Screw Fasteners Research Articles

The Automated Threaded Fastening Based on On-line Identification

The principle of the thread fastenings have been known and used for decades with the purpose of joining one component to another. Threaded fastenings are popular because they permit easy disassembly for maintenance, repair, relocation and recycling. Screw insertions are typically carried out manually. It is a difficult problem to automat. As a result there is very little published research on automating threaded fastenings, and most research on automated assembly focus on the peg-in-hole assembly problem. This paper investigates the problem of automated monitoring of the screw insertion process. The monitoring problem deals with predicting integrity of a threaded insertion, based on the torque vs. insertion depth curve generated during the insertions. The authors have developed an analytical model to predict the torque signature signals during self-tapping screw insertions. However, the model requires parameters on the screw dimensions and plate material properties are difficult to measure. This paper presents a study on on-line identification during screw fastenings. An identification methodology for two unknown parameter estimation during a self-tapping screw insertion process is presented. It is shown that friction and screw properties required by the model can be reliably estimated on-line. Experimental results are presented to validate the identification procedure.

An axisymmetric model for thread forming in polycarbonate and polypropylene screw and boss fasteners

AbstractPlastic boss and screw fasteners are an economical means of securing automotive components, such as instrument or body panels. However, new materials and/or suboptimal design present challenges to the boss/screw effectiveness. Failure of a boss/screw can result is loss of functional performance or increased squeak and rattle. Failure is often controlled by what occurs during the initial thread‐forming process. Thus, the goal of this paper is to develop an FEA model to elucidate the thread‐forming process so that we can facilitate subsequent design and/or process optimization, and understand potential failure modes. The FEA must accommodate nonlinear couplings, such as large strain and heat transfer. Heat generation is present in the forms of interfacial shear heating and plastic work associated with the large deformation of the interface between the boss and screw. Strain rate‐dependent materials are included using the Eyring theory for plastic flow of polymeric materials. Results of the model are presented and compared to experimentally determined torque curves and temperatures. Polym. Eng. Sci. 44:1498–1508, 2004. © 2004 Society of Plastics Engineers.

Model Based Monitoring Strategies for Threaded Assembly System

Threaded fastenings are a common assembly method, accounting for over a quarter of all assembly operations. Threaded fastenings are popular because they permit easy disassembly for maintenance, repair, relocation and recycling. A model based approach for monitoring screw fastenings is presented in this paper. The proposed strategy can be integrated with a mechatronics based approach for automating threaded assembly systems. The screw insertion process is typically monitored using the torque vs insertion depth signature signals. For a screw insertion with a given set of parameters, the insertion signature signal will have a unique identity and by comparing the on line signature signals with the ideal signature signal, the integrity of the joint can be predicted. The approach adopted in this study is to use an analytical model developed by the authors to predict ideal insertion signals. However, this model requires various fixed process parameters as input, and it is not always possible to know these parameters in advance with sufficient accuracy. Hence the focus of this paper is the on-line parameter estimation during threaded assembly. A methodology for estimating four unknown parameters of a general self-tapping screw insertion is presented. The approach is based on the Newton Raphson Method (NRM). It is shown that up to four parameters required by the model can be reliably estimated in real time, from on-line torque signature signals. Experimental results are presented to validate the estimation procedure.

Splitting Failures in Trapezoidal Steel Roof Cladding

High wind events such as hurricanes and storms often cause severe damage to crest-fixed thin steel roof claddings. Past research on wind damage has shown that low cycle fatigue cracking of steel roof sheeting around the fastener holes has been the reason for the premature pull-through failures of roof claddings under fluctuating wind forces. Such a situation will be at its worst if the roof sheeting is already split at the fastener holes. An inspection of trapezoidal steel roofs has shown that roofing has been split in the transverse direction due to overtightening of screw fasteners accidental or caused by poor workmanship. Once split, even slightly, the roofing can survive only a few cycles of wind uplift loading. Therefore an investigation using laboratory experiments and finite element analyses was carried out to study the splitting behavior of two commonly used high tensile steel trapezoidal roof claddings. The analytical and experimental results agree reasonably well and present a good understanding of the splitting behavior of trapezoidal roof claddings. This paper presents the details of this investigation and the results.

