Tribological Surface Degradation and Tooth-Profile Changes in Oil-Immersed and Dry-Running Timing Belts Under Real Operating Conditions

Belt-in-Oil (BIO) timing belts operate under repeated tooth engagement, cyclic deformation, elevated temperatures, and continuous contact with engine oil. This study comparatively evaluated the surface degradation and tooth-profile geometry of BIO and dry-running timing belts used under real operating conditions. Two BIO belts (S1: 80,000 km; S2: 120,000 km) and one dry-running belt (S3: 80,000 km) were analysed using 2D and 3D digital optical microscopy. Sample S1 showed localised material loss and exposure of fibrous structural elements, mainly near the tooth grooves and roots. More extensive delamination-like damage was observed on S2, whereas S3 exhibited localised crack-like surface damage. Mean profile depths measured at six circumferential locations were 2.477 ± 0.439 mm for S1 and 2.178 ± 0.272 mm for S3, corresponding to mean relative deviations of −23.8% and −37.8%, respectively, from the catalogue reference geometries. The results indicate that local surface degradation and tooth-profile geometric deviation represent distinct condition characteristics. Because the belts originated from different engine designs and material systems, the independent effects of oil exposure and mileage cannot be isolated.