Abstract:
An improved bushing for a heavy duty hydraulic hammer provides increased bushing life and provides cooler operation for the hammer. A Polymeric bushing is positioned near the lower end of the tool holder of a hammer surrounding the tool. The polymeric bushing is protected by a steel ring below the polymeric bushing preventing foreign objects from impacting upon the polymeric bushing.

Description:
FILED OF THE INVENTION 
     This invention relates to an improved bushing engaging the tool in an impact hammer and more particularly to a low friction bushing protected from foreign objects in use. 
     BACKGROUND OF THE INVENTION 
     Hydraulic impact hammers are heavy equipment used in mining, construction, demolition, roadwork, quarrying, and similar applications. Such hammers range in size from units weighing less than 200 lbs. to units weighing more than 15,000 lbs. These hammers are used to break up concrete, rock, ore, and the like. They are also sometimes used to remove surface portions from an underlying substrate. 
     Hydraulic impact hammers generally comprise a housing having a central cylinder. A piston is contained within the cylinder. The top end of the cylinder communicates with or forms part of a gas chamber. The bottom end of the cylinder communicates with a bore in a tool holder which is connected to the housing. The tool holder contains a tool, such as a chisel point, which will strike a workpiece. A valve is attached to the housing and controls the flow of hydraulic fluid into the cylinder. The hydraulic fluid forces the piston up compressing the gas in the gas chamber. When the piston reaches a certain height, the hydraulic fluid is allowed to exit the portion of the cylinder below a piston seal and the piston is brought down forcefully to strike the tool. The tool is thus sharply hammered and in turn impacts upon the substrate desired to be broken or modified. The operation of the valve causing the piston to travel upward and allowing it to travel downward is automatic. The piston reciprocates rapidly resulting in numerous forceful blows of the tool against the substrate. The impact frequency, the number of impacts per minute, of a hydraulic impact hammer ranges from about 200 to over 2000 impacts per minute. Each impact involves significant amounts of energy. At high impact rates, large amounts of energy are used and/or dissipated. Because of the large amounts of force and energy involved, hydraulic impact hammers must be built robustly. 
     Hydraulic impact hammers create dust, debris and obstructions by their operation. Breaking-up of concrete results in dust, flying stones and particles of concrete and exposed reinforcing rod (rebar). The dust is created at the working end of the hammer as are the flying stones and other particles. The dust and flying particles can be moving quickly and can penetrate between working parts causing wear and interfering with operation. Rebar is often encountered in breaking up concrete. As it is long, often relatively slim, and tough, it can cause damage. The environments in which hydraulic impact hammers are used are often difficult environments. 
     One mechanism developed to compensate for the difficult environment described and the intrusion of particles into the lower end of the hammer is the use of bushings. The tool holder, holding the tool in place at the bottom of the hammer does not bear directly against the tool. Rather, a cylindrical bushing is fitted into a recess in the bottom end of the tool holder and surrounds the tool. Such bushings are sometimes made of steel and sometimes made of nylon or another polymer. The bushings do not move with respect to the tool holder. Therefore, there is no wear or very little wear around the bushing. The tool moves within the bushing. Wear occurs between the tool and the tool bushing. This wear is aggravated by the intrusion of dust and particles of concrete. Moreover, impacts from standing rebars and the like often damage the bushing. The bushing is replaceable. Thus, while the bushing and tool wear the tool holder and rest of the hydraulic impact hammer are protected. 
     Additionally, when one uses a steel bushing and a steel tool, frictional forces generate significant heat where the tool rubs against the tool bushing. This heat can interfere with lubrication and aggravate wear on the tool and tool bushing. 
     This arrangement requires that the operator check the tool and tool bushing for wear periodically. When the tool bushing is worn excessively, the hammer must be taken out of service, the tool removed and the tool bushing removed from the tool housing. After the tool bushing and tool are replaced, the hammer can be placed back in service. Significant maintenance costs and down time are thereby incurred. 
     Plastic, often nylon, bushings have been used in place of steel bushings. Such bushings fail frequently. One mechanism of failure is cracking of the bushings. Once the bushing starts to crack, it quickly deteriorates and must be replaced. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved bushing and tool housing structure which overcomes many of the above referred to problems, minimizes wear, extends bushing life, and is easy to maintain. In accordance with the present invention, there is provided a tool holder assembly for a hydraulic impact hammer in which the lower end of the central bore contains a polymeric or plastic bushing adapted to surround a tool and a metallic bushing protector below the bushing having an inside diameter slightly larger than the inside diameter of the bushing. 
