Standard fasteners are divided into ten categories, and the selection should be determined based on the usage occasion and function of the standard fasteners. The editor has listed eight common misconceptions in the selection process, depending on how many of them you have chosen.
- Replacing Fine Teeth with Coarse Teeth
There are many important connections on the machine, such as transmission shafts, and most of the bolts are fine threads. If there are missing parts during maintenance, some maintenance personnel will use coarse threaded bolts instead, which should be avoided because fine-toothed bolts have a larger inner diameter, smaller pitch and outer angle, high strength, good self-locking performance, and strong ability to withstand impact, vibration, and exchange loads. Once coarse threaded bolts are used as substitutes, they can easily become loose, detached, or broken and even cause mechanical accidents.
- Pore mismatch
The bolts on the machine that bear lateral loads and shear forces, such as transmission shaft bolts and flywheel bolts, should have a transitional fit with the bolt holes, and the assembly should be sturdy and reliable and can withstand lateral forces. Some people need to pay more attention to inspection during the assembly process and continue to install when there is a large gap between the bolts and bolt holes, which is prone to bolt loosening or cutting accidents.
- Thickening nuts to 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 connection. However, in fact, the thicker the nut, the more uneven the load distribution between the threads of each circle, and the more easily the connecting parts become loose.
- One mother with multiple pads
After installation, there may be situations where the bolt is too long, so that someone may install an additional spring washer on the bolt. In this case, the spring washer may break due to uneven force during the strengthening process, resulting in a decrease in the pre-tightening force of the bolt and the possibility of eccentric load, reducing the reliability of the bolt connection.
- The tighter the tightening, the better
Many staff members have the misconception that bolts should be “better tightened than loosened”, so they deliberately increase the tightening torque, resulting in bolt slippage. In addition, some important bolts need to be tightened with torque. In contrast, others use adjustable wrenches to tighten them for convenience, resulting in insufficient torque, leading to bolt loosening and even mechanical failure.
- Excessive gasket size is not a problem
Sometimes, there is a need for washers of suitable size, and some workers may replace them with washers with larger inner diameters. 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 easily loose.
- Improper locking
Important bolts should be locked with anti-loosening devices after assembly, which can be explained in four situations. If a split pin is used for locking, it is not allowed to use a thin or half-piece open lock for locking. If a spring washer is used for locking, it is not allowed to use a washer with an opening offset that is too small; If a locking plate is used for locking, it is not allowed to lock the locking plate at the edges and corners of the nut; If double nut locking is used, thinner nuts should not be installed outside.
- False firmness
If the bolts, nuts, or threads are rusted, or impurities such as scale and iron filings are present, they must be cleaned before assembly; Any impurities such as burrs and sediment on the joint surface of the connecting parts should also be removed. Otherwise, when tightening the bolts, it may appear to be tightened on the surface, but in reality, the connecting parts are not truly tightened. Under such false firmness, if vibration, load impact, and temperature changes occur, the bolts will quickly loosen.