Eight Common Misunderstandings of Fasteners

  1. Use coarse teeth instead of fine teeth

There are many important connecting parts on the machine, such as the drive shaft, and most of the bolts are fine thread. If there are missing parts during maintenance, some maintenance personnel will use coarse bolts instead that is not correct. Because the inner diameter of the fine pitch 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 impact, vibration, and exchange loads is relatively strong. Once replaced with coarse-threaded bolts, it is prone to loosening or detachment, disassembly, and even cause mechanical accidents.

2. the pores are not matched

The bolts on the machine that bears the lateral load and shear force, such as the drive shaft bolts and flywheel bolts, are matched with the bolt holes as transitional fits. 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 continues to install when there is a large gap between the bolt and the bolt hole, which is prone to bolt loose or cut off accidents.

3. Thicken nuts increase 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 coupling. But in fact, the thicker the nut, the more uneven the load distribution between the threads of each circle, and the easier the coupling is to loosen.


4. one female with multiple pads

After the installation is complete, sometimes the bolt is too long, so someone installs an additional spring washer on the bolt. In this case, the spring washer will break due to uneven force during the strengthening process, which will cause the bolt to be pre-tensioned. If the tightening force is reduced, eccentric load may be generated, which reduces the reliability of the bolt connection.

5. the tighter the better

Many workers have such a misunderstanding: that the bolts should be "tighter rather than loose", so they deliberately increase the tightening torque, resulting in bolt slip. In addition, for some important bolts that need to be tightened with torque, some people use adjustable wrenches to save trouble. As a result, the torque is insufficient, causing the bolts to loosen, and even causing mechanical failure.

6. the washer is too big and does not get in the way

Sometimes there is a lack of washers of the right size. Some workers will use washers with larger inner diameters instead. In this case, the contact area under the bolt head with the washers is small, which will reduce the bearing pressure or locking force of the washers. There are vibrations and shocks, and bolts are easily loosened.

7. improper locking

Important bolts should be locked with the anti-loosening device after assembly. Here are four cases. If a split pin is used for locking, it must not be locked with a thin split lock or half-piece split lock; if a spring washer is used for locking, it must not be used with a washer that has too small an opening error; if a lock plate is used for locking, the lock plate must not be locked. At the edges and corners of the nut; if double nuts are used for locking, do not install thinner nuts on the outside.

8. the surface looks being tightened

If the bolts, nuts, or threads are rusted, or impurities such as scale, iron filings, etc., must be cleaned before assembly; burrs, sand, and other impurities on the joint surface of the coupling must also be removed. Otherwise, when the bolts are tightened, they appear to be tightened on the surface, but in fact, the couplings are not really tightened. Under such false solidity, the bolts will loosen quickly if vibration, load shock, and temperature change occur.