Abstract:
A rod pushing and pulling machine includes at least one hydraulic cylinder having a front end thereof engagable with a reaction surface at an entry opening of a existing pipeline or borehole, a spindle assembly, and a dual vise assembly. The spindle assembly includes a frame, a spindle shaft rotatably mounted in the frame, a distal end of the spindle shaft being threaded for engagement with a mating thread of a rod, a drive system for rotating the spindle shaft in threading and unthreading directions, the spindle frame being secured to a rear end of the hydraulic cylinder for pushing or pulling of a rod string engaged to the spindle shaft upon extension or retraction of the hydraulic cylinder, and a support assembly for the spindle shaft. The support assembly includes a set of roller bearings rotatably supporting the spindle shaft, a radial flange on the spindle shaft, and a load flange secured to the spindle frame positioned to engage the radial flange, whereby the radial flange comes into engagement with the load flange during pulling operation to prevent rotation of the spindle shaft during pulling operation, and leaves engagement with the load flange during pushing operation so that the spindle shaft may rotate during pushing operation supported by the roller bearings. The dual vise assembly has two pairs of separately actuable jaws positioned to grip a rod nearest the spindle shaft and a rod adjacent the rod nearest the spindle shaft.

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 10/916,117, filed Aug. 11, 2004, U.S. Pat. No. 7,140,806 to Wentworth et al. 
    
    
     TECHNICAL FIELD 
     The invention relates to underground pipe bursting and replacement systems of the static type which operate by pushing or pulling a string of rods to which a bursting head or other tooling is attached. 
     BACKGROUND OF THE INVENTION 
     Pipe bursting is a well known process that brings enormous potential for the efficient and unobtrusive replacement of buried pipelines. Currently the there are two widely used but separate systems used to accomplish pipe bursting. The choice of the system is most often dependent on the type of utility being upgraded. 
     Gravity sanitary sewer systems are made up of interconnected pipes buried at depths from (4) to (40) foot beneath the surface. These systems make use of ‘manholes’ to provide access for maintenance and cleaning of the interiors of the pipes. It is advantageous to minimize the damage and potential need for replacement of these manholes during the bursting operation. To do that requires practicing the method described in U.S. Pat. No. 6,299,382. That method calls for use of a pneumatic actuated tool, hydraulic winch with the guide cable passed through the existing pipe, and a front-mounted bursting head. Hence, primarily for that reason, most gravity sewer pipe bursting is done with pneumatic tools. 
     Potable water pipes are also widely in need of replacement. These systems, by the nature of the fact that they are pressure fed and therefore independent of the effects of gravity, tend to be buried at shallow depths in moderate climates. In addition, they do not have manholes, unlike the gravity sewer systems. For these reasons, water pipes are typically burst with a machine that requires an access pit at each end of the job. In the situation where there is no manhole present, having two access pits is not necessarily disadvantageous. The machine that fits this description is called a static system. Using significant hydraulically actuated force applied through a rod string, the tooling used on a static system splits the existing pipe and expands the surrounding soil. This style of bursting has four major components: 
     A. Tooling. This subsystem performs the function of cracking the pipe, expanding the adjacent soil with a conical form and a lastly provides a means of attachment of the product pipe to the rear of the tooling. 
     B. Rod String. The rods, threaded at each end, are engaged end to end into a string. This string transmits the pull force between the hydraulic pulling unit and the tooling. 
     C. Hydraulic power pack. This subsystem exists purely to provide pressurized hydraulic flow for operation of the pulling unit. The power pack may even be a hydraulic excavator configured to power auxiliary equipment as needed. 
     D. Downhole Unit. This is generally the most complex part of the machine; it entails the greatest amount of mechanism and complication of any of the four components. Hydraulic cylinders are employed to cyclically stroke a rod engagement system. The rod engagement may be through the threaded end, or by a mechanism that grips the rod outer surface or engages features on the outer surface. The engagement system-must grip or engage the rod and apply thrust force in one direction, while sliding freely along the length of the rod in the opposite direction. This system must have the capability of being shifted relative to direction of operation so that rods may be added to, or removed from the string. An optional subsystem in downhole unit is a device to aid in the threading and unthreading of rods. 
