Abstract:
A ground piercing tool of the invention includes an elongated tubular housing having front and rear openings, a head including an anvil secured in the front opening of the housing, a striker disposed for reciprocation within an internal chamber of the housing to impart impacts to a rear impact surface of the anvil for driving the tool through the ground, a reciprocation mechanism for reciprocating the striker, and a tail assembly mounted in a rear end opening of the housing that secures the striker and reciprocation mechanism in the housing. A plastic tube forms the body of the housing, and the plastic tube is secured to the head. The reciprocation mechanism is preferably an air distributing mechanism that uses compressed air to reciprocate the striker. Such a tool can be used to install an underground pipe by welding a front end of the replacement pipe coaxially to a rear end of the plastic tube, operating the tool over a run to pull the pipe into the ground, and when the tool has reached the end of the run, disconnecting the tool from the replacement pipe.

Description:
TECHNICAL FIELD 
     This invention relates to impact tools and methods of use of such tools, particularly self-propelled pneumatic ground piercing tools. 
     BACKGROUND OF THE INVENTION 
     Self-propelled pneumatic tools for making small diameter holes through soil are well known. Such tools are used to form holes for pipes or cables beneath roadways without need for digging a trench across the roadway. These tools include, as general components, a torpedo-shaped steel body having a tapered nose and an open rear end, an air supply hose which 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. The striker impacts against the front wall (anvil) of the interior of the tool body, causing the tool to move violently forward into the soil. The friction between the outside of the tool body and the surrounding soil tends to hold the tool in place as the striker moves back for another blow, resulting in incremental forward movement through the soil. Exhaust passages are provided in the tail assembly of the tool to allow spent compressed air to escape into the atmosphere. 
     Most impact boring tools of this type have an air distributing mechanism which utilizes a stepped air inlet. The step of the air inlet is in sliding, sealing contact with a tubular cavity in the rear of the striker. The striker has radial passages through the tubular wall surrounding this cavity, and an outer bearing surface of enlarged diameter at the rear end of the striker. This bearing surface engages the inner surface of the tool body. 
     Air fed into the tool enters the cavity in the striker through the air inlet, creating a constant pressure which urges the striker forward. When the striker has moved forward sufficiently far so that the radial passages clear the front end of the step, compressed air enters the space between the striker and the body ahead of the bearing surface at the rear of the striker. Since the cross-sectional area of the front of the striker is greater than the cross-sectional area of its rear cavity, the net force exerted by the compressed air now urges the striker backwards instead of forwards. This generally happens just after the striker has imparted a blow to the anvil at the front of the tool. 
     As the striker moves rearwardly, the radial holes pass back over the step and isolate the front chamber of the tool from the compressed air supply. The momentum of the striker carries it rearward until the radial holes clear the rear end of the step. At this time the pressure in the front chamber is relieved because the air therein rushes out through the radial holes and passes through exhaust passages at the rear of the tool into the atmosphere. The pressure in the rear cavity of the striker, which defines a constant pressure chamber together with the stepped air inlet, then causes the striker to move forwardly again, and the cycle is repeated. 
     In some prior tools, the air inlet includes a separate air inlet pipe, which is secured to the body by a radial flange having exhaust holes therethrough, and a stepped bushing connected to the air inlet pipe by a flexible hose. These tools have been made reversible by providing a threaded connection between the air inlet sleeve and the surrounding structure which holds the air inlet concentric with the tool body. The threaded connection allows the operator to rotate the air supply hose and thereby displace the stepped air inlet rearwardly relative to the striker. Since the stroke of the striker is determined by the position of the step, i.e., the positions at which the radial holes are uncovered, rearward displacement of the stepped air inlet causes the striker to hit against the tail nut at the rear of the tool instead of the front anvil, driving the tool rearward out of the hole. See, for example, Wentworth et al. U.S. Pat. Nos. 5,025,868 and 5,337,837. 
     Expanders are tapered, ring-shaped shells that fit over the tapered nose portion of an earth boring tool in order to widen the hole made by the tool as it passes through the ground. In this manner, a 4 inch diameter tool may be used to make a 6 or 8-inch diameter hole. The tool is often sent through to make an initial bore, and then sent through a second time with the expander in order to widen the existing hole and/or crack an existing pipe. According to a known method, a plastic pipe may be attached to the back of the expander with the above described reversible tool inside the pipe so that the pipe is installed as the tool moves through the soil, with or without additional widening of the bore. 
