Abstract:
A water pressure operated post-hole digging tool. The tool includes a vertically elongate pipe with an enlarged mandrel member having a generally circular cross section mounted on its lower end, and an attachment for connecting the upper end of the conduit to a source of water pressure. The water pressure is discharged through an open end at the bottom of the mandrel member, so that soil is dislodged at the bottom of the post-hole bore. The flow of water then passes upwardly through the annular cavity around the sides of the mandrel member so as to scour out of the sides of the bore. The lower end of the mandrel member is tapered so as to accelerate the flow of water as this passes into the annular cavity around the body, and upper taper slows the flow as it exits the cavity and flows up the excavated bore. The mandrel member may be formed of solid plastic material.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/083,225, filed Apr. 27, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     a. Field of the Invention 
     The present invention relates generally to apparatus for digging postholes and other small-diameter holes in the earth, and more particularly, to a hand-held posthole digger which forms bores in the earth by application of water pressure. 
     b. Related Art 
     Posthole diggers are used to bore comparatively small diameter holes in the earth, for installation of fence posts, and other posts and supports of various kinds. For purposes of convenience, the term “fence post”, as used in this description will be understood to include all such supports and similar members, which are installed or otherwise placed so as to extend a few feet into the earth. 
     Although fence posts are, of course, widely used, they are particularly prevalent in the agricultural industries. For example, a typical livestock operation or orchard will have many hundreds or even thousands of fence posts, used for fences, to support trees or vines, and so on. Since the posts must be replaced or moved from time-to-time, the time and labor involved in installing fence posts becomes a major concern for many farmers, ranchers, orchardists, and so on. 
     Prior art posthole digging tools, however, have remained notoriously slow, labor-intensive, and inefficient in use. Manual posthole diggers have generally taken one of two forms: The most common comprises a pair of shovel-shaped blades which are joined together by a hinge, at the end of a pair of long poles; the operator thrusts the tool downwardly into the ground and then brings the blades together to withdraw a scoop of dirt. These tools are slow and tiring to use, and all the more inefficient due to their tendency to let dirt fall back into the hole as it is being withdrawn. 
     A second form of manual posthole digger uses a pair of auger like blades which define a bucket-like structure at the lower end of the tool, into which the dirt is scooped as the tool is rotated by the operator; after a few turns, the tool is lifted from the ground, dumped and the process is repeated. While superior in some respects to the type of tool described above, this latter form does not work well in rocky earth, and in any event remains an highly labor-intensive device. 
     Powered posthole diggers generally take the form of a long, screw-like auger mounted to a power head. Portable types are usually powered by a gasoline engine, and are comparatively fast in action. However, they are expensive, complicated, unreliable, and generally require two men in order to hold the power head against the torque which is generated by the bit during operation. These problems, especially the additional labor costs, represent serious drawbacks in commercial use. 
     Other types of posthole diggers are available which are designed to be mounted to a vehicle, such as a tractor or BOBCAT™ loader or similar vehicle for example. These devices are commonly driven using the vehicle&#39;s hydraulic system, and have the advantage of requiring only one operator. Again, however, they tend to be very expensive. Moreover, because they are mounted to the vehicle, this means that the vehicle must be maneuvered each time another hole is to be made, and in many instances there will not be enough room in which to maneuver the vehicle, as around trees or other obstacles. Once again, these drawbacks can present serious problems in a commercial operation, especially in the case of an orchard or a vineyard where many posts and other supports need to be placed in and amongst the plants themselves. 
