3-Minute Guide To Ordinary Bolts Vs. High-Strength Bolts

Bolts can be categorized into ordinary bolts and high-strength bolts. Taking an M16*70 bolt as an example, the letter "M" indicates a standard metric thread specification, the number "16" denotes a nominal diameter of 16mm, and the nominal length is 70mm (excluding the nut). Based solely on these specifications, it is impossible to determine whether the bolt is ordinary or high-strength, so further inspection is required.

Generally, bolts with a performance grade of 8.8 or higher are classified as high-strength bolts (which can be further divided into hex head type and twist-off type, depending on the design). Those below grade 8.8 are considered ordinary bolts.

So, what are the differences between high-strength bolts and ordinary bolts? They can be distinguished based on their manufacturing materials, strength grades, load-bearing characteristics, and applications.

(1) Different Raw Materials:

High-strength bolts are made from high-strength steel, such as No. 45 steel or 40 boron steel, for the bolt rod, nut, and washer.

Ordinary bolts are typically made from Q235 steel.

High-strength bolts undergo heat treatment after manufacturing to enhance their strength and hardness, whereas ordinary bolts generally do not require heat treatment.

(2) Different Strength Grades:

High-strength bolts commonly use grades 8.8s, 10.9s, and 12.9s, with 10.9 being the most prevalent.

Ordinary bolts have lower strength grades, usually 4.4, 4.8, 5.6, and 8.8.

Explanation of Strength Grades:
Taking 8.8S as an example:

The first number (8) indicates the nominal tensile strength of the bolt material: 8 × 100 = 800 MPa.

The second number (8) represents the ratio of yield strength to tensile strength (0.8), meaning the yield strength is 800 × 0.8 = 640 MPa.

The letter "S" denotes a standard type.

For ordinary bolts, such as grade 4.8:

The tensile strength is 4 × 100 = 400 MPa.

The yield strength is 400 × 0.8 = 320 MPa.

(3) Different Load Transfer Mechanisms:

High-strength bolts apply pre-tensioning and rely on friction between the connected components to transfer external forces.

Ordinary bolt connections transfer shear forces through the bolt rod's shear resistance and the bearing pressure on the hole wall. The preload generated by tightening the nut is negligible.

(4) Different Usage Scenarios:

Ordinary bolts can be reused.

High-strength bolts are typically used for permanent connections and cannot be reused.

Installation Methods:

High-strength bolts are pre-stressed bolts. Friction-type bolts require a torque wrench to apply the specified preload, while bearing-type bolts require the removal of the spline head.

Ordinary bolts only need to be tightened with a standard wrench.