Effect of lubricant viscosity grade on mechanical vibration of roller bearings
Introduction
Depending on some aspects, lubrication in mechanical systems can occur in different regimes: full film, mixed or boundary lubrication. Full film lubrication can be further divided into elastic-hydrodynamic lubrication (EHL), which occurs in non-conformal contacts under high pressure, and hydrodynamic lubrication (HD), occurring under low pressure and usually in conformal contacts [1], [2], [3].
Among the group of mechanical components operating under EHL condition, there are the rolling bearings. This machine element type is one of those more sensitive for development of faults related to lubrication deficiency. According to technical publications of rolling bearing manufacturers [5], [6], from the total of faults found in this type of component, 50–80% are related to deficient lubrication, resulting from inadequate lubricant use, lack or excess of lubricant, lubricant aging, and presence of solid or liquid contaminant. In the face of high percentage of rolling bearing failures, the development of techniques for detection and diagnosis of faults in rolling bearings, due to lubrication deficiency, is a fundamental contribution to the preservation of machine precision.
In terms of monitoring of rolling bearing performance, vibration measurements are among the most used techniques. Nowadays, a lot of works on detection of localized defects in rolling bearing elements through vibration analysis can be found in the literature [7], [8], [9]; conversely, references on the detection of lubrication-induced faults are still found to be less. Among these, there is one [10] pointing out that when a rolling bearing is inadequately lubricated, its vibration response is similar to that of a system submitted to a random excitation. In the case of systems with low damping, the predominant components of such response would correspond to the natural frequencies of the rolling bearing. On the other hand, according to Berry [11], frequency spectra of vibration signals for inadequate lubrication condition are characterized by three or four peaks in the frequency bands from 900 to 1600 Hz, corresponding to natural frequency bands of the rolling bearing. In addition, he affirms that these frequency bands are also seen under adequate lubrication condition; although the vibration magnitudes are much smaller in this case.
In the same context, another study [12] concludes that vibration energy of a bearing depends on surface irregularities, external loadings, running speed and lubricant viscosity. For instance, it was observed in experimental tests that the influences of lubricant viscosity on vibration response depended on speed: for a bearing under large load and low running speeds, increase in lubricant viscosity causes reduction in vibration energy. In contrast, at high running speeds, vibration energy is high when lubricant viscosity is high.
Furthermore, Massouros [13] cites that, in plain bearings under boundary lubrication condition, the journal rotation results in impacts among the micro asperities of the sliding surfaces at contact, producing vibrations, both normal and tangential, to the sliding direction.
All the mentioned works reinforce the already known complexity concerning the analysis of vibration phenomena occurring in mechanical contacts. The present work intends to contribute for improving knowledge on the relationship between vibration phenomena and lubrication in bearings. This is done by studying how a change in lubricant viscosity can affect the mechanical vibration of a rolling bearing by means of basic procedures of vibration analysis.
Section snippets
Methodology
The tested rolling bearings were of NU205 type, presenting the geometry shown in Fig. 1.
Mineral oil without additive was used as lubricant. Three different viscosity grades were tested, ISO 10 (V1), ISO 32 (V2) and ISO 68 (V3), with the purpose of obtaining the vibration response related to different lubrication regimes in the bearing element contacts. The two higher viscosity grades are recommended for bearings operating in the selected test conditions, according to the manufacturers
Oil temperature
Fig. 3 shows oil bath temperature as a function of test time, for the three tested viscosities. A gradual increase in temperature can be observed during the first hour of test and a trend of stabilization in the second hour. This behavior is repetitive for the three tested viscosity grades. According to technical publications of rolling bearings manufacturers [5], both increase and stabilization of temperature occur with any type of rolling bearing, and the stabilization is due to equilibrium
Conclusions
Roller bearings were tested in order to verify differences in vibration response when lubricated with different viscosity oils. The vibration behavior was studied in two main frequency bands and by using a tribological parameter (λ factor). The main conclusions are:
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Changes in lubrication regime of roller bearings due to change in oil viscosity grade could be detected by vibration monitoring. In the tests with ISO 32 and ISO 68 viscosity grades, lubrication regime was of full film type. With the
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