     Further in accordance with the invention, a recess is provided in the lower end of the tool holder of a hydraulic impact hammer and a cylindrical polymeric bushing is contained in the recess. The polymeric bushing has an inside diameter slightly larger than the outside diameter of the tool being supported. A robust steel ring is pressed into the recess below the polymeric bushing. This steel ring has an inside diameter slightly larger than the bushing so it does not engage against a tool but protects the bushing from impact by flying debris, upstanding rebar or the like. 
     Still further in accordance with one aspect of the invention, the bushing protector is an add-on device for existing hammer designs in which a steel ring is pressed into the tool holder and is held in place by an interference fit. 
     Further in accordance with another aspect of the invention, the bushing protector is part of an original design in which the bushing protector is machined from the same workpiece as the tool holder and is integral with the tool holder. 
     Yet further in accordance with the invention, the polymeric bushing is generally cylindrical in shape and is retained in the tool holder by the bushing protector. Still further in accordance with the invention, a cylindrical polymeric bushing is provided with a generally axial gap which allows the bushing to be compressed to have a smaller outside diameter, inserted through the bushing protector and expanded within a recess in the tool holder where it will engage a tool. 
     Still further in accordance with the invention, the gap in the cylindrical tool is skewed with respect to the axis of the tool. 
     Still further in accordance with the invention, the diameter of the opening through the bushing protector is at least one-quarter (¼) inch larger than the diameter of the polymeric bushing central opening. 
     Yet further in accordance with the invention, the diameter of the bushing protector central opening is between about one-eighth (⅛) inch (2 millimeters) and one-half (½) inch (12 millimeters) larger than the diameter of the central opening of the polymeric bushing. 
     Still further in accordance with the invention, the bushing protector is at least one-half (½) inch (12 millimeters) long in the axial direction. 
     Yet further in accordance with the invention, the bushing protector is between one-half (½) inch (12 millimeters) and three (3) inches (75 millimeters) long in the axial direction. 
     It is the principal object of the present invention to provide a tool holder assembly for a hydraulic impact hammer in which the components last longer, are less prone to failure, produce less heat, and are reasonably maintained. 
     It is another object of the present invention to provide a polymeric tool bushing for an impact hammer which is protected from impact and abrasion by foreign bodies. 
     It is still another object of the present invention to provide a tool bushing for a hydraulic impact hammer which will be easily installed and replaced. 
     It is yet another object of the present invention to provide a tool holder assembly for a hydraulic impact hammer which minimizes the heat produced by movement of the tool within the tool holder. 
     It is still another object of the present invention to provide a tool holder assembly for a hydraulic impact hammer which increases the service life of tool bushing parts thereby increasing uptime of the hammer. 
     It is still another object of the present invention to provide a tool bushing assembly for a hydraulic impact hammer which is easy and inexpensive to maintain, and reduces the operating costs of the impact hammer over its life. 
