From generator equipment to automation systems, from transportation systems to leisure and sports gear, from test and measurement instruments to medical devices, and from digital products to children’s toys, threaded fasteners are used in the assembly of virtually all these items. Threaded joints offer numerous advantages, but they also have their limitations. One of the key challenges with threaded connections is self‑loosening. The issue of self‑loosening has long captivated product designers, manufacturing specialists, engineers, as well as maintenance and occupational safety professionals—quite understandably, since it directly impacts safety.

We’ve all experienced this: screws or nuts loosen—whether on a bicycle or on a pair of glasses. So, what causes this problem? And how can we prevent it from happening?

To achieve this, we need a deeper understanding of how joints function and how the threads of fasteners interact. We must also analyze certain stress factors to fully grasp the mechanisms underlying loosening. Only then can we identify the optimal solution to minimize or prevent loosening. During tightening, friction acts between the fastener’s threads and the bearing surfaces, and this friction helps to prevent the fastener from loosening once it has been tightened. So if friction can constrain the joint, why does it still loosen?

Connection

Under ideal conditions, the joint interface itself can withstand dynamic loads and prevent loosening. Fastener torque must achieve a specified preload, and bolted joints are subjected to forces exclusively in tension. Dynamic loading, however, can lead to loosening. To mitigate this, the design should account for preventing relative sliding of assembled components caused by lateral loads. In this regard, the clamping length becomes particularly critical. Fasteners with a clamping length less than five times the thread diameter will not exhibit resilient rebound; they lack elasticity and have poor vibration resistance. Whenever feasible, the joint design should be modified to achieve a clamping length of at least five times the thread diameter. Such bolted connections offer greater resilience and improved vibration‑damping performance. Nevertheless, in many practical applications, this approach is difficult to implement.

External load

To prevent loosening, designers must identify the external loads acting on the joint. These loads determine the stability of the frictional forces at the mating surfaces and whether additional preventive measures are required. External loads are classified into two categories: static loads and dynamic loads.

Dynamic load

Dynamic loads can arise in several different ways—whether from the equipment itself or from the surrounding natural environment. When external loads are applied, thread friction and bearing‑surface friction help maintain a tight connection. In the case of standard‑mechanism threads, friction exists only on one side of the thread, while the opposite side remains gap‑filled. If the dynamic load is sufficiently large, thread friction drops significantly, leaving only the friction between the screw head or nut and the bearing surface to prevent the joint from loosening.

Static load

If the joint is designed to withstand only static loads, loosening typically does not occur. However, on ocean-going cargo ships, the cargo is subjected to continuous vibration during transit. Vibrations generated by heavy-duty diesel engines cause all the cargo onboard to vibrate as well, potentially leading to loosening of threaded fasteners. Loosening, rotation, or even loss of screws or nuts can result in the complete disintegration of the assembled structure.

Friction

It is typically defined as the coefficient of friction (CoF). Friction depends on the materials and coatings selected. Some materials, such as stainless steel and aluminum, inherently determine the magnitude of the CoF, whereas steel usually requires additional surface treatments to establish its friction characteristics. In most cases, friction‑modifying additives in coatings are used to control the CoF, thereby reducing variations in tightening torque and, in turn, clamping force. This creates a dilemma: while these additives help ensure the desired clamping force and maximize fastener preload, they also lower the CoF, increasing the risk of loosening under dynamic loading. When fasteners operate at their maximum allowable clamping force and the thread flanks and bearing surfaces are subjected to significant external loads, they can better withstand dynamic stresses. However, this behavior does not occur in certain applications, such as when tightening soft materials like plastics or aluminum; in such cases, additional anti‑loosening measures are required.

Head shape

Flange/Washer‑Head Type: Using a head with a large bearing surface, such as a flange or washer head, increases friction between the head and the mating surface. This also reduces the contact pressure on the bearing surface, minimizes thread stripping, and generates additional friction to protect the threaded joint from dynamic loading.

Serrated‑surface/ribbed‑flange head: Employing a serrated‑surface or ribbed‑flange head on the bearing surface enables a locking function. During tightening, the serrated or ribbed surface grips the mating part’s surface, creating a high‑strength metal‑to‑metal lock. However, this type of locking can damage the mating surface, particularly painted finishes, so caution is advised. If a serrated design is used on the screw’s bearing surface, the corresponding nut should also feature a serrated bearing surface to ensure a high‑friction contact interface. Washers should not be used in conjunction with parts that have serrated surfaces.

