1,721,061 research outputs found
Effect of friction, surface treatment and adhesive-reinforcement on the behavior of threaded connections. Effetto dell'attrito, dei trattamenti superficiali e dell'incollaggio sul comportamento dei collegamenti filettati
Full text linkThis study aims at analyzing the phenomena which characterize threaded connections. This type of joint has been known for thousands years (one of the first applications is attributable to Archimedes) allows transforming a torque into an axial force, typically used to lock two or more components thanks to the friction forces that arise between the connected bodies. One of the main strengths of this type of coupling is the possibility to assemble and disassemble the assembled parts; in addition, it can be applicable to almost any type of material. However, the joint system is characterized by a low efficiency: only a modest amount of the tightening torque is actually converted into axial force. In addition, the obtained axial force is typically affected by a large scatter. This behavior is due to the combined effect of two different friction effects that arise between the under-head of the screw (or nut) and the surface of the elements to be locked in the first case, and between the screw threads and female screw in the second one. The relationship between tightening torque and preload axial force is very sensitive to the effect of the various friction coefficients that rule the tribological behavior of the joint:, these coefficients have a significant variability due to the processes used to manufacture the components, and to the different tightening parameters, such as the presence of lubrication or the tightening speed. Threaded connections do have a wide array of applications, both in the field of civil engineering and in mechanics. For instance, in the automotive compartment, due to a steadily growing strive towards weight reduction and increased automation of assembly procedures, an accurate knowledge of the performance of threaded connections has recently become critical in order to achieve the safety of the junction at an adequate production cost. In order to control a threaded connection, it is necessary to focus on the three main factors that characterize its behavior: the coefficients of friction at tightening, the self-relaxation phenomenon (decrease of axial preload in the first few seconds after tightening, due to sinking of the load bearing area in the clamped parts), self-loosening (loss of axial preload, typically due to the action of external shearing forces). In the following, literature review will be presented first, then it will be shown an experimental investigation on the effect of solid lubrication on the friction coefficients, carried out for repeated tightenings and different surface treatments. Moreover, the effect of the number of engaged threads on the performance of an anaerobic threadlocking adhesive will be examined. The need for these analyses arose from the study of an in-field failure occurred in a pinion shaft of a transmission for earth moving machines, which, as it will be shown in the following, probably failed due to a combination of the aforementioned typical issues of threaded components. The research has the aim of increasing the knowledge about the effect of lubrication on the different phenomena that contribute to determining the performance of a bolted connection
Impact of bolt positioning on the stiffness of angular support brackets
Full text linkBolts and screws are usually preferred over other joining techniques when assembly and disassembly operations are required. However, they add extra weight to the system, so it is essential to reduce their size and weight by optimizing bolt parameters. Additionally, the stiffness of bolted members is very crucial; those with low stiffness may affect the correct functioning of the other connected components. This study focuses on the impact produced by bolt positions, specifically their center of gravity and spacing among them, on the stiffness of angular L-shaped brackets using Finite Element Analysis. Bending and torsional loads were applied to the free end of the member and deformations were recorded under these loads. The results revealed that a triangular bolt pattern is recommended for three bolts, whereas for four or more bolts, the pattern should be uniform to avoid stress concentrations. Moreover, the center of gravity and bolt spacing have a significant impact on stiffness. By optimizing bolt spacing and the center of gravity, a combination of six bolts can be replaced with four bolts of the same size while maintaining the same stiffness
Tribological properties of bolts depending on different screw coatings and lubrications: An experimental study
No full textThis paper is focused on the experimental determination of the frictional properties of bolts, following the ISO 16047 International Standard. The campaigns involved M14 X 2 8.8 class hexagonal head steel screw. Different Design of Experiment techniques were applied to investigate the effects of screw coating and lubrication, along with their interaction. The effect of lubrication was then investigated in further details, comparing two different lubricants and the outcomes of partial lubrication on the underhead or in the threads only. The results indicate that both surface coating and lubrication are highly significant. Regarding lubrication, a ceramic paste by Interflon proved to be highly effective at reducing friction, in particular if applied at the underhead rather than in the threads
