Preload Loss Research Articles

A comparative study on the effectiveness of system parameters in monitoring pre-load loss in bolted joints

This article presents the monitoring of bolt loosening in a single bolted lap joint between two steel beams. The bolt loosening is initially studied using system modal parameters for various boundary conditions. Natural frequency and modal damping ratio in lower modes were not found to be reliable. Transmissibility function was seen to have a more reliable trend to identify the state of the joint. An in-house setup was developed to produce loosening in the bolted joint. Testing configuration was optimized by structural analysis. Strain measurements were carried out using Fiber Bragg-grating sensors while sinusoidal load was applied on the test structure. The results have been used to discuss a viable approach to identify the health of, or recognize a loosening trend, in a bolted joint.

Viscoplastic deformation behaviour of preloaded stainless steel connections

Preloaded bolted connections made of stainless steel are not commonly used in stainless steel structures as their application is not allowed by the execution standard EN 1090–2 and the design standard EN 1993-1-4 for stainless steel structures unless otherwise specified, respectively unless their acceptability for a particular application has been demonstrated from test results. This restriction is mainly caused by three facts: firstly, it is feared that due to the viscoplastic deformation behaviour of stainless steel, severe preload losses have to be expected, secondly, neither stainless steel bolting assemblies for preloading nor tightening procedures exist on which could have been relied and thirdly, galling and seizure of stainless steel bolting assemblies lead to problems on site. These three questions, beside others, were treated in the frame of the European RFCS-research project “Execution and reliability of slip resistant connections for steel structures using CS and SS” SIROCO. Some of the results are presented in this contribution with the main focus on the loss of preload of preloaded stainless steel bolted connections. The main conclusion is that preloaded stainless steel bolted connections can be treated similar to those made of carbon steel with regard to preload losses as they show comparable magnitudes of preload losses.

Slip factors for slip-resistant connections made of stainless steel

Stainless steels are becoming more and more popular as a construction material in both building and civil engineering structures, because of their high material strength, ductility and corrosion resistance. In this frame, the execution of slip-resistant connections made of stainless steel is necessary, e. g. in footbridges, facades etc. As no design regulations exist in any code, special experimental testing has to be performed in each case. Stainless steel alloys are thought to suffer more than carbon steels from time dependent viscoplastic deformation at room temperature. It could lead to higher preload losses and consequently to lower slip factors than achieved for carbon steels with comparable surface treatment. However, no evidence of this behaviour can be found in literature for preloaded bolted connections. For this reason, slip factors for the various stainless steel grades have to be determined experimentally if they want to be used in the steel structures. In the frame of the European RFCS-research project SIROCO, the preloading behaviour of stainless steel bolted connections as well as the slip-resistant behaviour of slip-resistant connections made of austenitic, various duplex and ferritic stainless steels have been investigated. In this paper, the results of slip factor tests on four stainless steel grades are presented and the influence of different surface treatments and the preload level on the slip factor of stainless steel slip-resistant connections is discussed. Finally, slip factors for various stainless steel grades with different surface treatments are proposed for a future implementation in the next revision of EN 1090-2.

Geometric accuracy long-term continuous monitoring using strain gauges for CNC machine tools

The accuracy retentivity has become one of the most important performance index of CNC machine tools, which are affected by many complicated factors, such as stress-induced deformations, bolt preload loss, wear, and the manufacturing process itself. The long-term continuous monitoring can bridge the wide gap between the accuracy retentivity and the manufacturing processing of machine tools. In this research, a geometric accuracy monitoring approach using the discrete strain gauges is proposed. The machine basis is firstly simplified based on the structural characteristics and stress condition. And, the straightness is evaluated through the reconstructed strain field of machine tool basis. Finally, to verify the validity of our method, a vertical machining center has been monitored for 7 months using the established wireless monitoring system.

Time–temperature-dependent response and analysis of preload relaxation in bolted composite joints

Although mechanically fastened joints are generally considered a mature technology, significant problems still exist with their use, among which the preload loss in pretensioned bolts is a major one. This paper has investigated two key issues in structural durability design for mechanically fastened composite joints, i.e. temperature–time-dependent behavior for viscoelastic preload relaxation and prediction method for its long-term performance. Towards this purpose, a two-phase experimental program was conducted consisting of short- and long-term joint relaxation tests. Phase I monitored the short-term fastener preload relaxation (up to 36 h), with a special self-made bolt strain sensor. Phase II utilized the time–temperature superposition principle to establish a method of using short-term relaxation curves to construct the long-term master curve (accelerated test). It was found that there was a significant loss of bolt preload when temperature and initial preload increased, and the relaxation rate of composited joint was much larger than that of the metallic joint. Based on the strong temperature dependence of bolt relaxation, accelerated characterization was conducted for long-term behavior. Furthermore, to quantitatively relate the relaxation data to a model equation, a new preload prediction model was developed based on the creep total-strain theory. Comparisons of the prediction model and accelerated test results indicated that the improved model provided better accuracy than existing models such as Shivakumar–Crews’s and Hook-Norton’s models for describing the long-term preload behavior.

