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TECHNICAL FIELD 
     This invention relates to directional boring, particularly to an apparatus and method for bursting an existing pipeline or boring a non-linear hole. 
     BACKGROUND OF THE INVENTION 
     Directional and non-directional boring apparatus for making holes through soil are well known. A directional borer generally includes a series of drill rods joined end to end to form a drill string. The drill string is pushed or pulled through the soil by means of a powerful hydraulic device such as a hydraulic cylinder. A spade, bit or chisel configured for boring is disposed at the end of the drill string, which may include an ejection nozzle for water to assist in boring. In general, the direction of boring is controlled by selectively rotating a boring head having an angled face. During rotation, the borer continues straight, whereas when pushed without rotation the boring head moves in the favored direction. See Malzahn U.S. Pat. Nos. 4,945,999 and 5,070,948, and Cherrington U.S. Pat. No. 4,697,775 (RE 33,793). The drill string may be pushed and rotated at the same time as described in Dunn U.S. Pat. No. 4,953,633 and Deken et al. U.S. Pat. No. 5,242,026. 
     In one variation of the traditional boring system, a series of drill string rods is used in combination with a percussion tool mounted at the end of the series of rods. The rods can supply a steady pushing force to the impact tool and the interior of the rods can be used to supply the pneumatic borer with compressed air. See McDonald et al. U.S. Pat. No. 4,694,913. This system has, however, found limited application commercially, perhaps because the drill string tends to buckle when used for pushing if the bore hole is substantially wider than the diameter of the drill string. 
     A variety of systems are now known for the installation of underground pipes, particularly for the replacement of an existing deteriorated pipe. In one widely practiced method, a pneumatic impact boring tool is sent through the existing pipeline such that the head of the tool, which may be provided with blades that apply intense local pressure to the existing pipe, fractures or splits the existing pipe. See, for example, Streatfield et al. U.S. Pat. Nos. 4,720,211, 4,738,565 and 4,505,302. A replacement pipe, typically made of plastic such as HDPE, can be drawn along behind the boring tool. This process has proven effective commercially because it bursts the old pipe and replaces it with a new pipe at the same time. However, the system relies on a pneumatic impact tool, which in turn requires an air compressor. Exhaust from the impact tool is vented into the interior of the replacement pipe, which is unacceptable for certain types of pipe installations, such as gas and water lines. 
     Directional borers are less effective for pipe bursting, especially for hard to burst pipes like cast iron, because the steady pushing force of the drill string lacks the impact power of a pneumatic impact boring tool. Thus, in some instances, a directional borer or winch is used to pull a pneumatic impact tool through an existing pipeline in order to burst the existing pipe and pull in the replacement pipe. These alternatives are effective but require considerable equipment and manual labor. A need remains for a boring system that can avoid the need for a pneumatic impact tool and still provide cyclic impacts suitable for pipe bursting operations, rock breaking, and the like. 
     SUMMARY OF THE INVENTION 
     An impact boring tool according to the invention has a tubular housing and a head mounted at a front end of the housing. The head preferably includes a chisel configured for breaking loose rocks and stones encountered in soil. A striker is disposed within the housing for delivering an impact force to the head, either directly or through one or more intervening parts. A trigger mechanism causes the striker to deliver an impact to the head and chisel only when an external force exerted on the impact tool in its lengthwise direction exceeds a predetermined level. This predetermined level generally coincides with a maximum effective amount of pushing or pulling force for moving the tool through the ground, which force is exerted by an external device such as a directional boring machine. 
     According to a preferred form of the invention, the impact tool includes a head mounted for limited longitudinal movement relative to a housing, a chisel shaft connected to the head, and a trigger shaft engaging the chisel shaft. The trigger shaft initially retains a striker in a rearward position against the action of a striker spring. Upon relative movement between the head and the housing, the trigger mechanism activates to release the striker to apply an impact force to the head through the trigger shaft and chisel shaft. 
