Self pressurizing squeeze film damper

A fluid damping structure is provided that includes an inner annular element, an outer annular element, a first outer seal, a second outer seal, an inner seal, a damping chamber, a supply plenum, a fill port, and a plurality of fluid passages. The plurality of fluid passages is disposed in at least one of the inner annular element or the inner seal. The fluid damping structure is configured such that one or more of the fluid passages is disposed in an open configuration when a local damping fluid pressure within the damping chamber is less than a local damping fluid pressure in an adjacent region of the supply plenum, and the one or more of the fluid passages is disposed in a closed configuration when the local damping fluid pressure within the damping chamber is greater than the local damping fluid pressure in the adjacent region of the supply plenum.

BACKGROUND

1. Technical Field

This disclosure relates generally to rotating shaft damping structures and more particularly to rotating shaft fluid damping structures.

2. Background Information

Gas turbine engines are often configured to include a fan section, a low pressure compressor section, a high pressure compressor section, a combustor section, a low pressure turbine section, a high pressure turbine section, a low speed spool, and a high speed spool. The fan section may be configured to drive air along a bypass flow path, while the compressor section drives air along a core flow path for compression and communication into the combustor section then expansion through the turbine section. The low speed spool and the high speed spool are mounted for rotation about an engine central longitudinal axis relative to an engine static structure via several bearing systems. The low speed spool generally interconnects the fan section, the low pressure compressor section and the low pressure turbine section. The high speed spool generally interconnects the high pressure compressor section and the high pressure turbine section. The combustor section is disposed between the high pressure compressor section and the high pressure turbine section.

Under normal operating conditions, a shaft section of a spool (e.g., a shaft section of the high speed spool) will rotate without significant vibration. Under certain operating conditions, however, a spool shaft section may be subject to cyclical, orbital motion which can lead to undesirable vibration. Such cyclical, orbital motion may be the product of temporary thermal bowing of the spool shaft section as a result of a thermal gradient within the engine. Once the thermal gradient sufficiently dissipates, the temporary bowing dissipates and the spool shaft section restores itself to normal operating condition.

As will be appreciated by those skilled in the art, the existence of an imbalance in a shaft section may result in a greatly increased demand on the bearing components to restrain the movement of the rotating member or shaft and to transfer the lateral forces induced by the imbalance into the machinery mounting structure. As will be described herein, this type of imbalance may exist with a gas turbine engine shaft. It should be noted, however, that this type of rotating shaft imbalance may exist in other types of machinery other than in a gas turbine engine.

One method of reducing the aforesaid lateral forces and attendant stresses on the bearings is the use of a fluid damping structure (sometimes referred to as “fluid squeeze damper”) between the outer portion of the shaft bearing race or housing and the supporting engine case. The fluid damper structure is a hydrodynamic system wherein a continuously flowing stream of damping fluid (e.g., oil) is supplied to an annular volume formed between the non-rotating outer bearing race (or housing) and the engine support case for the purpose of absorbing and reducing the transverse movement induced by shaft imbalance, temporary or otherwise. The damping fluid, which may be supplied from a lubricating system (e.g., a gas turbine engine lubricating system), fills the annular volume and subsequently exits the annular volume and is collected and passed to a recovery system (e.g., including a scavenge sump or the like). Existing systems for providing a pressurized damping fluid to fluid damping structures that we are aware of are problematic. U.S. Pat. No. 5,344,239 discloses a squeeze film damper with annular end plenums.

SUMMARY OF THE DISCLOSURE

According to an aspect of the present disclosure, a fluid damping structure is provided that includes an inner annular element, an outer annular element, a first ring seal, a second ring seal, a first outer annular seal, and a second outer annular seal. The inner annular element has an outer radial surface and a plurality of annular grooves disposed in the outer radial surface. The outer annular element has an inner radial surface. Each inner ring seal has a first lateral surface, a second lateral surface, an inner diameter surface, and an outer diameter surface. A damping chamber is defined by the inner annular element, the outer annular element, the first inner ring seal, and the second inner ring seal. A first lateral chamber is disposed on a first axial side of the damping chamber, and is defined by the inner annular element, the outer annular element, the first inner ring seal, and the first outer annular seal. A second lateral chamber is disposed on a second axial side of the damping chamber, and is defined by the inner annular element, the outer annular element, the second inner ring seal, and the second outer annular seal. A plurality of fluid passages are disposed in at least one of the inner annular element or the inner ring seals. The fluid damping structure is configured such that one or more of the fluid passages is disposed in an open configuration when a local damping fluid pressure within at least one of the lateral chambers exceeds a local damping fluid pressure in an adjacent region of the damping chamber, and the one or more of the fluid passages is disposed in a closed configuration when the local damping fluid pressure within at least one of the lateral chambers is less than the local damping fluid pressure in the adjacent region of the damping chamber.

According to another aspect of the present disclosure a gas turbine engine is provided that includes at least one rotor shaft extending between a compressor section and a turbine section, at least one bearing compartment disposed to support the rotor shaft, the bearing compartment having at least one bearing, at least one fluid damping structure as described above, and a lubrication system configured to provide a fluid flow to the fluid damping structure.

In any of the aspects or embodiments described above and herein, the inner annular element and the outer annular element may be radially spaced apart from one another and disposed about an axially extending centerline, and the outer diameter surface of each inner ring seal may be in contact with the inner radial surface of the outer annular element, and each inner ring seal extends a distance into one of the annular grooves.

In any of the aspects or embodiments described above and herein, the plurality of annular grooves may include a first inner annular groove and a second inner annular groove, each inner annular groove having an inner side surface and an opposing outer side surface and a width that extends there between. The plurality of fluid passages may be disposed in the inner annular element, extending between the inner side surface of each inner annular groove and the outer radial surface of the inner annular element.

In any of the aspects or embodiments described above and herein, the passages may be are disposed uniformly around a circumference of the inner annular element.

In any of the aspects or embodiments described above and herein, the first outer annular seal and the second outer annular seal may be ring seals, and each outer annular seal has an outer diameter surface, and the outer diameter surface of each outer annular seal is in contact with the inner radial surface of the outer annular element, and each outer annular seal extends a distance into one of the annular grooves.

In any of the aspects or embodiments described above and herein, at least some of the plurality of fluid passages are disposed in each first inner ring seal and each second inner ring seal, each of which passages extends between the inner diameter surface and the second lateral surface of the respective inner ring seal.

