BACKGROUND OF THE INVENTION Typically, symptoms of unstable shaft operation occur in the bearings in case of high sliding velocity and low bearing load. As a result anti-vibration bearings find their application in steam turbines and other rotating machines.

The elliptic bearing called also"lemon bearing"is also well known as a stabilising one. It comprises a bipartite bearing bush bisected by a horizontal plane. Each of bush parts has a cylindrical sliding lobe. The lobes have sliding surfaces shaped cylindrically and with axes lying on opposite sides of the horizontal plane dividing the bearing bush. While operating, two oil film wedges at opposite sides are formed maintaining shafts stability and preventing its vibration. However, the features of this bearing worsen significantly when the load direction varies from the direction perpendicular to the dividing plane.

Hydrodynamic bearing having three sliding surfaces taking each 120° in extent is theoretically a more advantageous one. However, manufacturing and assembly difficulties cause that this bearing type has found no technical application so far.

Hydrodynamic bearing known from the U. S. Patent No. 3,680,932 for supporting shafts of large machines like turbines and generators has a bipartite bearing bush divided by a horizontal plane. In one of alternative bearing design versions the bottom bush part has its sliding lobe extended through an arc of about 180° and the top bush part has two sliding lobes, each of about 90° in extent. The sliding lobes are separated by axial grooves lying along the width of the sliding surface.

Lubricating oil is supplied under pressure to each of these grooves. Each lobe has on its sliding surface a circumferentially extended recess, tapered along the arc. The width of the recess is less than the width of the sliding surface. It is placed in the middle of the surface. The recess begins from the adjacent axial groove and there is its deepest part. The recess'depth decreases in the direction of shaft rotation. The recesses have their inner surfaces shaped cylindrically and their axes do not coincide with the bearing bush central axis. But typically they are still placed either on the horizontal or on the vertical middle plane of the bearing. Different alternative designs of the bearing have also circumferential grooves on its sliding lobes enabling oil flow from one axial groove to another.

Having three sliding lobes with tapered recesses the bearing has improved stabi- lising features when compared with the"lemon bearing". However, its start up features become worse when the bearing is heavy loaded. Especially when the tapered recess covers a big part of the lobe surface, large recess area causes decrease of the hydrostatic oil lifting efficiency (using oil under high pressure) and increases risk of seizure when the machine is starting up or coasting. Taking into consideration the design of the bearing consisting of two bearing bush parts and the present manufacturing technology status one may imply that in reality only a bearing having two sliding lobes can be produced. It would have features similar to those of the"lemon bearing". All this caused that the bearing known from the U. S. Patent No. 3,680,932 did not widespread.

DESCRIPTION OF THE INVENTION The presented invention relates to the hydrodynamic journal bearing having three sliding lobes with improved stabilising features when operating under high sliding speed. The bearing can be heavy loaded in start up and in coasting processes and may commonly be used to substitute the"lemon bearing"in existing ma- chines. Especially in steam turbines.

The bearing has three sliding lobes covering three separate parts of the bearing bush. The bottom bush part takes 180° in extent and the two other parts take each 90° in extent. Sliding surfaces of the lobes are shaped cylindrically. To the advantage they all have equal radii. But at least in two of them the radii have their origins not lying on the bearing axis and they have different eccentricity amounts. At least one sliding surface axis is lying eccentrically on a plane including the bearing axis. But the plane is turned around the bearing axis by amount of a in the shaft direction of rotation.

Eccentricities of sliding surface axes form tapered slots between the sliding sur- faces and the shaft surface, taking all together more than a half of the total sliding surface circumference in extent. As a result hydrodynamic pressure appears in the slots and this increases the bearing load capability in the gravity direction and in perpendicular directions, too. Computational simulations have shown that also the vibration damping ability improves and that the critical turning speed value increase to the advantage.

The bearing having its bush divided in segments of 180° and twice of 90° is char- acterised by simplified manufacturing and assembly technology when compared with known bearings having three sliding surfaces. The bearing design with a bottom bush part of 180° in extent allows its application in existing bearing nests of present operating turbines where"lemon bearings"have been used.

BRIEF DESCRIPTION OF THE DRAWINGS The design of the bearing can be explained on the example shown in fig. 1 with its cross section. Fig. 2 shows bearing's longitidunal section and fig. 3 shows schematically the geometry of three sliding lobes and of their sliding surfaces.

The bearing bush consists of three segments 1,2 and 3. The bottom segment 1 takes 180° in extent and the upper segment 2 on the right and the upper segment 3 on the left take each 90° in extent. Segments 1,2 and 3 are joined together by fitted bolts 4. The segments of the bearing bush are covered by sliding lobes 5,6, 7 with specially shaped sliding surfaces 8,9,10. Sliding surfaces 8,9,10 are shaped by radii R having equal lengths and having origins at points 11,12,13

which lie eccentrically to the geometrical centre of the bearing bush and have different eccentricities s and are placed on lines 15,16 and 17. Lines 15,16 and 17 go through centre point 14 of the bearing and are created by rotation of sym- metry axes 18,19,20 of sliding surfaces arcs 8,9,10 by anglea in the rotation direction of the shaft 21. Thus the origin 11 of radius R1 of sliding surface 8 is placed at the distance s1 from centre 14 of the bearing bush and is rotated by angle a1 starting from symmetry axis 18 of sliding lobe 8 in the rotation direction of the shaft 21. Origin 12 of radius R2 of sliding surface 9 is placed at the dis- tance s2 from centre 14 and is rotated by angle a2 in the rotation direction of the shaft 21. Similarly, origin 13 of radius R3 of sliding surface 10 is placed at the distance s3 from centre 14 and rotated. Lubricating oil is supplied to sliding sur- faces through hole 22, through three oil pockets 23 and through circumferential groove 24, which connects pockets 23.

Amounts of sliding surface angular extension may vary in wide range from 30° to 180° for segment 1 and from 25° to 90° for segments 2 and 3 and depend on requirements to the bearing and the operating conditions.

The set of geometrical quantities-radii R and polar coordinates, a-depends on requirements posed on bearing features like high critical turning speed, high damping abilities for vibrations and loading capabilities during starting up and coasting processes. They also depend on the length to width ratio of the bearing, bearing clearance, oil viscosity and the shafts turning speed. The optimum bear- ing design for specified applications may be found by calculations made using computational programs based on hydrodynamic lubrication theory.