Abstract:
A sealing device for a hydraulic assembly wherein hydraulic fluid is contained in working chamber ( 53 ) formed between body ( 52 ) and thrust member ( 51 ) of the assembly. The device comprises annular seal ( 63 ) with opposed sealing faces which are urged into sealing engagement between body ( 52 ) and thrust member ( 51 ) which have convergent sealing faces. The device may also have a pressure relief valve ( 100 ) tapped into the over-stroke end of chamber ( 53 ) to protect seal ( 63 ) from over-stroke damage comprising porous body ( 101 ) which allows fluid to bleed from chamber ( 53 ) and allows seal ( 63 ) to pass the tapping point without obstruction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/598,363, filed on Jul. 15, 2008, which is hereby incorporated by reference and which claims the benefit of PCT Application Serial No. PCT/AU2005/000253, filed Feb. 25, 2005, and Australian Patent Application Serial No. 2004900922, filed Feb. 25, 2004. 
    
    
     FIELD OF INVENTION 
     This invention relates to high temperature seals for hydraulic assemblies and in particular seals which are suitable for hydraulic fasteners and nuts. 
     BACKGROUND OF THE INVENTION 
     Hydraulically tensioned nuts, washers and similar fasteners provide a means by which a stud or bolt can be tensioned by hydraulically actuating the nut or washer to exert a tensile force on the stud or bolt. These nuts and washers often operate under extreme pressure and temperature. 
     Hydraulic nuts or similar fasteners are typically pretensioned mechanically and thereafter hydraulic pressure is applied to a chamber within the fastened structure to generate an hydraulic force which applies an axial tensile load to a stud or nut engaged by the fastener. A locking collar may be provided to retain the tension after relieving the chamber of pressure. 
     Seals for use with hydraulic pressure devices are typically made of elastomeric material such as nitrile rubber or polyurethane. The ways in which these seal against the passage of fluid pressure can be divided into two types referred to herein as primary and secondary mechanisms. The primary mechanism of sealing acts during the initial application of fluid pressure. As this pressure increases, the elastomeric seal is deformed and forced into a position where the seal bridges the gap to be sealed, hereinafter referred to as the “extrusion gap”, in order to establish a secondary seal. 
     It is typical of hydraulically activated piston/cylinder arrangements that as the operating pressure increases, the cylinder walls expand radially causing a proportional increase in the extrusion gap between piston and cylinder. A limiting factor in the operation of hydraulic nuts is the effectiveness of their seals. Factors such as high pressures, high temperatures, service life under adverse conditions, limit their field of application and effectiveness. If these factors become extreme, either singularly or in combination, the materials which are commonly used as sealing agents fail. Failure occurs when there is flow or movement of the seal material into the extrusion gap under pressure and/or temperature and sealing is lost. 
     In extreme temperature/pressure applications, such as in electricity generators and nuclear power plant reactors, it is critical that seals do not fail as loss of tension applied to the studs or bolts for example in a generator housing or at a pipe flange joint, as such failure could result in a catastrophic disaster. U.S. Pat. No. 6,494,465 (Bucknell) (=International Application PCT/AU97/00425=International Publication WO 98/00660) discloses a range of hydraulic seals for hydraulic assemblies capable of operating at high temperatures. The seals incorporate lips which provide low pressure sealing between for example, a piston and a cylinder, and which are configured to move across the gap to be sealed at higher pressures with a base angled on a slope or a cup shape nestled into a groove. The seals may be formed of elastomeric material and/or thin sheet metal. 
     The seals of U.S. Pat. No. 6,494,465 have been used in many successful installations of high temperature, hydraulically tensioned fasteners in the electricity generation and nuclear power industries. However, experience has shown that there is a need for different types of sealing arrangements for fasteners, especially in response to specific operational requirements. 
     It is therefore an object of the present invention to provide high temperature seals for hydraulic assemblies such as fasteners which have improved sealing characteristics able to tolerate extreme factors such as high pressures and/or high temperatures. It is a further object of the invention to provide seals which achieve a greater extended service life under such adverse conditions or at least provide an alternative to prior art seals. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a sealing device for an hydraulic assembly wherein hydraulic fluid is contained in a working chamber formed between the body and the thrust member of the assembly comprises an annular seal with opposed sealing faces which are urged into sealing engagement between the body and the thrust member which have convergent sealing faces. 
     Preferably the device also comprises an annular mating spring clip retained in the body or in the thrust member of the assembly which bears against a non-sealing face of the annular seal to ensure primary sealing engagement between the body and the thrust member. 
