Guide to Fastener Thread Types: Selection and Applications
In industrial equipment, construction projects, and daily maintenance work, bolts, nuts, and screws are among the most commonly used connection components. Although these fasteners appear simple in structure, their correct selection directly determines the safety, reliability, and service life of equipment. In practical applications, many connection failures are not caused by insufficient fastener material strength or poor product quality, but rather by incorrect thread selection or improper thread matching.
Threads are the core structural feature that enables fasteners to perform their connection function. Different thread types vary significantly in dimensional standards, thread angles, pitch sizes, and application fields. For example, forcing an imperial thread bolt into a metric thread nut cannot create a reliable connection. Instead, it may damage the thread surfaces and cause component failure. Similarly, self-tapping screws designed for metal sheets and thread-forming screws used for plastic housings have completely different structural designs and installation methods.
This article provides a comprehensive introduction to common thread types used in bolts, nuts, and screws, including standardized systems such as Unified Thread Standard (UTS), ISO metric threads, British Standard Whitworth (BSW) threads, and ACME trapezoidal threads, as well as special thread types including machine threads, spaced threads, self-tapping threads, and thread-forming screws. It also explains basic thread structures, dimensional parameters, identification methods, and practical selection considerations to help engineers correctly identify thread types and avoid connection failures caused by improper matching.
Before selecting the correct thread type, it is important to understand the basic differences between bolts, nuts, and screws, the function of threads in fasteners, and the fundamental forms of external and internal threads.
Bolts, nuts, and screws are the most widely used threaded fasteners in industrial applications.
A bolt is generally used together with a nut. Through rotational movement, the bolt generates clamping force to secure two or more components together. A nut is a component with internal threads that engage with the external threads of a bolt to achieve a locking connection. A screw can usually be installed directly into a pre-threaded hole or used for self-tapping installation.
Although these fasteners all serve the purpose of connecting and fixing components, they differ in structural design and application methods. Bolts mainly depend on cooperation with nuts and are commonly used where repeated assembly and disassembly are required. Screws are designed to directly engage with the connected material and are often used in applications where installation space is limited or where an additional nut is unnecessary.
Threads are the key functional area that enables fasteners to achieve tightening and locking performance. Threads extend helically along the shank of a fastener. Through rotational movement, they create mechanical advantage and convert rotational force into axial linear movement.
This movement allows screws, nuts, or bolts to move relative to a fixed component, generating clamping force and creating a secure connection between parts.
Different thread designs are optimized according to application requirements, including material type, connection method, load conditions, and installation environment. Because the threaded section directly affects the strength and reliability of a fastener, understanding thread types is essential for proper selection.
All fastener threads can be divided into two basic forms: external threads and internal threads.
External threads are located on the outside surface of a component, such as the threads on bolts and screws. Internal threads are located inside holes or nuts, such as nut threads or threads created through tapping processes.
Although external and internal threads share the same basic structural parameters, their diameter definitions are different.
For external threads, the minor diameter refers to the distance between two opposite thread roots, while the major diameter refers to the distance between two opposite thread crests.
For internal threads, the minor diameter refers to the distance between two opposite thread crests, while the major diameter refers to the distance between two opposite thread roots.
To achieve proper engagement, external and internal threads must be accurately matched in terms of diameter, pitch, and thread profile.
Understanding thread structure and dimensions is essential for identifying and selecting the correct fastener.
The main structural elements of threads include the crest, root, thread depth, flank, and thread angle. Important dimensional parameters include pitch, threads per inch, major diameter, and minor diameter.
The thread crest is the highest point of the thread profile, while the root is the lowest point located at the bottom of the thread groove.
Thread depth refers to the distance between the crest and root. The flank is the angled surface that forms the thread profile, and the thread angle is the angle between the two thread sides.
These elements determine the geometry and mechanical performance of a thread.
For example, thread angle affects load capacity and self-locking ability. Thread depth is related to pitch, and in many standard thread designs, the depth is approximately half of the pitch value.
Thread dimensions are mainly defined by pitch, threads per inch (TPI), major diameter, and minor diameter.
Pitch is the distance between two adjacent thread crests and is commonly used for metric thread measurement in millimeters.
Threads per inch indicates the number of complete threads contained within one inch of thread length and is mainly used in imperial thread systems.
Although pitch and threads per inch describe similar thread spacing characteristics, they use different measurement methods. Therefore, the correct system must be confirmed before selecting fasteners.
Most standardized screws, bolts, and nuts are identified by outside diameter combined with pitch or threads per inch. Thread angle and depth are mainly used to define the specific thread standard.
Thread sizes are generally expressed by diameter, pitch, and length.
In the Unified Thread Standard (UTS) system, fasteners smaller than 1/4 inch are commonly identified using numbering systems such as No.0 to No.12.
In the ISO metric system, the letter “M” represents metric threads, followed by the nominal diameter in millimeters. For example, M8 indicates an 8 mm diameter metric thread.
