BACKGROUND OF THE INVENTION Turbochargers for gasoline and diesel internal combustion engines are devices known in the art that are used for pressurizing or boosting the intake air stream, routed to a combustion chamber of the engine, by using the heat and volumetric flow of exhaust gas exiting the engine. Specifically, the exhaust gas exiting the engine is routed into a turbine housing of a turbocharger in a manner that causes an exhaust gas- driven turbine to spin within the housing. The exhaust gas-driven turbine is mounted onto one end of a shaft that is common to a radial air compressor mounted onto an opposite end of the shaft. Thus, rotary action of the turbine also causes the air compressor to spin within a compressor housing of the turbocharger that is separate from the exhaust housing. The spinning action of the air compressor causes intake air to enter the compressor housing and be pressurized or boosted a desired amount before it is mixed with fuel and combusted within the engine combustion chamber.

The common shaft extending between the turbine and compressor is disposed through a turbocharger center housing that includes a bearing assembly for: (1) facilitating shaft rotation; (2) controlling axially directed shaft thrust effects and radially directed shaft vibrations; (3) providing necessary lubrication to the rotating shaft to minimize friction effects and related wear; and (4) providing a seal between the lubricated assembly and the turbine and compressor housings. The common shaft as used in turbocharger applications is known to have shaft-rotating speeds on the order of 60,000 to 80,000 rpm. Under such operating conditions it is imperative that the bearing assembly provide sufficient lubrication to the shaft to minimize the extreme friction effects that take place at such high rotating speeds, thereby extending shaft service life.

A thrust bearing is installed in the turbocharger center housing and is used to control the amount of axially directed thrust, or thrust load, that is imposed on the turbine shaft. The thrust bearing can either be hydrodynamic or non-hydrodynamic.

As used herein, the term"hydrodynamic"is understood to refer to pumped oil migration, or diverted oil flow, across an axial face of a thrust bearing, and the term "nonhydrodynamic"is understood to refer to a type of thrust bearing that is not designed to pump oil across an axial thrust bearing face or surface.

Hydrodynamic thrust bearings known in the art comprise an annular body that is disposed within the turbocharger bearing housing, around the turbine shaft. Such bearing includes an axially-facing frontside surface that is positioned adjacent an opposed housing member surface, and that includes a number of grooves or channels that are disposed radially across the frontside surface from an inside bearing diameter to an outside bearing diameter. Each such groove is separated by a surface section or pad.

The frontside surface, comprising the grooves and pads, is designed to distribute lubricating oil thereover in the following manner. Fresh oil is pumped into an inlet end of each groove adjacent the frontside surface inside diameter, and migrates from that groove circularly over the pad and radially towards the frontside surface outside diameter. As the oil is moved over the pad it is heated by the thrust bearing surface and moved towards an adjacent groove. The heated oil enters the adjacent groove, that was also an oil supply groove for an adjacent pad, and travels radially outwardly along the groove to the frontside surface outside diameter, where the heated oil exits the thrust bearing. In this manner, oil is circulated over each thrust bearing pad, via each groove that acts in both an oil supply and oil collection capacity.

Hydrodynamic thrust bearings provides a thrust load capacity that is dependent on the operating temperature of the oil film disposed between the thrust bearing and the adjacent housing member axial surface. It is a well known fact that the thrust load capacity for such bearings is inversely proportional to the oil film temperature across the bearing. It has been discovered that the above-described hydrodynamic thrust bearings do not provide a maximum degree of thrust load capacity because of the high oil-film temperatures that are experienced across the bearing. A reduced thrust load capacity has an adverse impact on turbocharger service life as is allows undesired turbine shaft axial play that causes premature turbine bearing and seal wear.

It is, therefore, desirable that a hydrodynamic thrust bearing be constructed that has an improved thrust load capacity when compared to conventional hydrodynamic thrust bearings. It is also desired that such thrust bearing be capable increasing the thrust load capacity without adversely impacting other performance areas of the thrust bearing itself and the bearing assembly. It is further desired that such thrust bearing be capable of fitment with existing turbocharger devices without extensive redesigning.

BRIEF DESCRIPTION OF THE DRAWINGS The details and features of the present invention will be more clearly understood with respect to the detailed description and the following drawings, wherein: FIG. 1 a illustrates a schematic front end view of hydrodynamic thrust bearing constructed according to principles of this invention; FIG. lb illustrates a schematic front end view of a second embodiment of the hydrodynamic thrust bearing constructed according to principles of this invention; and FIG. 2 illustrates a cross-sectional side view across section 2-2 of the hydrodynamic thrust bearing of FIG. 1.

