Abstract:
A hydraulic actuator for use in a work vehicle includes a tube including a first end, a second end and an inner surface; a plug with a portion of the plug disposed inside the tube and a portion of the plug extending outside the tube; a weld extending around the perimeter of the plug and coupling the first end of the tube to the plug, the weld forming a fluid-tight seal between the plug and the tube; a seal abutting against the plug and the inner surface of the tube and being spaced from the weld; an end plug affixed to the second end of the tube to enclose and seal the second end of the tube, the end plug defining a rod opening; a piston configured to be slidingly supported within the tube, the piston including a retract face, an extend face oppositely disposed from the retract face and a lip protruding from the extend face; and a piston rod affixed to the piston, the piston rod extending out of the tube through the rod opening.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 10/037,405, filed Dec. 21, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates generally to hydraulic actuators. More particularly, it relates to welded hydraulic actuators formed of a tubular portion and an end cap or plug that is welded to the actuator to enclose one end of the actuator.  
         BACKGROUND OF THE INVENTION  
         [0003]    Hydraulic actuators are used in a wide variety of industrial applications. One of the more common uses is as actuators on work vehicles. Work vehicles, such as agricultural tractors, road graders, telehandlers, skid steer loaders, and mobile drilling rigs, use either single- or double-acting hydraulic actuators to move various components of the work vehicle and to move implements attached to the work vehicle with respect to the vehicle and with respect to each other.  
           [0004]    A common method of manufacturing these actuators is to machine and polish the inside of a tube, such as a cylindrical tube. A plug or end cap is machined to enclose one end of the tube through which fluid will be introduced or removed from the actuator. The plug is partially inserted into the cylindrical tube, clamped in a rotational welding machine, and rotated in that machine while a circumferential weld is made that bonds one end of the tube to a portion of the plug.  
           [0005]    To ensure that the plug and the cylindrical tube are properly aligned during the welding process, the plug is usually provided with a pilot portion on one end that is inserted into the tube. This pilot portion has a smaller diameter than the remainder of the plug portion and the junction between the pilot portion and the remainder is formed as a planar or conical shoulder. To assemble the tube and plug, the pilot portion is inserted into the tube until the shoulder on the plug abuts an end face of the tube. The weld is formed between the end face of the tube and the abutting shoulder portion of the plug.  
           [0006]    A common failure mode for such welded actuators is that of weld failure. Hydraulic pressure acting against the inside surface of the tube creates hoop stress, which tends to cause the tube to expand or increase in diameter. The plug, on the other hand, is typically made of a rigid, solid piece of steel that does not expand when hydraulic fluid presses against its internal surfaces. As a result, a bending stress is created right at the weld joint coupling the tube and the plug. The tube expands radially when pressure is applied. The plug does not expand. Since the junction between the tube and the plug is the circumferential weld joint, it is the circumferential weld joint where the stress is at a maximum.  
           [0007]    One way of avoiding failures at the tube-to-plug joint has been to provide a more flexible coupling. For example, rather than employing a weld to join the tube and plug, many actuators, especially smaller actuators, use a thread joint between the tube and plug. In these actuators, a pilot portion of the outside diameter of the plug is threaded, and a corresponding inside portion of the end of the tube is also threaded. To couple the two together, the threads on the outside of the plug are engaged with the threads on the inside of the tube and the two are threaded together. When hydraulic fluid under pressure is introduced into the actuator, the tube expands slightly due to the hoop stress generated by the fluid. Since the bond between the tube and the plug is a thread joint, the tube is free to expand slightly thereby slightly increasing the gap between the tube and the plug. This non-restrictive joint allows slight expansion of the tube to occur without additional stresses of a joint trying to restrain it. In this manner, the tube is made stronger. In addition, by eliminating the weld joint, the “cast” portion of the actuator, the actuator is made much more resistant to stress generally.  
           [0008]    Of course, since the tube is permitted to expand with respect to the plug, a gap between the two along the thread joint is created. This gap, although small, provides a fluid leakage path. Fluid inside the actuator will leak out of the actuator along this thread joint. For this reason, a fluid tight seal that is relatively flexible is placed between the plug and the tube. In smaller actuators, this may be nothing more than a wrapping of thin Teflon® tape around the external threads on the plug. For larger actuators, however, such as those that have an area greater than about ½″ in diameter, seal, such as an O-ring, is typically placed in a circumferential or otherwise peripheral groove in the plug before it is inserted into the tube. The O-ring extends circumferentially around the diameter or perimeter of the plug and abuts both the plug and the tube providing a generally fluid-tight seal between the two that prevents fluid in the actuator from leaking out between the threads on the plug and the mating threads on the tube. When the tube in these threaded cap arrangements are pressurized with hydraulic fluid, it expands. The O-ring, however, is selected to be sufficiently pre-loaded to maintain contact with the internal walls of the tube even when it expands slightly because of hoop stress.  
