Coarse teeth:
Do not need to be marked pitch, such as M8, M12-6H, M16-7h, etc., mainly used for connecting threads, compared with fine thread, due to large pitch, thread angle is also large, self-locking is poor, usually more with spring washers With the use of: large pitch, deep tooth type, the strength of the body parts is also a big advantage is easy to disassemble, complete with matching standard parts, easy to interchange.
Fine teeth:
The pitch must be marked to show the difference from the coarse thread. The characteristics are the opposite of the coarse teeth. It is provided to supplement the special requirements that the coarse thread can't meet. The fine thread also has a pitch series, which is mainly used in the metric system of the hydraulic system. Pipe fittings, mechanical transmission parts, thin-walled parts with insufficient strength, internal parts subject to space restrictions, and shafts with high self-locking requirements.
To determine whether the thread is coarse or fine, first determine the use of the lower thread. Then use an caliper to measure the length of n pitches. After dividing by n, calculate the thread table.
Thick high strength, fine seal. The same level guarantees that there is no difference in load (slightly higher in fine teeth). When tightening, the preloads of the two are close to each other. The anti-loosening effect of the fine thread is better than that of the coarse teeth when the anti-loosening measures are not taken, and it is generally used for thin-walled parts and parts with relatively high anti-vibration requirements.
. When making adjustments, the fine thread is more advantageous. The disadvantage of the fine thread is that it is not suitable for applications where the tissue is too thick and the strength is poor. When the tightening force is too high, it is easy to slide.
Fine thread and coarse thread screws are used for fastening purposes. Fine thread screws are generally used for locking thin-walled parts and parts requiring relatively high vibration resistance. The self-locking performance of the fine thread is good, so the ability to resist vibration and prevent loosening is strong, but due to the shallow depth of the thread, the ability to bear greater tension is worse than that of the coarse thread. The strength of the fine thread is lower than that of the standard thread (coarse thread). The standard thread is actually to take the principle of equal strength, ie the strength of the standard nut thickness dimension is the same as the strength of the screw, from the point of view of strength. This design is the most reasonable.
So the standard thread is the most widely used. However, it also has the disadvantage that the self-locking thread is inferior to the fine thread, so spring washers or other self-locking devices must be installed where vibration is severe. Fine thread is generally not used for large force work, it is mainly used for positioning, adjustment, fixed and so on. Its feature is that the outer diameter of the nut can be much smaller than the standard thread, and is especially suitable for thin-walled cylindrical parts. Fine thread is widely used in various optical instruments. In addition, the self-locking of the single-head fine thread is very good, and it can be used to accurately adjust the blade that can be controlled. There are many similar instruments used in the micrometer. The disadvantage of the fine thread is that the thread is easy to break, affecting the installation and removal. A slight dimensional error may cause the screw and the nut to be damaged at the same time.
As shown in Figure 1, coarse thread applications
The coarse thread series (M / UNC / United Nations Resident Coordinator) is generally used for mass production of threaded fasteners such as bolts, screws, nuts, etc. It is uniquely superior in situations requiring rapid assembly or disassembly and certain savage operations that are prone to rust and damage. However, its self-locking properties are inferior to those of fine teeth and superfine teeth threads.
2, fine thread applications
The fine thread series (MF / UNF / UNRF) is often used where high fastener strength is required. Since the fine thread is lighter than the coarse thread and the thread diameter is smaller, the thread strength is higher than that of the coarse thread with the same nominal size. In addition, fine threading can be used for fine-tuning applications.
3, the application of ultra fine thread
The ultra-fine thread series (Emergency Unit / UNREF) is particularly suitable for applications requiring fine-tuning, such as adjusting screws.