Unveiling The Misunderstandings in Fastener Selection: Are You Trapped?

Common Misunderstandings in the Selection of Fasteners

Standard fasteners are divided into ten categories. When selecting, they should be determined according to the occasions and functions of the standard fasteners. The editor lists the common misunderstandings in the selection. See how many of them you have.

1. Using coarse threads instead of fine threads

There are many important connectors on the machine, such as the transmission shaft, 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 fine thread bolts have a larger inner diameter, smaller pitch and external angle, high strength, good self-locking performance, and strong ability to withstand impact, vibration and exchange loads. Once coarse thread bolts are used as substitutes, it is easy to loosen or fall off, break, and even cause mechanical accidents.

2. Pore mismatch

The bolts on the machine that bear lateral loads and shear forces, such as transmission shaft bolts and flywheel bolts, have a transition fit with the bolt holes. The assembly should be strong and reliable, and able to withstand lateral forces. Some people do not pay attention to the inspection during the assembly process, and continue to install when there is a large gap between the bolt and the bolt hole, which can easily cause the bolt to loosen or cut off.

3. Thicken the nut to increase the reliability of the connection

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. But in fact, the thicker the nut, the more uneven the load distribution between the threads, and the easier it is for the connection to loosen.

4. One nut with multiple washers

After the installation is completed, sometimes the bolt is too long, so some people install spring washers on the bolts. In this case, the spring washers will break due to uneven force during the solidification process, thereby reducing the preload of the bolts, and may also produce eccentric loads, reducing the reliability of the bolt connection.

5. The tighter the better

Many staff have such a misunderstanding: they think that bolts should be "tighter than loose", so they deliberately increase the tightening torque, resulting in bolt slippage. In addition, some important bolts that need to be tightened with torque are tightened by some people for the sake of saving trouble with a movable wrench, resulting in insufficient torque, causing the bolts to loosen, and even causing mechanical failures.

6. Washers that are too large are not a problem

Sometimes, when washers of suitable size are lacking, some staff members will use washers with larger inner diameters 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. If there is vibration and impact in the working environment, the bolts are easy to loosen.

7. Improper locking

Important bolts should be locked with anti-loosening devices after assembly. Here are four situations to explain. If cotter pins are used for locking, do not use overly thin cotter locks or half-piece cotter locks; if spring washers are used for locking, do not use washers with too small opening distances; if lock plates are used for locking, do not lock the lock plates at the corners of the nuts; if double nuts are used for locking, do not install thinner nuts on the outside.

8. False firmness

If the bolts, nuts or threads are rusted, or there are impurities such as scale and iron filings, they must be cleaned before assembly; burrs, sand and other impurities on the joint surface of the connector must also be removed. Otherwise, when tightening the bolts, it looks like they are tightened, but in fact the connection parts are not really tightened. Under such false firmness, the bolts will quickly loosen if vibration, load impact and temperature changes occur.