Two bearings normally support each section of the turbine shaft, although, with solid couplings, some designs only use one bearing between cylinders in order to save length and bearing losses. Plain white-metalled journal bearings are invariably used because of their high loading capacity, reliability and absence of wear due to hydrodynamically generated films of lubricating oil.
These films are formed automatically, when running, by a high pressure wedge of oil between the whitemetal and the shaft — the maintenance of which ensures that no metal-to-metal contact is made. The oil is continuously fed into the wedge by frictional drag and leaks away axially towards the edges of the bearing.
The white metal surface is either cast into a mild steel liner to form a bearing shell, which is supported in the bearing body, or cast directly into the bearing body itself. Both types are in use in modern UK turbines (Fig 1.121).
All bearing assemblies are split in halves on the horizontal centreline and held together by bolts, the halves being dowelled together to assure precise location on assembly.
The bearings are normally spherically seated in their pedestals on four pads under which shims are placed to facilitate precise horizontal and vertical alignment of the shaft line. The spherical seating feature assures that, on assembly, the bearing will automatically align to its correct axial tilt; this is necessary because, when coupling faces are set and bolted flat together, the outer bearing on each shaft needs to be higher than the inner bearing due to the natural bending catenary adopted by each shaft under its self weight (for further detail of alignment refer to Section 7 of this chapter). The spherical seat is restrained in normal operation.
Typical large turbine bearings are up to 530 mm in diameter and have length/diameter (L/D) ratios in the order of 0.5 to 0.7. Generator bearings tend to be rather longer, with L/D ratios of 0.6 to 1.0 to take account of the heavier generator rotor in relation to the weight of individual turbine rotors. In this way, a typical load on projected bearing area is kept below about 15 kPa.
Two main white metal profiles are in common use in modern UK turbine plant; these are known as elliptical and three-land respectively.
The elliptical bore is produced by first machining a circular bore with shims in the bearing horizontal half joint and then assembling the bearing without the shims. This results in typical clearance ratios (diametral clearance/diameter) of 0.001 vertically and 0.0015 horizontally. Oil is fed into the bearing via lead-in ports at two diametrically opposite points on the horizontal centreline.
The three-land design has three separate bearing surfaces, or lands, of equal width but of different arc lengths; one land in the bottom half and two in the top half.
There is an oil supply groove with lead-in at the beginning of each land, and a drain groove at the end. Recirculation of oil is limited by an axial strip of bearing surface between each drain groove and the following supply groove. The three-land bearing is generally of circular bore with a clearance ratio of about 0.0013. This design is more resistant to low frequency whirl (see later).
Oil is supplied, to cool and lubricate the bearings, at about 1 bar and 30-40°C, from the main turbine lubricating-oil pump. Each bearing also has a separate high pressure (300 bar maximum) jacking oil supply which is injected at the bottom of the bearing. This lifts the journal in the bearing when starting from rest, thereby preventing wear and reducing the starting torque required from the turning-gear drive motor.