     Two known pit launch static bursting machines are known commercially as the McLaughlin pit launch and the Vermeer PL8000. These are low force (10,0000 lb) pulling machines having a hole approximately 8″ in diameter in the front of the machine to accept small backreamers into the machine. When this is done, the vise floats (moves) with the spindle to allow the tooling to enter the machine. Since the pulling force was low, the hole did not present major problems with respect to soil entry or shoring area. With higher pulling forces, providing a hole in the front shore plate of the machine becomes problematic because soil will tend to enter the machine through the hole. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an improved static bursting system. According to one aspect of the invention, the system provides for plain bearing pullback without rotation, while allowing rotation during payout. Rod string rotation must be a feature of this design available during rod payout. In many applications where an existing line has collapsed, the rod string or winch rope cannot be passed through the collapsed section successfully. A system that rotates the drill string makes it possible to guide the unit through the collapse as rod is added using either a non-directional drill bit or a drill head as typically used in horizontal directional drilling (HDD). 
     While working through this collapsed section, modest axial thrust in the range of 1000 to 40,000 lb may be applied to the bit through the drill string during rotation. This allows the bit to displace material with the option of delivering drill fluid through the hollow drill string to float the soil out of the existing pipe. The thrust must be applied to the drill string through a bearing. Roller bearings of this capacity are moderately but not excessively large and costly. 
     Pullback is the process wherein rod is removed from the drill string, shortening the string. The tooling is progressively pulled through the existing pipe, cracking the pipe and expanding the local soil. During the process, the rod string is not rotated. Forces applied to the drill string are in the range of 60,000 to 250,000 lb, significantly greater than during rod payout. Bearings of this capacity are very large and costly. To avoid the encumbrance of these bearings, according to the invention, a plain load bearing flange is used instead. This bearing will not support rotation during pullback and will in fact cause the rotation motor to stall should rotation be attempted when high axial pullback forces are applied. To achieve a successful design in this style, the shaft should be free to float a short distance between being loaded on the payout direction against the roller bearing and being thrust against the plain bearing load flange in the pullback direction. This float may be small in magnitude, e.g. between 0.05 and 0.25 inch. 
     It is advantageous but not necessary to preload the shaft against the roller bearing when no load is applied to the rod string. This preload can be modest, in the 500 to 2000 lb range. It is best achieved with a preloaded spring, the spring is depressed a short percentage of its design travel in the installed condition. This preload does not change as axial thrust in the payout direction is applied. As axial thrust in the pullback direction is applied, the modest spring force is overcome and the spring compresses further. This allows axial movement of the shaft relative to the bearings. As the shaft moves through the short distance per the design, it soon contacts the plain bearing load flange. 
     According to the invention, the drill string is allowed to rotate during payout to drill through obstructions within a collapsed pipe. During pullback, the unit is unable to rotate the rod string, therefore rendering tooling such as back reamers that function with rotation unusable. Pullback tooling is therefore limited to conical expanders, blades and other devices that perform hole and pipe expansion via axial movement only. 
     The invention further provides a “bungee vise” that aids in extending or retracting the drill string. During both the payout and pullback phases of a bursting job, the movement of the rod is stopped momentarily to add or remove the last rod of the string. During this moment, there are residual forces applying an elastic load to the rod string. During payout, that elastic load may be due to an arced path that the existing pipeline follows, or it might be due to an obstruction encountered at the front of the rod. Because the rod string is small in diameter compared to the existing host pipe, any load will cause an imperceptible buckling that will disappear should the load be released. The buckling uses up a small percentage of the length of the rod string, as little as nothing or as great as 12 inches. Unloading of the rod during the period when the rod is stopped would cause the rod to thrust back this distance, resulting in location issues for rod thread up and causing wasted travel every time the process is repeated. 
     During pullback, the process is similar but opposite. The elastic load applied to the tooling by the product pipe will produce residual load even when the rod string has been halted and work done by the tooling has halted. If the rod is not secured while the last rod is being removed, then the string will be pulled by the elastic pipe forces back into the bore. This distance can be anywhere from nothing to 3 feet depending on the soil conditions. 
     In order to overcome these potential problems with residual load, a vise of the invention is configured to grip the rod string and provide frictional force due to high hydraulically induced clamping forces. In addition to the frictional force, should the residual loads be exceptionally high, the gripping jaws are configured to encounter a shoulder on the rod after a brief amount of axial slippage. This slippage is best kept to a minimum, preferably in the range of 0.10 to 0.25 inch, in order to limit damage to the gripped surfaces on the rod and jaw. 
     The vise is called on to do another task, that of restraining the rod string from rotating while the last rod is being rotated with significant torque to either add it to or remove it from the drill string. While the threading operation is in process, the vise holds the residual axial load of the string while simultaneously preventing the string from rotating as torque is applied to add or remove another rod. 
