You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
[0001]    This invention relates to a cushion sub-assembly of a type for absorbing vibrations being transmitted to a drill head by a drill pipe during the drilling of a bore hole. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is known to include a cushion sub-assembly below the drill head of a drilling rig and through which the turning force of the drill head is transmitted to the uppermost drill pipe section to thereby rotate the drill string within the bore being drilled. Such cushion sub-assemblies have taken a number of different forms, the design of which takes into account the features of the drilling operation in which the cushion sub-assembly will be used. 
         [0003]    For example, it is common in a drilling operation to force the drilling bit into engagement with the bottom of the bore and to achieve the cutting action due to rotational motion only of the drill string and drill bit. A cushion sub-assembly designed for this type of drilling must be capable of absorbing a significant portion of both the torsional and axial vibrations resulting from the cutting action of the bit. 
         [0004]    In downhole hammer drilling, compressed fluid is transmitted to a hammer means at the bit location and this results in additional axial vibration. Cushion sub-assemblies used for this type of drilling have therefore needed to be capable of transmitting a drilling torque as well, despite the cushion sub-assembly being primarily designed to prevent the transmission of high axial vibration forces to the drilling head. Existing cushion sub-assemblies are not satisfactory as vibration is still carried through to the rotary head. 
         [0005]    A further problem with existing cushion sub-assemblies of this type is their tendency to wear very quickly. The internal components of the cushion sub-assembly, particularly those in contact with the reciprocating piston, require regular maintenance, refurbishment, and eventual replacement. Refurbishment of existing cushion sub-assemblies is time consuming and expensive due to the number of parts requiring maintenance. Accordingly, there is a clear commercial benefit in having a cushion sub-assembly that boasts an increased lifespan, and which is designed to be quick, simple and inexpensive to refurbish. 
       OBJECTS OF THE INVENTION 
       [0006]    It is an object of the present invention to overcome at least some of the above-mentioned problems or provide the public with a useful alternative. 
         [0007]    There is a need for a cushion sub-assembly having both axial and torsional vibration absorbing characteristics capable of effectively handling the types of forces experienced in modern drilling techniques. 
         [0008]    There is also a need for a cushion sub-assembly having a longer wear life than is otherwise achievable with existing cushion sub arrangements whose design and internal arrangement facilitates quick, simple and inexpensive refurbishment/replacement. 
         [0009]    The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form or suggestion that the prior art forms part of the common general knowledge in Australia. 
       SUMMARY OF THE INVENTION 
       [0010]    In one aspect, the present invention provides a cushion sub-assembly for connection to a drill string to absorb vibrations in the drill string, the cushion sub-assembly including a main body defining a chamber therein, a piston member disposed within the chamber capable of axial movement at least partially within the chamber between a first and second position, at least one axially extending piston rod disposed inside a respective axial bore of the piston member, the at least one piston rod preventing relative rotation between the piston member and the main body while permitting axial movement of the piston member between the first and second positions, and a vibration absorbing means at opposite ends of the chamber for engagement by the piston member when in either of the first or second positions for absorbing axial vibrations, the configuration of the at least one piston rod inside the piston member, and engagement of the piston member with the vibration absorbing means serving to minimise transmission of axial vibrations between the piston member and the main body. 
         [0011]    In one embodiment, the piston member includes a piston shaft formed integrally therewith, the piston shaft being of a stepped down diameter. 
         [0012]    The piston member may further include a plurality of radially disposed respective axial bores extending through the piston member, the bores being disposed between the outer periphery of the piston member and the piston shaft for accommodating a plurality of the piston rods. By including piston rods running inside the piston member, not only has it been found that axial vibration is reduced, but once the cushion sub-assembly has reached the end of its wear life, relatively minimal work is required to refurbish the cushion sub-assembly for re-use. Typically, after prolonged use, the bores extending axially through the piston member become elongated and the piston rods themselves become significantly worn. Thus, in order to refurbish the cushion sub-assembly, the bores need to be bored out and made circular again, and the piston rods require replacement with rods having an appropriately larger diameter. However, in embodiments of the invention, the main body of the cushion sub-assembly does not require refurbishment or replacement because the piston rods are disposed inwards of the casing. 
         [0013]    In one embodiment, the main body includes a casing member defining the chamber and a back head mounted to the casing member at an upper end thereof, the back head including a first connection means for rigid connection to a driving portion of the drill string. 
