CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and all the benefits of U.S. Provisional Application No. 61/978,213, filed on April 11, 2014, and entitled "Foil Thrust Bearing For Oil Free

Turbocharger," which is incorporated herein by reference.

BACKGROUND

Field of the Invention

The invention relates to a turbocharger with an improved thrust bearing and more particularly, to an improved foil thrust bearing for oil free turbochargers.

Description of Related Art

Foil thrust bearings are used in a wide variety of high speed rotor applications. Foil thrust bearings, also labeled as air or gas foil bearings, are especially known for their use in rotor bearing systems for turbochargers. The durability of foil thrust bearings under extreme temperature and speed conditions make them very attractive for the turbocharger industry. One of the main advantages of foil thrust bearings is the elimination of lubricating oil that is typically required by traditional rotor bearing systems.

Turbochargers are used in engines to deliver air to the engine intake at a greater density than would normally be possible in an aspirated engine configuration. Turbochargers include a turbine wheel that is connected to a compressor wheel by a turbocharger shaft. The turbine wheel provides rotational power to drive the compressor wheel of the turbocharger. The turbocharger shaft is supported by a bearing system within a central bearing housing disposed between a turbine housing and a compressor housing. The bearing system may include a combination of journal and thrust bearings to support radial and thrust loads imposed upon the turbocharger shaft from the rotational motion.

Turbochargers generally require the use of oil-lubricated bearing systems. The turbocharger bearing system is connected to the engine oil system where engine oil is supplied to the bearing housing of the turbocharger to lubricate the bearings. Oil is discharged from the bearing housing and returned to the engine oil system. Oil lubricated turbochargers are prone to oil seal failures where oil can be discharged into the engine exhaust system increasing undesirable engine emissions.

SUMMARY

Foil thrust bearings are self-acting hydrodynamic bearings which use ambient air as the working fluid to lubricate bearing surfaces. Typical foil thrust bearings include any number of layers of a top foil, a bump foil and oftentimes a backing plate. The bump foil usually includes one or more individually positioned corrugated, spring-like compliant pads or each corrugated, spring-like compliant pad can include a supporting element attached to a thin bearing sheet. Air film builds up as a result of viscous air flow driven by the moving member which can be a shaft in journal bearings or a thrust disk in thrust bearings. In the case of thrust bearings, with relative movement of the rotating thrust disk, the foil thrust bearing is supported by a thin fluid film generated between the thrust disk and the top foil of the foil thrust bearing. The corrugated, spring-like compliant pads act to allow the foil thrust bearing to deflect and conform, to some extent, to the rotating rotor bearing elements that may become misaligned, unbalanced and thermally/mechanically distorted.

In contrast, the durability of oil-lubricated turbochargers is heavily dependent upon the engine lubrication. At cold start and/or operation under very cold conditions, delivery of the oil to the turbocharger can be delayed. Oil-lubricated turbochargers generally employ low viscosity oils that operate under extremely high engine temperatures to aid in reducing friction that may be caused by contact of rotating rotor bearing elements. However, the use of low viscosity oil has been found to hinder the effectiveness of oil seals as well as have been found to adversely affect the turbocharger bearing system stability and durability.

An exemplary embodiment of a rotor bearing system for a rotary machine such as a turbocharger can include a foil thrust bearing assembly disposed circumferentially about a rotating turbocharger shaft which connects a turbine wheel to a compressor wheel of the turbocharger. The foil thrust bearing assembly can include a circular thrust disk having a first foil thrust bearing combination disposed on one side thereof and a second foil thrust bearing combination disposed on a second opposing side thereof. The first and second foil thrust bearing combinations may each include a circular top foil member and a circular bump foil member. The circular top foil member can include a plurality of tapered non-corrugated pad elements and the circular bump foil member can include a plurality of tapered corrugated pad elements designed to allow each of the first and second foil thrust bearing combinations to flex, move and/or bend with respect to potential deformation, warping, or distortion associated with the rotational movement of the turbocharger shaft and/or other bearing system parts connected thereto. Both the circular top foil member and the circular bump foil member may each include corresponding step surfaces intended specifically to aid in installing and positioning the foil thrust bearing assembly within the bearing housing.