テーパ座面によるボルト・ナット締結体のゆるみ防止(加工・その他)

テーパ座面によるボルト・ナット締結体のゆるみ防止(加工・その他)

Strength reduction in screw fasteners resulting from outdoor exposure

It is well known that corrosion can weaken structures over time, but because corrosion rates can differ significantly from location to location, it is difficult to predict the amount of weakening that can occur for a particular structure used outdoors in a given location. Over the years, salt fog testing has been used to help predict how a structure might degrade over time resulting from exposure. The most common tests are based upon MIL-STD 810 “Environmental Engineering Considerations and Laboratory Tests,” and ASTM B117 “Standard Practice for Operating Salt Spray (Fog) Apparatus.” Although these standards give guidelines on conducting accelerated tests, they provide no insight as to the real world exposure level corresponding to a test condition, i.e. an acceleration factor. Small screw fasteners that had been exposed in service for two years in a marine environment were visually compared to similar fasteners exposed to varying lengths of time up to 22 days, under accelerated weathering conditions as specified in MIL-STD-810, and ASTM B117. A salt fog apparatus was used for the exposure testing. An acceleration factor for this service was determined, and found to be consistent with corrosion rate research conducted for other marine environments. Research of corrosion rates in other marine environments found that they can vary significantly, and are most strongly dependent on temperature, wind speed and direction, and rainfall. Mechanical testing of the screws removed from service was compared to those subjected to the accelerated weathering conditions. Testing was conducted with a servo-hydraulic test machine fitted with custom grips designed to load the screws in their as-installed configuration. Both the screws returned from two years of service, and screws subjected to the salt fog for various amounts of time up to 22 days were tested in this fashion. In addition, standard single lap shear testing was conducted on several screws exposed to the salt fog for various amounts of time up to 22 days. The amount of time for the screw strength to drop below an acceptable level was determined.

Strength Reduction in Screw Fasteners Resulting From Outdoor Exposure

Strength Reduction in Screw Fasteners Resulting From Outdoor Exposure

Cyclic Pull-Out Strength of Screwed Connections in Steel Roof and Wall Cladding Systems using Thin Steel Battens

Summary When crest-fixed thin steel roof and wall cladding systems are subjected to wind uplift or suction loading, local pull-through or pull-out failures occur prematurely at their screwed connections. During high wind events such as storms and hurricanes these localised failures then lead to severe damage to buildings and their contents. In recent times, the use of thin steel battens/purlins has increased considerably. This has made the pull-out failures more critical in the design of steel cladding systems. Recent research has developed a design formula for the static pull-out strength of screwed connections in steel cladding systems. However, the effects of fluctuating wind uplift or suction loading that occurs during high wind events are not known. Therefore a series of cyclic wind uplift/suction tests has been undertaken on connections between thin steel battens made of different thicknesses and steel grades, and screw fasteners with varying diameter and pitch. Tests revealed a significant reduction to pull-out strength caused by fluctuating wind loading. Simple design equations and suitable recommendations were developed to take into account this strength reduction. This paper presents the details of the cyclic tests and the results.

Optimizing study of SLBs for higher anti-loosening performance

The authors had earlier proposed the concept of an SLB (step–lock–bolt), which has some steps on the lead helix, with an objective of developing an anti-loosening screw fastener. The SLB produced had eight steps in a pitch and 45% flat part in which the lead angle is nil. It was found that the SLB does not easily loosen when it was fastened with a clamping force of 16±4 kN , because it suppresses the bolt torsion which is one of the causes of loosening. In this paper, a newly produced SLB, which has 10 steps and 50% flat part, is reported. The aim of developing this screw fastener is to prevent loosening even when a high clamping force cannot be applied, which is the case when wooden or rubber plates are fastened. The importance of the number of steps in a pitch and the ratio of the flat part in the anti-loosening performance of the SLB are discussed. The matching of the hardness of bolt and nut for preventing loosening is also discussed. The contact state between the bolt screw thread and nut is investigated carefully. As a result, the newly produced SLB prevents loosening even when the applied clamping force is 4 kN. The higher ratio of the flat part makes the SLB exhibit better performance.