         [0014]    In another embodiment, the piston shaft includes a second connection means for rigid connection to a driven portion of the drill string. 
         [0015]    Preferably the first position of the piston member corresponds with the upper end of the main body, and the second position of the piston member corresponds with the lower end of the main body. 
         [0016]    The vibration absorbing means may be in the form of at least one pad of elastomeric material. Further, the vibration absorbing means at the upper end of the main body may include three pads of elastomeric material, wherein the piston member engages a bottom of the three pads when in the first position. In this embodiment, the lower-most pad of elastomeric material has a first thickness and has a first grade strength, the middle pad has a second thickness which may be substantially equal to the first thickness and a second grade strength which may be substantially greater than the first grade strength, and the upper-most pad of elastomeric material has a third thickness which may be substantially thinner than the first thickness and has a third grade strength which may be substantially greater than the second grade strength. 
         [0017]    In another embodiment, the vibration absorbing means at the lower end of the chamber includes a single pad of elastomeric material of high strength, wherein the piston member engages the single pad when in the second position. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    Once or more embodiments of the invention are described below with reference to the accompanying drawings, which are incorporated in and constitute a part of this specification. In the drawings, 
           [0019]      FIG. 1  illustrates an exploded, perspective view of a cushion sub-assembly in accordance with a first embodiment of the present invention. 
           [0020]      FIG. 2  illustrates an alternate exploded, perspective view of the cushion sub-assembly of  FIG. 1 ; 
           [0021]      FIG. 3   a  illustrates an axial cross-sectional view of the cushion sub-assembly of  FIGS. 1 and 2 ; 
           [0022]      FIG. 3   b  illustrates a cross-sectional view of the cushion sub-assembly of  FIGS. 1 and 2 , taken along line A-A in  FIG. 3   a.    
           [0023]      FIG. 4  illustrates a cross-sectional view of the cushion sub-assembly of a second embodiment of a cushion sub-assembly. 
       
    
    
       [0024]    Wherever possible, the same reference numbers will be used throughout the drawings and the following description to refer to the same and like parts. 
       DESCRIPTION OF EMBODIMENTS 
       [0025]    Referring to the drawings, the cushion sub-assembly is generally denoted by the reference numeral  10 . Cushion sub-assembly  10  has a main body  12  including an outer housing or casing member  14 , the casing member  14  having an outer cylindrical surface  16  and an inner cylindrical surface  18 , and which together with a bottom surface  20  and a back head  22 , form an inner closed chamber  24 . 
         [0026]    The back head  22  includes a pin connection  26  in the usual form of a tapered threaded portion concentrically and integrally formed with the back head  22 . Disposed within the main body  12  is a piston member  28  formed integrally with a cylindrical shaft  30  which projects downwardly through a circular opening  32  in the bottom surface  20 , as well as through a circular opening  34  in a connecting piston seal plate  36 . Within a lower end  38  of the shaft  30  is formed a box connector  40  formed by the usual tapered threaded opening. 
         [0027]    While the present description refers to the back head  22  and the outer housing  14 , it is to be appreciated that the sub-assembly cushion  10  need not be orientated in the position shown in the drawings. However, normally the pin connection  26  is connected directly to the output of the rotary drive of a drilling rig (not shown), this being referred to herein as the drive portion of the drill string. In such an arrangement, the box connector  40  is directly connected to the upper most section of the actual drill string, hereinafter referred to as the driven portion of the drill string. 
         [0028]    Housed inside the chamber  24  are a number of components, including a plurality of elongated piston rods  42 , a lower vibration damper or shock pad  44 , and three upper vibration dampers or shock pads  46 ,  48  and  50 , which are described in further detail below. 
         [0029]    The piston member  28  is disposed within the casing member  14 , and has a top surface  52  and an outer cylindrical surface  54  which is of substantially the same diameter as the inner cylindrical surface  18  of casing member  14 , so as to be axially slidable within the casing member  14 . In the embodiment illustrated, the shaft  30  of the piston member  28  is integrally formed with the piston member, and is of smaller diameter so as to form a lower piston surface or shoulder  56 . The surface  20  defining the circular opening  32  through which the shaft  30  passes is provided with annular grooves which receive casing seals  58  and a casing wear strip  60 . The casing seals  58  are used to keep grease (not shown) in the chamber  24 . The piston seal plate  36  also includes a dust seal  92  which contacts the shaft  30  and thereby prevents dirt from entering inside the chamber  24 . 
