DRILL STRING WITH INTERFERENCE FIT COUPLINGS AND COUPLERS THERFOR

Abstract: A threaded end of a drill rod is formed from a sleeve to one end of the body. The sleeve has a threaded bore to receive a threaded end of an adjacent component and is secured to the body of the rod by an interference fit between a bore and spigot on the two components.

Description: DRILL STRING WITH INTERFERENCE FIT COUPLINGS AND COUPLERS THEREFOR FIELD OF THE INVENTION The present invention relates to the securing of components in a drill string. BACKGROUND OF THE INVENTION In percussion drilling applications drill rods or components are joined together to provide a drill string, which is used to transmit impact energy generated by a drill hammer to a drill bit. A joint mechanism has to be provided to connect the rod(s) to the drill hammer, establish connection between each pair of adjacent rods, and to connect the drill string to the drill bit. This is typically done by employing a reusable coupling containing internal threads at either end. The internal threads are compatible with external threads located on each end of the drill rods. One disadvantage of this coupling design is that an excessive amount of time during the drill string setup must be expended in threading each threaded rod end to the intermediate threaded coupling and that two threaded sections are subjected to the drilling forces. In order to simplify the setup of a drilling string, the female half of the coupling sleeve can be made an integral part of one end of the rod, while the other end of the rod contains a corresponding threaded male half. These modified drill rods are typically referred to as "male-female" or M/F rods in the prior art. A joined pair of M/F rods has better energy transmission characteristics due to a shorter threaded joint length. Since the female half of the M/F drill rods is significantly greater in diameter but much shorter than the rod itself, it is not economical to machine down from the female outside diameter to the nominal rod diameter. Conventionally, the M/F rods are made through friction or inertia welding of a finished female connector to one distal end of the rod, and a male threaded connector is machined at the other end. One disadvantage of this approach is that welding produces a heat affected zone adjacent to the weld plane joining the rod and the female connector. Of special concern is a softened zone located a few millimeters perpendicularly from the weld place, wherein the material of the rod experiences a high temperature tempering due to the welding process. If post heat treatment is not employed to this zone, mechanical properties such as material strength of the softened zone is significantly reduced. One method to rectify the loss of mechanical properties as to carburize the welded rods. However, a disadvantage of carburization is that it precludes any possible pre-treatments, such as corrosion coating on the ends of the drill rod to be welded. A further disadvantage of carburization is that the welded section of the drill rod can become unnecessarily for certain applications, which could result in increased notch sensitivity and consequently could cause premature rod breakage . It is therefore an object of the present invention to provide a secure connection between adjacent components in a drill string of either the percussive or the rotary type in which the above disadvantages are obviated or mitigated. SUMMARY OF THE PRESENT INVENTION A drill string according to the present invention has at least first and second drill string components secured to one another. The first component has a body portion which is slid into an held by an interference fit within a bore of the second component . The internal diameter of the bore of the second component is less than the external diameter of the body portion of the first component prior to securing the two components together with one another to produce the interference fitting when the two components are secured. According to an aspect of the present invention, there is provided a drill string component coupler where that coupler has first and second bores opening in opposite directions of the coupler. At least the first bore is defined by a smooth interior wall. The second bore may be defined by either a smooth or a threaded interior wall. BRIEF DESCRIPTION OF THE DRAWINGS The above as well as other advantages and features of the present invention will be described in greater detail according to the preferred embodiments of the present invention in which; Figure 1 shows a drill string. Figure 2 is an enlarged view of a portion of the drill string of Figure 1. Figure 3 is an unassembled view of Figure 2. Figure 4 shows a method of fabrication a coupling shown in Figures 2 and 3. Figure 5 is a further embodiment of for use in the drill string of Figure 1. Figure 6 is a sectional view of a further embodiment of rod. Figure 7 is longitudinal section of a further embodiment of coupling. Figure 8 is a graph showing the selection of parameters for the coupling for Figures 1 and 2. Figure 9 is an exploded perspective view of part of a drill string assembly according to a further preferred embodiment of the present invention. Figure 10 is an exploded perspective view of part of a drill string assembly according to still a further preferred embodiment of the present invention. Figure 11 is a side view showing the lower end of a drill string assembly in its working position according to yet a further preferred