In mechanical assembly and daily maintenance, bolts and nuts fastening may seem simple, but there are many overlooked misconceptions that can lead to unstable equipment operation, loose connections, and even accidents. Today, we will uncover these common screw tightening misconceptions and help everyone operate correctly to ensure a secure and reliable connection.
Pore mismatch
The fit between bolts and nuts and holes of bolts and nuts is the foundation of fastening. If the pore is too large, the bolts and nuts will loosen in the hole of bolts and nuts, unable to provide sufficient clamping force, resulting in unstable connection parts and easy vibration and displacement. If the pore size is too small, it will make it difficult to screw in the bolts and nuts, requiring excessive torque, which may cause thread damage or fracture of bolts and nuts. Therefore, when machining holes of bolts and nuts, it is necessary to strictly follow the standard dimensions and tolerance requirements to ensure that the fit gap between the bolts and nuts and the hole is appropriate and achieve a good connection effect.
The tighter the tightening, the better it will be.
Many people believe that the tighter the bolts and nuts are tightened, the more secure the connection will be, but this is not the case. Overtightening bolts and nuts can cause excessive tensile stress on bolts. When the yield strength of the material is exceeded, the bolts may undergo plastic deformation or even fracture, especially under high temperature, high pressure, or vibration conditions, which are more prone to problems. The correct approach is to use a torque wrench to tighten the bolts and nuts according to the specified torque based on the material, specifications, and connection requirements of the bolts and nuts, ensuring that they can provide sufficient clamping force without exceeding its bearing limit.
One bolt and nut, multiple flat pads
It is not advisable to use multiple flat washers on the same bolts and nuts. The main function of a flat washer is to increase the contact area between the nut and the connected object, disperse pressure, and protect the connection surface. But if too many flat washers are used, it will cause changes in the relative position of the nut and bolt, affecting the meshing effect of the thread and reducing the reliability of the connection. Meanwhile, excessive flat washers can also increase the complexity and difficulty of assembly, which is not conducive to quick installation and disassembly. In general, only one flat washer is needed for each bolt and nut to meet the requirements.
Replace fine teeth with coarse teeth or vice versa
Coarse thread and fine thread have their own applicable scenarios in design. Coarse thread has a larger pitch, deeper tooth profile, higher strength, and better self-locking performance, making it suitable for general mechanical connections and applications that can withstand large loads. Fine thread has a smaller pitch, shallower profile, smaller thread angle, better self-locking performance, and higher connection accuracy. It is commonly used for connections that require precise adjustment or to prevent loosening in vibration environments. If coarse teeth are randomly used instead of fine teeth or vice versa, it may lead to poor thread fit, affecting the reliability and accuracy of the connection. Therefore, when selecting thread types, they should be based on specific usage requirements and design specifications, and cannot be replaced arbitrarily.
Use flat pads and spring pads that do not match the bolts and nuts
The specifications and materials of flat pads and spring pads should match the bolts and nuts. If the size of the flat pad or spring pad is too large or too small to fully fit the bolt head or nut, it will affect its pressure distribution and anti-loosening effect. If the material does not match, it may cause the gasket to fail prematurely, unable to provide sufficient support or elasticity. For example, in high-strength bolts and nuts connections, the use of low-strength flat or spring washers may cause deformation or fracture due to insufficient strength, thereby affecting the stability of the connection. Therefore, when selecting flat pads and spring pads, it is important to ensure that their specifications, materials, and bolts and nuts match, and to choose according to standard requirements.
Inappropriate locking, when using double nut locking, install a thinner nut on top
Double nut locking is a common anti-loosening method, but its installation sequence and method are specific. The correct approach is to tighten the thicker nut below and the thinner nut above. Thicker nuts can provide a larger support surface and stronger clamping force, while thinner nuts are used to lock thicker nuts and prevent them from loosening. If a thinner nut is installed on top, due to its insufficient strength and rigidity, it is prone to deformation and loosening during the force process, thereby reducing the locking effect and causing the nut to loosen.
