Chris M. Ettles

The thrust bearing duty in a pump-turbine generator can be quite arduous, since the pad support system must be symmetrical about the center of the pad, yet the oil-film must converge adequately for either direction of rotation. Special care must be taken with large machines since the thermal and elastic deformation of the pads will increase nonlinearly with size B of the pad, for example, as B² when thermal deformation is considered. However from first principles, the thickness of the oil film will increase with only the square root of size B^½. Poorly shaped films can develop when a design standard is scaled-up to larger sizes. Three options for the thrust bearing design of a particular pump-turbine were considered: (a) “semihard” supports for the pads such as a spring-disk insert, (b) “piston-type” supports in the back of the pads, which are machined to form shallow pistons that fit into recesses, allowing the pads to be supported hydrostatically, and (c) a symmetric arrangement of coil springs. In this instance, an upper limit of thrust bearing temperature was specified. Penalties would incur if this were exceeded. It is shown using a design code (GENMAT) that the best performance is achieved with a spring support (option c), arranged to give a convex film shape in the direction of sliding, and a slightly concave film in the radial direction. This is achieved by limiting the extent of the spring pack in the circumferential direction so that there are unsupported “overhangs” at the lead and trail edges. The radial concavity is arranged by having the spring pack extend edge-to-edge in the radial direction. The bearing has performed very well since commissioning. The original machining patterns are untouched after thousands of reversals under load. The pads appear as new.

The modernization of hydro-generators can involve the analysis of many different manufacturer’s designs of thrust bearings. Recent designs of bearing in common use are very reliable, but when failures do occur, it is often with older machines and within the first few minutes of start-up. This paper is a result of general design studies of various thrust bearing configurations subjected to transient operating conditions. It is shown that transient effects can induce an ‘overshoot’ of thermal deformation which can become unstable, leading to ‘thermal ratchetting.’ Examples are given of pads of various manufacturer’s bearings that have been subjected to this mechanism. Results from operating turbines, basic studies and measurements of the thermal bending of plates indicate that a peak deflection occurs well before thermal equilibrium is attained. The peaking phenomenon may be obscured in some designs or in cases where the run-up is gradual. The beneficial effects of using an oil-lift system during start-up are described. During shut-down it is important that the contact of hot, crowned pads against the runner be prevented. Minimum times for operation of the lift system are suggested, based on the thickness of the pads.

The use of PTFE-faced pads in large vertical axis hydro-generators was pioneered in Russia in the 1970s, prompted by a series of failures of conventional babbitt-faced bearings. Some advantages claimed include higher specific loading, lower power loss and the omission of oil-lift facilities. There is strong interest in the Industry concerning this material, but limited data are available on actual performance. Some results from extensive testing of PTFE-faced pads are given, for two sizes of pad. These are compared directly size-for-size with results for babbitt bearings of nominally the same area. The power losses for the two types of bearing were found to be almost identical. Some of the effects observed during testing are described and discussed, including the effect of creep. The test results are compared with predictions using the GENMAT analysis software. A method of allowing for creep in numerical modeling is discussed.