Abstract:
A piston-type accumulator includes a separating piston ( 13 ) displaced axially within a piston housing ( 1 ). The piston separates a fluid side ( 15 ) of the accumulator from its gas side ( 23 ), and has comprises two sealing areas ( 17, 25 ) offset relative to each other in the axial direction. The sealing areas are arranged on the circumference of the piston and are displaced along the inner wall surface of the accumulator housing. The accumulator housing ( 1 ) is provided with a ventilation device ( 31 ) between the sealing areas ( 17, 25 ) for discharging leakage passing through the sealing areas ( 17, 25 ).

Description:
FIELD OF THE INVENTION 
   The present invention relates to a piston-type accumulator comprising a separating piston axially displaceable inside a piston housing. This piston separates a fluid side of the accumulator sealing areas axially offset from each other on its circumference piston housing. 
   BACKGROUND OF THE INVENTION 
   Piston-type accumulators are provided with a large number of designs. The part of the piston housing, surrounding the separating piston and extending axially, is in the form of a cylindrical tube. For this reason, the piston-type accumulator is often also termed a cylindrical accumulator. The sealing areas on the circumference of the separating piston customarily are formed of annular or O-ring seals which are seated in external circumferential grooves axially offset from each other in the separating piston. 
   Very high requirements are set, such as that of operation over wide temperature ranges, for example, between −40° C. and 150° C., from the viewpoint of the operating capacity of such cylindrical or piston-type accumulators. Test stand experiments show that accumulators do not function satisfactorily with respect to long-term behavior, since gas often overflows toward the oil or fluid side. Such behavior is not acceptable in the case of accumulators which are to perform a safety function, especially if the accumulators involved are used in conjunction with hydraulic braking systems. Overflow of gas into the hydraulic braking system could result in malfunction or even failure. 
   SUMMARY OF THE INVENTION 
   Objects of the present invention are to provide a piston-type accumulator which retains its sealing capacity even under extreme conditions and over long periods of operation and which ensures that gas cannot reach the fluid side under any operating conditions. 
   According to the present invention, these objects are attained by a piston-type accumulator in which the piston housing has, at a point situated between the sealing areas of the separating piston, an overflow feature for discharging leakage media overflowing the sealing areas. 
   The overflow feature, positioned between the sealing areas on the gas side and the fluid side, ensures that media cannot overflow from the gas side to the fluid side or in the opposite direction, even if molecules of the media adjacent to the separating piston make their way through the otherwise tight sealing system on the circumference of the separating piston. Because of the small molecules of the medium situated on the gas side, nitrogen in most cases, some penetration of the sealing rings provided on the separating piston cannot be completely eliminated, even if the surface on the inside of the cylindrical tube of the piston housing has been subjected to the most precise machining. Because of the requirement, by definition, of very little friction between the piston seal and the inside of the cylindrical tube during piston movement, high surface pressure cannot be provided thereat to avoid leakage. Even in the event of passage of leaks of hydraulic fluid, for example, through the piston seal from the fluid side, the hydraulic fluid leakage cannot penetrate the gas side because of the overflow feature. 
   The piston-type accumulator of the present invention is suitable, in particular, for applications in which safety requirements must be met, particularly for braking systems. Prevention of the passage of gas molecules to the fluid side is of decisive importance in this situation. 
   A vent opening drilled through the wall of the piston housing may be provided as an overflow feature. 
   In one advantageous embodiment, the discharge end of the vent opening communicates with a collecting chamber receiving the leakage media. As a result, leakage media are discharged to the exterior only after the collecting chamber is filled. The discharge of the collecting chamber can have a normally closed valve. This valve can be opened by the pressure prevailing in the collecting chamber. Automatic discharge occurs when a predetermined pressure builds up, after the collecting chamber has been completely filled. 
