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TECHNICAL FIELD 
   The invention relates to pneumatic ground piercing tools, and in particular, to a moveable chisel head assembly for pneumatic impact tool. 
   BACKGROUND OF THE INVENTION 
   Pneumatic impact ground piercing tools have been commercially useful products for decades. Self-propelled pneumatic ground piercing tools are used to install pipelines, power lines and information transmission cables such as fiber optics installed beneath the ground with a minimal amount of surface disruption. These tools include, as general components, a torpedo-shaped body having a tapered nose and an open rear end, an air supply hose that enters the rear of the tool and connects it to an air compressor, a piston or striker disposed for reciprocal movement within the tool, and an air distributing mechanism for causing the striker to move rapidly back and forth. 
   In the case of hard or rocky ground, it is often desirable to utilize pneumatic ground tools that incorporate movable bits or chisels at the tapered nose section of the tool to concentrate the striking force. For example, U.S. Pat. No. 6,273,201 to Randa et al., issued Aug. 14, 2001, the contents of which are incorporated herein for all purposes, discloses a reciprocating (front) head mole with a moveable chisel head that is axially independent of the remainder of the mole. Randa et al. facilitates transfer of striker energy directly to the leading end of the mole thereby improving productivity in hard ground. 
   In many cases, impact moles are started from pits dug in the earth. The mole is launched when the air valve supplying the mole with compressed air is opened, actuating the striker to begin impacting. The front end of the mole is forced against the sidewall of the launch pit until the mole penetrates the earth far enough so that sufficient friction force is produced between the mole body and the soil to hold the mole in position against the pneumatic reaction forces generated as the striker reciprocates. 
   Launching larger diameter pneumatic impact tools, for example in the range of 4″ diameter, tend to be considerably more difficult to start than smaller tools with diameters in the range of 2″. As the striker impacts the chisel and then the anvil, it generates a reaction force that first tends to move the movable head or chisel of the tool forward, then pull the tool body along behind. The striker then moves rearwardly in preparation for the next stroke. The difficulty arises as the striker reverses its direction and move forward for the next impact under the action of compressed air in the rear pressure chamber. The reaction force from this operation tends to move the tool body rearwardly. During normal operation when the mole is fully engaged in a borehole, friction between the surface of the tool body and the surrounding soil absorbs this reaction force, allowing the tool to make net forward progress through the ground. However, when the mole is first launched and only the head is engaged by the soil, the reaction forces generated by reciprocation of the striker can cause the movable head to lose engagement with the soil and requires the operator to manually apply an opposing force until the mole has penetrated the earth far enough so that friction between the mole and the soil holds the mole body in place. In soft soil, the friction between the mole body and the soil may not be sufficient to hold the mole in place, making start-up unusually difficult. 
   Most prior movable chisel-type ground piercing tools have used a metal spring or springs to bias the chisel in a rearward direction to return the chisel to its starting position after being impacted by the striker and partially absorb reaction forces during the forward stroke of the striker that would otherwise tend to make the tool body to move backward, especially during startup. For example, U.S. Pat. No. 5,095,998 to Hesse et al., issued Mar. 17, 1992, the entire contents of which are incorporated by reference herein for all purposes, discloses such an arrangement. However, the use of springs in this application raises issues of durability and design. Pneumatic impact moles normally operate at a relatively high impact frequency, typically in the range of 250 to 600 impacts per minute. Assuming an average travel rate of 1 foot/minute and 300 foot of boring per day, an impact mole may be subjected to 50 million impacts per year. Under these conditions, a spring is subject to fatigue fractures. 
   SUMMARY OF THE INVENTION 
   A ground piercing tool according to the invention includes an elongated tubular tool housing with a front anvil having a lengthwise bore through the anvil. A striker reciprocates within an internal chamber of the housing to impart impacts to a front impact surface of the anvil for driving the tool forwardly through the ground. A chisel including a front head and a rearwardly extending chisel shaft slides within the bore of the anvil, the chisel being movable between a rearward most position at which a rear end portion of the chisel shaft protrudes from the bore of the anvil to receive an initial impact from the striker and a forward most position at which the striker impacts on a rear impact surface of the anvil. A distributing mechanism reciprocates the striker in response to a supply of compressed fluid. A front chamber defined by the housing and chisel shaft decreases in volume as the chisel moves forward relative to the housing, and the distributing mechanism includes passages that conduct compressed fluid to the front chamber, which front chamber is configured to form a gas (air) spring using such compressed fluid. 