A second exemplary embodiment may include a circular top foil member having a plurality of non-corrugated pad elements and a circular bump foil member having a plurality of tapered corrugated pad elements. The plurality of tapered corrugated pad elements of the circular bump foil member; however, includes a plurality of split partial-circumferential segments having a plurality of cuts/spaces formed therebetween.

A third exemplary embodiment may include a circular top foil member having a plurality of tapered non-corrugated pad elements and a circular bump foil member having a plurality of tapered corrugated pad elements. The plurality of tapered non-corrugated pad elements of the circular top foil member may include step surfaces; however, the plurality of tapered corrugated pad elements of the circular bump foil member does not include step surfaces. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a partial side view of a rotor bearing system with a foil thrust bearing assembly in a first configuration.

Figure 2 is a partial side view of a rotor bearing system with the foil thrust bearing assembly in a second configuration.

Figure 3 is an enlarged side cross-sectional view of the foil thrust bearing assembly of

Figures 1 and 2.

Figure 4 is an exploded perspective view of a circular top foil member and circular bump foil member of the foil thrust bearing assembly.

Figure 5 is an enlarged partial perspective view of a first embodiment of the circular bump foil member of the foil thrust bearing assembly.

Figure 6 is an enlarged partial perspective view of a first embodiment of the circular top foil member of the foil thrust bearing assembly.

Figure 7 is an enlarged partial perspective view of a second embodiment of a circular bump foil member of the foil thrust bearing assembly. Figure 8 is an enlarged cross-sectional view of the circular top foil and circular bump foil members of the first and second embodiments assembled in an exemplary configuration.

Figure 9 is an enlarged cross-sectional view of the circular top foil and circular bump foil members of the first and second embodiments assembled in an alternate configuration.

Figure 10 is an enlarged partial perspective view of a third embodiment of a circular bump foil member of the foil thrust bearing assembly.

Figure 11 is an enlarged partial perspective view of a third embodiment of a circular top foil member of the foil thrust bearing assembly.

Figure 12 is an enlarged cross-sectional view of the circular top foil and circular bump foil members of the third embodiment assembled in an exemplary configuration.

DETAILED DESCRIPTION

Referring to Figures 1 and 2, a rotor bearing system 10 for an oil-free turbocharger (not shown) includes a foil thrust bearing assembly 12 which provides axial load support to a turbocharger shaft 14. The foil thrust bearing assembly 12 includes a circular thrust disk 16 having a first 18a and a second 18b foil thrust bearing combination disposed on each side thereof. The turbocharger shaft 14 connects the turbine wheel 20 and the compressor wheel 22 for rotational movement. The foil thrust bearing assembly 12 is disposed circumferentially about the turbocharger shaft 14. The rotor bearing system 10 further includes first 24a and second 24b foil journal bearing assemblies for reducing friction of rotating parts of the turbocharger by supporting radial loads.

The foil thrust bearing assembly 12 is capable of being arranged in a variety of positions with respect to the first 24a and second 24b foil journal bearing assemblies. For instance, Figure 1 details the foil thrust bearing assembly 12 positioned between the second foil journal bearing assembly 24b and the compressor wheel 22. However, the foil thrust bearing assembly 12 can also be positioned between the first 24a and second 24b foil journal bearing assemblies, as shown in Figure 2. In either instance, the foil thrust bearing assembly 12 provides enhanced load capacity and damping capabilities as well as provides improved thermal management to aid in avoiding bearing seizure and thermal instability.

