reverse circulation hammers), which is mainly related with the capacity of the hammer to seal the hole and prevent the leakage of pressurized fluid and rock cuttings to the annular space formed between the hammer and the wall of the hole and between the drill string and the wall of the hole.
 
     It should be noted that the rate of penetration, durability of the hammer, pressurized fluid consumption, reliability of the hammer and deep drilling capacity are factors that have direct incidence in the operational cost for the user. In general, a faster and reliable hammer having a useful life within acceptable limits will always be preferred for any type of application. 
     Pressurized Fluid Flow Systems 
     Different pressurized fluid flow systems are used in hammers for the process of supplying the front chamber and the rear chamber with pressurized fluid and for discharging the pressurized fluid from these chambers. In all of them there is a supply chamber formed inside the hammer from which, and depending on the position of the piston, the pressurized fluid is conveyed to the front chamber or to the rear chamber. In general, the piston acts as a valve, in such a manner that depending on its position is the state in which the front and rear chambers are, these states being those previously indicated: supply, expansion-compression and discharge. 
     At all times the net force exerted on the piston is the result of the pressure that exists in the front chamber, the area of the piston in contact with said chamber (or front thrust area of the piston), the pressure that exists in the rear chamber, the area of the piston in contact with said chamber (or rear thrust area of the piston), the weight of the piston and the dissipative forces that may exist. The greater the thrust areas of the piston, the greater the force generated on the piston due to a certain pressure level of the pressurized fluid and greater the power and energy conversion efficiency levels which can be achieved. 
     All the prior art pressurized fluid flow systems described in the following paragraphs are described with regard to the solutions for controlling the state of the front and rear chambers of a DTH hammer. The examples described refer to normal circulation hammers but they are equally applicable to reverse circulation hammers. 
     Type A Flow System, Represented by U.S. Pat. Nos. 4,084,646, 5,944,117 and 6,135,216 
     The designs described in these patents comprise a cylinder mounted inside the outer casing, the cylinder creating a fluid passageway between the outer surface of said cylinder and the inner surface of the outer casing. This fluid passageway extends along the rear half of the piston and ends in the supply chamber, which is partially defined by the outer sliding surface of the piston, near its middle point, and the inner surface of the outer casing. The provision of this cylinder requires the use of a dual outer diameter piston, the outer diameter of the same being greater at its front end and smaller at its rear end where the cylinder is placed. 
     The region where the piston&#39;s outer diameter changes, i.e. where there is a shoulder on the outer sliding surface of the piston, is subject to a pressure equal in average to the supply pressure of the hammer. Therefore, on each cycle the net work exerted by this region on the piston is null, i.e. it does not contribute with the energy transfer process to the piston, resulting in a reduced rear thrust area. 
     Moreover, in the normal or reverse circulation hammers with this type of flow system, an air guide is provided for controlling the discharge of the rear chamber, the air guide being a tubular element coaxial with the piston and the outer casing and located at the rear face of the rear chamber. Also, a footvalve is provided in order to control the discharge of the front chamber, the footvalve being a hollow tubular element coaxial with the piston and the outer casing and emerging from the rear face of the drill bit, known as impact face. 
     The above requires the use of a piston with a central bore, the bore extending along its entire length and interacting with the air guide and with the footvalve, This central bore reduces even more the rear thrust area and the front thrust area of the piston, which causes as a result a cycle of even less power. 
     Moreover, the alignment of the cylinder is a frequent problem in this type of design, which if is notaddressed, induces dissipative forces that drain power from the hammer&#39;s cycle. 
     Type B Flow System, Represented by U.S. Pat. Nos. 5,984,021, 4,312,412 and 6,454,026 
     The designs described in these patents comprise a pressurized fluid supply tube (inside of which the supply chamber is generated), which extends from the rear face of the rear chamber and is received inside a central bore in the piston. This bore extending along the whole length of the piston. 
