Dynamic Synthesis of a Timing Device with Pulse Loading

Abstract

This paper describes the experimental investigations which were conducted to verify existing theoretical vibration-amplitude predictions for centrally-preloaded, squeeze-film-bearing-supported rigid rotors. Very good agreement was obtained with all aspects of these predictions, including the degree of pressurization needed to achieve full-film lubrication and the complicated bistable operation behaviour. By developing the theoretical model for the more general case of a rigid rotor supported by unequal squeeze-film bearings at the ends, it is shown how existing theoretical data, derived on the basis of a symmetric rotor with symmetric motion, are readily applicable to the case of a rotor supported by squeeze-film bearings at one end only. The experimental rig consisted of such a rotor, and, although the results are based on a given bearing having a length-to-diameter ratio of 0.167, and did not allow for any significant variation in the radial support stiffness, nor for variation in the bearing dimensions, and the rotor speed did not exceed 7500 rev/min, it is concluded that the theoretical data predictions are valid in general. Their validity is questionable, however, should conditions be such that turbulence and/or unaccounted-for cavitation effects become significant. Squeeze-film bearing behaviour under such conditions warrants further investigation.

Slider-crank mechanisms are often used as timing devices, for example, to open and close valves at specified times. Such mechanisms are subject to pulse loads and their design is difficult. Here, a slider-crank mechanism with a flexibly-attached slider subjected to pulse loading is synthesized for position co-ordination. The pulse loading is expressed in the form of a Fourier series so as to simplify the analysis. A worked example is given to illustrate the synthesis procedure.