室温下C/C复合材料紧固件拧紧特性

本文研究了C/C复合材料紧固件在不涂防松胶（简称不涂胶）和涂防松胶（简称涂胶）两种状态下的拧紧特性。试验测试了两种状态下C/C复合材料紧固件拧紧力矩与预紧力的关系,并对两种状态下紧固件摩擦系数和拧紧力矩分配关系进行了深入分析。结果表明:涂胶与不涂胶相比,相同拧紧力矩作用下,螺栓预紧力增大20%~82%,且不涂胶状态下预紧力与拧紧力矩线形关系较好;不涂胶和涂胶两种状态下,螺纹副摩擦系数分别为0.41、0.35,端面摩擦系数分别为0.59、0.41,涂胶后两对摩擦副摩擦系数均降低;另外,涂胶和不涂胶两种状态下施加拧紧力矩在端面摩擦副、螺纹摩擦副和附加力矩中比例分配规则略有不同。      

防松胶对螺纹连接预紧力影响试验研究

鉴于螺纹在涂胶前后或涂不同种类防松胶、施加相同拧紧力矩时产生的螺栓预紧力差别较大,对空间光学遥感器装配中常用防松胶对螺纹连接预紧力影响进行定量试验研究,结果表明,在螺纹上涂硅橡胶D04(L)与不涂胶相比,相同拧紧力矩时螺栓预紧力减小50%~70%;涂厌氧胶乐泰243、相同拧紧力矩时螺栓预紧力增大46%~98%。某类防松胶使用前需对其对螺纹连接预紧力影响进行定量测试,据测试结果调整螺栓的拧紧力矩达到预期预紧力;在对预紧力精确控制的连接区域,力矩扳手紧固方式不再适用。     

螺栓拧紧过程中工况对复合材料连接结构夹紧力的影响

针对复合材料结构螺栓连接,拧紧工况的不同会改变端面和螺纹间的摩擦系数,从而影响到扭矩系数,使夹紧力随拧紧力矩变化的趋势十分复杂.首先对螺栓连接过程进行理论分析,得到了夹紧力和复合应力状态下螺栓屈服夹紧力的表达式.然后以某型号螺栓为对象进行试验研究,在规定的目标预紧力下,分别测得了不同润滑条件、垫圈条件和拧紧速度下的拧紧...     

振动工况下螺栓连接自松弛机理研究

为研究振动工况下螺栓连接自松弛机理,利用ANSYS参数化语言建立考虑螺纹的三维螺栓连接有限元模型,用降温法加载预紧力,进行螺栓连接横向振动瞬态分析;研究横向激励幅值、初始预紧力、螺纹啮合面、螺栓头及螺母承压面以及连接物之间结合面的摩擦因数等对螺栓连接自松弛影响。结果表明,横向振动时完全滑移先发生于螺纹啮合面处;横向激励幅值越小、初始预紧力越大、螺纹啮合面及螺栓头、螺母承压面摩擦因数越大,螺栓连接自松弛越不易发生;激励幅值一定时连接物间结合面摩擦因数对自松弛无影响,但摩擦因数越大,发生横向振动所需剪切载荷越大。研究结果对理解螺栓连接自松弛、指导防松设计具有重要意义。     

螺纹摩擦对螺栓轴向力-伸长量特征曲线的影响

基于超声波技术,通过对螺栓进行拉伸试验,研究了不同外载下螺栓轴向力-伸长量特征曲线的差异,分析了螺纹摩擦对螺栓轴向力-伸长量特征曲线的影响。结果表明:螺栓的单向拉伸与扭转拉伸因受到的载荷方向不同而导致其屈服点不同;螺栓前期拧紧标定过程中螺纹摩擦对螺栓轴向力有很大影响,会影响后续子系统螺栓夹紧力的大小;不同的螺纹摩擦状态导致拧紧过程中螺栓轴向应变速率不同,会产生不同的特征曲线,且螺纹摩擦因数越大,扭转应力就越大,螺栓屈服点就越低;若能保证轴向拉伸速度相同,不管是涂层之间的正常螺纹摩擦还是使用机油降低摩擦,获得的特征曲线斜率是相同的;无涂层的金属间摩擦会导致螺纹尖端产生塑性变形,形成的特征曲线弹性段斜率有较大差异。     

螺栓拧紧试验台的研制

螺栓拧紧试验台通过测定螺栓所受的拧紧力矩和预紧力 ,从而确定螺栓的力矩系数。本文对试验台的工作原理、结构设计、试验结果及测力传感器静态标定作了具体分析和介绍。     

基于横向振动试验的螺纹联接临界松动研究

横向振动试验是判断螺纹紧固件防松性能的主要方法,该文根据横向振动试验原理制定了试验方案,获得参数组合对旋转松动的影响规律,这些参数包括初始预紧力,紧固件尺寸、材料,摩擦系数,载荷大小、方向、频率,装配结构。在此基础上,通过分析预紧力衰减量、衰减百分比及衰减曲线,找到螺纹联接临界松动条件。按照该方法,可得到更多参数组合的试验数据,从而得到更全面、准确的临界松动条件,为工程实际提供设计依据与参考。     

