# Evaluation of the Variability of Vibration Measurement Results in Rolling Bearing Quality Control
## 3 Materials and Methods
Also visible above the bearing under test is the vibration sensor mounting assembly [23]. On the basis of the obtained signal, the RMS value of vibrations in three filtered frequency bands is calculated: low frequency range 50-300 Hz, medium frequency range: 300-1800 Hz and high frequencies 1800-10,000 Hz. The rolling bearing generates a very wide spectrum of vibrfigation frequencies. Separating the signal into three separate bands firstly increases the chance of detecting abnormal  
Figure 1: Automatic measurement of bearing vibrations on a production line. (**a**) general view of the stand, (**b**) view of the measuring head [23]. ities in the bearing under investigation, and secondly helps to pinpoint the presumed cause of increased vibration, as each band is assigned different potential faults. For example, in the low-frequency band we can detect ball diameter difference, ring roundness defect, in the medium-frequency band damage of a ball or inner raceway and waviness errors of mating surfaces, while in the high-frequency band such defects as too high surface roughness of a ball or raceway, contamination and lubrication errors are revealed. The measurement time should be no less than 0.5 s (for 1800 rpm), and the measurement itself should be performed after the readings have stabilised (the variation in the result should be caused by fluctuations and not by the unstable dynamic state of the bearing associated with the start of ball and raceway mating or by insufficiently distributed lubricant yet). A special industrial unit has been introduced for the evaluation of vibration levels. The value of the quantity Anderon is determined by the limiting frequencies of the bands used. This unit is closely related to the rotational speed used (in industrial measurements, the rotational speed of the shaft, and therefore in effect the inner ring of the bearing, is strictly defined and is 1800 rpm):  
\[1\;[\mathrm{And}]\;=\;2\pi\cdot 30\sqrt{\log_{2}\frac{f_{h}}{f_{l}}[\frac{ \upmu m}{s}]} \tag{1}\]  
where: \(f_{h}\)--highest frequency in the band, \(f_{l}\)--lowest frequency in the band. Some companies check the vibration of bearings that have already had grease applied. Measuring greased bearings reflects the actual future behaviour of the manufactured bearing. However, some companies test bearing vibrations before applying a portion of grease. In this case, the mating surfaces are only covered with a thin layer of oil. This is preferable because if the vibration level of the unlubricated bearings does not exceed the critical level, it will even less so after the application of the lubricant (the lubricant additionally dampens the vibrations). There are usually three vibration measuring stations at the end of each production line. Two of these are typically working and are used to measure all finished products about to leave the plant. A bearing leaving the line is automatically placed on one measuring device and, after measuring one side of the bearing, is placed on the other in a reverse manner. Measuring two sides of the bearing on two stations is simpler to automate. This also reduces the inspection time on the line. In many cases the line clock of vibration measuremnt is approximately 8 s. A third device at the end of the line is used to recheck (manually) any rejects from the automatic inspection. In addition, there are additional manual devices in the laboratories of industrial companies already used for in-depth inspection of bearings, checking new developments or statistical studies. Examples of laboratory devices are presented in Section 4. In order to illustrate the resulting difficulties when comparing several systems, an experiment involving a series of 50 measurements of the same bearing type 6205 ZZ, on the same side, at the same point on the outer ring's circumference, on three different measurement stations, was conducted. The tested systems were built by three different entities and differ in some design solutions.