Knowledge: Self-loosening

Validating and optimising your bolted joint design delivers safety and performance improvements, alongside streamlining costs. There is now a powerful new vibration test methodology using controlled shear force loading. It is significantly more advanced than traditional – and flawed – displacement amplitude-driven Junker Testing. This new approach enables you to test your bolted joint for resistance to rotational self-loosening under real-world conditions.

Abstract. Vibration can be desirable, such as when it is used to make music. More often, however, vibration is undesirable. It can introduce stress into mechanical systems and create fatigue that decreases service life. It can also loosen fasteners. This article discusses the root causes of the self-loosening of fasteners, then two practical examples illustrate the benefits and possible applications of vibration testing of fasteners, according to either standardised testing protocols or customised testing strategies simulating real-life conditions of the assembled parts. The requirements and protocols of the international standard ISO 16130, the USA national aerospace standard NASM 1312-7, the German national standard DIN 25201-4 B and its predecessor DIN 65151 are compared, with insights on their most suitable applications. Finally, the article describes a lean methodology to design a meaningful testing protocol.

Abstract. Are the Junker Test and standards based on it, DIN 65151, DIN 25201 and ISO 16130 fundamentally flawed for validating the integrity of bolted joints?

Did you know that the number of published scientific papers related to fastener self-loosening has grown 800% since 2013? Data from ResearchGate, ScienceDirect and others, all indicate the rotational self-loosening of fasteners is of increasing interest, and possibly concern, to an ever-growing number of researchers around the world. Read more about trends in scientific papers […]

What happens when fasteners become loose and how to prevent it? Fasteners self-loosening is the result of a combination of causes. Materials relaxation and embedding reduce the preload in bolted joints, in turn allowing vibration to initiate repeated transverse movement in the joint. Fasteners will come loose and fail, leading to equipment failures and dramatic […]

Fasteners in bolted joints subjected to transverse loading conditions will loosen, unless the correct torque, a locking element or both are applied. Understanding the self-loosening behavior of fasteners under vibration conditions is an essential element of the fastener and locking element product design process undertaken by fastener manufacturers. Downstream fastener users such as original equipment manufacturers (OEMs) require an understanding of how bolted joints will perform during actual service, which may include transverse loading conditions. Here Vibrationmaster CEO Morten Schiff uses an example to provide a step-by-step explanation and demonstration of how to perform a vibration test.

When a fastener or bolted joint fails, the results can be catastrophic and also sometimes tragic. The impact of the failure might be an isolated component malfunction, or an aircraft dropping from the sky. Whatever the impact, any fastener or bolted joint failure puts the pressure on the fastener manufacturer, the original equipment manufacturer (OEM), the operator and their insurers to determine the root cause and, ultimately, liability. Underpinning this process is the work of forensic engineers who apply fastener and bolted joint tests to find out what went wrong, so a solution can be developed to ensure it never happens again. Vibrationmaster’s Morten Schiff and Bolt Science’s Bill Eccles explain how forensic fastener and bolted joint testing methodologies can be used to determine failure modes.

No-one used to ask for the test reports on fasteners that had been vibration tested to determine their self-loosening behavior. They were simply never required. Vibrationmaster CEO Morten Schiff explains the new reporting requirements according to the DIN 25201, and how this new format can be used globally.

Testing plays a key role when developing new fastener and locking elements in response to customer requirements. The choice of testing enables fastener manufacturers’ Development Engineers to confirm that the new product performs according to specification. Downstream fastener consumers are constantly searching for innovative solutions to prevent self-loosening, a phenomenon first empirically investigated by Dr. Gerhard Junker in the 1960s. The result was the Junker test to determine self-loosening behavior, which has played a key role in the development of Unbrako’s new Durlok locking wedge washer.

The number of wind turbine-related accidents globally reached a record high of 160 in 2011, including 11 fatalities. Turbine failure can also have a serious impact on energy supply security. Studies into wind turbine reliability show that vibration fatigue is the third most common reason for wind turbine failure. When analysing the root causes of failures, the suitability of design methods and component standards has been called into question. Introducing standards such as DIN 25201 may prevent vibration-induced fastener self-loosening, thus cutting turbine failure rates and resulting in reduced accident numbers and fatalities, increased energy security and lower operation and maintenance costs.