Some Effects of Start-Up and Shut-Down on Thrust Bearing Assemblies in Hydro-Generators

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.