Bolt-nut Connections Research Articles

Numerical identification of position-dependent friction coefficients from measured displacement data in a bolt-nut connection

Friction is a complex system affected by microscopical effects and multidisciplinary phenomena. Coulomb's simple friction model with a constant friction coefficient cannot account for all these tribological effects. Nevertheless, this model is still widely utilised for calculations of mechanical applications. In order to reflect the importance of friction as a parameter for functionality, we need more realistic and sophisticated calculations. This is particularly relevant for bolt-nut connections, which serve as motivating example for our study. Our approach is to introduce position-dependent friction coefficients by dividing the contact surface into different friction areas, each characterised by a constant friction coefficient. These coefficients are then adapted to measured displacement data. To this end, we develop a numerical parameter identification tool. The tool combines calculations in Ansys Mechanical, an established Finite Element software, and Microsoft's Visual Basics for Applications for optimisation purposes. We verify the parameter identification tool using the simple model of a block on a planar surface. Within this test scenario, the algorithm converges and provides a good approximation of the friction coefficients. Subsequently, we apply parameter identification to the model of a bolt-nut connection. We perform optical measurements to acquire experimental displacement data. The parameter identification tool demonstrates its functionality. Finally, we discuss future modifications of the procedure, that will enable more realistic and reliable results.

Anti-loosening mechanism of pitch difference bolt nut connections based on Junker loosening test

Bolt nut connections are essential elements and widely used in industry. The authors have verified that introducing a slight pitch difference between the bolt and the nut may improve the anti-loosening performance and fatigue life of the bolt nut connections. Besides, the effect of residual torque of pitch difference bolt-nut connections on the loosening resistance performance was discussed. In this study, the Junker loosening test was investigated experimentally and analytically to confirm the anti-loosening performance of pitch difference bolt nut connections directly. The FEA results are in good agreement with the experiments. It is confirmed that, for pitch difference bolt nut connections, the threads contact status is different from the common bolt nut connections, and the torque between the thread contact surface friction is the main reason of anti-loosening performance improvement of the pitch difference bolt nut connections. In addition, the results showed that if the pitch difference is large enough, the nut loosening angle equals the tightening angle for each vibration, and nut loosening does not occur. If the pitch difference is relatively small but just large enough to provide a prevailing torque, with increasing of the vibrations number of the nut, the loosening speed decreases gradually and finally the clamping force remains steady, which means the bolt nut connections retain anti-loosening performance to some extent.

Investigation of loosening resistance based on Junker loosening test of bolt nut connections with pitch differences

Investigation of loosening resistance based on Junker loosening test of bolt nut connections with pitch differences

Consideration of optimum pitch difference for different diameters of bolt nut connections with pitch difference

The pitch difference nut can be used in automobiles and other industries to improve anti-loosening and fatigue strength with low cost. In this paper, based on the suitable pitch difference α experimentally obtained previously for JIS M12 and M16, the suitable pitch difference is discussed for other bolt nominal diameters. Here, the overlap thread length is calculated from the suitable α obtained for M12. Then, the same value of is assumed to estimate the suitable α for other diameters. The validity of this theory is confirmed through calculating the prevailing torque for other diameter by applying FEM analysis.

How to improve both anti-loosening performance and fatigue strength of bolt nut connections economically

In this paper, how to improve both anti-loosening and fatigue strength with low cost is newly proposed for bolt-nut connection. The authors’ previous study clarified that the most suitable pitch difference α=α1 for the fatigue strength is smaller than the most suitable pitch difference α=α2 for the anti-loosening. In this paper, toward improving both fatigue strength and anti-loosening, a larger nut height coupled with pitch difference is studied through experiment and FEM analysis. It is found that with increasing the nut height, the suitable pitch difference for the anti-loosening α=α3 decreases without changing the suitable pitch difference for the fatigue strength. Therefore, a larger nut height with the pitch difference α=α3≈α1<α2is suitable for improving both fatigue strength and anti-loosening.

Effects of root radius and pitch difference on fatigue strength and anti-loosening performance for high strength bolt–nut connections

The fatigue strength improvement and anti-loosening performance are studied experimentally and analytically for JIS M16 bolt–nut connections. Three different root radii are considered coupled with three different pitch differences. The enlarging the bolt root radius improves the fatigue limit of the bolt by more than 30% because both stress amplitude and mean stress can be reduced. Furthermore, suitable pitch difference improves the fatigue limit by more than 25%. This is because under no pitch difference the crack initiation always occurs at No.1 or No.2 threads close to the bolt head causing the final failure; however, under a suitable pitch difference the crack initiation occurs at No.6 or No.7 threads far away from the bolt head. Good anti-loosening performance can be expected for the bolt–nut connections having enlarged root radius because the prevailing torque [Formula: see text] = 19 Nm and the residual prevailing torque [Formula: see text] are not smaller compared to other special bolt–nut connections.