Operator-induced compressive axial forces during implant gold screw fastening

Statement of problem. The gold screw of an implant is put under tension during fastening. An increase in operator-induced compressive axial force during fastening may diminish screw tension, lower the friction between the screw threads, and allow for increased tightening torque. Purpose. This study was undertaken to assess and compare the compressive axial forces and torques placed simultaneously on implant gold screws by persons with varying degrees of expertise. Material and methods. A calibrated electric torque driver was used to fasten implant gold screws. Three groups of operators with various levels of implant experience (faculty [F; n = 4], prosthodontic residents [R; n = 4], and undergraduate dental students [S; n = 6]) were asked to repeatedly tighten and loosen a new gold abutment screw into a standard 3.75-mm diameter Brönemark abutment. Compressive axial forces during torquing were assessed over the tightening time by means of a miniature load cell adapted to the electric torque driver. Each operator repeated the experiment 3 times after calibration. Within operator and between operator reliability were evaluated. Results. Loosening compressive axial forces were always higher than tightening compressive axial forces, and peak torque was less on loosening than tightening. Faculty placed a smaller range of forces on the screws (mean = 3.29 N, SD ± 1.45 N) than did the residents (mean = 2.74 N, SD ± 1.96) or the students (mean = 3.01 N, SD ± 2.54). Conclusion. The clinical experience of operators seems to influence their application of compressive axial force during gold screw tightening. Less torque during unfastening of gold screws seems to be related to increased axial loading. (J Prosthet Dent 2001;86:15-9.)

Theoretical modelling of the self-tapping screw fastening process

The results of a theoretical study of the self-tapping screw insertion process are presented. Fundamental concepts of engineering mechanics are employed to analyse the axial torque required to complete a general self-tapping screw insertion operation. Equations for the screw insertion torque as a function of screw, hole and material properties are presented. Experimental results for torque signature signals are compared with the predictions of the theoretical model, confirming the validity of the model. The theoretical model provides a basis for developing automated monitoring and control strategies for self-tapping screw fastenings.

Mechatronic strategies for torque control of electric powered screwdrivers

The paper presents the results of a study on using mechatronics to enhance the performance and reliability of small electric powered screwdrivers (SEPS). A mechatronic solution to control the final tightening torque replacing the mechanical clutch of a purely electromechanical SEPS is presented. Torque estimation methods based on motor armature current measurements are integrated with an SEPS driven by a direct current motor. Strategies for controlling the final tightening torque of the screw fastening process are developed. The control strategies are tested in accordance with the ISO 5393 standard. It is shown that the mechatronic controller gives comparable performance to a SEPS with a mechanical clutch. The mechatronic solution has the advantage of reduced cost, size and complexity. The paper demonstrates the application of low-cost mechatronics in industrial power tools.

An experimental investigation of torque signature signals for self-tapping screws

The results of an experimental study on the self-tapping screw insertion process are presented. A test rig designed and constructed for measuring the torque signature signals during screw insertions is described. Experimental results for torque signature signals during general self-tapping screw fastenings are given graphically, confirming the five-stage variation with screw insertion depth predicted by an earlier theoretical study. The experimental data provide a basis for the further development of empirical and theoretical models of the self-tapping screw fastening process, in order to formulate and put into practice automated monitoring and control strategies.

Protective Approach against Looseness of Screw Fastening due to Impingement of Stress Wave.

Experience has shown that loosening occurs, when stress wave and vibration are emitted to the screw fastening system, even if the tightening is sufficient. In order to clarify the cause of the looseness, the stress wave emitted from bolt shank which constitutes the screw fastening system is made to impinge to engagement conjunction of screw thread, and the problem of the loseness has been studied experimentally. It was proven that seperating the engagement of screw thread of the bolt and nut by the stress wave was one of the causes of the loseness. This paper studies the looseness by emitting stress wave from both of the bolt and the nut. Noticing that soft materials have buffer effect for the impact, the prevention of the looseness is studied by using the soft washer in the screw fastening system.

422 軸直角方向より外力を受けるときのねじのゆるみ挙動に関する研究 : 軸直角半径方向回転外力下でのゆるみ現象について(GS-11 機械要素(2))

Explanation of loosening phenomena of screw fastener subjected to cyclical transverse load has still remained obscure. The present authors carried out experimental studies in the case of the screw fastener subjected to a rotating radial load, and obtained the following facts. Loosening is slightly dependent on the rotating speed. When the radial load rotates anticlockwise, increase in its magnitude beyond some level yields complete loosening. This level is given by the critical force for the nut bearing surface to slip. When the radial load rotates clockwise, loosening is stooped to remain some bolt stress, and this bolt stress is proportional to the load level.

