Abstract:
A seal assembly having a first seal ring of a generally annular shape and defining radial and circumferential directions, and a second seal ring positioned in facing relation to the first seal ring. One of the first seal ring and the second seal ring includes a plurality of hydropads. Each hydropad has an inner edge oriented substantially circumferentially, an outer edge oriented substantially circumferentially and spaced radially outward from the inner edge, a leading edge interconnecting the inner edge with the outer edge, and a trailing edge interconnecting the inner edge with the outer edge. Both the leading edge and the trailing edge are substantially straight and are positioned at the same angle relative to a radial axis passing through a mid-point of each edge, respectively.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 08/878,433 filed on Jun. 18, 1997, and issued as U.S. Pat. No. 5,941,532 on Aug. 24, 1999, which claims the benefit of U.S. Provisional Patent Application No. 60/020,153 filed on Jun. 20, 1996. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to the field of aerospace housing and shaft assemblies, and more specifically to seals for providing a barrier between a housing and a shaft in an aerospace application. 
     BACKGROUND OF THE INVENTION 
     Mechanical face seals are commonly used to provide a seal between a stationary housing and a rotating shaft. Such seals include a rotating ring mounted on the shaft and a stationary ring mounted on the housing. Either the stator or the rotor is biased toward the other to provide a biased seal therebetween. 
     A typical seal design for inhibiting process fluid, whether liquid or gas, from escaping from a housing along a rotating shaft includes two seals in fluid communication with an intermediate chamber containing a buffer fluid. One seal pumps the buffer fluid having a certain pressure across the seal between a stator and rotator into the housing containing the process fluid. The process fluid in the housing has a lower pressure than the buffer fluid in the intermediate chamber. The other seal pumps the buffer fluid to an environment external to the housing, such as ambient, which is at a pressure lower than the buffer fluid in the intermediate chamber. 
     To accomplish this pumping, each seal includes spiral grooves on either the face of the stator or rotor. The grooves are angled relative to the radius and circumference of the rotating shaft, and when the rotator is rotating, the grooves pump the buffer fluid across the seal. This pumping of the high-pressure buffer fluid toward the lower-pressure external environment or process fluid inhibits the loss of the process fluid from the housing. U.S. Pat. No. 5,375,853 discloses a seal design of this type. 
     In another design, grooved face seals are used in pumps to provide a seal between a high-pressure gas (e.g., a combustible gas) and the ambient atmosphere. In this situation, two seals are commonly used. A grooved inner seal pumps the high pressure gas to an intermediate chamber, and a grooved outer seal pumps from the intermediate chamber to the atmosphere. The intermediate chamber routes the high-pressure gas to a flare stack where the pumped gas is burned. The amount of high-pressure gas that is lost through the outer seal is thereby minimized. An example of this type of seal is disclosed in U.S. Pat. No. 5,217,233. 
     SUMMARY OF THE INVENTION 
     The present invention recognizes that grooved face seals are useful in applications other than in a fluid pump. For example, it has been discovered that grooved face seals provide significant benefits in terms of reduced heat generation and longer seal life when used in aerospace applications, such as on gear boxes, starters, constant speed drives and integrated drive generators. In fact, in some situations, significant benefits can be achieved by using grooved face seals to replace existing non-grooved seals. Surprisingly, this also has been found to work in situations where the seal is exposed to atmospheric pressures substantially less than 14.7 psia, such as when the seal is being used in an aerospace application (e.g., an aircraft) at altitude. In addition, the present invention has discovered a cost-effective method for forming the grooves on the seal rings, thereby making the grooved seals applicable to a larger number of applications where cost may be an issue. Furthermore, a unique and simple hydropad configuration has been developed. 
     In one aspect, the present invention provides an aerospace housing and shaft assembly comprising a housing, a shaft rotatably mounted within the housing, a seal assembly operatively positioned between the housing and the shaft, and low pressure air (e.g., atmospheric air) having a pressure substantially less than 14 psia and being positioned on one side of the seal assembly. The seal assembly includes a first seal ring mounted on the housing, and a second seal ring mounted on the shaft in facing relation to the first seal ring. One of the first seal ring and the second seal ring includes a hydropad. Preferably, the hydropad is positioned to pump from the inner diameter toward the outer diameter. For example, the low-pressure air can be positioned on the inner diameter of the seal assembly, in which case the hydropad would pump the low-pressure air. In a preferred embodiment, the low-pressure air has a pressure less than about 10 psia, and more preferably less than about 5 psia. 
     The present invention also provides a method of producing a seal assembly having a seal ring. The method comprises the steps of positioning the seal ring in alignment with a media blaster, placing a template between the seal ring and the media blaster, and forcing media from the media blaster, through the openings in the template, and into contact with the seal ring. The seal ring can then be mounted in a seal assembly. In one embodiment, ceramic beads are used as the blast media. The positioning step can include the step of mounting the seal ring on a holder. Preferably the holder is rotated while the media is being forced at the seal ring. To obtain better results, a timer can be used to insure that the blast time is consistent. 