     Accordingly, there exists a need for a posthole digger which is fast and efficient in operation, and which requires only a single operator, but which is also easily portable and convenient to use. Moreover, there exists a need for such a posthole digger which is simple and inexpensive in construction, and highly durable and reliable for extended use in a commercial operation. 
     SUMMARY OF THE INVENTION 
     The present invention has solved the problems cited above, and is a portable tool for formation of postholes and other bores by application of water pressure. 
     The tool includes a vertically elongate conduit having an enlarged mandrel member mounted on its lower end, and means for connecting the upper end of the conduit to a source of water pressure. The water pressure is discharged through an open end of the conduit at the bottom of the mandrel member, so that soil is dislodged and carried in the flow upwardly along the sides of the mandrel member, and then discharged out of the bore at the surface of the ground. 
     The mandrel member may be a vertically elongate, three-dimensional member which is generally circular in horizontal cross-section. The lower tip of the mandrel member may have a conical downward taper for penetrating into the ground, and for distributing the flow outwardly and upwardly over the body of the member. The body of the mandrel member may be generally cylindrical in form, and may have a diameter approximately equal to the diameter of the bore to be formed. The upper tip of the mandrel member may have an upward conical taper, over which the flow passes into the bore and to the surface. 
     The mandrel member may have a central bore through which the vertical pipe extends axially. The mandrel member may be detachably mounted to the pipe member so as to permit the mandrel member to be interchangeably removed and replaced with mandrel members of different sizes or shapes. 
     The tool may also include a valve for controlling the flow of water to the lower end thereof, and the control valve may be a manually operated ball valve. The tool may also include at least one transversely extending handle member for enabling the operator to exert a downward pressure on the tool. 
     The means for connecting the upper end of the vertical conduit to the source of water pressure may be a flexible hose for permitting the operator to manually maneuver and position the tool. The flexible hose may include a reinforced section where this is attached to the conduit. 
     The conduit may suitably be formed of pipe, such as galvanized steel pipe. The mandrel member may suitably be formed of dense plastic material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational view of a posthole digging tool in accordance with the present invention, showing the manner in which the tool is used to bore a hole into the ground, the ground being shown cut away for purposes of illustration; 
     FIG. 2 is an exploded view of the hole digging tool of FIG. 1, with this being disassembled to show the manner in which mandrel members of different sizes/shapes are interchangeably mountable on the lower end of the tool; 
     FIG. 2A is an elevational view of a mandrel member having a different shape that is interchangeably mountable on the lower end of the tool shown in FIG. 2; 
     FIG. 3 is an exploded view of the tool of FIGS. 1-2, showing the component parts of the assembly; 
     FIGS. 4A and 4B are, respectively, elevational and end views of a first size of mandrel member which is mountable on the lower end of the tool shown in FIGS. 1-3; 
     FIGS. 5A and 5B are, respectively, elevational and end views of a second size of mandrel member which is mountable on the tool of FIGS. 1-3; and 
     FIG. 6 is an elevational view of a tool in accordance with a second embodiment of the present invention, showing this with a tapered mandrel member on its lower end and a spray-containing skirt which is mounted around the main shaft of the tool. 
    