   In the case of an embodiment as a “supertight” piston-type accumulator, an additional third sealing area positioned even closer to the gas side may be provided, in addition to the sealing areas between which the overflow feature is positioned. Since passage of leakage components from the fluid side is possible, the piston seal forming the third sealing area and normally the piston seal forming the second sealing area as well would run dry. In one advantageous exemplary embodiment of the present invention, a supply device is provided in the separating piston for delivery of a free-flowing lubricant to the circumferential section of the separating piston positioned between the second and third sealing areas of the separating piston. Such lubricant may be a high-viscosity oil, such as a mineral oil or a free-flowing lipid. In addition to the lubrication achieved by this configuration, as a result of which piston friction is reduced and the service life extended, an additional blocking or sealing effect is obtained because of the high viscosity of the substance delivered. 
   Other objects, advantages and salient features of the present invention will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring to the drawing which form a part of this disclosure: 
       FIG. 1  is a side elevational view in section of a piston-type accumulator according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The piston-type accumulator shown in the drawing has a piston housing  1  with a cylindrical tube  3  closed on one end by an end wall  5  integral with the cylindrical tube  3  and on the opposite end by a sealing cover  7 . In the example illustrated, the sealing cover  7  is fastened to the cylindrical tube  3  by a snap ring  9 , and is sealed on the inner wall of the cylindrical tube  3  by an O-ring  11 . Snap ring  9  could be replaced by welding of the sealing cover to the cylindrical tube  3 . 
   A separating piston  13  is mounted in the cylindrical tube  3  for axially displacement. Separating piston  13  is sealed off from the superfinished inner wall of the cylindrical tube  3  by three piston seals axially offset from each and forming a first, second, and third sealing areas on the circumference or lateral peripheral side of the separating piston  13 . The piston seals are each represented by sealing rings seated in circumferential grooves in the separating piston  13 . First sealing ring  17 , nearest the fluid side  15  of the piston-type accumulator, forms the first sealing area. A passage  19  with a connecting sleeve  21  effects connection to an associated hydraulic system (not shown). 
   A second sealing ring  25  is seated in a circumferential groove in the separating piston  13 , and is provided as the second sealing area, axially offset from the sealing ring  17  forming the first sealing area, in the direction of the gas side  23  adjacent to the other side of the piston. A third sealing area is axially offset even further in the direction of the gas side  23 , is formed by a third sealing ring  27 , and is seated in a circumferential groove in separating piston  13 . 
   The path of the stroke of the separating piston  13  inside the cylindrical tube  3  is limited to a desired operating stroke length by mechanical stops (not shown) in the cylindrical tube, or, as an alternative, by controlling the pressure relationships of fluid side  15  and gas side  23 . The gas charging pressure of gas side  23  may be adjusted by a charging connection  29 . In the area of the cylindrical tube  3 , which extends over the entire operating stroke length of the separating piston  13  between the first and the second sealing area, that is, between sealing ring  17  and sealing ring  25 , a vent opening  31  extends through the wall of the cylindrical tube  3 . Vent opening  31  is formed as an overflow feature permitting discharge of leakage media. A recess in the shape of an annular groove  33  with beveled side edges extends over the entire circumference of the inner wall, and is in the inner wall surface of the cylindrical tube  3  in the area of the interior outlet or inner end of the vent opening  31 . The recess  33  also prevents shearing off of the seals  25  and  27  during assembly when these seals are pushed over the opening  31 . Together with a recess  37  in the circumference or lateral peripheral side of the separating piston, an inner chamber communicating with the vent opening  31  is formed into which leakage media may enter should migration of fluid molecules through the sealing ring  17  occur or should the sealing rings  27  and  25  be penetrated by the small gas molecules of the charging gas on the gas side  23 . Any such leakage media pass through the vent opening  31  into a collecting chamber  39  with which the outer end of the vent opening  31  communicates. 
