Pre-tightening Torque Research Articles

Investigation into fatigue failure of cable brackets and vibration suppression methods

The cable bracket is one of the most important components of high-speed EMUs, as it supports the cables of sensors. The fatigue failure of the bracket can severely affect the operation of the vehicle. In this study, the dynamic response of the cable bracket, including dynamic stress and acceleration, is obtained through field tests. The results show that the modal vibration of the bracket excited by high-frequency excitation in the wheel–rail system is an important cause of fatigue failure of the bracket. Rail welded joints in the track and polygonal wear of the wheelset further exacerbate the vibration and of the bracket. A model of the virtual vibration table is proposed to simulate the actual excitation experienced by the bracket. The effects of the cantilever beam length, steel plate thickness, and pre-tightening torque of the bolts on the dynamic stress of the bracket are studied using the virtual vibration table. Several vibration suppression methods are proposed. The results indicate that the dynamic stress of the bracket caused by the rail welded joints can be effectively reduced through the adjustment of the pre-tightening torque of the bolts or the appropriate reduction of the cantilever beam length. The findings of this study can serve as a reference for suppressing vibration and preventing fatigue failure of the bracket.

Experimental and numerical investigations on tensile failure mechanism and load-bearing capacity of expansion anchors under pull-through failure mode in HPSFRC

Pull-through(PT) is a unique failure mode of torque-controlled expansion(TCE) anchor after being pulled out from concrete. No universal design formula is available for predicting tensile bearing capacity of anchor of PT mode. In normal concrete(NC), PT failure usually occurs at large embedment depths(Hef), which is uncommon in the field and received less attention than the concrete breakout failure. However, due to the improved mechanical performance of the substrate, the PT mode dominates for anchors in high-performance steel fiber reinforced concrete(HPSFRC) substrate (even at small Hef). In this study, to study the PT mode in HPSFRC, the results of 34 PT failures from previous tests, including testing scenarios with different anchor diameters(D), Hef, and fiber contents(vf), are analyzed. The cracking morphology and load-bearing capacity distribution of PT mode are elaborated. Further, finite element models(FEM) of anchor pullout behavior in HPSFRC is established, which accurately identified different failure modes. The calculated Nu by FEM agree well with the tested Nu. The difference in damage domain between different failure modes is mainly reflected in the unloading stage. Parametric analyses were conducted on pre-tightening torque(Tini), Hef, D, sleeve length(Lsleeve), anchor cone width, anchor cone length(Lcone), and vf to study their influences on Nu of PT mode. Except for Lsleeve and Lcone, which do not affect Nu, changes in other factors can cause linear or nonlinear changes in Nu. The non-linear relationship between Nu and experimental parameters is formulated. Accordingly, a new prediction model that can accurately predict Nu of PT mode for anchors in HPSFRC is developed, of which calibration factors are derived using probability functions.

Investigations on the modal vibration caused by bolted joint interface contact in the rotor-AMBs systems: Modelling and experimentation

Rotor-active magnetic bearings (rotor-AMBs) systems nowadays have been widely used in fluid machinery and the impeller and the shaft are commonly connected by bolted joint. However, when a certain pre-tightening force is applied to the bolted joint and the interface contact between the shaft and the impeller is formed, causing flexible modal vibrations when rotor is levitated. The mechanism of this modal vibration needs to be revealed to ensure stable operation of the machine. In traditional modelling, the effect of the interface contact is equated to an additional stiffness matrix by massless spring units, whose certain contact stiffness is uniformly distributed over the interface. Particularly, the calculation of contact stiffness and the effect of AMBs are neglected, resulting in the inaccurate response compared to experiments. Based on traditional modelling, we consider that there is partial separation in the contact interface due to the AMBs supporting and a novel additional stiffness matrix related to contact status is proposed by calculating the real-time contact area. The contact stiffness is calculated by microscopic contact model based on fractal theory and surface topography measurement. Finally, numerical simulation shows that the interface contact influences the system robustness and the unstable mode is excited. The increase of pre-tightening torque and contact radius both will increase the steady vibration amplitude, with the former being the major contributor. Moreover, the influence and order of the vibration are quantitatively validated by the experiments.