     In order to control the supply of the front chamber and of the rear chamber with pressurized fluid and control the discharge of the rear chamber, the supply tube interacts with bores and undercuts inside the piston. 
     Undercuts on the outer sliding surface of the piston and on the inner surface of the outer casing complement the piston&#39;s control of the state of the chambers. Further, the discharge of the front chamber is controlled by a footvalve formed in the drill bit (U.S. Pat. Nos. 5,984,021 and 4,312,412) or alternatively by a front portion of the piston of smaller diameter that interacts with a piston guide (U.S. Pat. No. 6,454,026). This last solution can also be used as an alternative to the footvalve in the Type A flow system and in the rest of the flow systems which will be described hereinafter. 
     The presence of bores across the piston weakens the impact strength of this part of the hammer and implies a more complex manufacturing process. From this point of view, hammers with the Type A flow system have a stronger piston and a simpler manufacturing process than the hammers with the Type B flow system. In addition, the creation of the supply chamber inside the supply tube produces a delay in the initiation of the flow when the supply of pressurized fluid to the chambers is enabled, due to the distance between the former and the latter. The bores also cause an increment in the passive volumes of the chambers, being the main consequence of this a rise in the consumption of pressurized fluid and a reduction in the energy efficiency conversion in the thermodynamic cycle. 
     In the particular case of hammers that have a piston with a front portion of smaller diameter that interacts with a piston guide, the front thrust area of the piston is highly reduced due to the fact that a sufficiently large impact area is still required in order to withstand the stress generated by the impact, thus taking away surface from the front thrust area. 
     Moreover, the provision of a supply tube requires the use of a piston having a central bore extending along its entire length, resulting in the effects on power already mentioned for the Type A system. 
     Type C Flow Systems, Represented by the U.S. Pat. No. 4,923,018 
     The design described in this patent has three different sets of supply passages built in the outer casing. The first set of passages end at the inner surface of the outer casing and create a supply chamber between the outer sliding surface of the piston and the inner surface of the outer casing. The second and third sets of passages allow for the flow of pressurized fluid from the supply chamber toward the front chamber and toward the rear chamber respectively. In order to control the supply of pressurized fluid to the front chamber and to the rear chamber, the supply chamber interacts with recesses in the outer sliding surface of the piston and with the second and third sets of passages in the outer casing, while the discharge of the front chamber and the rear chamber are respectively controlled with the use of a footvalve and an air guide (refer to the Type A flow system applied to a normal circulation hammer). 
     The main disadvantages of this design is the addition of passive volume due to the presence of the second and third sets of passages and the fact that these passages significantly reduce the useful life of the outer casing which is largely dependent on the thickness of its wall. Also, the provision of an air guide and footvalve requires the use of a piston having a central bore extending along its entire length, resulting in the effects on power already mentioned for the Type A system. 
     Type D Flow System, Represented by U.S. Pat. Nos. 5,113,950 and 5,279,371 
     In the designs described in these patents a supply chamber is provided in the rear end of the piston, the designs have similar characteristics to the Type A and Type B flow systems. The Type D flow system uses a central supply tube as in the Type B flow system, but differs from the latter in that the supply chamber is not created inside the supply tube. Instead, similarly to the Type A flow system, the supply chamber is created and acts on a portion of the rear end of the piston. In this manner the supply tube performs the function of helping to convey the pressurized fluid toward the supply chamber and does not participate in its creation. All this produces as a consequence a reduction in the piston&#39;s rear thrust area. Moreover, the need to discharge the rear chamber requires the use of a piston with a central bore that emerges on the front face of the same, thus reducing even more the rear thrust area and the front thrust area of the piston, which results in a cycle of even less power. 
     Further, in U.S. Pat. No. 5,113,950 the presence of recesses and bores through the piston weaken the impact strength of this component. 
