Surface Treatment Of Rail Vehicle Fasteners

Bolt connections are widely used in rail transit vehicles. Reliable bolt connections in rail transit vehicles ensure the safe operation of trains. TB/T 3246 is adopted, which provides a reliable theoretical basis for urban rail transit.

Bolted connections will deform or creep under the influence of variable loads, vibrations, and even working temperatures, which will cause the bolt connection to loosen due to the decrease in preload, greatly increasing the alternating stress in the bolts compared to when they are not loose, and generating impacts during the force process, causing fatigue damage to the threads of the bolted connections.

In the design of rail transit vehicles, different anti-loosening measures are taken according to the different risk levels and tightening occasions of the bolted connection parts. The commonly used anti-loosening forms are friction loosening, mechanical anti-loosening, riveting anti-loosening, and structural anti-loosening.

The surface treatment of bolts can directly affect the reliability and service life of the connection function. The torque coefficient K is a combination of the friction coefficient between the internal and external threads, the friction coefficient between the bolt or nut support surface and the pressure-bearing surface of the fastened part and the fastener; it is related to many factors such as the surface treatment, tensile strength, geometric tolerance, thread accuracy, roughness of the pressure-bearing surface of the fastened part, and stiffness of the fastener, among which surface treatment is a key factor. The torque coefficients of different surface treatments vary greatly, sometimes by nearly 1 times.

The main corrosion of bolted connections is contact corrosion, and a surface coating with a controllable range should be selected. In some parts of the vehicle, due to daily maintenance, such as the anti-roll torsion bar bolts and the bottom cover plate connection bolts under the vehicle, due to daily car washing, the cleaning fluid will stay on the bolts near the lower edge of the skirts on both sides, making the surrounding area of ​​the bolt connection often wet; or due to the extremely harsh operating environment, such as the connection bolts on the vehicle bogie, the existing method of applying isolation grease is to apply specific isolation grease to the exposed surroundings of the entire bolt connection after the bolt installation reaches the specified preload, so as to protect the bolt connection from the influence of the environment and ensure driving safety.

At present, the main types of surface coatings for rail transit vehicle fasteners include electrogalvanizing, powder zincizing, Dacromet, Teflon coating, etc.