           [0009]    Nevertheless, a problem with threaded actuators is that forming the threads is an expensive operation relative to welding. Roger Mickelson recognized a need in the art for a welded actuator that reduced bending stress, and disclosed an improved hydraulic actuator that provides the low cost and ease of manufacture of a welded actuator yet reduces the longitudinal tensile forces on the weld to increase the actuator&#39;s longevity in U.S. Pat. No. 6,637,315, issued Oct. 28, 2003, and U.S. patent application Ser. No. 10/037,405, filed Dec. 21, 2001. U.S. Pat. No. 6,637,315 and U.S. patent application Ser. No. 10/037,405 are incorporated herein by reference.  
           [0010]    A problem presented by inserting a seal into an actuator having a welded plug is that the seal cannot be placed into the tube along with the plug, as in an actuator having a threaded connection between the plug and the tube. Rather, to prevent damage to the seal during the welding operation, the seal must be inserted after the plug is welded to the tube. Therefore, even with the advances in the art represented by U.S. Pat. No. 6,637,315 and U.S. patent application Ser. No. 10/037,405, there is a need for an apparatus and method that make assembly of the hydraulic actuator having a welded tube and plug simpler and therefore less costly.  
         SUMMARY OF THE INVENTION  
         [0011]    According to one aspect of the invention, a hydraulic actuator for use in a work vehicle comprises a tube including a first end, a second end and an inner surface; a plug with a portion of the plug disposed inside the tube and a portion of the plug extending outside the tube; a weld extending around the perimeter of the plug and coupling the first end of the tube to the plug, the weld forming a fluid-tight seal between the plug and the tube; a seal abutting against the plug and the inner surface of the tube and being spaced from the weld; an end plug affixed to the second end of the tube to enclose and seal the second end of the tube, the end plug defining a rod opening; a piston configured to be slidingly supported within the tube, the piston including a retract face, an extend face oppositely disposed from the retract face and a lip protruding from the extend face; and a piston rod affixed to the piston, the piston rod extending out of the tube through the rod opening.  
           [0012]    According to another aspect of the invention, a method of manufacturing a hydraulic actuator for a work vehicle, in which the actuator includes a tube and a plug, comprises the steps of inserting a portion of the plug into a first end of the tube; forming a weld between the plug and the first end of the tube around the entire perimeter of the plug and tube to form a hydraulic-fluid-tight junction between the plug and tube; inserting a seal into a second end of the tube such that the seal is coaxial with the plug; inserting a piston assembly into the tube; coupling a second plug to a second end of the tube, the second end being oppositely disposed to the first end; and supplying hydraulic fluid to retract the piston assembly into the tube and to force the seal into an abutting relationship with the plug.  
           [0013]    According to yet another aspect of the invention, a method of manufacturing a hydraulic actuator for a work vehicle, in which the actuator includes a tube and a plug, comprises the steps of: inserting a portion of the plug into a first end of the tube; forming a weld between the plug and the first end of the tube around the entire perimeter of the plug and tube to form a hydraulic-fluid-tight junction between the plug and tube; inserting a seal into a second end of the tube such that the seal is coaxial with the plug; inserting a piston assembly into the tube; and forcing the piston assembly into the tube to seat the seal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The present invention will become more fully understood from the following detailed description, taken in conjunction with the accompanying drawings, wherein like reference numerals refer to like parts, in which:  
         [0015]    [0015]FIG. 1 is a partial cut away view of a hydraulic cylinder in accordance with the present invention;  
         [0016]    [0016]FIG. 2 is a cross-sectional view of the plug of FIG. 1 taken along the longitudinal axis of both the tube and the plug;  
         [0017]    [0017]FIGS. 3A and 3B are charts and partial sectional views of a cylinder lacking the seal between the tube and the plug, and a cylinder having the seal disposed between the tube and the plug showing the longitudinal tensile and hoop stresses generated within the cylinder along a path “P” that extends along the inner surface of the tube and through the plug of these cylinders at the same depth; and  
         [0018]    [0018]FIG. 4 is an alternative embodiment of the cylinder of FIGS.  1 - 2  in which the O-ring seal illustrated in those FIGURES is replaced with a polymeric adhesive seal.  