   In one aspect, the distributing mechanism includes a fluid inlet tube mounted in the bores of the anvil and striker includes a radial port and the chisel shaft has a radial passage therein that conducts compressed fluid from the radial port of the inlet tube to the front chamber which is configured to form an air spring using the compressed fluid. The invention further provides an improved mechanism for removably securing a movable chisel to the tool body, which mechanism uses a jamb nut mounted between the chisel head and the anvil. These and other features of the invention are described further in the detailed description that follows. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts, and in which: 
       FIG. 1  is a longitudinal sectional view of a pneumatic ground piercing tool according to the invention showing the position of the striker at the moment it contacts the chisel shaft; 
       FIG. 2  is a longitudinal sectional view of the pneumatic ground piercing tool of  FIG. 1  showing the orientation of the chisel and striker after the striker has impacted the chisel shaft; 
       FIG. 3  is a longitudinal sectional view of the pneumatic ground piercing tool of  FIG. 1  illustrating the position of the striker upon completion of the impact stroke; 
       FIG. 3A  is an enlarged portion of  FIG. 3  illustrating seals between the striker of the ground piercing tool and a fluid supply tube passing through a bore in the striker; 
       FIGS. 4–6  are partial, enlarged sectional views corresponding to  FIGS. 1–3 , respectively, wherein the forward section of the tool is illustrated in greater detail; 
       FIG. 7  is a sectional view of a pneumatic ground piercing tool according to the invention with a shortened air inlet tube; 
       FIG. 8  is a cross sectional view of the striker of  FIGS. 1–3  taken along line A–A′ of  FIG. 1 ; 
       FIG. 9  is a lengthwise sectional view of a further embodiment of the invention with vent passages; and 
       FIG. 10  is a lengthwise sectional view of another embodiment of the invention using a valve in place of a air supply tube. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and are not to delimit the scope of the invention. 
   According to the invention, a moveable bit pneumatic ground tool is provided with a variable volume forward chamber that is pressurized with a fluid such as compressed air to form an air spring. The air spring offsets a substantial fraction of the reaction force generated when the striker of the tool is accelerated during the forward stroke of the striker. Reducing the reaction force in this manner substantially reduces the amount of force that must be applied by the operator as the tool is launched and reduces the tendency of the bit to break lose from the surrounding soil and/or to move backwards in the borehole. 
   Referring now to  FIGS. 1–4 , a pneumatic ground piercing tool  10  having a movable chisel assembly according to the invention includes an air distributing mechanism  12  for reciprocating a striker  14  disposed within elongated tubular tool housing  16 . Air distributing mechanism  12  includes a reversing mechanism actuated by rotating the air supply hose in a manner known in the art. A preferred air distributing mechanism for use in the present invention are exemplified in U.S. Pat. No. 5,603,383, Feb. 18, 1997, the entire contents of which are hereby incorporated by reference herein. Compressed air is supplied through a hose  11  to air distributing mechanism  12 , which causes striker  14  to reciprocate within housing  16 . 
   Housing  16  is cylindrical and is swaged or machined to a reduced diameter nose  20  at its forward end. However, the anvil may instead be threadedly secured in a threaded front opening of the housing, eliminating reduced diameter nose  20  and use of a swaging process to produce it. Striker  14  slides within housing  16  to deliver forward impacts to a movable chisel shaft  18  and to an anvil  22  press-fitted into the forward end of housing  16 . Anvil  22  is preferably a steel tube that fits closely within the front end opening of housing  16 ; however, “anvil” as used herein also refers to the corresponding portion of a one piece tool body, or a separate piece that is threadedly secured into the housing as described above. A frustoconical front end portion  26  of anvil  22  thereof has an outer surface that engages a like-shaped inner surface of nose  20  of housing  16  to retain anvil  22  in housing  16 . 
   Referring now to  FIG. 4 , anvil  22  includes a central bore  28  with a large diameter forwardly opening section  30 , intermediate tapered transition  31  and a small diameter rearwardly opening section  32 . A tubular bushing  34  includes a threaded end portion  38  that is screwed into threads on the inside surface of large diameter forward section  30  of bore  28  to secure bushing  34  in place. A round jamb nut  40  is threaded onto end portion  38  of bushing  34  forward of anvil  22 . Jamb nut  40  has four blind holes  41  on its side set 90 degrees apart that permit use of a spanner to tighten nut  40  against the front face of anvil  22 . Clamp loading produced by tightening nut  40  prevents the threaded engagement between bushing  34  and bore  28  of anvil  22  from loosening during tool use. The head assembly can be removed by first loosening jamb nut  40  and then unscrewing bushing  34  from bore  28 . 
