Common Types of Bolts and Selection Methods (Part 1)

Bolts are the most common type of threaded fasteners. Their heads come in various shapes (circular, hexagonal, square, etc.) to secure the bolt, while the threaded portion at the other end is connected to a nut. Bolts are primarily made of high-strength alloys, with a few special-purpose cases (such as electrical insulation or corrosion resistance) using non-metallic materials like resin or nylon. However, a high level of strength is still required.

Types of Bolts Head and Their Applications

Bolts come in various head shapes, each serving a distinct purpose depending on the environment and function of the application.

Hexagonal Head Bolts: These are the most commonly used type of bolt, especially in heavy-duty applications. The six-sided head allows for easy tightening with a wrench or socket. They are suitable for use in construction, automotive, and machinery.

Square Head Bolts: These bolts have a four-sided head and are often used in older or traditional constructions. They offer a larger surface area for the wrench and provide better resistance to loosening under vibration.

Round Head Bolts: Typically used in aesthetic applications, these bolts offer a smoother, more rounded appearance but are less practical for heavy-duty applications.

Countersunk and Semi-Countersunk Bolts: These bolts are designed for flush installation, where the head sits neatly within the material’s surface, creating a smooth finish. This is commonly seen in furniture, cabinetry, and aerospace applications.

T-Head Bolts: Often used in adjustable equipment or where frequent assembly and disassembly are necessary, T-head bolts provide a broad surface area for a wrench, making them easier to install and remove.

Specialty Types of Bolts

There are also specialized bolts designed for unique situations where general-purpose bolts might not suffice.

Expansion Bolts: These are used for fastening in materials that do not have high inherent strength, such as brick or concrete. The bolt expands when installed, creating a secure fit.

Anti-Loose Bolts: These bolts are designed to prevent loosening due to vibrations. Features such as locking mechanisms, serrated threads, or nylon inserts ensure that the bolt remains tightly secured.

Foundation Bolts: These are large bolts used to anchor structures to concrete foundations, offering enhanced tensile and shear strength. These are critical for heavy-duty industrial machines or construction projects.

Bolt Sizes and Dimensions

Another important aspect of types of bolts is their size and dimensions. The size of a bolt is determined by its diameter, length, and thread pitch.

Standard Bolts: These bolts follow international or national standards and are available in various sizes. The most common standards are those based on the ISO (International Organization for Standardization) or ANSI (American National Standards Institute).

Metric Bolts: Bolts with a diameter and thread pitch specified in metric units, often used in European or global applications.

Imperial Bolts: These bolts use inches for their diameter and thread pitch, commonly used in the United States and the UK.

Bolts come in a wide variety and can be classified in different ways:

• According to thread type: Fine thread, coarse thread, imperial thread.
• Material: Carbon steel bolts, stainless steel bolts, copper bolts, aluminum bolts, resin bolts, and nylon bolts.
• Bolt head shape: Hex head bolt, square head bolt, round head bolt, countersunk head bolt, semi-countersunk head bolt, bolt with hole, T-head bolt, pan head bolt, countersunk bolt, large flat head, cylindrical head, spherical cylindrical head, etc.
• Performance grade: High-strength bolts, high-precision bolts.Ø  Special functions: Expansion bolts, anti-loose bolts, foundation bolts, etc.
• According to screw slot type: Cross slot, hex slot, square slot, plum slot, H-slot, square hole slot, internal triangle slot, etc.Therefore, different bolts have different shapes and functions. Depending on the specific application, engineers need to have clear goals and selection methods when choosing bolts.

Thread Type Selection Method:

Coarse thread and fine thread bolts are both used for fastening, but they have different applications. Coarse thread, also known as standard thread, has a larger pitch, while fine thread has a smaller pitch.

Coarse thread bolts are characterized by high strength, good interchangeability, and adherence to standards. In general, coarse threads are often the optimal choice. However, compared to fine threads, coarse threads have a larger pitch and thread angle, resulting in poorer self-locking characteristics, especially in vibrating environments where additional measures such as anti-loosening washers or locking devices may be required. The advantage of coarse threads lies in easy assembly and disassembly, complete compatibility with standard components, and ease of interchangeability. Additionally, there is no need to specify the pitch during labeling.