         [0019]    [0019]FIG. 5 is a cross-sectional view of a piston having a lip formed on one end according to another embodiment of the invention;  
         [0020]    [0020]FIG. 6 is a cross-sectional view of a hydraulic cylinder according to the embodiment shown in FIG. 5; and  
         [0021]    [0021]FIG. 7 is a detail view of a seal disposed in a groove of an end cap according to the embodiment shown in FIGS. 5 and 6. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    Referring now to FIG. 1, a hydraulic actuator, and in particular a hydraulic cylinder  100 , is shown comprising a tubular portion  102  welded to a plug  104  at weld joint  106 . Since the tube and plug are joined together by weld joint  106 , they form an integral tube and plug assembly  108 . A piston assembly  110  includes a piston  112 , which is disposed inside of and sealingly engages with inner cylindrical wall  114 , and piston rod  116 , which is fixed to piston  112 . Piston  112  and inner wall  114  are so dimensioned as to permit piston  112  to slide within tube portion  102  while maintaining a hydraulic-fluid-tight seal between the outer surface of piston  112  and inner wall  114 . A second plug  118  is threadedly engaged with the distal end of tube portion  102  located away from plug  104 . Threads formed on the outer surface of second or threaded plug  118  engage mating threads formed on the inner surface of tube portion  102  at the distal end of tube portion  102 .  
         [0023]    In the preferred embodiments, the hydraulic actuator is a hydraulic cylinder comprising a cylindrical tubular portion  102 , a piston  112  having a circular cross section, and other components having circular cross sections. However, it will be appreciated that other cross sections will work equally well, and that the invention is not limited to any particular cross-sectional shape. Thus, the tubular portion may have a rectangular, elliptical or triangular cross section, as may the other components.  
         [0024]    Hydraulic cylinder  100  is a double-acting cylinder having two ports  120  and  122  located at opposing ends of tube portion  102 . Port  120  may be formed in second plug  118  to permit hydraulic fluid to flow into and out of the cylinder. Alternatively, it may be formed in the tube itself at a location generally adjacent to plug  118 . Port  122  may be formed in plug  104  to permit fluid to flow into and out of the cylinder. An O-ring  124  or other seal (but preferably an O-ring) is disposed in a circumferential or peripheral groove  126  in the outer circumferential surface  128  of plug  104 . O-ring  124  preferably has a radial thickness of between 0.020 and 0.250 inches. More preferably it has a radial thickness of between 0.040 and 0.180 inches. Even more preferably it has a radial thickness of between 0.060 and 0.150 inches.  
         [0025]    Plug  104  is configured as two integrally formed portions: an eye portion  130  that extends outward away from weld joint  106  and a pilot portion  132  that extends inward into tube portion  102  from weld joint  106 . Outer circumferential surface  128  and circumferential groove  126  are formed in the pilot portion  132  of plug  104 .  
         [0026]    Referring now to FIGS. 2 and 7, plug  104  and the end of tube portion  102  are shown in greater detail. As shown by cross-hatching, plug  104  is an integral body. It has an eye  200  formed in eye portion  130  with a longitudinal axis  202  that is perpendicular to and intersects longitudinal axis  204  of plug  104  itself. A counter bore  206  is formed in pilot portion  132  extending from free inner surface  208 , through pilot portion  132  and into eye portion  130 . Counter bore  206  is preferably coaxial with plug  104  sharing the same longitudinal axis  204 . An intersecting bore  210  is formed in eye portion  130  extending between outer circumferential surface  212  of eye portion  130  into counter bore  206  which it intersects. An inner surface of bore  210  has internal threads  214  configured to engage hydraulic line or coupling. Intersecting bore  210  defines port  122  previously identified in FIG. 1. As shown in FIG. 7 and discussed in more detail below, plug  104  has a groove  126  formed around the perimeter of the end of pilot portion  132 . The groove  126  is formed such that a lip or overhang  127  extends radially outward from the end of pilot portion  132 .  