   Chisel shaft  18  is slidably mounted in tubular bushing  34  with a small diameter rear end  36  of the chisel shaft extending through the small diameter rearwardly opening section  32  of bore  28 . Chisel shaft  18  is slidable in bushing  34  between the position shown in  FIG. 4  where the rear end  36  of the shaft protrudes through anvil  22  and the position shown in  FIG. 5  where rear end  36  is inside the anvil. Chisel shaft  18  includes a forward threaded end  42 , a central body portion  44  that passes through bushing  34  and an enlarged diameter sealing shoulder  46 . Enlarged diameter sealing shoulder  46  is rearwardly tapered to small diameter rear end  36  of shaft  18  so as to match the inside profile of bore  28 . A seal bearing  48  extends around the outer circumference of shoulder  46  to provide a gas tight seal between shoulder  46  and the inside wall of bore  28 . Similar seal bearings  49  are disposed between rear end  36  of bit shaft  18  and the small diameter section  32  of bore  28 , and between central body portion  44  and bushing  34 . 
   As illustrated, a stepped chisel head  50  is mounted on the forward threaded end  42  of chisel shaft  18 . Chisel head  50  includes an annular wall  53  that forms an axially extending central opening  52 . A smaller diameter hole  54  extending forwardly from central opening  52  includes interior threads for securing chisel head  50  onto threaded end  42  of chisel shaft  18 . Opening  52  is sized to receive the forward end  56  of bushing  34  and a seal  58  extending around the circumference of forward end  56  of bushing  34  provides a gas tight seal between bushing  34  and the inside wall of central opening  52 . 
   As best illustrated in  FIG. 3 , a fluid supply tube  60  extends from a central bore  62  formed in chisel shaft  18  into a coaxially extending bore  66  that passes through striker  14  to a variable volume rear striker chamber  64 . Bore  66  is configured to allow striker  18  to slide over tube  60  as striker  18  reciprocates. A seal  70  prevents leakage between tube  60  and bore  66 . Supply tube  60  is preferably formed from a resilient plastic material and is secured in chisel shaft  18  by means of a suitable adhesive and/or by molding the tube to the contour of bore  62 . 
   Referring to  FIGS. 4–6 , supply tube  60  is formed with radially extending ports  72  that communicate with an annular space  74  between the supply tube and the inside wall of bore  62 . One or more second ports  76  extend from annular space  74  through chisel shaft  18 , opening into an annular variable volume forward chamber  78  formed between anvil  22 , chisel shaft  18  and bushing  34 . Tube  60  along with radial port  72 , annular space  74  and second port  76  form a fluid conduit or passage from rear striker chamber  64  to forward chamber  78 , allowing the chamber to be pressurized with compressed air from the rear striker chamber. 
   Referring to  FIGS. 1–3 , when tool  10  is launched, air distributing mechanism  12  supplies compressed air to rear striker chamber  64 , accelerating the striker forward (left to right). The force accelerating striker  14  to the left simultaneously accelerates tool housing  16  to the right. During launch, the operator must compensate for this force by holding the tool against the wall of the launch pit.  FIGS. 1 and 4  show tool  10  at the instant when striker  14  contacts rear end  36  of chisel shaft  18 . 
     FIG. 2  illustrates the position of chisel head  50  and chisel shaft  18  after striker  14  has impacted chisel shaft  18 . Shaft  18  and chisel head  50  have been driven forward by striker  14  until the rear end  36  of shaft  18  is completely within bore  28  of anvil  22 . The forward movement of chisel shaft  18  relative to anvil  22  and tool housing  16  opens gap  80  between chisel head  50  and jamb nut  40 . Striker  14  then impacts anvil  22 , driving tool housing  16  to the left and closing gap  80 . Simultaneously, air distributing mechanism  12  reverses the flow of compressed air from rear striker chamber  64  to forward striker chamber  82 , accelerating striker  14  from right to left. 
   As striker  14  is accelerated from right to left, a corresponding reaction force accelerates tool housing  16  from left to right, tending to drive housing  16  out of the borehole. As striker  14  moves from left to right, air distributing mechanism  12  vents forward striker chamber  82  ( FIG. 3 ) to atmosphere, stopping the rearward motion of the striker at the position shown in  FIG. 3  at which time the cycle is repeated. 
   Referring again to  FIG. 1 , if during the forward stroke of striker  14 , the operator is unable to compensate for the reaction force accelerating the tool housing  16  to the right as striker  14  is accelerated to the left, housing  16  will move to the right, opening gap  80  (as illustrated in  FIG. 5 ) between chisel head  50  and jamb nut  40 . If gap  80  opens to the maximum possible width, bushing  34  impacts shoulder  46  of chisel shaft  18  in the manner of a slide hammer, causing undesirable effects. Chisel head  50  and possibly housing  16  may break free of the frictional forces holding the chisel head and housing in the bore before striker  14  impacts chisel shaft  18 . If the frictional forces holding chisel  50  in the borehole are overcome, chisel head  50  may be pulled rearwards from the borehole (right to left), undoing the work accomplished during the previous cycle of striker  14 . 