Mechanism of bolt breakage in deep mining roadway under dynamic load and advanced strengthening support technology

To reveal the problem of local instability caused by bolt breakage and failure in deep mining roadway under impact dynamic loads, and explore strengthening support technologies that can reduce bolt breakage and improve the stability of roadway support bodies. This article adopts theoretical analysis, numerical simulation, and on-site industrial experiments to conduct research, investigate and obtain the fracture failure characteristics of bolt in deep mining roadway, quantitatively characterize the deformation pressure of roadway surrounding rock and dynamic load of overburden fracture, establish the mechanical criterion of bolt fracture, and analyze the influence law of different anchoring factors on bolt force under static load and dynamic and static load combination. The results show that when the anchor angle is different, the range under dynamic load increases by 169.3 % compared with static load, when the anchor length is different, the range increases by 863.3 % relatively, and the influence is significant; while the anchor support resistance increases by 11.6 % at different times, and the range decreases by 41.5 % at different pre-tightening torque. Then, based on this criterion, the influence law of bolt failure on the stability of surrounding rock is analyzed, and the optimal support scheme is obtained by simulation research, that is, bolt cable +U steel support scheme. After adopting this scheme for support, the deformation of surrounding rock is reduced by about 60 %, and the maximum concentrated stress of surrounding rock of roadway is reduced by 15.0 %. Based on this, the support parameters of the on-site roadway were optimized and a strengthening support scheme was determined, which includes “roof bolt surface support reinforcement + anchor cable combined with U-shaped steel deep reinforcement, and two strong bolts advanced reinforcement”. The on-site strengthening support practice showed that the bolt fracture rate was reduced by 87.3 %, the deformation of the surrounding rock was effectively controlled, and the stability was significantly improved.

Analysis of the Support Failure Mechanism Caused by Bolt Pre-Tightening Force Loss

The pre-tightening force loss (PTFL) of bolts is an important but underestimated cause of roadway instability. In mine anchorage systems, the actual pre-tightening force of bolts is only 50% to 80% of the design value. Through a case study at Xiahuo Coal Mine, it was found that the essential causes of PTFL are the increasing friction coefficient between supporting units controlled by factors such as pre-tightening torque levels, pre-tightening cycles, and surrounding rock roughness. This study investigates the behavioral characteristics of PTFL and its influence on surrounding rock failure in roadways. This research reveals a linear correlation between pre-tightening force and torque, with an average torque conversion coefficient of approximately 0.19. However, the PTFL increases with higher levels of pre-tightening torque, increasing pre-tightening cycles, and rougher surrounding rock conditions. For every 30 N·m increase in pre-tightening torque, the PTFL increases by approximately 1.67 kN. Reducing the PTFL can expedite the stress redistribution process and shorten the self-stabilizing period of the roadway by approximately 10 days, thereby enhancing the active supporting potential of bolts.

Investigation of the effect of pre-tightening forces on bolted connection for FRP-steel joints

This paper experimentally and numerically investigates the effect of pre-tightening forces on bolted connection for double-lap FRP-steel joints, considering varied levels of pre-tightening forces. In the experimental phase of this study, six bolted double-lap pull-pull specimens with varying pre-tightening degrees (preloaded of 0, 3, 6, 7, 8, and 9 N·m, respectively) were examined. Test results indicated that all specimens exhibited bearing failure of FRP plates and the load capacity was significantly influenced by the pre-tightening forces applied through bolts. The highest loading capacity (27.1 kN) was recorded for the specimen with a preloaded of 6 N·m. In order to study the stress distribution of FRP plate for the bolted specimens, three-dimensional finite element (FE) models were conducted. Based on the FE analyzing results, the pre-tightening coefficient Pc (the coefficient of the preload increment per pre-tightening torque of bolts) of 750 m−1 was concluded and further proved by the analysis of the stress in contact zones for specimens preloaded by 0, 3, and 6 N·m.

