# Evaluation of the Variability of Vibration Measurement Results in Rolling Bearing Quality Control
## 1 Introduction  
The term \"vibration measurement\" is very general and requires further specification in terms of the recorded physical quantity, measurement method and evaluation parameter. Firstly, it is necessary to clarify the physical quantity that will represent the vibrating motion. As in the case of any other motion, vibration is characterised by a certain displacement, velocity or acceleration. Therefore, whenever the term \"vibration measurement\" is used, it must be interpreted as a measurement of one of the aforementioned quantities. The choice of the quantity representing the vibrating motion should be based on the range of frequencies emitted by the tested vibrating object. Then, it is necessary to determine whether the measured vibration is relative or absolute. Relative vibration is rolling bearing vibration measured relative to a fixed reference point. Absolute rolling bearing vibration can be recorded by using a vibration sensor fitted directly to the vibrating object. The sensor vibrates in the same manner as the bearing does and allows the vibration to be measured without the need for a reference point. Bearing vibration can be measured in a manner that involves contact, or a manner that is contactless. Contact methods of measuring rolling bearing vibration involve the vibration sensor being in contact with the vibrating object. Contactless methods include measurement methods during which the vibration signal recording device has no physical contact with the tested object. The bearing vibration evaluation parameter is a quantity calculated based on a raw signal. It is necessary to choose a parameter that will contain the maximum amountof useful information in terms of the research problem under consideration. There are many vibration evaluation parameters, including peak value, peak-to-peak value, average vibration value, the vibration's RMS value, kurtosis, skewness, peak coefficient, and aspect ratio. Furthermore, artificial intelligence methods have recently been used to evaluate the research results [1]. Bearing vibration measurements used in quality control aimed at detecting defective products should be regarded as a different problem from diagnostic measurements aimed at monitoring and detecting bearing damage and wear. Bearing vibration measurements related to these two aspects differ both in their objectives and in the methods used. In production measurements, a bearing straight from production is tested. It may only have discrete imperfections, which diagnostic models do not take into account. In production, electrodynamic sensors measuring vibration velocity are used and vibration is measured in a relative (indirect) manner. The bearing is left unbuilt and subjected to a relatively light axial load, with the RMS value of the vibration recorded in three frequency bands at a reference speed that strictly adheres to the recommended standards. Diagnostic measurements, on the other hand, concern bearings at various stages of life, which are often damaged or worn out. Piezoelectric sensors measuring vibration acceleration are used and vibration measurement is carried out in an absolute manner. The tested bearing is built up and the load can be varied both axially and radially. A variety of analysis methods are used in diagnostics, with spectral analysis being the predominant technique and the speed depending on the specific application. The vibration measurement result will be substantially affected by the recorded physical quantity, the measurement method and the given parameter. The issue of selecting these three parameters is universal and concerns the broad meaning of vibration measurement. However, in practice, there are additional metrological conditions and obstacles. Knowledge of the numerous issues related to the elusive dynamic quantity of bearing vibration allows for the identification of the highest number of bearing vibration measurement uncertainty components possible.