     A further aspect of the invention relates to the thrust cylinders. In a static bursting system, the rod thrust or pullback is normally applied to the rod string via actuation of hydraulic cylinders. Normally, hydraulic cylinders are designed in a manner where flexible hydraulic hoses are plumbed to the cylinder body through ports in the cylinder wall. Pressurized hydraulic fluid is fed to the cylinders through these ports and the cylinder rod is extended or retracted relative to the cylinder body as a function of which port the fluid is supplied to. 
     In the commonplace configuration just described, most mobile hydraulic equipment is assembled with the cylinder body fixed or pinned to the frame of the machine. Further, the rod, not the cylinder body, is permitted to extend or retract relative to that same frame. This works well in most cases as the flexible hoses do not have to move any appreciable distance during movement of the rod. Should the rod end be pinned to the frame and the cylinder end with hoses attached be allowed to move, this would not be the case. 
     Moving hoses are prone to abrasive wear, leaking and pinching in machine features. Also, the slack in the hoses that must be accommodated in the retracted condition would make them prone to being snagged on other machine components and possibly torn out of the ports at either end. In the case of the machine used as an example herein, the travel of the cylinders is 46.5″ from fully retracted to fully extended. In this case, successfully accommodating moving hoses over that length would prove difficult. 
     Conventional cylinders as described above with a rod extending through one end of the cylinder have forces that are not equal in the extension and retraction directions. The area of the cylinder bore is always greater than the area of the cylinder bore less the cross sectional area of the rod. This is well understood in hydraulic cylinder application and allows the design of the cylinder to be tailored with variation in rod size should the retraction direction not be the primary work direction. A larger rod diameter causes the rod side of the cylinder to have a small area and therefore permits rapid retraction for a given hydraulic pump flow rate. A bursting machine using the concepts disclosed herein uses the more powerful extension direction to pull rod back, and the less powerful direction to thrust rod in the payout direction. The cylinders each have a rod that is large in comparison to the cylinder size, thus there is no compromise on performance of the machine due to either using cylinders in the ‘wrong’ direction, or using cylinders available through an industrial catalog that would have a small rod diameter. 
     Conventional static bursting machines are configured with the cylinder body stationary and the rod attached to the moving carriage. This carriage serves to grip or propel the rod string in the direction chosen by the operator. According to the invention, the cylinders are configured with the cylinder body attached to the carriage and the rod anchored at the front of the machine where they are loaded against the shoring plate. While the rod size in these cylinders is large in comparison to the cylinder body, it is still smaller than the cylinder body. By reversing the normal orientation, the tooling at the end of the rod string may be pulled into the machine and ‘docked’ between the cylinder rods. This would still be possible in the conventional orientation that has been described, however it should be understood that the machine would require greater overall width. This increased width has the potential to encumber operation and will require additional weight, added pit excavation, and greater difficulty in machine placement should there be other utilities located adjacent to the host pipe being burst. 
     The combination of the relatively large rod diameter coupled with the desire not to feed the hydraulic cylinders through flexible hoses that move with the bodies creates an opportunity to feed the cylinders through drilled longitudinal passages in the rod. The hydraulic hoses are attached to the rod at the rod end which is anchored to the frame or shore plate. These dual passages are drilled the length of the rod to provide both ingress and egress of the hydraulic fluid to the cylinder cavities. These passages eliminate any need for the hoses to move with the carriage. 
     Another option according to this aspect the invention that is used in the example below works with the reverse cylinder configuration to narrow the machine further. Offsetting the cylinders such that the cylinders are positioned diagonally relative to the frame of the machine, above and below the spindle shaft will orient the rods so that docked tooling may be removed from between the rods while still allowing the operator to stand close to the center line of the machine. This position close to the center line becomes important when the operator is loading or removing rods manually into/from the docking area along the centerline. 
     The invention further provides a collapsing rod cradle made to support and align a rod when it is added to or removed from the rod string. The design of this support or cradle becomes complicated when applied to a bursting machine such as that described previously and further having a spindle that applies the pulling or thrusting force via direct threaded attachment to the rod string. The rods are added or removed in a zone that is between the vise and the spindle frame face. Further, during this operation it is necessary for the front end of the rod to reside in the vise, the back half of the rod must sit in a cradle that is in the vicinity of the spindle face. During the cycle of traversing the spindle from right to left, the zone where the rod was added is now ‘compressed’ until the spindle frame nearly touches the vise. 
     While this right to left spindle frame movement is intended to move the rod string, it also results in the spindle frame occupying the volume where the rod cradle was performing its function of supporting the rod. Once the rod is tied into the rod string by making up the thread between the last and next to last rods, as well as between the last rod and the spindle, the cradle is no longer needed. It would be possible to use a cradle mechanism that collapses into the area below the spindle frame as the frame moves from right to left, and reset into position as the frame moved left to right. Such a design has been used in the past on machines such as the Vermeer PL8000 directional boring machine. In this case, spoil or contamination bound to enter into the machine and fall into the hull would impair the free movement of the device. 