     The present invention also recognizes that hydropads do not need to be spiral in shape. More specifically, the hydropads can include an inner edge oriented substantially circumferentially, an outer edge oriented substantially circumferentially and spaced radially outward from the inner edge, a leading edge interconnecting the inner edge with the outer edge, and a trailing edge interconnecting the inner edge with the outer edge. Both the leading edge and the trailing edge are substantially straight and oblique to a radial direction. Preferably, to simplify the design, the inner edge and the outer edge are also both substantially straight. In one embodiment, the leading edge is positioned at an angle relative to the radial direction passing through a mid-point of the leading edge, and the trailing edge is positioned at the same angle relative to the radial direction passing through a mid-point of the trailing edge. 
     A more detailed description of a specific embodiment of the present invention is illustrated and described below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the use of hydropad seals in various applications in a gas turbine engine. 
     FIG. 2 is a partial cross-section of a rotating shaft positioned in a stationary housing. 
     FIG. 3 illustrates a front view of a mating ring having hydropads. 
     FIG. 4 is an enlarged view of a hydropad. 
     FIG. 5 is a front view of a template used to form the hydropads on a mating ring. 
     FIG. 6 illustrates a partial cross section of a mating ring holder and blasting assembly. 
     FIG. 7 is an enlarged section of the mating ring holder. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates the use of hydropad seals in an aerospace gas turbine engine  10 . It has been discovered that hydropad seals can be used in a variety of positions within the engine  10 , and can be used to replace standard (non-hydropad) seals. In FIG. 1, the engine  10  employs use of the hydropad seals as compressor inlet seals  11 , compressor/drive seals  12 , interstage seals  14 , turbine seals  16 , and gearbox seals  18 . Uses also include accessory seals such as constant speed drives, alternators, starters, generators, de-oilers, fuel pumps, hydraulic pumps, gearboxes, main shafts and fuel control seals (not shown). Hydropad seals provide virtually leakage free operations at temperatures ranging up to about 600 degrees Fahrenheit. The hydropad seals operate with a shaft speed up to 120,000 rpm for small sizes, and can be designed to handle reverse pressures. The hydropad seals can also operate in virtually any fluid, liquid or gas. 
     FIG. 2 illustrates a cross-section of a rotating shaft  20  positioned within a stationary housing  22 , and a seal assembly  24  mounted therebetween. The seal assembly  24  includes two seal rings: a mating ring  28  mounted on the shaft  20  and a seal nose  30  mounted on the housing  22 . A hydropad  26  is formed on the mating ring  28 . The mating ring  28  is typically a ductile material, such as hardened steel, but instead can be composed of tungsten carbide or silicon carbide, or can be steel with a ceramic coating. 
     FIG. 3 illustrates a front view of the mating ring  28  having a plurality of hydropads  26 . The dashed lines represent an inner circumference  34  and an outer circumference  36  which define the position of the seal nose  30  relative to the hydropads  26 , generally called a sealing interface area  38 . As illustrated in FIG. 3, the hydropads  26  overlap the inner circumference  34  of the sealing interface area  38  on the mating ring  28 . A sealing dam region  40  defines the region from the outer circumference  36  of the sealing interface area  38  to the outer edge of the hydropad  26 . 
     In FIG. 2, a metal bellows  32  is positioned between the housing  22  and a seal nose  30 , and allows for axial movement of the seal nose  30 . When the shaft  20  is not rotating, the mating ring  28  contacts the seal nose  30  at the sealing interface area  38 . A working fluid  42  (e.g., oil) is present outside (i.e., on the outer diameter of) the mating ring  28 . Air  44  is positioned on the inner diameter of the seal. The air can be filtered by a filter  45  as schematically shown. The air  44  is preferably at atmospheric pressure, which is substantially less than 14.7 psia (standard absolute pressure at sea level) in the case of an aircraft flying at altitude. As used herein, the phrase “substantially less than 14.7 psia” means that the atmospheric air is what would be experienced by an aircraft flying at altitude. The seal of the present invention has been successfully tested to pressures of about 2 psia. 
     When the shaft  20  is rotating, the hydropads  26  force the air  44  between the mating ring  28  and the seal nose  30  to thereby cause a small gap to develop between the sealing interface area  38  and the seal nose  30 . As the air  44  is pressurized, a barrier is created inhibiting working fluid  42  from passing through the sealing interface area  38 . When the shaft  20  is not rotating, the sealing dam region  40  seals the working fluid  42  on the outside of outer circumference  36  of the sealing interface area  38 . Because the hydropads  26  do not extend across the entire top surface of mating ring  28 , the separation of the lubricating fluid region  38  from the air region  44  is accomplished. 
