Thread differences

Difference between coarse and fine threads: The pitch sizes differ; coarse threads have a larger pitch, while fine threads have a smaller one； 1. The helix angle of fine-pitch threads is smaller, which facilitates their self-locking effect. Therefore, fine-pitch threads are generally used in applications where prevention of loosening is required. 2. Fine-pitch threads have a small pitch; for the same thread length, they accommodate more threads. This helps to reduce fluid leakage, making them suitable for applications where sealing is required. 3. For coarse-thread screws, the number of threads per unit length is smaller; each thread has a larger cross-sectional size, resulting in better load-bearing capacity. They are more suitable for withstanding high tensile and impact forces. 4. Fine-pitch threads also have the advantage of a small pitch, which allows for fine adjustments. Coarse-thread bolt threads have better fatigue resistance and are easier to assemble and disassemble frequently ； Fine-toothed types have strong self-locking capacity, a large bottom diameter, and high strength under static loads. Additionally, the tightening torque for fine threads is somewhat lower. There is a certain relationship between the steel grade and the preload capacity; at the same preload level, the torque required for fine threads is smaller. Generally, coarse-threaded threads are used; fine-threaded threads are used when structural factors such as high connection strength or small dimensions at the connection point require it. For threads of the same size, fine-pitch threads have higher strength than coarse-pitch threads. For example, for an M8 thread of grade 3.6, the guaranteed load is 6590 N; whereas for a fine-pitch M8×1 thread, the guaranteed load is 7060 N. Fine-threaded threads have higher self-locking properties than coarse-threaded threads, and they also generate greater pre-tension. In situations with high temperatures and significant vibrations, fine-threaded threads can be used in critical components; however, this is not a universal rule, and the choice should be based on specific circumstances. In the design of pressure vessels, fine-threaded threads are generally used in Class 2 vessels, typically to secure internal components of the vessel, but they are rarely used in parts that are frequently opened, such as manholes. Coarse-thread screws are widely used. In practice, fine-threaded threads should be used as little as possible, and the bolt diameter can be increased appropriately. The main reason is the issue of tolerances. Fine threads are used when high loads are involved. However, when working with fine threads, it’s necessary to remove any burrs or debris from the threads; otherwise, they can easily seize up, making installation difficult. The choice depends on the application: in situations where requirements aren’t very stringent, coarse threads should be used instead, as their manufacturing precision is lower. Places where fine threads are used are generally: 1. In areas where preload is required, since fine-threaded threads have a higher self-locking property than coarse-threaded threads, the preload is also greater. 2. In applications where high precision is required, such as the threads on seals. 3. The design is limited, such as when a compact structure is required. 4. When used as a transmission component in industrial design, fine-pitch threads can be employed for precision transmission shafts that require sealing.