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Core Differences of 11 Thread Types (Part I)

[Abstract]:This article elaborates on the core characteristics, performance differences and applicable scenarios of 11 mainstream thread types, providing comprehensive technical support for the selection and application of fasteners.

In the fastener industry, thread selection directly determines the reliability, stability and service life of structural connections. Fastening scenarios vary significantly across applications, ranging from general mechanical assembly, aerospace precision fitting and household equipment fixation to sealed connection of industrial pipelines. Working conditions impose distinct requirements on thread structure, mechanical strength and sealing performance. Among dozens of mainstream thread specifications in the industry, eleven types stand out for their versatile applicability and distinctive performance characteristics, serving as essential knowledge for industry practitioners. This article systematically elaborates on the structural features, core properties, applicable scenarios and key differences of these eleven thread types to support accurate identification and scientific selection.

Before analyzing individual thread types, it is necessary to clarify the core classification dimensions, which form the fundamental logic for thread differentiation. Threads can be categorized by profile, standard system, function, rotation direction and thread number. The thread profile determines load-bearing capacity and sealing performance, including triangular, trapezoidal, buttress and rectangular shapes. Standard systems cover metric, imperial and unified national standards corresponding to different national and industrial specifications. Functional classification includes fastening threads, transmission threads and sealing threads. Rotation directions consist of right-hand (standard) and left-hand (special scenarios) threads. Thread types include single-start (general applications) and multi-start (high-efficiency transmission). The following comparison of eleven thread types is conducted based on these core dimensions.

I. Standard Coarse Thread (M) — Basic Universal Fastening Thread

As the most widely used metric thread, the standard coarse thread is coded M with a 60° triangular profile, serving as the fundamental fastening thread in the fastener industry. It features a large pitch, isosceles triangular cross-section and standard single-start right-hand design (custom left-hand threads available for special applications).

Core Performance: It features simple manufacturing processes, low cost and basic fastening and load-bearing capacity with convenient assembly and disassembly. The large pitch leads to a relatively large thread lead angle, enabling efficient manual and mechanical installation. However, it delivers relatively weak vibration resistance and is prone to loosening under long-term cyclic vibration conditions.

Application Scenarios: Applicable to almost all non-precision fastening scenarios for general machinery, including mechanical equipment housing fixation, building structure connection and household product assembly. Common specifications range from M1.6 to M64, matching bolts and screws of various strength grades, making it the most widely used thread type in the industry.

Core Differences: Its primary advantages lie in universal compatibility and cost-effectiveness, making it the priority choice for scenarios without special sealing or transmission requirements. Compared with metric fine threads, it features a larger pitch, smaller load-bearing area and poorer vibration resistance.

II. Standard Fine Thread (M×P) — Advanced Anti-Vibration Precision Thread

Also a 60° metric triangular thread, the standard fine thread is coded M×P (P represents pitch and must be marked explicitly). Its core feature is a smaller pitch and denser tooth profile compared with coarse threads of the same nominal diameter.

Core Performance: The smaller pitch and increased thread engagement area enable uniform load distribution and significantly improved vibration resistance, effectively preventing loosening under continuous vibration. The small lead angle provides excellent self-locking performance for precision adjustment applications. Nevertheless, fine threads require higher manufacturing precision, resulting in slightly higher costs and lower assembly efficiency than coarse threads.

Application Scenarios: Widely adopted in precision machinery assembly such as instrumentation and machine tool guide fixation, vibration-intensive working conditions including automotive chassis and engineering machinery components, thin-wall part connections with reduced substrate weakening, and basic sealing scenarios relying on multi-tooth engagement. Typical specifications include M8×1, M10×1.25, commonly used in high-end manufacturing fields.

Core Differences: Distinguished from coarse threads by smaller pitch, superior vibration resistance and higher precision for precision and anti-vibration scenarios. Unlike pipe threads, it has no dedicated sealing structure and only meets basic low-grade sealing requirements.

III. Non-Sealing Pipe Thread (G) — Low-Pressure Pipeline Connection Thread

Coded G, the non-sealing pipe thread is an imperial triangular thread with a 55° profile. It features an isosceles triangular tooth shape with rounded crests and roots, designed exclusively for non-sealing pipeline connections.

Core Performance: With mature manufacturing technology and easy assembly and disassembly, it provides reliable positioning for pipeline connections. Since it has no inherent sealing structure, sealing performance depends entirely on external auxiliary components such as sealing rings and PTFE tape. It is classified into Grade A/B internal threads and Grade A external threads to match different precision requirements.

