1,720,991 research outputs found
Accelerated life testing in mechanical design
Get PDFThe introduction of new products on the market is a time-consuming process, which typically includes both design and testing phases. Often, the experimental validation phase significantly affects the overall process time. Indeed, in many industrial sectors, the product development procedure is based on trial and error methodologies. Intermediate validation tests are performed on full-scale physical prototypes and, based on their outcome, the design is updated (in the case of a negative result) or validated (if the result is positive). The efficiency of this method in terms of time-resources is notoriously sub-optimal.
To improve the efficiency of this process it is, for example, possible to exploit accelerated test methodologies, which consist in subjecting the product to test conditions that exceed its actual working conditions. In this way, a reduction in the number of cycles necessary to bring the component to final failure can be achieved, with obvious beneficial effects on the efficiency of the process. Another way to accelerate the test is to switch from testing the top-level assembly to performing tests on subassemblies or individual components.
It is however mandatory, to ensure that the results obtained with these test methods are useful for the design, to take appropriate precautions. For example, it is essential to preserve the original failure mode of the component. To do this, it is necessary, among other things, to know the relationship between the boundary conditions of the entire assembly and the stresses of the individual components. In the present paper, the methodology described above is illustrated with reference to its application to locking industry components (demonstrator).
Several experimental tests have been carried out, in order to characterize the fatigue life and wear resistance of the materials involved in the construction of the demonstrator.
Numerical FEM models were also developed to determine the stresses of the sub-assemblies and components of the demonstrator during the test phase.
Combining the experimental results with the numerical ones, it was possible to develop an analytical model. Such model allows estimating the endurance of the demonstrator when subjected to accelerated tests. The model has shown a good correlation with experimental results.
The principles underlying this procedure can be applied, without any loss in terms of generality, to many sectors of the industry
Steel screws on aluminium nuts: Different engagement ratio tapped threads compared to threaded inserts with a proper tolerance choice
No full textHigher engagement length over screw diameter ratios (ER), are recommended to avoid thread shearing off in applications, where high strength steel screws are tightened into nuts of materials with lower strength. The aim of this study was to determine the frictional coefficients of bolts for three different engagement ratios, to be compared to those of a commercial threaded insert, whose use is also suggested for the aforementioned application. The tests were run over five repeated tightenings, on M6 class 12.9, black oxidized screws in lubricated conditions. Retightening does not affect friction, whereas ER does and the insert exhibits the highest friction coefficient. The ER threshold to avoid thread stripping was also estimated by experimental, numerical and analytical methods
Coating effect on the fatigue strength of a free cutting steel
No full textThis paper deals with the effect of electrodeposited zinc and nickel coatings on the fatigue strength of a free cutting steel.
This class of materials is frequently used in the mass production, due to their good machinability, in order to reduce the
production costs. Moreover, free cutting steels often operate under high wear conditions, for instance in sliding components. Therefore, suitable coatings are usually applied to improve their tribological response. In addition, in many
industrial applications, free cutting steels must withstand significantly high fatigue loads, even in corrosive environment.
In order to assess the fatigue response, S–N curves and endurance limits at run-out of specimen sets with different
features were determined. The tests involved samples with different coatings and plain geometry for comparison purposes. The experimental results, properly set up by tools of Design of Experiment, were statistically processed and
compared. The outcome was that no significant difference in terms of fatigue strength between the three specimens set
was encountered. It implies that coatings can be suitable to enhance wear resistance without detrimental effects
on fatigue
Effects of aging temperature and humidity on the response of medium and high strength threadlockers
No full textWear behavior of electrodeposited nickel coating on ZP5 zinc alloy
No full textNickel coating on zinc alloy is quite promising for the achievement of an additional wear improvement in corrosive environment. To investigate this point, block-on-ring tests were carried out. Blocks measuring 5 × 5×20 mm3 were obtained from real locking components, made of ZP5 EN 12844 and coated by Zn/Cu1Ni5s ISO 1458. As counter material, an AISI 304 with a 40 mm diameter cylindrical geometry was used. Three load levels (5, 10 and 15 N) and three coating thickness levels (low: 5 to 10 μm; medium: 10 to 15 μm; high: 15 to 20 μm) were considered. The sliding distance was initially set to 500 m. Then, since the coating was worn out very quickly during the test, the sliding distance was set to 150 m. The results, in terms of volume loss due to wear, were processed using the statistical tool of two-way ANOVA and Fisher test. It was assessed that the wear rate is strongly affected by the applied load. The coating thickness does not significantly affect the wear
Influence of the interference level and of the assembly process on the shear strength of Loctite 648 anaerobic adhesive
