Rolling contact bearings are amongst the most important machine parts, because they can transfer forces and movements. They are used in almost all drive engineering areas. Their designs are always dependent on the use. To ensure a calm and persistent use of the rolling contact bearings, greases and oils must be applied. The lubricants are mixed with additives, which give the lubricants additional properties.

When using additives, the properties of the lubricant can be changes so significantly, that the usage of the rolling contact bearings become unfavourable and the long-term handling is endangered. The rolling contact bearings are therefore tested at a test station and are tested with new oils. On the one hand, acoustic measurements and on the other hand abrasion tests are performed.

DIN norms, like the DIN ISO 15242-1 up to DIN ISO 15242-3, dictate testing methods that determine the maximal noise emission in the bearings. Din norms, like the DIN 51819 are used to determine the wear rate when using different lubricants on cylinder roller thrust bearings. Different test rigs are used in these DIN norms.

Depiction 1: Process diagrams of the Optimzer4D-software. Above a bumpy bearing race can be detected after a run-time of six hours, with high values in friction and acoustically audible frequencies. The bearing test was done with only a little wear protection additives. Below is a bearing race after six hours with the same approach. It shows less friction and lower noise.

Monitoring with QASS Optimizer4D

The built-in sensors of the testing stations are for analysing the bearing surface or strong noise generation when rolling the rolling body. The frequencies which develop are scarcely audible but mostly inaudible to the human ear. Especially the inaudible signals are interesting for evaluating the bearings. Specific frequency areas ≥ 50 kHz describe friction noise, unstable bearing races, and the development of early damages. Both, microphones and piezo-electrical elements are used. The commonly used acceleration sensors are too inertial and not built to detect these kinds of highly frequent signals. Especially since they can occur in short intervals of a few milliseconds. Acceleration sensors detect the smallest movements, which can be started by impulses, which mostly come from damages on the bearing’s surface, which, however, have already been there for a long time. The door has been shut after the horse has bolted.

QASS Optimizer4D uses vibration sensors to detect the inaudible frequencies. The style of the sensors allows listening to the whole frequency spectrum, which is then sampled highly frequent with 100 MHz. The simultaneous data acquisition leads to every change in the bearings becoming visible. The vibration sensors convert the inaudible sounds into electrical currents, which simultaneously are shown as time-amplitude-signals. The signal is directly transformed using a mathematical formula (Fast-Fourier-Transformation), so that additional information becomes available. The signal is depicted in such a way that the assignment of frequencies to their corresponding amplitudes in the corresponding point of time is possible. The therewith composed spectra give an extensive summary of the quality of the bearings and lubricants.

Through this technology, friction noise and the smallest roughness on the surface of bearings can be detected. The friction noise is related to the used lubricant and highly dependent on the used additives.

Depiction 1 shows a comparison between two lubricants during an FE-8 testing, which was done on the basis of a basic lubricant. The lubricants differ only in the combination of their additives. In this case, the proportion of the wear protection additives is different. The test was done in an FE-8 test rig under the same axial pressure and temperature.