Prevailing torque and residual prevailing torque of Bolt-Nut connections having slight pitch difference

In this study, by applying three-dimensional FEM, the nut screwing process is analyzed to obtain the prevailing torque Tp confirming anti-loosening. Since the nut unscrewing properties after the nut tightening is more important to prevent the nut self-loosening, the nut unscrewing is also analyzed to obtain the residual prevailing torque Tp u newly defined in this process. Both results of Tp and Tp u are in good agreement with the experimental results. With increasing the pitch difference, the residual prevailing torque Tp u increases as well as the prevailing torque Tp .

Three-dimensional finite element analysis for prevailing torque of bolt-nut connection having slight pitch difference

In a vast industrial field, the bolt-nut connection is widely used and unitized as an important machine component. In order to improve safety and for cost reduction, cheap bolt-nut connections with excellent anti-loosening performance are always needed. In this paper, the slight pitch difference is considered between the nut and bolt threads to improve the anti-loosening by focusing on the prevailing torque Tp. In the first place, during the nut screwing process, the bolt axial force Fa caused by the pitch difference is examined because previously Fa was studied by the axisymmetric FEM simulation as a substitute of the prevailing torque Tp. Next, the prevailing torque Tp is directly analyzed by changing the pitch difference through the 3D FEM simulation. The obtained prevailing torque needed for the nut rotation is in consonance with the measured value experimentally. By considering manufacturing error sometimes included, finally, the real pitch difference or of the nut is identified from the number of nut thread in contact with the bolt thread, which is measured experimentally. Then, the correctness of this α is confirmed through the prevailing torque obtained by 3D FEM simulation.

An Experimental Research on the Effect of Connection Types on Frame using Cold-Formed Steel

The growth of construction technology, innovations using cold-formed steel materials have begun to be developed in ordinary earthquake resistant buildings. In addition, those materials lighter and cheaper instead use conventional steel material. In building construction, the weakest part of the structure is the connections. Many cases of building failures occur due to collapse in the connection of structures. In this research an experimental study was carried out on frame structure using cold-formed steel with variations in the connection of beam-column given lateral loads as a representation of earthquake loads to determine the type of effective connection on the structural performance. Specimens in this study are one bay two stories frame with variations used are the type of connection of bolt-nut and self-drilling screw. Loading test is performed by giving lateral load at the upper end of one of the frame sides with a four steps cyclic load. From experimental test results found that in the push cycle, frame with bolt-nut connection has greater lateral load than model with self-drilling screw connection. In contrast to the pull cycle of model with bolt-nut connection has smaller lateral load compared to model with self-drilling screw connection. Frames with bolt-nut connections have better ability to dissipate energy compared to models with self-drilling screw connections. Compared to initial stiffness, the model with bolt-nut connection has greater decrease than model of self-drilling screw connection. However, in term of stiffness behavior, model with bolt-nut connection is more rigid compared to the model with self-drilling screw connection.

Identification of real pitch difference in bolt-nut connection to improve anti-loosening performance and fatigue strength

In a wide industrial field, the bolt-nut joint is unitized as an important machine element and widely used. To ensure the safety of the structure and reduce cost, low-cost bolt-nut connections with good anti-loosening performance and high fatigue strength are always required. In our previous papers, the effect of pitch difference between a bolt and nut was studied. Under a small pitch difference, anti-loosening performance is not enough. Under a large pitch difference, fatigue strength is not good. Therefore, in order to improve both anti-fatigue and anti-loose performance, the choice of suitable pitch difference is very important although the manufacturing errors always appear in bolt-nut connection. Until now, the result of experiment and analysis without manufacturing error but is not match. Since even small manufacturing errors may affect anti-fatigue and anti-loose performance, in this paper how identification of real pitch difference is discussed after the pitch difference nut is produced. Also, the accuracy of the identified pitch difference is confirmed. By using the confirmed error, the machining accuracy may be improved to ensure the nut has good performance.

New Bolt Tightening Method with High Accuracy Utilizing Real-time Measurement of Nut Factor

Torque control method is widely used when tightening bolt-nut connections owing to its easy operation. However, it usually involves a fair amount of scatter in bolt preloads. In contrast, elastic angle control method can provide bolt preloads with higher accuracy, though its tightening operation requires longer working hours and higher costs. In this study, a new tightening principle is proposed by combining the two methods, which achieves greater accuracy and ease of operation simultaneously. The proposed method can precisely estimate the critical value of “nut factor” in the course of tightening operations, thereby modifying the objective torque into the proper one enabling the attainment of the target bolt preload. Tightening equipment, which was manufactured for trial based on the proposed principle, successfully tightened the target bolts with scatter staying within 5% of the objective bolt preload.

