01
Data Acquisition
First, raw image data of the tooth flanks in the IR spectrum is acquired under various load conditions. These datasets form the basis for the subsequent difference calculation.
Watch videoAutor: Sadler Fränz, edevis Gmbh
Automated wear pattern inspection
Gearboxes are key components in power transmission within mechanical engineering, automotive engineering and power generation. Deviations in tooth flank contact lead to increased wear, noise and power losses – sometimes with serious consequences for reliability and service life. This is where the thermographic contact pattern inspection developed by edevis comes in: it records load-induced temperature patterns on tooth flanks non-contact and without the use of additives, thereby ensuring that power transmission takes place precisely at the defined contact point. The thermographic contact pattern inspection can be integrated into the manufacturing process as an automated quality control measure.
Fig. 1 Conventional contact pattern inspection using ink paste
Introducion
The quality of the gearing determines the efficiency, smoothness and service life of a gearbox. This relationship is particularly evident in electric mobility: without the masking background noise of the internal combustion engine, gearing defects become immediately apparent acoustically and are instantly noticed by both drivers and quality inspectors. Yet its relevance extends far beyond the powertrain of passenger cars. Whether it be commercial vehicle gearboxes under continuous load, wind turbines with their extreme operating cycles, or industrial gearboxes in process manufacturing – wherever torque needs to be transmitted reliably, precise wear pattern inspection is essential.Conventional testing methods reach their limits here. Manual dye application yields subjective results, is time-consuming and is difficult to integrate into automated production lines. Thermographic contact surface inspection bridges this gap: it measures temperature patterns on the contact surfaces non-contact, reproducibly and in real time, thereby making the contact surface accessible for fully automated 100% inspection without the need for chemical additives or subsequent cleaning.
Background
To date, particularly in the case of hypoid gearboxes, tooth contact testing has been carried out by applying dye and then assessing the results manually. To do this, a thin layer of dye (dye paste or varnish) is applied to the tooth flanks to be tested. When the gears are subsequently brought into mesh, the paint is removed at the points of contact through pressure or abrasion. The resulting image – for example, the displacement of an oil-resistant paste on the gearing – shows where the flanks are actually in contact. A specialist then visually assesses whether the position and size of the impressions correspond to the target condition.In the event of deviations, adjustments are made – for example, by retroactively shifting the gears relative to one another (e.g. using shims) or by proactively adjusting the manufacturing parameters. Furthermore, all paint residues must be removed without leaving any traces prior to commissioning.
However, the traditional contact pattern inspection using paint application has several disadvantages:
Each inspection process requires the application of dye, the joining of the parts and subsequent cleaning, entailing increased time and labour costs
The assessment is visual, often based on a cursory inspection. The result depends on the inspector's experience and interpretation.
The mark shows only the aggregate contact pattern; often, due to time constraints, the contact point is inspected on only a partial segment of the gear. Local deviations at individual contact points may be overlooked.
Manual ink application is difficult to integrate inline into automated production; 100% inspections are practically impossible to implement.
Uneven or incomplete ink application can distort the image. These limitations create a need for a more objective, efficient method, which is where thermographic contact pattern inspection comes in.
Fig. 2: Layout of the measuring cell with camera and drive unit
Thermographic pattern inspection
In thermographic contact pattern analysis, the contact pattern of a gear pair is visualised using the resulting thermal images. Under a defined mechanical load, the friction between the tooth flanks (rolling-sliding) generates a characteristic thermal pattern. This pattern is captured by an infrared camera for both the leading and trailing flanks, accurately mapped to the tooth geometry, and the position and shape of the contact pattern are then analysed.
System architecture and measurement setup
The test system consists of an automated load test bench with a drive unit, a thermal imaging camera and associated sensor technology (see Fig. 2). The gearbox under test is fitted with torque transducers between the input and output sides and operated at a defined torque. The infrared camera is aligned so that it captures the meshing tooth flanks of a gear. The synchronised control and evaluation software coordinates all components. This ensures that the camera records defined sequences whilst the drive motors are controlled accordingly. At the same time, the software processes the captured thermal images; the measurement data is analysed in real time and thus provides a relevant control variable for upstream manufacturing and assembly processes.