In any of the aspects or embodiments described above and herein, the passages may be disposed uniformly around a circumference of the respective inner ring seal.

In any of the aspects or embodiments described above and herein, the fluid damping structure may include a first annular plenum disposed in the inner radial surface of the outer annular element aligned with the first lateral chamber, a first lateral chamber port providing fluid communication into the first annular plenum, a second annular plenum disposed in the inner radial surface of the outer annular element aligned with the second lateral chamber, and a second lateral chamber port providing fluid communication into the first annular plenum.

In any of the aspects or embodiments described above and herein, the fluid damping structure may include a first annular plenum disposed in the inner radial surface of the outer annular element aligned with the first lateral chamber, and a second annular plenum disposed in the inner radial surface of the outer annular element aligned with the second lateral chamber.

In any of the aspects or embodiments described above and herein, the fluid damping structure may include a first lateral chamber port disposed within the inner annular element providing fluid communication into the first annular plenum, a second lateral chamber port disposed within the inner annular element providing fluid communication into the second annular plenum, and a damping chamber port disposed within the inner annular element providing fluid communication into the damping chamber.

In any of the aspects or embodiments described above and herein, the fluid damping structure may include a first lateral chamber port disposed within the inner annular element providing fluid communication into the first lateral chamber, a second lateral chamber port disposed within the inner annular element providing fluid communication into the second lateral chamber, and a damping chamber port disposed within the inner annular element providing fluid communication into the damping chamber, and the engine may be configured to provide the fluid flow from the lubrication system to the fluid damping structure through the damping chamber port, and is configured to permit fluid flow to exit the fluid damping structure through the first lateral chamber port and the second lateral chamber port.

In any of the aspects or embodiments described above and herein, the fluid damping structure may include a first lateral chamber port disposed within the inner annular element providing fluid communication into the first lateral chamber, and a second lateral chamber port disposed within the inner annular element providing fluid communication into the second lateral chamber, and the engine may be configured to provide the fluid flow from the lubrication system to the fluid damping structure through the first lateral chamber port, and is configured to permit fluid flow to exit the fluid damping structure through the second lateral chamber port.

In any of the aspects or embodiments described above and herein, the fluid damping structure may include a first lateral chamber port disposed within the inner annular element providing fluid communication into the first lateral chamber, a second lateral chamber port disposed within the inner annular element providing fluid communication into the second lateral chamber, and a damping chamber port disposed within the inner annular element providing fluid communication into the damping chamber, and the engine may be configured to provide the fluid flow from the lubrication system to the fluid damping structure through first lateral chamber port and the second lateral chamber port, and to permit fluid flow to exit the fluid damping structure through the damping chamber port.

According to another aspect of the present disclosure, a fluid damping structure is provided that includes an inner annular element having an outer radial surface, an outer annular element having an inner radial surface, a first outer seal, a second outer seal, and an inner seal, each of which seals is engaged with both the inner annular element and the outer annular element, a damping chamber defined by the inner annular element, the outer annular element, the inner seal, and the second outer seal, a supply plenum disposed on an axial side of and contiguous with the damping chamber, the supply plenum defined by the inner annular element, the outer annular element, the first outer seal, and the inner seal, a fill port in fluid communication with the supply plenum and a source of damping fluid, and a plurality of fluid passages disposed in at least one of the inner annular element or the inner seal. The fluid damping structure is configured such that one or more of the fluid passages is disposed in an open configuration when a local damping fluid pressure within the damping chamber is less than a local damping fluid pressure in an adjacent region of the supply plenum, and the one or more of the fluid passages is disposed in a closed configuration when the local damping fluid pressure within the damping chamber is greater than the local damping fluid pressure in the adjacent region of the supply plenum.

According to another aspect of the present disclosure, a fluid damping structure is provided that includes a damping chamber, a supply plenum, and a fill port. The damping chamber is defined by an inner annular element, an outer annular element, an inner seal, and a first outer seal. The supply plenum is disposed on an axial side of, and contiguous with, the damping chamber. The supply plenum id defined by the inner annular element, the outer annular element, the inner seal, and a second outer seal. The fill port is in fluid communication with the supply plenum and configured to be in fluid communication with a source of damping fluid. The damping fluid structure is configured so that a first flow of damping fluid from the supply plenum to the damping chamber is subject to a first fluid flow resistance, and a second fluid flow of damping fluid from the damping chamber to the supply plenum is subject to a second fluid flow resistance, which second fluid flow resistance is greater than the first fluid flow resistance.

The foregoing features and the operation of the present disclosure will become more apparent in light of the following description and the accompanying drawings.

DETAILED DESCRIPTION

It is noted that various connections are set forth between elements in the following description and in the drawings (the contents of which are included in this disclosure by way of reference). It is noted that these connections are general and, unless specified otherwise, may be direct or indirect and that this specification is not intended to be limiting in this respect. A coupling between two or more entities may refer to a direct connection or an indirect connection. An indirect connection may incorporate one or more intervening entities.

Referring now to the FIGURES, to facilitate the description of the present disclosure a two-spool turbofan type gas turbine engine20is shown (e.g., seeFIG. 1). This exemplary embodiment of a gas turbine engine includes a fan section22, a compressor section24, a combustor section26, a turbine section28, and an engine lubrication system in fluid communication with one or more fluid damping structures. The fan section22drives air along a bypass flow path “B” in a bypass duct, while the compressor section24drives air along a core flow path “C” for compression and communication into the combustor section26then expansion through the turbine section28. Although a two-spool turbofan gas turbine engine is described herein to facilitate the description of the present disclosure, it should be understood that the present disclosure is not limited to use with two-spool turbofans as the teachings may be applied to other types of machinery with rotating shafts; e.g., a gas turbine engine with a three-spool architecture, a high speed turbocharger that may be used in an automotive application, or a ground based gas turbine engine application that may be used in a power generation application or a land based vehicle, etc.

The exemplary engine20shown inFIG. 1includes a low speed spool30and a high speed spool32mounted for rotation about an engine central longitudinal axis “A” relative to an engine static structure36via several bearing systems38. It should be understood that the location, number, and characteristics of bearing systems38may vary to suit the particular applications.

