Up Time Magazine- February/March 2008

vibration upload All Cracked Up Examining the Vibrational Effects of a Cracked Shaft by Dr. Zhuang Li & Dr. Suri Ganeriwala A shaft crack is one of the most common defects in a rotor system and early detection of this failure mode is a very serious matter. In this study, shaft cracks were simulated and analyzed using SpectraQuest’s rotor Machinery Fault Simulator™ (MFS). A series of experiments were conducted to observe the behavioral changes of the cracked shaft in critical speed, 1X and 2X frequency responses. The experimental results were found to be consistent with the theoretical prediction of the shaft crack. A shaft crack is a slowly growing fracture of the rotor. If undetected in an operating machine, as a crack grows, the reduced cross section of the rotor will not be able to withstand the dynamic loads applied to it. When this happens, the rotor will fail in a fast brittle fracture mode. The sudden failure releases a large amount of energy that is stored in the rotating system, and the rotor will fly apart. This kind of failure may cause serious injury or even death to anyone unfortunately standing near the machine at that moment. Obviously, shaft crack detection is a very serious matter, and machines that are suspected of having a crack must be treated with the utmost caution. Cracks are initiated in the shaft in regions of high local stress. Shafts are subjected to large-scale stresses due to bending, torsion, static radial loads, constrained thermal bows, thermal shock, and residual stresses from heat treatment, welding and machine operations. All of these stresses combine to produce a local stress field that changes periodically. In a small, local region where stresses exceed the maximum that the material can withstand, a crack will form in the material. If the cyclic stresses are sufficiently high, the leading edge of the crack will slowly propagate so that the plane of the crack is perpendicular to the orientation of the tensile stress field. The orientation of this stress field is determined by the type of stress (bending or torsional) and by geometric factors. If the rotor is subjected only to simple bending stresses, then the stress field will be oriented along the long axis of the rotor, and the crack will propagate directly into and across the rotor section, forming a transverse crack. The pure torsional stress will produce a tensile stress field that is oriented at 45° relative to the shaft axis. A crack in this stress field will propagate into the rotor and tend to form a spiral on the shaft surface. Figure 1 shows these two types of cracks. In most rotor systems, the stress field contains a mixture of bending and torsional stress. Bending stress, however, is usually the dominant component, thus the crack will usually propagate into the rotor as, basically, a transverse crack. Maximum Tensile Stress Transverse Crack Torsion Crack Maximum Tensile Stress Figure 1 - Transverse and Torsion Cracks Shaft bending stiffness is related to the shaft cross-section area. As a crack propagates across the shaft, the remaining cross section becomes smaller, and the bending stiffness of the shaft decreases. This will reduce the resonance frequency as well as the critical speed of the rotor system. The reduction in shaft stiffness also causes the rotor to bow more in response to a static or dynamic load, such as a rotating unbalance. The bow is likely to change over time. As a result, it will change the effective location and magnitude of the heavy spot, which will consequently change the 1X rotor response. If a rotor with a crack has a steady, unidirectional radial load, then a strong 2X response may appear when the rotor is turning at half of any resonance speed. As a breathing crack involves both the closing and opening in one revolution, the rotor will respond at the 2X frequency. If a resonance exists at twice running speed, then the 2X vibration will be amplified. In this study, a series of tests were carried out on a SpectraQuest MFS with cracked shafts to observe their behavioral changes, including the critical speed, 1X and 2X frequency responses, compared with an intact shaft. Experimental Setup Three experiments were designed to approach the objective of this study. They were: 78 february/march 2008

Table of Contents Feed for the Digital Edition of Up Time Magazine- February/March 2008

Up Time Magazine- February/March 2008 Contents Upfront Upclose: Lessons Learned from the Walter Reed Story Information Technology: Parenting and Asset Performance Infrared: It's Alive! Lubrication: A Sense of Change Maintenance Management: PdM, Standardized Motor Testing: Being Well Connected Precision Maintenance: How to be Marketable Reliability: The Voyage of Wells Dairy Ultrasound: Two is Better Than One Vibration: Diagnosing a Cracked Shaft Upgrade: A Different Kind of Journey

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