    
     DETAILED DESCRIPTION 
     a. Overview 
     FIG. 1 shows a posthole digging tool  10  in accordance with the present invention, as this is being used to form a hole  12  in the ground  14 . 
     As can be seen, the tool  10  includes an elongate, comparatively small-diameter primary pipe  16  which serves as the main shaft member of the assembly, with a mandrel member  18  being mounted on its lower end. The pipe extends through the mandrel member to an opening  20  at the lower end, and the upper end of the pipe, in turn, is connected to a supply of water pressure, via flexible hose segment  22 . A ball valve  24  controls the flow of water through the pipe, and a handle  26  extends laterally from one side of the pipe to enable the operator to exert a downward pressure on the assembly. 
     In operation, water pressure is supplied to the assembly through the flexible hose segment  22 , in the direction indicated by arrow  28 . With valve  24  open, the water passes down the length of the main pipe  16 , and is discharged through the opening  20  at the bottom of the mandrel member, as indicated by arrows  30  in FIG.  1 . The comparatively high-pressure/high-velocity flow at the discharge opening breaks up and dislodges the soil in the area below the mandrel tip, fluidizing and suspending the material in the water. 
     The lower end of the mandrel member is provided with a conical taper  32  which helps the mandrel penetrate into the ground, and also directs the flow of water/mud (which has been deflected by the bottom of the hole) in an upward and outward direction, so that the flow passes upwardly and is distributed more or less evenly around the cylindrical main body  34  of the mandrel. This forces the flow into an annular channel  36 , which scours the walls of the hole so as to form a more-or-less smooth-walled, constant-diameter bore; the relatively narrow, constricted annular passage increases the velocity of the flow in this area, thereby enhancing the scouring action. This also eliminates any need to twist or turn the tool in order to form a round, symmetrical bore. 
     The upper end of the mandrel is provided with an upward taper  38  so that the flow above the mandrel opens back into the bore, which slows the flow somewhat so as to reduce the amount of splashing/spray at the surface. The flow passes upwardly through the bore, carrying the mud and other suspended soil materials with it, and discharges out of the top of the hole over the surface of the ground, as indicated by arrows  40 . Once the hole has reached the desired depth, the tool is removed and repositioned to form the next bore. 
     The tool of the present invention has proven capable of digging fence postholes very rapidly, in a matter of just a few minutes, using water pressures as low as 30-40 psi. This allows the tool to be used with comparatively low-pressure water systems, such as in some municipal and well water systems. Moreover, for commercial/agricultural applications, much higher water pressures can be used, e.g., on an order of 150 psi: For example, water pressure can be supplied to the tool from a PTO-driven sprayer pump on a tractor or other vehicle, making for very rapid formation of postholes. Unlike the conventional vehicle-mounted posthole diggers described above, however, the connection to the vehicle is via a flexible hose, so that the operator is free to move the tool about and into confined areas, such as amongst trees, vines, and so on. 
     Moreover, as is shown in FIGS. 2 and 2A, the tool can be converted quickly and easily to form bores having different diameters, depending on the size of post to be installed or other factors. As can be seen, the main vertical pipe  16  of the tool has a detachable lower segment  42  which extends through the central bore in the mandrel member and threadingly engages a coupling  44  on the lower end of the upper pipe segment. Detaching and withdrawing the lower pipe segment  42  permits the mandrel  18  to be interchangeably replaced with mandrels  46  of different sizes or shapes, e.g., mandrels of smaller or larger diameters. To install the second mandrel, the detachable pipe segment is slipped through the central bore of the mandrel and then reattached to the main pipe section; the coupling  44  and a flange  48  on the bottom of the detachable pipe section act as upper and lower stops to hold the mandrel member in place and prevent it from sliding longitudinally along the shaft. 
     b. Components/Assembly 
     The exploded view in FIG. 3 shows the individual components of an exemplary tool assembly in accordance with the present invention. This embodiment differs slightly from that shown in FIGS. 1-2, in that the main pipe extends continuously through the bore of the mandrel member, with a detachable retainer being threaded on the lower end rather than the lower section of the pipe itself being detachable. 
     Accordingly, the main pipe  16  includes an elongated pipe nipple  50  having a lock ring  52  mounted partway along its length. The mandrel member includes a central bore  54  through which the lower section  56  of the pipe (below lock ring  52 ) extends in a close-fitting, but sliding engagement. The length of the lower section  56  is such that the threaded lower end  58  of the pipe protrudes just beyond the bottom of the mandrel member, for attachment of a pipe nut  60  which secures the mandrel member to the pipe. In some embodiments a conical metal nozzle may be used in place of or in conjunction with the pipe nut  60  to provide the tool with enhanced penetration and wear characteristics. 
     The upper end of the long nipple  50 , in turn, is threaded into one leg of a pipe tee  62 . A comparatively short (e.g., 6″) nipple  64  is threaded into the transverse leg of the tee so as to form the handle  26 , with a pipe cap  66  being threaded onto the end of the handle. 
     The ball valve  24  is mounted to the second leg of the tee by a close nipple  68 . The ball valve is provided with a control lever  70  which permits convenient manual operation of the valve; as is conventional in such valves, the handle is preferably attached so that the valve is open when the handle is aligned with the direction of flow, and closed when the handle is turned to the transverse position. 
     Another comparatively short pipe nipple  72  connects the ball valve  24  to one end of the flexible supply hose  22 , using a hose coupling  74 . The opposite end of the supply hose is provided with a second hose fitting  76  for attachment to the hose from the pressure source (not shown). The flexible supply hose renders the tool more maneuverable and easier to use for a single operator; in a preferred embodiment, the flexible supply hose may be about a foot long, and provided with reinforcement/protection (such as wound wire sheath, for example) for additional strength and durability. 
     The exemplary embodiment which is shown in FIG. 3 employs ¾″ galvanized steel pipe as its main structural material, with an exemplary parts list being as follows: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 PN 
                 PARTS LIST 
                 QTY 
               