In reference to Figures 3 and 4, the first 18a and second 18b foil thrust bearing combinations each include circular top foil members 26a, 26b and circular bump foil members 28a, 28b, respectively. The circular top foil member 26a of the first foil thrust bearing combination 18a is disposed adjacent a first side 16a of the circular thrust disk 16 and the circular top foil member 26b of the second foil thrust bearing combination 18b is disposed adjacent a second side 16b of the circular thrust disk 16. When the turbocharger is not rotating, circular top foil members 26a, 26b are spaced from the circular thrust disk 16 such that a clearance 30 is formed therebetween. Circular bump foil members 28a, 28b are positioned adjacent a first bearing housing portion 32a and a second bearing housing portion 32b, respectively. Each circular bump foil member 28a, 28b includes a plurality of tapered corrugated pad elements 34 (see figure 4) having a plurality of lands 36a, 36b and grooves 38a, 38b. Grooves 38a, 38b are positioned adjacent the circular top foil members 26a, 26b, and the lands 36a, 36b are positioned adjacent the first bearing housing portion 32a and a second bearing housing portion 32b, respectively. The lands 36a, 36b and grooves 38a, 38b are arranged at a predetermined angle ( i) with respect to the outer diameter 44 of the circular bump foil members 28a, 28b and at a predetermined angle ( 2) with respect to the lands 36a, 36b and grooves 38a, 38b of an adjacent tapered corrugated pad element 34. The lands 36a, 36b and grooves 38a, 38b of the tapered corrugated pad elements 34 provide variable spring stiffness to the circular bump foil members 28a, 28b in both the axial and radial directions. The tapered corrugated pad elements 34 bias the first 18a and second 18b foil thrust bearing combinations away from the first 16a and second 16b sides of the circular thrust disk 16 to help prevent contact therebetween when the turbocharger is not rotating or is rotating at very low speeds.

In further reference to Figures 3 and 4, each circular bump foil member 28a, 28b includes a first outer ring portion 40 having an inner diameter 42 and an outer diameter 44. Openings 46 are formed in the outer diameter 44 of the first outer ring portion 40 for receipt of pins (not shown) to affix the foil thrust bearing assembly 12 to the first bearing housing portion 32a and second bearing housing portion 32b. The circular bump foil members 28a, 28b may include any number of tapered corrugated pad elements 34; however, 3-8 tapered corrugated pad elements 34 have been found to work well. The plurality of tapered corrugated pad elements 34 is spaced circumferentially about and is connected to the inner diameter 42 of the first outer ring portion 40 by first support members 48. First support members 48 include a step surface 50. Step surface 50 extends away from the first outer ring portion 40 in a direction towards the first bearing housing portion 32a and/or second bearing housing portion 32b. First support members 48 are disposed a predetermined distance/height slightly out of plane with respect to the first outer ring portion 40. Consequently, the plurality of tapered corrugated pad elements 34 is also disposed at a predetermined distance/height slightly out of plane with respect to the first outer ring portion 40. The plurality of tapered corrugated pad elements 34, being out of plane with respect to the first outer ring portion 40, extend from the step surface 50 and are positioned adjacent to the first bearing housing portion 32a and/or second bearing housing portion 32b. Step surface 50 aids in positioning the circular bump foil members 28a, 28b with respect to the circular top foil members 26a, 26b and also assists with positioning the foil thrust bearing assembly 12 within the first 32a and second 32b bearing housing portions.

Circular top foil members 26a, 26b include a second outer ring portion 52 that correlates with the first outer ring portion 40 of the circular bump foil members 28a, 28b. Second outer ring portion 52 includes an inner diameter 54 and an outer diameter 56. Openings 58 are formed in the outer diameter 56 of the second outer ring portion 52 for receipt of pins (not shown) to affix the foil thrust bearing assembly 12 to the circular bump foil members 28a, 28b and to the first bearing housing portion 32a and a second bearing housing portion 32b. Circular top foil members 26a, 26b include a plurality of tapered non-corrugated pad elements 60 that are spaced circumferentially about and connected to the inner diameter 54 of the second outer ring portion 52 by second support members 62. The circular top foil members 26a, 26b may include any number of tapered non-corrugated pad elements 60; however, 3-8 tapered non-corrugated pad elements 60 have been found to work well. Second support members 62 include a step surface 64. Step surface 64 extends from the second outer ring portion 52 in a direction away from the first 16a and/or second 16b sides of the circular thrust disk 16. As such, second support members 62 are disposed a predetermined distance/height slightly out of plane with respect to the second outer ring portion 52. The plurality of tapered non-corrugated pad elements 60 is located in the same plane as the outer ring portion 52. Second support members 62, being out of plane with respect to the outer ring portion 52, are positioned adjacent to the first support members 48. Step surface 64 is designed to aid in positioning the circular top foil members 26a,