     The tool body of the pneumatic impact tool used in such pipe pulling operations is the single largest and most expensive component of the tool, weighing over 1000 pounds in a large diameter tool. Known tool bodies include a long tubular housing and a tapered nose, which may be integral with the housing or a separate assembly secured to it. The former are made by machining or by swaging as described in Wentworth et al. U.S. Pat. No. 5,487,430, issued Jan. 30, 1996. The present invention provides a pneumatic impact tool specially adapted for pipe pulling operations in which the traditional tubular steel tool body is eliminated. 
     SUMMARY OF THE INVENTION 
     A ground piercing tool of the invention includes an elongated tubular housing having front and rear openings, a head including an anvil secured in the front opening of the housing, a striker disposed for reciprocation within an internal chamber of the housing to impart impacts to a rear impact surface of the anvil for driving the tool through the ground, a reciprocation mechanism for reciprocating the striker, and a tail assembly mounted in a rear end opening of the housing that secures the striker and reciprocation mechanism in the housing. According to the invention, a plastic tube forms the body of the housing, and suitable means are provided for securing the plastic tube to the head, which is made of steel or a material of comparable strength. The reciprocation mechanism is preferably an air distributing mechanism that uses compressed air to reciprocate the striker, but a hydraulic or electric reciprocating mechanism could also be used. 
     According to another aspect of the invention, a method using the foregoing tool is provided for installing an underground pipe. This method includes the steps of welding a front end of the replacement pipe coaxially to a rear end of the plastic tube, operating the tool over a run to pull the pipe into the ground, and when the tool has reached the end of the run, disconnecting the tool from the replacement pipe, preferably by sawing through either the plastic tube or the replacement pipe at the weld or a location proximate the weld. 
     Other 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. 1 is a lengthwise sectional view of an apparatus according to the invention; 
     FIG. 2 is a lengthwise sectional view of the apparatus of FIG. 1, fitted with an expander for pulling an oversize replacement pipe; 
     FIG. 3 is a cross-sectional view taken along the line  3 — 3  in FIG. 1; and 
     FIG. 4 is a schematic diagram of the tool of FIG. 1 used to carry out a pipe bursting and replacement method according to the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to FIGS. 1-3, a pneumatic impact tool  10  includes, as main components, a tool body  11 , a striker  12  for impacting against the interior of body  11  to drive the tool forward, an air distributing mechanism in the form of a stepped air inlet conduit  13  which cooperates with striker  12  for supplying compressed air to reciprocate striker  12 , and a tail assembly in the form of a valve anchor  14  which allows exhaust air to escape from the tool and secures conduit  13  to body  11 . 
     Tool body  11  of the invention comprises a cylindrical plastic tubular housing (tube)  21  and a bursting head  22  having a forwardly tapered outer surface  23 . Plastic tube  21  is made of a high-strength, highly rigid plastic such as high density polyethylene (HDPE), whereas bursting head  22  is normally made of a rigid, impact-resistant metal such as steel. Plastic tube  21  has an outer diameter of from about 4 to 24 inches and a thickness of from about 0.3 to 2.0 inches, the required thickness being less for smaller diameter pipes. In particular, to accommodate the air pressures commonly used in such tools, the thickness of the pipe should about 8% the outer diameter of the tool. Screws  25  set flush in aligned holes  26 ,  27  in a tubular rear portion  28  of head  22  and a front end portion of tube  21 , respectively, removably secure bursting head  22  to tube  21 . Other suitable means for securing head  22  to tube  21  may also be employed, for example a threaded connection, an adhesive, welding, friction-based pulling devices such as a steel mesh that tightens about the surface of the plastic tube, and other alternative mechanical fasteners. 
     Striker  12  is disposed for sliding, back-and-forth movement inside of tool body  11  forwardly of conduit  13  and valve anchor  14 . Striker  12  comprises a cylindrical rod having a rearwardly opening recess  32 . A front bearing land  33  and a rear plastic bearing ring  36  disposed in an annular groove  37  in the outer periphery of striker  12  support it for movement along the inner surface of tube  21 . Annular land  33  is interrupted by a pair of external grooves or flats  38  at the front of striker  12  which conduct compressed air to the front of the striker  12 . 