False fastening, rusting of threads, direct locking due to impurities not cleaned or replaced
Before assembly, if there are oil stains, rust, impurities, and other contaminants on the threaded surface of bolts and nuts or nuts, it will affect the normal engagement of the threads and lead to false tightening. Although it appears to be tightened on the surface, the effective contact and clamping force between the bolt and nut are insufficient, which can easily lead to loosening or even detachment during use. Therefore, before tightening the bolts and nuts, it is necessary to clean the threads to remove oil, rust, and impurities, ensuring that the threads are clean and smooth. If necessary, new bolts and nuts or nuts can be replaced to ensure the reliability of the connection.
By understanding these common tightening misconceptions of bolts and nuts, we can avoid making mistakes in practical operations and ensure that every connection is firm and reliable.
How deep should the nut be screwed in properly? Why should the bolt extend 2 to 3 times beyond the nut?
The appropriate depth of nut insertion usually refers to ensuring that the threads can fully engage when the nut is matched with the bolt, while providing sufficient clamping length to ensure the reliability of the connection and the required preload force. There is no unified standard answer for how deep to screw in, as it depends on multiple factors, including but not limited to the following.
1. Bolt size: Different specifications of bolts (such as M10, M12, etc.) have different effective thread engagement lengths.
2. Material properties: The type of connected material (such as steel, cast iron, aluminum, plastic, etc.) will affect the required thread engagement length and whether additional safety margins are needed.
3. Mechanical performance requirements: High-strength connections may require more thread engagement to ensure sufficient load-bearing capacity and prevent thread failure due to excessive stress.
4. Industry standards: Different industry standards may specify the minimum bolts and nuts in depth for nuts, for example, the general principle mentioned above is 1.5 to 1.6 times the outer diameter of the thread, but this is not applicable in all cases.
5. Bolt hole depth: The depth of the bolt hole must be greater than the total length of the bolt after being screwed in to ensure that the bolt does not penetrate the connected part.
6. Safety considerations: For specific occasions, such as aviation, automotive, and other industries, there may be stricter requirements for bolts and nuts in depth and preload force.
In practice, when screwing in the nut, it should be ensured that the threaded part of the bolt can at least enter the threaded hole of the connected part to achieve complete thread contact, and the appropriate tightening degree should be determined according to design requirements and engineering practice. For certain situations, such as nuts in blind holes, it is also necessary to consider the distance between the bottom of the nut and the bottom of the blind hole to avoid damaging the workpiece or affecting the effective connection of the thread. In general, the screwing depth of the nut should be determined based on relevant design specifications and standards, while ensuring the connection function and safety.
In various situations, the depth of the insertion of bolts and nuts may be very different.
For self tapping bolts and nuts, they are mainly used for self drilling and fixing in materials. In this case, the screwing depth should usually be controlled between 1 and 2 times the thread diameter to ensure that the bolts and nuts can be firmly fixed in the material, while avoiding excessive screwing that may cause material damage or breakage of bolts and nuts.
For wood bolts and nuts, they are mainly used to fix wood. Due to the relatively soft texture of wood, the screwing depth can be slightly deeper, generally reaching 1.5 to 2 times that of the wood material. This ensures that the bolts and nuts can tightly adhere to the wood and provide sufficient support.
For machine bolts, the depth of tightening usually needs to be determined based on the thread length. Generally speaking, the screwing depth should be at least 1.5 times the thread length to ensure that the bolt can firmly connect two or more components.
In addition to the type and material of the bolts and nuts, the actual application situation and material also have an impact on the depth of screwing in. For example, when screwing in hard materials, it may be necessary to reduce the screwing depth appropriately to avoid excessive force causing breakage of bolts and nuts or material damage. When screwing in soft materials, the screwing depth can be appropriately increased to improve the fixing effect.
In general, determining the depth of the insertion of bolts and nuts requires comprehensive consideration of multiple factors. In practical operation, it is recommended to make judgments based on specific situations and follow relevant standards and specifications for operation. If you are unsure of the specific screwing depth, it is recommended to consult a professional or consult relevant materials for more accurate guidance. Additionally, it is important to conduct experiments to determine how to proceed.
