The high demands placed on the reliability and quality of rolling bearings are making ultrasonic testing an increasingly important part of quality assurance. The primary material, cylindrical bearing rings, tapered bearings, spherical roller bearings, ball bearing rings, rolling elements, cylindrical rollers, tapered rollers, barrels, rollers and balls are tested.
Raw material testing in the form of a purity test with ultrasound according to SEP 1927
Before the component is manufactured, the starting material (flat or bar material) is subjected to a macroscopic degree of purity measurement. Typical defect classes for the used steel are KSR 0.3 – 0.5 mm.
Fig. 1: PROline ultrasonic inspection system for the inspection of bars. The ultrasonic inspection is carried out with three probes as spiral scan. During the inspection, the probes move along a common axis along the component and thus cover the entire volume of the bar material.
Fig. 2: Test result display and evaluation of a purity test according to SEP 1927 using a ScanMaster ultrasonic testing system
Outgoing goods inspection by ultrasonic testing of the functional and core area of the rolling bearings
In production, highly stressed bearing components such as railway or wind turbine bearing rings and rolling elements are ultrasonically tested to ensure that they are free of defects during production. Conventional and innovative ultrasonic testing methods, such as phased array technology, are used. In addition, ultrasonic tests on components are carried out in the test laboratory to accompany long-term load tests and are correlated with the results of the loads for component optimisation.
The ultrasonic testing is carried out contactless in immersion technology. Corresponding testing systems have several linear axes that move the test head in space and a turntable that centers the component and rotates it at high speed. The component is thus moved past one or more probes over 360°. The probe follows the geometry of the component in such a way that the angle of incidence into the material or the angle of incidence on the functional area (near the running surface) remains the same over the height of the bearing ring. In this way, scan plans for almost any geometry can be run automatically. Regarding the sensitivities, a distinction is made between the core area and the functional area, whereby the latter is not very deep and directly adjoins the running surface.
The operator/evaluator can evaluate the results in the form of imaging display lists. Any existing defects can be evaluated with regard to their size and depth. The components are usually inspected with a vertical beam on the surface or alternatively with a vertical beam on the rear-wall surface.
Fig. 3. spherical roller bearing, C and D image of the volume aperture online during testing, amplitudes and sound paths, 0 degrees, indications > KSR 0.3 mm in the volume
In order to be able to detect inclined indications which do not reflect the ultrasound, one works in parallel with the back wall shading. Here the constancy of the reflected echo of the running surface is monitored. In case this echo becomes smaller, there is a shadow effect caused by an oblique defect lying in front of the running surface.
In production, testing is mainly carried out perpendicularly to the scanning surface or perpendicularly to the functional surface and, in addition, the rear wall shadowing is monitored. Thus, high inspection sensitivities of KSR 0.3 – 0.5 mm can be realized. The defect resolution is limited by the microstructure indications of the material (background noise). By means of intelligent online evaluations, defects can be detected in the functional range up to 0.5 mm to the surface at any angle of incidence.
Monitoring rolling bearings during their life cycle by means of recurring ultrasonic testing
Manufacturers of highly stressed components are subject to high quality requirements. Downtimes caused by premature component defects lead to considerable costs for the customer and possibly also for the manufacturer. The reliability and service life of these components therefore have a decisive influence on their economic viability.
In wind turbines in particular, there is the problem of premature fatigue of the bearing rings installed. This is caused by the phenomenon known as white etching cracks (WEC). White etching cracks are irregular microstructural changes below the material surface, which only become apparent after installation and continuous high loads. These microstructural changes cause cracks and spalling to develop underneath and on the surface of the components, leading to premature bearing failure. Recurring ultrasonic inspections of wind turbine bearings and rolling elements that have been removed in the meantime allow WEC damage to be detected at an early stage and cost-intensive downtimes to be reduced.
Individually adjustable inspection programs offer the possibility of inspecting defects at different angles, such as 0°, 45°, 60° and 70° to the running surface. Depending on the expected defects, an optimized sonication is performed so that the expected material damages are sonicated at 90° to their extension as far as possible.
In addition to test frequencies of 5 – 10 MHz, high-frequency ultrasonic tests of 25 – 50 MHz are also performed close to the surface from the bearing ring running surface.
Figure 4 shows the C-scan of a preventive ultrasonic inspection of a bearing ring. The first small white etching cracks can be seen below the running surface. No damage can be detected from the outside at this stage.
Fig. 4. C-scan of WEC damage of a bearing ring. The damage under the track is larger than 1mm
As an alternative to conventional inspection technology, the use of phased array ultrasonic inspection systems is also useful in some cases. Here, probes with a high number of oscillator elements (e.g. 32, 64 or 128) are used and controlled electronically in parallel as well as with multiplexers. This allows different sound fields without hardware changes as well as the testing of larger areas at the same time and/or focusing in different depth positions as well as variable angles of incidence during the test. This opens up new possibilities in inspection technology, but also increases costs.
Fig. 5. phased array ultrasonic immersion inspection of a bearing ring taken with a synthetic focused 128-element PAUT probe. The figure shows the zoomed view of two measured circular disk reflectors (flat bottom holes) with 0.5 mm diameter, located at least 0.5 mm away from the bearing rolling surface of a spherical bearing. Due to the bearing geometry and the position of the circular discs, the reflectors are detected with different sound paths
Inspection systems for ultrasonic testing of rolling bearings
We supply inspection systems for the inspection of the pre-material, the finished components as well as for the recurring ultrasonic inspection of components in operation. They have been developed for both the laboratory and networked production (Industry 4.0).
We also offer the possibility of testing components in service as an extended workbench or as a production facility networked with production in series.
Conventional ultrasonic inspection systems with one to four channels are used, as well as phased array systems with multi-element probes, which allow the complete bearing surface or a large part of the bearing surface to be inspected with one rotation.