Lock nuts and other types of screw locks

In industrial applications, inconspicuous details often determine operational reliability, maintenance and product service life – including screw locking devices. Whether in mechanical engineering, material handling or robotics, a loosened screw can have costly consequences. In this blog, we take a real-world look at locknuts, introduce alternative locking methods such as locknuts, adhesives and some interlocking solutions, and analyze the effectiveness of screw locks using a Junker test.

Why do screws loosen?

Screws hold components together, transfer forces, and secure moving components. However, screws may unintentionally loosen during operation. This may manifest itself as increased wear or play in the joint, but can also have serious consequences.

Typical causes of unintentional loosening are:

• Vibrations and micromovements: In dynamic systems, such as robots or conveyors, repeated vibrations lead to a gradual "backing out" of the screw, especially on unsecured or poorly preloaded joints. The role that the coefficient of friction plays in this is explained in our article on the Friction and friction coefficient determination of friction values of materials.
• Settling processes and material flow: On thermal or mechanical stress, the contact surfaces deform slightly. This reduces the preload force and the screw is no longer tight.
• Fluctuating temperatures: Different expansion coefficients of screw and component material can lead to stress reduction and loosening, for example in systems that are cyclically heated and cooled.
• Missing or incorrect assembly: Insufficient torque, lack of screw locks, mounting of old or damaged fasteners, or uneven surfaces are classic mounting errors that can lead to the long-term loosening of a screw.

When do you need screw locks?

Threaded joints are essential in almost all industrial applications. They ensure that components remain securely and permanently joined to each other. However, threaded joints are subject to numerous loads during operation. These include vibrations, shock loads, thermal expansion and settling phenomena. But changing mechanical forces can also cause screws to loosen gradually.

Self-loosening of screws can have serious consequences. In the least serious case, it leads to increased maintenance and unexpected downtime. However, in safety-critical systems - such as when attaching robot grippers, support rollers or guards - a loosened screw can also lead to property damage, production downtime or endanger people.

The screw lock therefore has the primary task of maintaining the preload force of the joint, i.e. the force with which the screw clamps the components together. This is the only way to keep the joint reliable and operationally reliable. A suited screw lock not only protects the joint itself, but also increases the service life of components, reduces maintenance costs and thus makes a key contribution to system safety.

Objectives of Screw Locks

A wide variety of screw locks are used to permanently prevent the unintentional loosening of screws. However, their purpose goes beyond just gripping – they perform several key technical functions that are critical to the safety and reliability of the connections.

Screw lock to prevent self-loosening of screws

In continuous operation, threaded joints are often subject to dynamic forces: Vibrations, impacts, shocks, or changing loads. Particularly in applications such as robotics, material handling or rotating assemblies, the risk is high that a screw will loosen itself over time. An effective anti-rotation feature prevents this unintentional twisting of the screw, thus protecting the mechanical integrity of the joint.

Screw lock to maintain the preload force

The preload force is the clamping force generated by tightening the screw, which holds the joined parts together. As soon as a screw loosens, this preload force is lost. This leads to play in the joint, which in turn can lead to increased wear and potential joint failure. A suited screw lock ensures that the preload force remains constant over longer periods of time, for example by compensating for settling phenomena or blocking unwanted rotational movements.

Screw locks to prevent maintenance and retightening

In many industrial plants, regular manual retightening of screws is time-consuming and costly, especially in difficult-to-reach areas or under harsh environmental conditions. Effective screw locks reduce or eliminate this effort. By using permanent locking solutions, maintenance cycles can be extended, downtime reduced and service costs lowered.

Types of screw locks

Screw locks can be roughly classified into the following categories: friction-locking, interlocking, substance-to-substance locking joints.

