There are examples, in which in place of conventional oil lubricating bearings, gas bearings are used as bearings for journal shafts of gas turbines, compressors, etc., which rotate at high speed. In the case where working temperature is high as in gas turbines, thermal expansion and thermal deformation are increased to cause a problem that hydrodynamic journal bearings, of which bearing surfaces opposed to a journal shaft are composed of rigid members and which include a tilting pad and a herringbone groove type hydrodynamic bearings, are hard in terms of reliability to be put to practical use. Therefore, there have been developed and adopted foil hydrodynamic journal bearings, of which bearing surfaces are composed of an elastically compliant foil sheet to be able to absorb thermal deformation, thermal expansion, manufacturing error, and assembly error of respective parts, etc.
In addition to the feature described above, foil hydrodynamic journal bearings are stable in high-speed rotation since they elastically support and damp a journal shaft. For example, the literature (TRIBOLOGY TRANSACTIONS, Vol. 43-4 (2000), pages 795–801) describes the historical development of foil hydrodynamic gas bearings.
According to the disclosure of the literature, foil hydrodynamic journal bearings are generally composed of bearing sliding surfaces and spring members, which elastically support the bearing sliding surfaces, and enhanced in bearing performance by an increase in flexibility and damping quality of the bearing. Such foil hydrodynamic journal bearings are mainly classified into leaf type ones and bump type ones according to the structure of spring members, which elastically support bearing sliding surfaces.
Leaf type foil hydrodynamic journal bearings comprise a plurality of leaf foils arranged round a journal shaft with a bearing gap therebetween in a manner to partially overlap one another on a bearing circle, and a non-cylindrical bearing shape obtained by overlapping of the foils is made use of to generate dynamic pressure upon rotation of the shaft. Also, as an axle load acts, the leaf foils are deformed in a manner to follow the journal shaft, and the bearing is enhanced in damping capacity by friction, which is caused by the deformation to act on the leaf foils, so that it is possible to expect an effect of reduction in rotational vibrations of a journal shaft.
On the other hand, bump type foil hydrodynamic journal bearings comprise a substantially circular-shaped top foil arranged round a journal shaft with a bearing gap therebetween, the top foil being elastically supported on a bearing holding member (housing) through a bump foil, which is formed to be corrugated in shape. When a load acts on the journal shaft, the top foil is deformed in a manner to follow the journal shaft, which causes strain on the bump foil and causes slip on contact surfaces between the top foil and the bump foil to give bearing damping by virtue of friction. Besides, multiple winding type foil hydrodynamic journal bearings have been also proposed.
Furthermore, in order to improve the bearing performance for rotational vibrations or the like, an improvement in stiffness and damping characteristics is made by providing an intermediate layer (mid-foil) to make a plurality of layers, as by means of a method of inserting a further foil between a top foil and an elastic support member (for example, a bump foil), and by contriving the shape and material of the elastic support member. However, foil hydrodynamic journal bearings having the above feature involve a problem that parts are large in number, precision machining is demanded in manufacture, and since handling is difficult to make, a minute work is made necessary at the time of mounting the parts in an assembly stage to be unsuitable for mass production.
Since in case of a lubricant being gaseous, a lubrication film formed between a bearing sliding surface and a journal shaft upon rotation is as small as several microns or less, minute irregularities are generated on the bearing sliding surface due to being discontinuously supported by a bump foil to make it hard to form an ideal wedge film shape, which constitutes a basis for formation of a lubrication film, with the result that the bearing loading capacity is lowered.
Hereupon, as in a hydrodynamic gas journal bearing described in JP-A-9-14262, there has been proposed an arrangement, in which a vibration damping material (vibration-proof rubber, macromolecular gel, or powder) is provided between a cylindrical-shaped foil and a bearing holding member (housing).