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Common Misunderstandings In The Selection Of Bolt Fasteners

Nov 23, 2020

Replace fine teeth with coarse teeth: there are many important connectors on the machine, such as transmission shaft, and most of the bolts are fine thread. In case of missing parts, some maintenance personnel will use coarse thread bolts instead, which should be avoided. Because of its large inner diameter, smaller pitch and outer angle, fine thread bolt has high strength, good self-locking performance, and strong ability to withstand impact, vibration and exchange load. Once the coarse tooth bolt is replaced, it is easy to loose or break, and even cause mechanical accidents.


Mismatch of holes: the bolts on the machine that bear lateral load and shear force, such as transmission shaft bolt and flywheel bolt, are transitional fit with bolt hole, and assembly should be firm and reliable, and can bear lateral force. 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 is prone to bolt loosening or cutting accident.


Thickened nuts increase connection reliability: some people mistakenly believe that thickened nuts can increase the number of working turns of threads and improve the reliability of connectors. But in fact, the thicker the nut is, the more uneven the load distribution among the threads is, and the more easily the connector becomes loose.


One mother with more pads: after the installation, sometimes the bolt will be too long, so someone will install additional spring washer on the bolt. In this case, the spring washer will be broken due to uneven force in the process of firming, which will reduce the pre tightening force of the bolt, and may produce eccentric load to reduce the reliability of the bolt connection.


The tighter the screw, the better: many staff members have such a misunderstanding: they think that the bolt should be "rather tight than loose", so they deliberately increase the tightening torque, resulting in bolt slip. In addition, some important bolts need to be tightened with torque, but some people use spanner to screw them because of the convenience. As a result, the torque is insufficient, resulting in bolt loosening and even mechanical failure.


If the washer is too large, it will not hinder: sometimes there is a lack of washer with appropriate size, and some staff will replace it with a washer with a larger inner diameter. In this case, the contact area between the washer and the bolt head 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 easy to loosen.


Improper locking: locking device should be used to lock important bolts after assembly. There are four cases to explain. If the split pin is used for locking, it is not allowed to lock with too thin opening lock or half piece opening lock; if the spring washer is used for locking, the opening stagger of the washer shall not be too small; if the locking plate is used for locking, the locking plate shall not be locked at the edges and corners of the nut; if the double nut locking is adopted, the thinner nut shall not be installed outside.


False firmness: if the bolts, nuts or threads are rusty, or scale, iron filings and other impurities, they must be cleaned before assembly; the burrs, sediment and other impurities on the joint surface of the connecting parts should also be removed. Otherwise, when tightening the bolt, it seems that it is tightened on the surface, but in fact, the connector is not really compressed. Under such false firmness, the bolt will loosen quickly in case of vibration, load impact and temperature change.