Systematic analysis of factors that cause loss of preload in dental implants

Screw loosening is the most common factor associated with dental implant failure. One of the major cause for screw loosening is the “loss of preload”. Several factors including screw geometry, material properties particularly stiffness, surface texture and condition of mating surfaces, degree of lubrication, rate of tightening, integrity of joint etc.Objective:This review analyses the factors that are responsible for the loss of preload.Material and Methods:Screw geometry, Implant- Abutment Connection type (external hexagon platform, morse taper), Material properties viz Stiffness, Resilience, Materials viz gold, titanium, titanium alloy, Surface texture of the abutment screw, Condition of mating surfaces, Lubrication, Torque value, Rate of tightening (10, 20, 35N and retorque after 10mins) are taken into consideration in this study. The MEDLINE-PubMed database was searched from September 2016 to 10 years previously. Several journals were hand searched and from cross references. The outcome analysed are the factors that are responsible for loss of preload.Results:The search yielded 84 articles. After excluding duplicated abstracts and applying the inclusion and exclusion criteria, 36 studies were eligible for analysis. The result shows that loss of preload can occurs depending upon the type of material used, torque method, torque sequences, abutment connection type, influence of lubrication, abutment collar length. However we detected some potential limitations in the studies selected, mainly a minimum number of samples used for the study. Hence we suggest further studies to guarantee an excellence in methodological quality.Conclusion:Based on the available data it can be summarized that the knowledge of preload loss must be known for the clinicians to avoid such screw loosening and subsequent implant failure.

Some factors affecting the loosening failure of bolted joints under vibration using finite element analysis

Finite element analysis has been regarded as an effective research method for analyzing the loosening failure of bolted joints under vibration. However, there exist some factors, which influence the accuracy and reliability of loosening results, thus determining the explanations of the loosening mechanism. In this study, a 3D finite element model of a typical bolted joint was built to investigate the effects of several different factors on the loosening under transverse vibration loading. These influencing factors include preload generation, vibration parameter, and material model. Based on the simulation results, it was found that applying the method of pretension element to generate preload instead of the actual method of torque was reliable and efficient. For the vibration parameter, it showed that the decrease rate in preload was higher for a larger vibration amplitude. But once the bearing surface reached complete slip, the loosening rate would keep constant. This was because the thread surface at that time reached a sticking state. Vibration frequency was proved to have no effect on the loosening behavior. This result demonstrated that the quasi-static assumption for vibration frequency was reasonable. Additionally, it also indicated that plastic material models only affected the preload loss in the initial several vibration cycles and had no influence on the loosening rate of preload after several vibration cycles. Finally, experiments were conducted to confirm qualitatively the results obtained based on finite element analysis.

Preload losses in stainless steel bolting assemblies

AbstractStainless steel material is a suitable choice for modern steel constructions as it has a high resistance to corrosion combined with high material strength and ductility. Furthermore, its use leads to significant reductions in maintenance. In this frame, bolted connections made of stainless steel components become more and more important to enhance the application of stainless steel not only to small parts of steel structures but also to complex structures. Whereas non preloaded stainless steel bolted connections are already widely used, according to EN 1090‐2, the application of preloaded stainless steel bolting assemblies is not allowed unless otherwise specified. If they shall be used, they shall be treated as special fasteners and a procedure test is mandatory. Also EN 1993‐1‐4 requires that their acceptability in a particular application has to be demonstrated from test results. These restrictions are mainly caused by two facts: firstly, the viscoplastic deformation behaviour of stainless steel which might result in not negligible preload losses in the bolting assemblies themselves and secondly, the gap of knowledge regarding suitable tightening parameters and procedures for stainless steel bolting assemblies to secure a required preload in the bolting assemblies and to avoid galling. To solve these questions, research activities have been carried out in the frame of the European RFCS‐research project ”Execution and reliability of slip resistant connections for steel structures using CS and SS“ SIROCO. The present contribution gives an initial insight into the viscoplastic deformation behaviour of stainless steel bolting assemblies which were achieved in SIROCO which shows that preloaded bolted stainless steel connections can be treated similar to those made of carbon steel.