     For the aforementioned reason, the cradle of the invention is preferably designed in not only a telescoping manner, but is also engineered to follow a path during retraction that would cause it to move away from the rod as it is retracted. This distance gained between the cradle and the rod helps prevent the rod upset from hanging up on the cradle as the relative axial movement occurs. Any entanglement between the cradle and rod could result in a damaged cradle mechanism. 
     In contrast to the known static pulling machines mentioned above, the machine of the invention uses a front shore plate with a slotted opening to provide good shoring area and limit soil entry into the machine. The shore plate is removed prior to entry of the tooling into the machine through a relatively large (15.5″ diameter) front hole. This is a unique feature when the floating vise is combined with a removable shore plate. These and other aspects of the invention are further described in the detailed description which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, where like numerals denote like elements: 
         FIG. 1  is a perspective view of a rod pushing and pulling machine according to the invention; 
         FIG. 2  is a perspective view of an auxiliary shore plate used with the machine of  FIG. 1 ; 
         FIG. 3  is a perspective view of the spindle assembly, jaw assembly and cylinders of the machine of  FIG. 1 , with the cylinders extended and the jaw assembly in its front position; 
         FIG. 4  is the same view as  FIG. 3 , with the cylinders retracted; 
         FIG. 5  is the same view as  FIG. 3 , with the cylinders extended and the jaw assembly in its rear position (the “final docking” position); 
         FIG. 6  is a perspective view of the jaw assembly of the preceding figures; 
         FIG. 7  is a top view of the jaw assembly of  FIG. 6 ; 
         FIG. 8  is a side view of the jaw assembly of  FIG. 6 ; 
         FIG. 9  is a front view of the jaw assembly of  FIG. 6 ; 
         FIG. 10  is a top view of the spindle assembly of the preceding figures with cradle extended; 
         FIG. 11  is a top view of the spindle assembly of the preceding figures with cradle retracted; 
         FIG. 12  is a side view, in section, taken along the line  12 - 12  in  FIG. 10 ; 
         FIG. 13  is the same view as  FIG. 12 , showing the cradle in its retracted position (line  13 - 13  in  FIG. 11 ); 
         FIG. 14  is a sectional view taken along the line  14 - 14  in  FIG. 10 ; 
         FIG. 15  is a top view of a thrust cylinder of  FIG. 1 , in an extended position; 
         FIG. 16  is a side view of the thrust cylinder of  FIG. 15 ; 
         FIG. 17  is a sectional view taken along the line  17 - 17  in  FIG. 16 ; 
         FIG. 18  a top view of the rod shown in  FIGS. 15-17 ; 
         FIG. 19  is an enlarged sectional view of the seal carrier shown in  FIG. 17 ; 
         FIG. 20  is an enlarged sectional view of rod seal shown in  FIG. 17 ; 
         FIG. 21  is enlarged side view of the piston and seals of  FIG. 18 ; 
         FIG. 22  is an exploded view of the cradle assembly of the machine of  FIG. 1 ; 
         FIG. 23  is a side view of a rod section used in the invention; 
         FIG. 24  is a lengthwise sectional view of the rod shown in  FIG. 23 ; and 
         FIGS. 25 and 26  are side and end views, respectively of the load flange of  FIG. 14 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a downhole machine  10  of a pipe bursting machine of the invention. A spindle assembly  9  including a spindle frame  12  is shown with its sheet metal cover in place. Spindle frame  12  traverses right-to-left a distance equal to 40% of the overall length of the entire machine. The spindle shaft  115  of spindle assembly  9  is connected to a rod string  11  by a threaded joint in the end of a spindle shaft extension  20  which is made as a separate part for ease of replaceability. 
     Force to perform the pipe bursting operation is applied to spindle frame  12  via a pair of hydraulic cylinders  26 . A cylinder rod  14  of each cylinder  26  is attached to a front shore plate  25 . Shore plate  25  is placed against the access pit wall and the face of the existing or host pipe. A rod box  31  stores rods to add to or remove from rod string  11 . When rod box  31  is full, tabs  47  are rotated upwards and a lifting hook is engaged. Box  31  is then replaced with a box full of rods or an empty box, as the situation demands. Box  31  sits on a tray  46 . Tray  46  holds box  31  in position to facilitate easy manual rod placement into or away from a rod cradle  18 . A front access door  48  is removed to extract or replace rods. Tray  46  is removable for transport. When tray  46  is removed, the mate to eye  49  is exposed. This pair of eyes  49  facilitate lifting the entire lower unit  10  into or out of the access pit. 
