This invention relates to a bearing structure for machinery which operates at high rotational speeds.
It has been difficult to design satisfactory bearing systems for high-speed rotating systems. One particular example is in bearing systems for turbochargers which have caused difficulties due to the very high speed of rotation of the rotor assembly, combined with the extreme light weight of the rotor components. "Shaft whirl" or "Shaft whip" has always been a specific problem that needed to be overcome before the turbocharger as used today could attain sufficient durability to become practical and usable on internal combustion engines. In addition, the turbocharger must be an extremely low cost device so that it becomes economically feasible for use on high volume diesel and gasoline engines such as are used in motor vehicles, farm tractors, etc.
Devising a stable bearing system that is simple in construction, rugged enough to withstand rough handling and easily serviced in the field in case of trouble has been the object of much time and effort over the past years. A number of bearing systems of this general type have been and are currently being used in turbochargers. Some of these systems are disclosed in U.S. Pat. Nos. 3,096,126; 3,056,634; and 3,390,926.
It has been found that providing a floating sleeve bearing of substantially heavy wall thickness allows the shaft to rotate very stably at high speed. In such a situation, the outer oil film between the bearing and the stationary housing is of much larger area than the inner film between the bearing and the rotor shaft, thereby resulting in the frictional drag being greater between the bearing and stationary housing than between the shaft and bearing. Thus, the bearing rotates at a speed much less than the speed of the shaft. The two fluid films provide a cushion for unbalance forces as well as a cushion against an external force acting on the housing. The total clearance existing by virtue of the two fluid films gives the rotor greater freedom to find and rotate about its center of mass leading to stable operation at the extreme high speeds at which turbocharger-rotating assemblies must run.
According to the present invention, a thrust and journal bearing assembly is provided which comprises two sleeve bearings located within a housing and disposed between rotatable members. Annular bearing surfaces are provided by the housing for engaging the inner end faces of the sleeve bearings.
The bearing assembly of the present invention results in a less costly combined journal and thrust bearing in which the stability of the bearing system is maintained. The improved bearing assembly also provides both radial and longitudinal bearing support for the rotating assembly whereby the reduced speed of rotation of the bearings relative to the rotor provides a reduced relative speed between the rotating surfaces of the rotor and the mating thrust surfaces on the bearing. This reduced relative speed leads to reduced losses compared to existing systems where the thrust load of the rotating assembly is carried on a stationary surface attached to the stationary housing.
In the improved assembly oil under pressure is introduced through passages to the outer-bearing surfaces and provides lubrication for this surface. In addition, the oil is carried through the sleeve bearings to feed the thrust surfaces on one or both ends of the sleeve bearing.
Preferably, each bearing includes a plurality of radial holes extending therethrough for introducing the lubricating oil to the inner-bearing surfaces. In order to provide an adequate oil supply to the thrust surfaces on the ends of the bearing, each bearing preferably includes axial holes extending therethrough and communicating with the radial holes carrying the pressurized oil. Thus, a direct supply of cool oil is introduced to all four thrust surfaces. Where reference is made to axial holes throughout the specification and claims, it is not intended to be limited to holes along or parallel to the axis of the bearing but is intended as well to mean holes or grooves extending in an non-axial direction from a radial hole to an end face of the bearing.
The thrust or end face surfaces of the bearings may be of a variety of types; for example, they may carry radial grooves desirably of V-shaped cross-section wherein the sides of each groove form an included angle of approximately 150.degree.. The axial oil holes extending through the bearing may communicate with some or all of the end face grooves. Alternately, however, the axial holes through the bearings may communicate with the portions of the end face disposed between the radial grooves. In the latter construction, at least part of the surface portion between each pair of radial grooves may be inclined to the axis of the bearing thus providing a tapered wedge-shaped thrust bearing surface. If desired, the axial holes may be positioned so as to communicate with the tapered wedge thrust-bearing surface at or near the area of maximum clearance so that the oil provides a thrust-bearing surface with maximum load-carrying capacity.
As a further alternative construction, axial grooves might be provided in the outer and inner journal surfaces of the bearing for the purposes of carrying oil to the thrust-bearing surfaces. This latter construction, however, reduces the area of the bearing surfaces. Using axial holes through the bearings allows oil to be fed to all thrust-bearing surfaces without any loss in the journal-bearing area and consequently without any loss in the load-carrying capacity of a given size bearing.
Although the present invention is not limited to particular rotational machinery, it is especially suitable for use in turbochargers.