Pull-Out Strength of Steel Roof and Wall Cladding Systems

When steel roof and wall cladding systems are subjected to wind uplift/suction forces, local pull-through/dimpling failures or pull-out failures occur prematurely at their screwed connections. During extreme wind events such as storms and hurricanes, these localized failures then lead to severe damage to buildings and their contents. An investigation was therefore carried out to study the failure that occurs when the screw fastener pulls out of the steel battens, purlins, or girts. Both two-span cladding tests and small-scale tests were conducted using a range of commonly used screw fasteners and steel battens, purlins, and girts. Experimental results showed that the current design formula may not be suitable unless a reduced capacity factor of 0.4 is used. Therefore, an improved design formula has been developed for pull-out failures in steel cladding systems. The formula takes into account thickness and ultimate tensile strength of steel, along with thread diameter and the pitch of screw fasteners, in order to model the pull-out behavior more accurately. This paper presents the details of this experimental investigation and its results.

An anti-loosening screw-fastener innovation and its evaluation

It is widely known that screw fasteners loosen sooner or later, even so-called anti-loosening screw fasteners available in the market do not prevent loosening, as reported in an earlier paper. The purpose of this paper is to present the development of a real anti-loosening screw fastener that has a strong resistance against loosening without sacrificing any of the advantageous features of conventional screw fasteners. Before introducing the development, the mechanism of the loosening process of screw fasteners is investigated. It is shown that the loosening of screw fasteners is caused by two factors. One is relative slip between the bolt and nut screw threads, the slip causing torsion in the bolt. The other is relative slip between the bearing surface of the bolt or nut and the surface of the fastened material, the slip causing slackening of the torsion. The prevention of loosening should be possible if either one of these causes is eliminated. Aiming at eliminating the first cause, the torsion, a new screw named `The Step-Lock Bolt' (SLB) is developed. The developed bolt has eight steps in which the lead angle degree is nil at the circumference of the thread. The efficacy in preventing the initiation and progress of loosening is examined by applying a force cyclically at right angles to the center line of a bolt. The results show that whilst a conventional fastener lost the fastening force within 1000 cycles, the developed SLB does not lose the fastening force until over 30 000 cycles. It may thus be concluded that the SLB is a practical means of preventing screw fastener loosening.

Review of Current Test Methods for Screwed Connections

The pull-through/local dimpling failure strength of screwed connections is very important in the design of profiled steel cladding systems to help them resist storms and hurricanes. The current American and European provisions recommend four different test methods for the screwed connections in tension, but the accuracy of these methods in determining the connection strength is not known. It is unlikely that the four test methods are equivalent in all cases and thus it is necessary to reduce the number of methods recommended. This paper presents a review of these test methods based on some laboratory tests on crest- and valley-fixed claddings and then recommends alternative tests methods that reproduce the real behavior of the connections, including the bending and membrane deformations of the cladding around the screw fasteners and the tension load in the fastener.

Fuzzy logic control of automated screw fastening

In this paper, we present an intelligent control strategy for automated screw fastening. In automated assembly processes, there are often found dedicated stations for various types/sizes of screw fastening. Problems found in current processes include cross-threading, screw jamming, slippage and the need to apply precise torque. The intelligent controller developed here supervises the integrated process of an electric driver mounted on a robotic positioning system to fasten screws. The new scheme controls continuously the motion and driving stages to avoid process-caused failures, to achieve a desired precise torque and to detect bad parts at early stages of the assembly. Initial simulation results are encouraging.

Development of Antiloosening Screw Fasteners.

The purpose of this paper is to present the development of effective antiloosening screw fasteners which are very resistant to loosening without sacrificing any of the advantageous features of conventional screw fasteners. Two factors cause loosening of screw fasteners. One is relative slip between the bolt and the nut screw thread. The other is relative slip between the bearing surface of the bolt or nut and the surface of fastened material. Prevention of loosening could be realized if either one of these causes were eliminated. In order to eliminate relative thread slip, we developed a new screw called the "step-lock bolt". This bolt has 8 steps in one helical circumference. Bolts of size M8 with the new shape were made by the rolling process. The performance of the bolts fastened with conventional nuts was examined and the efficacy in preventing loosening was discussed. The results show that a step-lock bolt fastened with a force of 12kN or higher does not loosen.