A new vibratory response-based method for determining the optimal assembly tightening torque of the high-frequency piezoelectric ultrasonic transducer

Pre-tightening torque has a significant impact on the performance of a bolt-clamped high-frequency piezoelectric ultrasonic transducer widely used in wire bonding. To develop a vibratory response-based method for the determination of optimal pre-tightening torque, the vibration amplitude signals of the transducer under different excitation conditions were monitored, and the characteristic parameters of the response, involving the resonance frequency, settling time within the initial transient response, and vibration amplitude of the steady-state stage, were extracted. Based on a series of in-depth tests, analyses, and discussions, the influences that the pre-tightening torque has on the vibratory response properties were investigated. Then, the experimental measurement and the random forest method were combined to determine the optimal pre-tightening torque. The performance test results show that the developed method can give the optimal pre-tightening torque according to the desired vibrational response properties, and it is feasible, effective, and reliable as well as improves the adaptability and flexibility of the high-frequency ultrasonic transducer in the application.

An experimental study of FRP truss side plate joint

Ultra-high performance concrete (UHPC) slab - fiber reinforced polymer (FRP) truss hybrid beam has been proved as an effective and advanced bridge system. However, the limited shear strength and the significantly lower transverse properties compared with its longitudinal counterparts result in low rigidity and load carrying capacity of bolted FRP truss joints, which governs the overall performance of the UHPC slab- FRP truss hybrid beam. A FRP truss side plate joint was proposed which integrates a steel U-shaped gusset plate (SUGP) to reinforce the traditional bolted joint. The proposed joint outperformed the conventional bolted joints in terms of the failure mode, load carrying capacity, and stiffness. The load carrying capacity of the proposed joint can be improved by up to 63% and the ultimate deflection was reduced by up to 33%. The effects of a variety of key design parameters, including adhesive, bolt diameter, pre-torque, and length and thickness of the gusset plate, on the performance of the SUGP joint were evaluated. Test results showed that: (i) the adhesive between the pultruded FRP profiles and the steel gusset plate could increase both the initial stiffness and ultimate load carrying capacity of the steel SUGP joint; (ii) length and thickness of the steel gusset plate did not result in notable difference in the behavior of the steel SUGP joint for the range of parameters investigated herein; and (iii) application of pre-tightening torque within the thread of the bolts slightly improved the load carrying capacity but significantly improved the ductility of the steel SUGP joint.

Preload measurement of steel-to-timber bolted joint using piezoceramic-based electromechanical impedance method

Steel-to-timber bolted connections play an essential role in enabling glulam column and beam to work together. However, these connections are prone to bolt looseness due to shrinkage and creep effects of timber material during a long service life, which may cause consequent connection failures and structural safety hazards. Recently, detection and evaluation of bolt looseness have gained considerable attention, whereas the existing vision-based method is challenging to inspect early bolt looseness, and the percussion-enabled approach is susceptible to being affected by harsh environmental noises. Thus, in this study, the authors proposed a pre-tightening torque measurement method for steel-to-timber bolted connection based on variations of electromechanical impedance (EMI) of designed Lead Zirconate Titanate (PZT) sensors. A theoretical formula is derived first based on the piezoelectric equation and boundary conditions to illustrate the feasibility of using EMI variations to reflect looseness levels. Moreover, a sensor that can be easily integrated into the steel-to-timber connection and work as a washer is designed and fabricated. Consequently, the environmental performance against temperature and humidity of designed sensors is investigated by preliminary tests. Based on preliminary tests results, a frequency range that showed lower environmental sensitivity is selected as the excitation signal in the following bolt looseness measuring experimental studies. Experimental results well validate the effectiveness of EMI method in the monitoring of pre-tightening torque of steel-to-timber bolted connections, showing a great potential to inspect preload looseness of glulam structures in field applications.