embodiment of the present invention . Figure 12 is a sectional view through the lower region of a drill string assembly according to another preferred embodiment of the present invention. Figure 13 is a sectional view through the lower end of a drill string assembly according to yet another preferred embodiment of the present invention. Figure 13a is a sectional view through the lower end of a drill string assembly slightly modified from that shown in Figure 13. Figure 14 is a sectional view through the bottom end of drill string assembly showing the connection between a drill rod and the bit of the assembly according to a preferred embodiment of the present invention. Figure 15 shows the drill rod and bit from the assembly of Figure 14 when released from one another. Figure 16 is an exploded perspective view of part of a drill string assembly according to yet a further preferred embodiment of the present invention. DETAILED DESCRIPTION ACCORDING TO THE PREFERRED EMBODIMENTS OF THE PRESENT INVENTION IN WHICH: Referring to Figure 1, the components of s drill string 10 includes a drill bit 11 drill rods 12 and coupling 14 to connect the rods 12 and bit 11 end to end. Connectors 14 transmit torque and impact between an adjacent components, either a pair of drill rods 12, or drill bit rod 12 while at the same time permitting the string t 10 to be taken apart once a drilling operation has been completed. A bore 17 extends through each of the rods 12 and couplings 14, 16 to provide a passageway for fluid transfer along the drill string 10 and has an internal diameter sufficient to permit adequate flushing. Each of the rods 12 and bit 11 includes a body 13. A spigot 19 is located at one end of each of the bodies 13 and in the case of the rods 12, a threaded portion 26 is located at the other end 15. As can be seen in Figures 2 and 3, the couplings 14 include a sleeve 16 which has a machined bore 18 to 01/31162 - 6 receive the spigot 19 of the body 13 and a threaded bore 20 which is to receive the threaded end 15 of the body 13 of the adjacent component. An inner surface of the bore 20 contains a threaded section 24 compatible with a threaded section 26 of the end 15. The internal diameter b of the bore 18 is accurately machined to be of a smaller dimension than that of the spigot the end 19, at the same temperature. This difference in diameters provides interference fit between the spigot 19 and the sleeve 16 sufficient to transmit torque and tensile load along the drill string 10. A typical example of this interference fit is between 0.1% and 0.4% of the external diameter of the rod 12 more preferably 0.2% to 0.3%. The selection of the interference fit between the bore 18 and the rod 12 must provide sufficient integrity without imposing undue stress in the coupling that could lead to premature failure. The diameter Λ b' of bore 18 is a function of the diameter Λ a' of the bore 17. Λ i.e. b=ha. If k is the ratio of the nominal outside diameter λ c' of the sleeve 16 and Λ a' is the nominal diameter of the internal bore 17, i.e. k=c/a, then the value of h for maximum retention against longitudinal loads is given by The value of h for maximum retention against drilling torque is given by As can be seen from the table below and the graph of Figure 8, similar, though not identical values h x and h 2 are available over the commonly used values of k, i.e. 1.5 to 10. Thus by selecting appropriate values of the diameter of bore 17, ( a' ) and the diameter of the sleeve 16, c' for the bore characteristics an optimum value of the bore diameter, b, can be determined. Typically, the value of k is selected to be in the range 3 to 5 with a corresponding value of h of 2 to 4. The axial overlap between the spigot 19 and sleeve 16 can very according to specific application but is typically in the range lc to 5c and preferably in the range 3c to 4c. The connection according to the present invention may be used with rods of various nominal diameters including both thin-walled and thick-walled tubes. In an embodiment for rods having a nominal diameter of 32mm the diameter of the bore 17 is 12mm the outside diameter of the sleeve 16 is 45mm the internal diameter of the bore 18 is 30.8mm and the axial overlap is 75mm. To assemble the sleeve 16 on the rod 12, the sleeve 16 is heated over a specified time to enlarge the bore 18 due to thermal expansion of the sleeve material. This process temporarily eliminates the interference fit between spigot 19 and the sleeve 16 and, once the respective spigot 19 diameters are within a specified tolerance, the bore 18 is able to receive the spigot 19. It is recognized that either the sleeve 16 can be enlarged by heating or the spigot 19 can be decreased by cooling, in order to temporarily eliminate the interference fit. An example of temperature differential is in the range 400°F to 1100°F, typically 700°F. The upper limit is maintained below the temperature at which metallurgical changes may occur in the rod. In reference to Figure 4 a heating apparatus although a furnace or induction heating will provide a more uniform heat source is used to temporarily enlarge the initial internal diameter b, shown in ghosted view, to a diameter slightly larger than that of the diameter of the spigot. As shown in Figure 2, the spigot 19 is then received by the bore 18 and the coupling is maintained until the bore 18 cools sufficiently to induce an interference fit between the sleeve 16 and spigot 19. At this stage, a substantially uniform clamping