Causes of gear failures and diagnostic methods

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Causes of gear failure and diagnosis methods: Operating under high loads for extended periods of time and poor lubrication can all cause various types of damage to the gear teeth. Based on the nature of such damage, it can be classified into the following four categories: (1) Wear ； (2) Surface fatigue ； (3) Plastic deformation ； (4) Tooth fracture. Tooth damage is a serious gear failure; once it occurs, the operating conditions deteriorate sharply. Gears that cause the aforementioned faults can generate significant vibration and noise, and certain characteristics can be reflected in the vibration and noise spectra; therefore, by analyzing these spectra, it is possible to diagnose gear faults. Gear failures are divided into long-cycle failures and short-cycle failures. Large-cycle faults refer to faults characterized by the rotational frequency of the gear as their fundamental frequency. Such as eccentricity, cracks, and broken teeth, etc.; some of them affect the spectrum in the form of errors, while others affect the response in the form of sudden changes in stiffness. Short-period faults refer to those characterized by the tooth engagement frequency as their fundamental frequency, such as welding, pitting, and wear; they generally affect the spectrum in the form of phase errors. Its basic characteristics are as follows: (1) For gears with large-period faults, the amplitude at the rotational frequency and its harmonics increases as the fault worsens, whereas for gears with small-period faults, the amplitude at the meshing frequency and its harmonics increases as the fault worsens. (2) Gears with large-period faults generate a family of sideband frequencies at the meshing frequency and its harmonics, spaced by the rotational frequency of the faulty gear, and these sidebands intensify as the fault worsens. (3) For gears with large-period faults, the amplitude at the meshing frequency and its harmonics has little relation to the fault, whereas for gears with small-period faults, the amplitude at the rotational frequency and its harmonics is independent of the fault. Diagnostic methods for gear failures: 1. The method of observing energy at specific frequencies. The normal spectrum of a gear mechanism is generally fixed, and the energy levels do not change much during operation. Typically, the energy from the normal spectrum combined with the energy from noise is used as a criterion to distinguish fault signals. 2. Spectrum amplitude comparison method: When there are many frequency components and the amplitudes vary greatly, it is difficult to compare the amplitudes of specific frequencies on the spectrum diagram; even slight changes in frequency can result in significant variations in the spectral lines. To observe this situation and determine the fault, the maximum of three values method is used. The amplitude corresponding to the frequency value at three consecutive points is selected as the largest value; this envelope is used as a criterion to determine gear faults – values above this envelope indicate a fault, while values below it indicate normal operation. 3. Sideband method: In noise testing, the frequency components obtained from the spectrum are often quite diverse. When there is a fault with the gear, modulation may occur; this is reflected in the spectrum as the appearance of numerous sidebands near the meshing frequency. The more severe the fault, the more side spectral lines appear and the higher their amplitude.