Slip‐resistant bolted connections of stainless steel

AbstractThe use of stainless steel components can lead to a significant reduction of maintenance costs compared to a structure executed in carbon steel. Because of its high material strength, ductility and corrosion resistance stainless steels are becoming more and more popular as a construction material in both building and civil engineering structures. Consequently slip‐resistant bolted connections made of stainless steel are becoming more important. Slip‐resistant bolted connections are used in joints where slip is not acceptable (because they are subject to reversal of shear load or any other reason) or in joints that are subject to cyclic shear load (to improve the fatigue class of the connecting plates). Existing design codes/standards do not specify slip factors for surface treatments of stainless steel grades, the minimum values of slip factors for common surface treatments/coatings that are specified in EN 1090‐2 are exclusively valid for carbon steels. One of the reasons for this is that stainless steel alloys are thought to suffer more than carbon steels from time dependent behaviour (creep and relaxation) at room temperature. This could lead to higher preload losses and consequently to lower slip factors than used for carbon steels with comparable surface treatment. However, no evidence of this can be found in literature. Creep and relaxation are stress dependant phenomena and the stresses in the components of preloaded bolted connections are locally highly non‐uniform. Therefore, slip factors of different stainless steel grades have to be determined by experiments to investigate the effects of time dependant material behaviour. In this paper the results of slip factor tests on four stainless steel grades are presented and the influence of surface treatments and the preload level on the slip factor of stainless steel slip‐resistant connections is discussed.

01.20: Preloading of stainless steel bolting assemblies

ABSTRACTIn modern steel constructions, the use of stainless steel becomes more important, especially in corrosive environments. But: the use of stainless steel bolting assemblies is restricted to non preloaded assemblies by EN 1090‐2 due to several reasons. Stainless steel tends to creep and relaxation which might result in severe preload losses in bolting assemblies. In addition, the susceptibility of stainless steel to galling respectively cold welding causes problems in bolting assemblies because of undefined coefficients of friction. Herewith, qualification procedures are necessary for every possible application scenario. Within the European RFCS‐research project “Execution and reliability of slip resistant connections for steel structures using CS and SS” (SIROCO), stainless steel components and stainless steel bolting assemblies made of austenitic, super duplex, duplex and lean duplex materials have been investigated in tightening tests according to EN ISO 16047 and EN 14399‐2. Furthermore, relaxations tests of bolted connections with bolts according to EN ISO 4014 and EN ISO 4017 have been carried out. The presented paper intends to provide an initial insight into the behaviour of stainless steel bolts based on first results achieved from SIROCO. Regarding the tightening tests, in a first step the evaluation was chosen similar to EN 14399‐2 which specifies essential properties and characteristics of carbon steel bolting assemblies for preloading which should be fulfilled for tightening. The objective of the investigations will be to define a secure preload level and a corresponding tightening procedure for preloaded stainless steel bolting assemblies. The tightening tests are compared with results of the well‐known carbon steel preloading systems HR according to EN 14399‐3 and HV according to EN 14399‐4. Within this contribution, the so far achieved results of the tightening and relaxation tests will be summarized and discussed. For now, the susceptibility to galling and the tendency to creep and relaxation can be confirmed even though galling can be significantly reduced by the use of suitable lubricants which could be determined by experimental testing. Furthermore, secure preload levels could be defined depending on the stainless steel grade and strength class.

A year-long monitoring of preloaded free-maintenance bolts – Estimation of preload loss on BobTail bolts

Abstract In order to allow an increase in the height of the towers to exploit stronger and more stable wind profiles, a welded-free friction connection was proposed for the new generation of wind towers. This geometry requires a reliable estimation of the bolts’ performance over time and a free maintenance bolt is demanded since no access to the outside of the tower is possible. This paper illustrates the monitoring of such a bolted connection prototype with focus on the behaviour of free maintenance BobTail® bolts, which allows the tightening from the inner side of the tower. The monitoring and identification procedure for the 1-year monitoring of the force and the temperature in a prototype is presented. A calibrated nonlinear Hammerstein-Wiener model is used to withdraw the temperature effect from the analysis allowing for a long-term estimation of the preload losses. This model is able to simulate the force with less than 2.5% of error. The significant part of the preload loss was foreseen within the first two years. Furthermore, the influence of the tightening temperature on the bolts loss was discussed and, as expected the lower the temperature during tightening, the lower the long term preload losses.

Preload loss of abutment screws after dynamic fatigue in single implant-supported restorations. A systematic review.

