Basil A. Housari

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.

A Proposed Model for Creep Relaxation of Soft Gaskets in Bolted Joints at Room Temperature

A mathematical model is proposed for predicting the residual clamp load during creep and/or relaxation in gasketed joints. An experimental procedure is developed to verify the proposed model for predicting the gasket relaxation under a constant compression, gasket creep under a constant stress, and gasket creep relaxation. To study gasket creep relaxation, a single-bolt joint is used. The bolt is tightened to a target preload and the clamp load decay due to gasket creep relaxation is observed over time under various preload levels. Experimental and analytical results are presented and discussed. The proposed model provides an accurate prediction of the residual clamp load as a function of time, gasket material, and geometric properties of the gasket. A closed form solution is formulated to determine the initial clamp load level necessary to provide the desired level of a steady state residual clamp load in the joint, by taking the gasket creep relaxation into account.

Formulation of Elastic Interaction Between Bolts During the Tightening of Flat-Face Gasketed Joints

A novel mathematical model is proposed for studying elastic interaction in gasketed bolted joints. The model predicts the tension changes in tightened bolts due to the subsequent tightening of other bolts in the joint. It also predicts the final clamp load distribution after the completion of joint tightening. The model is used to investigate the effect of various factors on the elastic interaction phenomenon; factors include the gasket thickness, bolt spacing, fastener preload level, and tightening sequence of various bolts. Experimental verification is provided for the validation of the mathematical model. Experimental and analytical results are presented and discussed. The proposed model provides a good prediction of the final clamp load in the joint. Moreover, the proposed model may be used to determine the level of initial bolt tension in each bolt that would be necessary to achieve the desired level of uniform clamp load in the joint at the initial assembly.

A Proposed Model for Predicting Clamp Load Loss Due to Gasket Creep Relaxation in Bolted Joints

An improved mathematical model is proposed for predicting clamp load loss due gasket creep relaxation in bolted joints, taking into consideration gasket behavior, bolt stiffness, and joint stiffness. The gasket creep relaxation behavior is represented by a number of parameters which has been obtained experimentally in a previous work. An experimental procedure is developed to verify the proposed model using a single-bolt joint. The bolt is tightened to a target preload and the clamp load loss due to gasket creep relaxation is observed over time under various preload levels. The experimental and analytical results are presented and discussed. The proposed model provides a prediction of the residual clamp load as a function of time, gasket material and thickness, bolt stiffness, and joint stiffness. The improved model can be used to simulate the behavior of creep relaxation in soft joints as the joint stiffness effect is considered. Additionally, a closed form solution is formulated to determine the initial clamp load level necessary to provide the desired level of a steady state residual clamp load in the joint, by taking the gasket creep relaxation into account.

Development of Tightening Specification for Post Yield Angle Control Tightening Strategy

There are several tightening strategies used for bolted joints in the industry, such as torque control, angle control, yield control, and stretch control. The most common strategy is torque control due to the ease of use, simple tooling, and availability of tabulated torque and average corresponding clamp force values for common fastener sizes and classes.For some special applications, angle controlled tightening strategy is used where maximum fastener strength utilization and more consistent clamp load are required. Unlike torque control tightening strategy, angle control tightening is more complicated and requires more advanced tooling. In addition, there are no tabulated values that provide the angle control tightening parameters for a specific fastener material and size. The tightening specification is rather developed on a case by case basis.This paper provides a method for developing tightening specification for angle control tightening. It also provides a method to calculate the maximum expected final torque which can be used to select the proper tool size. The maximum expected final torques are tabulated in the paper for most commonly used fasteners. Finally, the paper provides a criterion for detecting parts and/or joint quality issues when angle control tightening is used.Copyright

Effect of Coating and Lubrication on the Vibration-Induced Loosening of Threaded Fasteners

This study provides an experimental and theoretical investigation of the effect of the bearing friction coefficient and the thread friction on the self-loosening of threaded fasteners that are subjected to cyclic transverse service loads. Coating and lubrication affect the thread and the underhead friction of the fastener, which affects the loosening rate when it is subjected to transverse loads. A mathematical model was developed to evaluate the self-loosening behavior in threaded fasteners when subjected to cyclic transverse loads. An experimental procedure and test set up are designed in order to collect real-time data on the rate of preload loss per cycle as well as the rotational angle of the bolt head during its gradual loosening. The values of the coefficients of friction under the bolt head and between the threads were changed in the mathematical model to monitor their effect on the loosening rate. Experimentally, the friction coefficients are modified by changing the coating or the lubrication applied to the fasteners. One coating and one solid film lubricant are used, namely, phosphate and oil coating and Olefin and Molydisulfide lubricated bolts. The theoretical and experimental results are presented and discussed.Copyright

Development of Tightening Torque for Self Tapping and Thread Rolling Fasteners

Torque is the most common tightening strategy used to control clamp load when bolts are tightened, since a direct relationship between torque and clamp load exists. For unified and metric fasteners used in non-critical applications, there are torque tables available in the literature that provide a guideline as to what torque level is recommended to be used based on fastener size, thread pitch, and material class. For critical applications, the tightening specification is usually developed based on individual cases. For self tapping and thread rolling screws, the tabulated torque values available for machine threaded screws cannot be used. If the torque spec is not developed carefully for such joints then problems such as stripping the threads or torqueing out before seating the screw may be encountered. This is mainly due to the fact that these types of screws are very sensitive to many variables, such as thread type, hole size, coating, joint surface finish, tapped material thickness, and other factors. For such fasteners, the torque spec should be developed experimentally based on test data for the actual joint. This paper provides an accurate procedure for developing tightening specifications for joints that use self tapping and thread rolling screws. Additionally, it provides a criterion to determine whether the joint design is robust enough for assembly or not.Copyright

A Proposed Model for Predicting Residual Clamp Load in Gasketed Bolted Joints

An improved mathematical model is proposed for predicting the residual clamp load in gasketed bolted joints, taking into consideration gasket creep relaxation behavior, bolt stiffness, and joint stiffness. The gasket creep relaxation behavior is represented by a number of parameters which has been obtained experimentally in a previous work. An experimental procedure is developed to verify the proposed model using a single-bolt joint. The bolt is tightened to a target preload and the clamp load loss due to gasket creep relaxation is observed over time under various preload levels. The experimental and analytical results are presented and discussed. The proposed model provides a prediction of the residual clamp load as a function of time, gasket material and thickness, bolt stiffness, and joint stiffness. The improved model can be used to simulate the behavior of creep relaxation in soft joints as the joint stiffness effect is considered. Additionally, a closed form solution is formulated to determine the initial clamp load level necessary to provide the desired level of a steady state residual clamp load in the joint, by taking the gasket creep relaxation into account.Copyright

Study of the Effect of Hole Clearance and Thread Fit on the Self-Loosening of Threaded Fasteners

This study provides an experimental and theoretical investigation of the effect of hole clearance and thread fit on the self-loosening of tightened threaded fasteners that are subjected to a cyclic transverse service load. An experimental procedure and test set up are developed in order to collect real-time data on the rate of clamp load loss per cycle as well as the loosening rotation of the bolt head. Three levels of hole clearance are investigated; namely, 3%, 6%, and 10% of the bolt nominal diameter. For the commonly used 2A thread fit for a selected bolt size, three classes of the nut thread fit are considered; namely, 1B, 2B, and 3B. A simplified mathematical model is used for the analytical investigation of the effect of the hole clearance and thread fit on threaded fasteners self-loosening. The experimental and theoretical results are presented and discussed.© 2006 ASME