In contrast to coarse threads, fine thread bolts have a smaller pitch, making them more conducive to self-locking and anti-loosening. With more threads, they can reduce the effects of leakage, achieving a sealing effect. In precision applications, stainless steel screws with fine threads offer precise control and adjustment. However, fine thread bolts have lower tensile strength compared to coarse threads, and their threads are more susceptible to damage. It is not recommended to disassemble and reassemble fine threads multiple times, as this can lead to damage to both the screw and the accompanying nuts or fasteners. Fine thread bolts must specify the pitch during labeling to distinguish them from coarse threads.

Fine threads exhibit good self-locking performance and are often used in thin-walled parts and components with high anti-vibration requirements. However, due to the shallow depth of the threads, fine threads have a lower capacity to withstand greater tensile forces compared to coarse threads. Fine threads are not suitable for use in materials with excessively coarse structures and low strength, as excessive tightening force can lead to thread slippage. Fine thread bolts are primarily used in metric fittings for hydraulic systems, mechanical transmission components, thin-walled parts with insufficient strength, internal components constrained by space limitations, and shafts with high self-locking requirements. When anti-loosening measures are not taken, fine threads provide better anti-loosening effects than coarse threads and are generally used in thin-walled parts and components with high anti-vibration requirements. For adjustment purposes, fine threads have distinct advantages.

Bolt Thread Types and Their Applications

The types of bolts vary based on their thread type as well.

Coarse Thread Bolts: Coarse thread bolts are characterized by their larger threads, providing strength and stability. These bolts are often used in applications where quick assembly is needed, and the components are not subjected to heavy vibrations.

Fine Thread Bolts: Fine thread bolts, with smaller threads, offer better resistance to vibration and provide superior holding strength for precision applications. They are typically used in mechanical and precision engineering.

UNC/UNF Threads: UNC (Unified National Coarse) and UNF (Unified National Fine) are two different types of threads commonly used in North America, each suited to different applications based on the material and load requirements.

Material Selection:

Bolts are primarily made of three materials: carbon steel, stainless steel, and copper.

Low Carbon Steel: Includes A3 steel (Q235 high-line), SWRCH6A, 8A, 18A, 22A (1022). (Mainly used for products with no hardness requirements, such as 4.8-grade bolts, 4-grade nuts, and small screws.)

Medium Carbon Steel: Commonly referred to as 35# and 45# steel. Internationally known as 1035 and SWRCH35K. (Mainly used for 8-grade nuts, 8.8-grade bolts, and 8.8-grade hexagonal products.)

High Carbon Steel: C% > 0.45%. Rarely used in the market.

For alloy Steel, contains alloy elements such as silicon, manganese, chromium, nickel, vanadium, molybdenum, etc. 12.9-grade screws mainly use chromium-molybdenum alloy steel, such as SCM435. Alloy elements are added to ordinary carbon steel to enhance certain special properties. Examples include 35CrMo (SCM345), 35CrMo (SCM440).

For stainless Steel, mainly divided into martensitic stainless steel, austenitic stainless steel, and ferritic stainless steel. Performance grades: 45, 50, 60, 70, 80 (SUS200 series, SUS302, SUS304, SUS316, SUS410, etc.).

Imported materials, mainly from Japan, include SUS302, SUS304, and SUS316. For copper, commonly used materials are brass and zinc-copper alloys. Standard components mainly use H62, H65, H68 copper.

Carbon Steel Bolts: These are the most common and cost-effective bolts, suitable for many general-purpose applications.

Stainless Steel Bolts: Stainless steel bolts are highly resistant to corrosion, making them ideal for outdoor or marine environments.

Copper Bolts: Known for excellent conductivity, copper bolts are used in electrical applications.

Nylon Bolts: Made from plastic, these bolts are lightweight and resistant to corrosion, but they are not suitable for heavy-duty applications.

In addition to metals, there are many non-metallic materials available for making bolts.Non-metallic materials include plastics, rubber, synthetic fibers, ceramics, etc. Polymer materials have many advantages, such as abundant raw materials, low density, good elasticity within a suitable temperature range, and good corrosion resistance. The main drawback is that they are prone to aging, and many materials have poor flame retardant. Overall, non-metallic materials may have advantages in certain special-purpose applications.