force is maintained between the spigot 19 and sleeve 16. The full effect of the clamping force is attained upon reaching a thermal equilibrium between the components. The clamping force between the sleeve and the shaft acts to enhance the frictional resistance between the body 13 and sleeve 16, which inhibits relative displacement there-between, either axial or circumferential, during rotation of the drill string 10. In assembly of the drill string 10, a threaded connection is established between the bore 20 of the sleeve 16 and the threaded end 15 of the body 13. Once the drilling operation is completed, the drill string 10 may be disassembled by unthreading the end 15 from the bore 20 and the sleeve 16 may be reused for subsequent reassembly of the drill rods 12, 13. In a further embodiment of the present invention, as shown in Figure 5, a sleeve 16a has a threaded bore 20a at one end and a spigot 38 at the other end. The one end of a body 13a has a machined bore 42 of a diameter 44, which is smaller than that of a diameter 40 at the same temperature. A coupling between the components 16a and 13a is accomplished by heating the one end in conjunction with the operation procedure described above. Alternatively, the sleeve 16 may be provided with two machined bore 18 allowing a heat shrink connection to each component. Such an arrangement may be useful for a semi permanent connection, such as between a drill bit and a threaded adapter. It is appreciated that other geometrical shapes for the sleeve ends and the rod ends may be used. As shown in Figure 6, the body 13b is octagonal with rounded corners although other polygonal or oval sections may be utilized. It is recognized that simultaneous heating and cooling of respective components 14, 16 separately may also be employed to facilitate the interference fit coupling there-between. A further arrangement is shown in Figure 7 in which a pair of rods 12c each have a pair of recesses 42c. A bridging member 48 of larger diameter then the recesses 42c is fitted with the recesses 42c to interconnect the rods 12c. Each of the rods 12 may then be threaded at one end for attachment in a drill string. In each of the above embodiments, the connection is exemplified between a pair of drill rods. Any components of a drill string may be connected in this manner including a drill bit that can be secured to a threaded adapter. Early described Figure 2 shows the connection of drill rods of a drill string assembly by means of a coupler. The coupler has a threaded connection to one rod and an interference fit with the other rod. Figure 9 shows a slightly different coupling arrangement. More specifically, an assembly of drill string components generally indicated at 51 comprises a first drill rod 53 and a second drill rod 57. Drill rod 53 includes at one of its ends a spigot 55 with a spigot 59 being provided at one end of drill rod 57. These two drill rods are secured to one another by means of a coupler generally indicated at 61. Coupler 61 includes back to back socket like bores 63 and 65 separated by a full bridge 67. Bridge 67 includes a central opening 69 to allow the flow of fluids through the coupler along the two rods when they are connected to one another. In this particular case, both of the bores in the coupler have interior smooth walls to receive the externally smooth walled spigots 55 and 59 of the two drill rods. Therefore, in this set up the coupler provides an interference fit for each of the rods. Once again that interference fit it achieved by heating the coupler sufficiently to enlarge the two socket bores such that they slideably receive the two spigots from the rods. Once the coupler cools down, the metallic construction of the coupler causes it to shrink and provide the interference fit with the two rods. Figure 13 shows a somewhat similar arrangement comprising part of a drill string assembly generally indicated at 115. In this assembly, two spigots 119 and 120 are held end to end with one another through an interference fit provided by means of a coupler or sleeve 117. In this particular coupler, the socket like bores to opposite ends of the coupler for receiving the two spigots are continuous throughout the coupler rather than having a full bridge between bores . Figure 13a shows a further coupler 116. This coupler includes a threaded socket 118a and a smooth walled socket 118b without a bridge between the sockets. This will be referred to as a semi bridge coupler for use in accordance with the present invention. Figure 10 of the drawings shows a drill string assembly generally indicated at 71 comprising a drill rod 73 and a drill rod guide generally indicated at 79. In this particular embodiment drill rod guide 79 has multiple functions. It acts as a centering guide for the drill string and it further acts as a drill hole clearing member. In addition, it acts as a coupler between components of the drill string assembly. More particularly, guide 79 has socket like bores 83 and 85 to opposite ends of the guide. Socket 83 is a threaded bore to receive the threaded spigot 75 of rod 73. Socket 85 on the other hand is a smooth walled bore to receive the smooth walled spigot 87 of a component to be secured by an interference bit within bore 85. Provided externally of the guide, are a plurality of tooth like members 81. These turned tooth like members are separated from one another by troughs 82. As the drill string including the drill bit (not shown in Figure 10) rapidly percusses it