Common misunderstandings and selection principles of fasteners

Fasteners are widely used and are generally referred to as standard parts in the market. There are not only many types of fasteners, but also a large demand. In the process of use, we need to select the appropriate types to fasten and connect the mechanical parts according to the specific actual situation. Through understanding, we will share with you the common misunderstandings about fasteners and the principles for selecting various types of fasteners, hoping to help you.
Eight common misunderstandings of fasteners
1、 Replace fine teeth with coarse teeth
There are many important connecting parts on the machine, such as transmission shaft, and most of the bolts are fine thread. If parts are missing during maintenance, some maintenance personnel will use coarse bolts instead, which should be avoided. Because of the large inner diameter, small pitch and external angle, high strength, good self-locking performance, and strong ability to withstand impact, vibration and exchange load of fine-tooth bolts. Once the coarse thread bolt is used instead, it is easy to loosen, detach and break, and even cause mechanical accidents.
2、 Pore mismatch
Bolts on the machine that bear lateral load and shear force, such as transmission shaft bolts and flywheel bolts, are transitional fit with bolt holes. The assembly should be solid and reliable, and can bear lateral force. Some people do not pay attention to the inspection during the assembly process, and still 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 nut increases connection reliability
Some people mistakenly believe that thickened nuts can increase the number of working turns of threads and improve the reliability of connectors. But in fact, the thicker the nut, the more uneven the load distribution between the threads of each ring, and the easier the coupling is to loosen.
4、 One female with multiple pads
After installation, sometimes the bolt will be too long, so someone will install additional spring washer on the bolt. In this case, the spring washer will break due to uneven stress during the fastening process, which will reduce the pre-tightening force of the bolt, and may also generate eccentric load, reducing the reliability of the bolt connection.
5、 Tighten as tight as possible
Many staff members have the misconception that the bolts should be "rather tight than loose", so they purposely increase the tightening torque, resulting in bolt slippage. In addition, some important bolts that need to be tightened with torque, while some people use a spanner wrench to tighten them because they are trying to save time, resulting in insufficient torque, resulting in loose bolts and even mechanical failure.
6、 The washer is too large to be in trouble
Sometimes there is a lack of washers of proper size, and some workers will use washers with larger inner diameter to replace them. In this case, the contact area between the bolt head 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 bolt is easy to loosen.
7、 Improper locking
Important bolts should be locked by anti-loosening devices after assembly, which are described in four cases. If cotter pin is used for locking, it is not allowed to use too thin opening lock or half opening lock for locking; If spring washer is used for locking, it is not allowed to use the washer with too small opening offset; If locking is adopted, the locking tab shall not be locked at the edges and corners of the nut; If double nut locking is adopted, the thinner nut shall not be installed outside.
8、 False firmness
If the bolts, nuts or threads are rusted, or scale, iron filings and other impurities, they must be cleaned before assembly; Impurities such as burr and sediment on the joint surface of the connecting piece shall also be removed. Otherwise, when tightening the bolt, it seems to be tightened on the surface, but in fact, the coupling is not really pressed. Under such false firmness, if there is vibration, load impact and temperature change, the bolt will loosen quickly.