Engineers and mechanics often struggle to select the right type of bolt for the load and complex installation environment. Using the wrong bolt can lead to structural failure, increased maintenance costs, and safety hazards. This article provides a detailed classification of bolt types in mechanical engineering, from materials to strength grades, helping you optimize your selection. Mastering this in-depth knowledge will ensure absolute durability and safety for all projects
Classification of Bolts by Head Shape and Application
The shape of the bolt head determines the tightening tool and intended use. In mechanical engineering, the following three types are the most common:
External Hex Bolt:
This is the most common type of bolt, usually manufactured according to DIN 931 (partial thread) or DIN 933 (full thread) standards.
The hexagonal head allows the use of wrenches or sockets to create a large tightening torque, ideal for prefabricated steel structures, heavy industrial machinery installations, and main load-bearing systems.
The partial thread type (DIN 931) is preferred in shear-resistant joints because the unthreaded body has a solid cross-section, increasing shear resistance.
Socket Head Cap Screw:
Usually conforming to DIN 912 standard, this type of bolt has a cylindrical head with a recessed hexagonal hole inside.
A major advantage is their use in tight spaces where wrenches cannot reach. When tightened, the bolt head sinks to or below the surface of the material, providing high aesthetics and safety (no snagging).
They are crucial in CNC machine manufacturing, molds, and equipment requiring high precision.
Round Head Square Neck Bolts (Carriage Bolts):
Characterized by a smooth, round head with a square neck section underneath.
When installed (usually through a square or round hole in soft materials like wood), the square neck grips the material tightly, preventing the bolt from rotating when the nut is tightened. They are commonly used to connect thin metal sheets or to connect metal to wood.
Classification by Material and Strength Grade
Alloy Steel Bolts:
This is the choice for load-bearing applications. Strength grade is indicated by two numbers such as 4.8, 5.6, 8.8, 10.9, 12.9.
How to read: For example, a strength grade 8.8 bolt:
The first number (8) x 100 = 800 MPa (Minimum tensile strength).
The second number (.8) x 10 = 80% (Ratio between yield strength and tensile strength).
Yield strength = 800 x 80% = 640 MPa
Grades 8.8, 10.9, and 12.9 are high-strength bolts that must be made from alloy steel (such as SCM435, 40Cr) and undergo heat treatment (quenching and tempering) to achieve the required hardness and toughness.
Stainless Steel Bolts:
This option doesn't focus on extremely high load bearing capacity (strength is usually equivalent to grade 4.8 or 6.8) but rather on corrosion resistance.
Stainless Steel 304 (SUS 304): The most common type. Excellent rust resistance in normal environments, outdoors, and food processing plants. This is the cost-effective choice for applications requiring cleanliness and durability.
Stainless Steel 316 (SUS 316): A higher grade containing molybdenum. It has superior corrosion resistance, especially in acidic, chemical, and seawater environments. Any machinery located near the sea or in a chemical plant must use Stainless Steel 316 to avoid maintenance costs.
To ensure that bolts used in mechanical manufacturing meet technical standards, engineers need to conduct rigorous quality checks upon receiving goods. Basic inspection steps include visual inspection (no cracks, breaks, uniform plating), thread accuracy measurement using a thread gauge, and dimensional verification using an electronic caliper.
However, visual inspection alone cannot determine the most crucial factor: material and strength grade. Engineers cannot tell whether a bolt is 8.8 or 10.9 grade based solely on its appearance. This is why choosing a reputable supplier with a clear commitment to quality is essential.
At THINH PHAT VUNG TAU TRADING SERVICE CO., LTD., we understand that the quality of input materials determines the safety of an entire system. With the slogan "QUALITY BUILDS REPUTATION", we are not just a trading company but also a technical partner, committed to providing:
Standard Products: We supply a full range of hexagon bolts (DIN 931/933), countersunk hexagon bolts (DIN 912), and stainless steel bolts (304, 316L).
Transparent Strength Grades: We are committed to supplying high-strength steel bolts (8.8, 10.9) with the correct material (SCM435, C45...) and heat treatment processes.
Technical Consulting: Our team is ready to advise engineers and purchasing managers on selecting the correct material (304 or 316 stainless steel) and strength grade (8.8 or 10.9) to optimize costs while ensuring technical safety.
Understanding the technical characteristics of various types of bolts in mechanical fabrication is key to ensuring the quality and longevity of the project. Whether it's standard hexagonal bolts, corrosion-resistant stainless steel bolts, or high-strength bolts, selecting the right material and strength grade based on load is essential. Mastering this knowledge helps engineers and mechanics minimize risks, optimize costs, and improve assembly efficiency.
