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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.
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:
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.
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.
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.
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.
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.
Screw locks can be roughly classified into the following categories: friction-locking, interlocking, substance-to-substance locking joints.
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 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 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
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.
• 1 - Upper nut
• 2 - Lower nut
• P1, P2 - Horizontal forces
• P3 - Preload force
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.
U-nuts or U-clip locking nuts are special fasteners that can be applied directly to components without a counter thread.
• Friction ring (1)
• acting compressive force P
• Reaction force P’
U-Nuts® is a registered trademark of Fuji Seimitsu Co.
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.
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.
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.
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.
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.
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.
