Stefano Fini

The influence of lubrication on the frictional characteristics of threaded joints for planetary gearboxes

Planetary gearboxes generally consist of a ring gear, or gear body, connected with the input and output flanges by means of several screws, equally spaced along the diameter. The ring gear is manufactured with steel, whereas the flanges are usually made of cast iron. These screws must provide axial preload between the parts, allowing the assembly withstanding the breakaway torque given by the difference between the output and input torque applied to the gearbox. For a given screw geometry, the axial preload can be calculated, provided that the friction coefficients in the thread and in the underhead are known. Most often, the tightening torque is the only parameter being controlled during assembly and service operations. Hence, it is mandatory to know the friction coefficients of the joint. These depend, among others, on the hardness, roughness and texture of the mating surfaces, as well as on the lubrication state of the joint. In fact, the addition of a lubricant modifies the tribological behavior of the joint, thus the wearing evolution of the surfaces across repeated tightening operations. The present work tackles the following two aspects: (i) the characterization of the preloading force–tightening torque relationship on the actual component by means of a dedicated specimen, (ii) the evaluation of the influence of lubrication on the evolution of the frictional characteristics of the joint across several re-tightening operations. The present work has been carried out by means of both numerical finite element analyses and experimental stress analysis techniques.

Analysis of the Influence of Fretting on the Fatigue Life of Interference Fitted Joints

The present paper deals with the fatigue behaviour of shaft hub interference fitted joints. The investigation is carried out by means of a down-scaled ad-hoc specimen, which has been developed by the authors in order to perform accelerated fatigue tests. The specimen is similar to those suggested by standard ISO 1143, but it consists of two parts, joined together by shrink fitting. In a previous work, the authors compared the performances of a plain specimen manufactured according to ISO 1143 and those of the shrink fitted specimen. Both the plain and the shrink fitted specimens were made of C40 EN 10083 low carbon steel. The outcome of that research was that shrink fitting determines a degradation of the fatigue response of the assembly, with respect to those of the plain specimen. Moreover, such decrease cannot be predicted by means of FEA alone, since it is partly due to fretting phenomena. In fact, fretting takes place on the mating surface between the shaft and the hub, and especially in the vicinity of the end of the contact. The present paper deals with the observation of failed and survived specimens by means of optical and SEM microscopes, in order to determine the actual tribological characteristics of the contact surface. For instance, the amplitude of the sliding zone observed experimentally is compared with that given by FEA for different choices of the contact formulation. Since fretting is often associated with the presence of secondary fatigue cracks, which do not propagate, the authors set up an experimental method for locating the secondary cracks prior to sectioning the specimen for microscope observation.Copyright

Design of a cutting head for a crosscutting machine

The present paper deals with the structural analysis and the re-design of a cutting head for an automatic crosscutting machine. The machine is already marketed, and can process rectangular section wooden slats. The principal goal of this work is to develop a new mechanism capable of enhancing the productivity of the machine by around 80%. The work has been carried out by means of both numerical finite element analysis tools and analytical models. In fact, a secondary aim of the research is to define an analytical model which can capture the dynamic behavior of the device: this tool will be helpful to the design engineer in order to save costs associated with the development of future head designs.

A user-friendly computational algorithm for the structural analysis of wrapping machine rotating rings

Wrapping machines usually consist of a two- or a four-column frame, supporting a huge rotating ring, connected to a pre-stretch 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.

A numerical and experimental approach to the design and failure analysis of a pinion shaft for wheel loaders

The present paper deals with the failure analysis and consequent design tips regarding a pinion shaft belonging to a differential gear for offroad machinery applications, namely wheel loaders. The motivations of the study arise from some in-field failures, which resulted in the fracture of the pinion shaft, originated at an external groove located at the end of its threaded portion. The issue has been tackled by means of analytical, numerical and experimental tools. The observed failures have been demonstrated to be due to the in-service loosening of a ring nut, whose function is to preload the tapered rolling bearings, which support the shaft. The weak points of the shaft design are highlighted and possible approaches to overcome the observed occurrence are presented at the end of the study.

