Pneumatic ground piercing tool with movable chisel head

A ground piercing tool has an elongated tubular tool housing and a striker that reciprocates within an internal chamber of the housing. The striker impacts a front impact surface and drives the tool through the ground. An air distributing mechanism causes the striker to reciprocate in response to a supply of compressed fluid. An anvil providing a front impact surface is disposed in a front end opening of the tool housing. The anvil has a rear frustoconical portion that engages a like-shaped forwardly tapering inner wall of the housing and a lengthwise bore that has an enlarged diameter portion. A chisel is slidably disposed in the bore of the anvil and has an outwardly opening annular groove. A spring is confined in compression in a space defined by the annular groove of the chisel and the enlarged diameter portion of the anvil bore. The spring urges the rear end of the chisel to protrude rearwardly from the bore of the anvil a predetermined distance. The spring compresses as the striker delivers an impact to the chisel and moves the chisel forward until the striker contacts a rear end of the anvil.

TECHNICAL FIELD OF THE INVENTION
 The invention relates, in general, to pneumatic impact tools and, in
 particular to a movable chisel head for a pneumatic impact tool.
 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 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. 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 movement through the soil.
 Some pneumatic tools incorporate movable bits or chisels at the tapered
 nose section of the tool to more easily penetrate hard ground. Although
 this concentration of force is useful for penetrating obstructions, total
 tool displacement per impact is reduced. This inefficiency causes slower
 tool speeds when the tool is not penetrating hard ground conditions. To
 the extent the movable chisel and bit mounted on it move outwardly from
 the tool body during impact, there is also a tendency for soil to enter
 the gap behind the bit between the bit and tool body and cause the chisel
 to become jammed. Spektor U.S. Pat. No. 5,031,706 describes using a
 resilient gasket confined under compression between the movable head and
 chisel adapter as a means of preventing soil from entering behind the
 chisel.
 Some prior movable chisel tools have also relied on elaborate front end
 structures which are susceptible to breakage and more difficult to
 assemble and disassemble than tools wherein the anvil of the tool does not
 move. Another has the disadvantage of delivering direct impacts to the
 rear end of an anvil that is threadedly secured in a front end opening of
 the tool body, damaging the threaded connection and/or making it difficult
 to disengage the anvil when the chisel requires replacement. See, for
 example, U.S. Pat. No. 4,462,468.
 In normal operation, the chisel is spring-biased to a position at which its
 rear end protrudes beyond the read end or impact surface of the anvil. If
 the chisel encounters an obstruction and is not moved all the way forward
 as a result of receiving an impact from the striker, then all of the
 striker's energy continues to be transferred to the chisel with each
 repeated forward stroke until the rear end of the chisel is flush with the
 impact surface of the anvil. The present invention provides a movable
 chisel of simple but durable design that allows the tool to penetrate hard
 ground and maintain tool performance better than other movable chisels
 currently in commercial use.
 SUMMARY OF THE INVENTION
 According to a first aspect of the invention, a ground piercing tool
 includes an elongated tubular tool housing, a striker disposed for
 reciprocation within an internal chamber of the housing to impart impacts
 to a front impact surface for driving the tool forwardly through the
 ground, and an air distributing mechanism that reciprocates the striker in
 response to a supply of compressed fluid. An anvil disposed in a front end
 opening of the tool housing, which anvil includes a rear, outer
 frustoconical portion which engages a like-shaped forwardly tapering inner
 wall of the housing whereby the anvil is held in the housing. The anvil
 has a lengthwise bore therein, and a rear end of the anvil defines the
 front impact surface for the striker. A chisel is slidably disposed in the
 bore of the anvil, which chisel is movable between a rearwardmost position
 at which a rear end portion of the chisel protrudes from the bore of the
 anvil to receive an initial impact from the striker, and a forwardmost
 position at which the striker can impact directly on the rear impact
 surface of the anvil. A spring may be confined in a space between the
 chisel and anvil to cause the rear end of the chisel to protrude
 rearwardly from the bore of the anvil a predetermined distance. The spring
 compresses as the striker delivers an impact to the chisel and moves the
 chisel forward until the striker contacts the rear end of the anvil.
