Thread profile
Thread profile refers to the outline shape of a thread in a cross-section through the thread axis. It is one of the most important characteristic parameters of a thread and directly determines its fit, load-bearing capacity, and transmission performance. Different types of threads have different profiles, with common examples including triangular, trapezoidal, rectangular, and serrated threads. Each profile has specific geometric parameters and applications. Understanding the characteristics and design requirements of thread profiles is crucial for thread design, manufacture, and selection.
Triangular threads have an isosceles triangle profile, with a typical flank angle of 60° (regular threads) or 55° (pipe threads). They are the most widely used thread type. Regular threads (flank angle 60°) are available in coarse and fine pitches. Coarse threads have a larger pitch and higher strength, making them suitable for general connection applications; fine threads have a smaller pitch and better self-locking properties, making them suitable for thin-walled parts or applications requiring precise adjustment. Pipe threads (flank angle 55°) are available in cylindrical and tapered pipe threads, primarily used for pipe connections. Their profile features a 1:16 taper (tapered pipe threads), offering excellent sealing properties and enabling tight connections without the need for additional seals. Triangular threads have a larger profile and higher root strength, capable of withstanding significant axial and radial forces. However, their transmission efficiency is lower, making them generally not used for power transmission, but primarily for tightening connections.
Trapezoidal threads have an isosceles trapezoidal profile with a 30° flank angle. They are the primary thread type used for transmitting power and motion and are widely used in equipment such as machine tool lead screws, jacks, and presses. Trapezoidal threads are characterized by flat crests and roots, and a 15° flank angle. Compared to triangular threads, they offer advantages such as higher transmission efficiency (typically 0.7-0.8), increased strength, and improved machinability. Trapezoidal threads have coarse and fine pitches. Coarse threads are used for transmitting high power, while fine threads are used for precise transmission. According to national standards, the basic parameters of trapezoidal threads include major diameter, pitch diameter, minor diameter, pitch, and crest clearance. The pitch diameter is the primary parameter determining the thread’s fit, and the threads are classified as internal or external. The fit clearance is controlled by the pitch diameter tolerance. Trapezoidal threads have a high profile, high root strength, can withstand high axial loads, and can adjust the clearance after wear, resulting in a long service life.
Rectangular threads have a rectangular tooth profile and a tooth angle of 0°, making them a highly efficient thread type. Their tooth tops and roots are flat, and their sides are parallel, making them suitable for power transmission applications such as lathe screws and screw presses. The advantages of rectangular threads include high transmission efficiency (up to 0.8-0.9), approximately 10%-15% higher than trapezoidal threads, a simple structure, and ease of processing. However, their disadvantages are also significant: low root strength, no inclination angle on the tooth flanks, and a lack of self-locking properties. They are prone to loosening during transmission, requiring additional anti-loosening devices. Furthermore, rectangular threads have a large clearance, resulting in low transmission accuracy and difficulty adjusting the clearance after wear. Consequently, they are rarely used in precision transmission applications. Currently, rectangular threads have been gradually replaced by trapezoidal threads and ball screws, but they still have applications where transmission efficiency is more critical and precision is less demanding.
A buttress thread has a sawtooth profile with a flank angle of 3° on the working surface and 30° on the non-working surface. It is a unidirectional thread type used primarily for applications subject to unidirectional axial loads, such as crane jackscrews and press screws. Its characteristics include a small working flank angle for high transmission efficiency, and a large non-working flank angle for high root strength, enabling it to withstand significant unidirectional axial forces. Its profile height is between that of a triangular and trapezoidal thread, with flat crests and roots, offering excellent processability. Buttress threads are available in both metric buttress threads (GB/T 13576-1992) and imperial buttress threads. Metric buttress threads have a standard pitch and standardized flank angle parameters, facilitating design and manufacturing. However, since buttress threads can only withstand unidirectional loads, they cannot be used in applications subject to bidirectional forces, resulting in a relatively limited range of applications.
The design of a thread profile requires comprehensive consideration of factors such as application requirements, processing technology, and economic efficiency. When designing a thread profile, the first step is to determine the profile type based on the intended use: triangular threads are preferred for fastening connections; trapezoidal threads or ball screws are preferred for transmitting power and motion; and serrated threads are preferred for threads designed to carry unidirectional forces. Secondly, the profile’s geometric parameters, such as the thread angle, pitch, crest height, and root height, must be determined. These parameters must comply with national standards to ensure interchangeability and processability. For non-standard threads, profile parameters must be designed based on specific application requirements, taking into account process feasibility. Furthermore, the surface quality of the thread profile is also important. The surface roughness Ra value on the tooth side is generally 1.6-3.2μm to reduce friction and wear and increase thread life. Through appropriate thread profile design, threads can achieve excellent processability and economic efficiency while meeting application requirements.
