Any machine equipped with anti-friction bearings and ge […]
Any machine equipped with anti-friction bearings and gears will exhibit high frequency vibration to some extent. Rotary screw air compressors are an example of this type of machine. Single stage rotary screw compressors are designed with two rotors, each with four to six lobes. The rotor rotates in the opposite direction, and the lobes almost touch, forming a 'screw' that compresses the air. The compressor is generally driven by an electric motor connected directly to the male rotor, which has gears in the coupling and, in turn, the gears in the female rotor. The gears are synchronized so that the rotor never touches.
Given this mechanical configuration, several different high frequency vibrations can occur or will occur:
• Frequency of rotor mesh equal to the number of lobe angles of the outer rotor multiplied by the engine operating speed. Therefore, this frequency will be synchronized with the engine operating speed.
• The gear engagement frequency is equal to the number of gear teeth multiplied by the engine operating speed. In the case of a multi-stage rotary screw compressor, there will be more than one set of gear gear frequencies. This vibration will also be synchronized with the engine operating speed.
• The frequency of the bearing depends on the specific bearing used in the machine. Normally, the bearing frequency will not be synchronized with the engine operating speed. The internal and external ethnic frequencies will be the most common frequencies.
• Other frequencies: may be caused by machine resonance or compressed air flow.
Standard alarm methods on machines with natural vibration, such as rotary screw air compressors, often do not work even when they work normally. The general level of vibration is sometimes insensitive to small changes in vibration levels caused by bearing degradation. This is due to the large amount of vibration that occurs naturally in rotary screw compressors. Each measurement activates the usual high frequency bearing inspection tool or technology, which in turn is caused by gear and rotor vibrations. This can lead to 'unpleasant' alerts.
Taking these problems into account when using standard alarm techniques, the spectral data of rotary screw air compressors is often compared. Spectral data is generally compared with previous measurements and reference measurements. Given the many measurement points and the amount of different frequencies that may occur, this can be a long and daunting task. One technique to accelerate this process is to compare the percentage of synchronous and asynchronous vibrations between two measurements. Many vibration software programs calculate this data for each spectrum. The deterioration of the bearing is usually caused by asynchronous vibration. In other words, it is not an integer or harmonic multiple of the machine's operating speed.