Bolt thread accuracy and installation adaptability
- What are the thread accuracy grades of bolts and their applicable scenarios?
Ordinary precision (6g) is the most commonly used, with a large thread tolerance range, which can meet the connection needs of ordinary railways, low cost, and high tolerance to hole position deviation during installation (±0.3mm), suitable for non - key parts such as sleepers and fishplates. Precision grade (5g) has small thread tolerance and high dimensional accuracy, with a fitting gap ≤0.1mm during installation, suitable for key connections of high - speed railways and heavy - haul railways, such as turnout bolts, which can ensure uniform preload and reduce loosening risk. Rough grade (8g) has the lowest accuracy, only used for temporary connections or parts with small stress, such as fixing railway ancillary facilities, not suitable for the main track structure due to poor connection reliability and easy loosening. Special precision grade (4h) is used in scenarios with extremely high connection accuracy requirements, such as bolts connecting bridges and tracks, which can control preload deviation within ±2% to ensure long - term stability, but the processing cost is high.
- What installation problems can insufficient thread accuracy cause?
Installation difficulty is a common problem. Low thread accuracy will cause "sticking", that is, the bolt is loose and tight when screwed in, or even cannot be fully screwed in, requiring forced knocking, which may damage the thread. The sticking rate of 6g precision bolts in ordinary railways is about 1%, while that of 8g is 5% - 8%. Preload is uneven. Threads with insufficient accuracy have small contact area with nuts, and stress points are concentrated. The preload deviation of the same group of bolts may exceed 15%, leading to uneven stress on rails and affecting track stability, which is more sensitive to heavy - haul railways. Connection loosens quickly. Low - precision threads have large fitting gaps, and are prone to relative sliding under vibration loads. The preload attenuation rate is 2 - 3 times that of precision threads. 6g bolts in ordinary railways attenuate by 10% in 3 months, while 8g reaches more than 25%. Accelerated thread wear. Threads with insufficient accuracy have irregular tooth profiles, increasing friction during meshing, shortening service life by 30% - 50%. Bolts for frequent disassembly (such as maintenance) wear more obviously.
- How to detect the accuracy of bolt threads?
Using thread gauges is a standard method. The go gauge can be screwed in smoothly without loosening, and the no - go gauge is screwed in no more than 2 threads (about 3mm for M24 bolts), then the accuracy is qualified. 6g precision bolts must 100% pass gauge detection. Measure the thread pitch diameter, measure with a three - wire method on a thread micrometer. The pitch diameter deviation must be within the specified range (such as M24 bolt 6g grade pitch diameter 19.95 - 20.05mm), otherwise it is judged as unqualified. Check the thread profile, observe with a projector. The profile angle deviation (60°±0.5°) and profile half - angle deviation (30°±0.5°) must meet the standards. Excessive deviation will affect meshing quality and cause uneven stress. Detect surface roughness. The thread surface Ra ≤1.6μm. Excessive roughness (Ra＞3.2μm) will increase friction coefficient and cause preload fluctuation. Lathe or rolling process is needed to ensure roughness.
- What are the requirements for the fitting accuracy between bolt threads and nuts?
Thread accuracy grades must match. 6g bolts are usually matched with 6H nuts, and 5g bolts with 5H nuts. If the grade difference is too large (such as 6g bolts with 8H nuts), the fitting gap will increase by more than 0.2mm, leading to connection loosening and accelerated preload attenuation. The pitch diameter fitting gap should be controlled at 0.05 - 0.2mm. Too small gap will cause installation difficulty, and too large gap will lead to loose connection. The pitch diameter gap between M24 bolts and nuts is usually 0.1 - 0.15mm to ensure easy installation and sufficient force transmission. Profile consistency is important. The sum of profile angle deviations of bolts and nuts must be ≤1°, otherwise additional stress will be generated, accelerating thread wear. Check with a profile template to ensure shape matching. The thread contact area must be ≥70%, that is, more than 70% of the meshing thread teeth are in close contact. Detect by coloring method. Insufficient contact area will cause local stress concentration and reduce bearing capacity.
- What should be noted when matching bolts and nuts of different materials?
Steel bolts and steel nuts are the most common. The hardness difference between them should be controlled at 50 - 100HV, such as 8.8 grade bolts (HV320 - 360) with 8 grade nuts (HV240 - 280), to avoid seizing due to too close hardness. Apply grease during installation. When stainless steel bolts (304) are matched with steel nuts, an insulating gasket must be added on the contact surface to prevent electrochemical corrosion. Because the potentials of stainless steel and carbon steel are different, direct contact in humid environments will generate current, accelerating rust, which is more important in coastal areas. When high - strength bolts (10.9 grade) are matched with high - strength nuts, the same grade materials must be used, such as grade 10 nuts, otherwise the nuts are easy to deform, leading to preload reduction. Turnout bolts strictly follow this requirement. Composite material bolts (such as glass fiber) need to be matched with nuts of the same material. Because the expansion coefficients of composite materials and metals are different, additional stress will be generated when the temperature changes, leading to thread loosening, which is only suitable for low temperature difference environments.