     A circular front impact surface  29  of striker  12  impacts against an impact surface  31  formed on the inside of bursting head  22  as shown in FIG. 1. A number of rear radial holes  42  in striker  12  allow communication between recess  32  and an annular space  43  between striker  12  and tube  21 . Annular space  43  and flats  38  comprise the front, variable pressure chamber of the tool. 
     Stepped air inlet conduit  13  includes a flexible hose  51  and a tubular bushing  52 . Hose  51 , which may be made of rubberized fabric, is secured to a fitting  55  threadedly coupled to a front end opening of a central tubular inner sleeve or hub  54  of valve anchor  14 , which is in turn coupled by a fitting  57  to a further length of hose  53  which ultimately connects tool  10  with the air compressor. The inner end of hose  51  is secured to a rear end of a fitting  58 , which is threadedly coupled to a rear end opening of bushing  52 . An axial bore  56  which extends through hub  54 , hose  51 , and bushing  52  allows compressed air to pass from hose  53  through recess  32 . 
     The cylindrical outer surface of bushing  52  is inserted into recess  32  in slidable, sealing engagement with the wall thereof. Recess  32  and the adjoining interior space of stepped conduit  13  together comprise a rear, constant pressure chamber which communicates intermittently with the front, variable pressure chamber by means of holes  42 . Bushing  52  may, if needed, have a pair of front and rear plastic bearing rings  59 A,  59 B disposed in respective annular peripheral grooves to reduce air leakage between bushing  52  and the wall of recess  32 . 
     Valve anchor  14  is considerably simpler in structure than tail assemblies employed in comparable known tools. As shown in FIG. 3, four thin, flat spokes  61  connect hub  54  to an annular outer sleeve  62 . Sleeve  62  is in close contact with the inner surface of tube  21  and is removably secured thereto by countersunk screws  63  positioned at positions midway between adjoining spokes  61 . Four sets of screws  63  set 90° apart are used with four spokes  61  set 90° apart. However, other arrangements may be employed, and alternate means for fastening may be used as described above in connection with screws  25 . In the illustrated embodiment, spokes  61  include outer, forward extensions  64  which permit sleeve  62  to be located forwardly of hub  54  for convenience of installation and to provide a broader surface for engaging the inner surface of plastic tube  21 . The external surface of outer sleeve  62  may be barbed or roughened to provide better engagement with the inside of tube  21 . Exhaust from the front pressure chamber passes between spokes  61  and exits through replacement pipe  80 A. 
     Bursting head  22  may optionally include forwardly directed blades or sharp edges to aid in pipe bursting, as known in the art. Head  22  may comprise a single piece, or may include a central rod  71  having a frustoconical rear end portion that is seated in a tapered rear end opening  72  of a central bore  73  in head  22 . A front locking ring  74  is threadedly coupled to the outside of a frontwardly protruding portion of rod  71 . Screws  76  are adjusted to push against the front face of head  22 , putting rod  71  under lengthwise tension. The frontwardly protruding portion of rod  71  has an eye  77  for attachment of a towing cable. Rod  71  may be replaced as needed; eye (or clevis)  77  occasionally breaks. The rear end of rod  71  is slightly spaced from the rear wall of head  22 , so that rod  71  does not receive direct impacts from striker  12 . 
     A replacement pipe  80 A having the same outer diameter as tool body  11  is attached by any suitable means to a rear end portion of body  11 , most preferably by welding to form a weld  79 . This is particularly advantageous where tool body  11  is itself a section of the same HDPE pipe  80 A to be installed. To form an effective weld, the tool body and replacement pipe must be made of compatible materials, and would normally be made of the same material. In this manner, towing of the replacement pipe can be carried out without an adapter or similar device for pulling the replacement pipe along. However, a towing connector of a type known in the art may be employed if desired. Tool  10  as such can be provided by sawing off a suitable length of a replacement pipe and installing the striker  12 , air inlet conduit  13  and valve anchor  14  by drilling holes and mounting screws as described above. 
     FIG. 2 illustrates an alternate use of tool  10  wherein the tool is used to install a replacement pipe  80 B having a larger diameter than tool body  11 . An expander collar  81  is fitted to and held against rearward movement by a tapered rear portion  82  of head  22 . A rearwardly opening counterbore  83  receives a front end of the replacement pipe  80 B, which is secured thereto by two or more sets of screws  84  in the same manner as head  22  is secured to plastic tube  21 . Alternate means for fastening may be used instead of screws  84  as described above in connection with screws  25 . Differently sized expanders may be used so that tool  10  can be adapted to various replacement pipe sizes. Pipe  80 B may be any common commercially available plastic pipe, such as PVC, and need not have the high strength that tube  21  has. 