Nut

If increasing friction on the bearing surface by using a serrated face is not considered, a nut‑locking method can also be employed. However, improper use of the nut often leads to loosening as well. The term “effective torque‑type locknut with all‑metal or non‑metal inserts” can itself be misleading, since these fasteners are typically referred to as “locknuts.” By incorporating a nylon insert or deforming the metal threads, loss of friction at the joint is prevented. Nevertheless, even with such locking mechanisms in the threads, the nut may still rotate and loosen. The effective torque‑generating element never extends along the entire height of the nut; the locking effect is confined to only a few thread pitches at the nut’s top, while the remaining threads remain unsecured.

When dynamic loads are sufficient to reduce the preload on the joint interface, the locking mechanism can prevent nut loosening, but it cannot prevent further loss of preload under slight rotational motion; once the preload is lost, it cannot be restored. This may lead to misalignment of the joint and ultimately to fatigue failure. Furthermore, when using torque‑controlled locknuts, reusability must be taken into account, as repeated use will progressively degrade the locking performance.

Threaded locking method

Polymer coating

The locking element for non-metallic torque‑controlled threads is polyamide. Polyamide is a thermoplastic resin that softens at temperatures above 120°C, and within this range the effective torque is lost. Torque‑controlled threads are typically coated with polyamide at specific points by first applying a fine powder to the designated heated thread area. Following high‑temperature treatment, the powdered layer on the thread surface instantly softens and adheres to the threads. This ultimately forms a flexible polyamide patch on the thread, which becomes trapped in the mating thread during screw tightening. Once loosened, the screw will no longer continue to rotate due to vibration.
If the screw is rotated during coating, a 360° polyamide coating can be applied. This not only locks the thread but also provides a sealing effect—making this type of screw particularly important for installing sealed containers. The polyamide coating can be applied directly to the functional areas, namely the points where the external or internal threads engage. The coating thickness can be adjusted to some extent, thereby achieving the desired torque‑bearing performance. Typically, 2–3 thread turns at the end of the thread are left uncoated, ensuring smooth engagement when the screw is tightened into the mating component. Torque‑controlled screws are commonly used as adjustment screws.

Adhesive Coating

Using “adhesive coating” can also eliminate thread clearance between standard screws and nuts or internal threads. During assembly, this product is applied into the thread gaps; as it cures, it prevents the thread flanks from slipping against each other, maintaining friction and enabling the screw/nut to resist vibrational forces. The full curing process may take up to 72 hours, but in most cases, the joint locks within a short time. Therefore, to ensure reliable locking, avoid adjusting the fastener after assembly. For the same reason, adhesive‑based locking should be used only once; if disassembly is required, the fastener must be discarded. A 360-degree adhesive coating also provides excellent sealing performance. It is important to note that the “bonding strength” should match the strength of the fastener itself. If the adhesive‑based lock is excessively strong, it may damage the hardware during removal.

washer

Washers are part of the fastener family, yet they do not provide direct clamping force. Improper use of washers is widespread and often compromises joint strength or increases the risk of loosening in dynamic applications.

Flat washer

When using a flat washer—provided it is properly selected—the goal is to reduce the surface pressure on softer materials, thereby minimizing the loss of clamping force due to settlement. The bearing surface of the washer is typically larger than that of the screw and/or nut. A larger contact diameter results in greater frictional resistance; therefore, during tightening, the bolt head should always rotate against the washer rather than the washer rotating against the clamped components. The washer protects softer materials, thus reducing the risk of loosening under dynamic loading. Select the appropriate washer hardness based on the fastener’s strength class. If the material of the contact surface is too soft to adequately support the bolt head, choosing an incorrect washer hardness can increase the likelihood of loosening.

Split lock washer

Most people mistakenly believe that using split lock washers can reduce the risk of rotational loosening. However, in most cases, this assumption is incorrect. The purpose of split lock washers is to minimize the loss of clamping force during seating. Therefore, when used properly, they can lower the risk of loosening caused by dynamic loads. Yet, users often overestimate the strength of these washers, leading to increased loosening under seating and/or dynamic loading. It is well known that split lock washers can only achieve a clamping force comparable to that of 5.8‑grade fasteners—i.e., at the fastener’s ultimate load capacity. When paired with such fasteners, split lock washers help reduce clamping‑force loss, thereby mitigating the risk of loosening under dynamic loading.

In addition, the edges of most split lock washers feature a mechanical locking mechanism that, when used on soft material surfaces, helps enhance the locking performance.