26b with respect to the circular bump foil members 28a, 28b and to help with positioning the foil bearing assembly 12 within the first 32a and second 32b bearing housing portions. An area 66 of the circular top foil members 26a, 26b, outlined in Figure 4 with dotted lines, define the surface of the circular top foil members 26a, 26b that mate with and contact the first 16a and second 16b sides of the circular thrust disk 16 when the turbocharger is rotating at high speeds. First and second outer ring portions 40 and 52 of the circular bump foil members 28a, 28b and circular top foil members 26a, 26b, respectively, are not bearing surfaces, but are required for installation of the foil thrust bearing assembly 12 within the first bearing housing portion 32a and a second bearing housing portion 32b. Referring to Figure 5, the plurality of tapered corrugated pad elements 34 are supported on the first outer ring portion 40 of the circular bump foil members 28a, 28b by first support members 48. The plurality of tapered corrugated pad elements 34 has a wider base 68 and tapers radially inwardly towards a smaller inner portion 70. Smaller inner portion 70 is positioned adjacent the turbocharger shaft 14. By virtue of the tapered shape of the plurality of tapered corrugated pad elements 34, first support members 48 also taper from the wider base 68 of the plurality of tapered corrugated pad elements 34 radially inward towards the smaller inner portion 70. First support members 48 are connected to the first outer ring portion 40 by step surface 50 which provide the plurality of tapered corrugated pad elements 34 with the increased structural strength, flexibility, deformation, deflection and resistance necessary to have a controlled response with respect to movement of the rotating parts of the rotor bearing system 10. The ability of the plurality of tapered corrugated pad elements 34 to have a controlled response with respect to movement of the rotating parts of the rotor bearing system 10 minimizes the loss of load capacity and increases load damping by providing variable spring stiffness in both the axial and radial directions.

In reference to Figure 6, the plurality of tapered non-corrugated pad elements 60 are supported on the second outer ring portion 52 of the circular top foil members 26a, 26b by second support members 62. The plurality of tapered non-corrugated pad elements 60 has a wider base 72 and tapers radially inwardly towards a smaller inner portion 74. Smaller inner portion 74 is positioned adjacent the turbocharger shaft 14. Due to the tapered shape of the plurality of tapered non-corrugated pad elements 60, second support members 62 also taper inwardly from the wider base 72 of the plurality of tapered non-corrugated pad elements 60 radially inward towards the smaller inner portion 74. Second support members 62 are connected to the second outer ring portion 52 by step surfaces 64a and to the plurality of non-corrugated pad elements 60 by other step surfaces 64b. Step surfaces 64a, 64b provide the plurality of tapered non-corrugated pad elements 60 with the increased structural strength, flexibility, deformation, deflection and resistance necessary to have a controlled response with respect to movement of the rotating parts of the rotor bearing system 10. The ability of the plurality of tapered non-corrugated pad elements 60 to have a controlled response with respect to movement of the rotating parts of the rotor bearing system 10 minimizes the loss of load capacity and increases load damping by providing variable spring stiffness in both the axial and radial directions. Referring to Figure 7, a second embodiment of the circular bump foil member 128 includes a plurality of tapered corrugated pad elements 134 having a plurality of split partial- circumferential segments 134a-134d. Each split partial-circumferential segment 134a-134d includes a plurality of cuts/spaces 135 formed therebetween. The plurality of tapered corrugated pad elements 134 are spaced circumferentially about and connected to an inner diameter 142 of a first outer ring portion 140 of the circular bump foil member 128. Circular bump foil members 128 may include any number of tapered corrugated pad elements 134 and any number of split partial-circumferential segments 134a-134d; however, 3-8 tapered corrugated pad elements 134 and 3-6 split partial-circumferential segments 134a-134d have been found to work well. The split partial-circumferential segment 134a-134d configuration allows each individual split partial-circumferential segment 134a-134d to perform independently of the other when responding/ reacting to movement of the rotating portions of the rotor bearing system 10. The ability of the plurality of split partial-circumferential segments 134a-134d to act independently of one another provides for a more defined, concentrated and controlled response of compliance radially about the circumference of the turbocharger shaft 14. The plurality of split partial- circumferential segments 134a-134d increases the load capacity and are also designed to significantly reduce thermal instability generated by hot spots by being able to accommodate edge leakage, thermal gradient and heat generation of the localized effects of fluid film pressure.