BackgroundTo carry out a systematic literature review of the causes of preload loss of the abutment screws, of internal and external connection implants, tightened to different torque values and subjected to cyclic loading.Material and MethodsA systematic search was conducted in PubMed, EMBASE, and Cochrane Library databases with reference to in vitro studies in which internal and external connection implants were subjected to cyclic loads to determine the degree of loosening of the abutment screws after loading.ResultsThe reviewed studies tested distinct implant connections (mostly externally hexed, and morse taper) subjected to diverse cycles (from 16667 to 1 million), with loads ranging from 0-400 Nw, using screws of different materials and designs that were tightened into torques between 20-45 Ncm, Accordingly after loading the percentage of torque loss ranges between 16.1% to 39%.ConclusionsMost of the studies indicate that the internal connection, together with the morse taper, best resists cyclic loading in terms of screw loosening in single-tooth implants. Key words:Dental Implants, Dental Implant-Abutment Design, Torque, In Vitro Techniques, Systematic Review.

An investigation of preload relaxation behaviour of three zinc- aluminum alloys

Zinc alloy castings are usually assembled together or mounted by screwed steel fasteners, and are tightened to a predetermined torque to develop the required tensile preload in the fastener. Due to relaxation processes in the castings, creep may cause a partial preload loss at an elevated temperature. The equipment used for load relaxation tests consists of a loadmonitoring device, an oil bath, and a data-acquisition system. A load cell monitoring device is used to monitor the load loss in an ISO-metric M6*1 steel screw set into sand castings made from alloys No. 3, No. 5 and No. 2 and tightened to produce an initial preload of 6 kN. The castings were held at constant temperature in the range 80 - 120°C in an oil bath. The oil bath maintains the desired test temperature throughout the experiment. All tests were conducted for periods of up to 160 h. For all alloys, the initial load loss was high, decreasing gradually with time, but not ceasing. The load loss increased rapidly with test temperature, and almost all of the relaxation curves approximated to a logarithmic decay of load with time. Alloy No. 2 had the best resistance to load loss, with No. 5 next and No. 3 worst at all temperatures. The lower resistance to relaxation of alloy No. 3 was mainly due to the lower relaxation strength of copper-free primary dendrites, whereas in alloys No. 5 and No. 2, the higher copper contents contribute greatly to their relaxation strength in the form of second-phase particles.

A quantitative investigation on vibration durability of viscoelastic relaxation in bolted composite joints

Time-dependent behavior and factors affecting preload relaxation in a carbon/epoxy composite bolted joint under resonance were studied. The effect of viscoelasticity of composite material on bolt relaxation was studied quantitatively through modal analysis from the perspective of energy dissipation and stiffness degradation. Damping ratio and resonance frequency were utilized to characterize the effects of preload relaxation on structural dynamic response. The loss of preload was found to decrease with increasing initial preloads over a 10 h vibration fatigue. However, an increase in preload loss occurred as exciting frequency increases. Vibration fatigue damage was found to result in decaying stiffness and amplitude responses of the bolted joints, along with an increase in damping ratio. As a proof-of-concept study, a beam-like specimens with and without bolted joints were comparatively excited to ascertain their respective dynamic responses; results revealed that relaxation in bolted joints could be attributed to the conjunct mechanisms between viscoelastic behavior of polymer matrix composites and interface friction for different contact surfaces, where such relaxation behavior was mainly due to viscoelasticity of the joint materials.

Investigation of a friction-grip connection in laminated glass

Friction-grip bolted connections are theoretically well suited for the introduction of in-plane loads in glass structures because they distribute the load over a relatively large area and thus avoid critical stress concentrations. However, the performance of friction-grip connections for laminated glass may be jeopardised over time due to viscoelastic behaviour of the interlayers and gaskets used. More specifically, the latter may lead to preload losses in the connection. However, only a few technical papers have dealt with the use of friction-grip connections in laminated glass. Experimental tests were thus performed to study the time-dependent behaviour and the slip resistance of an existing connection. The latter consisted of a polyvinyl butyral laminated glass plate bolted to steel skew plates with preloaded bolts. Different materials (e.g. styrene butadiene rubber, nylon and polyoxymetheylene(-copolymer)) were investigated for the gasket, used to avoid direct contact between glass and steel. Specimens with and without a compression ring laminated between both glass panels were compared. The investigation very clearly demonstrated the importance of gasket design as well as the negative effect of interlayer creep on the performance of the investigated friction-grip connection.

