Patent Publication Number: US-2020284351-A1

Title: Slider seal

Description:
GOVERNMENT LICENSE RIGHTS 
     This invention was made with Government support awarded by the United States. The Government has certain rights in this invention. 
    
    
     FIELD 
     The present disclosure relates generally to gas turbine engines and, more particularly, to slider seal assemblies used to prevent leakage of gases through openings in casings and casing-like structures within gas turbine engines. 
     BACKGROUND 
     Gas turbine engines typically include a fan section, a compressor section, a combustor section and a turbine section. During operation, air is pressurized in the compressor section and mixed with fuel and burned in the combustor section to generate hot combustion gases. The hot combustion gases are then communicated through the turbine section, where energy is extracted from the hot combustion gases to power the compressor section, the fan section and various other loads occurring within or proximate a gas turbine engine. 
     A core engine of a gas turbine engine typically includes the compressor, the combustor and the turbine, each of which is surrounded by a core engine casing. The core engine casing may serve to separate a core flow path, in which gases at high pressure and high temperature flow, from a bypass flow path, in which gases at substantially lower temperature and pressure flow. Other casing-like structures may exist within a gas turbine engine and serve, among other things, to define different flow paths, providing separate passageways for gases flowing at different temperatures and pressures within the gas turbine engine during operation. 
     Gas turbines engines may also include numerous components, such as fuel fittings, bleed ports, conduits, probes, pins and the like, that are required to extend through openings in the core engine casing or through the other casing-like structures described above. Seal assemblies are required to seal the openings and to minimize or prevent leakage through the openings, particularly where the openings extend through a casing separating gases existing at different pressures and temperatures. The seal assemblies typically include a housing and a metallic seal plate. The housing is mounted to a surface of the casing, the seal plate is affixed adjacent to the housing and the component extends through openings in the seal plate and the housing to provide a seal therebetween. 
     Slider seals are free floating seals that may be used in gas turbine engines at locations where the various components described above may be required to extend from one side of a core engine casing or other casing-like structure to the other. The seals may be configured to allow the various components described above to translate and rotate in several dimensions with respect to the casing through which they extend, which is usually fixed in space with respect to the gas turbine engine. For example, as a casing and the components extending therethrough change shape or relative position due to increased temperatures during operation, the slider seals allow for relative movement in several dimensions between the casing and the components, thereby limiting gases from one side of the casing to leak toward the other side of the casing. 
     SUMMARY 
     A seal assembly is disclosed. In various embodiments, the seal assembly includes a housing; a seal member configured for disposition within the housing, the seal member comprising a tube section defining an inner tube surface configured to receive an interface component and having a length and an inner tube surface radius that is substantially constant along the length; and a retaining ring configured to retain the seal member within the housing and to confine translational movement of the seal member with respect to the housing to two degrees of freedom. 
     In various embodiments, the seal member includes a plate section connected to the tube section. In various embodiments, the plate section extends circumferentially about the tube section. In various embodiments, the retaining ring is configured to confine translational movement of the plate section with respect to the housing to the two degrees of freedom. 
     In various embodiments, the interface component includes an outer component surface configured to slidably engage the inner tube surface along the length. In various embodiments, the outer component surface includes a radial arc portion configured to slidably engage the inner tube surface along the length. In various embodiments, the radial arc portion defines a radial arc-length that extends circumferentially about a central axis of the interface component. In various embodiments, the radial arc portion defines a radius of curvature that is substantially constant along the radial arc-length. In various embodiments, the radius of curvature is substantially equal to the inner tube surface radius along the length. 
     In various embodiments, the tube section defines a central axis and the interface component includes a radial arc portion configured to enable the interface component to translate in a direction along the central axis with respect to the tube section. In various embodiments, the radial arc portion defines a radius of curvature that is substantially constant along a radial arc-length and is configured to enable the interface component to rotate in three degrees of freedom with respect to the housing. 
