In the past 50 years, advances in gear technology have been driven by energy and aerospace, specifically by the gas turbine, jet engine, wind, hydro turbines, and the helicopter. During this time improved manufacturing techniques and material control have extended gear life by as much as 80 percent, in some instances. In such applications, where the sounds and vibrations can be in sympathy with the gear, failures that can occur are attributed to fatigue cracking.
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Fatigue failures are one of the most confusing of all materials phenomena. In most standard listings that provide references to types of gear fatigue failure, the most insidious and destructive—namely resonance—is not even listed. One reason is probably that the destruction is so massive that the cause is difficult to identify. In large, heavy, slow-running gears, such failures are almost nonexistent. Gears with a high power and speed relative to their size, and light weight, experience resonance because the shaft speeds can excite the natural frequencies, especially for high-speed, lightweight gears such as thin webbed bevels. Gears may be designed with consideration of resonant frequencies, which may make them heavier and more costly. Fortunately there are certain tests available that can identify the natural frequencies and the resulting stress levels. Several methods, such as siren or bang tests, are available to determine a gear’s natural frequencies and modes. When it is known that the frequencies are in the operating excitation range, the designer has two choices: modify the design, or provide damping. Damping is very effective in reducing resonant frequencies, and a popular method is to fit one or more damping rings into a machined groove in the gear’s rim. The ring centrifugally expands, altering its mass.
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In 1963 serious tooth breakages in helicopter transmissions were attributed to gear resonance conditions with insufficient damping. In R.J. Drago and F.W. Brown’s paper 80-C2/DET-22, reference is given to a helicopter gear that exploded during operation because one of the resonant frequencies coincided with an excitation frequency. Crane traveling drives have been subject to the sudden fracture of one or more bevel gear teeth, normally the high-speed stage. Sometimes the fracture occurs long after it has been in service, and at other times this happens almost immediately. This failure pattern is associated with resonance. In these drives torsional vibrations arise from the pulsating torque of the gear/shaft/coupling. These vibrations are increased when the bevel gear’s mesh frequency approximates the shaft/coupling frequency. In automotive transmissions drive rattle is excited by the angular acceleration caused by the fluctuating torque output. Since the resonance cannot be avoided, it is only diminished by improvements in the gear layout. In a temper mill, after replacing gears the gear mesh frequency resulted in chatter marks. Data was gathered with the use of modern vibration analysis. It was learned that the vibration from the gear mesh is amplified as it goes through resonance between 200 to 300 rpm. And this energy was being transferred through the strip to the mill rolls.
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Drives deemed “critical” require torsional analysis at the earliest design stage to identify potentially damaging resonances. When not carried out at the start the drive may require dampening devices to reduce vibration-induced stresses. Vibration analysis is also used to monitor the vibrations with transducers and computers. The causes can then be determined in time to avoid a catastrophic failure. In many instances, but not all, there is a time period that permits an orderly shutdown due to the slow rate of crack propagation. When there is a time period prior to failure when the cracked tooth is transferring some of the load to adjacent teeth. There are several signal-processing methods that provide gear vibration analysis in the major areas of predictive maintenance, and fatigue testing. AGMA paper P.159.05, “Identification and Correction of Damaging Resonances in a Gear Drive,” includes an investigation of an air turbine starter gear.