     Tie down loops  35  are used in transport to secure the lower unit  10  to a truck bed or trailer. A storage box  53  holds the operator&#39;s manual. A cover  27  protects a large pilot-controlled hydraulic valve (not shown). This valve facilitates the high hydraulic flows required to actuate the main thrust cylinders  26 . The pilot flows that control the valve are metered at a control station  37  by the machine operator. Direction and flow rate of the main thrust cylinders  26 , as well as spindle motor direction, are controlled at station  37 . 
     Four height adjustment legs  34  are provided at the four corners of the machine  10 . A hydraulic cylinder  41  of each leg  34  is secured to an outer frame  33  of leg  34 . Frame  33  is bolted to main hull  23  which contains the majority of the working components of the lower unit  10 . Extension of cylinder  41  moves inner leg  45  down, forcing foot  39  against the pit bottom. Foot  39  is free to pivot about a pin  43 . Similar height adjustment legs  34  are located at all four corners of the machine  10 . The cylinders  41  are actuated by the operator at hydraulic control station  29 . 
     At the front of the unit, shore plate  25  has notches  55  on its upper edge for mounting an auxiliary shore plate  19  thereon ( FIG. 2 ). Auxiliary shore plate  19  doubles up over shore plate  25  with its main face forward. A downwardly opening slot  21  allows the rod string to pass through plate  19  as well as allowing the auxiliary shore plate  19  to be lifted off rod string  11  when nearing completion of the bursting job. This removal becomes necessary so that the tooling may be drawn through the large center hole  28  in shore plate  25  surrounding the rod string  11 . Tabs  24 , located on both sides of plate  19 , fit into notches  55  to assure alignment and proper height of slot  21 . A series of slots  22  near the upper edge of plate  19  allow it to be lowered into or raised out of the pit. 
       FIG. 3  is an isometric view from the same vantage as  FIG. 1 , however it differs in that all external components of machine  10  have been removed. Spindle frame  12  is supported vertically by track rollers  17 . Two track rollers  17  are visible; they in fact exist at all four corners of the frame  12 . Track rollers  17  may be those available from Torrington Manufacturing, effectively small steel wheels with an internal needle roller bearing. In this view, cylinder body  16  is visible throughout its length. Rod cradle  18  is shown fully extended with a crotch  30  aligned with shaft extension  20 . Cylinder rod  14  is also fully extended, making the area for rod placement and removal of rods between shaft extension  20  and rod string  11  easy to see. 
     A vise assembly  15  is shown with rod string  11  clamped in one of two jaw sets  72 ,  73 . Serrations  51  on jaws  72 ,  73  can clamp on an added rod to apply torque. Vise  15  is further guided and restrained by cylinder rods  14  which pass through cylindrical sleeves  63  forming ends of the frame  36  supporting vise  15  for movement along cylinder rods  14 . Shoulders  13  at the front ends of cylinder rods  14  are mounted to and react in thrust against shore plate  25 . Hydraulic ports  57  and  61  on each rod  14  are used to connect flexible hydraulic supply hoses to feed the thrust cylinders  26  made up of rod  14  and cylinder body  16 . Hydraulic control valve  59  sequences the operation of the jaws in vise  15 . 
       FIG. 4  is the same set of components as  FIG. 3 , however rods  14  have been fully retracted into cylinder bodies  16 . With shoulders  13  attached to shore plate  25  (such as by bolts) and the shore plate  25  further bolted to hull  23 , the result of retracting rod  14  is actually to move cylinder bodies  16  and attached spindle frame  12  closer to shore plate  25 . In this position, vise  15  is very close to spindle frame  12 , leaving no room for rod cradle  18 . Rod cradle  18 , partially visible behind vise  15 , has retracted into spindle frame  12  with its supporting arms  101  inside spindle frame  12  and crosspiece  102  against the spindle frame  12 . Rod string  11  is now in position to be threaded to spindle extension  20 . This is accomplished by clamping the forward set of jaws  72  in vise  15  against rod string  11  (operation shown completed) and rotating the spindle extension  20  in the appropriate direction. 
     Referring to  FIG. 5 , rod  14  is then fully extended from cylinder body  16 . Vise  15  is pulled along with spindle frame  12  from its normal working position. This is accomplished by engaging jaw set  72  against rod string  11  and extending rod  14  to move the entire assembly of frame  12  and vise  15  to the right. This position is desirable when tooling must be pulled into hull  23  for final docking as explained hereafter. Vise  15  is more completely visible in  FIG. 6 . Sleeves  63  are hollow, permitting them to be centered on rods  14 . This provides torque reaction when the rod string is tightened, as well as permitting sliding along rods  14  when room must be made for docking of the tooling as per  FIG. 5 . 