The gas turbine engine20diagrammatically depicted inFIG. 1is one example of a high-bypass geared aircraft engine. In other examples, the gas turbine engine20may have a bypass ratio that is greater than about six (6), with an example embodiment being greater than about ten (10), the geared architecture48may be an epicyclic gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3 and the low pressure turbine46may have a pressure ratio that is greater than about five (5:1). In one disclosed embodiment, the gas turbine engine20bypass ratio is greater than about ten (10:1), the fan diameter is significantly larger than that of the low pressure compressor44, and the low pressure turbine46has a pressure ratio that is greater than about five (5:1). The low pressure turbine46pressure ratio is pressure measured prior to the inlet of the low pressure turbine46as related to the pressure at the outlet of the low pressure turbine46prior to an exhaust nozzle. The geared architecture48may be an epicycle gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3:1. It should be understood, however, that the above parameters are only exemplary of one or more embodiments of a geared architecture engine and that the present disclosure is applicable to other gas turbine engines including direct drive turbofans.

FIG. 2is a simplified cross-sectional view of a portion of a gas turbine engine. The engine portion includes a rotor shaft60(e.g., a shaft section of a high speed spool), a bearing62, a bearing housing64, a stator structure66, and a fluid damping structure68. In the non-limiting embodiment shown inFIG. 2, the bearing housing64is mounted to a cage structure69disposed adjacent thereto. As will be described below, the bearing housing64may be subjected to forces that cause a cyclical, orbital motion (sometimes referred to as a “whirl”) of the bearing housing64. The cage structure69permits some amount of elastic motion of the bearing housing64(e.g., the “whirling”) in response to the aforesaid forces. The fluid damping structure68embodiment shown inFIG. 2includes a damping chamber70, a first lateral chamber72, and a second lateral chamber74. A source76of damping fluid (e.g., oil) is provided to the fluid damping structure68by, for example, the engine lubricating system. The rotor shaft60is rotatable about an axis of rotation Ar. The bearing62includes roller elements78(e.g., spherical balls) disposed between an inner race80and an outer race82. The present disclosure is not limited to any particular bearing configuration. The bearing inner race80is mounted on the rotor shaft60and therefore rotates with the rotor shaft60. The bearing outer race82is not fixed with the rotor shaft60and does not rotate about the axis of rotation Ar. The bearing housing64is nonrotating (i.e., it does not rotate about the axis of rotation Ar) and has an outer radial surface84, an inner radial surface86, a first lateral surface88, and a second lateral surface90. It should be noted that the geometric configuration of the bearing housing64shown inFIG. 2and described herein is an example of a structure for supporting the bearing62and forming a portion of the fluid damping structure68. The present disclosure is not limited to this particular embodiment. As will be described below, the bearing housing64may be subjected to forces that cause a cyclical, orbital motion (sometimes referred to as a “whirl”) of the bearing housing64, but such whirling motion is not considered to be rotation about the rotor shaft axis of rotation Ar. The inner and outer radial surfaces84,86extend generally between the first and second lateral surfaces88,90. The bearing outer race82is engaged with the inner radial surface86of the bearing housing64. The stator structure66includes a cylindrical inner radial surface92that is spaced radially apart from the outer radial surface84of the bearing housing64.

In the embodiments shown inFIGS. 2, 6, 6A-6C, 7, 7A, 9, 11 and 11A, at least two pairs of seals extend between the bearing housing outer radial surface84and the inner radial surface92of the stator structure66. In the embodiment shown inFIGS. 2, 6, 6A, 7 and 7A, the fluid damping structure68includes a pair of outer seals (e.g., first outer seal94, second outer seal96) and a pair of inner seals (e.g., first inner seal98, second inner seal100). The aforesaid inner and outer seals may be any type of seal that is capable of providing the sealing function in the fluid damping structure68. For example, the outer seals94,96may be ring type seals and the inner seals98,100may be ring type seals. To facilitate the description herein, the inner and outer seals64,96,98,100are described herein as ring type seals, but are not limited to this type of seal. The inner seals98,100are disposed axially between the outer seals94,96; e.g., the first inner seal98is disposed axially between the first outer seal94and the second inner seal100, and the second inner seal100is disposed axially between the first inner seal98and the second outer seal96. The inner and outer seals94,96,98,100are spaced axially apart and extend circumferentially about the axis of rotation Arof the rotor shaft60.

Now referring toFIGS. 3 and 3A, in some embodiments each outer seal94,96has an outer diameter surface102disposed at a diameter “D1”, an inner diameter surface104disposed at a diameter “D2” (D1>D2), a first lateral surface106, a second lateral surface108, and a thickness110. The first and second lateral surfaces106,108extend between the outer diameter surface102and the inner diameter surface104. The thickness110of each outer seal94,96extends between the lateral surfaces106,108.

Now referring toFIGS. 4 and 4A, in some embodiments each inner seal98,100has an outer diameter surface112disposed at a diameter “D3”, an inner diameter surface114disposed at a diameter “D4” (D3>D4), a first lateral surface116, a second lateral surface118, and a thickness120. The first and second lateral surfaces116,118of each inner seal98,100extend between the outer diameter surface112and the inner diameter surface114. The thickness120of each inner seal98,100extends between the lateral surfaces116,118.

In the inner seal98,100embodiments shown inFIGS. 3 and 4, the lateral surfaces116,118of the inner seals98,100may be planar and substantially parallel one another. In some alternative embodiments, each inner seal98,100may include a plurality of passages that extend between the inner diameter surface114and the second lateral surface118. For example,FIG. 5diagrammatically illustrates an inner seal98,100embodiment having passages122in the form of troughs122, extending between the inner diameter surface114and the second lateral surface118. These troughs122are configured to allow passage of damping fluid through an annular groove, under certain circumstances as will be described below. The troughs122are not limited to any particular geometric configuration. The troughs122may all have the same geometric configuration, or there may be one or more troughs122having a first geometric configuration, one or more troughs122having a second geometric configuration, etc. The trough122embodiment shown inFIG. 5breaks through the edge formed at the intersection of the inner diameter surface114and the second lateral surface118of the inner seal98,100. In alternative embodiments, the troughs122may extend between the inner diameter surface114and the outer diameter surface112, thereby creating a passage between the two surfaces112,114. The troughs122are spaced apart from one another, distributed around the circumference of the respective inner seal98,100. In the embodiments shown inFIGS. 5 and 5Athe troughs122are uniformly distributed around the circumference of the inner annular groove126,128; i.e., each trough122is spaced apart from an adjacent trough122by an equal angular separation.FIG. 5shows the eight troughs122disposed around the circumference of the respective inner seal98,100, each disposed forty-five degrees from the adjacent troughs122. The present disclosure is not, however, limited to circumferential uniformly distributed troughs122.