Application Scenarios: Suitable for low-pressure pipeline systems including water supply and drainage pipelines, compressed air pipelines and general hydraulic circuits that do not rely on thread self-sealing. Common specifications include G1/2 and G3/4 for different pipe diameters.

Core Differences: Unlike sealing pipe threads that achieve self-sealing via structural fit, non-sealing pipe threads require external sealing accessories. Compared with metric standard threads, it adopts an imperial 55° profile to adapt to imperial pipeline systems with non-interchangeable specifications.

IV. Sealing Pipe Thread (R, RC, NPT) — Specialized High-Pressure Sealing Thread

Designed specifically for high-pressure sealing applications, sealing pipe threads realize self-sealing through precise profile fitting without additional gaskets. They are mainly divided into 55° imperial sealing threads (R, RC) and 60° American standard sealing threads (NPT).

1. Imperial Sealing Pipe Thread (R/RC)

It features a 55° profile with rounded crests and roots, including tapered external threads (R) and tapered internal threads (RC). Tight conical surface fitting achieves reliable sealing. It delivers high sealing stability, excellent vibration resistance and adaptability for medium and high-pressure pipelines with a standard 1:16 taper requiring strict precision control. Typical applications include hydraulic systems, fuel systems and gas pipelines for engineering machinery and automotive fuel circuits.

2. American Standard Sealing Pipe Thread (NPT)

With a 60° triangular profile and 1:16 taper, NPT threads achieve dual sealing through flank meshing and conical surface compression. It supports higher sealing pressure and better corrosion resistance than imperial sealing threads, suitable for high-temperature and high-pressure environments. Its unified national specifications are incompatible with metric and imperial standard threads. It is widely used in high-end sealing scenarios such as aerospace fuel pipelines, petrochemical high-pressure pipelines and refrigeration systems.

Core Differences: R/RC and NPT threads differ in profile angle (55° vs 60°) and standard system (imperial vs American) with completely non-interchangeable specifications. Compared with non-sealing G threads, they feature inherent self-sealing performance for high-pressure scenarios without external sealing parts.

V. Trapezoidal Thread (Tr) — Precision Transmission Mainstream Thread

Coded Tr, the trapezoidal thread is a dedicated transmission thread with a 30° isosceles trapezoidal profile. Its structural advantages lie in the wide tooth cross-section, large load-bearing area and stable precision transmission performance.

Core Performance: It provides excellent axial load-bearing capacity, smooth and impact-free linear transmission, and adjustable wear compensation gaps for extended service life. However, its complex manufacturing process leads to higher costs, and it is not suitable for high-speed transmission conditions.

Application Scenarios: Ideal for mechanical structures requiring precise axial transmission, such as machine tool lead screws, elevator lifting mechanisms, screw presses and hydraulic cylinder piston rods. Common specifications include Tr40×7 and Tr50×10 with customizable pitches for different transmission efficiency requirements.

Core Differences: Different from standard fastening M threads, trapezoidal threads prioritize transmission functions rather than fastening, with optimized profile structure for superior load and transmission performance. Compared with buttress threads, its symmetrical profile supports bidirectional axial load bearing, while buttress threads only adapt to unidirectional loads.

VI. Buttress Thread (B) — Unidirectional Heavy-Duty Transmission Thread

Coded B, the buttress thread adopts an asymmetrical trapezoidal profile with a 3° load-bearing working angle and a 30° non-working angle, featuring extreme unidirectional load capacity and high transmission efficiency.

Core Performance: The small working angle ensures uniform contact stress distribution to withstand tremendous unidirectional axial loads, while the large non-working angle reduces frictional resistance and improves transmission efficiency. Its main limitation is unidirectional load adaptability, failing to fit bidirectional force scenarios, with relatively high manufacturing precision requirements.

Application Scenarios: Extensively applied in unidirectional heavy-load transmission equipment such as crane lifting mechanisms, press slider driving systems and injection molding machine clamping structures. Common specifications include B40×10 and B60×12 for heavy-duty mechanical transmission systems.

Core Differences: Distinguished from trapezoidal Tr threads by its asymmetrical profile and superior unidirectional heavy-load capacity, while trapezoidal threads support balanced bidirectional load bearing. Unlike standard fastening threads, it is structurally optimized for heavy-load transmission rather than structural fixation.