     A slider seal assembly for a gas turbine engine is disclosed. In various embodiments, the slider seal assembly includes a housing configured for mounting to a casing within the gas turbine engine; and a seal member configured for disposition within the housing, the seal member comprising a tube section defining an inner tube surface configured to receive an interface component and having a length and an inner tube surface radius, the seal member retained within the housing and configured to confine translational movement of the seal member with respect to the housing to two degrees of freedom. 
     In various embodiments, the tube section defines a central axis and the interface component includes a radial arc portion configured to enable the interface component to translate in a direction along the central axis with respect to the tube section. In various embodiments, the radial arc portion defines a radius of curvature that is substantially equal to the inner tube surface radius along a radial arc-length and is configured to enable the interface component to rotate in three degrees of freedom with respect to the tube section. 
     In various embodiments, the seal member includes a plate section connected to the tube section. In various embodiments, a retaining ring is configured to retain the seal member within a recessed opening within the housing. In various embodiments, a seal ring is disposed within a seal ring groove that extends into the radial arc portion. 
     A gas turbine engine is disclosed. In various embodiments, the gas turbine engine includes a casing; and a slider seal assembly extending through the casing and configured to secure an interface component and enable three degrees of translational freedom and three degrees of rotational freedom of the interface component with respect to the casing. The slider seal assembly includes a housing, a seal member configured for disposition within the housing, the seal member comprising a tube section defining an inner tube surface configured to receive the interface component and having a length and an inner tube surface radius that is substantially constant along the length, and a retaining ring configured to retain the seal member within the housing and to confine translational movement of the seal member with respect to the housing to two degrees of freedom. 
     In various embodiments, the tube section defines a central axis and the interface component includes a radial arc portion configured to enable the interface component to translate in a direction along the central axis with respect to the tube section. In various embodiments, the radial arc portion defines a radius of curvature that is substantially equal to the inner tube surface radius along a radial arc-length and is configured to enable the interface component to rotate in three degrees of freedom with respect to the tube section. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter of the present disclosure is particularly pointed out and distinctly claimed in the concluding portion of the specification. A more complete understanding of the present disclosure, however, may best be obtained by referring to the following detailed description and claims in connection with the following drawings. While the drawings illustrate various embodiments employing the principles described herein, the drawings do not limit the scope of the claims. 
         FIG. 1A  is a schematic view of a gas turbine engine, in accordance with various embodiments; 
         FIG. 1B  is a schematic view of an outer casing of a gas turbine engine, in accordance with various embodiments; 
         FIGS. 2A and 2B  are cutaway perspective and side views of a slider seal assembly, in accordance with various embodiments; 
         FIGS. 3A and 3B  are cutaway perspective and side views of a slider seal assembly, in accordance with various embodiments; and 
         FIGS. 4A and 4B  are cutaway perspective and side views of a slider seal assembly, in accordance with various embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description of various embodiments herein makes reference to the accompanying drawings, which show various embodiments by way of illustration. While these various embodiments are described in sufficient detail to enable those skilled in the art to practice the disclosure, it should be understood that other embodiments may be realized and that changes may be made without departing from the scope of the disclosure. Thus, the detailed description herein is presented for purposes of illustration only and not of limitation. Furthermore, any reference to singular includes plural embodiments, and any reference to more than one component or step may include a singular embodiment or step. Also, any reference to attached, fixed, connected, or the like may include permanent, removable, temporary, partial, full or any other possible attachment option. Additionally, any reference to without contact (or similar phrases) may also include reduced contact or minimal contact. It should also be understood that unless specifically stated otherwise, references to “a,” “an” or “the” may include one or more than one and that reference to an item in the singular may also include the item in the plural. Further, all ranges may include upper and lower values and all ranges and ratio limits disclosed herein may be combined. 
