Whether it’s imbalance, misalignment or cavitation: how to discover the cause of vibrations in your machines and how to eliminate them.
There are various causes for dangerous vibrations in machines and systems with moving masses: imbalance, misalignment, roller curvatures, problems with the fundament, cavitation, gear errors, electrical or magnetic interferences, just to name a few. Vibration measurement and analysis requires a lot of fundamental knowledge and experience. That doesn’t mean you need to have a PhD in physics or become an expert in vibration measurement. A competent service partner like Off-Highway Powertrain Services will handle these issues.
Phase measuring as a first step
Vibrations on large rotating machines like turbines, generators, compressors or large compactors are especially critical. If vibrations amplitudes rise to a dangerous level, serious and costly damage are the result. Typical variables for monitoring in such machines are absolute bearing vibrations, relative shaft vibrations and axial shaft movement, these are measured with sensors.
Vibrations occur in different phase positions. Only by measuring the respective phase position is it possible to detect the causes of vibrations. Using analysis software will enable a more comprehensive, in-detail analysis of the cause of vibrations.
Web-based vibration monitoring
Vibration measurement is web-based nowadays. Data collection and monitoring devices are integrated as IIoT devices into the Cloud and feature dynamic input channels and integrated, web-based visualization and analysis software. Configuration and online display of vibrations and parameters is done in the web browser via analysis charts and diagnostic functions.
In hazardous areas or in the case of machines in different locations, wireless monitoring systems are in use. Such systems are able to measure machine vibrations and temperature synchronously and monitor the machine’s condition. It provides the users with a real-time monitoring of their machines and allows detection of faults well in advance – this is what Condition Monitoring essentially means.
Vibration measurement via Motor Current Signature Analysis
Motor Current Signature Analysis is based on the fact that mechanical defects like bearing defects, imbalances or cavitation can be detected in the motor current and subsequently, can also be assessed by precise current and voltage measurements. Such CMS systems are suitable for most machines with electric motors and generators. To conduct a measurement, a motor is connected with current probes and voltage converters. An evaluation device records the measurement, puts together a machine analysis and generates a report, detailing all symptoms of damage and the machine’s condition.
Different causes of vibrations
- Imbalance: We feel imbalances as a vibration of turning rotors. An angle grinder vibrates because the motor or the disc has some imbalance. Our cell phones vibrate because they are purposely constructed with a small motor that has some imbalance. These vibrations are generated by centrifugal forces of a mass which is unevenly distributed on the rotor. Imbalance in the context of machines is the deviation of a shaft’s rotational axis from the optimal alignment.
- Parallel shaft misalignment: Couplings connect shafts of different machines and machine components. If the shafts are horizontally and vertically offset, this will result in increased vibration amplitudes. These vibrations will occur on the two connected machine or machine parts in opposite phases. More than 50% of malfunctions in rotating machines are attributable to misaligned shafts.
- Belt problems: Apart from couplings, transmission belts are another way to connect rotating parts. Faulty belts can be detected by looking at the belt running frequency. This should generally be smaller than half the rate of the slowest belt shaft speed. Belt faults are often due to resonance excitation of the active and passive strands.
- Roller curvature: Roller curvature is a factor that is given only little attention in bearing calculations, since bearings are often placed at the end of a shaft or axis where no significant bending occurs anymore. Nevertheless, there is a considerable number of cases where they should not be neglected. Relative wave vibration can be detected with contactless displacement sensors.
- Cavitation: Cavitation is the spontaneous formation of vapor bubbles in rapidly flowing liquids, where the pressure in the liquid is lower than the vapour pressure of the flowing liquid. This may cause a problem in turbo-machines when cavitation occurs on fast-moving components. As soon as the pressure in the liquid exceeds the vapour pressure of the liquid again, these vapour bubbles implode and disintegrate. As a result, pressures of up to several thousand bar can occur which cause severe damage on components.
- Gear tooth errors: Low noise is an important design objective in the development of gears and a significant quality criterion. Possible causes for elevated noise are faulty materials, deficient thermal treatment or internal stress due to elevated heat generation during gear grinding.
- Loose motor base: If mounting screws on the motor base or flange plate come loose, vibration behaviour will be adversely affected.
- Electrical or magnetic interferences: Spark discharge due to potential formation occurs in frequency-controlled electrical motors via rolling bodies between rotor and stator. These discharge currents may cause damage in the tracks and are responsible for premature bearing failures.
