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308 PROCEEDINGS OF THE 29TH TURBOMACHINERY SYMPOSIUM De Laval ran into difficulties when attempting to patent his ideas in Germany, as the patent office did not accept the mathematics in the patent description. Because of this, De Laval had to build a hand-driven demonstration model for the German patent office that duplicated his cane experiments, but with a metal shaft. The model could show how the rotor ran smoothly despite being heavily unbalanced, after accelerating through the critical speed. Dunkerely conducted extensive studies in 1894, analyzing rotordynamic behavior considering the rotor as a flexible elastic body and bearings as simple supports. In 1919, H. H. Jeffcott, a well-known English dynamist, started studies in rotordynamics to examine the effect of unbalance on whirl amplitudes and on bearings. His insightful paper entitled “The Lateral Vibration of Loaded Shafts in the Neighborhood of a Whirling Speed—the Effect of Want of Balance” (Jeffcott, 1919), forms the basis of what most rotating machinery engineers are taught today. The basic rotordynamic equation that starts many papers and texts of rotordynamics originated with him. As a result of Jeffcott’s superb analysis, turbomachinery designers started producing high-speed machinery. In the 1920s, several manufacturers went to flexible shaft designs with lighter rotors operating well above the first critical speed. This resulted in some severe instability problems. GE encountered a series of instability related failures in blast furnace air compressors. The blowers were seen by shop engineers to sustain “violent fits of vibration” and during these fits, the shaft would vibrate at low frequency, which shop engineers called “shaft whipping.” The problems were initially attributed to poor balance but when it became evident that this was not the underlying cause, the investigation was put under Dr. B. L. Newkirk (1876 to 1964) of GE Research Laboratory. After a series of observations and experiments, Newkirk determined that during the violent whirling, the rotor centerline would precess at a rate equal to the first critical speed (Newkirk, 1924). If the rotor’s speed was further increased above its initial whirl speed, the whirl amplitude would increase, leading to eventual rotor failure. After detailed experimental tests, he discovered the following facts (Gunter, 1966): • The onset of whirling or whirl amplitude was unaffected by refinement of rotor balance. • Whirling always occurred above the first critical speed. • The whirl threshold speed could vary widely between machines of similar construction. • The precession or whirl speed was constant regardless of the unit rotational speed. • Whirling was encountered only with built up rotors (disk press fits). • Increasing foundation flexibility would increase the whirl threshold speed. • Distortion or misalignment of the bearings would increase stability. • Introducing damping into the foundations would increase the whirl threshold speed. • Increasing the axial thrust bearing load would increase the whirl threshold speed. • A small disturbance was sometimes required to initiate the whirl motion in a well-balanced rotor. A theory for the source of the vibration was provided by A. L. Kimball (1924), who suggested that the forces normal to the plane of the deflected rotor could be produced by the hysteresis of the metal undergoing alternate stress reversal cycles. Newkirk concluded that these forces could be caused by disk shrink fits. Thus started the study of shaft whirling or self-excited instabilities. Newkirk showed that the condition for rotor stability was dependant on several parameters such as bearing characteristics, bearing support, rotor flexibility, as well as external forces and torques acting on the system. While modern analytical rotordynamic codes and techniques are highly sophisticated today, rotor instability still remains an elusive problem, especially with high-pressure ratio reinjection machines where the aerodynamic forces, rotating stall, and balance piston seal forces can have substantial influence on compressor stability and vibration. Bearings Associated with the history of high-speed turbomachinery were advancements of bearing technology. McHugh (1998) provides an excellent overview of the history of this field. The industrial revolution (1750 to 1850) focused attention on the need for effective and reliable bearings. The widespread use of steam engines and railways made the study of lubrication more important. The most popular lubricant at that time was olive oil. James Watt recommended its use in his steam engines, and Osborne Reynolds’ classic paper deals with olive oil. As early as the 1400s, Leonardo da Vinci recommended the use of a bearing material alloy consisting of three parts (30 percent) of copper and seven parts (70 percent) of tin. The amount used in today’s whitemetal is 80 percent. Isaac Babbitt (1799 to 1862) patented a tin-based material for steel shells in 1839. Pioneering Experiments of Beauchamp Tower In 1883, an English engineer by the name of Beauchamp Tower discovered full film lubrication. Tower was an established researcher and was chosen by the British Institution of Engineers to study friction in journal bearings. Tower constructed a special test rig to measure friction on a gunmetal half-bearing six inches long. The bearing rested on a horizontal shaft with a four inch diameter journal. A vertical load could be applied to the bearing to simulate loading. The test shaft was driven by a steam engine designed by Tower. The device was designed to measure the friction between the journal and the half-bearing by the restraining torque necessary to keep the half-bearing from rotating. As was the practice at that time, different kinds of lubricants were provided through a hole drilled at the top of the journal. Tower decided to place the lower half of the bearing in a bath of oil. He soon discovered that this arrangement produced a considerable hydrodynamic pressure and that the shaft was actually floating on a film of oil. Tower then proceeded to make accurate measurements to measure the oil pressure along the bearing length and circumference. His classic experiments clearly showed that the film of oil completely separated the journal and the bearing and carried the applied load. His seminal results were presented to the Institution of Engineers in a series of reports (Tower, 1883, 1885). It should be mentioned that Petrov in Russia independently developed similar findings during the same time. Some of Tower’s results are shown in Figure 75. Figure 75. Beauchamp Tower’s Experimental Results in Measuring Oil Film Pressure.PDF Image | THE HISTORICAL EVOLUTION OF TURBOMACHINERY
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