3-Minute Guide: Regular Bolts Vs. High-Strength Bolts

Bolts can be categorized into regular bolts and high-strength bolts. Taking an M16×70 bolt as an example:

"M" indicates a standard metric thread.

"16" is the nominal diameter (16mm).

"70" is the nominal length (70mm, excluding the nut).

The specification alone doesn't distinguish between regular and high-strength bolts-further inspection is needed. Typically:

High-strength bolts have a grade of 8.8 or higher (e.g., 8.8S, 10.9S, 12.9S).

Regular bolts are below grade 8.8 (e.g., 4.4, 4.8, 5.6).

Key Differences

Material

High-strength bolts: Made from hardened steel (e.g., 45# steel, 40B steel) and heat-treated.

Regular bolts: Typically use Q235 steel without heat treatment.

Strength Grades

Example: 8.8S

First "8": Tensile strength = 8×100 = 800 MPa.

Second "8": Yield-to-tensile ratio = 0.8 → Yield strength = 640 MPa.

"S": Standard type.

Regular bolt (e.g., 4.8): Tensile strength = 400 MPa, yield strength = 320 MPa.

Load Transfer Mechanism

High-strength bolts: Rely on friction between clamped surfaces (pre-tensioned).

Regular bolts: Transfer shear via bolt shank resistance and hole-wall bearing.

Reusability

Regular bolts: Reusable.

High-strength bolts: Generally permanent (friction-type require torque wrench; shear-type snap off after installation).

Installation

High-strength bolts: Require calibrated tools (e.g., torque wrench).

Regular bolts: Tightened with standard tools.