It should be emphasized that when split lock washers are used in conjunction with heat‑treated fasteners of grade 8.8 (Class 5) or higher, their anti‑loosening effectiveness is either negligible or nonexistent. The washer’s elasticity is too weak, and its edges fail to engage the hardened surface of the fastener. Moreover, when paired with higher‑performance fasteners, the washer may undergo plastic deformation or even fracture under high loading, thereby posing an even greater risk.

Serrated lock washer

The title itself is misleading. This type of washer serves only to facilitate electrical conductivity. It is commonly used in grounding applications, such as automotive batteries. The serrated portion of the washer is often regarded as a locking feature, but in reality, its design typically cannot withstand the assembly‑induced stresses. This leads to a higher risk of galling and, consequently, greater loosening. Even though the serrations can increase friction on softer materials, the surface hardness of most fasteners—particularly those with a strength class of 8.8 or higher—remains too high to effectively achieve metal‑to‑metal locking. In certain cases, for example when paired with fasteners of strength class 6.8 or lower, the washer’s limited contact pressure and clamping force may provide some degree of locking.

Ribbed Lock Washer

Ribbed lock washers have ribs on at least one side. Together with the friction of the thread, they increase friction on the bearing surface, preventing the screw and/or nut from loosening due to spontaneous rotation.

The design of the ridge (or tooth) enables these washers to self‑lock onto the clamped workpiece while also anchoring against the bearing surface of the bolt or nut, thereby resisting reverse forces. Similar to conical spring washers and split lock washers, ribbed lock washers are used to reduce the risk of embedment. Like plain washers, ribbed lock washers come in various sizes, with their functions differing according to their shape. The outer diameter of a ribbed spring lock washer is approximately equal to the bearing surface diameter of the screw and/or nut, and both sides feature ribbed patterns.

Rip-Lock-type ribbed spring lock washers feature a larger outer diameter and are available with either oversized clearance holes or slotted holes. The screw head or nut rests on the upper surface of the ribbed side, while the underside of the washer is smooth. The washer’s large diameter generates sufficient friction at the clamping interface to prevent rotation.

NORD-LOCK washers

Nord-Lock is a specialized serrated locking washer. These washers are always used in pairs to achieve a locking effect. The outer surface features ribbed patterns that can even bite into exceptionally hard materials, while the inner surface has finely machined inclined surfaces. When the screw or nut is tightened, the inclined surfaces of the two washers engage tightly, creating a secure connection between them. If vibration causes the screw to loosen and rotate, the upper washer will turn slightly relative to the lower washer. Because the angle of the washer’s inclined surface exceeds the thread lead angle, this effectively increases the clamping force, continuing to prevent rotational loosening. Nord-Lock washers can be reused multiple times, and the required clamping force can be achieved with a slightly higher tightening torque. Please refer to the relevant product information in the Bossard catalog.

The original Nord‑Lock locking washers cannot prevent energy release in the same way as the conical ribbed locking washers. Recently, Nord‑Lock has introduced a new conical ribbed washer that, in addition to its standard performance, also helps reduce energy release. In summary, these ribbed locking washers possess the following characteristics:

Must be used in conjunction with bolts/nuts to achieve secure locking.

It can be used with hardened fasteners. However, only Nord-Lock washers are suitable for use with 12.9‑grade fasteners.

Self-tapping locking screw

By eliminating play in the threads, self-tapping locking screws do not loosen under dynamic loading (vibration). Typically, there is clearance between the threads of a screw and nut. However, when a self‑tapping screw is driven into a workpiece, the internal thread is formed by cold extrusion, leaving no thread clearance.

Even under severe vibration, the threaded surfaces do not experience relative slippage. The existing thread friction is fully preserved, eliminating the need for additional locking devices. Moreover, the anti‑vibration and self‑locking performance remains intact after disassembly and reassembly. Self‑tapping lock screws eliminate the requirement for nuts and can be installed in either blind or through holes. Thanks to their excellent resistance to vibration and loosening, design engineers have successfully deployed these fasteners in machinery and equipment subjected to intense vibrational loads.

Self-tapping locking screws can be used on low-carbon steel, light alloys, and most plastics; different material types require the selection of corresponding thread profiles.

Summary

There is no one-size-fits-all solution applicable to every joint design. Design engineers must calculate or estimate the likelihood of joint loosening. By taking into account factors such as applied loads, materials, safety requirements, design specifications, reusability, and assembly considerations, they can select the most appropriate application‑specific solution.