Abstract:
A seal assembly which operates efficiently at high altitudes and low surface speeds includes a first seal ring, a second seal ring, and hydropads. The first seal ring is of a generally annular shape and defining radial and circumferential directions. The second seal ring is positioned in facing relation to the first seal ring and rotatably mounted relative to the first seal ring about an axis of rotation. A plurality of hydropads formed on one of the first seal ring and second seal ring provides a lift force that varies about the circumference of the mating ring.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     FIELD OF THE INVENTION 
     The present invention generally relates to the field of and shaft assemblies, and more specifically to hydrodynamic seals for providing a barrier between a housing and a shaft. 
     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, or rotor, mounted on the shaft and a stationary ring, or stator, 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 radially pumps the buffer fluid having a certain pressure across the seal between a stator and rotor 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 radially 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 radial 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 rotor is rotating, the grooves radially pump the buffer fluid across the seal. The rotor must be rotated at a speed sufficient to generate a lift force that overcomes the hydraulic and mechanical forces biasing the rotor and stator toward each other in order to create a gap between the rotor and stator through which the buffer fluid is pumped. 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 radially pumps the high pressure gas to an intermediate chamber, and a grooved outer seal radially 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. 
     These types of seals are also used in aerospace applications, such as disclosed in U.S. Pat. Nos. 5,941,532 and 6,257,589, which are assigned to the assignee of the present application and fully incorporated herein. When used in an aerospace application, these seals have a limited operating window in which they are effective. At low surface speeds and high altitude conditions (with very low ambient pressures) it has been discovered that the seals may not generate sufficient lift to overcome hydraulic and mechanical closing forces, resulting in contact between the sealing faces and degraded performance. A need exists for seals which can more effectively operate in low surface speeds/high altitude conditions. 
     SUMMARY OF THE INVENTION 
     The present invention provides a seal assembly including a first seal ring of a generally annular shape and defining radial and circumferential directions. A second seal ring is positioned in facing relation to the first seal ring and rotatably mounted relative to the first seal ring about an axis of rotation. A plurality of hydropads formed on one of the first seal ring and second seal ring provides a lift force that varies about the circumference of the mating ring. In one embodiment, hydropads define an eccentric ring about the axis of rotation. In another embodiment, an outer circumference of the hydropads is eccentric relative to said axis of rotation. In yet another embodiment, each of the hydropads of the plurality of hydropads has a nominal center, and the nominal centers are circumferentially spaced about a point offset from the axis of rotation. 
     A general objective of the present invention is to provide a seal assembly having hydropads that can operate efficiently in high altitudes and with low surface speeds. This objective is accomplished by providing a seal having a lift force which varies about the circumference of the mating ring which allows a squeeze film effect to axially space the first and second rings in high altitudes and at low surface speeds. 
     These and other aspects of the invention are not intended to define the scope of the invention for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, and not limitation, preferred embodiments of the invention. Such embodiments do not define the scope of the invention and reference must be made therefore to the claims for this purpose. 
    
    
     
       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 with the seal nose engaging the mating ring; 
         FIG. 3  is a partial cross-section of a rotating shaft positioned in a stationary housing with the seal nose lifted off the mating ring; 
         FIG. 4  illustrates a front view of a mating ring having hydropads; 
         FIG. 5  illustrates a side view of the mating ring of  FIG. 3 ; and 
         FIG. 6  is an enlarged view of a hydropad. 
     
    
    