    
    
     These and other objects of the present invention will become apparent to those skilled in the art from the following description taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 shows a hydraulic impact hammer, partially cut away, in accordance with the first embodiment of the present invention; 
     FIG. 2 shows the tool holder assembly and associated element seen in FIG. 1 in cross-section and in greater detail; 
     FIG. 3 shows the lower portion of the tool holder seen in FIGS. 1 and 2 with a second embodiment of the invention showing alternate tool bushing and tool holder structures; 
     FIG. 4 shows a tool bushing as shown in FIG. 2 in a top plan view; 
     FIG. 5 shows the tool bushing of FIG. 4 in a side view cross-section taken along the line  5 — 5  of FIG.  4  and an associated bushing protector; 
     FIG. 6 shows a bushing and bushing protectors in cross-section similar to that seen in FIG. 5 with the addition of a slot in the bushing; 
     FIG. 7 shows a top view of the tool bushing as seen in FIG. 6 showing a slot with radial side walls; 
     FIG. 8 shows a top view of a bushing as seen in FIG. 6 with a slot having skewed side walls; 
     FIG. 9 shows an alternate construction of a bushing usable in the invention in plan view; 
     FIG. 10 shows another embodiment of a bushing usable in the invention seen in FIGS. 1 and 2; 
     FIG. 11 is a top view of an alternate lower bushing holder and lower bushing usable in the invention; and, 
     FIG. 12 shows the bushing holder and bushing of FIG. 11 in a side view cross-section taken along line  12 — 12  of FIG.  11 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to the drawings wherein the showings are made for the purposes of illustrating preferred embodiments of the invention only and not for purposes of limiting the invention, FIG.  1  illustrates a heavy duty hydraulic impact hammer  10  in accordance with the invention. The hammer will normally be mounted in a bracket which supports the hammer and allows connection to an excavator. The bracket elements are conventional and not illustrated. It should be remembered in the following description that hydraulic impact hammers are large and heavy weighing up to about 15,000 lbs. and more. The hydraulic impact hammer  10  includes a main housing  12 , a gas head  14 , a piston  16 , a tool holder assembly  18 , a tool  20 , and a valve  22 . The impact hammer  10  also includes replaceable sleeves  24  forming a cylinder  28  and seals  26  between the piston  16  and the main housing  12 . The sleeves  24  and seals  26  are replaceable and ease maintenance of the product. The valve  22  comprises several parts, the operation of which is briefly described hereinafter. All of these elements, except the novel tool holder assembly  18 , to be described herein below, are described in publications available to those skilled in the art. They will not be described in great detail herein. For those who wish additional information, reference should be made to publications such as the NPK Hydraulic Hammer Service Manual published by NPK Construction Equipment, Inc. of 7550 Independence Drive, Walton Hills, Ohio 44146. 
     The above-described major elements are held together by tie-rods  32 . The tie-rods threadably engage a tool holder  34  and pass upwardly through a spacer  36 . The tie-rods  32  pass through openings in the main housing  12  and exit through the top of the gas head  14 . Four tie-rod nuts  38  are threaded onto the exposed ends of the tie-rods  32  and tightened to hold the entire hydraulic impact hammer  10  together. 
     The piston  16  is generally cylindrical with a portion of enlarged diameter  42  near its center. Hydraulic fluid ports communicate with the interior of the cylinder  28  containing the piston  16 . The upper of these ports  44  communicates with the cylinder  28  above the enlarged center of the piston  42 . The lower of these ports  46  communicates with the cylinder below the enlarged center portion of the piston  42 . Hydraulic fluid enters the cylinder through the lower port  46 , drives the piston upwardly and compresses the gas contained in the gas head  14 . After the piston reaches a certain height, the valve  22  allows hydraulic fluid to exit the portion of the cylinder  28  below the enlarged center of the piston  42 , flow through the valve  22  and into the portion of the cylinder  28  above the enlarged center of the piston. The piston moves down forcefully driven by the compressed gas in the gas head  14  and strikes the tool  20 . The valve returns to its configuration directing high pressure hydraulic fluid through the lower port  46  and again drives the piston upwardly in the cylinder  28 . This series of events occurs rapidly. The frequency of impacts upon the tool in a typical hydraulic impact hammer is several hundred beats per minute. The above-described operation is conventional. 
     As described above, the piston  16  moves up and down forcefully several hundred times per minute. When it moves down, it strikes the tool  20  causing the tool to strike a workpiece and/or move down forcefully within the tool holder assembly  18 . The tool  20  is subject to a variety of forces in use. The tool  20  will normally be applied to and in contact with a workpiece. However, the axis of the tool  20  will not always be directly perpendicular to the workpiece. Thus, reactions from impacts against a workpiece will often include large radial components as well as axial components. Even when the tool  20  is directly perpendicular to a workpiece, reaction forces will vary as the workpiece is broken up. Thus, the tool  20  will reciprocate axially within the tool holder assembly subject to both large axial forces and impacts and large radially directed forces. 
     Conventionally, bushings are provided in the tool holder assembly  18 . The tool  20  slides axially making contact with these bushings rather than the tool holder  34  itself. Steel bushings are used in many hammers as steel is a robust material. However, the sliding friction of the tool upon the bushings creates significant heat and wear. Grease fittings are provided in conventional tool holder assemblies  18 . The application of grease lessens the friction. However, heat and the difficult environment in which hammers operate interfered with the ability of grease to minimize wear and friction. 
     When a bushing becomes worn out of specification, it is removed from the tool holder and a new bushing pressed into place. The lower bushing, being closest to the point of impact of the tool  20 , suffered the most severe wear. 