     FIG. 4 illustrates a method of the present invention using tool  10 . Prior to the bursting and replacement operation, the rear end of tool body  11  is welded directly to the leading end of replacement pipeline  80 A to be installed. In the alternative, pipe  80 B is inserted into the gap between expander  81  and tool body  11  and secured to expander  81  with screws  84 . Air hose  53  is fed through pipe  80 A or  80 B and connected to tool  10  for supplying compressed air to operate tool  10 , which is then positioned at the entrance to the existing borehole or pipeline in a front (launch) pit  91 . 
     At a rear (exit) pit  92 , a pulling apparatus  93  is positioned near an exit opening of an existing pipeline  98 , such as a 4-inch diameter clay pipe, to provide a pulling force on a steel cable  97  which extends in the axial direction through the interior of existing pipeline  98 . Cable  97  may be threaded by hand using a fiberglass rod through pipeline  98  and is attached to eye  77 . Cable  97  may comprise a steel cable, chain, rope, or other similar device. Pulling apparatus may be a truck-mounted winch, but is preferably a portable winch of the type described in co-pending U.S. Ser. No. 08/888,893, filed Jul. 7, 1997, the contents of which are incorporated by reference herein. 
     Pulling apparatus  93  is then operated to provide a continuous pulling force on tool  10 . Tool  10  is placed with head  22  in contact with an entrance opening of the existing pipeline or borehole  98 , and then operated in forward mode. Tool  10  moves progressively through the existing pipeline  98  as shown. Head  22  bursts (shatters or slits) pipeline  98  while tool  10  pulls replacement pipe  80 A into position. Use of pulling apparatus  93  is optional, but is preferred because the pulling force in the lengthwise direction of pipeline  98  prevents tool  10  from going off course, and the extra pulling force prevents the tool from stalling during long runs (over  50  feet) due to the weight of the trailing plastic pipe. 
     Exit pit  92  may comprise an existing underground structure such as a manhole or basement. Once tool  10  emerges into pit  92  far enough so that the replacement pipeline can be separated, the compressed air supply is turned off. For the embodiment of FIG. 1, tool body  11  is sawed off from pipe  80 A and then withdrawn from the pit  92 . For the embodiment of FIG. 2, the tool must emerge into the pit far enough so that screws  84  of expander  81  can be removed. Expander  81  is then removed in the exit pit  92 . 
     Tool  10  can be pulled out of pipe  80 B in the forward direction, or if the greatest outer diameter of head  22  is less than the inner diameter of pipe  80 B, tool  10  can be withdrawn rearwardly by pulling on air hose  53  or a cable trailing from the rear end of tool  10 . The latter option may be best for situations where the exit pit is narrow, or a man-made obstacle (such as the diameter of a manhole opening) prevents tool  10  from being withdrawn from the exit pit. Even under such conditions, tool  10  can usually be withdrawn forwardly out of the exit pit because it is relatively short, e.g., 36 inches long, in comparison to conventional pneumatic ground piercing tools that include a full size steel body and a reversing mechanism. 
     Tool  10  cycles rapidly in comparison to conventional pneumatic ground piercing tools because the valve diameter is large relative to the weight of the striker. A conventional 12-inch diameter pneumatic uses a striker that weighs about 900 pounds and cycles about 250 times per minute, whereas tool  10  of the present invention in one preferred embodiment uses a 250 pound striker that cycles 550 times per minute. A striker having a weight of from about 50-500 pounds that cycles from 300-800 times per minute is generally preferred. The weight of the striker increases with increasing tool diameter, such that a 50 pound striker is preferred for a 4-inch tool and a 500 pound striker is preferred for a 24-inch tool. When used by attaching the replacement pipe directly to the back of the tool body, the tool of the invention provides more power than a comparable metal-bodied tool that is inserted inside the replacement pipe and pulls the pipe along using a expander. In such a situation, the tool diameter is reduced relative to the diameter of the existing pipeline to be ruptured and relative to the replacement pipe. 
     It will be understood that the foregoing description is of preferred exemplary embodiments of the invention, and that the invention is not limited to the specific forms shown. Modifications may be made in the design and arrangement of the elements without departing from the scope of the invention as expressed in the appended claims.