also rotates. Guide 79 located above the drill bit has a maximum outside diameter approximating that of the bit to center the entire assembly within the ground opening at a location above the drill bit. In addition, the teeth 81 on the guide assist in keeping the ground opening clear from soft soil, loose rocks etc. The troughs 82 between the teeth 81 allow flushing fluids and the debris carried with those fluids to move upwardly along the drill string past the hole clearing guide. Figure 11 shows a modified drill string assembly generally indicated at 91. In this assembly, a drill rod 93 secured to a drill bit 95 is fitted with a hole clearing guide 97. Guide 97 which has the same function as earlier described guide 79 is secured directly to the main body of rod 93 by means of the interference fit of the present invention . Much of the description above relates to the use of a coupling with an interference fit for securing end to end drill rods with one another. Figures 14 and 15 show how this same interference fit can be used to secure a drill bit to a drill rod. More specifically, the lower end assembly generally indicated at 125 of a drill string comprises a drill rod 127 and a drill bit 131. Drill rod 127 includes a lower end smooth walled spigot 129 while drill bit 131 includes an upwardly facing hollow socket 133. This socket is provided with a smooth walled interior bore to receive spigot 129. As is the case with all of the earlier described interference fits the bore in socket 133 of the bit has an internal diameter less than the external diameter of the spigot prior to the bit being fitted with the spigot . However, upon heating of the bit, the socket expands allowing the non heated, non expanded spigot of the drill rod to be slid into the socket. The socket of the bit then cools off and shrinks onto the spigot providing an interference fit between the two. As will be appreciated it will at some point during or after the drilling operation be desirable to remove the drill bit from the drill rod. This is achieved by once again applying heat to the drill bit. However, in this case, the heat also will be applied to the drill rod. Therefore, in order release the bit from the rod the bit or at least the socket portion of the bit needs to have a different material construction with different expansion and contraction properties from the material which is used to make up the spigot of the drill rod. Specifically, the material in the drill bit must be one which has greater expansion capacity then that of the drill rod allowing the bit upon heating to be removed from spigot of the rod. There are in fact steel products known to the industry which do not expand when subjected to heat. These non expandable steels can therefore be used in the formation of drill rod 127. The drill bit on the other hand would be made from conventional bit forming material which would expand relative to the rod upon heating and allow the bit to be removed from the rod. In the earlier described embodiments using a coupler to secure components of a drill string one particularly suitable material for use in making the coupling is molychromium alloy. Although other metallic materials could be used this particular is less likely to be damaged or weakened by the heating process. In the case of the drill rods they would typically be made from iron having some chromium content . Another standard rod construction would comprise iron with both nickel and chromium content. It is to be understood that the above materials are only indicative of some that can be used in the construction with other metallic materials being equally suitable. It is to be understood that different components of the drill string will have different material constructions according to their function and location in the drill string. For instance some of the components which are more exposed to wear, atmospheric elements, etc., will be made from metals having higher strength and greater resistance to corrosion than other components in the string. This saves the cost of manufacturing the entire drill string from these more expensive metals. Figures 14 and 15 shown the string as including a guide 135. In this set up the guide, sleeved and secured by an interference fit on rod 127 locates directly above the drill bit. Additional guides, at spaced apart locations, may be provided above guide 135 over the length of the drill string. As earlier noted, it is known in the prior art to threadably connect male and female rods directly to one another without the use of a coupler. In accordance with the present invention, male and female rods can be connected directly to one another by means of an interference fit. This is shown in Figure 16 of the drawings in which part of a drill string assembly generally indicated at 135 comprises a female rod 137 having a socket 139 at one end to receive the spigot 143 of a male rod 141. The socket of the female rod is heated to expand the internal bore of the socket allowing the spigot to be slid into the socket and thereafter the socket cools down to shrink onto and provide an interference fit with the spigot . It should be noted that the female socket may be made of sufficient diameter to act as a string centering guide . Although various preferred embodiments of the invention have been described in detail it will be appreciated by those skilled in the art that variations may be made without departing from the sprit of the invention or the scope of the appended claims .

