Bolt Material Selection and Anti-Loosening Measures
Common ones include Q235 steel, which has low strength (tensile strength 375-500MPa) but good plasticity and toughness, suitable for parts with small stress, such as ordinary bolts for sleeper connection. No. 45 steel, after quenching and tempering, has medium strength (tensile strength 600-800MPa) and good comprehensive performance, widely used in medium-stress parts such as fishplate connection bolts. There are also high-strength bolts, such as 8.8-grade and 10.9-grade bolts, which are mostly made of alloy structural steel such as 40Cr, with high strength (8.8-grade tensile strength >=800MPa), capable of withstanding large loads, and suitable for key connection parts of high-speed and heavy-haul railways.
For connections with small stress, such as the fixing bolts of under-rail pads, Q235 steel can be selected, which is low in cost and can meet the requirements. For medium-stress parts, such as fishplate connection bolts of ordinary railways, bolts made of No. 45 steel after quenching and tempering are more suitable. In high-speed and heavy-haul railways, bolts that bear large tension and shear force, such as high-strength bolts of rail fasteners, need to be 8.8-grade and above high-strength alloy structural steel bolts to ensure the reliability of the connection.
The vibration generated during train operation will cause relative movement between the bolt and the nut, reducing the friction force of the threaded connection and gradually loosening. The alternating load generated by vibration will cause the bolt to be repeatedly stretched and compressed, causing elastic and plastic deformation of the bolt, which in turn leads to a decrease in preload. In addition, the processing accuracy of bolts and nuts is not high, the fit gap is too large, or the preload is insufficient during installation, which will also aggravate the loosening phenomenon.
- What are the common anti-loosening measures for bolts?
There are friction anti-loosening, such as using spring washers, which use the elastic deformation of the washer to maintain pressure between the bolt and the nut and increase friction; double nut anti-loosening, which prevents loosening by tightening two nuts to generate relative force between them; mechanical anti-loosening, such as using cotter pins with slotted nuts to fix the nut on the bolt to prevent it from rotating; and permanent anti-loosening, such as applying thread locking adhesive on the thread, which bonds the bolt and nut together after curing, suitable for parts that do not need to be disassembled.
A torque wrench can be used to directly measure the tightening torque of the bolt, and judge whether the preload meets the standard according to the conversion relationship between torque and preload. For bolts in important parts, an ultrasonic preload measuring instrument can be used to accurately calculate the preload by measuring the elongation of the bolt under the action of preload. Regularly sample the preload of bolts, and timely retighten the bolts with insufficient preload to ensure the tightness of the connection.