 Virtually all prior movable chisel ground piercing tools have used a
 spring. According to a further aspect of the invention, it has been found
 that the spring can be omitted and the chisel can be configured to return
 to a position at which a rear end of the chisel protrudes a short distance
 from the opening in the anvil. If the bit is held in place by the soil,
 the striker will impact the front anvil and can therefore drive the
 housing forward until it stops against the bit, thus resetting the chisel.
 If the bit is not secured by the soil, the striker impacts the chisel, and
 drives it forward through the already opened bore. Since the bore was
 already opened, very little energy is used in moving the bit and chisel.
 The striker will continue moving forward until it impacts the front anvil,
 driving the housing forward to catch up with the bit and chisel. In either
 case, the chisel is reset.
 A ground piercing tool according to this aspect of the invention having a
 housing, striker and air distributing mechanism as described above
 includes an anvil disposed in a front end opening of the tool housing. The
 anvil has a lengthwise bore therein, and a rear end of the anvil defines
 the front impact surface for the striker. The chisel is slidably disposed
 in the bore of the anvil and is movable between a rearwardmost position at
 which a rear end portion of the chisel protrudes from the bore of the
 anvil to receive an initial impact from the striker, and a forwardmost
 position at which the striker can impact directly on a rear impact surface
 of the anvil. The chisel has an enlarged diameter rear end portion or stop
 that is in close sliding contact with an enlarged rear end portion of the
 anvil bore such that the enlarged diameter rear end portion of the chisel
 engages a step at the front end of the enlarged rear end portion of the
 anvil bore when the chisel is in its forwardmost position. Most
 preferably, the step and a contact surface of the enlarged diameter rear
 end portion of the chisel have a forwardly tapering, frustoconical shape
 with a sufficient taper to cause the chisel to become temporarily locked
 in its forwardmost position during rearward tool operation.
 A bit is mounted on a front end portion of the chisel. The bit has an outer
 surface configured to engage the wall of a hole being bored so that as the
 striker first impacts the chisel and propels the chisel and bit forward,
 then impacts the impact surface of the anvil, and then moves rearwardly in
 preparation for another impact, the chisel and bit are propelled forward,
 increasing a gap between the bit and the front end of the housing.
 Thereafter, the housing is propelled forward, decreasing the gap between
 the bit and the front end of the housing and causing the chisel to assume
 its rearwardmost position without the aid of a spring. Removal of the
 spring improves the efficiency of operation because some of the striker's
 energy is lost in the process of compressing the spring on each impact.
 The invention further provides a ground piercing tool provided with a wear
 cover configured to fit over the nose of the ground piercing tool housing,
 which nose includes a reduced diameter cylindrical front end portion and a
 forwardly tapering portion rearwardly thereof. The wear cover comprises a
 cylindrical sleeve having a forwardly tapering inner surface that engages
 the forwardly tapering portion of the nose of the housing. It can be
 friction fit over the nose and replaced when necessary. The invention also
 provides a two-piece threaded bit system wherein the two bit sections are
 tightened against one another for enhanced security and the front end of
 the bit shaft is covered. A V-shaped notch with optional seal as described
 hereafter may be provided for preventing dirt from entering in behind the
 bit and jamming the movement of the bit and chisel. These and other
 aspects of the invention are described in detail below.

While various embodiments of the 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
 contexts. The embodiments discussed herein are merely illustrative of
 specific ways to make and use the invention and are not to limit the scope
 of the invention.
 DETAILED DESCRIPTION OF THE INVENTION
 Referring now to FIGS. 1-3, a pneumatic ground piercing tool 10 having a
 movable chisel assembly according to the invention includes an air
 distributing mechanism 11 for reciprocating a striker 12 disposed within a
 housing 13. Air distributing mechanism 11 includes a screw-reverse
 reversing mechanism actuated by rotating the air supply hose in a manner
 known in the art. Preferred air distributing mechanisms for use in the
 present invention are exemplified in U.S. Pat. Nos. 5,603,383, 5,505,270,
 5,487,430, 5,465,797, 5,199,151 and 5,025,868, the entire contents of
 which are hereby incorporated by reference herein. Compressed air is
 supplied through a hose to air distributing mechanism 11, which causes
 striker 12 to reciprocate within housing 13.
