The difference between coarse thread and fine thread of stainless steel bolts

Coarse thread:
No need to mark the pitch, such as M8, M12-6H, M16-7h, etc., mainly used for connecting threads. Compared with fine thread, the pitch is large, the thread lead angle is also large, and the self-locking property is poor. Generally, it is used with spring washers: the pitch is large, the tooth shape is deep, and the strength of the main body is greatly reduced. The advantage is that it is easy to disassemble and assemble, and the matching standard parts are complete and easy to interchange.

Fine pitch:

The pitch must be marked to distinguish it from coarse pitch thread. Its characteristics are exactly opposite to coarse pitch. It is stipulated to supplement the special use requirements that coarse pitch thread cannot meet. Fine pitch thread also has a pitch series, which is mainly used in metric pipe fittings of hydraulic system, mechanical transmission parts, thin-walled parts with insufficient strength, internal parts of machines with space restrictions, and shafts with high self-locking requirements, etc.

To determine whether the thread is coarse pitch or fine pitch, first roughly determine the purpose of the thread. If you are unsure, use a caliper to measure the length of n pitches, divide it by n, and then check the thread table. Coarse pitch has high strength and fine pitch has good sealing. The same level of guaranteed load is not much different (fine pitch is slightly higher). When used for tightening, the preload of the two is close. When no anti-loosening measures are taken, the anti-loosening effect of fine pitch thread is better than that of coarse pitch. It is generally used for thin-walled parts and parts with high vibration resistance requirements.

When used for adjustment, fine pitch thread has more advantages. Disadvantages of fine thread: It is not suitable for use on materials with excessively coarse structure and poor strength. When the tightening force is too large, it is easy to slip.

Fine thread and coarse thread screws are both used for fastening. Fine thread screws are generally used to lock thin-walled parts and parts with high vibration resistance requirements. Fine thread has good self-locking performance, so it has strong anti-vibration and anti-loosening ability, but due to the shallow thread depth, its ability to withstand greater tension is worse than coarse thread. The strength of fine thread is lower than that of standard thread (coarse thread). Standard thread actually adopts the principle of equal strength, that is, the strength of the standard nut thickness size is the same as that of the screw. From the perspective of strength, this design is the most reasonable.

Therefore, standard thread is the most widely used. However, it also has disadvantages, that is, the self-locking performance is worse than that of fine thread, so spring washers or other self-locking devices must be installed in places with severe vibration. Fine thread is generally not used for large force work. It is mainly used for positioning, adjustment, fixing, etc.

Its characteristic is that the outer diameter of the nut can be much smaller than that of the standard thread, which is particularly suitable for thin-walled cylindrical parts. For example, fine pitch threads are widely used in various optical instruments. In addition, the self-locking property of single-start fine pitch threads is very good, and it can be easily controlled for precise adjustment. It is widely used in measuring instruments such as micrometers.

The disadvantage of fine pitch threads is that the threads are easily damaged, affecting installation and disassembly. A slight dimensional error may cause the screw and nut to be damaged at the same time due to random buckling during installation.