         [0030]    As described above, the cushion sub-assembly  10  includes a plurality of elongated piston rods  42 . The piston rods  42  are of a circular cross-section and are stepped down in diameter towards the upper and lower ends of the rods. The lower ends of the rods  42  are stepped down in diameter a first time to accommodate the lower shock pad  44 , and then again so as to be accommodated within the axially extending and correspondingly shaped bores  62  in the lower surface  20  of the casing member  14 . Likewise, the upper ends of the piston rods  42  are stepped down first to accommodate three shock pads  46 ,  48  and  50  and to be received within axially extending grooves  64  formed in the back head  22 , and then again to receive rod seals or caps  66 , which are placed on the upper ends of the piston rods  42  to prevent movement thereof. 
         [0031]    The piston rods  42  extend the full length of the chamber  24 , and run inside bores  68  which extend axially through the piston member  28 . The purpose of the piston rods  42  is to prevent relative rotation of the casing member  14  and piston member  28 , while permitting axial movement of the piston member  28  relative to the casing member  14 . 
         [0032]    The fact that the piston rods  42  run inside the piston member  28  contributes to reduced vibration, as described in further detail below, but also ensures that the rods  42  and the piston member  28  are the only major components which require re-working/replacement when the cushion sub-assembly is at the end of its useful working life. Clearly, the refurbishment of the cushion sub-assembly  10  requires only the boring out of the piston member bores  68 , and replacing of the piston rods  42  with rods of larger diameter which correspond with the diameter of the newly formed bores  68 . The commercial benefits of using a cushion sub-assembly  10  which can be quickly, easily and inexpensively refurbished to a state that is suitable for re-use are clearly apparent. 
         [0033]    The piston member  28  is hollow so as to define an internal passageway  70  axially therethrough, the passageway  70  extending through the shaft  30  of the piston as well so as to be in fluid communication with the interior of a driven portion of the drill string connected by way of the box connector  40  to the piston member. The pin connection  26  of the back head  22  also has an internal passageway  72  extending therethrough so as to communicate with the section of a drill string which forms the drive portion, normally the rotary drive (not shown), connected to casing  14  of the cushion sub-assembly  10  by way of the back head  22 . 
         [0034]    A cylindrical sleeve member or air nozzle  74  has an upper end thereof disposed within a lower portion of the internal passageway  72  of the back head  22  and a lower end thereof disposed within an upper portion of the passageway  70  of the piston member  28 , the air nozzle  74  thus placing the internal passageway  72  in fluid communication with the internal passageway  70 . Accordingly fluid is free to flow through the cushion sub-assembly  10  from an upper driven member of the drill string to a lower driven member. The air nozzle  74  has a radially projecting annular flange  76  encircling its upper end. The flange  76  is retained against a shoulder  78  formed within an enlarged lower portion of the internal passageway  72  in the back head  22  by way of a retaining ring  80 . 
         [0035]    Two o-rings  82  are located above the flange  76  so to isolate the cylinder chamber  24  from the internal passageway  72 . At the upper end of the internal passageway  70  in the piston member there is also provided a plurality of annular grooves, the upper one of which contains a piston wear strip  84 , and the others being piston seals  86 , to isolate the cylinder chamber  24  from the internal passageway  72 . 
         [0036]    Located in the upper end of cylinder chamber  24  are three annular shaped piston shock pads  46 ,  48  and  50 , which prevent metal to metal contact between the top surface  52  of the piston member  28  and the back head  22 . Located in the lower end of cylinder chamber  24  is a single annular shaped piston shock pad  44 , which sits above surface  20  of the casing member  14  and prevents metal to metal contact between the shoulder  56  of the piston member  28  and the surface  20 . The shock pads serve to cushion axial vibration and may be formed of an elastomeric type material, such as a polyurethane. 