Now referring to the embodiment shown inFIG. 2, the damping chamber70portion of the fluid damping structure68is defined at least in part by the outer radial surface84of the bearing housing64, the inner radial surface92of the stator structure66, and inner seals98,100. The first lateral chamber72portion of the fluid damping structure68is defined at least in part by the outer radial surface84of the bearing housing64, the inner radial surface92of the stator structure66, the first outer seal94, and the first inner seal98. The second lateral chamber74portion of the fluid damping structure68is defined at least in part by the outer radial surface84of the bearing housing64, the inner radial surface92of the stator structure66, the second outer seal96, and the second inner seal100.

The outer radial surface84of the bearing housing64may include an annular groove for each of the inner and outer seals94,96,98,100. The present disclosure is not limited to embodiments having an annular groove for each of the inner and outer seals94,96,98,100; e.g., the bearing housing64may be configured to constrain, and provide one or more sealing surfaces for the outer seals, with a structure other than an annular groove. In the embodiments shown inFIGS. 6 and 7, the outer radial surface84of the bearing housing64includes a first outer annular groove124, a first inner annular groove126, a second inner annular groove128, and a second outer annular groove130. The aforesaid grooves124,126,128,130(also shown inFIG. 2) are axially spaced apart from one another, and the inner annular grooves126,128are disposed axially between the outer annular grooves124,130; e.g., the first inner annular groove126is disposed axially between the first outer annular groove124and the second inner annular groove128, and the second inner annular groove128is disposed axially between the first inner annular groove126and the second outer annular groove130. The aforesaid annular grooves extend into the outer radial surface84of the bearing housing64and each has a base surface132, an inner side surface134, and an outer side surface136opposite the inner side surface134. The annular groove base surface132is located at a depth from the outer radial surface84. Each annular groove124,126,128,130has a width138that extends between the opposing side surfaces134,136. As will be described below, the width138of a given annular groove is greater than the width110,120of the seal disposed within the respective groove so that the seal may translate axially within the groove. InFIGS. 6 and 7, all of the grooves124,126,128,130are shown as having the same geometric configuration and dimensions; e.g., rectangular having the same depth and width. The present disclosure is not, however, limited to all grooves124,126,128,130having the same geometric configuration and/or dimensions.

In some embodiments (e.g., seeFIGS. 6 and 7), the bearing housing64has a plurality of passages140engaged with each inner annular groove126,128, each passage140extending between inner side surface134of the respective groove and the outer radial surface84of the bearing housing64. The passages140are configured to provide a conduit for damping fluid through the inner annular groove126,128, under certain circumstances as will be explained below. The passages140are spaced apart from one another, distributed around the circumference of the respective inner annular groove126,128. The passages140may, for example, be uniformly spaced around the circumference of the respective inner annular groove126,128. The present disclosure is not, however, limited to uniformly distributed passages140. Each passage140is configured to extend into the respective inner annular groove126,128at a radial position at least in part exposed below the inner diameter surface114of the respective inner ring seal98,100as will be explained below.

FIGS. 6 and 7illustrate a non-limiting example of a passage140in the form of a trough that extends into the inner side surface134of the respective inner annular groove126,128, and extends into the outer radial surface84of the bearing housing64, thereby providing a fluid passage between inner side surface134of the inner annular groove126,128and the outer radial surface84of the bearing housing64. The troughs140are spaced apart from one another, distributed around the circumference of the respective inner annular groove126,128. In the embodiments shown inFIGS. 6 and 7, the troughs140are uniformly distributed around the circumference of the inner annular groove126,128; i.e., each trough140is spaced apart from an adjacent trough140by an equal angular separation.FIG. 8shows the eight troughs140disposed around the circumference of the respective inner annular groove126,128, each disposed forty-five degrees from the adjacent troughs140. The present disclosure is not, however, limited to circumferential uniformly distributed troughs140.

As indicated, the troughs140are configured to provide a passage for damping fluid out of, or into, the inner annular groove126,128, under certain circumstances. The troughs140are not limited to any particular geometric configuration. The troughs140may all have the same geometric configuration, or there may be one or more troughs140having a first geometric configuration, one or more troughs140having a second configuration, etc. The trough140embodiment shown inFIGS. 6-8breaks through the edge formed at the intersection of the inner side surface134of the respective inner annular groove126,128and the outer radial surface84of the bearing housing64. The geometric characteristics of the troughs140may be chosen to suit the application; e.g., sized to permit adequate fluid flow under the anticipated operating condition of the device. To be clear, the troughs140shown inFIGS. 6-8are non-limiting examples of a passage extending between the inner side surface134of an inner annular groove126,128and the outer radial surface84of the bearing housing64, and the present disclosure is not limited to this particular embodiment. As another example (e.g., seeFIG. 9), the passages may be apertures140A that extend between the inner side surface134of an inner annular groove126,128and the outer radial surface84of the bearing housing64. Such apertures140A would not break through the edge formed at the intersection of the inner side surface134of the inner annular groove126,128and the outer radial surface84of the bearing housing64.

As indicated above, the pairs of seals94,96,98,100extend between the bearing housing outer radial surface84and the inner radial surface92of the stator structure66. The outer diameter surface of each seal is typically biased against the inner radial surface92of the stator structure66and provides some amount fluid sealing there between. In the embodiment shown inFIGS. 6, 7, and 9, each of the inner and outer seals are received a distance into the respective annular groove; e.g., each inner seal98,100extends a distance into one of the inner annular grooves126,128, and each outer seal94,96extends a distance into one of the outer annular grooves124,130.

