Bearing failure is one of the most common faults in industrial machines. Proper condition monitoring is therefore of the highest importance. There are two main groups of bearing types:
Journal and plain bearings are in general tight-fitting cylinders, which encompass a shaft with some lubrication in between. The plane bearings are monitored with vibration, temperature and oil pressure as the most important parameters. Bigger radial bearings are normally monitored with a pair of displacement sensors, in order to measure shaft position, and shaft orbit and spectra are the major part of the vibration monitoring task.
High- or low-pressure, oil-lubricated journal bearings are normally used in specific applications and have advantages over rolling-element bearings:
- Relatively inexpensive
- Lower running friction
- Higher loading capacity
- Relatively smaller
- Have a large support damping
Due to clearances between bearing surface and the shaft journal, non-contacting sensors are required to provide information about shaft hydro-static and hydro-dynamic behaviour relative to the bearing. Uneven oil pressures in the bearing clearance can induce vibrations (oil-whip and whirl) and various configurations of journal bearings are available to reduce these effects.
Due to different orthogonal-axes bearing stiffness the sensors are mounted in pairs, at 90º to each other in the radial plane, to determine the shaft’s static position and dynamic movement during rotation. This configuration allows analysis of the shaft’s orbit during rotation to provide information and trace the source of problems. The orbit shape reveals load direction, existence of unbalance, bent shaft, misalignment, rubbing, oil-induced vibrations or other problems.
Rolling element bearings
Rolling element bearing covers the whole family of Roller and Ball bearings, and the monitoring method is similar for Radial bearings of both types. The condition monitoring is normally done with one radial accelerometer and the use of some special measurement techniques with envelope detection as most important. REBs with thrust loads need additional measurements in the axial direction.
Discrete faults in ball or roller bearings cause a series of impacts at a frequency determined by the location of the fault, e.g. outer race, inner race, rolling-element, etc. in the bearing.
At the initial stages when the fault is still microscopic the impulses are so short that the frequencies can extend up to 300 kHz. These impacts excite structural and other resonance, including the resonance of the accelerometer, and produce a series of bursts, with a frequency content dominated by these resonances. This bearing signal is masked by other background vibrations from the machine, and the basic problem is to find a frequency range where the bearing signal is dominant over background vibration.
The repetition rate is indicated better by analyzing the envelope of the bursts rather than the raw time-signature. It is possible to calculate the repetition frequency of the bursts knowing the bearing data and using simple classical mechanics, but this calculation assumes purely a rolling action, whereas there is really some sliding action as well. Therefore the equation should be regarded as only approximate. Amplitude modulations may also produce sidebands.
Modern predictive maintenance software programs have built-in bearing databases from various manufacturers to simplify these calculations, and the resultant bearing damage frequencies can be superimposed onto the spectrum or envelope curve as an additional bearing analysis aid.
Read more about typical damages at rolling element bearings.