     Preferably the annular seal is formed with a pair of annular sealing lips which are urged into sealing engagement between the body and the thrust member of the assembly at an initial low pressure, the remainder of the seal being urged into sealing engagement at higher pressures. 
     Preferably the seal is elastically deformed when it is placed in position so that it springs towards its original shape thus urging sealing engagement between the body and the thrust member. 
     Preferably the seal has a rounded heel which rolls under pressure to maintain sealing engagement. 
     In an alternative form the sealing device is provided with a pressure relief valve tapped into the over-stroke end of the chamber to protect the annular seal from over-stroke damage comprising a porous body which allows hydraulic fluid to bleed from the chamber and which allows the annular seal to pass the tapping point without obstruction. 
     Preferably the porous body is formed from sintered metal or porous ceramics. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To enable the invention to be fully understood, preferred embodiments will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  is a sectioned view of the components of an hydraulically assisted nut; 
         FIG. 2  is a sectioned view of the assembled hydraulic nut assembly of  FIG. 1 ; 
         FIGS. 2A and 2B  are cross sections of prior art seals; 
         FIGS. 2C to 2Q  are cross sections of seals in accordance with the present invention; 
         FIG. 3  is a sectioned view of an hydraulic nut in the full reset position′ fitted with the pressure relief device of the present invention; 
         FIG. 4  is a sectioned view showing the nut of  FIG. 3  in the over-straight condition; 
         FIG. 5  is an enlarged view of part of the nut of  FIG. 4 ; 
         FIGS. 6 and 7  are cross sections of distorted prior art seal lips; and 
         FIGS. 8 and 9  are cross sections of seals of  FIGS. 2F to 2Q . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 and 2  illustrate the components and assembly respectively of the prior art hydraulically assisted nut disclosed in U.S. Pat. No. 6,494,465, including a piston  51 , cylinder  52 , locking ring  59 , thrust washer  50  and a hydraulic working chamber  53 . Seals  63 ,  64  are provided in closed working chamber  53  between piston  51  and cylinder  52  in the manner described in U.S. Pat. No. 6,494,465. 
       FIGS. 2A and 2B  illustrate the generally V shaped prior art seals  63 ,  64 .  FIG. 2C  shows an interference seal  10  in accordance with the present invention, which is responsive to a slow injection of an hydraulic medium of low viscosity. It offers greater mobility than prior art seals  63 ,  64  as it has a greater angle as seal  10  is driven by pressure. There is a slight difference in angle between face  11  of seal  10  and face  71  of piston  51  which ensures that the thicker part of body  12  of seal  10  is driven against face  71  of piston  51  and cylinder  52 . This seal construction is effective in applications not requiring a slow pressure charging routine. 
       FIG. 2D  illustrates a similar construction where spring clip  81  on cylinder  52  ensures that primary sealing contact is made with the sealing faces.  FIG. 2F  is like the prior art designs of  FIGS. 2A and 2B  and has thin seal lips  113 ,  114  to receive an initial lower pressure and therefore produces two phases of sealing.  FIG. 2H  shows seal  210  with seal lips  213 ,  214 , which operate on the same principles but which can be pressed from sheet metal. 
     Seals  110 ,  210  and  310  of  FIGS. 2F, 2H and 2G , respectively are formed in shapes which ensure that a spring force is applied to the seals lips  113 ,  114 ,  213 ,  214 ,  313 ,  314  to provide primary sealing when seals  110 ,  210  and  310  are inserted in the working position. The secondary sealing action is activated by increasing the charge pressure. Seal  310  of  FIG. 2G  combines initial low pressure sealing of lips  313 ,  314  with a double ramp to force backup ring  320  to do most of the sealing work. In this arrangement, the seal function becomes more like that of a “V-packing” where multiple lips share the work. 
       FIG. 2E  illustrates seal  410  in which lip  413  in contact with the wall of piston  510  does virtually all the work of sealing. Seal  410  is made larger than the limiting dimensions of the seal groove and piston wall so that when it is fitted, it has a residual spring force to drive it against the wall. The lip  413  of seal  410  is allowed to flex and follow the expansion of cylinder  52  caused by the increasing charge pressure. Seal  410  is best used with relatively low pressures and minimum radial wall deflection. 
     The seals shown in  FIGS. 2J to 2Q  are of a quite different construction in that they are spring loaded on installation so that primary sealing is effected by the seal&#39;s attempt to return to its original shape. This is illustrated in  FIGS. 8 and 9  where the seal which is made in the shape shown in  FIG. 8  is inserted into position shown in  FIG. 9  so that it is forced inwards by the seal groove, and will therefore be forced against the adjacent outer cylinder walls. 