         [0037]    The three upper shock pads have different grade strengths. The lowest pad  46  is thick but has the lowest strength of the three pads, the middle pad  48  is of the same thickness but with increased strength, and the top pad  50  is the thinnest but the strongest. The pad at the lower end of the chamber  24  is of the same thickness as pad  46  and  48  but of the same strength as pad  50 . It has been found that in arranging the shock pads in layers of different size and strength as illustrated, as well as having the piston rods  42  run inside the piston member  28  as described above, allows axial vibration to be significantly decreased because force is absorbed across all three pads, and the final pad being the strongest ensures that minimal vibration is carried up through to the rotary head. The rod seals  66 , in cushioning the upper ends of the piston rods  42 , may also provide a form of torsional vibration damping. 
         [0038]    As the piston member  28  approaches its lower extreme position, its downward movement relative to the casing member  14  is cushioned by the lower shock pad  44  and then as it approaches its higher extreme position, its upward movement relative to the casing member  14  is cushioned by the upper shock pads  46 ,  48  and  50 . It can be seen, therefore, that the piston member  28  has relatively free or floating movement within a major portion of its axial travel intermediate the lower surface of shock pad  46  and the upper surface of shock pad  44 , while being prevented from axial rotation relative to the casing member  14  by way of the piston rods  42 . The extent of movement of the piston member  28  is approximately 100 mm. 
         [0039]    The back head  22  and piston seal cap  36  can be attached to the casing member  14  using any suitable connection means such as threaded bolt connections. The same applies to the connection between the retaining ring  80  and air nozzle  74 . The reference numeral  90  is used in connection with all of the bolts used. It is to be understood however that the present invention is not intended to be limited to a particular type of connection means. Prior to bolting, the chamber  24  is filled with grease (not shown), and where re-greasing is required, there is also provided a plug  88  extending through the side wall of casing member  14 . 
         [0040]      FIG. 4  illustrates a second embodiment of the cushion sub-assembly  10 . In this embodiment, there is a single annular shaped piston shock pad  94  as an alternative to three annular shaped piston shock pads  46 ,  48  and  50  illustrated in previous embodiments. Also, in this embodiment, the plug  88  (not shown on  FIG. 4 ) is disposed towards an upper section of the cushion sub-assembly  10 . Furthermore, in this embodiment, piston wear strip  84  is disposed below the top surface  52  of the piston member  28 . 
         [0041]    The cushion sub-assembly  10  provides a means of significantly reducing vibrations resulting from downhole hammer type drilling, which are typically carried up through to the rotary head and which may otherwise cause significant damage. During testing of various embodiments, the cushion sub-assembly  10  has been found to decrease axial vibrations by a factor of three to four. 
         [0042]    The invention further provides a cushion sub-assembly that is of a more efficient design than previously known sub-assemblies in that there are fewer parts, and the arrangement of parts, for example the fact that the piston rods  42  run inside the piston member  28 , means that fewer parts (such as the casing member  14  for example) require replacement. The design allows those parts which do become worn through use, to be easily refurbished or replaced within minimal time and with minimal expense. 
         [0043]    Further advantages and improvements may be made to the present invention without deviating from its scope. Although the invention has been shown and described in what is conceived to be the most practical embodiment, it is recognized that departures may be made therefrom within the scope and spirit of the invention, which is not to be limited to the details disclosed herein but is to be accorded the full scope of the claims so as to embrace any and all equivalent devices and apparatus.

Summary:
The invention discloses a cushion sub-assembly ( 10 ) for connection to a drill string to absorb vibrations in the drill string, the cushion sub-assembly ( 10 ) including a main body ( 12 ) defining a chamber therein ( 24 ), a piston member ( 28 ) disposed within the chamber ( 24 ) capable of axial movement at least partially within the chamber ( 24 ) between a first and second position, at least one axially extending piston rod ( 42 ) disposed inside a respective axial bore of the piston member ( 28 ), the at least one piston rod ( 42 ) preventing relative rotation between the piston member ( 28 ) and the main body ( 12 ) while permitting axial movement of the piston member ( 28 ) between the first and second positions, and a vibration absorbing means ( 44, 46, 48, 50 ) at opposite ends of the chamber ( 24 ) for engagement by the piston member ( 28 ) when in either of the first or second positions for absorbing axial vibrations, the configuration of the at least one piston rod ( 42 ) inside the piston member ( 28 ), and engagement of the piston member ( 28 ) with the vibration absorbing means ( 44, 46, 48, 50 ) serving to minimise transmission of axial vibrations between the piston member ( 28 ) and the main body ( 12 ).