Patent Application
20060104806
The present invention relates to gas turbine engines and, more particularly, to a seal assembly for use in gas turbine engines, gas turbine engine starters, and auxiliary power units.
Air turbine starters (ATS) may be used to initiate rotation of many relatively large turbine engines, including turbofan jet engines. Typically, the ATS is disposed within a housing and includes a turbine section and an output section. The turbine section includes a turbine wheel coupled to an output shaft. When high pressure fluid, such as compressed air, enters the ATS, it contacts the turbine wheel and causes it to rotate at a relatively high rate of speed. As the turbine wheel rotates, the output shaft also rotates, which in turn, causes the jet engine to rotate. The ATS output shaft may be rotationally mounted within the housing using one or more bearing assemblies. Preferably, a lubricant, such as oil, is contained within the housing to lubricate the output shaft and bearing assemblies.
In order to prevent the lubricant from leaking out of the output section into the turbine section, a seal assembly is typically provided between the two sections. The seal assembly may be a face seal that includes a rotor, a seal ring, and a seal case. The rotor is mounted on the turbine wheel output shaft and has an axially facing flange that extends radially away from the shaft. The seal case may have a neck and may be mounted to the ATS housing in the turbine section and surrounds the turbine wheel output shaft, while the seal ring is housed within the seal case. The seal ring has a spring pre-load that supplies force thereto to sealingly engage the seal ring with the rotor axially facing flange. An O-ring is typically mounted on the seal case in the space within which the seal ring is housed and is used to maintain conformal radial contact between the inner diameter of the seal ring and the outer diameter of the seal case neck. Thus, the O-ring acts as a secondary seal by deforming out of its original round shape at the zones of contact with the seal ring and seal case.
Typically, O-rings are made of an elastomer (rubber) and may vary in hardness. Material hardness may be measured by a durometer instrument, which may include a contact tip for pressing into the material. The resistance of the material toward indentation motion of the tip is then monitored. Durometer measurements are communicated using various Shore hardness scales, including, but not limited to Shore A, Shore D, Shore H, and Shore M scales. In many cases, Shore M scales are used to indicate the hardness of plastics. For instance, a material having a Shore M hardness value of below 55 is made of a relatively soft material, while a Shore M hardness value of above 80 is a relatively hard material.
In the past, O-rings having a Shore M hardness of between 55 and 80 have been used in conjunction with face seal assemblies. However, it has been found that during operation, some of these O-rings may become deformed or compression set due to prolonged exposure to heat. When this occurs, the O-ring may lose its original round shape if in its free state and may form permanent flat sections at the seal case and seal ring contact zones. As a result, if rotor axial movement occurs, the O-ring may supply an excessive amount of friction against the return motion of the seal ring. High axial O-ring friction may prevent the seal ring from properly sealing against the rotor. Consequently, lubricant may leak out of the ATS gearbox at the turbine section and/or output section and assembly maintenance and/or repair may need to be more frequently performed.
Identifying suitable elastomers that may be less likely to demonstrate excessive axial friction when compression set within the seal assembly has been somewhat difficult. In particular, two materials may have the same hardness durometer measurements, but one material may be better-suited for use in the seal assembly than the other material. Because there may be numerous materials that may have similar durometer values, identification of appropriate O-ring materials is currently performed by trial and error. This method of identification may be extremely time-consuming and does not take into consideration whether the elastomer properties may change over time and after exposure to friction.
Accordingly, it is desirable to a seal assembly that is less time-consuming to design and manufacture. It is also desirable for the seal assembly to supply an appropriate amount of friction against a seal ring to provent the seal ring from becoming dislocated from the seal rotor in the event of seal rotor axial movement. In addition, it is desirable to have a seal assembly that minimizes leakage at the turbine section and/or output section of the ATS. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.
The present invention provides a stator seal assembly including a seal case, an O-ring and a stator ring. The seal case has an inner peripheral wall, an outer peripheral wall, and an annular cavity formed therebetween. The O-ring is coupled to the seal case inner peripheral wall and disposed in the annular cavity, and is constructed of a material having stress versus strain ratio of less than about 5 ksi and a Shore M value of between about 30 and 50. The stator ring is disposed within the seal case annular cavity.
In one embodiment, and by way of example only, a seal assembly is provided that includes a seal case, an O-ring, and a stator ring. The seal case has an inner peripheral wall, an outer peripheral wall, and an annular cavity formed therebetween. The O-ring is coupled to the seal case inner peripheral wall and disposed in the annular cavity and comprises a material having stress versus strain ratio of less than about 5.0 ksi and a Shore M value of about 50. The stator ring is disposed within the seal case annular cavity.
In another embodiment, and by way of example only, a face seal assembly is provided that includes a seal rotor, a seal case, an O-ring, and a stator ring. The seal rotor is adapted to mount to a shaft and has an axially facing flange. The seal case has an inner peripheral wall, an outer peripheral wall, and an annular cavity formed therebetween. The O-ring is coupled to the seal case inner peripheral wall and disposed in the annular cavity and comprises a material having stress versus strain ratio of less than about 5 ksi and a Shore M value of between about 30 and 50. The stator ring is disposed within the seal case annular cavity and is configured to engage the axially facing flange.