     The primary forces are selected to suit the conditions and the seals are made from material of the required elasticity so that they deform when inserted to the shape required. The seals shown in  FIGS. 2J to 2Q  all use this spring loading principle to achieve primary sealing. This sealing action is then reinforced by the increasing internal pressure in cylinder  52 . The sealing force exerted against the wall is determined by the area of the seal which responds proportionately to the injected pressure. 
     Deformation of thin sections of seal elements under the effects of pressure and temperature decreases and often destroys the seal&#39;s integrity. Prior art seals with thin lips as shown in  FIGS. 2A and 2B  are required to maintain some spring pressure against the cylinder walls at all times. This means that a material of sufficient yield strength is selected so that the seal does not deform plastically in regions of high local stress. If the material strength is not sufficient permanent deformation can occur. This tends to happen progressively from thinner section to a point where there is sufficient thickness to balance the destructive force, so that when the seal lip is deformed in this manner, it can curl back from contact with the cylinder wall. 
     Increasing temperatures lower the effective strength of most materials and particularly that of engineering steels and a metal seal which is deformed in use will be difficult to return to service. Medium under pressure forces into the gap created at the thin edge and acts as a wedge to force the lower sections away from sealed contact with the cylinder walls. This problem with known seals is illustrated in  FIGS. 6 and 7 . 
     The innovative design of the “seal ring” seals of  FIGS. 2J to 2Q  solves this problem by the action of the charging fluid&#39;s pressure upon the opposed surface of the seal, which generates thrust forces to aid sealing on the critical faces. Such force is directly related to incremental pressure, and therefore, maintains the relationship required for sealing throughout the range. The problem of heat affecting thin sections and causing permanent deformations is resolved by the new designs having thick sectional areas. 
     Seals  510 ,  610  and  710  exhibiting these characteristics are illustrated in  FIGS. 2J, 2K and 2N . Seals  810  and  910  illustrated in  FIGS. 2L and 2M  show hollow versions of the seals  510  and  610  of  FIGS. 2J and 2K , but generally would have limited application in practice. Seals  910 ,  1010  of  FIGS. 2O and 2Q  show how the principles of the “spring ring” can be applied to thinner sections of materials. These can be made inexpensively and are generally sufficient for hydraulically assisted nut fasteners used at lower operating pressures. Seal  110  or  FIG. 2P  illustrates a version of the seal which can be made in a chevron form wherein the pressure will act to expand the seal&#39;s outer diameter and provide sealing against the wall of cylinder  52 . 
     It will be readily apparent to the skilled addressee that the selection of the material for the seals, the particular shape, size and configuration of the seals, will be dependent on the intended applications. Factors which will be significant in selecting the appropriate seal will include the operating temperatures and pressures of the hydraulic assemblies and the type and pressure of the charging medium. 
     A further factor which destroys seal integrity is overstroke, that is, during attempted operation, the seal travels beyond its practical working limit, resulting in failure and a dangerous burst release of high pressure fluid. To prevent such failure, it is desirable to introduce a bleed-off port into the construction of the hydraulically assisted fastener nut. Should the seal be forced to travel over its stroke limit, then this port minimises seal damage by allowing fluid to escape. However, the seal would be irreparably damaged even by its partial transit across the port since extreme internal pressures extrude the seal material as it passes, even scratching hardened steel surfaces. 
       FIG. 4  illustrate a bleed port  100  which accommodates a pressure relief device comprising a porous plug  101 . The inner face  102  of the plug  101  is profiled to conform to the adjacent sealing wall face  52 A so that seal  63  is not damaged as it moves over bleed port  100 . The material of the porous plus plug  101  is chosen to have high strength to provide support to seal  63  as it moves over bleed port  100 , and is porous to allow fluid  103  to migrate from pressure chamber  53  freely. As seal  63  moves across bleed port  100 , more material of porous plug  101  is exposed and the bled rate is increased. The density and relative porosity of plug  101  is chosen to provide appropriate strength and bleed rate for the application. Low cost materials of choice for plug  101  are sintered metal and porous ceramics but other materials may be suitable. 
     It will be readily apparent to the skilled addressee that porous plug  101  of the pressure relief device will protect seal  63  against damage if it moves from the full reset position shown in  FIG. 3  to the overstroke condition shown in  FIG. 4 . The porous bleed plug of the present invention can be applied to any hydraulic assembly where overstroke damage can occur to seals. 
     VARIATIONS 
     It will be realized that the foregoing has been given by way of illustrative example only and that all other modifications and variations as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of the invention as herein set forth. Throughout the description and claims to this specification the word “comprise” and variation of that word such as “comprises” and “comprising” are not intended to exclude other additives components integers or steps.