Predicting the Displacement Gain from the Mechanical Quality Factor in Ultrasonic Transducers

The displacement gain is the most important performance parameter for power ultrasonic transducers typically used for welding or cutting: it controls the proportional relationship between the displacement of the tool and the voltage or current input to the transducer, a key process parameter. However, due to the aging effects of the PZT piezoceramics typically used in these transducers, and other variables such as gradual preload loss or tool clamp wear, this displacement gain can drift over time causing a shift in process, and loss of machine-to-machine portability in mass production environments. The “re-calibration” of the displacement gain usually involves a time consuming procedure of standardized controlled tests, and/or measurements using an expensive device such as a laser vibrometer. However, elementary engineering vibrations theory asserts that the displacement gain should be proportional to the static displacement (i.e., 0Hz or DC) and the mechanical quality factor Qm at resonance, derived from a simple Bode plot, which is already familiar to most transducer designers. This research investigates the methods for obtaining the mechanical quality factor from Bode plots (e.g., constant current or constant voltage sweeps), and ring-down techniques using logarithmic decrement, based on their predictability for determining the displacement gain via the static displacement. The investigation focuses solely on welding transducers for semiconductor wire bonding which employ common hard PZT4 or PZT8 piezoelectric materials. Several other metrics are investigated such as impedance, capacitance and electro-mechanical coupling factor. The experimental and theoretical research methods include Bode plots, equivalent circuits, mechanical analogies and scanning laser vibrometry.

Evaluation of Shape Memory Alloy Bulk Actuators for Wear Compensation in Ball Screw Drives

In this paper we present an approach to reset a preload loss in ball screw drives which enables to compensate losses of positioning accuracy due to wear over lifetime. The main purposes are reduced maintenance, longer service life and improved workpiece quality by regenerating the desired preload conditions which leads to a higher profit for manufacturers. For this purpose, a novel shape memory alloy (SMA) bulk actuator module for ball screw drives is designed. The device is developed to be implemented in standard drives without major changes in component design. It is installed between flanged nut and counter nut and replaces a spacer usually needed to set a defined preload. To achieve the required actuator performance SMA bulk actuators are investigated in detail. This covers their basic design and activation concepts. A prototype is designed and experimentally investigated; measurements show the proof of the concept..

Influence of Cyclic Fatigue in Water on Screw Torque Loss of Long-Span One-Piece Implant-Supported Zirconia Frameworks.

It is still unclear whether four, six, or more implants should be used when restoring fully edentulous maxillae. This research evaluated the in vitro screw torque loss of zirconia frameworks supported by six implants and cantilevered zirconia frameworks supported by four implants. Computer aided design/computer-assisted machining was used to fabricate 10 one-piece frameworks. Standardized pressable porcelain crowns were fabricated and luted to the frameworks. Specimens were divided into two groups (n = 5): AO4, cantilevered 12-unit full-arch fixed dental prosthesis supported by four implants; AO6, 14-unit supported by six implants. An opposing mandibular dental arch was fabricated with bis-acrylic composite resin. Specimens were submitted to 200 N underwater cyclic load at 2-Hz frequency for 1 × 106 cycles in a controlled 37°C temperature. A digital torque gauge assessed the initial and postload screw removal torque. Linear mixed-effects model was used for statistical analysis (α = 0.05). Significant screw torque loss was found for AO6 after cyclic loading (before: 36.20%/after: 52.82%; p < 0.05). Group AO6 (36.20%) presented lower preload loss before the cyclic loadings compared with AO4 (60.10%) (p < 0.05). Cyclic loading and lower implant-to-replaced-units ratio do not seem to compromise screw stability compared with higher implant-to-replaced-units ratio; however, a steep drop in preload was found before cyclic loading for both groups.

AN EXPERIMENTAL STUDY ON THE RELAXATION OF BOLTS

Loss of pre-load with time, commonly known as ‘Relaxation’ is an established phenomena. Behaviour of a bolted joint depends upon the pre-load in the bolts in use, not the pre-load introduced by the mechanic. Loss of pre-load is expected due to the many factors such as embedment relaxation, gasket creep, elastic interactions, and vibration loosening or stress relaxation. In a gasketed joint, due to the gasket flexibility, relaxation is always substantial during preliminary passes, as 80 to 100% loss is not uncommon in almost all the bolts, resulting in a dynamic behaviour. Pre-load in a gasketed joint is stabilized and retained to certain extent in the final passes only. In a non-gasketed joint, due to no gasket and no rotation its static behaviour is concluded. This paper highlights the factors affecting the amount of relaxation with time and presents important considerations that can reduce this. Both the short and long term relaxations are recorded and a ‘best fit’ model for relaxation behaviour is derived.

20614 円すいころ軸受の予圧抜けに関する研究

20614 円すいころ軸受の予圧抜けに関する研究