Enrico Ciulli

Pneumatic stability of the integral aerostatic bearing: comparison with other types of bearing

Abstract The analysis of pneumatic stability of an integral aerostatic bearing subject to axial loads is carried out with the lumped parameter approach. The influence of various geometric quantities on the instability range has been analysed. Both single and opposed bearings are considered. A comparison with the Yates and separate effect configurations has also been carried out.

An investigation on thrust bearing tilting pads

Abstract The present work regards the first stage of a research activity aimed at the understanding of the phenomena involved in the working of thrust bearing tilting pads. Three pads of the same shape and dimensions with different pivot positions have been studied. An experimental investigation is carried out on the pads using a particularly versatile test rig. Simultaneous measurements of film thickness, pad tilt and frictional force are made; in particular the tilt angle is measured by optical interferometry with monochromatic light. Experimental results are compared with numerical data. A finite element method code is used to calculate the pressure field of the finite thrust pad. For a given pivot film thickness an iterative procedure yields the tilt angle using the condition of zero moment about the pivot. First results indicate the way to future investigations.

Static behaviour of an integral externally pressurized gas bearing — comparison with other types of bearing

Abstract The integral bearing consists of two adjacent journal and thrust parts with no intermediate exhaust. The influence of various geometrical parameters and of the supply pressure on the characteristic quantities of the integral bearing, both single and opposed, subject to axial loads, has been studied in order to give useful information for producing a better design. In particular, the convenience of using small orifices and radial clearance is pointed out. A comparison between the integral bearing and other configurations such as the separate effects and Yates bearings has been carried out, also proposing indications for the selection of the most appropriate configuration and pointing out the higher load capacity and stiffness of the opposed integral bearing compared to the others.

Numerical and experimental investigations for the evaluation of the wear coefficient of reverse total shoulder prostheses

In the present study, numerical and experimental wear investigations on reverse total shoulder arthroplasties (RTSAs) were combined in order to estimate specific wear coefficients, currently not available in the literature. A wear model previously developed by the authors for metal-on-plastic hip implants was adapted to RTSAs and applied in a double direction: firstly, to evaluate specific wear coefficients for RTSAs from experimental results and secondly, to predict wear distribution. In both cases, the Archard wear law (AR) and the wear law of UHMWPE (PE) were considered, assuming four different k functions. The results indicated that both the wear laws predict higher wear coefficients for RTSA with respect to hip implants, particularly the AR law, with k values higher than twofold the hip ones. Such differences can significantly affect predictive wear model results for RTSA, when non-specific wear coefficients are used. Moreover, the wear maps simulated with the two laws are markedly different, although providing the same wear volume. A higher wear depth (+51%) is obtained with the AR law, located at the dome of the cup, while with the PE law the most worn region is close to the edge. Taking advantage of the linear trend of experimental volume losses, the wear coefficients obtained with the AR law should be valid despite having neglected the geometry update in the model.

Effect of size and dimensional tolerance of reverse total shoulder arthroplasty on wear: An in-silico study

Although huge research efforts have been devoted to wear analysis of ultra-high molecular weight polyethylene (UHMWPE) in hip and knee implants, shoulder prostheses have been studied only marginally. Recently, the authors presented a numerical wear model of reverse total shoulder arthroplasties (RTSAs), and its application for estimating the wear coefficient k from experimental data according to different wear laws. In this study, such model and k expressions are exploited to investigate the sensitivity of UHMWPE wear to implant size and dimensional tolerance. A set of 10 different geometries was analysed, considering nominal diameters in the range 36-42mm, available on the market, and a cup dimensional tolerance of +0.2, -0.0mm (resulting in a diametrical clearance ranging between 0.04-0.24mm), estimated from measurements on RTSAs. Since the most reliable wear law and wear coefficient k for UHMWPE are still controversial in the literature, both the Archard law (AR) and the wear law of UHMWPE (PE), as well as four different k expressions were considered, carrying out a total of 40 simulations. Results showed that the wear volume increases with the implant size and decreases with the dimensional tolerance for both the wear laws. Interestingly, different trends were obtained for the maximum wear depth vs. clearance: the best performing implants should have a high conformity according to the AR law but low conformity for the PE law. However, according to both laws, wear is highly affected by both implant size and dimensional tolerance, although it is much more sensitive to the latter, with up to a twofold variation of wear predicted. Indeed, dimensional tolerance directly alters the clearance, and therefore the lubrication and contact pressure distribution in the implant. Rather surprisingly the role of dimensional tolerance has been completely disregarded in the literature, as well as in the standards. Furthermore, this study notes some important issues for future work, such as the validation of wear laws and predictive wear models and the sensitivity of k to implant geometry.