     Type E Flow System, Represented by U.S. Pat. Nos. 8,640,794 and 7,921,941 
     The designs described in these patents comprise a cylinder mounted inside the outer casing, the cylinder creating a supply chamber for supplying pressurized fluid to the front chamber and to the rear chamber of the hammer, and a discharge chamber for discharging pressurized fluid from the front chamber and from the rear chamber. In this design, the supply and discharge chambers are defined by respective recesses, disposed in series longitudinally, on the inner surface of the outer casing. 
     In these designs the flow of pressurized fluid into and out of the front and rear chambers is controlled solely by the overlap or relative position of the multiple outer sliding surfaces of the piston and the inner surface of the cylinder during the alternating movement of the piston. The former represents and advantage because no aligment problems must be expected as far as the piston only slides within the cylinder. But these designs require that the piston be provided with multiple fluid-conducting means for supplying pressurized fluid to the front chamber and to the rear chamber of the hammer, and for discharging pressurized fluid from the front chamber and from the rear chamber. These multiple fluid-conducting means represents a disadvantage as far as the presence of bores across the piston weakens the impact strength of this part of the hammer and implies a more complex manufacturing process. The bores also cause an increment in the passive volumes of the chambers, being the main consequence of this a rise in the consumption of pressurized fluid and a reduction in the energy conversion efficiency in the thermodynamic cycle. 
     In the following paragraphs the different known pressurized fluid flow systems are described for the specific case of reverse circulation hammers, with regard to the solutions for conveying the pressurized fluid discharged from the front chamber and from the rear chamber to the bottom of the hole, specifically to the periphery of the front face of the drill bit, for flushing of rock cuttings. 
     Type 1 Flow System, Represented by the U.S. Pat. Nos. 5,154,244, RE36,002(US), 6,702,045 and 5,685,380. 
     These patents describe a flow system where the pressurized fluid is conveyed from the rear end of the drill bit to the front end of the same by means of channels formed in a cooperative way by splines machined on the inner surface of the driver sub and splines machined on the outer surface of the drill bit shank, and with a ring or sleeve acting as sealing element, so as to create enclosed passages through which the pressurized fluid is discharged to the periphery of the front end of the drill bit. 
     In a variant of the former solution, described in U.S. Pat. No. 6,702,045, a flow system is shown where the pressurized fluid is conveyed from the rear end of the drill bit up to an intermediate point on the outside of the same by means of channels created on the outer surface of the drill bit. These channels cooperatively work with the splines of the driver sub to create enclosed passages. From this intermediate point the flow of pressurized fluid is deviated through bores in the driver sub to a passage formed between the outer surface of the driver sub and the inner surface of the sealing ring or sleeve in such a manner as to discharge the pressurized fluid at the peripheral region of the front end of the drill bit. 
     From the point of view of the control of the state of the front and rear chambers, commercial designs from these patents are of the Type A and Type D flow systems. As with the Type B flow system, a front region of the piston of smaller diameter that interacts with a piston guide is used as an alternative solution to the footvalve for controlling the discharge of the front chamber. The discharge of the rear chamber is controlled by means of an air guide that opens or blocks the flow of pressurized fluid from the rear chamber to a central coaxial channel formed between the inner sliding surface of the piston and the outer surface of the sampling tube, this passage extending from the rear chamber to the rear end of the drill bit. 
     The disadvantages of this flow system are the same ones as those associated with the Type A and Type D flow systems and, in particular, impact negatively the design of the drill bit in two aspects. The first one is the need for a multiplicity of manufacturing processes for producing the channels in the outer surface of the drill bit, which increases the manufacturing cost of the hammer. The second is that, due to the presence of these channels, the drag surface of the splines, which depend on the contact area of each spline individually and the total number of splines, can in some applications be insufficient. This last problem can be counterbalanced by lengthening the drill bit, but this implies increasing the cost of the hammer. 
