An isoviscous, isothermal model investigating the influence of hydrostatic recesses on a spring-supported tilting pad thrust bearing

Abstract

Tilting-pad hydrodynamic thrust bearings are used in hydroelectric power stations around the world, reliably supporting turbines weighing hundreds of tonnes, over decades of service. Newer designs incorporate hydrostatic recesses machined into the sector-shaped pads.With the aid of external oil pressurisation at low rotational speeds, oil film thickness is increased, thereby reducing friction and wear to the benefit of service life and reliability. It follows that older generating plants, lacking such assistance, stand to benefit from being retrofitted with hydrostatic lubrication systems. The design process is not trivial however – the need to increase the groove area to permit spontaneous lifting of the turbine under hydrostatic operation conflicts with the need to preserve performance of the original plane pad design. A haphazardly designed recess can change the pressure distribution of the oil film in such a way as to tilt the pad away from its optimum position. This may lead to reduced oil film thickness and increased temperature, which is concomitant with reduced mechanical efficiency and increased risk of damage to the bearing surfaces. It is therefore, inadvisable to ignore the presence of grooves in simulations. In this work, a numerical study of a sector-shaped pad is undertaken to understand how recess size and shape can affect the performance of a typical bearing. An isoviscous, isothermal model has been used in this instance because the operating conditions of the turbine in question were shown not to be severe enough to warrant the computational expense of a fully coupled thermoelastic hydrodynamic model.