               
                   
                   
               
             
             
               
                   
                 1 
                 Supply Hose 
                 1 
               
               
                   
                 2 
                 3/4 × 6 Nipple 
                 2 
               
               
                   
                 3 
                 3/4 Ball Valve 
                 1 
               
               
                   
                 4 
                 3/4 Close Nipple 
                 1 
               
               
                   
                 5 
                 3/4 Tee 
                 1 
               
               
                   
                 6 
                 3/4 Cap 
                 1 
               
               
                   
                 7 
                 3/4 × 48 Nipple 
                 1 
               
               
                   
                 8 
                 3/4 Lock Ring 
                 1 
               
               
                   
                 9 
                 4 Inch Mandrel 
                 1 
               
               
                   
                 10 
                 3/4 Pipe Nut 
                 1 
               
               
                   
                   
               
             
          
         
       
     
     It will be understood, however, that the assembly may be constructed using different materials and a different arrangement of components than the particular example which has been given above. For example, while galvanized steel pipe has many advantages in terms of strength, durability and corrosion resistance, other embodiments may be constructed of other types of metal/plastic pipe or other types of conduit. Similarly, other types of valves may be used in place of the ball valve in some embodiments. 
     As for the mandrel member, this may also be formed of any suitable material. A heavy grade of plastic, such as an extruded UHWM plastic “log”, cut to length, bored, and then lathed to form the tapered ends, is eminently suitable for this purpose in terms of strength and durability; also, the weight of the solid plastic material provides the tool with good balance and handling characteristics. It will be understood, however, that any suitable material may be used to form the mandrel member, such as various metals, plastics or molded resins, for example. 
     Suitable dimensions for two sizes of interchangeable mandrel members are shown in FIGS. 4A-4B and  5 A- 5 B. 
     The main body portion  82  of the mandrel  80  shown in FIGS. 4A-4B has a cylindrical radius R 1  of about 2.00″, and an overall length L 1  of about 22.0″. The upper and lower tapers  84  and  86  are each formed at an angle θ 1  of about 80°, (i.e., a 10° taper) with the radius rr 1  at the tip of the mandrel being about 0792″. The radius rr 1  of the axial bore  88 , in turn, is about 0.542″, so as to provide sufficient clearance for the mandrel member to slide onto the end of the lower end of the main pipe, but without creating excessive “slop”. 
     FIGS. 5A-5B, in turn, show the dimensions of a second size mandrel  90  which is mountable to the tool interchangeably with that shown in FIGS. 4A-4B. Exemplary dimensions for this size of mandrel are as follows: radius R 2  of main body  92  −1.00; overall length L 2  of mandrel  90  −22″; angle θ 2  of upper and lower end tapers  94 ,  96  −80°; tip diameter r 2  −0.613″; diameter rr 2  of bore  98  −0.425″. 
     Mandrel members having the dimensions described above have demonstrated excellent performance when used in common soils and when supplied with water pressures in the range described above. It will be understood, however, that mandrel members having other dimensions and/or shapes may be employed in the present invention, including, for example, that which is shown in FIG.  6 . 
     c. Additional Features 
     FIG. 6 shows a tool  100  in accordance with another embodiment of the present invention. As can be seen, this includes a mandrel member  102  which, rather than having a generally cylindrical form as in those described above, has a bulbous, downwardly-pointed head portion  104  at its lower end, and a long, gently tapered portion  106  at its upper end. This form of mandrel may be preferable for use in certain types of soils. 
     The tool shown in FIG. 6 also includes a “T”-shaped head  108  having first and second handles  110   a ,  110   b , which enable the operator to exert a strong downward pressure on the device. 
     Furthermore, the assembly includes an optional skirt-shaped spray shield  112 , which serves to protect the operator against water/mud exiting the top of the bore. As can be seen, the shield includes a conical fabric skirt  114 , which extends from a small-diameter upper ring  116  which fits closely about the pipe  16 , to a larger-diameter bottom ring  118  which is sized to fit over and around the top opening of the hole. A conical spring  120  inside the skirt biases the upper and lower rings apart so as to maintain the conical shape of the shield. 
     The upper ring  116  is in sliding engagement with pipe  16 , so that the spray shield is free to move up and down along the pipe while ring  118  rests on the surface of the ground. Thus, the spray shield can be slid to a lowermost portion in which it substantially surrounds the mandrel member, thereby protecting the operator as boring begins. Then, as the tool penetrates downwardly into the ground, the skirt slides up the pipe while the bottom ring maintains contact with the surface around the opening. A stop ring  122  is mounted around the pipe below ball valve  24 , to arrest vertical movement of the shield at this point; as the tool is depressed further into the ground, the spring  120  yields resiliently so that the skirt gradually collapses while maintaining the cover over the hole. 
     It is to be recognized that various alterations, modifications, and/or additions may be introduced into the constructions and arrangements of parts described above without departing from the spirit or ambit of the present invention.