     Type 2 Flow System, Represented by U.S. Pat. Nos. 5,407,021 and 4,819,746 
     U.S. Pat. Nos. 5,407,021 and 4,819,746 describe a flow system where the pressurized fluid is conducted from the rear end of the drill bit up to an intermediate point on the outside surface of the same by means of channels formed in a cooperative way by splines machined on the inner surface of the driver sub and splines machined on the outer surface of the drill bit shank. From this intermediate point the flow of pressurized fluid is deviated through mainly longitudinal bores created in the head of the drill bit in such a way as to discharge the pressurized fluid at the peripheral region of the front end of the drill bit. 
     The bit head has the further function of avoiding the escape of pressurized fluid through the annular space formed between the hammer and the wall of the hole and between the rods and the wall of the hole. 
     From the perspective of controlling the state of the front and rear chambers, U.S. Pat. No. 4,819,746 has a Type A flow system. 
     In both patents, as an alternative solution to the foot valve for controlling the discharge of the front chamber, a front portion of the piston of a smaller diameter is used that interacts with a piston guide, as described in the Type B flow system. 
     The discharge of the rear chamber is controlled by an air guide (U.S. Pat. No. 4,819,746) which opens or closes the flow of pressurized fluid from the rear chamber to a central coaxial channel formed in between the inner sliding surface of the piston and the outer surface of the sampling tube, which extends up to the rear end of the drill bit. 
     The disadvantages in this case (U.S. Pat. No. 4,819,746) are the same as those of the Type A flow system and the design of the drill bit is also negatively impacted in the same two aspects already mentioned for the Type 1 flow system plus a third aspect. This third aspect is given by the mechanical weakness induced on the drill bit as a result of the mainly longitudinal bores made on the head of the drill bit for channeling the pressurized fluid and discharging it at the peripheral region of the front end of the drill bit so as to produce a flow of pressurized fluid from the periphery along the front face of the drill bit towards the inside of the central coaxial passage of the hammer and the rods. 
     OBJECTIVES OF THE INVENTION 
     According with the issues and technical antecedents stated, it is a goal of the present invention to present a pressurized fluid flow system that, applied to a normal circulation hammer, provides a better performance than the normal circulation hammers of the previous art, and that combined with drill bit pressurized fluid channeling means adapted to said system, provide an improved DTH normal circulation hammer. Specifically and without sacrificing useful life, it would be desirable to have a normal circulation hammer improved in the following aspects:
         a high power and high efficiency in the energy conversion process, which implies a higher penetration rate.   a structurally simpler design and reduced manufacturing cost.   a high reliability and sturdiness.       

     BRIEF SUMMARY OF THE INVENTION 
     With the purpose of providing an improved pressurized fluid flow system for a normal circulation DTH hammer according to the above-defined goals, a solution has been devised that makes an efficient use of the cross-sectional area of the hammer and employs fewer parts and is simpler to manufacture. 
     The pressurized fluid flow system of the invention is characterized by comprising a set of equal diameter outer sliding surfaces for the piston thus avoiding failure of this part due to thermal cracks induced by friction between the piston and misaligned parts (air guide, supply tube, foot valve, etc.). Moreover, the piston does not have holes, channels or passages, making it a completely solid component. 
     Specifically, the pressurized fluid flow system of the invention is characterized by having a cylinder coaxially disposed in between the outer casing and the piston; and two sets of channels, a set of supply channels and a set of discharge channels, delimited by the outer surface of the cylinder and the inner surface of the outer casing. The set of supply channels is permanently filled with fluid coming from the source of pressurized fluid and connected without interruption to the outlet of said source. The set of discharge channels is permanently communicated with the bottom of the hole drilled by the hammer. The supply channels are disposed in parallel longitudinally with respect to the discharge channels overlapping longitudinally and both sets of channels are defined by respective sets of recesses on the cylinder outer surface. 
     The piston has a recess on its external surface that defines, in cooperation with the inner surface of the cylinder, a supply chamber. The supply chamber is permanently connected without interruption to the set of supply channels. In this way, the supply chamber is permanently filled with fluid coming from the source of pressurized fluid and connected without interruption to the outlet of said source. 
     The flow of pressurized fluid supplied into and discharged from the front and rear chambers is controlled solely by the overlap or relative position of the outer sliding surfaces of the piston with the inner surface of the cylinder. 