     A directional boring apparatus of the invention includes a drill string, a directional boring machine connected to a rear end of the drill string and capable of forcing the drill string through soil, and an impact boring tool as described above connected to a front end of the drill string. The invention further provides a method of directional boring using such a directional boring apparatus including the steps of pushing (or pulling) the impact tool forward through the ground using the directional boring machine, rotating the drill string while pushing or pulling it to move the tool in a substantially straight forward direction, ceasing rotation of the drill string while pushing or pulling to change the direction of travel of the tool, actuating the trigger mechanism when the pushing force exerted by the drill string exceeds the predetermined level, and re-setting the trigger mechanism after actuating it. The latter operating is preferably done by pulling back on the drill string as described hereafter. 
     Objects, features and advantages of the invention will become apparent from the following detailed description. It should be understood, however, that the detailed description is given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will hereafter be described with reference to the accompanying drawing, wherein like numerals denote like elements, and: 
     FIG. 1A is a cross-sectional view of a front portion of an impact tool according to the invention; 
     FIG. 1B is a cross-sectional view of the rear portion of the impact tool according to the invention shown in FIG. 1A; 
     FIG. 2 is a bottom view of the impact tool of FIGS. 1A and 1B; 
     FIG. 3 is a cross-sectional view along the line  3 — 3  in FIG. 2; 
     FIG. 4 is a front of the impact tool of FIGS. 1A and 1B; 
     FIG. 5 is a perspective view of the head of the impact tool of FIGS. 1A and 1B; 
     FIGS. 6,  7  and  8  are partial cross-sectional views showing the impact mechanism at various positions during operation; and 
     FIG. 9 is a schematic diagram of a directional boring system according to the invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIGS. 1A and 1B, an impact boring tool  10  according to the invention is designed to use a combination of axially applied constant force and sudden impact force to penetrate of the earth or crack an existing pipe. Tool  10  includes a head  12  mounted at the forward end of a tubular body or housing  28 , a tail nut  120  disposed in a rear end opening of body  28 , and an end cap  136  which serves to connect the tool to a string of rods as described hereafter. 
     Head  12  has an oblique front surface  15  which is provided with forwardly-facing knife edges  16  and  18 . An axial fastener-receiving hole  20  extends into head  12  from the forward end thereof. A rear portion of head  12  has a plurality of circumferentially spaced, longitudinal splines  26 . A front portion  30  of tool body  28  has a plurality of circumferentially spaced, longitudinal splines  36  which matingly receive splines  26  of head  12 . In this manner, head  12  is slidably supported on body  28  but is prevented from rotating relative thereto. 
     A cylindrical chisel shaft  42  is axially aligned with the body  28 . Chisel shaft  42  is positioned behind head  12 , and a frontwardly tapered front end  43  of chisel  42  is seated in a frontwardly tapering rear end portion  21  of hole  20 . A central threaded hole  44  at the forward end of chisel shaft  42  threadedly receives the stem of a bolt  45  that is inserted through hole  20  of head  12 . As the head of bolt  45  engages an annular, frontwardly facing wall  47  in hole  20 , head  12  and chisel shaft  42  become locked together. Bolt  45  is secured and removed by means of a hex-shaped socket  49 . 
     Chisel shaft  42  further comprises an elongated cylindrical midportion  46  which extends rearwardly to an enlarged rounded rear end portion  48 . A number of Belleville springs  56  (in one or more pairs) are situated within a rearwardly opening cavity  24  of head  12  and disposed around the tubular midportion  46  of chisel shaft  42 . During actuation of tool  10 , as described hereafter, Belleville springs  56  are compressed between an annular wall  58  situated at the bottom of cavity  24  and a front end cap  60 . 
     Front end cap  60  is disposed concentrically with tool body  28  in contact with midportion  46  of chisel shaft  42  at a location behind springs  56 . Cap  60  has a peripheral annular groove  64  which receives a seal ring  61  therein. Seal ring  61  prevents dirt from entering the inside of the apparatus during sliding movement of head  12  relative to body  28 . A plurality of radially spaced, stepped bolt receiving apertures  62  extend through cap  60  in the lengthwise direction of the tool body  28 . 