     Referring now to the drawings,  FIG. 1  illustrates a gas turbine engine  100 . In various embodiments, the gas turbine engine  100  includes a fan  102 , a low pressure compressor  104 , a high pressure compressor  106 , a combustor  108 , a high pressure turbine  110  and a low pressure turbine  112 . During operation, air is pulled into the gas turbine engine  100  by the fan  102 , pressurized by the low pressure compressor  104  and the high pressure compressor  106 , and then mixed with fuel and burned in the combustor  108 . Hot combustion gases generated within the combustor  108  flow through the high pressure turbine  110  and the low pressure turbine  112 , which extract energy from the hot combustion gases. In a two spool design, the high pressure turbine  110  powers the high pressure compressor  106  through a high speed shaft  114  and the low pressure turbine  112  powers the fan  102  and the low pressure compressor  104  through a low speed shaft  116 . While the foregoing describes a two-spool design, the disclosure contemplates and is applicable to other gas turbine engine designs, such as, for example, single-spool and three-spool designs. 
     A nacelle  118  is disposed circumferentially about an engine centerline axis A and surrounds the numerous components of the gas turbine engine  100 . A bypass flow path B extends between an inner surface  122  of the nacelle  118  and an outer engine casing  124 , which houses the low pressure compressor  104 , the high pressure compressor  106 , the combustor  108 , the high pressure turbine  110  and the low pressure turbine  112 . A portion F 2  of the incoming airflow (“AIRFLOW”) enters the gas turbine engine  100  and is communicated through a core flow path C, while the remaining portion F 1  of the incoming airflow is communicated through the bypass flow path B to provide thrust for powering the aircraft. 
     Referring now to  FIG. 1B , a plurality of slider seal assemblies  130  is illustrated mounted to the outer engine casing  124  of the gas turbine engine  100 . In various embodiments, one or more of the plurality of slider seal assemblies  130  may be mounted adjacent to the sections comprising the low pressure compressor  104 , the high pressure compressor  106 , the combustor  108 , the high pressure turbine  110  and the low pressure turbine  112 , or any other section of the gas turbine engine  100 . In addition, one or more of the plurality of slider seal assemblies  130  may be mounted adjacent the nacelle  118  (e.g., mounted on the inner surface  122  of the nacelle  118 ) and configured to seal piping providing compressed air bled from one of the low pressure compressor  104  and the high pressure compressor  106  to components housed within the nacelle  118 . In general, and as described further below, the one or more of the plurality of slider seal assemblies  130  is configured to extend through an opening within a casing separating flow paths containing gases existing at different pressures and temperatures and provide a seal against the disparate gases leaking from one flow path to another. 
     Referring now to  FIGS. 2A and 2B , a slider seal assembly  230  (or seal assembly), such as, for example, one of the plurality of slider seal assemblies  130  described above with reference to  FIG. 1B , is illustrated. In various embodiments, the slider seal assembly  230  includes a housing  232 , a seal member  234  and a retaining ring  236 . The seal member  234  includes a plate section  240  and a tube section  241 . In various embodiments, the seal member  234  may be constructed of metallic materials or non-metallic materials (e.g., ceramic matrix composite materials or the like). The housing  232  includes a recessed opening  238  for receiving the plate section  240  of the seal member  234 . The retaining ring  236  is received within a slot  247  of the recessed opening  238  to retain the plate section  240  within the recessed opening  238 . In various embodiments, the plate section  240  extends circumferentially about the tube section  241 . In various embodiments, the tube section  241  may extend along a length  243  from the plate section  240  and, as described further below, provides an interior surface configured to slidably engage a corresponding exterior surface of an interface component  250 . In various embodiments, the tube section  241  defines a central axis C 1  that extends along the length  243  of the tube section  241 . 
     The housing  232  includes an outer surface  242  and an inner surface  244  configured for mounting against a casing  224 . In various embodiments, the recessed opening  238  extends circumferentially from the inner surface  244  towards the outer surface  242 , thereby defining a cylindrical volume within which the plate section  240  may be received. A body portion  246  of the housing  232  generally surrounds the recessed opening  238  and at least partially houses the seal member  234  within the recessed opening  238 . A housing opening  248  having a first diameter D 1  extends through the housing  232  and is configured to receive the interface component  250 . The housing opening  248  extends into the recessed opening  238 , which defines a second diameter D 2  that is typically larger than the first diameter D 1 . 