Fig. 3: Procedure for thermographic wear pattern analysis
Objective assessment and identification of the load distribution
The process of analysing thermographic tread pattern inspections can be divided into four steps, as shown in Fig. 3. The aim is to obtain an objective and quantitatively measurable representation of the actual tread pattern from the thermograms captured.
02
Difference Calculation and Segmentation
In the next step, the measurements are compared. A difference calculation is used to eliminate background components, thereby revealing the temperature increases caused by contact. The difference images reveal the areas where the tooth flanks were actually in contact. This is followed by the segmentation of relevant areas: an algorithm identifies the contact areas within the region of interest (ROI) on the tooth flank and separates them from non-contact areas.
03
Rectification to the projected surface
As the tooth flank appears distorted due to its curvature and the camera perspective, the segmented contact image is mathematically rectified. This geometric projection transfers the detected contact image onto a flat surface that corresponds to the actual tooth flank geometry. This allows the contact image to be analysed metrologically and compared with CAD data or target contours.
04
Objective evaluation and feature extraction
In the final step, the relevant parameters are automatically calculated from the rectified surface. These include, in particular, the position of the centre of gravity of the contact point relative to the tooth flank. Based on defined target values and tolerance limits, the algorithm determines whether the gear meets the requirements.
Advantages of thermographic contact pattern inspection
Temperature data can be quantitatively analysed, ensuring repeatability and comparability. Thermography also achieves very high sensitivity and resolution: even subtle contact deviations become visible as temperature differences, with sub-millimetre precision relative to tooth size. In addition, thermography allows each tooth pair to be assessed individually rather than yielding only an overall impression. This allows conclusions about potential noise generation. Thermographic contact pattern inspection is significantly faster than the manual method – in preparation, measurement, and post-processing alike – enabling 100 % inspection in series production. Automation reduces staffing requirements and eliminates costs for consumables such as marking compound and cleaning agents.
Automation and Documentation
With thermography, every inspection result is automatically available in digital form and can be fully documented. This ensures traceability, provides a benchmark for upstream manufacturing steps, and makes trends visible over time. The automatic analysis of thermograms ensures consistent assessment against defined criteria: the algorithm determines whether each contact pattern lies within the defined tolerances. Taken together, this transforms contact pattern inspection from a manual procedure into a digitally integrated process step that also supports manufacturing optimisation.
Fig. 4 Live display of the thermographic results
Integration into existing processes
Thermographic load profile testing is suitable for in-line use and can be integrated into existing production lines. To test both flanks – the tensile flank and the compressive flank – the gearbox is loaded in both torque directions. This requires a drive concept capable of applying both driving and braking loads. If the tension and compression flanks are to be recorded separately, an additional handling system – such as a 6-axis robot – is required to position the camera for each flank direction. If only one flank is being examined, this handling step can be omitted. In enclosed or highly complex assemblies, design modifications or additional optical components such as mirrors or IR windows may therefore be necessary. As metallic surfaces reflect IR radiation and external heat sources distort the measurement image, the inspection area is thermally shielded. Targeted encapsulation of the measurement zone decouples the infrared measurement from external interference and ensures reproducible measurement conditions. Economic factors should also be taken into account. Due to the high inspection speed, integration is particularly worthwhile for medium to high production volumes or complex assemblies. For very small production runs or simple components, the conventional method may suffice. Typically, the thermography station replaces a manual test bench and can be installed in its place or, to save space, in locations where a conventional test bench would not fit. With early planning regarding installation space, cycle time and interfaces, the system runs stably and reliably once set up.
Fig. 5 External view of the test rig
Conclusion
Thermographic contact pattern inspection fills a gap that conventional dye-based methods are structurally unable to address: it delivers objective, reproducible results – with precise edge definition, no residue, and cycle times suitable for mass production. Where manual methods fail due to subjectivity, cleaning requirements, and limited automation, the edevis thermography system enables 100 % inspection under real-world load conditions.The technology is already in series production use in the automotive sector. As quality demands rise in commercial vehicles, industrial drives, and power generation, so does the range of applications – wherever torque must be transmitted reliably and reliability is critical.
Any questions about contact pattern inspection?
We are happy to support you in finding the right solution for your contact pattern inspection requirements. Whether for research, series testing or individual special applications, our thermography technologies offer a wide range of possible uses. In a non-binding discussion, we will work together to determine which system best fits your application.
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