In still another embodiment, and by way of example only, an air turbine starter is provided that includes a rotatable shaft and a seal assembly. The seal assembly is coupled to the rotatable shaft and includes a seal rotor, a seal case, an O-ring, and a seal ring. The seal rotor is coupled to the rotatable shaft and has an axially facing flange. The seal case has an inner peripheral wall, an outer peripheral wall, and an annular cavity formed therebetween. The O-ring is coupled to the seal case inner peripheral wall and disposed in the annular cavity, and comprises a material having stress versus strain ratio of less than about 5 ksi and a Shore M value of between about 30 and 50. The stator ring is disposed within the seal case annular cavity and is configured to engage the axially facing flange
Other independent features and advantages of the preferred seal assembly will become apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.
The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention. Before proceeding with the detailed description, it should be appreciated that the present invention is not limited to use in conjunction with a specific type of rotating machine. Thus, although the present invention is, for convenience of explanation, depicted and described as being implemented in an air turbine starter, it should be appreciated that it can be implemented in numerous other machines including, but not limited to, a solenoid, a control valve, a solenoid control valve, a hydraulic pump mechanical face seal, a water pump mechanical face seal, a petrochemical mechanical face seal, a pressure accumulator piston, a pressure transducer, or any other suitable machine or application.
Turning now to the description, a cross section view of an exemplary air turbine starter (ATS) that is used to initiate the rotation of a larger turbine, such as a turbofan jet engine, is depicted in
With reference to
The O-ring 146 is mounted on the second hub outer surface 162. Preferably, the O-ring 146 is toroidally-shaped and constructed of material capable of providing a sufficiently low amount of friction when used as a radial compression O-ring between the seal stator ring 140 and seal case 138 such that the spring preload of the spring washer 142 may overcome the O-ring friction when the seal stator ring 140 moves axially. In one exemplary embodiment, suitable materials have a Shore M value of between about 30 and 50, but also a suitable stress versus strain ratio (“compressive modulus”). The compressive modulus of a material may be measured in any one of numerous manners. In one example, the compressive modulus is measured by determining the force that is needed to compress a complete surface of a component, such as a rubber component in contact with the seal case 138 and the seal ring 140. Preferably, materials appropriate for constructing the O-ring 146 have a compressive modulus of less than about 5.0 ksi, more preferably between about 5.0 and 0.1 ksi, and most preferably about 1.4 ksi.
In other exemplary embodiments, other material specifications may be dependent on the particular application in which the O-ring 146 will be implemented. For example, the O-ring 146 in ATS 100 is preferably resistant to structural degradation due to exposure to oils. Thus, the O-ring is preferably constructed of a fluorocarbon, including, but not limited to perfluorocarbon, peroxide cured fluorocarbon, GLT fluorocarbon (available through DuPont Dow Elastomers of Wilmington, Del.), and GFLT fluorocarbon (available through DuPont Dow Elastomers of Wilmington, Del.). The O-ring elastomer could also be any of the available elastomers or products including but not limited to ethylene propylene, nitrile, butadiene, chloroprene, butyl, isoprene, silicone, fluorosilicone, other fluoroelastomers, such as Sifel™ (available through Shin-Etsu Silicones of America, Inc. of Akron, Ohio), Kalrez® (available through DuPont Dow Elastomers of Wilmington, Del.), Viton® (available through DuPont Dow Elastomers of Wilmington, Del.), Chemraz® (available through Greene Tweed of Kulpsville, Pa.), Fluorel (available through 3M Corporation of Minnesota), Omniflex™ (available through Saint-Gobain Performance Plastics Corporation of Garden Grove, CA), Aflas® (available through Asahi Glass Co. of Tokyo, Japan), polyurethane, polyester, or, in accordance with ASTM D1418 abbreviations, the elastomer compounds of ACM, AEM, CSM, EPDM, EPM, FKM, FEPM, FFKM, CO, ECO, CO, ECO, BR, CR, IIR, CIIR, IR, NBR, SBR, HNBR, XNBR, FVMQ, PMQ, PVMQ, MQ, VMQ, AU, and EU. In other exemplary embodiments, the O-ring 146 may exhibit certain properties after soaking in oil, for example, Mobil 254, for at least three hundred thirty-six (336) hours at 392° F. 110° F., such as having a compression set of less than about 20%, volume swell of less than about 12%, reduction in ultimate elongation damage of less than about 10%, and/or a reduction in tensile strength of less than about 10%.
Thus, there has now been provided a seal assembly that is less time-consuming to design and manufacture. Moreover, the seal assembly supplies an appropriate amount of friction against a seal ring, without become dislocated in the event of seal ring dislocation. In addition, the seal assembly minimizes leakage at the turbine section and output section of the ATS. The selection of the O-ring elastomer also uses a compression modulus of the elastomer and/or tensile modulus of the material, and/or shear (torsional or flexure/transverse) modulus of the material, instead of, or alternatively, in addition to, durometer hardness measurements for material selection in identifying which material may be used in low rolling friction applications such as mechanical face seals or solenoids or control valves.
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