     For channeling the pressurized fluid from the supply chamber to the front and rear chambers of the hammer, front and rear sets of recesses are provided on the cylinder. For channeling the pressurized fluid from the front and rear chambers to the set of discharge channels multiple discharge through-ports are provided in the cylinder. 
     Therefore, the flow of pressurized fluid into and out of the front and rear chambers takes place inbetween the inner surface of the cylinder and the outer surface of the piston. Further, the state of the front chamber and the rear chamber are controlled in the invention by the interaction of a single pair of components. 
     The above-mentioned configuration enables an optimal use of the cross sectional area of the hammer compared to prior art hammers. By disposing the set of supply channels longitudinally in parallel with the set of discharge channels it is possible to increase the front thrust area and the rear thrust area of the piston. 
     The front thrust area and the rear thrust area of the piston under the configuration of the invention are identical in size. Additionally, control of the discharge of the front chamber and the rear chamber by interaction between the piston and the cylinder makes it unnecessary to have either a foot valve or a front portion of the piston of smaller diameter interacting with a piston guide or an air guide for this purpose, thus avoiding the additional losses in the thrust areas as it occurs with the flow systems of the prior art. 
     Moreover, the fact that the flow of pressurized fluid into and out the front and rear chambers takes place inbetween the inner surface of the cylinder and the outer surface of the piston allows the latter to be completely solid, avoiding the need of holes, channels or passages that can weaken it, increase the front and rear chambers passive volumens, deteriorate the cycle efficiency and make the piston a more expensive part. 
     Even more, one or more flushing channels may be provided in the dividing walls that separates the set of supply channels and the set of discharge channels for permitting part of the flow of pressurized fluid available from the source of pressurized fluid to be discharged directly to the bottom of the hole, conforming in this fashion an assisted flushing system and enabling an increased deep drilling capacity without a noticeable reduction in the penetration rate. 
     The invention also refers to a normal circulation DTH hammer characterized by having the above-described pressurized fluid flow system and a drill bit in which the conventional central passage in the rear end thereof and the two or more passageways that converge to this central passage used in normal circulation hammers have been replaced by one or more flushing passages bored across the drill bit and extending from the channels which, as in the described Type 1 and Type 2 flow systems, are cooperatively formed by the splines on driver sub and on the drill bit shank, to the front face of the drill bit. This enables a simplified and sturdier drill bit for a normal circulation hammer. 
     By having the the invention a set of discharge channels delimited by the outer surface of the cylinder and the inner surface of the outer casing, i.e. adjacent to the inner surface of the outer casing, it is possible not only to divert the pressurized fluid flow exhausting from the set of discharge channels to the outside of the drill bit shank and towards the channels cooperatively formed between splines on the inner surface of the driver sub and splines on the outer surface of the drill bit shank, but also, the flow of pressurized fluid can then be discharged from these channels to the front end of the drill bit through the one or more flushing passages which are bored across the body of the bit and extend from said channels to the front face of the drill bit. 
     The mentioned characteristics of the bit plus the features previously described in relation to the pressurized fluid flow system strongly improve the reliability of the hammer. 
     For ease of understanding of the precedent ideas, the invention is described with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  depicts an elevation view of the normal circulation DTH hammer of the invention. 
         FIG. 2  depicts a bottom view of the normal circulation DTH hammer of the invention. 
         FIG. 3  depicts a longitudinal cross section view of the normal circulation DTH hammer of the invention specifically showing the disposition of the piston with respect to the outer casing, cylinder and drill bit when the front chamber is being supplied with pressurized fluid and the rear chamber is discharging pressurized fluid to the bottom of the hole. 
         FIG. 4  depicts a longitudinal cross section view of the normal circulation DTH hammer of the invention specifically showing the disposition of the piston with respect to the outer casing, cylinder and drill bit when the rear chamber is being supplied with pressurized fluid and the front chamber is discharging pressurized fluid to the bottom of the hole. 