Modeling and Experimental Research of One Kind of New Planar Vortex Actuator Based on Shape Memory Alloy

In order to alleviate the problems of complex structure and low reliability of traditional Shape Memory Alloy (SMA) rotary actuator, a planar vortex actuator (PVA) based on SMA material was proposed to directly output torque and angular displacement. Based on the calculation method of PVA and the constitutive model of the phase transition equation of SMA, the mechanical model is established, and the pre-tightening torque, temperature, output torque, and rotation angle are obtained. The relationship expression between the tests has verified the mechanical model. The results show that the relationship between the excitation temperature and the output torque, the coefficient of determination between the calculated value and the tested value, is 0.938, the minimum error is 0.46%, and the maximum error is 49.8%. In the relationship between angular displacement and torque, the coefficient of determination between the calculated value and the test value is 0.939, the maximum error is 58.5%, and the minimum error is 28.0%. The test results show that the calculated values of mechanical model and experimental data have similar representation form.

Identification of Pre-Tightening Torque Dependent Parameters for Empirical Modeling of Bolted Joints

The vibration characteristics of bolted structures are crucially affected by the pre-tightening torque. An approach for identifying the pre-tightening torque dependent stiffness parameters of bolted joints is proposed in this paper. Firstly, the interface of the bolted joint is characterized by the thin layer element with the isotropic material property, and the parameter value of the property is assigned relative to the distance from the center of the bolt; the influence of the bolt is ignored. Secondly, the model updating method is adopted to identify the parameters of thin layer elements using experimental data, and modal data under different values of pre-tightening torque in the range of 2 N·m~22 N·m are obtained; the torque wrench is used to determine the pre-tightening torque in the modal test. Finally, after identifying the material parameters using partial experimental data on pre-tightening torque range, the empirical equation of the interface parameters with the pre-tightening torque parameter is obtained by curve fitting and the rest of the experimental data are used to verify the accuracy of the fitted empirical equations. It is concluded that this method can obtain all the parameters of the equivalent thin layer elements within a certain range of pre-tightening torque, which can provide a reference for the empirical modeling of bolted structures, improve modeling efficiency and reflect the characteristic performance of real structural dynamics.

Study on the Load Conversion of Bolt-Nut Fasteners between Tightening and Service Conditions

At present, the common method to realize the pre-tightening effect of threaded fasteners is by applying an accurate tightening torque on them. However, when removing the tightening torque, the load characteristics will have a transient conversion, so that the pre-tightening loads will be significantly different from the tightening condition. For the aim of understanding the pre-tightening effect under the service condition, it is important to model the load conversion between tightening and service conditions. Based on the most basic mechanical equilibrium analysis, the load characteristics of bolt-nut fasteners under a round of the tightening and service conditions were studied. The torque equilibrium equations were built respectively, the load conversion between two conditions was analyzed and an approximate relationship between the applied tightening torque under the tightening condition and the pre-tightening torque under the service condition was obtained. In the meanwhile, a dynamic strain measuring system was proposed to accurately determine the pre-tightening loads in real time. Results verified that the thread torque and the bearing friction torque balance each other and act as pre-tightening torques under the service condition. The final pre-tightening loads can be roughly estimated to be larger than half of the difference in tightening torques by wrenches, but smaller than half of the sum of tightening torques, or can be accurately measured through dynamic strain measurements. • We study on bolt-nut fasteners , load characteristics under tightening and service conditions. • We model the relationship between the pre-tightening torque and the applied tightening torque. • We verify that the pre-tightening torque is apparently determined by the friction on the contacting interface.