     An enlarged head portion of a head nut  68  is disposed in a rearwardly opening recess  66  of front end cap  60 . A rear threaded portion of head nut  68  is secured in corresponding threads formed on the inside of body  28  near its front end opening. Bolts  69  are inserted into stepped apertures  62  and become seated therein as the threaded end of each bolt  69  engages a frontwardly opening threaded hole  71  in head nut  68 . Threaded holes  71  (e.g. six or eight such holes) are ranged in a circular, evenly spaced formation in alignment with apertures  62 . As bolts  69  are tightened, front end cap  60  becomes securely clamped to the front end of body  28 , and a clamp load is applied to head nut  68 . Head nut  68  has a sloped annular rear surface  76  positioned and dimensioned for engagement by a frontwardly tapering stop surface  50  of chisel shaft  42  to limit forward movement of the head-chisel shaft assembly relative to the tool body  28 . 
     A rearwardly extending central boss  77  of chisel shaft  42  has a rounded surface  54  positioned in contact with a circular front end recess  79  in a trigger shaft  78 . A trigger coil spring  80  is contained within and co-axial with tool body  28  behind trigger shaft  78 . Trigger spring  80  engages a inwardly directed annular flange  40  of tool body  28  and a flange  82  at the front end of trigger shaft  78 . This biases trigger shaft  78  toward the front end of the tool  10 . Flange  82  has a rearwardly facing surface  86  which engages the trigger shaft spring  80  and which extends at a slight angle (e.g., about 1°-20°) relative to an imaginary annular surface oriented at a 90° angle relative to the lengthwise axis of trigger shaft  78 . By this means, in addition to biasing the trigger shaft  78  forwardly relative to housing  28 , trigger spring  80  biases the trigger shaft  78  to pivot downwardly as shown in FIG.  1 A. 
     Trigger shaft  78  has a rearwardly tapering shoulder  92  that extends between a relatively large diameter forward portion  96  and a relatively small diameter rearward portion  98 . A boss  100  located at the extreme rearward end of trigger shaft  78  forms a step  101  which performs the trigger function as described hereafter. 
     A cylindrical striker  102  is coaxially disposed inside of tool body  28  rearwardly of flange  40 . Striker  102  is supported for sliding movement relative to housing  28  by a series of spaced plastic bearing rings  103  disposed in annular grooves  104 . The front end of striker  102  comprises a front, annular trigger shaft engaging surface  108  and a central trigger shaft receiving recess  110 . The rear end of striker  102  has a cylindrical spring receiving recess  112 . 
     A tail nut  120  is mounted in a rear opening of body  28  and secured therein by means of internal threads  121  which are engaged by external threads  116 . A seal ring  117  is set in an external annular groove  118  rearwardly of threads  116  to prevent contamination of the mechanism in a manner similar to seal ring  61 . Tail nut  120  has a forwardly facing, cylindrical spring receiving recess  122 . Recess  122  is coaxial with body  28  and also with a smaller diameter threaded aperture  132  which extends rearwardly from recess  122 . 
     A compression spring  134  is contained within recess  112  of striker  102  and recess  122  of tail nut  120 . Striker spring  134  is considerably stiffer than trigger spring  80  and biases striker  102  to a forward position as shown in FIGS. 1A,  1 B. An end cap  136  is positioned at the extreme rear end of the directional boring tool  10  and is retained by a bolt  137  inserted through a central opening  135  in end cap  136  and secured in threaded aperture  132 . 
     FIG. 9 illustrates a directional boring apparatus  140  according to the invention. End cap  136  of tool  10  is adapted to be secured to a front rod  141  of a string of rods  142  which extends from and is actuated by a directional boring machine  143 . The string of rods  142  is most commonly pushed into the ground or an existing pipeline by a hydraulic cylinder  144 . 
     In addition to pipe bursting, the present invention is also useful for creating new bores or widening existing bores in which no existing pipeline is present. Drilling mud or pressurized water may be injected through the interior of rods  140  to a rearwardly opening cylindrical recess  138  in end cap  136 . One or more radial holes  139  may be provided in end cap  136  so that the lubricating mud can be conducted to the outside of the tool body where it can flow along the tool body, a portion of the mud reaching head  12 . More advantageously, means can be provided for conducting the mud directly to head  12  so that more effective lubrication can be achieved with less wasted mud. For example, a channel (not shown) for conducting mud towards head  12  from hole  139  can be provided on the outside of tool body  28 . A battery-powered sonde (not shown) may be installed in an additional tubular housing attached to the rear of tool  10 . The sonde transmits a radio signal on depth and angular orientation in two axes in a manner known in the art to assist in guiding the tool through the ground. 