Influence of the engagement ratio on the shear strength of an epoxy adhesive by push-out tests on pin-and-collar joints: Part I: Campaign at room temperature
No full textThis paper focuses on an epoxy adhesive (LOCTITE 9466), which is particularly suitable for applications involving different materials and where a clearance is present between the adherents. The investigated subject is concerned with the effect of the Engagement Ratio (ER, coupling length over coupling diameter) on the shear strength of LOCTITE 9466 at room temperature. Motivations arise from the increasing interest in epoxy-adhesive joints in lightweight structures and from the consequent need for design data. Decoupling tests have been performed on pin-and-collar samples manufactured according to current Standards. The height has been adjusted in order to explore a sufficiently wide ER range at four different levels. The results have been processed by the tools of the Analysis of Variance and of the Fisher test to investigate the significance or the not significance of ER on the joint shear strength. The final outcome was that ER significantly affects resistance at a very high confidence level. This result has then been refined by the tool of orthogonality, in order to allocate the differences among the four levels of ER. This further analysis has shown that the joint strength is significantly enhanced, when ER exceeds 1 and assumes values around 1.3 or higher
Influence of the engagement ratio on the shear strength of an epoxy adhesive by push-out tests on pin-and-collar joints: Part II: Campaign at different temperature levels
No full textPrevious research led to the conclusion that the Engagement Ratio (i.e. the coupling length over the coupling diameter, ER) does not significantly affect the shear strength of an anaerobic adhesive (LOCTITE 648). Conversely, ER is effective on the response of an epoxy adhesive (LOCTITE 9466), with a beneficial effect for ER>1. The aforementioned campaigns have been performed at room temperature, whereas, the effect of ER combined to that of temperature is still unexplored. The subject of this paper consists in the experimental investigation of the impact of ER on the strength of LOCTITE 9466 at higher temperatures. Decoupling tests have been performed, considering three levels of temperature (40 °C, 60 °C and 80 °C). Pin-and-Collar samples have been prepared, considering four levels of ER. A fixture device has been designed, to prevent misalignments and to reduce heat dissipation during the pushing-out phase.
The statistical processing of the data led to the conclusion that ER retains its effectiveness up to the temperature of 40 °C with strength enhancement for ER beyond 1. Conversely, at the highest levels of temperature, a strength drop to approximately 44% occurs, and the effect of ER is no longer significant to compensate this decrease. Moreover, a highly significant negative interaction was detected between ER and temperature
A user-friendly computational algorithm for the structural analysis of wrapping machine rotating rings
No full textWrapping machines usually consist of a two- or a four-column frame, supporting a huge rotating ring, connected to a prestretch unit with film coil carrier. Stiffness is a key point of packaging machines, since it is strictly related to the accuracy of the wrapping task. It depends on the stiffness of the frame, which can be achieved by the four-column architecture, and on the ring constraint system. As a consequence, the ring structures are usually highly statically indeterminate. Nowadays, there is an increasing demand for higher rotational speeds and more reduced operation times. Therefore, an accurate structural analysis of the ring, considering its actual loading and constraints is more and more important. The structural analysis of the rotating ring is treated by many references; however, such a statically indeterminate constraining makes this problem highly complicated. The goal of this paper consists in the development of a general and original computational algorithm for the structural analysis of rotating rings. The results are collected in a user-friendly way in terms of normalized internal loads, so that they can be of a great help even for not expert users. This model has been experimentally validated and easily applied to case studies and optimization tasks
EFFECTS OF SCREW SURFACE TREATMENTS AND OF DIFFERENT LUBRICATIONS ON THE FRICTIONAL PROPERTIES OF BOLTS
No full textFATIGUE CHARACTERIZATION OF ALUMINIUM WELDED JOINTS
No full textThis contribution deals with the fatigue characterization of aluminium welded joints for
automotive applications. The base material is a vacuum casting alloy, whereas the joints have
been hand welded by means of the TIG process. The joint category is a butt-welded type, one
side only, full penetration without backing. The samples have been tested both in tensiontension
and in bending. Tests results have been processed according to the ISO 12107
standard: SN curves have been retrieved as well as the endurance limit of the joints at 5·106
cycles
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