1 Solid Mechanics, Department of Mechanical Engineering, Technical University of Denmark2 Department of Mechanical Engineering, Technical University of Denmark
Pockets are often machined in the surfaces of tilting-pad thrust bearings to allow for hydrostatic jacking in the start-up phase. Pockets and other recesses in the surfaces of bearing pads influence the pressure distribution and thereby the position of the pivot resulting in the most advantageous pad convergence ratio. In this thesis, a theoretical approach is applied in the attempt to quantify the influence of recesses in the pad surfaces. The recesses may be relatively deep and enclosed as is the case with pockets designed for hydrostatic jacking. Such recesses are characterized by low friction and a small pressure build-up. As in parallel-step bearings the recesses may also have a depth of the same order of magnitude as the oil film thickness. Such recesses are characterized by a strong pressure build-up caused by the reduction of the flow area at the end of the recess. Numerical models based on the Reynolds equation are used. They include the effects of variations of viscosity with temperature and the deformation of the bearing pads due to pressure and thermal gradients. The models are validated using measurements. Tilting-pad bearings of standard design are studied and the influences of the bearing length-to-width ratio, pad deformation and injection pocket size are quantified. Suggestions for the design of energy efficient bearings are given. The results show that correctly dimensioned, bearings with oil injection pockets have smaller friction coefficients than bearings with plain pads. Placing the pockets in the high-pressure zones close to the trailing edges of the bearing pads causes a substantial reduction in the friction coefficient. The design of the recess sizes and positions leading to the largest improvements is studied and design suggestions for various pad geometries are given. Parallel-step bearings theoretically have smaller friction coefficients than tilting-pad bearings. A design of a tilting-pad bearing is suggested which combines the benefits of the two types of bearings in a tilting-pad bearing with inlet pockets. This design results in a substantial reduction of the friction loss. Both this bearing and the bearing design with enclosed recesses in the high-pressure regions of the pads suffer from a higher sensitivity to the position of the pivot. The design of such bearing is therefore no trivial task.