   Tool  10  of the invention reduces the likelihood of these undesirable effects by compensating in part for magnitude of the reaction force with an air spring. The gas spring in forward chamber  78  is created when the chamber is pressurized through tube  60 . In order for gap  80  to open as striker  14  is accelerated forward, bushing  34  must move toward shoulder  46  of chisel shaft  18 , overcoming the pressure in forward chamber  78  as the volume of the chamber is reduced. The force required to overcome the pressure in forward chamber  78  substantially offsets the reaction force accelerating tool housing  16 , reducing the amount of force that must be applied by the operator. 
   For example, in the case of one tool having a body diameter of 2.2 inches and a piston (striker) diameter of 1.614 inches, the reaction force generated when the striker  14  is accelerated is calculated to be 155 lbs, assuming a compressed air pressure of 100 psig. The calculated force to overcome the pressure in forward chamber  78  is 83 lbs., resulting in a net force of 72 lbs required to hold tool housing  16  in place as striker  14  is accelerated from left to right during the forward stroke of the striker. Thus, the operator of tool need only compensate for 72 lbs of force rather than 155 lbs. The effect is magnified in the case of larger diameter ground piercing tools. Further, the reduction in the amount of force required to compensate for the reaction force is accomplished without the use of a metallic spring, alleviating the breakage and design problems associated therewith. 
   Turning to  FIG. 7 , in an alternate embodiment, a ground piercing tool  100 , is in all respects substantially identical to tool  10  of  FIG. 1 , with the exception of supply tube  102 . As illustrated, supply tube  102  extends only partially into chisel shaft  18 , eliminating the need for radially extending ports  72 . 
     FIG. 9  is a further alternative embodiment of the invention wherein a ground piercing tool  110  is substantially identical to tool  10  of  FIG. 1 , except that a special vent passage has been added. As the seals of the tool begin to leak, the effectiveness of the air spring is diminished due to pressure in the space behind shoulder  46  that counteracts the pressure in chamber  78 . Vent passages  111 – 113  are provided behind enlarged diameter shoulder  46  of bit shaft  18  to ensure that the pressure on the back side of this piston remains very low. Passage  111  extends radially through anvil  22  from the surface of tapered transition  31  to open onto one or more outwardly opening, frontwardly extending grooves  112  on the outside of anvil  22 . The ends of these grooves  112  communicate with an annular gap  113  between jamb nut  40  and housing  16 . Gap  113  is open to the atmosphere. 
   Maintaining low pressure on the back side of the shoulder  46  ensures that the pressure supplied to the front side of shoulder  46  applies the maximum amount of force in the rearward direction (to reset the bit shaft). This aspect of the invention can also be used in connection with known designs that use a coil spring (U.S. Pat. No. 5,095,998 cited above) rather than the air spring described herein. 
     FIG. 10  illustrates a further embodiment of the invention wherein tube  60  and related structures are omitted entirely. Instead, a central valve  121  is biased against a seat  122  by a relatively large, durable spring  123 . Valve  121  is mounted in central bore  124  of bit shaft  126 , sealing chamber  78 . During the portion of the cycle in which the front pressure chamber ahead of striker  14  is pressurized, such pressure pushes back valve  121  a short distance, slighting compressing spring  123  and opening the passages leading to chamber  78 . Chamber  78  then remains pressurized during the exhaust stage of the cycle because valve  121  closes under the action of spring  123  when the pressure ahead of striker  14  drops. This embodiment avoids the need to provide an air supply tube and thus may have better durability that the previous embodiments. While certain embodiments of the invention have been illustrated for the purposes of this disclosure, numerous changes in the method and apparatus of the invention presented herein may be made by those skilled in the art, such changes being embodied within the scope and spirit of the present invention as defined in the appended claims.

Summary:
A ground piecing tool includes a housing and an air distributing mechanism that reciprocates a striker to impact a chisel shaft in response to a supply of compressed fluid including a fluid inlet tube mounted in the bore of the striker having a radial port, a rear end of the inlet tube being in communication with the distributing mechanism, wherein the housing and chisel shaft cooperate to define a front chamber that decreases in volume as the chisel moves forward relative to the housing, and wherein the chisel shaft has a radial passage therein that conducts compressed fluid from the radial port of the inlet tube to the front chamber, which is configured to form an air spring.