     The illustrated tool  10  is designed for use with a directional boring machine that rotates and pulls or pushes the tool at the same time. The slanted surface  15  of head  12  causes the tool to deviate from a straight path when the tool is pushed but not rotated by the directional boring machine. When the boring machine both rotates and pushes, the tool moves in an approximate straight line because the angled face of the head spends essentially equal time facing each direction. 
     A known method of directional boring takes advantage of the foregoing to complete a bore beneath an obstruction such as a roadway or stream without having to dig entry and exit pits. The bore comprises three segments or stages. In the first stage, the borer head is directed down into the earth at an angle, often in a curved path. The rods of the drill string bend sufficiently to accommodate the changes in direction. At the second stage, the directional borer must round a corner in order to stop descending and move in a horizontal path beneath the obstruction. In the third stage, the borer must change directions a second time to assume an upward angle, so that the head eventually re-emerges from the ground on the other side of the obstruction. 
     The most difficult stage of this operation is the second change of direction. The effective pushing force of the directional boring machine becomes progressively less as the drill string becomes longer, and extra force is required to turn a corner even with a relatively small change in direction such as 20-50° from the horizontal. The borer most often becomes stuck at the second corner and must then be withdrawn. The problem is compounded if rocky soil or other hard obstacles are encountered far down the bore. 
     The present invention addresses the foregoing problem by providing a device which can crack through hard obstacles (e.g., stone, rocks, concrete) using a percussion force supplied by the above-described triggered impact mechanism. The invention can also supply the extra force required to turn a difficult corner during a boring run. The peak force supplied when both the impact mechanism and the directional boring machine are acting together is substantially greater than what the directional boring machine can exert alone, and the force exerted by the impact mechanism is substantially independent of how far the tool has traveled from the point of entry (i.e., the length of the drill string extending behind the tool.) The invention is thus useful as a system for assisting a directional boring system in steering especially when 300′ or more out and trying to make a turn. 
     The invention can also be used with a non-rotational pushing system such as the rod pushing machine sold under the trademark “Hydraburst” by Earth Tool Company. In such a case, a symmetrical or non-slanted head could be used. If a slanted head were used, steering would still be possible by alternately pushing and then manually rotating the series of rods. 
     The operation of the directional boring tool  10  is illustrated in FIGS. 6,  7  and  8 . FIG. 6 (and FIGS. 1A,  1 B)illustrate the directional boring tool  10  in an unloaded condition. A considerable gap may exist between Belleville springs  56  and front wall  58 . The rear boss  77  of trigger shaft  78  engages the front recess  110  of striker  102 , and the striker spring  134  is relatively uncompressed. The step  101  of trigger shaft  78  engages striker surface  108  at one edge as shown. 
     When tool  10  is under moderate load, the gap between Belleville springs  56  and wall  58  closes. Engagement between flange  40  of housing  28  and the sloped shoulder  92  of trigger shaft  78  pivots trigger shaft  78  upwardly against the action of the trigger spring  80 . However, the rear end of the trigger shaft  78  remains in engagement with the trigger shaft engaging surface  108  of striker  102 . The striker spring  134  becomes substantially compressed. 
     FIG. 7 illustrates the component parts of directional boring tool  10  just prior to actuation of the striker  102  to apply an impact force to head  12 . At this point, interaction between flange  40  and shoulder  92  pivots the trigger shaft upwardly until it is almost but not quite aligned with the trigger shaft receiving recess  110  of the striker  102 . The striker spring  134  is further compressed. 