   In the exemplary embodiment illustrated, collecting chamber  39  is formed by an annular element  41  seated on the outside of the cylindrical tube  3 . This annular element is a shaped element of plastic or sheet metal integrated with a flat outer annular surface or member  43  and set laterally in edge strips  45  extending vertically relative to it. The free ends of edge strips  45  rest on the exterior of the cylindrical tube  3  so that the annular surface or member  43  is kept equidistant from the exterior of the cylindrical tube  3 . The edge strips  45  are sealed off from the exterior of the cylindrical tube  3  by O-rings  47 . The collecting chamber  39  formed has an outlet to the exterior formed by an opening  49  which is opened or closed by a valve system. An elastic band  51  surrounds the annular surface or member  43  of the annular element  41 . The initial tension selected, which the band  51  applies to the annular surface or member  43 , is such that the band  51  is lifted from the opening  49  when a predetermined excess pressure is present in the collecting chamber  39  to discharge the leakage media present in the collecting chamber  39  into the environment. 
   In  FIG. 1 , the opening  49  forming the outlet is shown on the top side of the cylindrical tube  3 . For the overflow feature to extend upward no matter how the piston-type accumulator is mounted, the annular element  41  is rotatable on the cylindrical tube  3  to permit the opening  49  to be adjustable to the highest position. 
   In place of sealing the collecting chamber  39  off from the cylindrical tube  3  by the O-rings  47 , the ends of the edge strips  45  could be configured as sealing edges acting directly in conjunction with the cylindrical tube  3 . In place of a clear through vent opening  31 , an opening containing a porous plug-like insert might be provided, for example, by use of a plug of a porous sintered material. 
   An annular groove  53  communicates with a supply device for supplying a free-flowing lubricant, and is in the circumferential area of the separating piston  13  between the second and third sealing areas, that is, between sealing rings  25  and  27 . The separating piston  13  has, for this purpose, a concentric auxiliary cylinder  55  mounted in its interior, closed in the direction of the fluid side  15 , and open in the direction of the gas side  23 . An auxiliary piston  57  is introduced from the direction of the gas side into the auxiliary cylinder  55 . This auxiliary piston  57  has a circumferential piston seal  59  and is secured against escape from the auxiliary cylinder  55  by a snap ring  61 . The enclosed space situated between the auxiliary piston  57  and the closed end of the auxiliary cylinder  55  is filled with a supply of a free-flowing lubricant. A connecting channel  63  connects this lubricant supply space to the annular groove  53  on the circumference of the separating piston  13 . 
   The auxiliary piston  57  is spring loaded or biased by a helical pressure spring  65 . Spring  65  rests on a retaining plate  69  secured on the separating piston  13  by a snap ring  67 . Consequently, the auxiliary piston  57  is subject to the pressure of the gas side  23  and to the initial tension of the spring, so that the auxiliary piston  57  in the compartment containing the supply of lubricant generates a delivery pressure by which the lubricant is pressed into the annular groove  53 . The lubricant is a high-viscosity oil or a free-flowing lipid. As a result, a blocking or sealing effect is produced in the relevant areas of the separating piston  13 , in addition to lubrication of the sealing rings  25 ,  27 . Especially good long-term behavior of the piston-type accumulator is obtained, in particular complete safety from escape of the medium on the gas side  23  to the fluid side  15 , so that the piston-type accumulator of the present invention is especially well suited also for use in braking systems. 
   The elastic band  51  covering the opening  49  may also be replaced by another ring-shaped elastic element, such as one in the form of an O-ring or a ring rectangular in cross-section, or the like. 
   As is shown by the illustration, the possibility also exists of introducing into the interior of the housing a stop element  70  in the form of a bushing. This stop element  70  prevents the seal  17  from sliding into the groove  33  should the separating piston  13  return to a much higher position (not shown). The external circumference of the respective stop element  70  rests flush against the interior circumference of the piston housing  1 , and extends in an axial direction between the end wall  5  and one free end of the separating piston  13  when, as shown in the figure, this piston comes to rest against the stop element  70 . The stop element  70  is fixed by its inherent tension inside the piston housing  1  in its position as illustrated. The stop element may also be replaced by a projection or other stop means on the inside of the piston housing  1 . The configuration of the stop is selected such that the sealing means  17  cannot reach the groove  33  when the device is in operation. 
   While one embodiment has been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined in the appended claims.