         FIG. 5  depicts a longitudinal cross section view of the normal circulation DTH hammer of the invention specifically showing the disposition of the piston and the drill bit with respect to the outer casing and cylinder when the hammer is in flushing mode. The front set of recesses is depicted with a dashed line for a best understanding of its location respect to the piston. 
         FIG. 6  depicts an isometric view of the cylinder of the hammer of the invention. 
         FIG. 7  depicts a cross section view of the cylinder of  FIG. 6  for a best understanding of the different features of this element. 
     
    
    
     In all these figures, the flow system of the hammer has also been depicted with respect to the solution designed under the invention to convey the pressurized fluid to the bottom of the hole from the front chamber and rear chamber, in all the the modes and states, specifically to the front end of the drill bit for flushing the rock cuttings. The direction of the pressurized fluid flow has been indicated by means of arrows. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1 to 7 , a normal circulation DTH hammer is shown that comprises the following main components: 
     a cylindrical outer casing ( 1 ) having a rear end and a front end; 
     a driver sub ( 110 ) mounted to said front end of the outer casing ( 1 ) and having an inner surface ( 113 ) with splines ( 112 ) machined thereon; 
     a rear sub ( 20 ) affixed to said rear end of the outer casing ( 1 ) for connecting the hammer to the source of pressurized fluid; 
     a piston ( 60 ) slidably and coaxially disposed inside said outer casing ( 1 ) and capable of reciprocating due to the change in pressure of the pressurized fluid contained inside of a front chamber ( 240 ) and a rear chamber ( 230 ) located at opposites ends of the piston ( 60 ), the piston ( 60 ) having multiple outer sliding surfaces ( 64 ,  67 ); and 
     a drill bit ( 90 ) slidably mounted on the driver sub ( 110 ), the sliding movement of the drill bit ( 90 ) limited by the drill bit retainer ( 210 ) and the drill bit supporting face ( 111 ) of the driver sub ( 110 ), the drill bit ( 90 ) comprised of a drill bit shank ( 95 ) at the rear end of the drill bit and a drill bit head ( 96 ) at the front end of the drill bit, the drill bit head ( 96 ) being of bigger diameter than the drill bit shank ( 95 ) and having a front face ( 99 ), the drill bit shank ( 95 ) having an outer surface ( 98 ) with splines ( 93 ) machined thereon; 
     channels ( 97 ) cooperatively formed between the splines ( 112 ) on the inner surface ( 113 ) of the driver sub ( 110 ) and the splines ( 93 ) on the outer surface ( 98 ) of the drill bit shank ( 95 ). 
     The pressurized fluid flow system of the invention includes a cylinder ( 40 ) that is coaxially disposed in between the outer casing ( 1 ) and the piston ( 60 ), the cylinder ( 40 ) having an inner ( 47 ) and an outer surface ( 48 ). 
     The rear chamber ( 230 ) of the hammer is defined by the rear sub ( 20 ), the cylinder ( 40 ) and the rear thrust surface ( 62 ) of the piston ( 60 ). The volume of the rear chamber is variable depending on the piston&#39;s ( 60 ) position. The front chamber ( 240 ) of the hammer is defined by the drill bit ( 90 ), the cylinder ( 40 ), the drill bit guide ( 150 ) and the front thrust surface ( 63 ) of the piston ( 60 ). The volume of the front chamber is also variable depending on the position of the piston ( 60 ). 
     The piston ( 60 ) has an annular recess ( 68 ) on its external surface that defines, in cooperation with the inner surface ( 47 ) of the cylinder ( 40 ), a fluid flow supply chamber ( 66 ). This fluid flow supply chamber ( 66 ) is respectively longitudinally limited at each end by the outer sliding surfaces ( 64 ,  67 ) of the piston. 
