Threaded connections may be subjected to all types of variable loads in working conditions, including extremely intense vibration and impact loads. Under the action of variable loads, the failure of threaded connections is usually caused by their own loosening and fatigue damage. In general, the life of anti-vibration loosening of threaded connections is much shorter than the fatigue life of their materials and structures. Long before fatigue damage, loosening failure of threaded connections due to loosening has occurred, or premature fatigue damage of connectors and connected parts due to loosening has occurred. The failure of threaded connections will affect the normal operation of products and equipment, and may even cause serious consequences. How to prevent the loosening of threaded connections is one of the important tasks in the development and design of threaded fasteners.
There are three basic types of anti-loosening methods for threaded fastener connections:
- Non-Detachable Anti-Loosening
This is an anti-loosening method that uses welding, bonding or punching riveting to change a detachable threaded connection into a non-detachable threaded connection. It is a very reliable traditional anti-loosening method. Its disadvantage is that threaded fasteners cannot be reused. And the operation is cumbersome. It is often used in some important occasions that require high reliability in anti-loosening without disassembly.
- Welding: The nut is welded to the bolt or the fastener to the workpiece.
- Bonding: Apply adhesives such as epoxy resin or anaerobic adhesive to the matching thread surface. After the adhesive is cured, it can firmly bond the matching threads to achieve the purpose of locking and preventing loosening. Different adhesives often have different locking capabilities. Fasteners coated with epoxy resin have a high bonding strength and cannot be disassembled. Fasteners coated with anaerobic adhesive can be disassembled, but the adhesive remaining on the thread surface after disassembly is difficult to clean, and the thread may be damaged, so the fasteners should not be used again.
- Punching/Riveting:The punch riveting method is used to cause local deformation of the bolt (or screw) and nut after tightening to prevent them from loosening. The connection is reliable and can be used for any connection that does not require disassembly.
- Anti-Loosening of Mechanical Fasteners
Mechanical fasteners are used to fix and pin the threaded parts to the connected parts or between the threaded parts to prevent loosening. The advantage of this method is reliable anti-loosening, and its anti-loosening reliability generally depends on the static strength or fatigue strength of the mechanical fastener (or the fastener itself, such as the slotted nut). Its disadvantages are that it increases the weight of the fastened connection, is difficult to manufacture and install, and cannot be installed flexibly, so the cost is relatively high. Due to its high anti-loosening reliability, it is still widely used in some important parts of mechanical products and aerospace products.
- Cotter Pins with Slotted Nuts: Cotter pins pass through the slots in the nut and the pin holes at the end of the bolt to directly lock the nut and bolt. They can be used in important moving parts in a loose connection state without tightening (i.e. without applying preload), such as the connection of the movable joints of the joystick in the cockpit of aerospace vehicles and vehicles. They can also be used in particularly important parts that require high reliability in preventing loosening under long-term severe vibration conditions. In this case, the nuts and bolts must be tightened with an appropriate preload, otherwise, in the loose connection that is not tightened, the cotter pins or nuts will suffer fatigue damage, causing the fasteners to loosen and fail. Such accidents often occur in many connections that are subjected to harsh working conditions.
- Tab Washer:Use a single or double steel washer to fix the nut and the connected part together or two nuts to each other. It is reliable to prevent loosening and can be used for anti-loosening connections in high-temperature parts. Commonly used in important parts of engine products
- Locking wire: Use steel wire to penetrate the small hole of the screw head or nut to connect several screws or nuts together and lock them. Although assembly is more troublesome, it is still used in important occasions, especially important parts of aerospace products, because it is reliable to prevent loosening. Can be used to prevent loosening of bolts or screws in groups
Anti-Loosening by Increasing Friction
The purpose of anti-loosening is achieved by increasing the friction between the threads or the bolt (screw) head and the nut end face or increasing the friction of both at the same time. This anti-loosening method is less reliable than the above two methods, but its biggest advantage is that it is not limited by the use space, can be repeatedly assembled and disassembled, can be assembled flexibly, and some of the fasteners (such as nylon ring locking nuts, all-metal locking nuts) have reached a very high level of anti-loosening reliability. Therefore, this anti-loosening method is most widely used in the machinery manufacturing sector and the aerospace field.
