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1. The holes of the screws and nuts do not match
Screws and nuts (such as drive shaft bolts, flywheel bolts) that are subjected to lateral loads and shearing forces on the machine should be fitted with the bolt holes for transitional fit, and the assembly should be strong and reliable, and can withstand lateral forces. Some people do not pay attention to the inspection during assembly, and the bolt is still installed when there is a large gap between the bolt and the bolt hole, resulting in a bolt loosening or cutting accident.
2. Substitute thick screws and nuts for thin
Important joints on the machine, such as drive shafts, fly shafts, etc., mostly use fine thread for their bolts. When parts are missing during maintenance, some people use coarse-toothed bolts instead. Due to the larger inner diameter, smaller pitch and outer angle of the fine-thread bolts, it has the advantages of high strength, good self-locking performance, etc., and has a strong ability to withstand shock, vibration and exchange loads; , causing mechanical accidents.
3. One screw and nut with multiple pads
During the installation, sometimes the assembled screws and nuts are too long, so some people install many spring washers on a bolt, so that the spring washers are not evenly stressed during the firming process, and some will break, making the bolts prearranged. The tightening force decreases; or an eccentric load is generated, which reduces the reliability of the bolt connection.
4. The washer of the screw and nut is too large
Sometimes due to the lack of suitable gaskets, some people use larger inner diameter gaskets instead. In this way, the contact area between the head of the bolt and the washer is small, the bearing pressure or locking force of the washer is reduced, and the bolt is easy to loosen under the action of vibration and impact load.
5. Thickened nut
Some people often mistakenly believe that thickening the nut can increase the number of working turns of the thread, thereby improving the reliability of the coupling. In fact, the thicker the nut, the more uneven the load distribution between the threads, and the easier it is to cause the coupling to loosen.
6. The torque of the screw and nut does not match
Many people think that the screw and nut should be "better tight than loose", so they consciously increase the tightening torque of the screw and nut, which results in the screw and nut slipping. It is easy to use an adjustable wrench to screw it, and the result is often loose due to insufficient torque, resulting in mechanical failure.
7. False solid screw
There are impurities such as rust stains, scale, iron filings, sand particles in the threads of bolts, nuts or screw holes, or impurities such as burrs, sediment and other impurities on the joint surface of the connecting piece, which are not removed before assembly. Blocking effect, the surface is tightened very tightly, and the torque value meets the requirements, but in fact the connecting piece is not really compressed; Although the torque of the bolt meets the specification, the workpiece is not compressed. Under such false firmness, vibration, load shock and temperature change often cause the bolt to loosen quickly.
6. Improper locking of screws and nuts
After the screw and nut are assembled, the anti-loosening device should be used to lock. When locking with a split pin, a common mistake is to lock with a thin split lock or a half-piece split lock; when a spring washer is used for locking, the common mistake is that the gap of the washer opening is too small and loses elasticity; locking with a locking plate Yes, the common mistake is to lock the lock plate at the corner of the nut; with double nut locking, the common mistake is to install the thinner nut on the outside and not tighten it tightly
