Standard fasteners are divided into ten categories, and the selection should be determined according to the use occasions and functions of standard fasteners. We have listed the common mistakes in the selection, and see how many you have hit.
1. Use coarse thread instead of fine thread
There are many important connecting parts on the machine, such as drive shafts, etc., and most of the bolts are fine threads. If there are missing parts during maintenance, some maintenance personnel will use coarse thread bolts instead, which should be avoided. Because the inner diameter of the fine thread bolt is larger, the pitch and outer angle are smaller, the strength is high, the self-locking performance is good, and the ability to withstand shock, vibration, and exchange load is relatively strong. Once coarse-threaded bolts are used instead, it is easy to loosen or fall off, break off, and even cause mechanical accidents.
2. The gap does not match
The bolts on the machine that are subjected to lateral loads and shear forces, such as drive shaft bolts and flywheel bolts, are transitionally fitted with the bolt holes, and the assembly should be solid and reliable and can withstand lateral forces. Some people do not pay attention to inspection during the assembly process and continue to install when there is a large gap between the bolt and the bolt hole, which is prone to bolt loosening or cutting accidents.
3. Thickened nut increases connection reliability
Some people mistakenly believe that thickening the nut can increase the number of working turns of the thread and improve the reliability of the joint. But in fact, the thicker the nut, the more uneven the load distribution between the threads of each circle, and the easier it is for the joint to loosen.
4. One bolt with multiple washers
After the installation is completed, sometimes the bolts are too long, so some people install additional spring washers on the bolts. In this case, the spring washers will break when the force is uneven during the firming process so that the bolts are pre-installed. If the tightening force decreases, the eccentric load may also occur, which will reduce the reliability of the bolted connection.
5. The tighter the screw, the better
Many staff has such a misunderstanding: that is, they think that the bolts should be "rather tight than loose", so they deliberately increase the tightening torque, which results in the bolts slipping. In addition, for some important bolts that need to be tightened with torque, some people use an adjustable wrench to tighten them to save trouble. As a result, the torque is insufficient, causing the bolts to loosen and even cause mechanical failures.
6. It is not a problem if the washer is too large
Sometimes there is a lack of gaskets of the right size, and some workers will replace them with gaskets with larger inner diameters. In this case, the contact area between the bottom of the bolt head and the washer is small, which will reduce the bearing force or locking force of the washer. If there is vibration and shock in the working environment, the bolt is easy to loosen.
7. Improper locking
Important bolts should be locked with anti-loosening devices after assembly. Here are four situations to illustrate.
If a cotter pin is used for locking, it shall not be locked with an overly thin cotter lock or a half-piece cotter lock;
If a spring washer is used for locking, it is not allowed to use a washer whose opening is too small;
If the locking piece is used for locking, the locking piece shall not be locked at the corner of the nut;
If double nut locking is used, the thinner nut must not be installed outside.
8. False solidity
If there is rust on the bolts, nuts, or threads, or impurities such as scale and iron filings, they must be cleaned before assembly; burrs, sediment, and other impurities on the joint surface of the joints must also be removed. Otherwise, when the bolt is tightened, it appears to be tightened on the surface, but in fact, the joint is not really pressed. With such a false solidity, the bolts will loosen quickly in the event of vibration, load shocks, and temperature changes.