     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. 
       FIGS. 2 and 3  illustrate 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  for rotation about an axis  38  of rotation substantially coaxial with an axis of rotation of the shaft  20  and a seal nose  30  mounted on the housing  22 . 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 . 
     The mating ring  28  and nose seal  30  may be fabricated from suitable materials known in the art, such as hardened steel, carbon, silicon carbide, carbon composites, ceramics, tungsten carbide, and a combination thereof. Preferably. the mating ring  28  is a ductile material, such as hardened steel. 
     A working fluid  42  (e.g., oil) is present outside (i.e., on the outer diameter of) the mating ring  28 . Air  40  is positioned on the inner diameter of the mating ring  28 . The air  40  can be filtered by a filter  45  as schematically shown. The air  40  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. However, the seal assembly  24  can be used at other altitudes, such as sea level, at which atmospheric pressure is above 14.7 psia without departing from the scope of the invention. 
     As shown in  FIG. 3 , when the shaft  20  is rotating, the hydropads  26  force the air  40  between the mating ring  28  and the seal nose  30  to create a small gap  56  between the mating ring  28  and the seal nose  30  forming a sealing interface between the mating ring  28  and seal nose  30 . As the air  40  is pressurized, a barrier is created inhibiting working fluid  42  from passing through the sealing interface. When the shaft  20  is not rotating, as shown in  FIG. 2 , the seal nose  30  engages a face  52  of the mating ring  28  and seals the working fluid  42  on the outside of outer circumference  36  of the hydropads  26 . Because the hydropads  26  do not extend radially across the entire face  52  of the mating ring  28 , the separation of the working fluid  42  from the air  40  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. 
     As shown in  FIG. 4 , a plurality of hydropads  26  are formed on the face  52  of the mating ring  28 . Each hydropad  26  formed on the mating ring  28  includes an inner edge  48  joined to an outer edge  50  by a leading edge  44  and a trailing edge  46 . The inner edges  48  of the hydropads  26  define an inner circumference that is substantially equidistant from the axis of rotation. The outer edges  50  of the hydropads  26  define an outer circumference, or ring, that is eccentric, i.e. has a center  54  offset relative to the axis  38  of rotation. Advantageously, this eccentric orientation of the hydropads  26  creates lift in an eccentric manner on the face  52  of the rotating mating ring  28 . 
     By applying a lift feature in an eccentric manner on the face  52  of the rotating mating ring  28 , a lift force which varies about the circumference of the mating ring  28  is generated. Since the mating ring  28  rotates, the lift force will also vary as a function of time. This varying lift force produces a pumping action (nutation of the faces) that allows the working fluid  42  between the mating ring  28  and seal nose  30  to generate a lift force as a result of a squeeze film effect. The squeeze film effect supplements the lift force created by the hydrodynamic nature of the pattern of the hydropads  26  at lower rotational speeds of the mating ring  28  and fluid densities. Advantageously, the working fluid  42  allowed into the sealing interface initially forms the gap between the mating ring  28  and seal nose and maintain the separation at high altitudes and low rotation speeds. However, as the mating ring  28  rotates, working fluid  42  that is brought into the sealing interface between the mating ring  28  and seal nose  30  from the outer diameter is urged radially outwardly by the radial pumping created by the hydropads to prevent the working fluid  42  from passing through the seal interface. 
     Since the working fluid film thickness will be lowest at the point where the hydropads  26  extend the least in the direction of radial pumping, preferably the mating ring  28  is grooved or chamfered at this point to introduce fluid into the sealing interface between the mating ring  28  and seal nose  30 , as shown in  FIGS. 4 and 5 . In the embodiment disclosed herein, the air  40  is pumped from the inner diameter of the mating ring  28  toward the outer diameter of the mating ring  28 . Accordingly, in the embodiment disclosed herein, a chamfer  58  is formed on the outer diameter of the mating ring  28 . Although forming the chamfer  58  on the mating ring  28  is preferred, the chamfer or groove can be formed on the seal nose  30  instead of, or in addition to, forming the chamfer  58  on the mating ring  28  without departing from the scope of the invention. Moreover, if air or other fluid is pumped from the outer diameter of the seal toward the inner diameter of the seal, preferably a groove or chamfer is formed on the inner diameter of the mating ring and/or seal nose. 
     Because the working fluid  42  in the sealing interface between the mating ring  28  and seal nose  30  has a higher viscosity than air alone, separation of the seal nose  30  from the mating ring  28  will occur at a lower speed than it would with only air  40  in the interface. This working fluid  42  in the sealing interface helps to reduce heat generation and wear at the interface. Also due the eccentric nature of the lift geometry, the stationary seal nose  30  will tend to tilt relative to the rotating face  52  of the mating ring  28  due to the varying lift force. This tilt will also in essence create a slider bearing geometry which will further supplement the hydrodynamic lift. 
       FIG. 6  is an enlarged view of a hydropad  26 . The preferred configuration of the hydropad  26  is such that the leading edge  44  and the trailing edge  46  diverge radially outwardly and are connected by an inner edge  48  and an outer edge  50 . The outer edges  50  of the hydropads  26  define an outer circumference of the hydropads  26  which is eccentric relative to the axis  38  of rotation. Preferably, the inner edge  48  and outer edge  50  are substantially straight and define a nominal center  60  midway between the inner edge  48  and outer edge  50 . The nominal centers  60  are circumferentially spaced about the point  54  offset from the axis  38  of rotation further defining the eccentric orientation of the hydropads  26 . 
     The preferred depth of the hydropads  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. The hydropads  26  can be formed on the mating ring using any method known in the art. One preferred method is disclosed in U.S. Pat. No. 6,257,589 which is fully incorporated herein by reference. 
     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.