It has become increasingly common to design turbomachinery, such as gas turbine engines, with concentrically mounted yet independently rotatable shafts. Because of the normal constraints on such concentric shafts, they are usually long and relatively flexible and subject to bending deflections at critical speeds. Examples of such turbomachinery, commonly referred to as two spool, can be found in U.S. Pats. Nos. 3,357,176, 3,680,309, and 3,589,132.
As a result of the shaft configurations mentioned above, support and/or bearing problems are frequently encountered with respect to these independently rotatable concentric shafts. Operating near or passing through critical shaft speeds creates considerable problems in shaft dynamics. Bending criticals, which are usually more severe than rigid body criticals, result in shaft eccentricities promoting unbalanced forces which, in turn, promote additional eccentricities.
Methods typically employed to resolve the above problems may include increasing shaft strength, decreasing bearing span, reducing operating speed range, dynamic "trim" balancing, and using a rolling contact midspan bearing and/or soft mounted end bearings. Each of these above methods imposes severe limitations upon the performance of the turbomachinery. For example, the limited space between the concentrically arranged, independently rotatable shafts makes it extremely difficult to provide a rolling contact bearing therebetween. In addition, it is difficult to provide lubrication to such rolling contact bearings in this location. Reference is made to U.S. Pat. No. 3,756,672 as an example of a soft mounted bearing directed toward a possible solution to the above-identified problem.
Fluid or gas bearings have recently been utilized in many applications for high speed turbomachinery. These fluid bearings generally comprise two relatively movable elements with a predetermined spacing therebetween. The spacing is filled with a fluid such as air, which, under dynamic conditions, forms a supporting wedge to provide sufficient support for a load by preventing contact between the two relatively movable elements. More recently, improved fluid bearings, particularly gas bearings of the hydrodynamic type, have been developed by providing foils in the space between the relatively rotatable bearing elements. Such foils, which are generally thin sheets of a generally compliant material, may be deflected by the hydrodynamic film forces between the adjacent bearing surfaces and the foils. The foils enhance the hydrodynamic characteristics of the fluid bearings and also provide improved operation under extreme load conditions when normal bearing failure might otherwise occur. Additionally, foil segments provide the added advantage of accommodating eccentricity of the relatively movable elements and further provide a cushioning and damping effect.
The ready availability of relatively clean process fluid, plant air or ambient atmosphere as the bearing fluid makes these hydrodynamic, fluid film lubricated, bearings particularly attractive for high speed turbomachinery. While in many applications these hydrodynamic or self-acting fluid bearings provide sufficient load carrying capacity from the pressure generated in the fluid film by the relative motion of the two converging surfaces, it is sometimes necessary to externally pressurize the fluid between the bearing surfaces to increase the load carrying capacity. While these externally pressurized or hydrostatic fluid bearings do increase the load carrying capacity they do introduce the requirement for an external source of fluid under pressure.
Examples of hydrodynamic and/or hydrostatic bearings assigned to the Assignee of this applications are: U.S. Pat. Nos. 3,215,479, 3,215,480, 3,366,427, 3,375,046, 3,382,014, 3,434,761, 3,434,762, 3,467,451, 3,511,544, 3,560,064, 3,615,121, 3,635,534, 3,642,331, 3,677,612.