Definition of fastener thread
A thread is a shape with a uniform spiral protrusion on the outer or inner surface of a solid.
There are three main types of threads: thread, self-tapping thread and self-drilling thread.
Machine thread: during assembly, drill a hole in the assembly to tap the thread, and the tapped internal thread is the same as the external thread of the screw, so the assembly is carried out with a small torque.
Self-tapping: during assembly, drill holes in the assembly first, without tapping internal teeth, and use a large torque for assembly.
Self-drilling thread: it can be directly used on the assembly, and the screw can be drilled and tapped to form the thread in one step.
Screw action of fastener
1. Fastening and connecting function: applicable to most screw products at this stage.
2. Transmission action (displacement action): for example, the micrometer used by QC to check dimensions.
3. Sealing function: such as the connection and sealing of pipelines.
Coarse thread and fine thread
The so-called coarse thread can be defined as standard thread; However, fine thread is relative to coarse thread. Under the same nominal diameter, the number of teeth per inch is different, that is, the pitch of coarse thread is large, while that of fine thread is small. That is to say, for 1/2-13 and 1/2-20 specifications, the former is coarse teeth and the latter is fine teeth. Therefore, it is expressed as 1/2-13UNC and 1/2-20UNF.
Coarse thread
Definition: The so-called coarse teeth actually refer to standard threads. Unless otherwise specified, fasteners such as stainless steel screws that we generally buy are coarse teeth.
The characteristics of coarse thread: it has high strength, good interchangeability and can be compared with standards. Generally speaking, coarse thread should be the best choice;
Compared with fine thread: due to the large pitch, high thread angle and poor self-locking, it is necessary to install check washer and self-locking device in vibration environment; Has the advantages of convenient disassembly and assembly, complete matching standard parts and easy interchangeability;
Note: It is not necessary to mark the pitch of coarse thread, such as M8, M12-6H, M16-7H, etc., and it is mainly used as coupling thread.
Fine thread
Definition: Fine teeth are just the opposite of coarse teeth, which are stipulated to supplement the special use requirements that coarse teeth threads can’t meet. Fine teeth threads also have a series of pitch, and the pitch of fine teeth is smaller, so its characteristics are more conducive to self-locking and anti-loosening, and the number of teeth can reduce the leakage and achieve the sealing effect. In some precision occasions, fine-tooth stainless steel screws are more convenient for precise control and adjustment.
Disadvantages: The tensile value and strength are lower than those of coarse teeth, and the thread is easy to be damaged. It is not recommended to disassemble and assemble for many times. The matching fasteners such as nuts may be equally accurate, and the size is slightly wrong, which may easily damage the screws and nuts at the same time.
Application: Fine thread is mainly used in metric pipe fittings of hydraulic system, mechanical transmission parts, thin-walled parts with insufficient strength, internal parts limited by space and shafts with high self-locking requirements, etc. When fine thread is marked, the pitch must be marked to show the difference from coarse thread.
How to choose coarse thread and fine thread?
Both coarse thread and fine thread screws are used for fastening.
Fine-tooth screws are generally used to lock thin-walled parts and parts with high requirements for vibration prevention. Fine thread has good self-locking performance, so it has strong anti-vibration and anti-loosening ability. However, due to the shallow depth of thread, the ability to bear greater tension is worse than that of coarse thread.
When no anti-loosening measures are taken, the anti-loosening effect of fine thread is better than that of coarse thread, and it is generally used for thin-walled parts and parts with high anti-vibration requirements.
When adjusting, fine thread has more advantages. Disadvantages of fine thread: It is not suitable for materials with excessively coarse structure and poor strength. When the tightening force is too large, it is easy to slip.
Post time: Nov-29-2022