In the embodiments shown inFIGS. 2, 6, 7 and 11, the fluid damping structure68is configured to provide damping fluid to the damping chamber70, the first lateral chamber72, and the second lateral chamber74. In the embodiment shown inFIG. 6, for example, the fluid damping structure68includes a fill port142disposed in the stator structure66that permits damping fluid to enter the damping chamber70through the inner radial surface92of the stator structure66. The fill port142is in fluid communication with the damping fluid source76(e.g., the engine lubrication system) and may include a one way check valve144(seeFIG. 2) that allows fluid to enter the damping chamber70but prevents fluid passage in the opposite direction. The embodiment shown inFIG. 6may further include a first lateral chamber port146and an annular plenum148disposed in the inner radial surface92of the stator structure66aligned with the first lateral chamber72, and a second lateral chamber port150and an annular plenum152disposed in the inner radial surface92of the stator structure66aligned with the second lateral chamber74. The annular plenums148,152are shown diagrammatically as having an arcuately shaped cross-section, but are not limited thereto. For example, inFIGS. 6, 6A, 7, and 7Athe annular plenums are shown disposed in an embodiment of the inner radial surface92that extends in a single plane across the first lateral chamber72, the damping chamber70, and the second lateral chamber. In alternative embodiments, the inner radial surface92may have a non-planar configuration wherein the geometry of one or both of the lateral chambers72,74differ from that of the damping chamber70, and thereby functions as an annular plenum. In still further alternative embodiments, the outer radial surface84of the bearing housing64may include an annular plenum or be configured to function as an annular plenum.

In an alternative embodiment shown inFIG. 7, the fluid damping structure68includes a first lateral chamber port146and an annular plenum148disposed in the inner radial surface92of the stator structure66aligned with the first lateral chamber72, and a second lateral chamber port150and an annular plenum152disposed in the inner radial surface92of the stator structure66aligned with the second lateral chamber74. One of the first or second lateral chamber ports146,150is in fluid communication with the damping fluid source76(e.g., the engine lubrication system) and may include a one way check valve (e.g., like that shown inFIG. 6) that allows fluid to enter the respective lateral chamber72,74but prevents fluid passage in the opposite direction.FIG. 7shows the first lateral chamber port146in fluid communication with the damping fluid source76.

In an alternative embodiment shown inFIGS. 11 and 11A, the fluid damping structure68includes a first lateral chamber port146and an annular plenum148disposed in the inner radial surface92of the stator structure66aligned with the first lateral chamber72, and a second lateral chamber port150and an annular plenum152disposed in the inner radial surface92of the stator structure66aligned with the second lateral chamber74. Both of the first and second lateral chamber ports146,150are in fluid communication with the damping fluid source76(e.g., the engine lubrication system) and may include a one way check valve (e.g., like that shown inFIG. 6) that allows fluid to enter the respective lateral chamber72,74but prevents fluid passage in the opposite direction. An exit port143extends through the stator structure66at a position aligned with the damping chamber.

In a gas turbine engine that is operating under “normal” conditions (e.g., in a constant RPM cruise mode), the fluid pressure within the damping chamber70is substantially consistent around the circumference of the damping chamber70. In an imbalanced condition (e.g., operating conditions such as a thermal gradient within an engine that exists at start up) however, a rotating spool shaft60may be subject to cyclical, orbital motion (i.e., “whirl”). This type of imbalanced condition and motion can create variations in fluid pressure within the damping chamber70(i.e., a dynamic pressure component that varies as a function of time and circumferential position). The variations in pressure may be considered as a pressure field representative of forces acting on the rotor shaft60during whirl by the film of damping fluid disposed between the outer radial surface84of the bearing housing64and the inner radial surface92of the stator structure66, around the circumference of the damping chamber70. The stator structure66is fixed, and the bearing housing64whirls with the rotor shaft60. The pressure field develops as the rotor shaft60whirls, resolving the net force acting on the rotor into components that align with the eccentricity and components that are perpendicular to the eccentricity. When a sufficient amount of rotor shaft60whirl occurs, the pressure field will include a region of positive pressure with respect to the circumferential mean of the pressure within the damping chamber70(i.e., a region of pressure greater than the circumferential mean pressure), and a region of negative pressure with respect to the circumferential mean of the pressure within the damping chamber70(i.e., a region of pressure less than the circumferential mean pressure). The circumferential differences in pressure within the damping chamber70cause the damping fluid to travel circumferentially within the damping chamber70.

To visualize the aforesaid pressure field, it is useful to “unwrap” the fluid pressure within the damping chamber70as a function of circumferential position as shown inFIG. 10. In this idealized representation, the circumferential mean pressure (sometimes referred to as the “DC” pressure) within the damping chamber is typically set by the characteristics of the damper fluid supply system and the leakage of the oil through the damping chamber seals. As the damping chamber seals approach ideal seals (i.e., no leakage), the DC pressure approaches the damping fluid supply pressure. The unsteady part of the pressure within the damping chamber (sometimes referred to as the “AC” pressure) amplitude, builds with whirl amplitude. The larger the whirl, the larger the AC pressure amplitude becomes. This idealized model works well conceptually until the zero-to-peak amplitude of the AC pressure causes the local pressure to fall below compartment pressure (denoted as PambientinFIG. 10). In some prior art squeeze film dampers, if the pressure within a damping chamber falls below Pambient, air within an engine compartment adjacent to the damping chamber may be drawn into the damping chamber from the adjacent engine compartment. Once air, or any gas, is entrained within the prior art damping chamber, the effectiveness of the squeeze film damper may be compromised. In some instances, if the damping chamber of a prior art squeeze film damper is isolated from any ingress of air from the adjacent engine compartment into the damping chamber, the damping fluid within the damping chamber may cavitate if the local pressure within the damping chamber is reduced below the vapor pressure of the damping fluid and thereby compromise the effectiveness of the squeeze film damper.

To decrease or avoid the possibility of an influx of gas (e.g., compartment air) into the damping chamber70of the present fluid damping structure68and/or damping fluid cavitation within the damping chamber70, the present fluid damping structure68is configured to “self-pressurize” the damping chamber70.