Effect of the Engagement Ratio and of Temperature on the Shear Strength of Epoxy Adhesive Bonded Aluminum Alloy Pin-and-Collar Joints

ABSTRACT Epoxy adhesives are widely used in industrial applications, as they are particularly suitable to bond many types of materials. Conversely, possible drawbacks may arise from the use under high temperature, which is likely to imply a drop of mechanical properties. Previous research indicated that the Engagement Ratio (ER), namely, the ratio between the joint length and its coupling diameter, has an effect on the shear strength of an epoxy adhesive applied to steel adherents. Moreover, the shear strength decreases for increasing temperature, with loss of any ER effect beyond the glass transition temperature. The present research is focused on EN AW 7075-T6 alloy adherents that are widely applied in lightweight constructions. The study has involved LOCTITE 9466 with experimental tests on Pin-And-Collar samples with ER varying from 0.4 to 1.7 over four levels (10 replications). The effect of temperature has also been assessed, by campaigns at room temperature and at 40°C, 60°C and 80°C. The results, also interpreted by an analytical model, indicate that keeping ER around 0.9–1 is advisable to optimise strength. Temperature leads to a shear strength drop, to a loss of ER effectivity and to higher scattering, when exceeding the glass transition temperature of the adhesive.

DMLS Built Maraging Steel Fatigue Response Investigated for Different Build Orientations and Allowance for Machining

This work derives its motivations from the increasing interest towards Additive Manufacturing and the lack of studies, mainly in the field of fatigue. The effect of build orientation and of allowance for machining on DMLS produced Maraging Steel MS1 has been assessed. The experimental results, properly set up by tools of Design of Experiment, have been statistically processed and compared. The outcomes were that, probably due to effect of the thermal treatment, machining and material properties, the aforementioned factors do not have a significant impact on the fatigue response. This made it possible to work out a global curve, accounting for all the result. Fracture surfaces have been carefully studied as well.

Effects of Machining and Heat and Surface Treatments on as Built DMLS Processed Maraging Steel

The main motivations for this study arise from the need for an assessment of the fatigue performance of DMLS produced Maraging Steel MS1, when it is used in the “as fabricated” state. The literature indicates a lack of knowledge from this point of view, moreover the great potentials of the additive process may be more and more incremented, if an easier and cheaper procedure could be used after the building stage. The topic has been tackled experimentally, investigating the impact of heat treatment, machining and micro-shot-peening on the fatigue strength with respect to the “as built” state. The results indicate that heat treatment significantly enhances the fatigue response, probably due to the relaxation of the post-process tensile residual stresses. Machining can also be effective, but it must be followed (not preceded) by micro-shot-peening, to benefit from the compressive residual stress state generated by the latter.

Analysis of Threaded Connections for Differential Gear Pinions

The present paper deals with the failure analysis of a pinion shaft belonging to a differential gear for offroad machinery applications, namely wheel loaders. The motivations of the study arise from some in-field failures, which resulted in the fracture of the pinion shaft, originated at an external groove located at the end of its threaded portion. The issue has been tackled by means of analytical, numerical and experimental tools. The observed failures have been demonstrated to be due to the in-service loosening of a ring nut, whose function is to preload the tapered rolling bearings, which support the shaft.Copyright

Experimental Characterization and Finite Element Modeling of Film Capacitors for Automotive Applications

As electronics keeps on its trend towards miniaturization, increased functionality and connectivity, the need for improved reliability capacitors is growing rapidly in several industrial compartments, such as automotive, medical, aerospace and military. Particularly, recent developments of the automotive compartment, mostly due to changes in standards and regulations, are challenging the capabilities of capacitors in general, and especially film capacitors. Among the required features for a modern capacitor are the following: (i) high reliability under mechanical shock, (ii) wide working temperature range, (iii) high insulation resistance, (iv) small dimensions, (v) long expected life time and (vi) high peak withstanding voltage. This work aims at analyzing the key features that characterize the mechanical response of the capacitor towards temperature changes. Firstly, all the key components of the capacitor have been characterized, in terms of strength and stiffness, as a function of temperature. These objectives have been accomplished by means of several strain analysis methods, such as strain gauges, digital image correlation (DIC) or dynamic mechanical analysis (DMA). All the materials used to manufacture the capacitor, have been characterized, at least, with respect to their Young’s modulus and Poisson’s ratio. Then, a three-dimensional finite element model of the whole capacitor has been set up using the ANSYS code. Based on all the previously collected rehological data, the numerical model allowed to simulate the response in terms of stress and strain of each of the capacitor components when a steady state thermal load is applied. Due to noticeable differences between the thermal expansion coefficients of the capacitor components, stresses and strains build up, especially at the interface between different components, when thermal loads are applied to the assembly. Therefore, the final aim of these numerical analyses is to allow the design engineer to define structural optimization strategies, aimed at reducing the mechanical stresses on the capacitor components when thermal loads are applied.Copyright