THE POWER OF SCATTERED LIGHT: CHECKING ROUNDNESS, WAVINESS, AND ROUGHNESS IN A SINGLE PROCESS

Bearings are precise machine components with extremely tight specifications when it comes to the roundness, waviness, and roughness of rolling elements and bearing rings. Production processes are subject to tolerances down to the submicron. There is a special measuring device for checking roundness and waviness, and another for surface roughness. Parts are tested randomly in the lab and this causes certain problems. To do justice to increasingly strict quality requirements, it is becoming more and more important to conduct a larger volume of random tests. But at the same time, it should not take up more time to do quality checking. So what happens next? Experts at the Würzburg-Schweinfurt University of Applied Sciences, the Steinbeis Transfer Center for Bearing Technology in Herzogenaurach, and OptoSurf from Ettlingen think they have found the solution: an optical measurement technique based on scattered light.

Their technology uses light deflected by surfaces. These deflections make it possible to gauge roundness, waviness, and roughness in a single sweep. To keep the work environment clean, measurements can be made automatically with scattered light and integrated into production processes such as honing and grinding. Cycle times are also shorter than the time taken with contact measurement methods. A team of researchers at Würzburg-Schweinfurt University of Applied Sciences, the Steinbeis Transfer Center for Bearing Technology, and OptoSurf have been examining measurement technology in more detail as part of a shared project sponsored by the Bavarian State Ministry for Science and the Arts and the European Union. The initiative also comes under a European Social Fund (ESF) project focusing on digital knowledge sharing in the field of innovative measurement technology for SMEs. The ESF project is being carried out at Würzburg-Schweinfurt University of Applied Sciences.

LIGHT SCATTERING TECHNOLOGY

Light scattering technology is an alternative approach to measuring and capturing the micro-geometry of engineered surfaces. The angle-resolved measurement of scattered light is based on the laws of light reflection and a model of “mirror facets.” When light strikes a rough surface, it is deflected by micro-angles[6]. This deflected light is transformed to the focal plane by the processes of Fourier optics. A detector is used to record the distribution of intensity, which corresponds to the frequency distribution of the scattering angle. This scattered light method is also capable of assessing the macro-geometry (form profile) of surfaces. The readings taken with scattered light measurement can be calibrated. Roundness and waviness can be equated to international standards. The optical result for surface roughness (Aq) is a new parameter that does not correlate with more generally used values such as Ra and Rz, but instead with the occasionally used value Rdq.

FOURIER ANALYSIS OF FORM PROFILES

One suitable method for predicting roughness is to analyze geometric profiles based on Fourier analysis, also because bearings are stimulated harmonically. “The underlying idea is that a track ball on the ring surface should stimulate vibrations in the bearing. There are different ways to stimulate the bearing: broadband excitation or harmonic excitation,” explains Dominik Helfrich, a director at the Steinbeis Transfer Center for Bearing Technology. Harmonic excitation is more pleasant because it just generates one tone, which can change during modulation[1].

The bearing industry usually measures shape and waviness in a precision measurement room, ideally with a shape tester. “It’s not recommended to do that in a production environment because vibrations can skew the measurements. It’s easy for those skewed measurements to result in expensive misinterpretations,” highlights Prof. Dr.-Ing. Stephan Sommer, director of the Steinbeis Transfer Center for Bearing Technology and Professor at Würzburg-Schweinfurt University of Applied Sciences. To determine roundness, a low-pass filter is used. This eliminates high-frequency constituents (waviness and roughness). To assess high-frequency constituents, the profiles are examined using fast Fourier transformation (FFT). Frequencies in the mid to high frequency range (>25 waves/cycle) often result in complaints due to the noise they create[1].The milling procedure of honing results in optimized surface treatment, improved roughness, improved absolute form deviation, and a reduction in the amplitudes of frequency spectrums. As a result, conducting a Fourier analysis of form profiles is a standard procedure in quality assurance.

LIGHT SCATTERING TECHNOLOGY IN THE BEARING INDUSTRY

Assessing quality in bearing production requires precise measurements. Tolerances are extremely tight and manufacturers have to produce large batches within short cycle times. Until now, using conventional measurement techniques has only made it possible to analyze samples in order to see if they are compatible with processes. This makes it easy to overlook random errors in the production process. As a result, it would be hugely beneficial to have 100% in-line monitoring processes. The experts at Steinbeis have been developing a test machine with experts at Würzburg-Schweinfurt University of Applied Sciences. Their device contains a scattered light sensor for measuring the form, waviness, and roughness of rolling elements. It can also be integrated into production lines. This makes it possible to offer 100% monitoring of the rolling path of outer rings. One typical defect encountered in the serial production of ball bearings is that the rolling path is not honed properly. When this happens in the area where rolling elements come into contact with the body of the roll, at some point there’s a good probability that the bearing will get loud. Such errors can only be discovered by chance during random inspections.

The project team ran tests with scattered light and defined 1,024 overlapping measuring points for scattered light all around the bearing ring. Well-honed surfaces had Aq values within the defined tolerances. Polished rings had Aq values significantly higher than the tolerances. This makes it possible to identify specific areas that have been badly honed. Similar results can be achieved by assessing the geometric form of the ring. With an inadequately honed bearing, the roundness profile will show signs of waviness in the area around the milled surface. The amplitude spectrum will also reflect this error and as a result, the ring will have defects and be removed after the declining tolerance curve is noticed during 100% checking. The test machine developed for the research project takes less than a second to measure and evaluate a bearing ring. One key prerequisite for taking measurements is a clean surface.

Another application area for scattered light technology is measuring the waviness of rolling elements on ball screw drives. Ball screws are a common component in the electronic power steering of modern cars. The surface quality of the rolling path plays a decisive role when it comes to the noise generated by the steering system, and a key aspect of this is the waviness of the area of the rolling element that comes into contact with the ball and ring. Final processing can eliminate waviness, which is typically caused by previous processes. One major challenge is the different functional emphasis of bearings during operation. If final processing only concentrates on one side of a bearing, it is not possible to conduct a meaningful evaluation with measured coordinates since both walls are measured. The scattered light device is able to swivel the sensor and specifically attribute the waviness of both walls to the rolling path.

The demand for high-quality rolling contact bearings is rising in the automotive industry. Scattered light provides a traceable measurement technology that calculates statistical roughness values (Aq) and is adaptable to different production methods such as honing and grinding. At the same time, it offers integrated macro-angles making it possible to ascertain geometric profiles. The technology is robust, fast, non-contact, and can be used in manufacturing areas to carry out 100% monitoring of production processes. As a result, the research team is completely won over by the idea!