Abstract:
An accumulator for a hydraulic system includes a polymer liner defining a cavity. A metal bellows assembly is housed in the cavity and separates the cavity into a first chamber and a second chamber, with the first and second chambers isolated from one another by the bellows assembly. A composite shell substantially encases the liner. The liner and shell are configured so that the first chamber receives hydraulic fluid from and delivers hydraulic fluid through an opening in the liner and the shell as the bellows assembly expands and compresses due to pressurized gas in the second chamber balancing fluid pressure changes in the first chamber. In one embodiment, the metal bellows assembly includes hydroformed bellows.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/102374, filed Oct. 3, 2008, which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The invention relates to a hydraulic accumulator, such as a hydraulic accumulator for a hydraulic vehicle, and a method of manufacturing a hydraulic accumulator. 
     BACKGROUND OF THE INVENTION 
     Hydraulic accumulators are energy storage devices that maintain a reserve of pressurized fluid to be provided to a hydraulic system when fluid pressure in the system drops. One type of hydraulic accumulator uses precharged gas that maintains pressure against fluid in the accumulator, forcing some fluid out of the accumulator and into a hydraulic system line when fluid pressure drops. When fluid pressure in the hydraulic system rises, fluid reenters the accumulator to maintain a reserve. Hydraulic accumulators help to balance pressure fluctuations in the hydraulic system. 
     SUMMARY OF THE INVENTION 
     An accumulator for a hydraulic system includes a polymer liner defining a cavity. A metal bellows assembly is housed in the cavity and separates the cavity into a first chamber and a second chamber, with the first and second chambers isolated from one another by the bellows assembly. A composite shell substantially encases the liner. The liner and shell are configured so that the first chamber receives hydraulic fluid from and delivers hydraulic fluid through an opening in the liner and the shell as the bellows assembly expands and contracts due to pressurized gas in the second chamber balancing fluid pressure changes in the first chamber. 
     In one embodiment, the metal bellows assembly includes hydroformed bellows. Hydroformed bellows may be less expensive than metal bellows made from separate metal discs welded to one another. Furthermore, bellows hydroformed from a metal tube result in no scrap metal, unlike welded bellows where the center of each disc is stamped out and removed. 
     Preferably, the first chamber (i.e., the fluid chamber) is open to and partially defined by the polymer liner and the second chamber (i.e., the gas chamber) is isolated from the liner by the metal bellows assembly. Configuring the accumulator with the gas inside of the bellows has several advantages. First, it allows the liner to be a relatively low cost polymer, rather than metal. A polymer liner is less desirable if the gas chamber is outside of the bellows, as polymers are generally not impervious to gasses. The present accumulator is designed to be maintenance-free for life, as a precharge of gas in the gas chamber will not require recharging. Second, it avoids the need to fully compress the bellows and therefore allows for the use of a formed instead of edge welded bellows. Third, for delivery of given fluid volume, it reduces the strain range (minimum to maximum extended length) of the bellows thereby improving bellows fatigue life. 
     Optionally, one or more guide features, such as polymer rings, are nested between the bellows assembly and the liner to substantially prevent contact of the bellows assembly with the liner, thus reducing fatigue wear. 
     In order to provide strength to the accumulator without adding undue weight, the composite shell may be a fiber reinforced composite with one or more of carbon, glass and aramid fiber in a binder base, such as a thermoplastic or thermoset resin. The fiber shell may be overwrapped on the liner after the bellows are inserted. An accumulator with a polymer liner and a composite shell typically requires a bladder to be used as the barrier (instead of bellows), as the bladder can be fit through an end opening in the liner and shell while bellows cannot collapse beyond their fixed diameter. To overcome this limitation, a multi-piece liner is used with a tubular center portion and separate end portions that are welded or otherwise connected with the center portion after installation of the bellows. 
     A method of manufacturing the hydraulic accumulator described above includes hydroforming metal annular bellows, securing metal end caps to opposing ends of the bellows, such as by welding, and then placing the bellows within a tubular polymer liner portion. Polymer end portions of the liner are then secured to opposing ends of the tubular portion by thermoplastic welding to enclose the bellows within the liner. The thermoplastic polymer liner is then covered with a composite material. 