     As best seen in FIG. 2, in accordance with the present invention, a tool holder assembly  18  is provided comprising a tool holder  34 , an upper bushing  52 , a lower bushing  54 , a tool retaining pin  56 , and a bushing protector  58 . The tool holder  34  generally surrounds the tool  20 . The tool retaining pin  56  engages in a recess  62  in the tool  20 . The tool retaining pin  56  prevents the tool  20  from falling out of the tool holder assembly  18  as it engages the top of the recess  62  when the tool is in the fully down position. The tool  20  is restrained from moving too far upwardly within the tool holder  18  by a shoulder  64  in the upper bushing  52 . 
     An enlarged generally cylindrical recess  70  is provided at the lower end of the tool holder  34 . The lower bushing  54  resides in this enlarged cylindrical recess  70  and surrounds the tool  20 . The lower bushing  54  is retained in place by a bushing pin  66  passing through an aperture in the tool holder  34  and a recess  68  in the lower bushing. As seen in FIG. 2, the lower bushing  54  is a polymeric material. Nylon or other tough plastic materials are preferred. The outside diameter of the lower bushing  54  is about equal to the inside diameter of the enlarged cylindrical recess  70 . If the lower bushing is unslotted, it is pressed into place. The lower bushing  54  has an inside diameter slightly larger than the outside diameter of the tool  20  allowing the tool to reciprocate within the bushing  54  while being supported by the bushing  54 . The bushing protector  58  is a ring of steel having an outside diameter substantially equal to the inside diameter of the recess  70 . The bushing protector  58  is pressed into the recess  70  in an interference fit. The inner diameter of the bushing retainer is slightly larger than the inside diameter of the lower bushing  54 . Therefore, the tool  20  does not bear against the bushing retainer  58  but rather rides on the lower bushing  54 . A steel on nylon bearing surface is provided at the lower extremity of the tool holder assembly  18  rather than a steel on steel engagement as in some conventional impact hammers. 
     The embodiment shown in FIG. 2 uses a tool holder  34  similar in shape to the tool holder used in some prior art hammers. Such prior art tool holders used lower bushings extended all the way to the bottom of the recess  70 . No bushing protector  58  was provided. The present invention can be used in such prior art hammers by removing the existing lower bushing and inserting a new nylon bushing  54  and steel bushing protector  58  in the existing tool holder  34 . A protected nylon bushing is thereby provided. 
     The relative dimensions of the tool  20 , the lower bushing  54  and the bushing protector  58  are important. In a hammer using an eight-inch diameter tool, the inside diameter of the lower bushing  54  is 204.2 millimeters (8.04 inches). The inside diameter of the steel bushing protector  58  is 210 millimeters (8.27 inches). An annular space  74  is present between the outside diameter of the tool  20  and the inside diameter of the bushing protector  58 . This annular space  74  has a thickness of about three millimeters (⅛ inch) around the entire circumference of the tool  20 . Thus, the tool bears against the polymeric lower bushing  54  but does not bear against the steel bushing protector  58 . Moreover, the polymeric lower bushing  54  is protected from damage by upstanding rebar or other materials which could crack and destroy it. Such materials are prevented from entering into contact with the bushing or the steel bushing protector  58 . Rebar and other similar materials are unlikely to penetrate through a three (3) millimeter (⅛ inch) gap. 
     The axial length of the bushing protector can vary according to the size of the hammer. However, the bushing must have the strength to withstand impact from obstructions. On the other hand, the bushing  54  must be near the bottom of the tool holder. A two (2) inch (50 millimeters) axial length is appropriate for an eight-inch hammer. The axial length may vary from one-half (½) inch (12 millimeters) to three (3) inches (76 millimeters) or more. 
     Applicants have found that this arrangement results in a significant reduction in the heat generated at the lower bushing when the hammer is in operation. Moreover, this arrangement provides a substantially lengthened lifetime for a polymeric bushing and a hammer. Steel bushings get hot and wear. Unprotected nylon bushings are prone to failure by cracking and disintegration caused by impact with rebar or other hazards during operation of the hammer. 
