Selecting the correct bolt strength grade requires a comprehensive consideration of the working load and environmental conditions. The core principle is to match the mechanical requirements with the service environment, avoiding "under-rated failure" or "over-rated backlash."
Your question is crucial-in engineering practice, a seemingly minor bolt selection error can trigger a chain reaction of structural risks. Especially in equipment design, maintenance, or project construction, scientific selection not only ensures safety but also optimizes costs. Below, I will outline a clear selection logic for you from two dimensions: load type and environmental conditions, combining standards and practical experience.
Determining the Strength Grade Based on Working Load
Working load is the primary factor determining the bolt strength grade. Different load types have vastly different effects on bolt stress.
Table
Load Type | Characteristics | Recommended Strength Grade | Description
Static Load (e.g., brackets, cabinet fixing) | Stable load, no vibration | 4.8 ~ 6.8 | Ordinary carbon steel bolts are sufficient, low cost
Dynamic Load/Impact Load (e.g., engines, stamping equipment) | Repeated stress, prone to fatigue | 8.8 ~ 10.9 | Requires high yield strength to resist plastic deformation, RAHT process (heat treatment followed by thread rolling) is recommended to improve fatigue life
Alternating Load (e.g., wind power, bridges) | Periodic tension and compression, high fatigue risk | 10.9 or 12.9 | Stress amplitude must be controlled, preload should reach more than 70% of yield strength to prevent loosening
Eccentric Load (e.g., cantilever structures) | Additional bending moment, stress concentration | 8.8 and above | It is recommended to increase the bolt diameter or use MJ threads to reduce stress concentration in the head-and-shank transition zone
