Design and experimental investigation of a nondestructive detector for bolt loosening length of railway track fasteners

Track fastener bolt loosening is inevitable and can lead to unsafe operating conditions. The WJ-8 fastener is widely applied in high-speed railway ballastless tracks in China, and as a result, there is enormous demand for detecting if these bolts are loose once installed. Traditional manual detection methods are inefficient due to reliance on sophisticated operations. Furthermore, existing detection methods based on computer vision, typical sensors, or three-dimensional (3D) reconstruction technology remain susceptible to environmental influences and are limited in certain applications. Therefore, we propose a nondestructive detector that consists of a probe, a dial indicator, and a spring mechanism in this work. The probe’s pivotal parameters were optimized, and then we constructed a prototype of the detector. A series of experimental tests of the detector on both laboratory and outdoor simulated tracks was subsequently conducted. The results show that the average relative error of the detector decreased significantly as bolt loosening length increased, and under 2.00% especially for bolt loosening length over 12.50 mm. In addition, the detector achieved an accuracy of ±0.61 mm and effectively resisted interference. Compared to other representative bolt loosening detection methods, the new detector performed better in terms of both relative error and accuracy. Hence, the detector may become a key tool for railway safety operation and maintenance due to its excellent detection performance and strong environmental adaptability.