Friction-locking screw locks

Friction-locking devices increase friction between the mated components to prevent the screw from loosening on its own. They act by preloading or additional clamping elements and are particularly common in dynamically loaded applications. Typical examples are:

• Spring washers
• Toothed lock washers
• Locking plates
• Clamping nuts
• Screws with prevailing-torque threaded section
• Use of two nuts (locknuts)

Interlocking screw locks

Interlocking devices mechanically block the rotational movement, for example by means of stops, claws, or other anti-rotation elements. They offer a very high degree of reliability against screws coming loose. Interlocking screw locks include, for example:

• Locking plates with circumferential edging
• Cotter pins through castle nuts
• Wire locking devices
• Serrations on screw head or bearing surface

Substance–to-substance locking screw lock

Substance–to-substance locking devices permanently connect the threaded parts by an adhesion or melting process. This method is particularly effective against vibrations and sealing problems, but usually requires a lot of effort during disassembly. Substance–to-substance locking devices include:

• Thread locking enamels
• other adhesives

Locknuts or double locknuts

This type of screw lock with two nuts (locknuts) is a proven mechanical method for preventing the self-loosening of a threaded joint. It is often used where robustness, releasability and purely mechanical screw locks are required. The nuts are secured by tightening (countering) two nuts against each other. This leads to a preload and an increase in friction on the threaded flanks of the screw and nut in the area of the two nuts. The friction between the contact surfaces of the two nuts caused by the preload prevents them from rotating with respect to each other, while the friction of the thread flanks prevents the nuts from rotating on the screw.

However, this method does not provide sufficient reliability against loosening the joint in all use cases. Use with vibrating joint may cause the lock to loosen. It must also be ensured that the screws have sufficient tension between them to prevent the unwanted loosening.

Securing with a locknut and securing with Hard Lock Nuts® or other double locknuts may appear similar at first. However, they are not directly comparable, since Hard Lock Nuts®, for example, in addition to the tension over the threaded flanks, also build up radial tension, which increases friction yet again.

When tightening the upper nut (1), the forces are automatically directed towards P₁. The horizontal forces increase during tightening until the upper and lower nuts (2) fit closely together as shown in Figure b. The nuts are properly secured by the wedging action. After tightening, the internal stress from the sum of the forces P₁, P₂ and P₃ is maintained against all external influences.

The screws or shafts must have thread precision JIS 6g (Class 2) for use with Hard Lock Nuts®. If the thread precision is different, the nuts may not fit properly. Although the outer diameter of the upper nut and the lower nut may become eccentric, or play may occur during assembly due to this structure, this will not affect operation.

Advantages of lock nuts

• In applications without heavy vibration or with low varying loads, the lock nut provides good and durable lock.
• Joints with lock nuts are purely mechanical and independent of temperature, chemicals, or aging and thus particularly robust and reliable.
• No special parts are required. Traditional nuts can be used. These are also available in high-strength versions with suitable hardening methods.
• The joint is releasable and reusable, as long as there is no damage to the thread.
• Lock nuts are well suited for maintenance-friendly designs with visibility or access to the nut.

Disadvantages of lock nuts

• The assembly is more elaborate than with simple nuts or locking elements, as two nuts must be tightened exactly against each other.
• More axial clearance is needed. This may limit the application in compact assemblies.
• In the event of incorrect sequence or insufficient torque, the locking effect may be completely absent and may result in an increased potential for error.
• This method is not vibration-proof in the sense of modern dynamic test standards (e.g. Junker test), unless additional measures are combined.

Locknuts or U-nuts

U-nuts or U-clip locking nuts are special fasteners that can be applied directly to components without a counter thread.

The integrated spring bracket or friction ring in the U-nut creates an additional friction force in the threaded area and provides a self-clamping effect. The friction ring exerts the compressive force P on the screw threads, causing the reaction force P’. This results in an additional frictional moment, preventing unwanted movement.

Advantages of U-nuts

• U-nuts can be quickly and easily installed, as they can be mounted on a component without special tools.
• The self-clamping construction with friction ring reliably prevents uncontrolled loosening, even during vibrations or dynamic loads.
• U-nuts allow for one-sided assembly, which makes work much easier, especially for hard-to-reach or thin-walled components.
• U-nuts are usually made of temperature-resistant metal materials and therefore function reliably even in high or low temperature environments.
• They are reusable, as long as they are not mechanically damaged, and thus provide an economical solution for many applications.
• The compact design reduces space requirements, especially when compared to classic nut-and-washer combinations.
• Misassembly is minimized because U-nuts can be positively positioned by their shape and clamping action.

Disadvantages of U-nuts

• U-nuts are primarily designed for thin-walled or edge-formed components, which limits their use to specific geometries.
• Under high axial forces or heavy tensile loads, U-nuts meet their load limits because they cannot transfer a high preload like classic threaded joints.
• Reusability is limited, as the clamping force or the friction ring may wear out with frequent installation.
• They do not provide an interlocking action, thus potentially requiring additional locking elements in extreme vibration or safety-critical applications.