 Housing 13 is cylindrical and is swaged or machined to a reduced diameter
 at its forward end. Striker 12 slides within housing 13 for delivering
 forward impacts to a movable bit shaft or chisel 14 and to an anvil 16
 mounted at the forward end of housing 13. Anvil 16 is a preferably a steel
 tube that fits closely within around front end opening 17 of housing 13. A
 tubular front end portion 32 of anvil 16 protrudes from opening 17. A
 frustoconical rear end portion 31 thereof has an outer surface that
 engages a like-shaped inner surface of the swaged front end or nose 19 of
 housing 13 to retain anvil 16 in opening 17. Anvil 16 is interference fit
 into housing 13 but the taper thereof does not lock anvil 16 therein.
 Chisel 14 slides within a central bore 15 of anvil 16, which bore 15 is
 coaxial with housing 13. A bit 18 is secured on a forward end portion 33
 of chisel 14 that protrudes from tubular front end portion 32 of anvil 16.
 Bit 18 is secured by pins 20 inserted through outwardly opening,
 semi-circular grooves 21 in chisel 14 and corresponding transverse
 through-holes in bit 18. Pins 20 may be solid pins as shown or
 spiral-wound roll pins.
 In this embodiment, chisel 14 has an outwardly opening annular groove 23 at
 an intermediate position between forward end portion 33 and an enlarged
 diameter rear portion 28. Annular groove 23 cooperates with a rear,
 enlarged diameter portion (counterbore) 25 of bore 15 to form a
 circumferential cavity 27 between chisel 14 and anvil 16 that confines a
 tubular spring 22, which may be either a coil spring or elastomeric
 sleeve. Spring 22 is confined under compression so that it biases chisel
 14 to a rearwardmost position relative to anvil 16 as shown in FIG. 2. For
 this purpose, spring 22 engages a rearwardly facing annular step 29 at the
 front end of counterbore 25 and a rear wall 30 of annular groove 23. Wall
 30 is also the front edge of enlarged diameter rear portion 28 of chisel
 14.
 A replaceable steel wear cover 24 is press-fit over nose 19 of housing 13.
 Cover 24 protects the forward end of housing 13 from excessive wear caused
 by rock and soil abrasion. Wear cover 24 is preferably a steel sleeve
 having a rearwardly flared (or forwardly tapering) inner profile whereby
 it fits closely onto the tapered portion of nose 19. The outer diameter of
 wear cover 24 is preferably less than or equal to the outer diameter of
 the tool housing 13.
 As shown in FIG. 2, in order to prevent loosening of anvil 16, it is
 preferred that the rearwardmost position of movable chisel 14 be limited
 by the point at which a rear end surface of bit 18 contacts the front end
 of housing 13. In this position, a small gap remains between bit 18 and
 cover 24, and also between a radially inner portion of bit 18 and the
 front end of anvil 16. For this purpose, a rearward opening 34 of bit 18
 includes an annular step 36 for engaging the front end of housing 13 as
 shown and an inner cylindrical recess 37 into which the front end of anvil
 16 slidingly fits. A rearwardly extending tubular skirt 39 on the outer
 periphery of bit 18 at the rear end thereof covers the gap that opens and
 closes between bit 18 and both of anvil 16 and housing 13, hindering soil
 from entering. The gap that does exist between cover 24 and skirt 39 is
 shallow and never fully closes.
 FIG. 2 illustrates the instant striker 12 impacts chisel 14. Prior to
 impact, chisel 14 has been reset to protrude a distance 26 from the rear
 end of anvil 16. Spring 22 compresses as striker 12 impacts chisel 14. The
 forward end of chisel 14 forms a pilot hole in the earth for bit 18, and
 bit 18 expands the pilot hole to the diameter of housing 13. Depending on
 ground conditions, bit 18 may be configured to expand pilot bore to the
 diameter of the housing, or wider, or smaller. Different heads with
 different major diameters may be used to optimize tool performance. If
 tool 10 is piercing a hard obstruction, chisel 14 will likely be forwardly
 displaced by a distance less than distance 26. Chisel 14 can then pierce
 obstructions more efficiently than a comparable tool lacking a movable
 chisel because chisel 14 and bit 18 transfer a greater force from striker
 12 than would be the case if the momentum of the striker were transferred
 via anvil 16 to the tool housing 13 and the parts it carries, including
 the air distributing mechanism 11. This effect is optimized if the mass of
 the bit and chisel are selected to provide an optimum coefficient of
 restitution relative to the mass of striker 12, as suggested by Spektor
 U.S. Pat. No. 5,031,706.
