A. O. Andrisano

Axisymmetric Mechanical Analysis of Ceramic Heads for Total Hip Replacement

An axisymmetric, mechanical analysis of conical press-fit ceramic heads is performed. The head strength and its fracture modes are assessed experimentally. The stress field is examined by finite element, strain gauge and photoelastic methods. An alternative head design, characterized by a cylindrical engagement with the stem, is analysed with the same techniques and its merits are explored.

Discussion on the design of a hip joint simulator

Hip joint simulators were developed for predicting, by attempting to duplicate in vitro physiological loads and motion, the wear rate that total hip replacements are likely to show in vivo. From a theoretical point of view, loading and motion cycles of hip joints could be closely reproduced by three rotation actuators and three force actuators. However existing devices have been designed assuming that some of these degrees of freedom are negligible, in order to reduce the complexity of the equipment. The present study singles out some preliminary indications on the design choices regarding the spatial configuration of loading and motion actuators. The aim is to define theoretically a simplified simulator but still able to apply the most physiologically realistic loading cycle to the specimen.

An Engineering Method for Comparing Selectively Compliant Joints in Robotic Structures

Large displacement compliant joints can substitute traditional kinematic pairs in robotic articulated structures for increasing ease-of-assembly, robustness, and safety. Nonetheless, besides their limited motion capabilities, compliant joints might be subjected to undesired spatial deformations which can deteriorate the system stability and performance whenever a low number of control inputs is available. In all these cases, it is convenient to select/design joint morphologies which enable a selectively compliant behavior, i.e., a low stiffness along a single desired direction. Within this context, this paper outlines an engineering method for quantifying the joints selective compliance by means of local and global performance indices. The approach is validated by comparing two beam-like flexures whose analytic solution is known from the literature. Finally, two joint morphologies, previously employed in the fabrication of robotic/prosthetic hands, are critically compared on the basis of the proposed criteria.

Structural Evaluation of Ceramic Femoral Heads: Effect of Taper Friction, Support Conditions and Trunnion Compliance

The outcome of a nonlinear finite element stress analysis of ceramic heads for artificial hip joints is presented. The analysis mainly covers the influence of taper friction, support conditions and trunnion modulus of elasticity on the hoop stress distribution at the surface of the head bore. The paper quantifies how much the maximum tensile stress decreases with increasing frictional coefficient, with stiffening of the support and with stiffening of the trunnion material. An appreciable rise of the maximum tensile upon unloading of the head is also shown for the case of cup support. The computational results are found in close correspondence with photoelastic measurements of taper pressures and encourage the use, for preliminary design purposes, of an approximate theoretical model retrieved from the literature.

Stiffness Evaluation and Vibration in a Tractor Gear

The problem of gear noise in vehicles has been intensively studied in the past; however, recently the interest about this problem grew because of great restrictions in the laws regarding noise level and the increase of international competition. One of the most important vibration and noise sources is transmission error that excites the gearbox as a dynamic system, the gearbox surfaces, and connected components; the external box radiates noise. However, the current understanding of gear vibration remains incomplete, even though there is general agreement about the nature of the phenomenon. Vibrations are due to several sources: torsion resonance, impulsive or cyclic fluctuations in drive torque, gear mesh transmission error, local component vibration responses and fluctuations in the output torque demand. The concept of a vibrating system made of two gears is generally modeled through two wheels linked by the teeth mesh stiffness. In its simplest form, this model can simulate the classical linear resonance, i.e. the resonant frequency of the system. However, more complex phenomena such as parametric instabilities can be an important source of noise. In the present paper vibration problems in the gears of an industrial vehicle are investigated through the use of perturbation technique. A suitable software has been developed to generate the gear profiles in order to evaluate global mesh stiffness using finite element analysis.Copyright

