Effect of Scratches on a Tilting-Pad Journal Bearing

Many industrial rotary machines are equipped with hydrodynamic journal bearings, such as centrifugal compressors, steam turbines, pumps and motors.

In hydrodynamic bearings a thin oil-film separates the shaft from the fixed part and theoretically completely avoiding contact between the solids. Since, in large machines for industrial applications, the shaft is a component of much greater value than bearings, the surface of the bearing is coated with a sacrificial white metal that can degrade in case of sliding contact, thus preserving the surface of the shaft. After some time from the installation, however, the surface of the bearings often presents imperfections and slight damages caused by the presence of particles and debris in the oil or by the start and stop of the operations, when the hydrodynamic mechanism is not well developed and the mixed lubrication it can happen. The presence of scratches on the bearing can lead to variations in the thickness of the oil film which, in turn, also causes significant variations in the hydrodynamic effects underlying the operation of this component. For example, a reduction in the minimum thickness of the oil film can lead to an increase in the local temperature, to local pressure peaks and, finally, to a deterioration - in some even dangerous situations - of the performance of the component.

At present, it is commonly believed that scratched bearings need to be replaced, but there are no standards that encode the urgency of replacing a damaged bearing based on the type of the detected damage (scratch position, depth, density, etc.). The pads are often replaced even if they could work safely or, on the contrary, it is possible to underestimate the danger of apparently secondary damages that actually compromise the safe operation of the machine.

Experimental data relating to scratches on the bearings of the tilting-pad journal bearing (TPJB) are extremely limited in the literature. Extensive experimental work was carried out to study the effect of artificial scratches on pads on the static and dynamic behaviour of a TPJB. The number of scratches, the depth and the axial position have been studied and the dynamic coefficients have been estimated as well. The experimental results will also be compared with the results of a numerical model.