Using the exemplary embodiment shown inFIG. 6as an example, as a gas turbine engine20is operated in a start-up mode (e.g., rotor shaft low rpms), damping fluid is fed into the damping chamber70via the fill port142extending through the stator structure66. Some amount of the damping fluid bypasses the inner seals98,100, enters and fills the first and second lateral chambers72,74. In a short period of time, some amount of the damping fluid within the first and second lateral chambers72,74will exit the lateral chambers bypassing the outer seals94,96and/or via the first and second lateral chamber ports146,150. The damping fluid exiting the lateral chambers72,74may exit into the adjacent engine compartment where it is collected and returned to the main lubrication system via a scavenging system. Hence, the damping fluid cycles through the fluid damping structure68during operation. The first and second lateral chamber ports146,150are configured to provide an appropriate amount of flow impedance so that the lateral chambers72,74remain filled with pressurized damping fluid during operation. In those embodiments that include an annular plenum148,152in communication with the respective lateral chamber port146,150, the annular plenum148,152assists in maintaining in circumferential fluid pressure uniformity within the lateral chamber72,74. Under normal conditions (e.g., no whirl), the fluid pressure within the damping chamber70is substantially uniform around the circumference of the damping chamber70. The substantially uniform fluid pressure is diagrammatically illustrated by the pressure values P1 and P2 in different regions substantially equally one another; i.e., P1≈P2, and the fluid damping structure68will continue to operate in this mode indefinitely and the circumferential pressure field within the damping chamber70will remain substantially uniform. In this mode, elevated relative fluid pressure within the damping chamber70will force the first lateral surface116of the each inner seal98,100into contact with the outer side surface136of the respective inner annular groove126,128and will provide fluid sealing there between albeit with some amount of leakage. In this configuration, the passages140(e.g., disposed within the bearing housing64or the passages122disposed in the lateral surface118of the inner seal98,100) may be described as being in a closed configuration since any fluid flow through the passages140,122remains within the damping chamber70and does not contribute to any damping fluid flow (e.g., leakage about the seal that may occur) between the damping chamber and one or both of the lateral chambers72,74.

Under circumstances wherein the rotor shaft60is experiencing a sufficient amount of whirl, an unsteady circumferential pressure field as described above will develop. In the high pressure region (e.g., P3—SeeFIG. 6A) of the circumferential pressure field, the fluid pressure within the damping chamber70will continue to force a first lateral surface116of the each inner seal98,100into contact with the outer side surface136of the respective inner annular groove126,128and provide fluid sealing there between. In the low pressure region (e.g., P4) of the circumferential pressure field, in contrast, the fluid pressure within the adjacent lateral chambers72,74will exceed the fluid pressure within the adjacent damping chamber70region. As a result and in that region, the inner seal98,100will deflect away from the outer side surface136of the respective inner annular groove126,128toward the inner side surface134of the aforesaid inner annular groove126,128. If the difference in pressure is great enough, the inner seal98,100second lateral surface118will be held in contact with the inner side surface134of the respective inner annular groove126,128(e.g., as shown in the bottom ofFIG. 6A). As a result, the passages140(e.g., disposed within the bearing housing64or the passages122disposed in the lateral surface118of the inner ring seal98,100) aligned with the damping chamber70low pressure region permit a flow of damping fluid from the respective lateral chamber72,74, through the inner annular groove126,128, and into the aforesaid region of the damping chamber70motivated by the difference in fluid pressure between the adjacent lateral chamber region and the damping chamber region.FIGS. 6B and 6Cshow enlarged views of this configuration. The aforesaid fluid flow locally into the damping chamber70causes a local increase in damping fluid pressure within the damping chamber70that helps prevent or eliminates the possibility of cavitation of damping fluid within the low fluid pressure region, and improves the performance of the fluid damping structure68. The damping fluid structure configuration having lateral chambers72,74adjacent the damping chamber70helps prevent or eliminates the ingress of compartment air into the damping chamber70. The eccentric whirling of the rotor shaft60and bearing housing64makes the above described creation of high pressure regions and low pressure regions a dynamic event that can be accommodated at any circumferential position by the present fluid damping structure68; e.g., the circumferentially distributed passages122,140. In this configuration, the passages140(e.g., disposed within the bearing housing64or the passages122disposed in the lateral surface118of the inner ring seal98,100) may be described as being in an open configuration since fluid flow through the passages140,122contributes to damping fluid flow (e.g., in addition to any leakage about the ring seal that may occur) between the damping chamber and one or both of the lateral chambers72,74. It should be noted that the pressure zones (e.g., P1 and P2, P3 and P4) are diagrammatically shown at opposite positions (e.g., 180° from one another) for illustrative purposes. High and low pressures zones may occur at various circumferential positions within the damping chamber, and therefore the diagrams should not be construed as limiting the performance of the present disclosure.

The fluid damping structure68embodiment shown inFIG. 7operates in a manner similar to that described above in terms of the fluid damping structure68embodiment shown inFIGS. 6 and 6A. The fluid damping structure68embodiment shown inFIG. 7, however, illustrates an alternative damping fluid travel path through the fluid damping structure68. As described above, in the alternative embodiment one of the first or second lateral chamber ports146,150is in fluid communication with the damping fluid source76(e.g., the engine lubrication system). In the diagrammatic illustration ofFIGS. 7 and 7A, the first lateral chamber port146is shown in communication with the damping fluid source76. In this configuration as a gas turbine engine20is operated in a start-up mode (e.g., rotor shaft low rpms), damping fluid is fed into the first lateral chamber72via the first lateral chamber fill port146. Some amount of the damping fluid bypasses the first inner seal98, enters and fills the damping chamber70, and subsequently bypasses the second inner seal100, enters and fills the second lateral chamber74. Similar to the description above, some amount of the damping fluid will exit the lateral chambers72,74bypassing the outer seals94,96. In this configuration, some amount of damping fluid may also exit the second lateral chamber74via the second lateral chamber port150. The damping fluid exiting the lateral chambers72,74may exit into the adjacent engine compartment where it is collected and returned to the main lubrication system via a scavenging system. The second lateral chamber port150may be configured to provide an appropriate amount of flow impedance so that the second lateral chamber74remains filled with pressurized damping fluid during operation.

Under normal conditions (e.g., no whirl), the fluid pressure within the damping chamber70is substantially uniform around the circumference of the damping chamber70. The substantially uniform fluid pressure is diagrammatically illustrated inFIG. 7(see alsoFIG. 11) by the pressure values P4 and P5 in different regions substantially equally one another (i.e., P4≈P5), and the fluid damping structure68will continue to operate in this mode indefinitely and the circumferential pressure field within the damping chamber70will remain substantially uniform. As described above, in this configuration the passages140,122are in a closed configuration.