Automatic fracture reduction with a computer-controlled external fixator

The reduction of fractures by means of an Ilizarovs fixator is obtained by successively shortening or lengthening the rods. This entails that all reduction operations of the fracture stumps be performed with a series of empirical attempts, requiring great experience and manual dexterity in the surgeon. Moreover this process involves a long exposure of both physician and patient to potentially harmful radiation due to the continuous checking of the intermediate positions on the X-ray image intensifier. In order to overcome these limits a new device has been conceived, based on the application of three stepper-motors on three-rods. Its basic principle is functionally very similar to Ilizarovs prototype. The relative motions between the two frames are carried out by controlling the three actuators with a computer, which processes the number of required steps on the basis of an algorithm, starting from a few inputs supplied by the surgeon. This article illustrates the functional kinematic study necessary for the complete automation of the reduction process. Also considered is the complex problem of the reduction trajectory definition, intended as a sequence of configurations of partial correction, obtained by formalizing in geometrical terms the empirical criteria followed by the orthopaedic surgeon in reducing fractures. Such a sequence is intended to be a suggestion for the surgeon who can visualize and possibly interact with the system to determine a trajectory harmless for the soft tissues surrounding the bone.

Human-centred design of ergonomic workstations on interactive digital mock-ups

Analysis of human-related aspects is fundamental to guarantee workers’ wellbeing, which directly limits errors and risks during task execution, increases productivity, and reduces cost [1]. In this context, virtual prototypes and Digital Human Models (DHMs) can be used to simulate and optimize human performances in advance, before the creation of the real machine, plant or facility. The research defines a human-centred methodology and advanced Virtual Reality (VR) technologies to support the design of ergonomic workstations. The methodology considers both physical and cognitive ergonomics and defines a proper set of metrics to assess human factors. The advanced virtual immersive environment creates highly realistic and interactive simulations where human performance can be anticipated and assessed from the early design stages. Experimentation is carried out on an industrial case study in pipe industry.

Virtual Prototyping of a Compliant Spindle for Robotic Deburring

At the current state-of-the-art, Robotic Deburring (RD) has been successfully adopted in many industrial applications, but it still needs improvements in terms of final quality. In fact, the effectiveness of a RD process is highly influenced by the limited accuracyof the robot motions and by the unpredictable variety of burr size/shape. Tool compliance partially solves the problem, although dedicated engineering design tools are strictly needed, in order to identify those optimized parameters and RD strategies that allow achieving the best quality and cost-effectiveness. In this context, the present paper proposes a CAD-based Virtual Prototype (VP) of a pneumatic compliant spindle, suitable to assess the process efficiency in different case scenarios. The proposed VP is created by integrating a 3D multi-body model of the spindle mechanical structure with the behavioural model of the process forces, as adapted from previous literature. Numerical simulations are provided, concerning the prediction of both cutting forces and surface finishing accuracy.

Dynamic Optimization of Spur Gears

This paper presents a global optimization method able to find gear profile modifications that minimize vibrations. A non linear dynamic model is used to study the vibrational behavior; the dynamic model is validated using data available in literature. The optimization method takes into account the influence of torque levels both on the static and the dynamic response. Therefore, two different objective functions are considered; the first one is based on static analysis and the second one is based on the dynamic behavior of a lumped mass system. The procedure can find the optimal profile modification that reduce the vibrations over a wide range of operating conditions. In order to reduce the computational cost, a Random-Simplex optimization algorithm is developed; the optimum reliability is also estimated using a Monte Carlo simulation. The approach shows good performances both for the computational efficiency and the reliability of results.Copyright

On Tire Monitoring Systems Temperature Compensation

Direct type [1] Tire Monitoring Systems supervise tire internal inflating pressure. The authors previously proved that Tire Monitoring must be focused on the tire (i.e.: vehicle) dynamical behaviour [2], real aim of the supervising action: then even the external absolute pressure must be taken into account. NHTSA studies showed improper warnings must be avoided in order to keep the driver confidence with the system; internal temperature fall down is the Tire Monitoring Systems’ improper warnings main cause. A new approach for optimal Tire Monitoring Systems temperature compensation related to external environmental temperature, able to avoid improper warnings, will be presented.