Sayed A. Nassar

An Experimental Study of Bearing and Thread Friction in Fasteners

An experimental procedure is proposed for studying the underhead and thread friction in fasteners. The effective bearing friction radius, the underhead friction coefficient, and the thread friction coefficient are experimentally determined for fasteners with standard hexagonal heads and for flanged head fasteners. Hence, greater accuracy has been achieved in determining the value of the torque components that are consumed in overcoming friction in threaded fasteners. This would lead to a more reliable torque-tension correlation and would enhance the safety and quality of bolted assemblies. A design of experiment procedure is presented in order to investigate the effect of fastener material class, the thread pitch, and the fastener size on thread friction coefficient. For the underhead bearing friction, an experimental model is presented in order to determine the effect of the radii ratio of the contact area on the bearing friction radius.

Bearing Friction Torque in Bolted Joints

Formulas are developed for calculation of the effective radius of the bearing friction forces on the rotating contact surface in threaded fasteners. These formulas provide a more accurate estimation of the underhead bearing friction torque component in threaded fastener applications. This enhances the reliability, safety, and quality of bolted assemblies, especially in critical applications. It is well known that the torque-tension correlation in threaded fasteners, and the resulting joint clamping force, is highly sensitive to friction torque components: under the turning head and between threads. This analysis focuses on the bearing friction torque component under the turning head of a threaded fastener. Furthermore, it analyzes the error contained in the current practice when an approximate value, equal to the mean contact surface radius, is used instead of the actual bearing radius. New formulas for the bearing friction radius are developed for a mathematical model of a bolted joint using four different scenarios of the contact pressure distribution under the rotating fastener head or nut. The effect of the radially varying sliding speed over the rotating contact surface is analyzed and compared with a constant-friction-coefficient scenario. Numerical results and error analysis are presented in terms of a single nondimensional variable, namely, the radii ratio between the outside and the inside bearing area.

Thread Friction Torque in Bolted Joints

In this paper, formulas are developed for the calculation of the effective thread friction radius in fasteners, in order to determine the thread friction torque component. Due to the lack of exact formulas in the literature, current practice uses the average value of the minor and major thread radii, as an approximation, for determining the thread friction torque component. Results provided by these formulas are compared with those given by the current practice that uses the average value of the minor and major thread radii, instead of the exact value. It is well known that the torque-tension relationship in threaded fastener applications is highly sensitive to the friction torque components: between threads, and under the turning fastener head or nut. Even moderate variations or inaccuracies in determining the friction torque components would significantly impact the fastener tension and the joint clamp load. High accuracy in the estimation of the friction torque components is critical, as it directly affects the reliability, safety, and the quality of bolted assemblies. This analysis focuses on the thread friction torque component. The new formulas for the thread friction radius are developed for a mathematical model of a bolted joint using five assumed scenarios of the contact pressure between male and female threads. Because of the fact that the variation in the sliding speed of various points on a thread surface is insignificant, a uniform thread friction coefficient is used in the analysts. However, a contact area weighted average value is used for the thread friction coefficient. Numerical results and error analysis are presented in terms of a single nondimensional variable, namely, the ratio between the major and minor thread radii.

Clamp Load Loss due to Elastic Interaction and Gasket Creep Relaxation in Bolted Joints

An experimental study is presented in order to determine the clamp load loss due to elastic interaction and gasket creep relaxation in bolted joints. Studied parameters include the gasket material and thickness, bolt spacing, tightening sequence, fastener grip length, and level of the fastener preload. The joint is composed of two steel flanges and a gasket made of styrene butadiene rubber or flexible graphite. The flanges are fastened together using M12x1.75 Class 10.9 fasteners. Force washers are used to monitor bolt tensions in real time. Four different gasket thicknesses of styrene butadiene rubber (1/16, 1/8, 3/16, and 1/4 in.) and two thicknesses of flexible graphite (1/16 and 1/8 in.) are considered. For the same bolt circle of the flange, the bolt spacing is varied by using a different number of bolts; spacing that corresponds to using three, five, or seven bolts is considered in this study. The effect of the tightening strategy is studied by using sequential, star, or simultaneous tightening patterns. Bolt tightening is accomplished by using either an electric digital torque wrench with various control options or by using a production-size multiple spindle fastening system that is capable of simultaneous tightening ofall fasteners. Experimental data is presented and analyzed, in order to study the effect of the various parameters on the clamp load loss due to the combined effect of elastic interaction and gasket creep relaxation at room temperature.

An Improved Stiffness Model for Bolted Joints

An improved stiffness model is proposed for bolted joints made of similar and dissimilar plates. A novel approach is used to obtain an expression for the effective area used for determining the joint stiffness. More accurate estimate of the joint stiffness provides a more reliable prediction of the joint behavior both during its initial assembly, as well as under subsequently applied tensile loads in service. The effect of the grip length-to-diameter ratio, joint sizes, underhead contact radii ratio, hole clearance, and plate material/thickness ratio are investigated. Experimental data are used for determining the envelope angle a in the proposed analytical model. Finite element modeling is used for evaluating the accuracy of the proposed stiffness model.