     The tubular polymer liner portion may be formed by extrusion. The polymer liner end caps may be formed by injection or compression molding around structural, load bearing polar rings. One of the bellows end caps is secured to one of the polymer end portions so that the other bellows end cap is movable within the liner toward an opening (i.e., a fluid port) through the liner and the composite. 
     Covering the liner with a composite may be accomplished by overwrapping the liner with a composite of carbon fiber, fiberglass or aramid fiber. Alternatively, a braided composite may be pulled over the liner. 
     The combination of metal bellows, a polymer liner, and a composite overwrap provides a low cost, maintenance free and lightweight hydraulic accumulator especially suitable for use in a hydraulic automotive vehicle, although its use is not limited to such. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective illustration in partial cross-section view of one embodiment of a hydraulic accumulator; 
         FIG. 2  is a schematic illustration in cross-sectional view of an end of the accumulator with a fluid port for connection to a hydraulic system, and showing a fluid retention system closed and the bellows in a fully extended position; 
         FIG. 3  is a schematic illustration in cross-sectional view of an opposing end of the accumulator with a gas port for initial charging of a gas chamber; 
         FIG. 4  is a schematic illustration in cross-sectional view of the accumulator end of  FIG. 2  with the fluid retention system opened and the bellows in a partially compressed position; 
         FIG. 5  is a schematic perspective illustration in partial cross-sectional view of another embodiment of a hydraulic accumulator; and 
         FIG. 6  is a flow diagram of a method of manufacturing the accumulators of  FIGS. 1-5 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, wherein like reference numbers refer to like components throughout the several views,  FIG. 1  shows a hydraulic accumulator  10  that is part of a hydraulic system  12 . The accumulator  10  is in fluid communication with the remainder of the hydraulic system, represented at  16 , via a fluid line  14 . As is readily understood by those skilled in the art, the accumulator  10  acts as an energy storage device to provide a reserve of hydraulic fluid to the remainder of the system  16  when pressure in line  14  drops. The accumulator  10  described herein is configured as a low cost, maintenance-free and lightweight accumulator, appropriate for a variety of hydraulic systems, including hydraulic automotive vehicles. 
     The accumulator  10  has a pressure vessel  18  that is a multi-piece polymer liner  20 ,  22 ,  24  with a composite shell  26  overlaying the liner  20 ,  22 ,  24 . The multi-piece liner includes a tubular portion  20  and two polymer end portions  22 ,  24 . Preferably, the tubular portion  18  is extruded polymer, while the end portions  22 ,  24  are injection molded polymer. The liner  20 ,  22 ,  24  lines an inner surface  29  (see  FIG. 2 ) of the composite shell  26 . 
     The liner  20 ,  22  and  24  and shell  26  define an interior cavity  28  in which is housed a metal bellows assembly  30 . The bellows assembly  30  includes metal bellows  32 , which are preferably hydroformed instead of welded, with metal end caps  34 ,  36  welded to either end of the bellows  32 . The bellows  32  and end caps  34 ,  36  may be any suitable metal, including metal alloys, such as stainless steel alloy 321, INCONEL®, marketed and sold by Special Metals Corporation of Huntington, W.V. 
     The bellows assembly  30  divides the cavity  28  into a first chamber  35  and a second chamber  37 . The first chamber  35  is defined by the volume of the cavity  28  outside of the bellows  32 , between the bellows assembly  30  and the liner  20 ,  22 ,  24 . The second chamber  37  is defined by the volume of the cavity inside of the bellows assembly  30 . The bellows assembly  30  is fixed at one end cap  36  (the end cap  36  secured to the end portion  24 ), with the end cap  34  free to collapse and expand within the cavity  28  toward the end portion  22  as described below. 