No full textThe aim of this research is to study the effect of the interference level and of the coupling procedure on the shear strength of Loctite 648 anaerobic adhesive. The tests involved shaft-hub joints with a quite large interference range and joined by a broad range of coupling strategies, which is one of the main issues of novelty of this research. 42CrMo4 tempered steel shaft specimens and 16CrNi4Pb hardening steel hub samples were used. Two factors were considered: the interference level and the assembly strategy. Four different levels of specific interference were analyzed up to a maximum value of 0.22%, performing five replications per level. Experimental trials were carried out with different coupling procedures: using press-fitted and shrink-fitted joints. Using the first procedure, a high level of interference induces scratches on the sample surfaces. These are conversely not produced by shrink-fitting. Some tests were devoted to friction coefficient estimation, thus also assessing the lubricating properties of the adhesive. Particularly, dry couplings and couplings and de-couplings with not yet cured adhesive were run to address this point. The results were processed by statistical methods, to assess the influence of the studied factors on the adhesive strength
Threaded fasteners with applied medium or high strength threadlockers: effect of different tightening procedures on the tribological response
Get PDFThreaded joints are widely used in the automotive industry, where threadlockers can be applied to prevent self-loosening and to possibly achieve a friction drop upon tightening. This study deals with this topic, investigating threadlocker tribological response before polymerization takes place. These lubricating properties are usually not considered in adhesive datasheets, but can be highly beneficial from the point of view of the achievable preload, thus preventing selfloosening. The topic was tackled experimentally, running an extensive campaign that involved a medium and a strong threadlocker (LOCTITE 243 and LOCTITE 270, respectively), used for tightening zinc-plated and black-oxidized screws. The testing procedure was inspired by a protocol being used by most manufacturers in the automotive field: in particular, the impacts of pre-tightening speed and of the time interval between pre-tightening and final tightening were investigated. The retrieved results indicate a lubricating effect of the medium threadlocker with achievable friction coefficients in the order of 0.11. Moreover, for both the threadlockers, a generally decreasing trend for the thread friction upon tightening for increasing time interval before tightening was observed. This is a particularly original outcome that could have remarkable implications for optimizing the assembly tasks of large sets of bolts
Experimental Measurement of the Shank Torque As a Function of the Stiffness and Frictional Characteristics of the Bolted Joint
No full textDuring tightening, the amount of torque given by the difference between the tightening torque, which is directly applied by the torque wrench, and the underhead torque, flows through the screw shank towards the threaded portion. This torque combines with the axial preload to bring about the overall stress state of the screw at tightening. Upon release of the torque wrench, a certain amount of the shank torque is released due to the elastic springback of the screw-plates system. In the literature [1], this phenomenon is just briefly treated by a few authors: they generally agree that approximately a half of the initial shank torque is released just a few seconds after torque wrench removal. This indication is given regardless of the frictional [2] and stiffness [3] parameters, which govern the joint. The present contribution aims at assessing, if there is any effect of the following parameters on the amount of shank torque being released after wrench removal: (i) the ratio between the torsional stiffness of the screw and of the plates, (ii) the friction coefficients in the underhead and in the thread. The experimentation has been run on a M20 8.8 class socket head screw, which has been instrumented by a double array of strain gauges, to simultaneously measure both the axial preload and the torque acting on its shank. Two different types of joined members have been examined: a cylindrical sleeve whose diameter is twice the screw diameter (compliant joint) and a rectangular plate whose transverse dimensions are more than ten times larger than the screw diameter (stiff joint). The underhead and thread friction coefficients have been controlled by properly selecting lubrication conditions. The main outcome of the work is that the torsional stiffness of the joined members does have an impact on the residual shank torque. A simple mathematical model has also been implemented, in order to predict the residual shank torque during the design phase
Comportamento tribologico di collegamenti filettati per applicazioni industriali
No full textIl presente gruppo di ricerca si occupa da anni di collegamenti filettati: questa breve sintesi parla di tre filoni portati avanti negli ultimi tempi. Il primo fa riferimento a collegamenti fra guide in acciaio e carcasse in lega di alluminio, frequentemente impiegati nelle più moderne macchine utensili, in cui viti in acciaio vanno a mordere su madreviti in lega di alluminio. Un secondo filone ha riguardato l’effetto di frenafiletti commerciali sulle prestazioni tribologiche di viti al serraggio. Infine, il terzo studio concerne l’applicazione di pallinature, per migliorare il comportamento all’attrito di viti per testa di biella, valutando anche l’effetto di serraggi multipli
A Methodology for the Lightweight Design of Modern Transfer Machine Tools
Get PDFThis paper deals with a modern design approach via finite elements in the definition of the main structural elements (rotary table and working unit) of an innovative family of transfer machine tools. Using the concepts of green design and manufacture, as well as sustainable development thinking, the paper highlights the advantages derived from their application in this specific field (i.e., the clever use of lightweight materials to allow ruling out high-consumption hydraulic pump systems). The design is conceived in a modular way, so that the final solution can cover transfers from four to 15 working stations. Two versions of the machines are examined. The first one has a rotary table with nine divisions, which can be considered as a prototype: this machine has been studied in order to set up the numerical predictive model, then validated by experimental tests. The second one, equipped with a rotary table with 15 divisions, is the biggest of the range: this machine has been entirely designed with the aid of the previously developed numerical model. The loading input forces for the analyses have been evaluated experimentally via drilling operations carried out on a three-axis CNC unit. The definition of the design force made it possible to accurately assess both the rotary table and the working units installed in the machine
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