Under circumstances wherein the rotor shaft60is experiencing a sufficient amount of whirl, an unsteady circumferential pressure field as described above will develop. In the high pressure region of the circumferential pressure field (e.g., P6 as shown inFIG. 7A,FIG. 11A), the fluid pressure within the damping chamber70will continue to force the first lateral surface116of the each inner seal98,100into contact with the outer side surface136of the respective inner annular groove126,128and provide fluid sealing there between. In the low pressure region of the circumferential pressure field (e.g., P7 as shown inFIG. 7A,FIG. 11A), in contrast, the fluid pressure within the adjacent lateral chambers72,74will exceed the fluid pressure within the adjacent damping chamber region. As a result and in that region, the inner seal98,100will deflect away from the outer side surface136of the respective inner annular groove126,128toward the inner side surface134of the aforesaid inner annular groove126,128. If the difference in pressure is great enough, the inner seal98,100will be held in contact with the inner side surface134of the respective inner annular groove126,128(e.g., as shown in the bottom ofFIG. 7A,FIG. 11A). As a result, the passages140(e.g., disposed within the bearing housing64or the passages122disposed in the lateral surface of the inner seal98,100) aligned with the damping chamber70low pressure region permit a flow of damping fluid from the respective lateral chamber72,74, through the inner annular groove126,128, and into the aforesaid region of the damping chamber70motivated by the difference in fluid pressure between the adjacent lateral chamber region and the damping chamber70region. As described above, in this configuration the passages140,122are in an open configuration.

The fluid damping structure68embodiment shown inFIGS. 11 and 11Aoperates in a manner similar to that described above in terms of the fluid damping structure68embodiment shown inFIGS. 6, 6A, 7, and 7A. The fluid damping structure68embodiment shown inFIGS. 11 and 11A, however, illustrates an alternative damping fluid travel path through the fluid damping structure68. As described above, in this alternative embodiment both of the first and second lateral chamber ports146,150are in fluid communication with the damping fluid source76(e.g., the engine lubrication system). In this configuration as a gas turbine engine20is operated in a start-up mode (e.g., rotor shaft low rpms), damping fluid is fed into the first lateral chamber72via the first lateral chamber fill port146, and into the second lateral chamber74via the second lateral chamber fill port150. Some amount of the damping fluid bypasses the first and second inner seals98,100, enters and fills the damping chamber70. Similar to the description above, some amount of the damping fluid will exit the lateral chambers72,74bypassing the outer seals94,96. In this configuration, some amount of damping fluid may also exit the damping chamber70via the exit port143. The damping fluid exiting the lateral chambers72,74may exit into the adjacent engine compartment where it is collected and returned to the main lubrication system via a scavenging system. The damping fluid exiting the damping chamber70via the exit port143may be passed directly to a scavenging system, or may pass into the adjacent engine compartment. The exit port143may be configured to provide an appropriate amount of flow impedance so that the damping chamber70remains filled with pressurized damping fluid during operation.

FIGS. 12 and 12Ashow another embodiment of the above described fluid damping structures68. In this embodiment, the fluid damping structure68includes a damping chamber70and a supply plenum160. The supply plenum160may be disposed on either side of the damping chamber70. The supply plenum160is configured (e.g., sufficient radial clearance and sufficient effective hydraulic diameter, etc.) to ensure that the local pressure within the supply plenum160remains above compartment pressure (i.e., the local pressure outside the fluid damping structure) under high whirl conditions. The fluid damping structure68is configured to have low fluid flow resistance for damping fluid entering the damping chamber70from the supply plenum160and high flow resistance for reverse flow from the damping chamber70back into the supply plenum160; i.e., a diodicitic configuration. In this embodiment, the fluid damping structure68includes a first outer seal162, a second outer seal164, and an inner seal166. The first outer seal162, second outer seal164, and inner seal166are spaced axially apart from one another and extend circumferentially about the axis of rotation Arof the rotor shaft60. The inner seal166is disposed axially between the first outer seal162and the second outer seal164. The damping chamber70is defined at least in part by the outer radial surface84of the bearing housing64, the inner radial surface92of the stator structure66, the inner seal166and the second outer seal164. The supply plenum160is defined at least in part by the outer radial surface84of the bearing housing64, the inner radial surface92of the stator structure66, the first outer seal162, and the inner seal166. As indicated above, the seals162,164,166may be any type of seal that is capable of providing the sealing function in the fluid damping structure68; e.g., ring type seals. As described above, the outer radial surface84of the bearing housing64may include a circumferentially extending annular groove for each of the first outer seal162, the inner seal166, and the second outer seal164; e.g., a first outer annular groove168, an inner annular groove170, and a second outer annular groove172. The inner annular groove170may be described as having a base surface232, an inner side surface234, and an outer side surface236opposite the inner side surface234(SeeFIGS. 12B and 12C). The aforesaid annular grooves168,170,172are axially spaced apart from one another, and the inner annular groove170is disposed axially between the first outer annular groove168and the second outer annular groove172. Each of the aforesaid annular grooves may be configured in the manner described above. The fluid damping structure68includes at least one fill port174in fluid communication with the supply plenum160and with a source76of damping fluid (e.g., oil from an engine lubricating system). The supply plenum160may include at least one annular plenum176. In the exemplary embodiment shown inFIGS. 12 and 12A, an annular plenum176is shown disposed in the outer radial surface84of the bearing housing64. Alternatively, the annular176plenum may be disposed in the inner radial surface92of the stator structure66, or annular plenums176may be disposed in both the inner radial surface92and the outer radial surface84. As will be described below, the annular plenum(s)176may assist in maintaining in circumferential fluid pressure uniformity within the supply plenum160.

In regards to selective fluid passage between the supply plenum160and the damping chamber, the fluid damping structure68of this embodiment may include a plurality of passages (e.g., troughs140, or apertures140A) in communication with the inner annular groove170that receives the inner seal166may, or may include a plurality of passages (e.g., troughs122) in the inner seal166, as described above (e.g., seeFIGS. 5 and 5A).

In regards to the fluid damping structure embodiment shown inFIGS. 12 and 12A, when a gas turbine engine20is operated in a start-up mode (e.g., rotor shaft low rpms), damping fluid is fed from the fill port174into the supply plenum160. Damping fluid will bypass the inner seal166, enter, and fill the damping chamber70until the circumferentially averaged fluid pressure within the damping chamber70and the supply plenum160are substantially equal.