Effect of Tightening Speed on the Torque-Tension and Wear Pattern in Bolted Connections

In an effort to enhance the reliability of clamp load estimation in bolted joints, this experimental study investigates the effect of tightening speed and coating on both the torque-tension relationship and wear pattern in threaded fastener applications. The fastener torque-tension relationship is highly sensitive to normal variations in the coefficients of friction between threads and between the turning head and the surface of the joint. Hence, the initial level of the joint clamp load and the overall integrity and reliability of a bolted assembly are significantly influenced by the friction coefficients. The effect of repeated tightening and loosening is also investigated using M12, class 8.8 fasteners with and without zinc coating. The torque-tension relationship is examined in terms of the nondimensional nut factor K. The wear pattern is examined by monitoring the changes in surface roughness using a WYKO optical profiler and by using a LECO optical microscope. A Hitachi S-3200N scanning electron microscope is used to examine the contact surfaces under the fastener head after each tightening/loosening cycle. Experimental data on the effect of tightening speed, fastener coating, and repeated tightening are presented and analyzed.

Effect of Thread Pitch and Initial Tension on the Self-Loosening of Threaded Fasteners

A mathematical model and an experimental procedure are presented to study the self-loosening phenomenon of threaded fasteners that are subjected to cyclic transverse loads. The study investigates the effect of thread pitch, initial bolt tension, and the amplitude of the external excitation on the loosening of a single-bolt joint. The rate of drop in the joint clamp load (fastener tension) per cycle, as well as the total number of cycles that would cause the complete loss of clamp load, are monitored. In the mathematical model, the differential equations of linear and angular motion of the bolt are formulated in terms of the system properties and the external cyclic transverse excitation. Numerical integration of the equation of angular motion provides the bolt rotation in the loosening direction, which causes the partial or full loss of the clamp load. An iterative MATLAB code is developed and used for the calculation of tension loss in the fastener tension due to the self-loosening. Analytical and experimental results are discussed.

Effect of Thread and Bearing Friction Coefficients on the Vibration-Induced Loosening of Threaded Fasteners

This study provides a theoretical and experimental investigation of the effect of the thread and bearing friction coefficients on the self-loosening of threaded fasteners that are subjected to cyclic transverse loads. The friction coefficients are varied by using different types of coating and lubrication. A phosphate and oil coating and an olefin and moly-disulfide solid film lubricant are used on the bolts tested. A mathematical model is developed to evaluate the self-loosening behavior in threaded fasteners when subjected to cyclic transverse loads. An experimental procedure and test setup are proposed in order to collect real-time data on the loosening rate (rate of clamp load loss per cycle) as well as the rotational angle of the bolt head during its gradual loosening. The experimental values of the friction coefficients are used in the mathematical model to monitor their effect on the theoretical results for the loosening rate. Experimentally, the friction coefficients are modified by changing the coating or the lubrication applied to the fasteners. The theoretical and experimental results are presented and discussed.

Clamp Load Loss due to Fastener Elongation Beyond its Elastic Limit

The amount of clamp load due to an externally applied separating force is determined for a boiled assembly in which the fastener is elongated past its proportional limit, while the clamped joint remained within its elastic range. After the initial tightening of the fastener, the joint is subsequently subjected to a tensile separating force, which further increases the fastener tensile stress into the nonlinear range. Such separating force will simultaneously reduce the clamping force in the bolted joint. Upon the removal of the separating service load, the bolted joint system reaches a new equilibrium point between the fastener tension and the joint clamping force. At the new equilibrium point, the fastener tension is reduced from its value at initial assembly, due to the plastic elongation of the fastener. The reduction in fastener tension translates into a partial-yet permanent-loss of the clamping load that may lead to joint leakage, loosening, or fatigue failure. A nonlinear model is established in order to describe the fastener behavior past the proportional limit of its material, and to determine the clamp load loss due to the permanent set in the fastener after the separating force has been removed. Two fastener materials with significantly different rates of strain hardening are used for modeling the behavior of the bolted joint system. The effect of three nondimensional variables on the amount of clamp load loss is investigated. The first variable is the stiffness ratio of the joint and the fastener. The second is the ratio of initial fastener tension to the fastener elastic limit, and the third variable is the ratio of the separating force to the force that causes joint separation to start. Analytical results are presented for a range of stiffness ratios that simulates both soft and hard joint applications. Experimental verification of the analytical results is presented.

Effect of Coating Thickness on the Friction Coefficients and Torque-Tension Relationship in Threaded Fasteners

This paper experimentally investigates the effect of coating thickness on the thread, bearing friction coefficients, and torque-tension relationship in threaded fasteners, as well as an investigation into the effect of coating thickness on surface roughness properties. The torque-tension relationship is highly sensitive to frictional changes. Two different coating thicknesses are investigated using two bolt thread pitch; test data are collected for a preselected level of bolt tension. The experimental setup collects real-time data on the tightening torque, bolt tension, and the corresponding reaction torque. Test data are used for calculating the thread and bearing friction coefficients, as well as the overall torque-tension relationship for two different coating thicknesses. The study would provide an insight into the variation in the torque-tension relationship, which is a key factor that significantly affect the reliability and safety of bolted assemblies in many mechanical and structural applications.