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Common misunderstandings and selection principles of fasteners


Fasteners are widely used and are often called standard parts on the market. Not only are there many types of fasteners, but also the demand is very large.In the process of use, we need to select the appropriate type for fastening and connecting mechanical parts according to the specific actual situation. The common misunderstandings and the principles of choosing various types of fasteners are hopeful to help everyone.
Eight common misunderstandings of fasteners
1. Use coarse teeth instead of fine teeth
There are many important connectors on the machine, such as the transmission shaft, etc. Most of the bolts are fine-threaded. If parts are missing during maintenance, some maintenance personnel will use coarse-tooth bolts instead, which should be avoided. Because the fine-diameter bolt has a large inner diameter, a small pitch and an external angle, high strength, good self-locking performance, and strong ability to withstand impact, vibration and exchange load. Once it is replaced with a coarse tooth bolt, it is easy to loosen or disengage, disassemble, or even cause a mechanical accident.
2. Poor pores
The bolts on the machine that are subjected to lateral loads and shear forces, such as drive shaft bolts and flywheel bolts, have a transitional fit with the bolt holes. The assembly should be strong and reliable, and can withstand lateral forces. Someone does not pay attention to the inspection during the assembly process, and the installation continues when there is a large gap between the bolt and the bolt hole, which is easy to loosen or cut off the bolt.
3. Thick nuts increase connection reliability
Some people mistakenly believe that thickening the nut can increase the number of working cycles of the thread and improve the reliability of the coupling. But in fact, the thicker the nut, the more uneven the load distribution among the threads of the circle, the easier the coupling is to loosen.
Four, one mother and more pads
After the installation is completed, sometimes the bolt is too long, so someone will install an additional spring washer on the bolt.In this case, the spring washer will be unevenly broken during the firming process, resulting in a pre-bolt Decreased tightening force may also produce eccentric loads, reducing the reliability of bolted connections.
5. The tighter the better
Many staff members have the misunderstanding that they think that the bolt should be “tightened rather than loose”, so they deliberately increased the tightening torque, which resulted in the bolt slipping. In addition, there are some important bolts that need to be tightened with torque, but some people use an adjustable wrench to save the trouble. As a result, the torque is insufficient, resulting in loose bolts and even causing mechanical failure.
Sixth, the washer is too large does not hinder
Sometimes a properly sized washer is missing, and some workers will use a washer with a larger inner diameter instead.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. There is vibration and shock, and the bolts are easy to loosen.
7. Improper locking
Important bolts should be locked with anti-loosening devices after assembly. There are four cases to illustrate here. If the cotter pin is used for locking, do not use a very thin cotter lock or a half-piece cotter lock to lock; if a spring washer is used for lock, the gap of the gasket opening should not be too small; if a lock plate is used for locking, the lock plate must not be locked At the corners of the nut; if double nut locking is used, the thinner nut must not be installed outside.
8. False and strong
If the bolts, nuts or threads are rusted, or scale, iron filings and other impurities, they must be cleaned before assembly; the joint surface of the coupling parts should also be cleaned of burrs, sand and other impurities. Otherwise, when tightening the bolt, the surface seems to be tightened, but in fact the coupling is not really compressed. Under such false solidity, if vibration, load shock and temperature change occur, the bolt will quickly loosen.