     The inner surface  244  of the housing  232  may include a curved portion that is configured to mate smoothly with a face  223  of the casing  224  to which the housing  232  is mounted. The actual dimensions of the curved portion may vary depending upon design specific parameters including, but not limited to, the shape of the face  223  of the casing  224  to which the housing  232  is configured for mounting. The housing  232  also includes one or more flanges  252  disposed radially outwardly from the body portion  246 . The one or more flanges  252  are disposed near opposite ends of the housing  232  (or equally spaced about the housing  232 ) and may include a fastener opening  254  for receiving a fastener to attach the housing  232  to the casing  224 . 
     The plate section  240  is received against an inner wall  255  of the recessed opening  238 . In various embodiments, the plate section  240  defines a plate opening  256  which, in various embodiments, has a diameter equal to an inner surface diameter  245  of the tube section  241 . The plate opening  256  is positioned adjacent the housing opening  248  of the housing  232  when received within the recessed opening  238 . In various embodiments, both the plate opening  256  and the inner surface diameter  245  define a third diameter D 3 . The third diameter D 3  of the plate opening  256  (or the inner surface diameter  245  of the tube section  241 ) is generally smaller than the first diameter D 1  of the housing opening  248 . The plate section  240  is configured to translate relative to the housing  232  to compensate for movement of the interface component  250  with respect to the housing  232  (or with respect to the casing  224  to which the housing  232  may be mounted). Referring to the coordinate system illustrated in  FIGS. 2A and 2B , the translation of the plate section  240  with respect to the housing  232  comprises generally two degrees of freedom, in the x-y plane. The retaining ring  236  generally prohibits the plate section  240  from translating in the z-direction with respect to the housing  232 . 
     Still referring to  FIGS. 2A and 2B , the seal member  234 , as described above, also includes the tube section  241  that, in various embodiments, extends from the plate section  240  in a direction substantially perpendicular to the x-y plane in which the plate section  240  is constrained to reside. The tube section  241  defines an inner tube surface  260 . In various embodiments, the inner tube surface  260  provides a surface configured to slidably engage an outer component surface  262  which, in various embodiments, may be considered an outer surface of the interface component  250 . In various embodiments, the outer component surface  262  is characterized by a radius of curvature  264  that swings through an angle  266 , thereby defining a radial arc portion  268  of the outer component surface  262  that extends along a radial arc-length  258 . In various embodiments, the radius of curvature  264  is substantially constant along the radial arc-length  258 . The radial arc portion  268  also extends in a generally circumferential fashion about a central axis C 2  of the interface component  250  and defines an outer diameter (or outer radius) substantially equal to the third diameter D 3  (or a radius substantially equal to one-half the third diameter D 3 ). A tolerance of one or two thousands of an inch 2.5 to 5.0 micrometers) provides sufficient gap to insert the radial arc portion  268  within the inner tube surface  260  and to maintain an interference fit sufficient to form a seal against a fluid or a gas flowing through the gap. While the disclosure refers to the tube section  241  as having a length  243  extending from the plate section  240 , the length  243  is more generally defined as a distance through which the interface component  250  is expected to translate (in the z-direction) within the tube section  241 . 
     The intersection of the radial arc portion  268  of the outer component surface  262  with the inner tube surface  260  provides for several additional degrees of freedom of relative movement between the interface component  250  and the housing  232 . For example, in various embodiments, the inner tube surface  260 , which extends in the z-direction at substantially constant diameter, enables the radial arc portion  268  to translate in the z-direction, thereby allowing three degrees of translational freedom with respect to the housing  232  (e.g., the two degrees of freedom in the x-y plane described above, together with the single degree of freedom in the z-direction just described). Further, if the radius of curvature  264  of the radial arc portion  268  is selected to be substantially equal to one-half the third diameter D 3  (or substantially equal to an inner tube surface radius), then the intersection of the radial arc portion  268  with the inner tube surface  260  will enable the interface component  250  to rotate with three degrees of rotational freedom with respect to the housing  232 . For example, the interface component  250  may rotate in a first direction  270  about the z-axis, in a second direction  272  in the x-z plane and in a third direction  274  in the y-z plane. Coupled with the three degrees of freedom of translational movement described above, the slider seal assembly  230  above described provides for a full six degrees of translational and rotational movement of the interface component  250  relative to the housing  232  (or relative to the casing  224 ) due to thermal or vibratory effects occurring during operation. 