Screw locking with adhesive

Screw lock with adhesive is a chemical method for preventing uncontrolled loosening of threaded joints. It is mainly used where vibrations, micro-motions, or leaks occur.

How does a screw lock with adhesive work?

The adhesive, usually an anaerobic thread locker enamel, is applied to the screw threads before the joint is closed. Once the screw is assembled and the adhesive is cut off from oxygen (anaerobic), curing begins through contact with the metal. The cured adhesive completely occupies the thread clearance, creating a friction lock and substance-to-substance locking joint, effectively preventing any relative movement between the screw and nut. There are different types of adhesive: from easily detachable for maintenance to high-strength for permanent connections.

An adhesive screw lock provides a highly effective and invisible locking solution, especially in dynamic and vibration-rich applications. It is particularly suited for permanent connections where the absence of maintenance is more important than dismantling.

Advantages of Screw Lock with Adhesive

• The adhesive is highly vibration- and shock-resistant, as the adhesive prevents any movement in the threads.
• Adhesive locking acts as a seal at the same time, which can be particularly beneficial in oil or pressure systems.
• The adhesive is distributed evenly in the thread and prevents corrosion by blocking moisture.
• No additional mechanical screw locks are necessary.
• Mounting is fast and easy, especially in mass production applications.

Disadvantages of screw lock with adhesive

• The joint is generally not releasable, or only releasable with difficulty, especially with high-strength adhesives. Heat exposure or special tools are then required for maintenance.
• The adhesives must cure. This can take several minutes to hours depending on the type. This delays assembly processes.
• A clean and grease-free surface is mandatory, as contamination affects adhesion. A LABS-free surface is therefore mandatory.
• The screw is often no longer reusable after loosening without intensive cleaning, as adhesive residue sticks to the thread. Both screw and internal thread must be thoroughly cleaned when reused.
• Adhesives are sensitive to temperature limits - excessive heat can cause a loss of the locking effect.

Other types of screw locks

In addition to nuts and screws with integrated anti-rotation and adhesives, there are a variety of additional mechanical locking elements that protect screws & nuts against uncontrolled loosening. Lock washers and locking pins are particularly common.

A locking pin is a metallic wire pin that is passed through a cross hole in a screw or shaft.

In combination with a castle nut whose slots allow access to the bore, the cotter pin is bent after insertion to permanently block the nut from unthreading. This type of lock provides positive locking protection against rotation as the nut cannot physically move without removing the cotter pin.

Spring washers are also part of the screw locks, but offer less protection against unintentional loosening than serrated tension washers or wedge lock washers.

Analyzing the effectiveness of screw locks

When selecting the correct screw lock, not only the question of whether a lock is present plays a role, but also how reliable it is in operation since many threaded joints lose their pre-load force over time. In order to objectively assess the actual effectiveness of screw locks, standardized test methods are required.

Various methods, from static torque measurements to real-world continuous oscillation tests, help to assess the locking effect under realistic conditions. Especially under dynamic loads, it is crucial how well a screw lock maintains the pre-load force over the long term. An internationally recognized method for testing under such conditions is the so-called Junker test.

Junker test

The Junker test is a standardized test method that measures the effectiveness of screw locks under lateral dynamic loading.

The Junker test involves mounting a threaded joint with a defined pre-load force in a test stand. A movement (vibration) then acts perpendicular to the screw axis while the pre-load force is continuously measured. The goal is to observe how much and how quickly this force decreases as a result of the movement.

The Junker test is considered to be the gold-standard for testing the vibration resistance of threaded joints. It places the screw under a realistic, dynamic load and objectively measures whether and when the joint loosens. The resulting axial force–vibration frequency diagram provides a clear representation of the loss of the measured axial force when the threaded joint is released by vibration.

It can be clearly seen that a screw secured with a Nord-Lock® wedge-lock washer loses some of the applied axial force at the beginning of the vibrations, but then retains it, while all others lose the axial force exerted by the thread.

For design engineers, the Junker test provides a reliable basis for comparing, evaluating, and specifically selecting different screw locks. It shows very clearly which systems perform under realistic conditions and which do not.