     Although the present embodiment illustrates a mating ring  28  rotating within the stationary housing  22 , it is also possible for the stationary housing  22  to rotate with the mating ring  28  in a fixed position. 
     FIG. 4 is an enlarged view of a hydropad  26 . The configuration of the hydropad  26  is such that a leading edge  44  and a trailing edge  46  diverge radially outwardly and are positioned at a constant angle relative to their respective radial axes. An inner edge  48  and an outer edge  50  are substantially straight and connect edges  44  and  46 . In an alternate hydropad configuration, the inner edge  48  and outer edge  50  can be curved with a center of rotation positioned toward the center of the mating ring. 
     The required depth of the hydropad  26  varies depending upon the application. The illustrated hydropads  26  consist of many shallow grooves at a given depth of approximately 0.0001 inches to 0.0025 inches, and at a fixed angle about the inner diameter of the sealing face. The depth, number of grooves and angle of the paths are fixed at fixed values and are chosen to meet the given operating conditions as necessary. 
     A method of forming a hydropad on a mating ring is also described, and is most clearly illustrated by FIGS. 5-7. The method utilizes a media blasting technique to form the hydropads  26  on the mating ring  28 . FIG. 5 is a front view of the template  56  that is positioned over the front face of the mating ring  28  during the blasting procedure. Hydropad openings  62  are positioned in the size and orientation necessary to form the desired hydropads  26 . 
     FIGS. 6 and 7 illustrate a mating ring holder  64  with a mating ring  28  positioned thereon. It is desirable to minimize the radial clearance between the mating ring  28  and the holder  64 . To facilitate this, o-rings  66  may be used to help center the mating ring  28  onto the holder  64 . The template  56  is positioned over the face of the mating ring  28 , and a clamping plate  68  and fastener  70  are used to secure the template  56  and mating ring  28  to the holder  64 . The hydropad openings  62  on the template  56  are utilized to define the areas for the formation of the hydropads  26 . 
     The mating ring holder  64  is positioned adjacent to a nozzle  74  of a media blaster  72  that is aligned toward the openings  62  in template  56 . The mating ring holder  64  positions the mating ring  28  and the template  56  a fixed and known distance from the nozzle  74 . The vertical and horizontal positioning of the gun assembly  72  is preset prior to the starting of blasting. A media stream  80  projecting a bead blast from the nozzle  74  is directed such that the media stream  80  impacts midway between the inner edge  48  and outer edge  50  of the hydropads  26 . 
     The media stream  80  can be composed of a variety of abrasive or peening materials, such as aluminum oxide or glass beads. It has been found that the use of ceramic beads is particularly advantageous since a more uniform result is achieved. It is believed that the use of ceramic beads results in a high amount of peening, as opposed to material removal. Peening reduces the amount of removed material being recirculated and blasted, and thus is believed to improve the consistency of the blasting operation. In addition, it is believed that peening results in fewer sharp peaks on the blasted part. Alternatively, the media can comprise a water slurry or air slurry or any other suitable material that can be blasted at the seal ring to cause formation of the hydropads, whether by material removal or material compression. 
     The axial positioning of the nozzle  74  is preset prior to the starting of the blasting operation. The bead blast pressure of the media stream  80  is also preset prior to starting of the blasting operation. Also, the mating ring  28  begins rotation before the bead blasting operation is started. The media blaster  72  is turned on simultaneously with a timer (not shown) which is used to indicate when the media blaster  72  is to be turned off. This method provides the greatest number of uniform mating ring hydropad depths. 
     The media blaster  72  can be rotated to direct the media stream  80  at an end of the template cutout holes  62  that form the hydropads  26 . More specifically, once the vertical, horizontal and axial locations are fixed, the nozzle  74  can be rotated such that the media stream  80  is directed at the outer edge of the openings  62 . The nozzle  74  can then be rotated to direct the media stream  80  at the inner edge of the openings  62 . It is contemplated that the gun assembly  72  can be set to rotate in many directions, as necessary for the particular application. 
     When the bead blasting operation is complete, the template  56  is unclamped from clamping plate  68 , and the mating ring  28  is removed. The clamping or staging surface of the fixture is then cleaned of bead media  80  prior to the installation of the next mating ring  28 . 
     In order to obtain the correct depth of the hydropad  26 , a polishing or lapping operation after bead blasting may be required. These operations truncate rough peaks of the bead blasted surface in the hydropad  26  and may dub edges of the hydropads. The actual depth of the hydropad  26  is better controlled by either changing the blasting time, the pressure or the distance of the gun assembly  72  from the mating ring  28 . 
     In determining the proper bead blasting settings prior to bead blasting, a set-up piece is used. Periodic monitoring of hydropad depths is recommended to ensure consistency. 
     The foregoing description of the present invention has been presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, and the skill or knowledge of the relevant art, are within the scope of the present invention. The embodiments described herein are further intended to explain best modes known for practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with various modifications required by the particular applications or uses of the present invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.