     Referring now to  FIGS. 3A and 3B , a slider seal assembly  330  (or seal assembly), such as, for example, one of the plurality of slider seal assemblies  130  described above with reference to  FIG. 1B , is illustrated. In various embodiments, the slider seal assembly  330  includes a housing  332 , a seal member  334  and a retaining ring  336 . The seal member  334  includes a plate section  340  and a tube section  341 . The housing  332  includes a recessed opening  338  for receiving the plate section  340  of the seal member  334 . The retaining ring  336  is received within a slot  347  of the recessed opening  338  to retain the plate section  340  within the recessed opening  338 . In various embodiments, the plate section  340  extends circumferentially about the tube section  341 . In various embodiments, the tube section  341  may extend along a length  343  from the plate section  340  and, as described further below, provides an interior surface configured to slidably engage a corresponding exterior surface of an interface component  350 . In various embodiments, the tube section  341  defines a central axis C 1  that extends along the length  343  of the tube section  341 . In various embodiments, the housing  332  is configured for mounting to a casing  324  having a face  323  that may, in various embodiments, exhibit either a flat surface or a curved surface. With the exception of the disclosure that follows, the constructional and operational characteristics of the slider seal assembly  330  are the same or similar to the constructional and operational characteristics of the slider seal assembly  230  described above with reference to  FIGS. 2A and 2B  and, therefore, are not repeated here. 
     Similar to the description provided above, the seal member  334  includes the tube section  341  that, in various embodiments, extends from the plate section  340  in a direction substantially perpendicular to the x-y plane in which the plate section  340  is constrained to reside. The tube section  341  defines an inner tube surface  360 . In various embodiments, the inner tube surface  360  provides a surface configured to slidably engage an outer component surface  362  which, in various embodiments, may be considered an outer surface of the interface component  350 . In various embodiments, the outer component surface  362  is characterized by a radius of curvature  364  that swings through an angle  366 , thereby defining a radial arc portion  368  of the outer component surface  362  that extends along a radial arc-length  358 . In various embodiments, the radius of curvature  364  is substantially constant along the radial arc-length  358 . The radial arc portion  368  also extends in a generally circumferential fashion about a central axis C 2  of the interface component  350  and defines an outer diameter (or outer radius) substantially equal to an inner tube surface radius  345  of the tube section  341 . Consistent with the constructional and operational characteristics above described with reference to  FIGS. 2A and 2B , the slider seal assembly  330  above described provides for a full six degrees of translational and rotational movement of the interface component  350  relative to the housing  332  (or relative to the casing  324 ) due to thermal or vibratory effects occurring during operation. 
     In various embodiments, a seal ring  380  is disposed within a seal ring groove  381  that extends into the radial arc portion  368  of the outer component surface  362 . The seal ring  380  is configured to reduce any gap that exists between the radial arc portion  368  of the outer component surface  362  and the inner tube surface  360 . In various embodiments, the seal ring  380  may include a chambered edge  382  on an upper surface of the seal ring  380  and on a lower surface of the seal ring  380 , toward an outer radial portion of the seal ring  380  configured to make contact with the inner tube surface  360 . In various embodiments, the chambered edge  382  enables the seal ring  380  to swing through the angle  366  or a portion thereof when the interface component  350  is rotated within the seal member  334  and, more particularly, enables the seal ring  380  to maintain contact with the inner tube surface  360  when the interface component  350  is rotated within the seal member  334 . 