Finite Element Analysis of Misalignment and Contact State on the Interface of Bearing Bore Under Preload Condition

The split bearing bores of diesel engine are generally bored after tightening bolts to install crankcase and cylinder block. Machining accuracy significantly affects the contact state during sliding bearing operation, thereby reducing the service life of bearing. For effectively suppressing misalignment of interface caused by deformation of bearing bore, this paper presents finite element model and experimental measurement. The theoretical analysis and numerical calculation of interface are carried out in this paper. The finite element models of bearing bore and interface between crankcase and cylinder block are established to quantitatively analyze the deformation characteristic and contact state. The results indicate that the contact pressure of interface increases gradually with increase of the pre-tightening torque. The sliding distance of interface is positively correlated with the contact pressure of interface. Furthermore, the direction and value of the principal stress are calculated by measuring the strain of nodes around the bearing bore. Differential forces on both sides of bearing bore lead to joint surface sliding. Finite element simulation results are compared with experimental results to verify the finite element model.

Nonlinear dynamic properties of the rotor-bearing system involving bolted disk-disk joint

Bolted joints are major components to connect the multiple stages of disks in the aero-engine, which directly influences the motion state of the rotor system. This paper studies the effect of some typical structure parameters on the time-varying bending stiffness of the bolted disk-disk joint through finite-element simulations. Based on the Lagrange’s equations, a two-node element used to represent the bolted joint is derived, which is called the joint element. Then, a mathematical model of the rotor system with a bolted disk-disk joint supported by ball bearings is established through the Timoshenko beam and the joint element. The dynamic model is numerically solved using the Newmark-β method. The largest Lyapunov exponent, three-dimension spectral plots, and frequency-response curves are employed to reveal the effect of the bending behavior on the rotor dynamics. Comparisons indicate that the structural parameter of the bolted joint has a slight influence on motion stability, critical speed, and amplitude corresponding to the critical speed. Finally, an experimental study is conducted through a bolted-disk joint rotor test rig with an electrical tightening wrench, showing that the increase of pre-tightening torque will lead to a decrease of the amplitude corresponding to critical speed due to the hardening effect. The modeling method proposed in the present work paves a way for modeling and analyzing of the rotor system with a bolted disk-disk joint.

Model-Based Analysis and Regulating Approach of Air-Coupled Transducers with Spurious Resonance.

As an essential characteristic of air-coupled transducers, electrical impedance can provide valuable information for quality control during manufacturing of transducers. It is also found feasible to directly read the optimal operating frequency from the impedance plots when the resonance is independent of the others. However, the spurious resonance emerges when two neighboring resonances are closely spaced, resulting in distorted impedance and ambiguous optimal operating frequency. In this paper, the electrical impedance of air-coupled transducers with spurious resonance is modeled using the Butterworth–Van Dyke (BVD) equivalent circuit. Then model-based sensitivity analysis is performed to evaluate the mutual interference between adjacent resonances. Based on the analysis results, the prestress method is proposed to regulate and suppress the spurious resonance by adjusting the equivalent parameters of the BVD model. Experimental study was carried out on the response of the electrical impedance and the vibration velocity of the transducer with spurious resonance to pre-tightening force. The results show that the spurious resonance disappeared when the pre-tightening force was initially loaded. Moreover, the vibration velocity of two main resonance peaks increases about 45.6% and 33.9% as the pre-tightening torque increases to 0.25 N∙m. Hence it is validated that the proposed prestress method is efficient to suppress the spurious resonance and improve the transducers performance.

Analysis on screw preload condition of acoustic beacon in high water pressure environment

The acoustic beacon is mainly assembled by using screw thread couplings between caps and shell. The products are always affected by heavy loads when operating under high water pressure. In this paper, to investigate the influence of screw thread couplings on acoustic beacon strength under deep water condition, the structural stresses and the distributions of the acoustic beacon with different pre-tightening torque of screw thread under particular water pressure is analysed by using finite element method. The variations of stresses which is relative to the pre-tightening torque of screw thread are also analysed and proposed as the form of line chart. The results indicate that the acoustic beacon strength can be improved by increasing the pre-tightening torque of screw thread, and the line chart can be used to determine the most suitable pre-tightening torque while assembling the products.