Each circular bump foil member 128 further includes a plurality of lands 136 and grooves 138. Grooves 138 are positioned adjacent the circular top foil members 126 and the lands 136 are positioned adjacent the first bearing housing portion 32a and the second bearing housing portion 32b. The lands 136 and grooves 138 of the tapered corrugated pad elements 134 are oriented radially and provide variable spring stiffness to the circular bump foil members 128 in both the radial and axial directions. The lands 136 and grooves 138 are arranged at a predetermined angle ( 3) with respect to the outer diameter 144 of the circular bump foil members 128 and at a predetermined angle (not shown) with respect to the lands 136 and grooves 138 of an adjacent tapered corrugated pad element 134.

The plurality of split circumferential segments 134a-134d decrease in length as the plurality of split circumferential segments 134a-134d taper radially inwardly from a wider base 168 towards a smaller inner portion 170. Shorter split circumferential segments 134a are positioned closest to the turbocharger shaft 14, while longer split circumferential segments 134d are positioned closest to the first outer ring portion 140. By virtue of the tapered shape of the plurality of tapered corrugated pad elements 134, first support members 148 also taper from the wider base 168 of the plurality of tapered corrugated pad elements 134 radially inward towards the smaller inner portion 170. A step surface 150 connects the support member 148 to the first outer ring portion 140. Step surface 150 extends away from the first outer ring portion 140 in a direction towards the first bearing housing portion 32a and/or second bearing housing portion 32b. First support members 148 are disposed a predetermined distance/height slightly out of plane with respect to the first outer ring portion 140. Thus, the plurality of split circumferential segments 134a-134d of the plurality of tapered corrugated pad elements 134 is also disposed at a predetermined distance/height slightly out of plane with respect to the first outer ring portion 140. The plurality of tapered corrugated pad elements 134, being out of plane with respect to the first outer ring portion 140, extend from the step surface 150 and are positioned adjacent to the first bearing housing portion 32a and/or second bearing housing portion 32b (shown in Figure 3). The circular bump foil members 128 of the second embodiment can be assembled with the circular top foil member 26a, 26b of the first embodiment (shown in Figure 6). The step surface 150 of the circular bump foil members 128 aids in positioning the circular bump foil members 128 with respect to the circular top foil members 26a, 26b and also assists with positioning the foil thrust bearing assembly 12 within the first 32a and second 32b bearing housing portions. Openings 146 are formed in an outer diameter 144 of the first outer ring portion 140 for receipt of pins (not shown) to affix the foil thrust bearing assembly 12 to the first bearing housing portion 32a and second bearing housing portion 32b (shown in Figure 3).

Referring to Figure 8, circular top foil members 26a, 26b and circular bump foil members 28a, 28b of the first embodiment and circular top foil members 126 and circular bump foil members 128 of the second embodiment are positioned with respect to one another. Step surface 64 of the circular top foil members 26a, 26b and 126, respectively of the first and second embodiments are positioned such that step surfaces 50, 150 of the circular bump foil members 28a, 28b and 128 align with the step surface 64 of the circular top foil members 26a, 26b and 126. As step surfaces 50, 150 of the circular bump foil members 28a, 28b and 128 align with the step surface 64 of the circular top foil members 26a, 26b and 126, second support members 62 of circular top foil members 26a, 26b and 126 are seated within first support members 48, 148, respectively of the circular bump foil members 28a, 28b and 128 of the first and second embodiments and assist with ensuring that the circular top foil members 26a, 26b and 126 are maintained in place with respect to circular bump foil members 28a, 28b and 128. Alignment of the step surface 64 of the circular top foil members 26a, 26b and 126 and the step surfaces 50, 150 of the circular bump foil members 28a, 28b and 128 also aids with positioning the foil thrust bearing assembly 12 within the first bearing housing portion 32a and second bearing housing portion 32b (shown in Figure 3). In this arrangement, grooves 38a, 38b and 138 of respective first and second embodiments are positioned adjacent the non-corrugated pads 60 of the circular top foil members 26a, 26b and 126. Lands 36a, 36b and 136 of respective first and second embodiments are spaced slightly from the non-corrugated pads 60 of the circular top foil members 26a, 26b and 126. The small space between the lands 36a, 36b and 136 of respective first and second embodiments and the non-corrugated pads 60 of the circular top foil members 26a, 26b and 126 allows for the circular top foil members 26a, 26b and 126 and the circular bump foil members 28a, 28b and 128 to move together in response to the expansion and/or contraction of the grooves 38a, 38b and 138 with respect to the potential deformation, warping, or distortion associated with the rotational movement of the turbocharger shaft 14, thrust disk 16 and/or other bearing system parts connected thereto. The pitch of the grooves 38a, 38b and 138 of the circular bump foil members 28a, 28b, 128 can be varied to adjust and/or modify the degree of compliance control with respect to the amount of movement and/or compliance required.