- Double Nuts:The traditional assembly method of double nuts is to tighten the inner nut first, then tighten the outer nut, and then reverse the inner nut, so that the threads between the heights of the two nuts produce a slight elastic deformation to obtain additional friction to prevent loosening. Practice has shown that this assembly method is not reliable for double nut anti-loosening. Its disadvantage is that when the inner nut is reversed, the fastening system is unloaded, the clamping force becomes smaller, and the anti-loosening ability is reduced. The new assembly method cancels the “reverse tightening of the inner nut” procedure, that is, first tighten the inner nut, then tighten the outer nut, and apply the same tightening torque to the two nuts, so that the clamping force of the fastening system can be maintained at a higher level. Domestic and foreign tests have shown that the anti-loosening ability of double nuts using the new assembly method is greatly improved. Among the various anti-loosening methods for threaded fasteners, it is one of the several anti-loosening methods with a higher vibration resistance life. Although the use of two nuts increases the weight, it has a simple structure, good anti-loosening effect, and can be used at high temperatures, so it is still used in some important occasions, such as anti-loosening of threaded connections of engines, etc.
- Serrated Flange Nuts and Bolts:The bearing surface under the nut and screw head is knurled or serrated. When the nut or screw is tightened, friction resistance is generated between the bearing surface and the connected part, especially when the “serrations” are embedded in the surface of the connected part, the locking is very firm. The tooth shape of the bearing surface serrations and the clamping force during tightening have a significant effect on the locking performance. A stable torsion-tension relationship and a sufficiently high clamping force are the prerequisites for this fastener to maintain its locking ability. Tests have shown that they have good anti-loosening properties. It cannot be used with washers, nor can it be used for threaded connections that cannot withstand high clamping forces or where the connected surface is sensitive to scratches and corrosion. When using this anti-loosening method, attention should be paid to the reasonable matching of hardness. Generally speaking, the hardness of the clamped part should be lower than the hardness of the fastener.
All-Metal Locking Nuts:The upper end of the nut body is closed with a non-circular end or a slotted end (the latter is also called a slotted beam nut) to partially deform the thread. After the bolt is screwed into the nut, the end of the nut expands outward, and the elasticity of the end is used to compress the thread pair laterally, eliminating the thread gap, increasing the thread friction, and firmly locking the bolt and nut together. The anti-loosening effect is good, and the slotted beam nut has better anti-loosening performance. When used with bolts with higher thread accuracy, the anti-loosening performance can be improved; when used with bolts with higher hardness corresponding to the hardness of the nut, the repeated assembly and disassembly service life of the connection can be significantly improved. Among all-metal locking nuts, the slotted beam nut has the best reusability. It can be used for any fastening connection part except the moving part.
- Nylon Insert Lock Nuts: A nylon ring is embedded in the upper end of the nut, and the inner diameter of the nylon ring is slightly smaller than the thread diameter. When the bolt is screwed in, the inner thread is squeezed out of the nylon ring, and the extremely elastic nylon material forms a large and stable friction resistance with the bolt, achieving reliable locking. It has good anti-loosening performance and can be repeatedly assembled and disassembled. It is suitable for use in places that are subject to severe impact and vibration. It can be used with any bolt from low precision to high precision; it can also be used with any bolt from low strength to high strength. The operating temperature is limited by the nylon ring material, generally -50～+100℃.
- Spring washers: Use the tension of the spring to provide locking for threaded connections. Its advantages are simple structure, low cost and easy use. Widely used to prevent loosening in unimportant parts of general electromechanical equipment. Since spring washers have poor anti-loosening effect, they are not suitable for use in parts that are subjected to intense impact and vibration; and electrogalvanized or cadmium-plated steel washers often produce delayed hydrogen embrittlement fracture, causing hidden dangers that are difficult to find and subsequent failure accidents. Therefore, spring washers are not used in important occasions.
- Toothed Lock Washers: When tightening nuts or screws, the teeth of the washers are flattened, increasing the friction resistance between the threads and the bearing surface, providing a locking effect for the threaded connection. Since the teeth are embedded in the screw head (or nut) and the surface of the connected parts, the damage caused by them increases the sensitivity to corrosion, and for fasteners or connected parts that are subjected to high stress, these damages may cause cracks. When subjected to large clamping forces, the teeth of the washers may crack or break.
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