     Referring to  FIG. 3 , the end cap  36  is secured to the end portion  24  via a bellow stem  40  which extends through a gas port  42  secured to a polar ring  44  molded into the end portion  24 . The bellow stem  40  has a center passage  46  extending therethrough. The bellow stem  40  is secured at an opening  41  in the end cap  36 . A one-way valve  48  is secured to the end of the bellow stem  40  and is openable to receive charging gas from a gas supply (not shown) to precharge the second chamber  37  with an inert gas. The valve  48  is covered by a cap  50  and sealed to the bellow stem  40  with a seal  52  so that the gas in the second chamber  37  may not escape. The second chamber  37  may also be referred to as a gas chamber. 
     Referring to  FIG. 2 , the end cap  34  supports a fluid retention mechanism  38  partially within a recess  60  formed in the end cap  34 . A collar  62  is secured to the end cap  34  and supports a guide plate  64  through which a poppet  66  slides. A retainer  68  is secured to one end of the poppet  66  and limits movement of the poppet  66  between the open position shown in  FIG. 4  and the closed position shown in  FIGS. 1 and 2 . As an alternative, the fluid retention system  38  may be mounted inside the fluid port  70  instead of the the bellows end closure  34  thus simplifying end closure  34 . 
     Referring to  FIG. 4 , a hydraulic fluid port  70  is secured at an opening  72  in the end portion  22  and the shell  26  with a polar ring  74  molded to the end portion  22 . A collar  76  helps secure the fluid port  70  and polar ring  74  to the end portion  22 . A ring seal  78  seals between the collar  76  and the polar ring  74 . A primary seal ring  80 , a primary seal retaining ring  81 , a secondary seal ring  82  and an O-ring seal  84  help to seal the fluid chamber  35 . 
     In the open position shown in  FIG. 4 , the poppet  66  does not cover an opening  86  through the fluid port  70 . Thus, fluid in the first chamber  35  is in communication with fluid in line  14  of  FIG. 1 , and flows to the remainder  16  of the hydraulic system  12  when fluid pressure in line  14  drops below the gas pressure in the gas chamber  37 , with the bellows  32  expanding toward the fluid port  70  to displace fluid from the chamber  35 . The second chamber  37  thus expands in volume as fluid is displaced from the first chamber, with gas pressure in the second chamber  37  falling as the bellows  32  expand. The extreme expanded position of the bellows  32  is shown in  FIGS. 1 and 2 . In the extreme expanded position, also referred to as the closed position, the poppet  60  contacts the seal ring  80  at the fluid port  70 , covering the opening  86 . The poppet  66  is held in position against the seal ring  80  by compression of a spring  90  positioned between the collar  62  and the poppet  66 . 
     The fluid retention mechanism  38  is configured so that in the extreme expanded position of the bellows  32  and the closed position of the poppet  66 , the end portion  34  does not contact the end cap  22 , so that the first chamber  35  has some minimum retained volume of hydraulic fluid to counteract the gas pressure in the second chamber  37  even when no external fluid pressure exists in line  14 , thus reducing the pressure differential between the chambers  35 ,  37  that can occur across the bellows assembly  30  to tolerable levels (i.e., levels that do not compromise the structural integrity of bellows assembly  30 ). Referring to  FIG. 1 , preferably the bellows  32  are configured with a stiffness that allows them to be collapsed to about one half of the fully expanded position shown in  FIG. 1 , to about line C. Thus, the extension ratio of the bellows  32 , i.e., the ratio of the expanded, maximum length in the closed position of  FIGS. 1 and 2  to the compressed, minimum length (length when compressed to line C) is about 2.0. 
     Optional guide features  90  are nested between the bellows  32  and the liner  20 ,  22 ,  24 . In this embodiment, the guide features  90  are rings that help keep the bellows  32  centered, preventing contact wear with the liner  20 ,  22 ,  24 . The guide rings  90  have apertures or other geometry that allows for fluid flow past the ring. Each guide ring  90  may also have a separate carrier  91  welded to the bellows  32 . As shown in  FIG. 2 , the end cap  34  is welded to the carrier  91 , which, in turn, is welded to the bellows  32 . 