         [0027]    Outer circumferential surface  128  of pilot portion  132  is spaced away from the inner circumferential surface  216  of tube portion  102 . In this manner, a gap “G” is provided between the two surfaces  216  and  128 . This gap, on the order of 0.001 to 0.020 inches, depending upon the tolerance stackups of the cylinder, is small enough to hold plug  104  and tube portion  102  in close alignment to permit accurate welding (indicated by weld joint  106 ), yet is large enough to permit plug  104  to be inserted into tube  102  without undue force. Such force, if the gap is too small, could cause plug  104  to jam when it is inserted into the open end of tube  102  prior to welding. O-ring  124 , weld joint  106 , inner surface  216 , and outer surface  128  define a sealed cylindrical cavity  220 .  
         [0028]    O-ring  124  is disposed in circumferential groove  126  and is sized such that it seals against groove  126  and also against the inner circumferential surface  216  of tube portion  102 . The O-ring is not provided to prevent leakage out of the cylinder, however, since weld joint  106  prevents fluid leakage. Weld joint  106 , as shown by dashed lines  218  extends circumferentially around the entire outer surface of tube  102  and plug  104 , thereby providing an integral metal seal between tube  102  and plug  104 . Weld joint  106  is comprised of metal from tube  102 , metal from plug  104 , and additional metal deposited during the welding process. Its microstructure is cast, and is not work-hardened. O-ring  124  is not positioned directly adjacent to weld joint  106 , but is spaced away from weld joint  106  by a distance “D”. Distance “D” is preferably between about 1 and about 0.1 inches. More preferably it is between about 0.5 and about 0.2 inches. Most preferably it is between about 0.4 and about 0.25 inches. Experiments conducted on welded cylinders using an O-ring such as that shown in FIGS. 1 and 2 illustrate the unusual and unanticipated results of applying an O-ring to a pilot portion of a welded cylinder.  
         [0029]    [0029]FIG. 3A illustrates a pair of exemplary hoop stress and longitudinal tensile stress curves for a prior art welded cylinder. In FIG. 3A, the solid curve  300  represents longitudinal tensile stress in the tube at its inner surface  216 . Dashed curve  302  represents the hoop stress within the cylinder caused by hydraulic fluid pressure. Note that the longitudinal tensile stress in tube portion  102  is quite low away from plug  104 . As one travels along tube portion  102  towards plug  104 , the longitudinal tensile stress begins to increase, indicating how weld joint  106  constrains the expansion of tube  102  when hydraulic fluid is applied inside cylinder  100 . The longitudinal tensile stress reaches a maximum in the vicinity of weld joint  106 . It rapidly falls off as we traverse path “P” into plug  104 . In a similar fashion, the hoop stress indicated by curve  302  is at a maximum in tube  102  and drops to near zero in weld joint  106 . Thus, the longitudinal tensile stress in cylinder  100  reaches a maximum at weld joint  106 . This high longitudinal tensile stress produces weld joint failure. Its effects are amplified by the fact that it is applied right at the root of a “crack”—the joint between the tube and the plug where the weld is formed. This region, while not formed by cracking but by welding, is a stress concentrator due to its very small radius of curvature right where the weld is formed.  
         [0030]    [0030]FIG. 3B illustrates the changes in cylinder stress provided by the use of O-ring  124 . As in the previous example shown in FIG. 3A, hoop stress  306  reaches a maximum in a portion of tube  102  located away from plug  104 . As one traverses path “P” through cylinder  100 , from left to right, the hoop stress drops dramatically, approaching zero, in the vicinity of O-ring  124 . At the same time, the longitudinal tensile stress indicated by curve  304  is at a minimum in tube  102  located away from plug  104  and increases to a maximum in the vicinity of O-ring  124 . Longitudinal tensile stress  304  decays to near zero moving along path “P” rightwardly from O-ring  124  to weld joint  106 . Thus, longitudinal tensile stress is at its maximum in the vicinity of O-ring  124  and is significantly reduced at weld joint  106 , as compared to the longitudinal tensile stress that would exist in the absence of O-ring  124 .  
         [0031]    This reduction of stress, or rather the transfer of stress from weld joint  106  to the vicinity of O-ring  124  is unexpected and anomalous. While O-rings have been provided in the past in cylinder plug grooves having threaded joints (rather than welded joints), their function has been to prevent leakage of fluid through a thread joint between the cylinder tube and the cylinder plug. They have not been used, nor is there any reason to use them, in hydraulic cylinders using a welded tube/plug joint, since the weld joint itself provides both mechanical connection and the leak proof seal.  