In reference to Figure 9, the circular top foil members 26a, 26b and 126 of the first and second embodiments, respectively, are flipped, with regards to the arrangement detailed in Figure 8, in order to increase the frictional damping of the rotor bearing system 10. In this arrangement, step surface 64 of the circular top foil members 26a, 26b and 126 and the step surfaces 50, 150 of the circular bump foil members 28a, 28b and 128 are not aligned with one another. Here, the circular top foil members 26a, 26b and 126 and the circular bump foil members 28a, 28b and 128 do not move together. Instead, in this alternate arrangement, the circular top foil members 26a, 26b and 126 and the circular bump foil members 28a, 28b and 128 move in opposite directions when loads are applied on the circular top foil members 26a, 26b and 126. More friction is generated due to the ability to have an increased relative movement between the circular top foil members 26a, 26b and 126 and the circular bump foil members 28a, 28b and 128. The increased relative movement between the circular top foil members 26a, 26b and 126 and the circular bump foil members 28a, 28b and 128 increases coulomb damping or the energy absorbed from the sliding friction. Therefore, the effectiveness of the rotor bearing system 10 is enhanced. Regarding Figure 10, a third embodiment of the circular bump foil member 228 includes a plurality of tapered corrugated pad elements 234 that are similar to the plurality of tapered corrugated pad elements 34 detailed in Figure 5 except the plurality of tapered corrugated pad elements 234 are not connected to a first outer ring portion 240 of the circular bump foil member 228 by a "supporting member". Rather, the plurality of tapered corrugated pad elements 234 are directly connected to the first outer ring portion 240 of the circular bump foil member 228 and does not include the use of a "step surface". First support members 248, instead, are connected at an end 250 of the plurality of tapered corrugated pad elements 234. Even though first support members 248, in this embodiment, are located at the end 250 of the plurality of tapered corrugated pad elements 234; the first support members 248 still function to provide the increased structural strength, flexibility, deformation, deflection and resistance necessary to have a controlled response of the plurality of tapered corrugated pad elements 234 with respect to movement of the rotating parts of the rotor bearing system 10. Each circular bump foil member 228 includes a plurality of lands 236 and grooves 238.

The lands 236 and grooves 238 of the plurality of tapered corrugated pad elements 234 are oriented radially and provide variable spring stiffness to the circular bump foil members 228 in both the radial and axial directions. The plurality of tapered corrugated pad elements 234 is spaced circumferentially about and connected directly to the inner diameter 242 of the first outer ring portion 240. The lands 236 and grooves 238 are arranged at a predetermined angle (not shown) with respect to the outer diameter (not shown) of the circular bump foil members 228 and at a predetermined angle (not shown) with respect to the lands 236 and grooves 238 of an adjacent tapered corrugated pad element 234. The plurality of tapered corrugated pad elements 234 taper radially inwardly from a wider base 268 towards a smaller inner portion 270 and is disposed in the same plane as the first outer ring portion 240. First support member 248 tapers as well from the wider base 268 towards the smaller inner portion 270. First support members 248 extend from the end 250 of the plurality of tapered corrugated pad elements 234 and are disposed a predetermined distance/height slightly out of plane with respect to the first outer ring portion 240. As such, first support members 248 extends away from the first outer ring portion 240 in a direction towards the first bearing housing portion 32a and/or second bearing housing portion 32b (shown in Figure 3), while the plurality of tapered corrugated pad elements 234 is positioned adjacent a circular top foil member 226 (shown in Figure 11). Circular bump foil members 228 may include any number of tapered corrugated pad elements 234; however, 3-8 tapered corrugated pad elements 234 have been found to work well. Openings (not shown) are formed in the outer diameter (not shown) of the first outer ring portion 240 for receipt of pins (not shown) to affix the foil thrust bearing assembly 12 to the first bearing housing portion 32a and second bearing housing portion 32b.