     The cylinder ( 40 ) has a set of supply ( 2 ) and discharge ( 3 ) channels defined by respective longitudinal recesses on its outer surface ( 48 ), the supply ( 2 ) and discharge ( 3 ) channels disposed around said surface ( 48 ) for in the first case conveying pressurized fluid from the rear sub ( 20 ) to the supply chamber ( 66 ) and therefrom to the front ( 240 ) and rear ( 230 ) chambers and in the second case discharging the pressurized fluid from the front ( 240 ) and rear ( 230 ) chambers towards the channels ( 97 ) formed between the driver sub ( 110 ) and the drill bit shank ( 95 ) and therefrom towards the bottom of the hole drilled by the hammer. When the hammer is operative, the first of these sets of channels is in permanent fluid communication with the source of pressurized fluid and it is filled with said fluid while the second of these sets of channels is directly communicated with the bottom of the hole. 
     The cylinder ( 40 ) has rear pressurized fluid intake ports ( 41 ) bored therethrough, which connect the supply channels ( 2 ) with a supply undercut ( 21 ) in the rear sub ( 20 ), and has elongated front pressurized fluid exit ports ( 42 ) bored therethrough, which fluidly and uninterruptedly communicate the set of supply channels ( 2 ) of the cylinder with the supply chamber ( 66 ), therefore permanently filling it with high pressure fluid. The cylinder ( 40 ) also has rear ( 43 ) and front ( 44 ) discharge ports bored therethrough, which allow the pressurized fluid to respectively flow from the rear chamber ( 230 ) and front chamber ( 240 ) into the set of discharge channels ( 3 ). 
     The cylinder ( 40 ) further has a front set ( 45 ) and a rear set ( 46 ) of recesses on its inner surface for allowing the pressurized fluid which flows from the rear sub ( 20 ) to the supply chamber ( 66 ) through the set of supply channels ( 2 ) to respectively divert part of the flow to the front ( 240 ) and rear ( 230 ) chambers in cooperation with the multiple outer sliding surfaces ( 64 ,  67 ) of the piston ( 60 ). 
     Control of the State of the Front Chamber ( 240 ) 
     When in the hammer cycle the impact face ( 61 ) of the piston ( 60 ) is in contact with the impact face ( 91 ) of the drill bit ( 90 ) and the drill bit ( 90 ) is at the rearmost point of its stroke, i.e. the hammer is at impact position (see  FIG. 3 ), the front chamber ( 240 ) is in direct fluid communication with the supply chamber ( 66 ) through the front set of recesses ( 45 ) of the cylinder ( 40 ). In this way, the pressurized fluid is able to freely flow from the supply chamber ( 66 ) to the front chamber ( 240 ) and start the movement of the piston ( 60 ) in the rearward direction. 
     This flow of pressurized fluid to the front chamber ( 240 ) will stop when the piston ( 60 ) has traveled in the front end to rear end direction of its stroke until the point where the front outer supply edge ( 73 ) of piston ( 60 ) reaches the rear limit of the front set of recesses ( 45 ) of the cylinder ( 40 ). As the movement of the piston ( 60 ) continues further in the front end to rear end direction of its stroke, a point will be reached where the front outer discharge edge ( 72 ) of the piston ( 60 ) will match the front limit of the front discharge ports ( 44 ) of the cylinder ( 40 ). As the movement of the piston ( 60 ) continues even further, the front chamber ( 240 ) of the hammer will become fluidly communicated with the set of discharge channels ( 3 ) through the front set of discharge ports ( 44 ) of the cylinder ( 40 ) (see  FIG. 4 ). In this way, the pressurized fluid contained inside the front chamber ( 240 ) will be discharged into the set of discharge channels ( 3 ) and from the set of discharge channels ( 3 ) it is able to freely flow out of the hammer through the channels ( 97 ) cooperatively formed between the splines ( 93 ) of the drill bit shank ( 95 ) and splines ( 112 ) of the driver sub ( 110 ), and through the flushing passages ( 92 ) of the drill bit ( 90 ) to the front face ( 99 ) of the drill bit ( 90 ). 