Claims: THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS: 1. A drill string having at least first and second drill string components secured to one another, said first component having a body portion which is slid into and held by an interference fit within a bore of said second component, said body portion of said first component having an external diameter and said bore of said second component having an internal diameter which is less than the external diameter of said body portion of said first component prior to securing the components together with one another. 2. A drill string as claimed in Claim 1 wherein said second component comprises a coupler for coupling said first component to a third component. 3. A drill string as claimed in Claim 2 wherein said body portion of said first component comprises a smooth walled spigot and wherein said bore has a smooth interior wall which slideably receives said spigot. 4. A drill string as claimed in Claim 3 wherein said coupler includes a second bore which receives a spigot of said third component. 5. A drill string as claimed in Claim 4 wherein said second bore is internally threaded and said spigot of said third component is externally threaded. 6. A drill string as claimed in Claim 4 wherein said spigot of said third component is held by an interference fit within said second bore of said coupler, said second bore of said coupler having an internal diameter and said spigot of said third component having an external diameter which is greater than the internal diameter of said second bore of said coupler prior to securing the third component with the coupler. 7. A drill string as claimed in Claim 6 wherein said spigot of said third component has a smooth exterior wall and said second bore of said coupler has a smooth interior wall. 8. A drill string as claimed in Claim 4 wherein said coupler includes a full bridge with a central fluid passage therethrough between said bore and said second bore . 9. A drill string as claimed in Claim 4 wherein said bore and said second bore of said coupler are continuous with one another through said coupler. 10. A drill string as claimed in Claim 1 wherein said first component comprises a rod of said drill string and wherein said second component comprises a guide secured by said interference fit onto said rod. 11. A drill string as claimed in Claim 2 wherein said first and third components comprise drill rods . 12. A drill string as claimed in Claim 2 wherein first and third components comprise a drill rod and a drill bit respectively. 13. A drill string as claimed in Claim 2 wherein said first and third components comprise a drill hammer and a drill rod respectively. 14. A drill string as claimed in Claim 1 wherein said first and second components comprise male and female drill rods. 15. A drill string as claimed in Claim 1 wherein said first and second components comprise a drill rod and a drill bit, the drill rod being made from a material with different heat expansion and contraction properties from that of said drill bit. 16. A drill string as claimed in Claim 1 wherein said first and second components comprise a drill hammer and a drill rod, said drill hammer having a different material construction with different heat expansion and contraction properties from that of said drill rod. 17. A drill string component coupler, said coupler having first and second bores opening in opposite directions of said coupler, said first bore being defined by a smooth interior wall. 18. A drill string component coupler as claimed in Claim 17 wherein said second bore is defined by a threaded interior wall. 19. A drill string component coupler as claimed in Claim 17 wherein said second bore is defined by a smooth interior wall. 20. A drill string component coupler as claimed in Claim 17 including a full bridge between said first and second bores . 21. A drill string component coupler as claimed in Claim 17 wherein said first and second bores are continuous with one another through said coupler. 22. A drill string component coupler as claimed in Claim 17 wherein said coupler also comprises a drill string guide .

Inventors: YAO JAMES JING, YAO, JAMES, JING
Applicants: BOART LONGYEAR INC, YAO JAMES JING, BOART LONGYEAR INC., YAO, JAMES, JING
Application year: 2000
Main classification: E21B17
IPC codes: E21B17