Regarding Figure 11 , circular top foil member 226 of the third embodiment includes a plurality of tapered non-corrugated pad elements 260 that are similar to the plurality of tapered non-corrugated pad elements 60 detailed in Figure 6 except the plurality of tapered non- corrugated pad elements 260 are not connected to a second outer ring portion 252 of the circular top foil member 226 by a "supporting member". Rather, the plurality of tapered non-corrugated pad elements 260 are directly connected to the second outer ring portion 252. In this instance, a step surface 264 extends from an end 276 of the plurality of non-corrugated pad elements 260 and second support members 262 extend therefrom. Step surface 264 extends from the second outer ring portion 252 in a direction away from the first 16a and/or second 16b sides of the circular thrust disk 16. Thus, the second support members 262 are disposed a predetermined distance/height slightly out of plane with respect to the second outer ring portion 252. The plurality of tapered non-corrugated pad elements 260 is; however, disposed in the same plane and at the same height as the second outer ring portion 252. Second support members 262 also function to provide the increased structural strength, flexibility, deformation, deflection and resistance necessary to have a controlled response of the plurality of tapered non-corrugated pad elements 260 with respect to movement of the rotating parts of the rotor bearing assembly 12.

The tapered non-corrugated pad elements 260 taper radially inwardly from a wider base 272 towards a smaller inner portion 274. Second support member 262 tapers as well from the wider base 272 towards the smaller inner portion 274. The circular top foil member 226 may include any number of tapered non-corrugated pad elements 260; however, 3-8 tapered non- corrugated pad elements 260 have been found to work well. Step surface 264, aids in positioning the circular top foil member 226 with respect to the circular bump foil member 228 and to help with positioning the foil bearing assembly 12 within the first 32a and second 32b bearing housing portions (shown in Figure 3). Openings 258 are formed in an outer diameter 244 of the first outer ring portion 240 for receipt of pins (not shown) to affix the foil thrust bearing assembly 12 to the first bearing housing portion 32a and second bearing housing portion 32b. First and second outer ring portions 240 and 252 of Figures 8 and 9, respectively, are not bearing surfaces, but are required for installation of the foil thrust bearing assembly 12 within the first bearing housing portion 32a and a second bearing housing portion 32b.

Referring to Figure 12, circular top foil member 226 and circular bump foil member 228 of the third embodiment are arranged with respect to one another. Step surfaces 264 of the circular top foil member 226 are positioned adjacent to the lands 238 of the circular bump foil member 228. As step surfaces 264 of the circular top foil member 226 align with the lands 238 of the circular bump foil member 228, support members 262 of the circular top foil member 226 become aligned with and are seated upon support members 248 of the circular bump foil member 228. Alignment of the support members 262 of the circular top foil member 226 with the support members 248 of the circular bump foil member 228 assist with ensuring that the circular top foil member 226 is maintained in place with respect to circular bump foil member 228 and further aids with positioning the foil thrust bearing assembly 12 within the first bearing housing portion 32a and second bearing housing portion 32b (shown in Figure 3). Grooves 238 are positioned adjacent the non-corrugated pads 260 of the circular top foil member 226 and lands 236 are spaced slightly from the non-corrugated pads 260. The lands 236 and grooves 238 of circular bump foil member 228 the circular top foil member 226 and the circular bump foil member 228 to move together in by expanding and/or contracting in response to deformation, warping, or distortion associated with the rotational movement of the turbocharger shaft 14, thrust disk 16 and/or other bearing system parts connected thereto. The pitch of the grooves 238 of the circular bump foil member 228 can be varied to adjust and/or modify the degree of compliance control with respect to the amount of movement and/or compliance required.