     Front faces  67  of vise frame  36  are configured to rest against the back of shore plate  25 . In doing so, they react against residual elastic pipe forces that may be present. Idler rollers  69  set on spaced vertical axles  70  keep rod string  11  centered relative to the vise and therefore to the rest of the machine. Rollers  69  are engineered so that shoulders on the rod will pass freely through them. A pair of cylinders  71  actuate clamping of jaws  72  and  73 . Another cylinder  65 , while the same size as cylinder  71 , is positioned to rotate jaws  73  about the axis of rod string  11 . This is done when jaws  73  are clamped and serrated surfaces  51  of jaws  73  grip the rod securely. Cylinder  65  breaks loose the threaded joint between rod string  11  and the endmost rod, allowing the endmost rod to be removed from the string. To loosen the threaded joint between rod string  11  and the endmost joint, jaws  73  turn approximately 30°, in any case less than 360°. This feature is only used to loosen threaded rod joints, never to tighten, because jaws  73  create very high torque relative to the spindle rotation drive motor. 
       FIG. 7  shows all of the jaws  72  and  73  from above. In this figure, jaws  72  are clamped on the available rod, while jaws  73  are open. In  FIG. 8 , a greater portion of cylinder  65  is exposed. In  FIG. 9 , idler rollers  69  are fully visible in profile, shown guiding and centering rod string  11 . This view demonstrates how cylinder  71  is positioned to provide clamp load on rod string  11 . 
     In  FIG. 10 , rod cradle  18  is shown fully extended. The four track rollers  17  are mounted at respective corners of rectangular spindle frame  12 , and a grease zerk manifold  139  is exposed through an opening in the top of frame  12 . Frame  12  includes a pair of front and rear walls  141 ,  142  having pairs of aligned openings  143 ,  144  therein in which cylinder bodies  16  are mounted, as well as internal structural members  146  on which various spindle assembly components are mounted as shown in  FIG. 14 . Openings  143 ,  144  preferably open laterally so that cylinders  26  can be removably mounted therein. Pairs of generally C-shaped holders  147 ,  148  are placed over the outside of cylinder body  16  and bolted to frame  12  to hold cylinders  26  in place. To hold cylinder bodies  16  stationary relative to frame  12 , openings  143  and front holder  147  engage an annular groove  77  on the outside of cylinder body  16 , discussed further in connection with  FIGS. 15-17  below. 
     As shown in  FIG. 12 , an arm  101  is configured to slide freely through a concentrically positioned center hole in bushing  103 . A collar  105  fixed to the outside of arm  101  limits outward travel of cradle  18  by bumping against the inner face of bushing  103 . A piston  107  provides the reaction force needed to hold cradle  18  in the proper position. Piston  107  is secured near the rear end of arm  101  behind collar  105  and slides freely within a tube  113 . Piston  107  is not located concentrically on arm  101 . In this manner, the angle between the axis of arm  101  and the axis of tube  113  will vary as cradle  18  is moved away back and forth through its range of travel, urged to extend by a gas spring  112  which is attached at one end to the inside of tubular arm  101  at the position of collar  105 . Cap  109  seals tube  113  at its rear end, and optional oiler  111  provides drip lubrication to the interior of tube  113 . 
     The change in angle causes cradle  18  to fall away from the bottom of the rod as arms  101  of cradle  18  are retracted into their respective tubes  113 . As shown in  FIGS. 12 and 13 , piston  107  biases the rear end of arm  101  downwardly relative to the opening through bushing  103 , causing the front end to which cradle  18  is attached to be lifted upwardly. This displacement lessens as the distance between piston  107  and bushing  103  becomes greater, causing cradle  18  to drop downwardly a slight distance as piston reaches the position shown in  FIG. 13 . In this position, the angle between the axis of arm  101  and axis of tube  103  is smaller that as shown in  FIG. 12 . 
     Referring to  FIG. 14 , a front end portion of the spindle shaft  115  is mounted in a front plain bearing  119 . Bearing  119  is contained in a bearing housing  121  that is bolted to the front face of spindle frame  12 . Bearing  119  is designed only for handling radial forces transmitted from shaft  115 . A rear end portion of spindle shaft  115  is supported by a set of tapered roller bearings  127  located in a housing  123  that is also bolted to spindle frame  12 . Tapered roller bearings  127  support shaft  115  in both thrust and pullback directions. However, bearings  127  are sized only to handle the magnitude of thrust developed by the machine in payout, in this example about 40,000 lb. During pullback, the capacity of bearings  127  would be greatly exceeded by the 250,000 lb. of pullback force that can be produced by the main thrust cylinders  26 . For this reason, the system has been designed to allow the shaft  115  to float, unloading the tapered roller bearings  127  in the pullback direction. 