     The above referred to dimensions are applicable over a broad range of hammer sizes. Hammers using smaller tools, for instance, a five-inch diameter tool, are supported in a lower bushing having a snug fit around the tool. The bushing protector  58  has an inside diameter selected to provide an annular space  74  having a thickness of approximately three millimeters (⅛ inch). The same relationship holds for hammers using larger diameter tools. Applicant provides a polymeric lower bushing  54  which fits snugly around the tool thereby providing a bearing surface and a bushing protector  58  having an inside diameter providing a small gap, preferably of about three millimeters (⅛ inch), between the steel bushing protector  58  and the tool  20 . 
     FIG. 3 illustrates another embodiment of the invention. FIG. 3 shows the lower extremity of a hammer as seen in FIG.  1 . The tool holder  134  is provided with recess  170  which is spaced upwardly from the lower end  172  of the tool holder  134 . The recess  170  has an inside diameter which is enlarged with respect to the inside diameter of the rest of the tool holder  134 . It receives a lower bushing  154  which has an internal diameter which snugly supports the tool  20 . The lower end  172  of the tool holder  134  has a circular opening  176  with an inside diameter about six millimeters (¼ inch) larger than the outside diameter of the tool  20 . This provides an annular space  74  identical to the annular space created by the bushing protector  58  in FIG.  2 . However, the lower end  172  of the tool holder  134  is integral to the tool holder  134  and provides the same functionality as the bushing retainer  58  seen in FIG.  2 . In effect, the tool holder  134  has an integral bushing protector  158 . 
     The lower bushing  154  is provided with a slot  180  extending from its upper end  182  to its lower end  184 . Should the bushing  154  require replacement, an operator will remove the tool retaining pin  56  and the tool  20 . If a bushing pin  66  is present, it will also be removed. The operator can then use tools to radially compress the lower bushing  154  and remove it through the circular opening  176 . A new bushing  154  can then be radially compressed by hand, inserted through the circular opening  176  and allowed to expand into the recess  170 . The lower bushing  154  is held in place by the integral bushing protector  158 . The slot  180  is skewed with respect to the axis of the tool  20 . This assures that the tool  20  is adequately supported in all radial directions. 
     FIG. 4 shows the bushing  54  seen in FIG. 2 in greater detail. FIG. 5 shows the bushing of FIG. 4 in side elevation partially cut away with the bushing protector  58  below it. The lower bushing  54  is cylindrical with a central bore  202  having a general uniform cross-section over its entire length. The top end  204  and bottom end  206  of the central bore are provided with rounded edges. The external surface  210  of the bushing  54  is generally cylindrical. The topmost portion  212  of the external surface  210  has a slightly reduced diameter which eases insertion of the bushing into the tool holder  34 . Three grooves  214  are spaced evenly around the periphery of the external surface  210  of the bushing  54 . The grooves  214  do not extend all the way to the bottom of the tool bushing  54 . They do extend all the way to the top of the tool bushing and also continue across the top surface of the tool bushing to the central bore  202 . A circumferential recess  68  is present around the entire periphery of the external surface  210  near the top end  204 . As seen in FIG. 2, the recess allows passage of a pin  66  which holds the lower tool bushing  54  in place. The circumferential recess  68  and pin  66  are not necessary in the present invention as a retaining function is accomplished by the bushing protector  58 . However, as existing units using steel bushings include bushing pins  66 , the recess allows easy retro fit of the present invention. 
     As best seen in FIG. 5, the steel bushing protector  58  is a ring-shaped structure having a bore with a general uniform diameter over its length. The outside surface of the bushing protector  58  is generally cylindrical. The top portion  220  of the bushing protector  58  has a reduced diameter to ease pressing the bushing protector  58  into the bottom of the tool holder  34 . The portion of the bushing protector  58  below the top portion  220  has a larger diameter and must be pressed into the tool holder  34  where it will be retained in interference fit. As can best be seen in FIG. 5, the diameter of the bore  222  of the bushing protector  58  is about six millimeters (¼ inch) larger than the diameter of the central bore  202  of the lower bushing  54 . This provides for the three millimeters (⅛ inch) annular space  74  described above. 
     FIG. 6 shows a bushing  154  and bushing retainer  58  identical in all respects to that seen in FIG. 5 save one. The bushing  154  of FIG. 6 is provided with a slot  180  extending from the bushing upper end  182  to the bushing lower end  184 . The slot  180  is skewed with respect to the axis of the tool  20  (not shown). The slot  180  is sufficiently wide to allow one to insert a screwdriver or other similar tool through the slot to pry up a portion of the bushing  154  so that it may be grasped and pulled inwardly. The two edges of the gap  186 ,  188  can be overlapped and the outside diameter of the lower bushing  154  can be made smaller than the circular opening  176  allowing one to remove a worn bushing and replace it with a fresh bushing. This can be done without removing the bushing protector  58 . 