Wear simulation of metal on metal hip replacements: an analytical approach

Hip replacement failure is mainly attributable to the implant wear. Consequently preclinical wear evaluations are extremely important. As experimental tests are attractive but highly cost/time demanding, several predictive models have been proposed mainly based on finite element simulations and for metal on plastic (MoP) implants. The aim of this study is to develop a mathematical wear model of metal on metal prostheses, revision of the previous one for MoP implants, developed by the same authors. The model, based on the Archard wear law and on the Hertzian theory, was applied to compare a total (THR) and a resurfacing (RHR) hip replacement under both in vivo and in vitro gait conditions. The results were in agreement with the literature predicting wear rates significantly higher for the RHR than for the THR. The effect of the boundary conditions on wear rates/maps was also investigated and the model limitations discussed.Copyright

Vibration Testing Procedures for Bone Stiffness Assessment in Fractures Treated with External Fixation

A bone healing assessment is crucial for the successful treatment of fractures, particularly in terms of the timing of support devices. However, in clinical practice, this assessment is only made qualitatively through bone manipulation and X-rays, and hence cannot be repeated as often as might be required. The present study reconsiders the quantitative method of frequency response analysis for healing assessments, and specifically for fractures treated with an external fixator. The novelty consists in the fact that bone excitation and response are achieved through fixator pins, thus overcoming the problem of transmission through soft-tissues and their damping effect. The main objective was to develop and validate a test procedure in order to characterize the treated bone. More than 80 tests were performed on a tibia phantom alone, a phantom with pins, and a phantom with a complete fixator. Different excitation techniques and input–output combinations were compared. The results demonstrated the effectiveness of a procedure based on impact tests using a micro-hammer. Pins and fixator were demonstrated to influence the frequency response of the phantom by increasing the number of resonant frequencies. This procedure will be applied in future studies to monitor healing both in in vitro and in vivo conditions.

Experimental study on circular eccentric cam–follower pairs:

Experimental verifications of cam–follower contacts are very important owing to the difficulties faced during a reliable simulation due to the continuous variation of load, speed and geometry of the lubricated contact. Some experiments have been carried out with a new apparatus, specifically designed and realised for investigation on cam–follower and gear teeth contacts, in order to test its capability to measure film thickness and contact forces. Circular eccentric cams have been used because they feature lower transient effects and comparison of the results with the theoretical/numerical ones is easier. The tests have been performed using cams with two different eccentricities and surface roughness, and two different followers, one made of steel and one made of glass. The behaviour of the cam–follower contacts at several different rotational speeds, ranging from 50 to 500 r/min, and different pre-loads have been investigated. Lubrication regimes ranged from boundary to complete, with most of the tests being performed under mixed lubrication conditions. Localised wear has occurred during some tests. Data of all contact force and moment components as well as of the cam shaft driving torque have been acquired at high acquisition frequency. Some methodologies for numerical data elaboration have been identified. Optical interference images have been correctly recorded at the desired frequency. The trends of the normal and friction forces measured in the different tests as well as the evaluated trends of the friction coefficient are presented in the paper. Some sample optical interference images are also shown. The results furnish encouraging indications about the capabilities of the experimental apparatus.

An Experimental Investigation on Aerospace Quality Gears Operating in Loss of Lubrication Condition

This paper shows the results of an experimental study carried out on spur gears for aerospace applications operating in loss of lubrication. The aim of this work was to establish a baseline for gear behavior under oil off conditions. A total amount of 40 tests were performed with gears made from 2 materials operating at different levels of sliding speed and contact pressure. In some cases the bulk temperature was measured to evaluate the heating of the running gear. A more relevant wear and heating of the gears was observed with the increase of the contact pressure rather than with the increase of the sliding speed. In all the tests the transmission was able to transfer the required power. However some tests were stopped before the required time because of the overcoming of the rig safety threshold. The results showed a different oil off time depending on the material.Copyright

Experimental Study on Wear and Fracture in Aeronautical Gear Transmissions

This work presents some of the results obtained from an extensive experimental test campaign performed at the Department of Mechanical Nuclear and Production Engineering (DIMNP) of Pisa University with the partnership of Avio Propulsione Aerospaziale S.p.A. This test campaign, that is still being performed, is concerned with the characterization of teeth damage in high performance gear transmissions, such as aeronautical ones, characterized by high levels of velocity and load. Damage detection and monitoring have been pursued during all the tests in order to achieve a better understanding of causes and evolution of teeth damage. Moreover, to investigate the effects of different parameters on teeth damage, tests with different load, velocity, teeth geometry, materials and surface finishing, have been performed. The surface conditions of the gear teeth have been investigated using in particular two different instruments: a computer-controlled stylus profilometer able to perform 3D investigations and a scanning electron microscope (SEM) recently provided by a software for surface reconstruction. Qualitative and quantitative changes in surface roughness have been detected. Besides the experimental activity, numerical analyses have been also started for finding theoretical criteria for surface damage prediction; some few obtained results are presented. Due to the vastness of the work performed, only a survey of the main results obtained till now is presented in the paper.Copyright