The FDM-A Method as Applied to Evaluate the Rolling-Element Spin and Misalignment of Bearing Support Elements

Andrzej Gębura, Tomasz Tokarski
2007 Research Works of Air Force Institute of Technology  
Some selected issues on the continuity of motion of rolling elements observed with the FDM-A 1 diagnostic method [1] have been given consideration. A formula for the ratio of angular velocities of the cage and the bearing journal during the process of perfect rolling (i.e. with no spins of rolling elements against the bearing race) has been proposed. The ratio has been labelled by the Authors as a 'coefficient of the rolling'. Included are results of the computer-aided simulation of how changes
more » ... in the geometry of some components of a rolling bearing affect dynamic properties thereof. A simple method to conduct preliminary checks of the correctness of the rolling motion of rollers of a rolling-element bearing has also been mentioned. This method has been successfully used by the Authors. Types of the rolling characteristics resulting from the tests with the FDM-A method have been described and correlated with particular types of mechanical failures to the rolling-element bearings and the compressor-turbine sub-system of a turbine engine. Andrzej GĘBURA, Tomasz TOKARSKI 148 of these high-speed engines. Lubrication, cooling, and general wear-and-tear problems appeared. The fast-moving rotor generates so very great centrifugal force that if -due to friction wear -considerable clearings appear, the rolling elements stop to be loaded as the rotational speed increases and they do not roll along the inner race of the bearing. This problem remains in complete opposition to that faced by designers of the 19 th century industrial revolution, i.e. the excessive loads (overloads) affecting the bearings of turbo-machines (fluid-flow machines) in ground-or water-based objects; various kinds of cracks and fractures, the seizing/galling, and other wear-effected traces resulted. On the other hand, the underloading of the rolling elements of any bearing often results in the angular-velocity reduction in these elements. Now, even a slight movement/manoeuvre of an aircraft may prove strong enough to cause both the forces of inertia and the gyrostatic moment generate a short-duration overload on the bearing. Since the rotational speed of the rolling elements has got reduced due to the bearing's underloading, they are forced to rapidly accelerate, usually with some considerable spin. Frequent repetitions of this phenomenon produce fast abrasive wear of particular rolling elements. The bearing's cage is the component severely exposed to dynamic-forcesinduced hazards. According to traditional design practice, the cage is intended to separate the rolling elements and not to transmit considerable forces. In the case the bearings remain commonly underloaded and the level of loading them rapidly changes, situation untypical of design 'tradition' appears: the bearing's cage is subject to impact forces. The from the non-uniform conditions of friction resulting kinematic diversification of the rolling elements induces impact forces in the rollingelement assembly. Energy fed into the cage during collision with a rolling element changes direction of its momentum to the circumferential and radial direction(s), and partially it is lost in some extra rolling-element spin against the bearing races. The effect is that the cage starts to oscillate between the rolling elements [2] . It also rotates around some different axis than that of the bearing. The stability of motion of the cage depends, to a considerable degree, on friction conditions directly at the rolling contacts and in areas where the cage contacts with the rolling elements. The Authors' experience [3] proves that these forces are great enough to induce cracking phenomena in the separating elements of the cage, and even disruption of its circumference. Such phenomena are often preceded with resonance. Untypical of earlier applications of the rolling-element bearings, they have provoked aircraft factories and aviation-dedicated organisations to start wide-scale research work to learn more about and monitor complicated processes that lead to failures to bearings in aircraft engines. The problem is a real challenge faced by designers, operators/maintenance engineers and air-accident investigators. Such research work is
doi:10.2478/v10041-008-0008-0 fatcat:4vgvsczd5ncivo5joxoxrkgynu