Self-loosening Of Bolted Joints Research Articles

Concept of radial slippage propagation triggering self-loosening and optimisation design of novel anti-loosening structures

Self-loosening of bolted joints can occur in a vibration environment, and it may induce bolt fatigue fracture with catastrophic consequences. It is essential to clarify the self-loosening mechanism, based on which novel anti-loosening thread structures can be developed. In this paper, we propose the concept of radial slippage propagation and provide new insights into the self-loosening process. The new theory states that the slippage along the radial direction of the thread surface induces more slippage areas (slippage propagation), and self-loosening occurs due to the dynamic evolution and propagation of contact states on the thread and bearing surfaces with an increase in the number of vibration cycles. Finite element analysis (FEA) was used to validate the propagation process of slippage areas on the thread surface. A novel bolted joint with step thread engagement was developed, which could prevent the occurrence of relative motion of the external and internal threads in the radial direction and thus block slippage propagation. A three-dimensional (3D) finite element model (FEM) of the novel thread structure was established, and a test specimen was manufactured using two special tools. FEA and experiments validated its superior anti-loosening and anti-fatigue performances, and the convenience of installation and removal. Experimental validation of the radial slippage propagation theory and the performance optimisation of the step-thread structure should be performed in the future.

Ultrasonic measurement of self-loosening in bolted joints

Self-loosening of bolted joints in response to vibration can lead to the catastrophic failure of a range of engineering components and structures. Many techniques employed to study this phenomenon focus on directly measuring the pre-load remaining in the bolt itself, and offer little insight into the behaviour of the clamped interface. In this study, a non-intrusive ultrasonic reflection-based technique is used to first characterize interface pressure in the joint, and then determine the rate at which relaxation occurs. A key advantage of the technique is that it does not require the modification of the contact conditions, and rather utilizes the spring-like behaviour of a rough surface interface when subject to ultrasonic excitation. A series of different bolt torques were investigated, along with the inclusion of both plain and spring washers under the bolt head. All test samples were subjected to an oscillating fixed–displacement vibration cycle. An initial rapid reduction in interface clamping pressure was observed, followed by a more steady-state period. Increasing bolt torque was seen to enhance joint integrity, whereas both the plain and spring washers showed little improvement. The spring washer was observed to extend the secondary steady-state phase of loosening, though as the majority of pre-load was removed prior to this period any change was largely unbeneficial.

2302 実機稼働下のねじ締結体ゆるみの絶対的評価に関する実験的研究 : 静的および動的荷重のかかるねじゆるみの評価(ねじの特性,一般講演)

Many papers have been published on the self loosening of bolted joints, making estimations of the locking effect from laboratory tests using a loosening test machine. However this kind of test method is only effective for the comparison of locking devices. In this paper we propose a method for estimating the absolute lock effect during actual machine operation ,that is construction machinery and material handling equipment. The authors found that there is a linear relation between axial tension and the time after the tightening on a log-log scale, in both static and dynamic loosening. Based on this relation, the authors propose a method for estimating the decrease of axial tension accurately after long hours of tightening by measuring the initial behavior as the bolt loosening evaluation chart.

Self loosening and strength estimation of bolted joint under machine operation. (1st Report, Absolote estimation on the self loosening).

Many papers have been published on the self loosening of bolted joints, making estimations of the lock effect from laboratory tests using a loosening test machine. However this kind of test method is only effective for the comparison of locking devices. In this paper we propose a method for estimating the absolute lock effect during actual machine operation. The authors found that there is a linear relation between axial tension and the time after the tightening on a log-log scale, in both static and dynamic loosening. Based on this relation, the authors proposed a method for estimating the decrease of axial tension accurately after long hours of tightening by measuring the initial behavior.

Study on self-loosening of bolted joints taking account of characteristics of impulsive friction.

In this paper, a method to make the self-loosening-limit diagram is described based on the theory of mechanism of the self-loosening. The method is presented taking account of the characteristics of impulsive friction on the interfaces of fastener estimated from the results of the experiment in which the coefficient of impulsive friction on the test pieces is measured. The diagrams obtained by the method agree with the experimental results. And after the examination and comparison of those diagrams, the following fundamental items are found effective to prevent the loosening: (1) the compressive force on the interfaces still remains, even when the tensile stress waves arised from the external impact come to the interface of fastened pieces. (2) the actual compressive stress on the pressure flank does not exceed the compressive yield point.