     Normally, the drill bit ( 90 ) is aligned to the outer casing ( 1 ) of the hammer by a drill bit guide ( 150 ) having discharge grooves ( 151 ) as shown in the Figures. In the current invention these discharge grooves connect the set of discharge channels ( 3 ) with the channels ( 97 ), so that the discharge of pressurized fluid flows through these discharge grooves ( 151 ) before reaching the channels ( 97 ) and thereafter flows through the flushing passages ( 92 ) of the drill bit ( 90 ). However, the invention is not limited to the use of a drill bit guide and alternative alignment solutions may be used with corresponding pressurized fluid discharge means. 
     Control of the state of the rear chamber ( 230 ) 
     When in the hammer cycle the impact face ( 61 ) of the piston ( 60 ) is in contact with the impact face ( 91 ) of the drill bit ( 90 ) and the drill bit ( 90 ) is at the rearmost point of its stroke, i.e. the hammer is at impact position (see  FIG. 3 ), the rear chamber ( 230 ) is in direct fluid communication with the set of discharge channels ( 3 ) through the rear set of discharge ports ( 43 ) of the cylinder ( 40 ) (see  FIG. 3 ). 
     In this way, the pressurized fluid contained inside the rear chamber ( 230 ) will be discharged into the set of discharge channels ( 3 ) and from the set of discharge channels ( 3 ) out of the hammer and to the front face ( 99 ) of the drill bit ( 90 ) in a similar fashion as with the pressurized fluid discharged from the front chamber ( 240 ). 
     This flow of pressurized fluid will stop when the piston ( 60 ) has traveled in the front end to rear end direction of its stroke until the rear outer discharge edge ( 70 ) of piston ( 60 ) reaches the rear limit of the rear set of discharge ports ( 43 ) of the cylinder ( 40 ). As the movement of the piston ( 60 ) continues further in the front end to rear end direction of its stroke, a point will be reached where the rear outer supply edge ( 71 ) of the piston ( 60 ) matches the front limit of the rear set of recesses ( 46 ) of the cylinder ( 40 ) (see  FIG. 4 ). As the movement of the piston ( 60 ) continues even further, the rear chamber ( 230 ) of the hammer will become fluidly communicated with the supply chamber ( 66 ) through the rear set of recesses ( 46 ) of the cylinder ( 40 ). In this way, the rear chamber ( 230 ) will be supplied with pressurized fluid coming from the supply chamber ( 66 ). 
     Flushing Mode Operation 
     If the hammer is lifted in such a way that the drill bit ( 90 ) stops being in contact with the rock being drilled and the drill bit&#39;s retainer supporting shoulder ( 94 ) rests on the drill bit retainer ( 210 ), the drill bit ( 90 ) will reach the front end of its stroke and then the hammer switches to its flushing mode. In this position the percussion of the hammer stops, hence leaving the impact face ( 61 ) of the piston ( 60 ) resting on the impact face ( 91 ) of the drill bit ( 90 ) (see  FIG. 5  for illustration of the flushing mode description while features ( 61 ) and ( 91 ) are shown in  FIG. 4 ), and the pressurized fluid is conveyed directly to the front end of the drill bit ( 90 ) through the following pathway: into the set of supply channels ( 2 ) through the supply undercut ( 21 ) of the rear sub ( 20 ) and the rear pressurized fluid intake ports ( 41 ) of the cylinder ( 40 ), and from the set of supply channels ( 2 ) to the set of discharge channels ( 3 ) through the front pressurized fluid exit ports ( 42 ) of the cylinder ( 40 ), through the rear chamber ( 240 ), and through the rear set of discharge ports ( 43 ) of the cylinder ( 40 ). From the set of discharge channels ( 3 ) the pressurized fluid is able to freely flow out of the hammer and to the front face ( 99 ) of the drill bit ( 90 ) in a similar fashion as with the pressurized fluid discharged from the rear and front chambers ( 230 ,  240 ) when the hammer is in drilling mode.