     Spring can  131  is loaded against taper roller bearings  127  by a coil spring  129  for small magnitudes of pullback, such as breaking or unthreading the rod joint. When the load increases above a threshold level such as 1000 lb, the spring  131  has compressed far enough that a flange  117  mounted on shaft  115  at an intermediate position along its length contacts the face of a load flange  125 . As shown in  FIGS. 25 and 26 , load flange  125  is preferably in the form of a wedge with its wide end bolted to and braced against frame  12 . The narrow end of load flange  125  has a cylindrical cutaway  126  to provide clearance for the spindle shaft  115 , and a counterbore the bottom of which forms a load bearing surface  128 . When flange  117  is in substantial contact with surface  128  of flange  125 , shaft  115  will not rotate due to the high friction induced. This is desired in that the machine is intended for static pipe bursting or other non-rotating pullback operations. Use of the plain bearing  119  and load flange  125  with flange  117  avoids the size and expense of a tapered roller bearing capable of handling 250,000 lb. 
     A sprocket  133  is torsionally keyed to shaft  115 . A roller chain (not shown) drives sprocket  133  under the operator&#39;s control to thread or unthread rods or turn small diameter tooling during payout. A water swivel  137  allows drilling fluids to pass to the hollow drill stem while being fed by a non-rotating hose. Locknuts  135  are used to secure sprocket  133  to shaft  115  in the axial direction. 
       FIGS. 15-21  show the structure of cylinders  26  in detail. Hydraulic port  57  at the distal end of rod  14  communicates with a flow passage  97  inside rod  14  which opens onto the piston side of rod  14 . Connecting the hydraulic fluid pressure source to port  57  while connecting port  61  to tank fills cylinder body  16  with fluid and extends rod  14 . Port  61  communicates with another lengthwise flow passage  99  which extends almost to the rear end of rod  14 . Passage  99  communicates with an outwardly opening annular groove  94  through a radial port  87 . Fluid in groove  94  enters the space on the rod side of a piston  90  mounted at the rear end of rod  14  through a series of cutaways  89  in piston  90 , retracting rod  14  when port  57  is connected to tank. 
     Piston  90  is mounted on the end of rod  14  by a steel lock ring  95 . A split nylon wear ring  93  mounted in an annular groove on the outside of piston  90  slides along the inside of cylinder  16 . Leakage between piston side and rod side is prevented by a urethane umbrella-type seal  96  mounted in another groove frontwardly from wear ring  93 . A seal carrier or cap  75  is secured by threads  83  to the front end of cylinder body  16 . Cap is supported on rod  14  by a nylon split bearing ring  81  and leakage is prevented by a series of nylon seals  82 . As discussed above, rod  14  has a large diameter relative to cylinder body  16 , making the annular space  88  on the rod side thin, so that only a small flow of fluid is required retract the cylinder in  FIGS. 15-17 . For this purpose, the cross section area of annular space  88  is from about 10% to 60% of the cross sectional area of the cylinder cavity. (This equates to a ratio of working surface area of from 10:1 to 1.67:1.) If annular space  88  is excessively thin (&lt;10% of the cross sectional area of the cylinder cavity), retraction of the cylinder will not be powerful enough. On the other hand, when it is too wide (exceeds 60% of the cross sectional area of the cylinder cavity) the cylinder begins to behave like a conventional hydraulic cylinder. 
       FIGS. 23 and 24  show a preferred form of drill rod  100  of the type used to make rod string  11 . Multiple rods  100  are joined together end-to-end to create a string  11  as long as 500 feet or more. Male thread  116  mates into the next rod&#39;s female thread  122 . An undercut  118  is provided for jaws  72  to grip. Should the axial load be high, the rod  100  may slip until a shoulder  120  contacts jaw  72 . Jaws  73  engage the outer surface of each rod  100  outside of female thread  122 . An axial bore  124  of rod  100  is optionally used conduct fluid from the downhole machine to the front of the rod string. Bore  124  also reduces the weight of rods  100  to facilitate manual handling. 