         [0032]    [0032]FIG. 4 illustrates an alternative embodiment of the cylinder of FIGS. 1 and 2 in which a different seal  400  is provided between tube portion  102  and plug  104  in pilot portion  132 . The embodiments shown in FIGS. 2 and 4 differ in one respect. In FIG. 4, circumferential groove  126  has been eliminated, together with O-ring  124 , and is replaced with seal  400 , which is preferably a polymeric sealant, that is disposed in gap “G” between inner circumferential surface  216  of tube portion  102  and outer circumferential surface  128  of pilot portion  132 . The preferred polymeric sealant is an anaerobic adhesive, which is preferably of a low viscosity sufficient to permit it to penetrate gap “G” by capillary action. Suitable sealants include “Thread Locker 290®,” “Loctite® 603,” or “Loctite® 609.” Each of these products are manufactured by Loctite Corporation.  
         [0033]    To ensure a good bond, cleaning of the surfaces of the gap is preferred. The material for cleaning the surfaces is preferably 1,1,1-trichlorethane or any of the alternatives or equivalents for the solvent that are currently used. Such hydrocarbon-based solvents are preferred since they dry residue free, thus providing a good seal between the surfaces  216  and  128 . A primer or surface activator such as “Primer 7471®,” (Loctite Corporation) may be used after cleaning to enhance the quality of the bond where the metals that form tube  102  and plug  104  are passive. Primer 7471® is also beneficial when the gap “G” between the tube and plug is greater than about 0.004 of an inch.  
         [0034]    Although the hydraulic cylinder according to the foregoing embodiments endured more cycles than known hydraulic cylinders, the foregoing hydraulic cylinder presented the difficulty of inserting an O-ring or other seal down the length of the tube portion until the seal seated in the groove in the plug. The seal preferably fits snugly within the tube portion, so the seal is difficult, if not impossible, to force down the tube portion by hand. In addition, the tube portion in many hydraulic actuators according to the foregoing embodiments is long—often longer than the average human arm—so even if one could force the seal down the tube portion, one could not reach far enough to seat the seal. Even if one could reach the end of the tube portion, one would encounter difficulty getting the O-ring to seat in its final position. Accordingly, there is a need for a welded hydraulic cylinder including a seal having the benefits of the foregoing embodiments, but being simple to manufacture.  
         [0035]    Another embodiment according to the invention achieves an improvement in the foregoing embodiments in that it is simpler to insert the seal in the welded tube portion. This embodiment is shown in FIGS. 5, 6 and  7 .  
         [0036]    [0036]FIG. 5 shows piston  112 , which preferably includes a stud bore  502  having a piston rod bore  504  on one end and a nut bore  506  on the other end. Rod bore  504  is formed into a retract face  509  of piston  112  and nut bore  506  is formed into extend face  510 . Stud bore  502  receives a threaded stud of piston rod  116  (shown in FIG. 1), piston rod bore  504  receives piston rod  116  itself, and nut bore  506  receives a nut that screws onto stud bore  502  to hold piston rod  116  to piston  112 . The assembled piston rod and piston form piston assembly  110  as shown in FIGS. 1 and 6. Of course, the way of connecting the piston to the rod is not essential to the invention, and other piston assemblies may be used.  
         [0037]    As shown in FIG. 5, retract face  509  of piston  112  is the face on which pressurized hydraulic fluid is applied to force the piston assembly further into the tube portion  102  until piston assembly  110  is fully retracted, which occurs when extend face  510  of piston  112  contacts plug  104 . Conversely, hydraulic fluid is applied to extend face  510  to force the piston assembly out of tube portion  102  until piston assembly  110  is fully extended, which occurs when retract face  509  of piston  112  contacts second plug  118 . As shown in FIGS. 1 and 6, the retract and extend faces are disposed oppositely to each other and are spaced along the tube longitudinal axis.  
         [0038]    Also as shown in FIG. 5, piston  112  preferably includes a lip  508  protruding from extend face  510  of piston  112 . Lip  508  is preferably integrally formed with the main body of piston  112 , and can be formed by milling or grinding away a portion of extend face  510 . However, the lip need not be integrally formed, and may be attached to the piston in an operation subsequent to forming the main body of the piston, such as by welding or adhering. The lip may also be formed of a material different from the main body of the piston, including, but not limited to, other metals, plastic or rubber. The lip allows O-ring or other seal  124  (shown in FIGS. 6 and 7) to be pressed into groove  126  formed in plug  104  during assembly of the hydraulic actuator. Therefore, the lip is preferably the same or substantially the same diameter or size as O-ring  124 . Of course, a seal other than an O-ring may be used, such as a gasket, without departing from the scope of the invention.  