     FIG. 7 shows a simplified top view of the bushing  154  seen in FIG.  6 . In the embodiment seen in FIG. 7, the slot edges  186 ,  188  are generally perpendicular to the slot inner and outer surfaces. Alternatively, the slot edges  186 ,  188  can be skewed or slanted with respect to the inner and outer surfaces as seen in FIG.  8 . The skewed or slanted edges  186 ,  188  make it easier for an operator to compress the bushing for removal. The width of the slot  180  is not critical. It should be wide enough when installed to allow one to remove a worn or cracked bushing. It should be narrow enough so that the proper support is provided for the tool in operation. 
     FIGS. 9 and 10 shows another embodiment of a bushing usable with the invention. With reference to FIG. 10, the bushing  254  is a slotted cylinder having a central bore  256  of uniform diameter over its entire length and a cylindrical outer surface  258  having a uniform diameter over its entire length. The bushing  254  is provided with a slot  280  which can be configured as seen in FIG. 7 or in FIG.  8 . The bushing of FIG. 10 is substantially less thick than the bushings seen in FIGS. 5 and 6. To compensate for this lessened thickness, the annular space  170  (FIG. 3) in which the bushing  254  is retained has a smaller diameter. The bushing can be molded into a cylindrical shape as seen in FIG.  10 . It is easily installed as it is relatively flexible because of its diminished thickness. The two edges of the gap  286 ,  288  are easily drawn into an overlapping relationship for installation or removal. 
     Alternatively, the bushing  254  seen in FIG. 10 can be fabricated as a sheet of polymeric material in the form of a trapezoid as shown in FIG.  9 . The two short edges  286 ,  288  of the trapezoid form the edges of the gap  280  when the sheet is rolled into a cylindrical form. The sheet is held in the appropriate cylindrical form when installed in the cylindrical recess  170 . 
     FIGS. 11 and 12 show another bushing usable with the invention. A metallic lower bushing holder  350  has external dimensions identical in all respects to a conventional metallic bushing. Thus, the metallic lower bushing holder  350  can be used as a direct field replacement for conventional hydraulic impact hammers using metallic bushings. The metallic lower bushing holder  350  fits into the enlarged cylindrical recess  70  (FIG.  2 ). The interior surface of the metallic lower bushing holder  350  consists of an upper cylindrical land  370 , a central cylindrical recess  372  and a lower cylindrical land  374 . The central recess  372  has an inside diameter slightly larger than the upper land  370  and lower land  374 . The central recess  372  accepts a polymeric bushing  254  identical in all respects to the polymeric bushing  254  shown in FIG. 10 or the variant shown in FIG.  9 . 
     The interior surface of the polymeric bushing  254  defines a central bore  256  which supports a tool  20  (FIG.  2  and dashed lines FIG. 12) in a bearing relationship. The upper land  370  and lower land  372  have diameters slightly larger than the central bore  256  diameter and do not engage the tool  20 . The lower land  374  has an internal diameter slightly larger e.g. ¼ inch (6 mm) than the diameter of the tool  20 . Only a small annular space  380  exists between the tool  20  and the lower land  374 . This annular space  380  is only about ⅛ (3 mm) wide. The lower land prevents rebar and other foreign matter from striking and damaging the polymeric bushing  254 . The lower land  374  need only be about ½ inch (12 mm) in axial length to perform its function, but can be longer. 
     The polymeric bushing  254  has a slot  280  which eases removal of a worn bushing  254  and installation of a new bushing  254 . The tool  20  is removed. A hand tool such as a screw driver is used to pry up the bushing  254  at the slot  280  and the bushing is grasped and removed. A new bushing  254  is compressed radially and placed in the recess  372 . The new bushing is allowed to expand into the recess  372  where it is retained by the lands  370  and  374 . The tool  20  is reinstalled and the hammer returned to service. 
     The invention has been described with reference to preferred embodiments. Obviously, modifications and alterations will occur to others upon a reading and understanding of this specification. For instance, a portion or all of the upper tool bushing may be nylon. It is intended to include such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.