 If soil is being pierced and no hard obstruction is present, chisel 14 may
 be forwardly displaced by the distance equal to (or greater than) chisel
 stroke 26 as shown in FIG. 3. In this case, residual impact force from
 striker 12 is transferred to anvil 16, and therefore moves the housing 13
 forward to close the gap between the back of the bit and front of the
 housing. The chisel is now reset to protrude a distance 26 from the rear
 end of the anvil 16, and the cycle can repeat.
 A tool 10 having a movable chisel with the foregoing structure is
 simplified in structure and avoids the use of a threaded connection at the
 front of the tool where the stresses from impact are greatest. If
 replacement of the movable chisel is necessary, pins 20 are tapped out and
 bit or expander 18 is removed. Upon removal of the striker 12 and air
 distribution mechanism 11 from the rear end of the tool 10, chisel 14 and
 can be loosened from engagement with housing 13 and removed.
 Referring now to FIGS. 4 and 5, a second embodiment of a tool 40 of the
 invention without spring 22 is illustrated. An enlarged diameter rear end
 portion 42 of a modified bit shaft or chisel 43 has a forwardly tapering
 step 44 that engages a rearwardly facing, forwardly tapering annular step
 45 in the bore of a modified anvil 46. The striker impact urges chisel 43
 forward until penetration resistance of the ground dissipates the impact
 force as described above, or end portion 42 engages step 44, assuming a
 countersunk position. Since the rear end of chisel 43 travels below the
 rear impact face of anvil 46, the total chisel stroke is greater than the
 distance 26, allowing the tool to make greater forward progress with each
 stroke.
 Chisel 43 is returned to the initial position shown in FIG. 4 as housing 13
 advances towards bit 18. If the bit is held in place by the soil, the
 striker can impact the front anvil and therefore drive the housing forward
 until it stops against the bit, thus resetting the chisel. If the bit is
 not secured by the soil, the striker impacts the chisel, and drives it
 forward through the already opened bore. Since the bore was already
 opened, very little energy is used in moving the bit and chisel. The
 striker will continue moving forward until it impacts the front anvil,
 driving the housing forward to catch up with the bit and chisel. In either
 case, the chisel is reset.
 As the remaining part of the momentum of striker 12 is transferred to anvil
 46, housing 13 and all the parts connected to it travel forward by a
 distance approximately equal to the stroke of chisel 43 between its
 extended and retracted positions. A spring for returning chisel 43 to its
 rearwardmost position is not needed. When the tool is in reverse mode and
 striker 12 is impacting against the tail nut instead of the anvil and
 chisel, the movable chisel assumes the forwardmost position shown in FIG.
 5, but returns to its normal starting position after forward travel
 resumes.
 FIGS. 6, 7 and 8 illustrate a third embodiment of a tool 60 according to
 the invention. Like tool 40, tool 60 lacks a spring and operates in
 substantially the same manner as tool 40, but is improved in several
 important ways. Housing 13 receives a front anvil 62 that has a modified
 enlarged diameter rear end portion 63 that tapers forwardly over some of
 its length to its front end. The angle of this front anvil taper is
 shallow enough (6.degree., or about 5 to 7 degrees) to temporarily lock a
 modified bit shaft 64 in the forwardmost position while the tool is in
 reverse operation as shown in FIG. 8, thus reducing detrimental blows to
 bit shaft 64. For this purpose, a rear end potion 69 of bit shaft 64 has a
 matching external taper as shown. Bit shaft 64 unlocks from this position
 upon the first impact from striker 12 in the forward mode of operation
 In this embodiment, the bit 68 is a two piece assembly that is attached to
 the bit shaft or chisel 64 by means of threaded connections. A rear bit
 section 71 is threaded onto a threaded, protruding front end portion 65 of
 bit shaft 64 until it stops against a shoulder 72 behind threaded end
 portion 65 but still protruding from anvil 62. A front bit section 73 is
 then threaded onto bit shaft 64, and rear bit section 71 is tightened
 against the front bit section 73 in a jam nut fashion, by any suitable
 means, such as external hex flats provided on each. Rear bit section 71
 moves forwardly a short distance as it is tightened against front section
 73.