         [0039]    As shown in FIG. 7, lip  508  of the piston is preferably shaped to interfit with groove  126  of the plug. When the piston is in the fully retracted position, as shown in FIG. 7, O-ring  124  is disposed within the groove  126  and interposed between the lip  508  and the surfaces of groove  126 . O-ring  124  is held in place by overhang  127 .  
         [0040]    To assemble the hydraulic actuator according to this embodiment, the piston assembly is assembled, and, separately, the tube and plug assembly is assembled. Thus, the tube and plug assembly is assembled so that, but for the absence of the piston assembly and second plug, the tube portion is ready to receive hydraulic fluid.  
         [0041]    Next, the O-ring is placed into the tube portion and forced along the length of the tube toward the plug a distance that is less than the entire working length of the tube portion. (The working length, or stroke, is the length between the threaded plug and the welded plug.) The piston assembly is then inserted into the tube portion and the threaded plug is attached to the tube portion, such as by screwing the threaded plug into the tube portion. Of course, the threaded or second plug may be welded or adhered, rather than screwed onto, the tube portion.  
         [0042]    At this point, the hydraulic actuator is ready to receive hydraulic fluid. However, the O-ring is not yet seated in the groove  126  of the plug. To seat the O-ring as shown in FIG. 7, hydraulic fluid is added under pressure to the side of the tube portion bounded by the retract face, thus forcing the piston assembly into the tube portion. Alternatively, rather than forcing the piston assembly with hydraulic pressure, a mechanical device can be used.  
         [0043]    As the piston is forced along the interior of the tube portion by the hydraulic fluid or mechanical device, the lip  508  of the piston eventually comes in contact with O-ring  124  and forces O-ring  124  into groove  126 . Preferably groove  126  is formed with a rim or overhang  127 , which retains the O-ring in groove  126 . Once the O-ring is seated in the groove, it remains seated regardless of whether the piston assembly is retracted into or extended out of the tube portion.  
         [0044]    It will be appreciated by one of ordinary skill in the art that the method can be used in any hydraulic actuator having a welded plug and a seal. Thus, it is not necessary to use the method with hydraulic actuators that include a piston having a lip and an end plug having a groove. We believe that either or both of the lip and groove can be omitted and the method of inserting the seal using the piston assembly to seat the seal will still be effective.  
         [0045]    It is surprising that the provision of an O-ring or sealant adjacent to a welded joint would reduce weld failures. First, cavity  220  that is being sealed is about 0.001 to 0.020 inches in thickness in some applications (i.e. the gap between the inner wall of the tube and the outer surface of the pilot portion) with a length of about 0.25 to about 0.75 inches (the longitudinal distance between the weld and the O-ring) and a circumference of about 8 to about 16 inches (for a cylinder inner diameter of about 2.5 to about 5 inches). The volume that is sealed between the weld and the O-ring might vary in a typical rage of applications between about 0.002 cubic inches and about 0.25 cubic inches. This range of volumes is so small compared with the length of the O-ring (8 to 16 inches) that the O-ring would seem to provide little resistance to tiny quantities of fluid passing the O-ring to fill the sealed-off volume. Once the sealed-off volume was filled with fluid, one might expect that the O-ring would no longer reduce stress near the weld, since any pressure in the cylinder would immediately be communicated through the O-ring to the sealed-off volume. Surprisingly, this does not happen even after repeated pressure cycling of the fluid in the cylinder. The hoop stress in the tube adjacent the sealed-off portion stays low and thus the bending stress applied to the weld joint is minimized.  
         [0046]    We are not sure of the mechanism that reduces stress in the tube between the O-ring and the weld that provides the benefits of the present invention. We believe it may be due to residual air trapped between the O-ring and the weld in the sealed-off volume. If air remains trapped in the sealed-off volume even after repeated pressure cycling, slight compression of the O-ring when the cylinder is pressurized will not raise the pressure in the sealed-off volume significantly. This mechanism would reduce hoop stress in the tube and reducing bending stress at the weld.  
         [0047]    We do not intend for the claims to be limited to this possible mechanism of operation. It is provided only as a possibility.  
         [0048]    While the embodiments illustrated in the FIGURES and described above are presently preferred, it should be understood that these embodiments are offered by way of example only. The invention is not intended to be limited to any particular embodiment, but is intended to extend to various modifications that nevertheless fall within the scope of the appended claims.