8 common mistakes in the selection of fasteners

1. Use coarse thread instead of fine thread. There are many important connecting parts on the machine, such as the transmission shaft, and most of the bolts are fine thread. If there is a missing part during maintenance, some maintenance personnel will use coarse thread bolts instead, which should be avoided. Because the fine thread bolt has a larger inner diameter, smaller pitch and external angle, it has high strength, good self-locking performance, and is more capable of withstanding impact, vibration and exchange loads. Once it is replaced with a coarse thread bolt, it is easy to loosen or fall off, break, and even cause a mechanical accident.

2. Pore mismatch
The bolts on the machine that bear lateral loads and shear forces, such as transmission shaft bolts and flywheel bolts, have a transition fit with the bolt holes. The assembly should be strong and reliable, and can withstand lateral forces. Some people do not pay attention to the inspection during the assembly process, and continue to install when there is a large gap between the bolt and the bolt hole, which is easy to cause bolt loosening or cutting accidents.

3. Thickening nuts to increase connection reliability Some people mistakenly believe that thickening nuts can increase the number of working turns of the thread and improve the reliability of the connection. But in fact, the thicker the nut, the more uneven the load distribution between the threads, and the easier it is for the connection to loosen.

4. One nut with multiple washers
After the installation is completed, sometimes the bolts are too long, so some people install spring washers on the bolts. In this case, the spring washers will break due to uneven force during the tightening process, which will reduce the preload force of the bolts and may also generate eccentric loads, reducing the reliability of the bolt connection.

5. The tighter the better Many staff have such a misunderstanding: they think that bolts should be "tighter than loose", so they deliberately increase the tightening torque, resulting in bolt slippage. In addition, some important bolts that need to be tightened with torque are tightened by some people for the sake of convenience. As a result, the torque is insufficient, causing the bolts to loosen and even cause mechanical failures.

6. Washers that are too large are not a problem. Sometimes there is a lack of washers of the right size, and some staff will use washers with larger inner diameters instead. In this case, the contact area between the head of the bolt and the washer is small, which will reduce the bearing pressure or locking force of the washer. If there is vibration and impact in the working environment, the bolts are easy to loosen.

7. Improper locking
Important bolts should be locked with anti-loosening devices after assembly. Here are four situations. If a split pin is used for locking, do not use a too thin split lock or a half-piece split lock; if a spring washer is used for locking, do not use a washer with too small an opening offset; if a lock plate is used for locking, do not lock the lock plate at the corner of the nut; if a double nut is used for locking, do not install the thinner nut on the outside.

8. False firmness
If the bolts, nuts or threads are rusted, or there are impurities such as scale and iron filings, they must be cleaned before assembly; burrs, sand and other impurities on the joint surface of the connector must also be removed. Otherwise, when tightening the bolts, it looks like they are tightened on the surface, but in fact the connector is not really tightened. Under such false firmness, the bolts will quickly loosen when vibration, load impact and temperature changes occur.