A complete fastener designation normally includes thread diameter, pitch, and fastener length.
The most common standardized thread systems include Unified Thread Standard (UTS), ISO/DIN metric threads, British Standard Whitworth (BSW), and ACME trapezoidal threads.
The Unified Thread Standard is an imperial thread system managed by ASME/ANSI. It uses a 60-degree thread angle and imperial measurement units.
UTS thread sizes are identified by diameter and threads per inch. For example, 10-32 indicates a diameter of 0.19 inches with 32 threads per inch, while 5/16-18 indicates a 5/16-inch diameter with 18 threads per inch.
UTS threads are divided into:
- UNC: Unified Coarse Thread
- UNF: Unified Fine Thread
- UNEF: Unified Extra Fine Thread
- UNS: Unified Special Thread
Thread fit classes range from Class 1 to Class 5. Classes 1 to 3 are commonly used, while Class 3 has stricter tolerance requirements.
External threads are identified with “A” and internal threads with “B”. For example, 2A represents a Class 2 external thread, while 2B represents a Class 2 internal thread.
ISO/DIN metric threads are the most widely used thread system worldwide. They are managed by ISO and are based partly on the German DIN standard system.
ISO 261 and ISO 262 define basic metric thread requirements, while ISO 965 specifies thread tolerances.
Metric threads use a 60-degree thread angle similar to UTS threads but use millimeters for dimensions.
A typical designation includes diameter and pitch, such as M1.6×0.2 Fine, which represents a 1.6 mm diameter thread with a 0.2 mm pitch.
Metric threads are generally considered coarse threads unless a specific pitch is provided.
The British Standard Whitworth thread was developed in 1841 and is one of the earliest national thread standards.
BSW threads use a 55-degree thread angle with rounded crests and roots. The thread depth is approximately 0.64 times the pitch.
Although BSW coarse threads are similar in size to UNC threads, they cannot be directly mixed because their profiles are different.
The fine version of BSW is called BSF, but it is less commonly used.
ACME threads were developed in the 1890s as an improvement over square threads.
They provide higher strength, better wear resistance, and easier manufacturing compared with square threads.
ACME threads use a 29-degree thread angle and a trapezoidal profile. They are commonly used in power transmission applications such as lead screws and mechanical actuators.
ACME threads are divided into general-purpose, centralizing, and stub types.
Machine threads are designed to work with nuts or tapped holes, while spaced threads form their own mating threads during installation.
Machine threads usually use a 60-degree profile and are widely used in industrial equipment requiring repeated assembly and high preload forces.
Coarse threads provide faster installation, better resistance to damage, and improved tolerance against contamination. Fine threads provide higher tensile strength, better vibration resistance, and improved performance in thin materials.
For example, 1/4-20 UNC represents a 1/4-inch diameter thread with 20 threads per inch. M8 represents an 8 mm metric coarse thread with a standard pitch of 1.25 mm.
Spaced threads, including self-tapping screws, are used in materials without pre-existing threads, such as wood, plastics, and thin metal sheets.
Self-drilling screws differ from self-tapping screws because they have drill-point structures that create holes and threads during installation.
High-low threads use two different thread heights to increase installation speed and reduce resistance. They are commonly used in concrete screws and similar applications.
Selecting the correct fastener thread requires consideration of material characteristics, operating conditions, and standard compatibility.
For steel and cast iron components, machine threads with nuts or tapped holes are commonly used. Thin metal sheets often use self-tapping or self-drilling screws. Wood and plastic materials generally require spaced threads because these materials allow threads to form directly during installation.
For ductile materials, thread-forming screws can provide reliable connections. However, brittle materials should avoid extrusion-based thread formation because it may cause cracking.
The thread standard must also match the equipment design. ISO metric threads should be selected for metric systems, while UTS threads should be used for imperial designs. Mixing different thread systems may result in damaged threads and unreliable connections.
For vibration environments, fine threads generally provide better resistance to loosening. For rapid assembly applications, coarse threads are usually preferred because fewer rotations are required.
Special attention should be paid to thread identification before installation. Diameter, pitch, and threads per inch must be verified. For critical connections, thread gauges should be used to confirm accuracy.
Different thread systems should also be stored separately and clearly labeled during inventory management to prevent incorrect use.
Fastener thread types are diverse, ranging from common UTS imperial threads and ISO metric threads to specialized ACME trapezoidal threads and customized non-standard designs. Each thread type has its own structural characteristics, performance advantages, and application areas. Understanding thread structures, dimensional parameters, and differences between thread systems helps engineers select the correct fasteners and improve connection reliability. During practical selection, factors such as material properties, operating environment, standard requirements, and assembly conditions should be considered together. Avoiding incompatible thread combinations is essential for preventing connection failures and ensuring the safe operation of industrial equipment and mechanical structures.
