This invention relates to a compliant hydrodynamic fluid film bearing and, in particular, to a foil bearing assembly having an increased load carrying capability.
As is well known, compliant hydrodynamic fluid film bearings are old in the art and are typically used in high speed, low load carrying applications. As disclosed in U.S. Pat. No. 4,300,806, some of the newer bearings utilize a pair of superimposed foil strips that are secured by a common joint in registration at one end to the inside wall of the bearing sleeve. The trailing end of the two piece sandwich is then circumferentially wrapped about the shaft. The foil unit includes an underlying resilient support foil, sometimes referred to as a bump foil, having a wave-like configuration with the peaks and valleys of the waves being aligned with the axis of the shaft. Overlying the bump foil is a flat top foil that serves as a flexible interface between the bump foil and the outer periphery of the rotor shaft. Rotation of the shaft produces a wedge shaped buildup of air between the top foil and the shaft. This, in turn, produces an increase in the hydrodynamic pressure of the entrained air that lifts the shaft away from the compliant foil sandwich thus allowing the shaft to turn on a cushion of air.
In U.S. Pat. No. 4,274,683 there is disclosed a compliant hydrodynamic bearing assembly wherein three parallel strips of bump foil are positioned beneath a single strip of top foil. In this arrangement, the two outboard strips of bump foil are made more compliant than the center strip thus providing the bearing greater freedom to deflect in conformity with a misaligned rotor. This arrangement also increases the bearing's load handling capacity in the misaligned situation. However, the use of a single strip of top foil gives rise to whirl problems when the axial length of the bearing exceeds some predeterminable dimension. It should be further noticed that each of the foil elements are attached to the bearing sleeve along a common axial joint. Load direction that moves the shaft in the direction of the joint should be avoided since load capability in this direction is reduced.
In many applications where a hydrodynamic fluid film bearing might be extremely attractive, high load carrying capabilities are required along with relatively whirl free performance. However, these two bearing characteristics are generally diametrically opposed when considering foil bearings. Any increase in the bearing length that is required for handling higher loads produces a corresponding decrease in the bearing's whirl stability. It should be also noted that the bearing's ability to accommodate misaligned shafts also decreases with the bearing length. Increasing the length of the bearing further gives rise to temperature related problems and/or failures in that the longer bearings are prone to generate and retain a substantial amount of heat. Accordingly, foil bearings have not heretofore found wide applications where high load carrying capability is a primary consideration.