When the oil-free turbocharger (not shown) is at rest or operating at very low speeds, the foil thrust bearing assembly 12 rests against the turbocharger shaft 14 and there is not clearance. During full operation, where rotation of the turbocharger shaft 14 is able to generate pressure sufficient for turbocharger operation, the foil thrust bearing assembly 12 is pushed away from the turbocharger shaft 14 resulting in the formation of a very small clearance therebetween. Air film, in turn, builds up as a result of viscous air flow driven by the thrust disk 16 of the foil thrust bearing assembly 12. Relative movement of the rotating thrust disk 16 over the circular top foil members 26a, 26b, 126 and 226 is supported by a thin fluid film of compressed air drawn into the plurality of tapered corrugated pad elements 34, 134 and 234 of the circular bump foil members 28a, 28b, 128 and 228 and generated between the thrust disk 16 and the circular top foil members 26a, 26b, 126 and 226. As the foil thrust bearing assembly 12 is pushed away from the turbocharger shaft 14 a load is applied to the plurality of tapered non-corrugated pad elements 60, 260 of the circular top foil members 26a, 26b, 126 and 226 causing the plurality of tapered non-corrugated pad elements 60, 260 of the circular top foil members 26a, 26b, 126 and 226 to make contact with the plurality of tapered corrugated pad elements 34, 134 and 234 of the circular bump foil members 28a, 28b, 128 and 228 thereby creating friction. The plurality of tapered corrugated pad elements 34, 134 and 234 of the circular bump foil members 28a, 28b, 128 and 228 being spring-like compliant pads, in response, deform axially and undergo a degree of axial and radial displacement/elastic deformation to achieve the desired stiffness and damping characteristics required to stabilize any contact, clearances or spaces between the foil thrust bearing assembly 12 and the turbocharger shaft 14. As a result, the hydrodynamic effects created allow the thrust disk 16 speeds to increase significantly and not be slowed due to friction, removing the rotating members from physical contact, reducing wear, optimizing load capacity and increasing efficiency.

The circular top foil members 26a, 26b, 126 and 226 and the circular bump foil members 28a, 28b, 128 and 228 can be spot welded or joined by another similar method to form a single bearing piece. Circular top foil members 26a, 26b, 126 and 226 and the circular bump foil members 28a, 28b, 128 and 228 can be formed from materials such as stainless steel, steel, copper, Inconel and other similar metals. In some instances, the top surface of circular top foil members 26a, 26b, 126 and 226 or the area 66 of circular top foil members 26a, 26b, 126 and 226 that mates with the thrust disk 16 can be coated with a solid lubricating coating such as a polyimide, graphite, polytetrafluoroethylene (PTFE), or molybdenum (MoS2). Additionally, the back surface of circular top foil members 26a, 26b, 126 and 226 and the front and back surfaces of the circular bump foil members 28a, 28b, 128 and 228 can be sputter coated with copper to enhance frictional damping and thermal conduction. These types of lubricant coatings provide additional lubrication until the turbocharger shaft 14 comes up to speed and is fully supported by the fluid film.

In other instances, a viscoelastic damping layer may also be coated or inserted between the circular top foil members 26a, 26b, 126 and 226 and the circular bump foil members 28a, 28b, 128 and 228 and/or between the circular bump foil members 28a, 28b, 128 and 228 and the bearing housing 32a, 32b to increase the damping capabilities of the foil thrust bearing assembly 12. Also, a shim foil can be inserted underneath the circular bump foils members 28a, 28b, 128 and 228 to provide a mechanical pre-load and/or to control the bearing clearance.

Although particular preferred embodiments of the invention have been disclosed in detail for illustrative purposes, it will be recognized that variations or modifications of the disclosed apparatus, including the rearrangement of parts, lie within the scope of the present invention.