Anti-Friction Bearing Systems for Jet Engines

Abstract

This paper presents an overview of anti-friction bearing technology for jet engines with an insight into the physics involved in bearing operations. Major issues affecting the lubricant performance have been discussed. Also presented is a quantitative assessment of parameters, viscosity, temperature and pressure, affecting system performance. Several methods to predict the performance of tribological systems have been highlighted. The information and analyses provided will give the engine design team an efficient evaluation of the bearing systems rectified for certain design parameters.

1. Introduction

A bearing system is an essential part of a turbo-machinery assembly, it guides/positions components and transmits loads while allowing smooth relative motion. Mechanical losses arising in a gas turbine jet engine incur 10 to 15% of fuel energy [1], these losses are, typically, dominated by the friction generated in bearings. Also, the main aerodynamic losses arising from the fan/turbine blade tip clearances directly depend on the allowances set by the bearing system. Therefore, a careful design/selection of a bearing system is very much of a fruitful task.

Friction is an embedded issue in existing problems across multiple engineering fields including dynamics and control, contact mechanics, aeromechanics, structural dynamics and fracture mechanics. However, the presented research is focused on the engineering discipline “tribology”. Tribology deals with the science and technology of lubrication that may achieve friction control and wear prevention of interacting surfaces. Tribological design abides two main principles; 1) prevent contact between rubbing surfaces and 2) regard the separating medium “the lubricant” as an integrated element of machine. A classic example is a ball-bearing or a cam-follower contact. This paper intends to present a critical assessment of tribological factors encompassing bearings.

2. Tribology

2.1 Friction

The friction phenomenon was first explored by Leonardo Da Vinci (1452-1519). He stated that the friction force is doubled as the mass is doubled and the force is independent of the area in contact [2]. The existence of friction was introduced into the world of science by the motion laws of Sir Isaac Newton (1643 -1727) particularly the one that stated ‘every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it’. The external forces were recognised to be resistive forces i.e. air drag, gravity and the force generated at a surface of contact. The law directly points towards the external forces, i.e. the frictional force that a body experiences in motion.

The frictional phenomenon was first studied experimentally by Guillaume Amontons (1663 – 1705) who discovered that frictional force is dependent on the surface characteristics of the contacting bodies. Later it was Charles Augustine de Coulomb (1736 – 1806) who modelled and formulated the frictional phenomenon based on numerous experimental tests.

2.1.1 Effect of Surface Profile – Roughness

The evolution in tribological sciences has shown that surface profile, particularly the micro/nano geometry, is the most important factor in determining the friction and wear of rubbing surfaces [2]. Therefore, it is vital to examine the surface topography of surfaces under consideration. Herein, four different surface profiles are examined where one is machine polished, two are manually polished using sand papers of different grit sizes to reduce surface roughness of the unpolished standard sample. Figure 1 shows 3-dimensional plots of surface heights for the samples. Unpolished disk samples have the highest surface roughness and the machine polished offers the lowest average roughness. Manually polished samples, P1200 and P600, have moderate roughness values, however, the skewness values are lower compared to the unpolished/machine polished samples, making them a better bearing surface as more lubricant is deposited in the valleys. Furthermore, the density of larger asperities around the surface is higher for the unpolished and machine polished disks compared to the manually polished disks………

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