A novel virtual material layer model for predicting natural frequencies of composite bolted joints

A novel virtual material layer model based on the fractal theory was proposed to predict the natural frequencies of carbon fiber reinforced plastic composite bolted joints. Rough contact surfaces of composite bolted joints are modeled with this new proposed approach. Numerical and experimental modal analyses were conducted to validate the effectiveness of the proposed model. A good consistence is noted between the numerical and experimental results. To demonstrate the necessity of accurately modeling the rough contact surfaces in the prediction of natural frequencies, virtual material layer model was compared with the widely used traditional model based on the Master-Slave contact algorithm and experiments, respectively. Results show that the proposed model has a better agreement with experiments than the widely used traditional model (the prediction accuracy is raised by 8.77% when the pre-tightening torque is 0.5 N∙m). Real contact area ratio A* of three different virtual material layers were calculated. Value of A* were discussed with dimensionless load P*, fractal dimension D and fractal roughness G. This work provides a new efficient way for accurately modeling the rough contact surfaces and predicting the natural frequencies of composite bolted joints, which can be used to help engineers in the dynamic design of composite materials.

Multi-objective optimization of experimental and analytical residual stresses in pre-stressed cutting of thin-walled ring using glowworm swarm optimization algorithm

In this study, an analytical prediction model was utilized to predict the residual stresses induced during pre-stressed cutting of thin-walled ring, and the results were validated by experiments. General multivariate regression predictive models were presented to characterize the relationship between machining parameters and key characteristic values of residual stresses. These predictive models were used to build multi-objective optimization models to select the optimal machining parameters in process planning decision. The objectives are to minimize the tensile surface residual stress and to maximize the maximum compressive residual stress and processing efficiency and be chosen as related to industrial applications. An adaptive step-size multi-objective glowworm swarm optimization (MOGSO) algorithm was employed in optimizing parameters including cutting speed, cutting feed and pre-tightening torque. Optimization results demonstrate the superiority of the improved algorithm over the traditional MOGSO. The optimum machining parameters calculated from the predicted results were represented in both objective function and decision variable spaces. Further experimental verification results verified the effectiveness of the optimization model.

Sealing failure and fretting fatigue behavior of fittings induced by pipeline vibration

Pipeline fittings containing ferrules are used to connect sections of aircraft hydraulic pipelines. Therefore, their reliability and stability are essential. The purpose of this investigation is to better understand and analyze the sealing failure mechanism of pipeline fittings produced by pipeline vibration. Based on accelerated tests, the sealing failure mechanism and fatigue life of fittings are discussed. The influences of stress and pre-tightening torque on fretting fatigue behavior are also considered. Results from tests using an electromagnetic shaker apparatus indicate that sealing failure in pipeline fittings is caused by fretting fatigue of the front ferrule. Moreover, the decrease of pre-tightening torque will lead to more serious fretting fatigue.

Failure analysis of bolted joint for the column of an aerial platform

This paper focuses on the identification of failure causes of a bolted joint used for the bedding of a primary column of an aerial platform. The tests conducted to determine chemical composition and mechanical properties have confirmed the 12.9 grade of the bolts. Also, mechanical properties tests and metallographic examination did not indicate any hidden defects caused by heat treatment (e.g. decarburisation etc.). On the other hand, visual and metallographic examination of cross-section as well as electron microscope scanning confirmed fatigue fracture of the bolts. Therefore, further analyses were done to check the dimensioning and the installation of the bolts. The effects on stress states in the bolts are presented from the standpoint of rated operating regimes, stiffness of joined elements, lubrication conditions of thread surfaces and temperature incompatibility between the bolts and elements being joined. Pre-tightening torque value incompatible with lubrication conditions was found to be the main factor that led to high stress values that initiated and later intensified the fatigue process which ended in fatigue failure of the bolts.