     Referring now to  FIGS. 4A and 4B , perspective and cross sectional views of a slider seal assembly  430  are provided, in accordance with various embodiments. Similar to the various embodiments above described, the slider seal assembly  430  includes a housing  432 , a seal member  434  and a retaining ring  436 . The seal member  434  includes a plate section  440  and a tube section  441 . The housing  432  includes a recessed opening  438  for receiving the plate section  440  of the seal member  434 . The retaining ring  436  is received within a slot  447  of the recessed opening  438  to retain the plate section  440  within the recessed opening  438 . In various embodiments, the plate section  440  extends circumferentially about a mid-section of the tube section  441 . In various embodiments, the tube section  441  may extend along a length  443 , with the plate section  440  extending radially outward from a mid-section of the length  443  and, as described above, provides an inner tube surface  460  configured to slidably engage a corresponding outer component surface  462  of an interface component  450 . In various embodiments, the tube section  441  defines a central axis C 1  that extends along the length  443  of the tube section  441  and the interface component  450  defines a central axis C 2  that extends along at least a portion of where the outer component surface  462  is configured for contact with the inner tube surface  460 . As with the slider seal assembly  230  and the slider seal assembly  330  described above, the central axis C 1  of the tube section  441  and the central axis C 2  of the interface component  450  are not required to be coaxial with one another, especially during operation when the interface component rotates with respect to the tube section  441 . 
     In various embodiments, the housing  432  is configured for mounting to a casing  424  having a face  423  that may, in various embodiments, exhibit either a flat surface or a curved surface. The interface component  450  may comprise any component extending from one side of the casing  424  to the other. For example, in various embodiments, the interface component  450  is one of a borescope plug, a fuel fitting, a bleed port, a conduit, a probe or the like. As described above, the slider seal assembly  430 , similar to the slider seal assembly  230  and the slider seal assembly  330  described above, seals the casing  424  at the interface component  450  and provides for a full six degrees of translational and rotational movement of the interface component  450  relative to the housing  432  (or relative to the casing  424 ) due to thermal or vibratory effects occurring during operation. 
     During normal operation of a gas turbine engine, such as, for example, the gas turbine engine described above with reference to  FIGS. 1A and 1B , there is typically a temperature and pressure difference between a first airflow F 1  (e.g., a first air flow passing through a bypass flow path B) and a second airflow F 2  (e.g., a second airflow passing through a core flow path C). The temperature difference between the first air flow F 1  and the second air flow F 2  may cause thermal growth mismatch between the bypass flow path B and the core flow path C. That is, various components or casings of the gas turbine engine may expand and contract in both the radial and axial directions relative to the engine centerline axis A because of the extreme temperature differences. The thermal growth mismatch may result in displacement of the interface component  450  with respect to the slider seal assembly  430  or, more particularly, the housing  432  of the slider seal assembly  430 . The slider seal assembly  430  is designed to compensate for the thermal growth mismatch and provide a seal for the pressure difference between the first airflow F 1  and the second airflow F 2 . 
     Benefits, other advantages, and solutions to problems have been described herein with regard to specific embodiments. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent exemplary functional relationships and/or physical couplings between the various elements. It should be noted that many alternative or additional functional relationships or physical connections may be present in a practical system. However, the benefits, advantages, solutions to problems, and any elements that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as critical, required, or essential features or elements of the disclosure. The scope of the disclosure is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” Moreover, where a phrase similar to “at least one of A, B, or C” is used in the claims, it is intended that the phrase be interpreted to mean that A alone may be present in an embodiment, B alone may be present in an embodiment, C alone may be present in an embodiment, or that any combination of the elements A, B and C may be present in a single embodiment; for example, A and B, A and C, B and C, or A and B and C. Different cross-hatching is used throughout the figures to denote different parts but not necessarily to denote the same or different materials. 
     Systems, methods and apparatus are provided herein. In the detailed description herein, references to “one embodiment,” “an embodiment,” “various embodiments,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. After reading the description, it will be apparent to one skilled in the relevant art(s) how to implement the disclosure in alternative embodiments. 
     Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. 112(f) unless the element is expressly recited using the phrase “means for.” As used herein, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. 
     Finally, it should be understood that any of the above described concepts can be used alone or in combination with any or all of the other above described concepts. Although various embodiments have been disclosed and described, one of ordinary skill in this art would recognize that certain modifications would come within the scope of this disclosure. Accordingly, the description is not intended to be exhaustive or to limit the principles described or illustrated herein to any precise form. Many modifications and variations are possible in light of the above teaching.