扭矩扳手在使用中的几点经验

虽然现在国产、进口力矩扳手的型号、样式各异 ,但由于工作原理相同或相似 ,所以在使用中有很多相同之处。我单位使用的大部分是美国ＳＮＡＰ -ＯＮ和ＰＲＯＴＯ公司生产的扳手 ,通过多年的使用和检定 ,我们总结出了一些力矩扳手的使用经验与常识。１ 力矩扳手只能用作安装紧固件（螺栓、螺母）时测量其安装力矩使用 ,绝不能作为拆卸工具去拧松已拧紧的紧固件。不能敲打、磕碰或作它用。使用时轻拿轻放 ,不许任意拆卸与调整。２ 在使用扭矩紧固件的场合尽可能带上护目镜 ,这样可以在突发情况下保护操作者的眼部。３ 为了保证工作人员正确使…     

螺纹联接的预紧与防松

螺纹联接作为目前应用最为广泛的一种联接方式，在工程机械各个行业发挥着重要的作用。但其预紧和松动问题一直是螺纹紧固件的最大难题。本文介绍了多种螺纹联接的预紧力控制方法和防松措施，从多方面针对性的阐述了解决螺纹联接质量的方法     

过盈配合和预紧力的混合作用对复合材料机械连接结构的影响及其机制

为研究钉孔过盈配合情况和紧固件预紧力及二者混合作用对复合材料连接强度的影响及其机制,提出了一种基于试验验证的有限元方法,在有限元模拟结果与两组不同配合复合材料与钛合金单剪双钉连接拉伸试验吻合良好的基础上,进一步模拟6种不同螺栓预紧力和7种不同钉孔配合模式的组合,共42种不同情况的结构承载能力。通过对模拟结果的分析发现：一定的干涉配合值和预紧力虽然可能导致钉孔周围在受载较小时提前出现较小破坏,但在载荷较大时反而可以改善孔周的接触条件,从而减小孔周的纤维破坏范围,并最终提高连接结构的破坏载荷。而且钉孔配合情况和螺栓预紧力对连接结构承载能力会相互影响,即在不同钉孔配合情况下所得到的紧固件预紧力优化值会有所不同,反之亦然。因此在进行复合材料机械连接结构承载能力优化时,必须同时考虑不同参数的影响,才能获得最优的结构设计。     

Loosening-resistance evaluation of double-nut tightening method and spring washer by three-dimensional finite element analysis

The mechanisms of loosening-resistance components are investigated within the framework of the three-dimensional finite element method (FEM). In this paper, we have evaluated the ability of double-nut tightening method (DN) and spring washer (SW) to resist self-loosening due to transverse loading. We have found that if locking state is properly achieved in the tightening process, DN shows significant loosening resistance regardless of the magnitude of locking force. It was observed in this case that thread surface on the upper nut retains stuck state even if bearing-surface undergoes complete slip. However, if the locking process is not performed properly, the ability to resist loosening completely disappears. On the other hand, it is shown that SW accelerates loosening rotation of nut. The stuck area on the contact surfaces is reduced to two corner edges of the SW and the rotational force around these edges thus drastically leads to loosening before complete bearing-surface slip.     

Tightening Tests and Friction Coefficients Definition in the Steering Shaft of Front Motorbike Suspension

Abstract: The steering shaft of front motorbike suspensions is, usually, tightened up the fork by means of a nut and a locknut. The aim of this study is to evaluate the actual relation between the bolt torque and the preloading force of the steering shaft by performing some tightening tests. The bolt torque is given by a torque wrench whereas the preloading force has been evaluated by means of a strain gauge located on the steering shaft. The steering shaft has a hollow section with an external thread so that the strain gauge has been applied on its internal surface. In relating the bolt torque to the bolt tension, the friction coefficients have been accurately calculated: the friction coefficient values increase notably after the first set of tightening tests mainly because of the spoiling of the contact surfaces.     

An experimental investigation of the bolt clamping force and friction effect on the fatigue behavior of aluminum alloy 2024-T3 double shear lap joint

In this article, the effect of bolt clamping force on the fatigue life of bolted double shear lap joints was investigated. To do so, fatigue tests were carried out on the bolt clamped double shear lap joint specimens made of aluminum alloy 2024-T3. These fatigue tests were conducted with applied torques of 0.25, 2 and 4 N m at different cyclic longitudinal load levels in un-lubricated and lubricated states. From these tests the stress–life (S–N) data for different clamping forces for un-lubricated and lubricated states were obtained. The results show that clamping force increases fatigue life compared to clearance fit specimens. In general, at higher tightening torque higher fatigue lives were achieved, however, below a certain load level the life improvement was discontinued because of fretting phenomenon. Also lubricating the parts of the specimens reduces the advantage of clamping force or torque tightening.     