     Upon the next incremental amount of forward movement of the housing  28  relative to the head  12  under the action of the drill string and the directional boring machine, further movement along the sloped shoulder  92  of the trigger shaft  78  moves step  101  off of surface  108  and brings trigger shaft  78  into alignment with recess  110  of the striker  102 . Striker  102  immediately moves forward under the action of the striker spring  134 . As trigger shaft  78  enters the trigger shaft receiving recess  110 , the striker  102  engages the trigger shaft  78 , applying an impact force to the head  12  through the trigger shaft  78  and the chisel shaft  42 . At this point, the component parts of the directional boring tool  10  are positioned as shown in FIG.  8 . Tool  10  is then reset by rearward movement of the drill string under the action of the directional boring machine. This returns the component parts of the directional boring tool  10  to the configuration illustrated in FIG.  6 . The operation of the trigger mechanism is comparable to that of a hand-operated machinists&#39; center punch. 
     In the overall operation of the directional boring tool  10 , the directional boring machine operates through a drill string to push the directional boring tool  10  through the earth. When the head  12  engages an obstruction for which the maximum force exerted by the drill string is insufficient to push past, forward movement of the head of the tool stops. Adjustment of the various springs included in the apparatus determines the threshold at which the impact mechanism will be triggered. In particular, Belleville springs  56  are provided in order to reduce the load required to trigger the impact mechanism relative to the total load placed on the tool  10 . For example, the maximum force exerted by the directional boring machine may be in the range from 4,000-5,000 pounds, but such a load would put excessive stress on the trigger mechanism and cause it to fail or shorten its life. In the illustrated embodiment, Belleville springs  56  reduce the effective force exerted on the trigger mechanism by as much as 3,000 pounds. Accordingly, the trigger mechanism can be designed to fire under a load of only 2,000 pounds, which corresponds to a load of 5,000 pounds on the tool head and body as exerted by the drill string. 
     The directional boring machine continues to apply force to tool  10  through the drill string, collapsing the front and rear sections of the tool body and causing splines  26 ,  36  to move together. As the applied force continues to increase, the component parts of tool  10  move from the configuration of FIG. 6 to the configuration of FIG.  7  and ultimately to the configuration of FIG.  8 . At this point, the trigger shaft  78  enters the trigger shaft receiving cavity  110  of the striker  102 , which allows the striker  102  to move forwardly under the action of the striker spring  134 . The striker  102  thus applies an impact force to the trigger shaft  78  which in turn applies the impact force to the chisel shaft  42  and hence to the head  12 , thereby destroying the obstruction. A series of blows can be imparted as needed. Once the obstruction has been demolished, the peak pushing force load no longer reaches the predetermined maximum, e.g., 4,000-5,000 pounds, and the tool continues to operate in the manner of a normal directional borer. 
     Striker  102  expends most or all of its impact force on the trigger shaft, and the impact is transmitted to the head of the tool as described above. Some remaining portion of the striker&#39;s energy may be expended as it impacts against flange  40 . However, the force of impact against flange  40  should be sufficiently low as not to damage the drill string. In the alternative, tool  10  can be configured so that striker  102  stops short of flange  40  during its impact stroke. 
     Although preferred embodiments of the directional boring tool  10  are illustrated in the drawings and described hereinabove, various modifications of the directional boring tool can be made within the spirit and scope of the invention. For example, the chisel shaft and the trigger shaft can comprise a single shaft which both secures the head for limited longitudinal reciprocation relative to the housing while allowing limited pivotal movement of the shaft to effect operation of the device. Also, the longitudinally offset surfaces comprising the trigger shaft engaging surface and the bottom of the trigger shaft receiving cavity can be located on the trigger shaft instead of on the striker, if desired. 
     Those skilled in the art will appreciate that although particular embodiments of the invention have been illustrated in the accompanying drawings and described in the foregoing detailed description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications, and substitutions of parts and elements without departing from the scope of the invention as expressed in the appended claims.

Summary:
An impact boring tool according to the invention suitable for using with a directional boring machine has a tubular housing and a head mounted at a front end of the housing. The head preferably includes a chisel configured for breaking loose rocks and stones encountered in soil. A striker is disposed within the housing for delivering an impact force to the head, either directly or through one or more intervening parts. A trigger mechanism causes the striker to deliver an impact to the head and chisel only when an external force exerted on the impact tool in its lengthwise direction exceeds a predetermined level. This predetermined level generally coincides with a maximum effective amount of pushing or pulling force for moving the tool through the ground exerted by an external device such as a directional boring machine.