     By designing the accumulator  10  with the fluid chamber (i.e., first chamber  35 ) outside of the bellows assembly  30  and the gas chamber (i.e., second chamber  37 ) inside of the bellows assembly  30 , the desired minimum volume of retained fluid is achieved with a much smaller extension ratio than would be possible if the fluid chamber were inside of the bellows assembly  30  and the gas chamber outside of the bellows assembly  30 . In that case, the bellows  32  would need to collapse to an overall height equal to the distance between the end cap  22  and the end portion  34  of  FIG. 2 . Because hydroformed bellows do not generally achieve as great a ratio of maximum length to minimum length as welded bellows (i.e., cannot collapse to as small a portion of their full extended length), the configuration of the gas inside of the bellows  32  and the fluid outside of the bellows  32  is especially suitable for the hydroformed bellows  32 . In comparison to an accumulator with welded bellows, for the same pressure range and volume ratio, the minimum collapsed length of the bellows  32  is longer, leading to higher bellows fatigue life. 
     Furthermore, by containing the gas in the second chamber  37  (inside of the bellows  32 ), a polymer liner  20 ,  22 ,  24  can be used regardless of its permeability to gas, as the gas is not in contact with the liner  20 ,  22 ,  24 . 
     The composite shell  26  encases the liner  20 ,  22 ,  24  to provide strength and integrity. The composite shell  26  is a composite of high strength yet lightweight fibers, such as carbon fibers, fiberglass, or aramid fibers in a binder base. The shell  26  may be filament or tape of the composite material wound around the assembled liner  20 ,  22 ,  24 . The combination of metal hydroformed bellows  32 , a polymer liner  20 ,  22 ,  24  and a composite shell  26  provides a low cost, maintenance-free and lightweight accumulator  10  suitable for many applications. 
     Referring to  FIG. 5 , another embodiment of an accumulator  110  is shown. The accumulator  110  also has hydroformed bellows  132  as part of a bellows assembly  130  contained in a cavity  118  defined by a multi-piece polymer liner  122 ,  124  (two-piece liner) and a composite shell  126  of similar material and construction as described above with respect to accumulator  10 . A first (fluid) chamber  135  is in fluid communication with an opening  186  in a fluid port  170  for providing a reserve of fluid to a remainder  16  of a hydraulic system, such as hydraulic system  12  of  FIG. 1 , through fluid line  14  with the bellows  132  expanding and contracting against gas pressure in a second (gas) chamber  137  inside of the bellows assembly  130 . A gas port  142  supports a one-way valve  144  for receiving gas from a gas supply to precharge the second chamber  137  to a desired gas pressure when a predetermined amount of fluid is in the first chamber  135 . The accumulator  110  does not have a fluid retention mechanism shown at the fluid port  170  to maintain a minimum fluid volume in the first chamber  135 , but a fluid retention mechanism such as mechanism  38  of  FIG. 1  could be incorporated into the accumulator  110 . 
     Referring to  FIG. 6 , a flow diagram illustrates a method  200  of manufacturing a hydraulic accumulator such as accumulators  10  and  110  described above. The method  200  is described with respect to accumulator  10 , but is not limited to manufacture of only accumulator  10 . In step  202  of the method, metal bellows  32  are hydroformed. Thus, the collapsible and extendable bellows  32  may be and preferably are one continuous piece, with no welding required of individual bellows to one another. 
     In step  204 , metal end caps  34 ,  36  are secured to the metal bellows  32 , preferably by welding. Before, after or contemporaneously with steps  202  and  204 , a tubular polymer liner portion  20  is extruded in step  206 . The bellows assembly  30  (i.e., the hydroformed metal bellows  32  with end caps  34 ,  36 ) is placed in the liner portion  20  in step  208 . Polymer end portions  22 ,  24  are secured to the liner portion  20  in step  210  to complete the liner. In step  212 , one of the metal end caps  36  is then secured to the polymer end portion  24 , such as by a gas port  42  and polar ring  44  with a bellows stem  40  secured through an opening  41  in the end cap  36 . Finally, in step  214 , the liner  20 ,  22 ,  24  is covered with a composite shell  26 , such as by overwrapping, filament winding or tape lay up. The completed accumulator  10  is now ready for gas precharging and connection with a fluid line  14  of a hydraulic system  12 . 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.