Evaluation of the Clamping Force of Bolted Joints Using Local Mode Characteristics of a Bolt Head

It is important to determine and control the clamping force of a bolted joint. Due to its simple setup, the torque control method is typically used to control the clamping force when tightening bolts. After tightening, hammer tests, ultrasonic techniques and methods employing sheet materials as sensors are often used. Many methods have been proposed, but using them to determine and control the clamping force during or after tightening bolts is labor intensive or expensive. Here we conduct impact tests with an impulse hammer combined with experimental modal analysis to determine the clamping force by interpreting the change in the local mode frequency of a bolt head in the high frequency region as a function of the clamping force. To demonstrate the applicability of our method, we also investigate its limits with regard to bolt sizes.     

Finite element investigations of bolt clamping force and friction coefficient effect on the fatigue behavior of aluminum alloy 2024-T3 in double shear lap joint

In this paper, the effect of bolt clamping force on the fatigue life of bolted double shear lap joints was investigated numerically. To do so, finite element simulation results were used to illustrate the trends occurred in experimental fatigue tests showing the effect of bolt clamping on improving the fatigue life of double shear lap joints. The results show that clamping force decreases the resultant longitudinal stress at the hole edge thus the fatigue life increases compared to clearance fit specimens. In general, at higher tightening torque longer fatigue lives were achieved, however, below a certain load level the life improvement was discontinued because of fretting occurrence. Also lubricating the specimens reduces the advantages of the clamping force.     

Experimental and numerical study of fatigue crack growth of aluminum alloy 2024-T3 single lap simple bolted and hybrid (adhesive/bolted) joints

In this paper, the effects of tightening torque (clamping force) on the fatigue crack growth rate and stress intensity factors in cracked single lap simple bolted and hybrid (adhesive/bolted) joints have been studied experimentally and numerically. To do so, series of fatigue crack growth tests for two different amounts of tightening torque in Aluminum alloy 2024-T3 pre-cracked joints have been carried out to record the fatigue crack growth and also the fatigue life of specimens. In the numerical part, finite element method was employed to obtain the stress intensity factors and also the effective stress intensity factor ranges for different crack lengths to explain the behavior of fatigue crack propagation. It was found that the hybrid joint has longer fatigue crack growth life compared to the simple bolted joint at a given bolt tightening torque. The results also showed that a higher bolt tightening torque provides improved fatigue crack growth life for both types of the joints.     

Effect of bolt clamping force on the fracture strength of mixed mode fracture in an edge crack with different sizes: Experimental and numerical investigations

To investigate the effect of bolt clamping force, resulting from torque tightening, on the mixed mode fracture (I and II) strength and effective geometry/loading factor of an edge crack with different lengths, experimental and numerical studies have been carried out. In the experimental part fracture tests were conducted on three batches of simple edge crack and bolt torque tightened with 3.5 and 7 N m edge crack at three different crack sizes of Poly methyl-methacrylate (PMMA) rectangular plate. The specimens’ fracture strength was obtained using a tensile test machine at different loading angles by employing a modified Arcan fixture. In numerical part, finite element simulations were employed to model the three test specimen batches at the three crack lengths to obtain their stress intensity geometry/loading factors. The results show that the bolt tightening torque significantly reduces the effective geometry/loading factor, and thus increases the joint fracture strength compared to bolt-less simple edge crack specimens. However, the bolt clamping effect on increasing the fracture strength was almost the same for different crack lengths.     

Investigation the effect of tightening torque on the fatigue strength of double lap simple bolted and hybrid (bolted–bonded) joints using volumetric method

In this research, the effect of the tightening torque on the fatigue strength of 2024-T3 double lap simple bolted and hybrid (bolted–bonded) joints have been investigated experimentally by conducting fatigue tests and numerically by implementing finite element analysis. To do so, three sets of specimens were prepared and each of them subjected to tightening torque of 1, 2.5 and 5 Nm and then fatigue tests were carried out under different cyclic longitudinal load levels. In the numerical method, the effect of the tightening torque on the fatigue strength of the considered joints has been studied by means of volumetric method. To obtain stress distribution around the notch (bolt hole) which is required for the volumetric method, nonlinear finite element simulations were carried out. In order to estimate the fatigue life, the available smooth S–N curve of Al2024-T3 and the fatigue notch factors obtained from the volumetric method were used. The estimated fatigue life was compared with the available experimental test results. The investigation shows that there is a good agreement between the life predicted by the volumetric method and the experimental results for different specimens with a various amount of tightening torques. The results obtained from the experimental analysis showed that the hybrid joints have a better fatigue strength compared to the simple bolted joints. In addition, the volumetric method and experimental results revealed that the fatigue life of both kinds of the joints were improved by increasing the clamping force resulting from the torque tightening due to compressive stresses which appeared around the bolt hole.