How To Choose The Right Fasteners To Ensure Safety And Reliability

How to choose the right fasteners to ensure safety and reliability

When a machine worth tens of millions suddenly stops running, it is usually just a screw worth a few cents that fails. Data released at the 2025 industrial exhibition in Berlin, Germany, shows that 37% of mechanical failures are directly related to errors in fastener selection. Choosing the right fasteners is not just as simple as tightening, it is related to the safety and life of the entire equipment.

First identify what types of fasteners you have on hand

Among the most common fasteners are bolts, which are usually used in conjunction with nuts and are used to connect two parts with through holes. As for screws, they can be screwed directly into the internal threaded holes of the workpiece without the need for nuts to cooperate. They are more common in electronic product casings and furniture assembly. There are threads on both ends of the stud. One end will be screwed into the body, and the other end is used to install other components. It can often be seen on the car engine block. Helical spring washers prevent nuts from loosening and are essential in vibrating machinery. Different types have different uses. How to choose suitable fasteners to ensure safety and reliability? Choosing the wrong type is equivalent to laying hidden dangers.

How automobile manufacturers choose riveting methods and welding materials based on metal composition ? Each vehicle uses about 3,500 fasteners, of which bolts account for the highest proportion. In aircraft manufacturing, the number of fasteners is even more staggering. A Boeing 787 uses more than 50,000 titanium alloy bolts and screws. In the construction industry, a large number of high-strength bolts are used to connect steel structures. The roof grid of Beijing Daxing Airport is fixed with 1.2 million sets of large hexagonal head bolts. In the furniture industry, self-tapping screws and embedded nuts are commonly used. IKEA sells more than 600 million sets of furniture fasteners worldwide every year.

Material selection determines fastener strength limit

Among fastener materials, steel is a very common material. Ordinary carbon steel is low in cost, has good strength, and is suitable for general environments. Stainless steel contains chromium, and an oxide film will form on its surface . How to choose riveting methods and welding materials according to the metal composition ? This film can resist moisture and weak acid and alkali corrosion, so it is suitable for use in chemical plants and seaside buildings. Copper fasteners have excellent electrical conductivity and are often used on terminal blocks of electrical equipment. Aluminum parts are lightweight, making it the material of choice in the aerospace industry. Titanium alloy is extremely strong and resistant to high temperatures. It must be used in high-temperature parts of aircraft engines.

For example, the tensile strength of ordinary high-strength bolts of grade 10.9 can reach up to 1,000 MPa, but the tensile strength of ordinary grade 4.8 bolts is only 400 MPa. Although stainless steel bolts are corrosion-resistant, their strength is generally lower than high-strength steel bolts of the same size. There is a chemical plant in Qingdao that once used ordinary carbon steel bolts to fix the reactor lid. After three months, the bolts rusted and broke, resulting in the leakage of toxic gases. And after using 316 stainless steel bolts instead, after five years of use under the same conditions, they are still as good as new. The selection of materials must be comprehensively judged by combining many factors such as temperature, humidity, corrosive media, etc. in the use environment.

Size and shape must match equipment needs exactly

The size of the device with fastening function covers the diameter, pitch, length and head shape of the thread and other related parameters. Unlike metric threads, which are measured in millimeters, inch threads are measured in inches. The two types of threads cannot be mixed with each other. Pitch refers to the distance between adjacent threads. Compared with coarse threads, fine threads can withstand greater preload force. However, they are more likely to slip during use. When selecting the corresponding length, you need to ensure that the screw can expose three to five threads after passing through the connector . How to choose the riveting method and welding materials according to the metal composition . If the screw length is too short, it will not be able to bear the force, and if the screw length is too long, it may hit the opposite part during the installation process.