Effect of pitch difference between the bolt–nut connections upon the anti-loosening performance and fatigue life

In this paper, the effect of a slight pitch difference between a bolt and nut is studied. Firstly, by varying the pitch difference, the prevailing torque required for the nut rotation, before the nut touches the clamped body, is measured experimentally. Secondly, the tightening torque is determined as a function of the axial force of the bolt after the nut touches the clamped body. The results show that a large value of pitch difference may provide large prevailing torque that causes an anti-loosening effect although a very large pitch difference may deteriorate the bolt axial force under a certain tightening torque. Thirdly, a suitable pitch difference is determined taking into account the anti-loosening and clamping abilities. Fourthly, fatigue experiments are conducted using three different values of pitch difference for various stress amplitudes. It is found that the fatigue life could be extended when a suitable pitch difference is considered Furthermore, the chamfered corners at nut thread ends are considered, and it is found that the finite element analysis with considering the chamfered nut threads has a good agreement with the experimental observation. Finally, the most desirable pitch difference required for improving both anti-loosening and fatigue life is proposed.

Fatigue Failure Analysis for Bolt-Nut Connections having Slight Pitch

Fatigue Failure Analysis for Bolt-Nut Connections having Slight Pitch

Accurate Bolt Tightening Using Model-Free Fuzzy Control for Wind Turbine Hub Bearing Assembly

In the modern wind turbine industry, one of the core processes is the assembly of the bolt-nut connections of the hub, which requires tightening bolts and nuts to obtain well-distributed clamping force all over the hub. This force deals with nonlinear uncertainties due to the mechanical properties and it depends on the final torque and relative angular position of the bolt/nut connection. This paper handles the control problem of automated bolt tightening processes. To develop a controller, the process is divided into four stages, according to the mechanical characteristics of the bolt/nut connection: a Fuzzy Logic Controller (FLC) with expert knowledge of tightening process and error detection capability is proposed. For each one of the four stages, an individual FLC is designed to address the highly non-linearity of the system and the error scenarios related to that stage, to promptly prevent and avoid mechanical damage. The FLC is implemented and real time executed on an industrial PC and finally validated. Experimental results show the performance of the controller to reach precise torque and angle levels as well as desired clamping force. The capability of error detection is also validated.

Improvement of performance of bolt–nut connections, Part II: Experimental investigation

In this work, bolt and nut geometries are modified for the improvement of fatigue life of the bolt–nut connections. The modifications are aimed at reducing the stress concentration at the thread roots by factors such as reducing the shank diameter, making axial hole in bolt, stepping the nut and their combinations. The effect of modifications is studied by experiment and simulation. The experimental results show that all types of modifications considered result in an improvement in the fatigue resistance of bolt–nut connections but to different extents. However, the effects of the two latter types are more significant. The most improvement is achieved for the case when all types of modifications are combined together. In this case, fatigue life can be increased by almost 100%. Numerical results also show that all types of modifications lead to the reduction of stress concentration.

Improvement of performance of bolt-nut connections, Part I: Simulations

The performance of bolt-nut connections can be improved by enhancing fatigue life of the connections. This can be accomplished by reducing the stress concentration in the threads of the connection. This investigation consists of two parts. In this part (part I), load distribution in threads of some ISO bolts is computed by three-dimensional numerical simulation and Stockley-proposed relations. The results show a close agreement between Stockley relations and the simulations for nearly all bolt sizes. The results indicate that stress concentration is nearly constant regardless of the bolt size. It is also found that the load percentage carried by the first thread varies from 35% for M6 and reaches to 58% for M20 and M30 ISO bolts. The results suggest that the rate of load distribution changes at a point of inflection, i.e. the rate after the inflection point diminishes as the bolt size decreases, whereas before this point, the trend of the rate is reversed. In part II (to be submitted separately), various techniques are employed for the reduction of stress concentration and enhancement of fatigue life of the connections. The techniques are evaluated by numerical simulations and fatigue tests.

Experimental results and finite‐element predictions of the effect of nut geometry, washer and Teflon tape on the fatigue life of bolts

ABSTRACTThe effect of nut geometry, curved spring washer and a sealing material (Teflon tape) on the fatigue life of M12 and M16 ISO bolts was investigated. This was accompanied by the study of the axial and bending stress distribution in threads by numerical simulation of bolt and nut connections using the finite‐element method. The experimental results showed that the highest fatigue life is achieved for a slotted tapered nut. The presence of a spring washer also increases the fatigue life, providing that the correct tightening torque is applied to produce a pretension in the bolt. The use of Teflon tape as a filling material between engaging threads of the bolt and nut is shown to significantly increase the fatigue life. On the whole, the highest fatigue life is obtained for a slotted tapered nut using washer. Four fracture mechanisms were observed for bolt–nut connections during the fatigue tests.

Fatigue strength of machined screw thread. Effects of loading modes in fatigue test.

The effects of various kinds of factors on the fatigue strength of machined screw thread connection generally used as the fastener of machine components are examined. Effects of materials (i. e. heat-treatment), fastening force, yielding pre-tension or pre-compression before the oscillating loading test and lubrication between the connecting surfaces are chosen as the factors. Fatigue tests are performed for both types of screw thread, and bolt-nut connections made of normalized or tempered S 45 C, and obtained results are investigated in detail. Consequently, the effects on the fatigue strength are clarified. Furthermore, the strain hardening caused at each step of the machining and loading process is discussed.