 Front bit section 73 is preferably configured as a cap with a rearwardly
 opening, threaded blind hole 74 by which it is mounted on bit shaft 64,
 and a front end projection or "false chisel" 75 that resembles the
 protruding front end of the chisel 14, 43 of the preceding embodiments. A
 pointed carbide stud 79 may be centrally mounted in a forwardly facing
 position on projection 75 to enhance the tool's ability to break hard
 obstructions. A tubular rear flange 76 of front section 73 extends into a
 forwardly opening recess or counterbore 77 of a threaded through-hole 78
 of rear bit section 71, leaving a slight clearance in the lengthwise
 direction as shown. This permits tightening in jam-nut fashion as desired
 but prevents soil from working inside the two-piece bit assembly. The
 clamp-loading of the bit sections 71, 73 provides for more secure mounting
 of the bit 68 onto bit shaft 64 and easier disassembly in comparison to
 pin mounting systems or threaded connections that do not apply an axial
 clamp load to threaded connection between bit and shaft, or in comparison
 to systems wherein a nut threadedly mounted in front of the bit holds the
 bit on.
 A modified wear cover 81 has a forwardly tapered edge 82 that cooperates
 with a rearwardly tapered edge 83 of the skirt 84 of rear bit section 71
 to form a V-shaped notch 86 that dislodges dirt more effectively than the
 arrangement of FIGS. 4 and 5. An annular groove 87 may be provided near
 the front end of housing 13 slightly ahead of notch 86 when edges 82, 83
 are in contact (FIG. 6) for a seal bearing 88 that helps prevent
 penetration of grit into the space behind bit 68. Seal bearing 88
 preferably lies flush with the outer periphery of nose 19 of housing 13
 and thus does not expand and contract in an attempt to fill notch 86.
 Operation of tool 60 is substantially the same as described above for tool
 40. FIGS. 6 and 7 show the two stages of striker impact, first against bit
 shaft 64 and then against anvil 62. During reverse operation as shown in
 FIG. 8, striker 12 is accelerated rearwardly, impacting the rear anvil or
 tail nut in a manner known in the art, which subsequently drives the tool
 rearward. In this reverse mode, the bit 68 and bit shaft 64 are
 accelerated rearwardly when rear end potion 69 of bit shaft 64 bottoms
 against the taper of the bore 89 of anvil 62.
 Tool 60 in accordance with the foregoing description thus provides further
 improvements in the structure of the anvil, bit and bit shaft. The bit is
 less prone to breakage and more readily disassembled than comparable known
 designs. The chisel or bit shaft is protected by a front end cap rather
 than directly exposed in the borehole. This facilitates repair in that the
 front end cap (bit section 73) can be removed and replaced without taking
 the rest of the tool or its front end apart, whereas bit shaft 64 cannot.
 Bit shaft 64 is also better protected from stress by the adoption of a
 semi-locking taper for use when the tool is operating in reverse.
 Various modifications of the preceding embodiments are within the scope of
 the invention. For example, the bit may have a variety of shapes suitable
 for digging in various conditions, such as blades, projections or splines.
 The anvil may be shortened so that it does not protrude from the housing,
 eliminating the need for a rearwardly opening stepped bore in the bit. A
 rock-breaking tungsten carbide stud, rounded or pointed like stud 79, may
 be mounted on the front end of the movable chisel to enhance the tool's
 ability to break a rock or other obstruction. While for convenience of
 manufacture it is desirable to make the anvil, chisel, housing and wear
 cover radially symmetrical as described, some portions of the mechanism
 could be asymmetrical. For example, instead of enlarged diameter rear end
 portion 28, the chisel could have a radial projection that slides in a
 corresponding groove in the anvil bore. These and other equivalents are
 within the scope of the invention as expressed in the appended claims.