Under normal conditions (e.g., no whirl; seeFIG. 12), the fluid pressure within the damping chamber70and the supply plenum160are substantially equal to one another, and are also substantially uniform around their respective circumference. The substantially uniform fluid pressure is diagrammatically illustrated by the pressure values P8 and P9 in different circumferential regions substantially equally one another, i.e., P8≈P9, and the fluid damping structure68will continue to operate in this mode indefinitely. In this mode, where the circumferentially averaged fluid pressure within the damping chamber70and the supply plenum160are substantially equal, the inner seal166is not relied upon for sealing between the damping chamber70and the supply plenum160. The first outer seal162and the second outer seal164, on the other hand, both function to seal and maintain fluid pressure within the respective supply plenum160and damping chamber70, albeit subject to some amount of leakage. Whatever damping fluid leakage occurs across the first and second outer seals162,164is replenished by damping fluid from the source76.

Referring toFIGS. 12A-12D, under circumstances wherein the rotor shaft60is experiencing a sufficient amount of whirl, an unsteady circumferential pressure field as described above will develop. In a high pressure region (e.g., see P10 inFIG. 12A, and seeFIG. 12B) of the circumferential pressure field within the damping chamber70, the fluid pressure within the damping chamber70will force a first lateral surface216of the inner seal166into contact with the outer side surface236of the inner annular groove170, creating a fluid and pressure seal there between; i.e., a portion of the inner seal is forced into contact with the outer side surface236of the inner annular groove170. In a low pressure region (e.g., P11; SeeFIGS. 12A, 12C, 12D) of the circumferential pressure field within the damping chamber70, in contrast, the local fluid pressure within the supply plenum160will exceed the local fluid pressure within the adjacent damping chamber70region. As a result and in that region, the inner seal166will be forced toward the inner side surface234of the inner annular groove170. If the difference in pressure is great enough, the second lateral surface218of the inner seal170will be held in contact with the inner side surface234of the inner annular groove170(e.g., as shown in the bottom ofFIG. 12Aand inFIGS. 12C and 12D); i.e., a portion of the inner seal is forced into contact with the inner side surface234of the inner annular groove170. As a result, the passages (e.g., passages140disposed within the bearing housing64(FIG. 12C), or passages122disposed in the lateral surface218of the inner seal166—FIG. 12D) aligned with the damping chamber low pressure region permit a flow of damping fluid from the supply plenum160, through the inner annular groove170, and into the aforesaid region of the damping chamber70motivated by the difference in fluid pressure between the adjacent supply plenum region and the damping chamber region. The aforesaid fluid flow locally into the damping chamber70causes a local increase in damping fluid pressure within the damping chamber70that helps prevent or eliminate the possibility of damping fluid cavitation within the low fluid pressure region, and improves the performance of the fluid damping structure68. The position of the supply plenum160contiguous with the damping chamber70helps prevent or eliminates the ingress of compartment air into the damping chamber70. The eccentric whirling of the rotor shaft60and bearing housing64makes the above described creation of high pressure regions and low pressure regions a dynamic event that can be accommodated at any circumferential position by the present fluid damping structure68; e.g., the circumferentially distributed passages122,140,140A. In this configuration, the passages140(140A,122) may be described as being in an open configuration since fluid flow through the passages140,140A,122contributes to damping fluid flow between the damping chamber70and the supply plenum160.

Some amount of damping fluid will exit the fluid damping structure68via leakage across the first outer seal162(exiting the supply plenum160) and the second outer seal164(exiting the damping chamber70). Damping fluid that exits the fluid damping structure68will pass into the adjacent engine compartment where it is collected and returned to the main lubrication system via a scavenging system. The first outer seal162and the second outer seal164are configured to provide an appropriate amount of fluid flow impedance so that leakage from the fluid damping structure68is held at an acceptable level.

The fluid damping structure68embodiment described above is configured to provide a low fluid flow resistance for damping fluid entering the damping chamber70from the supply plenum160(e.g., at circumferential regions where the local fluid pressure within the supply plenum160is equal to or greater than the local fluid pressure within the damping chamber70, damping fluid may pass through passages140,140A,122; hence relatively low resistance), and configured to provide relatively high fluid flow resistance for damping fluid flow from the damping chamber70back into the supply plenum160(e.g., at circumferential regions where the local fluid pressure within the damping chamber70is greater than the local fluid pressure within the supply plenum160, fluid passage through passages140,140A,122is not possible; hence, any fluid passage in these regions may occur, if at all, only based on seal leakage and therefore at a relatively high resistance). This diodicitic nature of the inner seal166creates a fluid damping structure68with a damping chamber70that may be described as self-pressurizing, and/or one configured to automatically correct circumferential pressure discrepancies that occur, with increasing whirl. By providing damping fluid at an elevated pressure into the supply plenum160(e.g., at a pressure greater than compartment/ambient pressure outside of the fluid damping structure68), the fluid pressure within the damping chamber70will always remain above the compartment/ambient pressure, effectively preventing any air ingestion across the damping chamber seals and into the damping chamber70.

The fluid damping structure68embodiment shown inFIGS. 12 and 12Adoes not require a check valve disposed in the damping fluid supply line. This embodiment can provide the functionality described above with or without a check valve in the supply line. A check valve can add additional cost and can be unreliable. Some embodiments of this fluid damping structure embodiment can also be implemented with a smaller axial dimension; e.g., only one supply plenum160is required, with fewer seals. The asymmetry of this fluid damping structure embodiment also improves the uniformity of the damping fluid flow pattern through the fluid damping structure68via leakage. The pressure coordination between the supply plenum160and the damping chamber70helps to avoid any fluid flow anomaly regions; e.g., regions with zero fluid flow. Some prior art squeeze film dampers are fed damping fluid directly from a supply source, and are consequently susceptible to supply pressure variations. In contrast, the present disclosure supplies damping fluid to the damping chamber70through a supply plenum160or lateral chamber. As a result, the present disclosure effectively decouples pressure oscillations that may occur within the damping chamber70from the damping fluid supply line, thereby reducing the potential and/or severity of coupled supply flow line/squeeze film damper dynamic oscillations.

While various embodiments of the present disclosure have been disclosed, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible within the scope of the present disclosure. For example, the present disclosure as described herein includes several aspects and embodiments that include particular features. Although these features may be described individually, it is within the scope of the present disclosure that some or all of these features may be combined with any one of the aspects and remain within the scope of the present disclosure. Accordingly, the present disclosure is not to be restricted except in light of the attached claims and their equivalents. In addition and as stated above, embodiments of the present disclosure are described in terms of a gas turbine engine application but are not limited to such application. Still further, the examples provided above describe that the damping fluid is provided from a lubrication system and returned via a scavenging system. The present is not limited to this type of damping fluid source and return.