SUMMARY OF THE INVENTION Hydrodynamic thrust bearings, constructed according to principles of this invention, comprise an axially-directed annular surface that is specially designed to promote oil migration thereacross in a manner that minimizes oil film temperature, thus maximizing thrust load capacity. Specifically, hydrodynamic thrust bearings of this invention comprise a series arrangement of three differently configured land sections across the bearing axial surface that are separated by an oil supply groove (on one side) and an oil return or collection grove (on an opposite side). Configured in this manner, oil is provided onto the series of land sections by the oil supply groove and is collected after passing over the series of land sections by the separate oil collection groove. The use of separate oil supply and collection grooves acts to minimize oil mixing, i. e., input oil mixing with the heated return oil, thereby reducing oil film temperature and increasing bearing thrust load capacity.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1 a, a hydrodynamic thrust bearing 10 of this invention has an annular body 12 comprising an inside diameter 14 and an outside diameter 16. The inside diameter is sized to accommodate placement of a turbine shaft (not shown), or alternatively a shaft bearing member (not shown), therein. The bearing includes an axially-directed face 18 that is designed to be positioned adjacent a turbocharger bearing housing member surface (not shown) to control the extent of axial thrust bearing displacement within the housing.

The axially-directed face 18 comprises a repeating serial arrangement of three land sections 20, wherein each serial arrangement defines a thrust bearing thrust pad.

An oil supply channel 22 is disposed along the bearing inside diameter along the axially-directed face 18, and an oil return or collection channel 24 is disposed-along the bearing outside diameter along the axially directed face 18. The series arrangements of the three land sections 20 extend radially across the axially-directed face 18 between the oil supply and collection channels.

Each series arrangement of lands/thrust pad 20 comprises, moving clockwise across FIG. la, a lower land 26, a ramp 28, and an upper land 30. The lower land 26 is an arc section of the thrust bearing face 18 that extends circumferentially across the face between an oil supply groove 32 and the ramp 28. The oil supply groove 32 is disposed a determined depth within the face 18 and extends radially from the oil supply channel 22 thereacross. In one bearing embodiment, the oil supply groove 32 extends radially across the face only a partial distance and not to the oil collection channel 24 (the embodiment shown in FIG. la). In another embodiment, the oil supply groove 32 extends radially across the face a complete distance to the oil collection channel 24 (the embodiment shown in FIG. lb). The oil supply groove 32 is designed to direct supply oil from the oil supply channel 22 to the lower land 26 for distribution across the thrust pad for forming a thrust-load bearing oil film layer thereon.

The ramp 28 comprises an arc section of the face that extends circumferentially across the face between the lower and 26 and the upper land 30. As best shown in FIG. 2, the ramp 28 is configured having an upwardly directed slope moving from the lower land to the upper land. Accordingly, oil that is passed to the ramp from the lower land is compressed against an adjacent planar surface of the housing before being passed to the upper land. This oil compression is caused by the

rotary action of the thrust bearing within the housing, and is necessary for forming a desired hydrodynamic effect between the bearing and the housing surface.

The upper land 30, as the name implies, comprises an arc section of the bearing face 18 that is positioned axially above both the lower lamp 26 and the, ramp 28. The upper land 30 extends circumferentially across the face from the ramp 28 to an oil return or collection groove 34. The oil return groove 34 is disposed a'depth within the face and extends radially inwardly a partial distance thereacross fro'm the oil return channel 24. The oil return groove 24 terminates before reaching the oil supply channel 22 to prevent the fresh supply oil from entering and mixing wilithe heated return oil collected from the thrust pads. Oil entering the oil return jgra. Xe 2A is directed radially therealong to the oil return channel 24 where it is collected ! and removed from the bearing.

Thus, configured in this manner, supply oil is provided from the oil supply channel 22 to each thrust pad 20 by a single oil groove 32 adjacent each thrust pad lower land 26. As the thrust bearing is rotated, the supply oil is whisked radially) and circumferentially across each thrust pad, forming a thin hydrodynamic film layers' between the bearing face and an adjacent housing surface. Heated oil is removes, from each thrust pad and collected within the oil collection channel 24 via the oil return groove 34 positioned adjacent each thrust pad upper land 34. As thel bearing 1S' rotated, oil is continuously provided to the thrust pads, used to create a desired hydrodynamic oil film layer thereon, and is removed from the thrust pads manner.

To prevent the unwanted migration of oil between each oil supply groove and an adjacent oil return groove, a separator 36 is disposed therebetween. The. separator 36 extends radially along the bearing face 18 from the oil supply channel 22'to'the oil return or collection channel 24 and, as best shown in FIG. 2, the separator is ; in'. the form of a rib that extends above each of the bordering grooves to act as a barrier to prevent oil from moving therebetween.

The use of separate oil supply and return grooves is an important feature of this invention as it prevents the relatively cooler supply oil from mixing together with the hot return oil during turbocharger operation, i. e., thrust bearing rotation, thereby both reducing the oil film temperature across the bearing face, and improving the thrust load carrying capacity of the bearing.