Shape design also plays a key role. The head of the countersunk head screw has a 120-degree tapered surface, which is suitable for occasions where surface flatness is required after installation, such as an alternative to aircraft skin riveting. Hexagonal head bolts can apply greater torque with the help of a wrench, and are suitable for mechanical bases that require higher connection strength. Hexagon socket head screws are suitable for assembly locations where space is limited and are used inside many precision instruments. There is a mold factory in Shenzhen that once mistakenly purchased M8x1.25 bolts for M8x1.0 bolts. During assembly, the threads got stuck. Five hundred sets of molds were reworked, resulting in a loss of more than 800,000 yuan. When checking thread specifications, the most reliable tool is the thread pass-and-stop gauge. It only takes a few seconds to determine whether it is qualified.

Quality certification is a hard indicator to distinguish good from bad

Only high-quality fasteners are certified by international standards. Common certifications include ISO 9001 quality management system and ISO 8978 special standard for fasteners. The automotive industry has a more stringent IATF 16949 certification, and suppliers need to conduct chemical composition analysis and mechanical performance testing on each batch of products. Aerospace fasteners must comply with NAS or SAE standards, and testing items include hardness, tensile strength, torque coefficient and fatigue resistance.

Judging from market data, the annual transaction volume of the global fastener market is approximately US$120 billion, of which counterfeit and shoddy products account for as high as 7%. A bridge in Jakarta, Indonesia, used uncertified high-strength bolts during construction. In the second year after the bridge was opened to traffic, more than 30 bolts broke, almost causing a major accident. Regular suppliers will provide batch traceable reports. This report covers the material heat number, heat treatment curve and torque test records. It is the safest approach to ask to see these documents when purchasing, and to randomly select samples and send them to a third-party laboratory for re-inspection.

Installation techniques directly affect the actual use effect

The correct installation method is to start by cleaning the threads. Oil, rust and burrs will all affect the accuracy of the torque. Use a torque wrench to tighten in two or three steps in a criss-cross sequence to ensure uniform stress on the connecting surface. In those situations where anti-loosening is required, applying thread locking glue or using spring washers and stop washers can effectively prevent vibration loosening. The pretightening force must be controlled within the range of 50% to 70% of the bolt's yield strength. If it is too tight, the thread will deform or even break.

During actual operation, the assembly line of the Shanghai Tesla Gigafactory was equipped with a high-precision electric tightening shaft, and the torque and angle data of each screw were uploaded to the cloud system. Maintenance personnel discovered that the error range of manual tightening is generally plus or minus 20%, but power tools can control it to within plus or minus 5%. The oversized bolts used to connect wind power towers must use hydraulic tensioners to accurately control the bolt elongation using hydraulic pressure. In household scenarios, even if there is no professional equipment, a pointer torque wrench worth tens of yuan can greatly improve reliability. After the installation is complete, draw a straight line with a marker on the nut and the workpiece. When checking later, you will be able to detect at a glance whether there is any looseness.

Regular inspection and maintenance ensure long-term stability of the equipment

During daily inspections, pay attention to four abnormal conditions, namely looseness, corrosion, deformation and cracks. Looseness can be identified by tapping and listening to the sound. If the fastener sounds crisp and loud, it is not loose. If the sound is dull and hoarse, it indicates that it is loose. Rusted parts need to be treated with rust remover. If the rust is severe, it must be replaced immediately. Deformation and cracks are difficult to detect with the naked eye. For key equipment, it is more reasonable to conduct magnetic particle inspection or color penetrant inspection every six months.

Curtain walls of buildings require a fastener sampling test every two years. Shenzhen Ping An Financial Center has to inspect more than 60,000 stainless steel bolts every year. The wheel bolts should also be replaced when replacing tires at the auto repair shop, as each disassembly and assembly will cause the threads to wear slightly. Pipe flange bolts in chemical plants that are at high temperatures must be replaced every time they are shut down for maintenance. The metal will creep under long-term stress under high temperature conditions, and the risk of continued use is quite high. Building a simple inspection form that records the date, location, and results of each inspection can help you discover patterns of potential problems.

Have you ever experienced equipment failure due to a loose screw? Feel free to share your stories in the comment area. Please like and forward this article so that more engineers who are engineers can avoid the pitfalls of model selection.