Relationship between bolt tightening torque and preload
- What is the standard tightening torque for M24 bolts?
The standard torque for 8.8 grade M24 bolts is 300-350N·m, and 10.9 grade is 400-450N·m. Too low torque leads to insufficient preload and easy loosening of bolts; too high may cause bolt yielding or even fracture. Use an adjustable torque wrench during installation to ensure it is within the standard range, with a deviation ≤±10%. High-speed railway bolts should be controlled according to the upper limit, and ordinary railways according to the median.
- What is the conversion relationship between torque and preload?
Generally, torque (T) = preload (F) × bolt diameter (d) × torque coefficient (k), with k value about 0.12-0.15. For example, M24 bolt (d=24mm), k=0.13, 300N·m torque corresponds to preload about 80kN (300=80000×0.024×0.13). In practice, k is affected by lubrication and must be determined through tests to ensure preload meets standards.
- How does lubrication affect the relationship between torque and preload?
After applying grease to threads, k value decreases to 0.08-0.1, and preload increases by 30%-40% under the same torque. Without lubrication, k value may rise to 0.2, and preload decreases significantly. The lubrication state must be unified during installation: either all greased or none, avoiding excessive preload differences. High-speed railway bolts are mostly coated with special grease to ensure consistency.
- How much does a 5% torque deviation affect preload?
A 5% torque deviation will change the preload by 5%-7% (due to k value fluctuation). For example, with a standard preload of 100kN, a 5% torque deviation may make the preload 93kN or 107kN. For bolts in curve sections and turnouts, the preload deviation must be controlled within ±5% to avoid loose connections or bolt overload. Calibrate torque wrenches regularly to ensure accuracy ±3%.
- How to avoid damaging bolts due to excessive torque?
Use wrenches with torque limit, which automatically slip after reaching the set value. Follow "progressive tightening" during installation: first tighten to 50% torque, then tighten to the standard value in two steps to avoid one-time overload. For old bolts, torque can be reduced by 10% because thread wear may increase k value. If bolt deformation or thread damage is found, replace immediately and do not continue using.