     Operation of downhole machine  10  according to the invention is as follows. A typical job will involve pushing a rod string out through an existing pipeline from the exit pit (where machine  10  is) to the entry opening in the pipeline, such as in a trench or manhole. To extend a rod string  11 , the machine  10  starts in the position shown in  FIG. 3 , but with no rod string  11  present. A rod  100  is removed from box  31  and placed in cradle  18  at crotch  30  with the male threaded end  116  facing shaft extension  20 , which has a female thread ( FIG. 14 ). The female end  122  is placed in rear jaws  73 , and jaws  73  are closed on it. The spindle assembly  12  is then operated to thread shaft extension  20  over male end  116 . Once this is done, jaws  73  are opened and spindle frame  12  is moved to the left by retraction of cylinders  26  to assume the position shown in  FIG. 4 . Jaws  72  are then operated to grip rod  100  at undercut  118 . The spindle shaft  115  and extension  20  are then rotated in reverse to unthread extension  20  from male end  116 . Cylinders  26  are then extended to move spindle frame  12  to the right to assume the position shown in  FIG. 3 , and the machine  10  is ready to accept another rod  100 . 
     The procedure for adding the second and subsequent rods  100  is the same as described above, except that jaws  73  are not closed on the female end  122  of the new rod  100 , and the male end  116  of the previous rod is positioned between jaws  73  as shown in  FIG. 3 . Instead, the female end of rod  122  is brought over male end  116  of the previous rod held in jaws  72 . When spindle assembly  12  is then operated to thread shaft extension  20  over male end  116  of the new rod, female end  122  of the new rod is threaded onto male end  116  of the previous rod at the same time. In the process of retracting the cylinders  26  to assume the position of  FIG. 4 , the entire rod string  11  is pushed forward. This process is repeated until the leading end of string  11  emerges from the entry opening. 
     Once the push out operation is complete, a bursting head or other tooling is mounted on the distal end of rod string  11  in preparation for pullback through the existing hole or pipeline. Such a bursting head preferably also pulls in a replacement pipe at the same time in a manner well known in the art. 
     Pullback starts with visel  5  closed on neck or undercut  118  as shown in  FIG. 4 . Jaws  72  are opened, and cylinders  26  are extended to move spindle frame  12  to the right, pulling the rod string  11  and bursting head with it. Once spindle frame  12  has reached the position shown in  FIG. 3 , jaws  72  are closed on the neck  118  of the second to last rod  100 , and jaws  73  are actuated by an automatic cycle that clamps female end  122  of the last rod  100  and rotates it a sufficient distance under the action of cylinder  65  to loosen the threaded joint; one-eighth to one-quarter turn is generally enough for this purpose. Jaws  73  are then opened and returned to their non-rotated position. Spindle shaft  115  is then rotated to unthread the last rod  100  the rest of the way from the second to last rod, with spindle frame  12  moving about an inch to the right during this process. Jaws  73  are then closed again on last rod  100 , and spindle assembly  9  is operated to unthread last rod  100  from shaft extension  20 . When this is done, jaws  73  are opened, and the last rod  100  may be manually removed and placed in storage box  31 . Rods  100  are sized to be lifted and handled by one person; in this embodiment, rods  100  weigh 52 pounds each. 
     The pullback steps are then repeated as required until the first rod  100 , having the bursting head attached thereto, is encountered. At this time, outer shore plate  19  is removed by attaching chains with hooks to openings  22 , exposing center hole  28 . Last rod  100  is then removed in a normal manner, resulting in a leading end portion  91  of bursting head  92  held in jaws  72 . Shaft extension  20  is then threaded onto bursting head  92 , and jaws  72  remain closed. Cylinders  26  are then extended, pulling back bursting head  92  through hole  28  into the position shown in  FIG. 5 . Vise assembly  15  travels back as well because it is locked to bursting head  92  by jaws  72 . Bursting head  92  is then unthreaded from shaft extension  20  and jaws  72  are opened, allowing bursting head  92  to be lifted out of the pit. 
     In the foregoing manner, the machine  10  of the invention can be used for pipe bursting and replacement. During the pushing out step, it may often be desirable to mount a drill bit on the leading end of the drill string in order drill a pilot bore through the ground, if there is no existing pipeline to follow. The drill bit may of the type having an angled steering face and can be steered with machine  12  using the well known push-to-steer, push-and-spin to bore straight ahead method. The drill bit might also be needed to drill through collapsed or block portions of an existing pipeline to be replaced. The machine of these invention is capable of performing these functions as well as pull back under much higher loads, without need for expensive high capacity roller bearings. 
     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. For example, while the invention has been discussed as a static bursting system, it is also possible to use a bursting or pipe splitting head capable of deliver cyclic impacts to the pipeline being burst.