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Engaged in fasteners for many years, do you know the difference between coarse and fine thread?

Feb 11, 2018

Coarse thread:


No pitch is required, such as M8, m12-6h, m16-7h, and so on, mainly to do connection thread use, and fine thread ratio, due to large pitch, thread rise angle is also large, self-locking on the difference, generally more with the spring washer with the use of: large pitch, tooth type is also deep, the strength of the body parts are also greatly reduced. The advantages are easy disassembly, with the complete set of standard parts, easy to interchange.


Fine thread:


Pitch must be marked, to show the difference with coarse teeth thread, characteristics and coarse teeth just the opposite, is in order to supplement the coarse teeth thread can not meet the special requirements of the specified, fine thread also has a pitch series, mainly used in the hydraulic system of the metric tube, mechanical transmission, insufficient strength of the thin-walled parts, Space-Restricted machine parts and self-locking requirements of higher shaft.



Determine whether the thread is coarse teeth or fine teeth, the first rough to determine the use of the thread, with an indefinite caliper to measure the length of the N-pitch, divided by the n calculation, then check the thread table.


The thick strength is high, the fine seal is good. The same level to ensure that the load difference is not small (slightly higher fine teeth), when fastening, the preload of both close load. The effect of fine thread is better than that of coarse tooth when it is not adopted, it is generally used in thin-walled parts and in the higher vibration-proof requirements.


When adjusting, fine tooth thread has more advantages. The shortcoming of fine thread: it is not suitable for the application of the material with excessive size and strength, so it is easy to slip when the tightening force is large.


Fine thread and coarse thread screw screws are used in the use of fastening, fine screw screws are generally used to lock thin-walled parts and vibration-resistant requirements of higher parts. Fine thread of the self-locking performance is good, so the ability to resist vibration and loose, but because of the depth of the thread teeth, the ability to withstand greater tensile strength than coarse tooth thread difference.


The strength of fine thread is lower than that of standard thread (coarse tooth), the standard thread actually adopts the principle of equal strength, that is, the strength of the standard nut thickness is equal to the strength of the screw, from the angle of intensity, such design is the most reasonable.


So the most widely used standard thread. But it also has the shortcoming, is the self-locking relative fine tooth thread to be inferior, therefore in the vibration intense place must install the spring washer or other self-locking device. Fine thread is generally not used for large force work, it is mainly used for positioning, adjustment, fixed and so on. It is characterized by the diameter of the nut can be relatively small size of the standard thread, especially suitable for thin-walled cylindrical parts, such as a variety of optical instruments widely used in fine tooth thread.


In addition, the self-locking of the single head fine thread is very good, used for precise adjustment can be controlled easily, similar to the measurement of micrometer used a lot of instruments. The disadvantage of fine thread is that the thread is easy to touch bad, affect the installation and disassembly, a slight size error may cause the installation when the screw and the nut at the same time damage.


1, coarse tooth thread application occasions


Coarse tooth Thread Series (M/UNC/UNRC) is generally used in the production of large quantities of threaded fasteners, such as bolts, screws, nuts, etc. In the request of rapid assembly or demolition and some prone to rust, damage of the brutal operation of the unique advantages. But its self-locking sex is inferior to fine tooth and superfine tooth thread.


2, fine tooth thread application occasions


Fine Tooth thread series (MF/UNF/UNRF) are usually used for occasions where fasteners are required to be high in strength. Because the fine thread is thicker than the coarse tooth thread and the diameter of the thread is large, the thread strength is higher than the coarse tooth thread with the nominal size. In addition, fine thread threads can be used for fine-tuning occasions.


Specifically as follows:


1. Where the need for preload, because the fine thread self-locking is higher than the coarse teeth thread, preload is also larger.

2. Where the high precision is required, such as the screw thread on the sealing part.

3. Design constraints, such as the requirements of the compact point.

4. As a fixture design transmission parts, can be used as precision shaft with seal requirements of the case with fine thread.


3, the application of ultra-fine tooth thread


Ultra-Fine Tooth thread series (UNEF/UNREF) is especially suitable for occasions where fine-tuning is required, such as adjusting screws.


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