Roberto Bassani

Nanotribology : tip-sample wear under adhesive contact

Abstract In this report, the irreversible variation of mass of the probe tip of an atomic force microscope (AFM) is considered from theoretical and numerical points of view through statistical methods. The tip–sample interaction due to the intermittent-contact operating mode of an AFM is modelled as a double-well potential where the wear mechanism, which reveals itself as mass sticking to the probe tip, is described as a transition between the two potential wells. We evaluate the interaction of a silicon nitride AFM/FFM tip with gold in order to compare the results with those obtained from previous experimental and numerical studies.

Hydrostatic systems supplied through flow dividers

Some flow-compensating devices for hydrostatic lubrication are described, in particular the tapered-spool dividers, and examples are presented of the behaviour of opposed and coplanar pads and of slides equipped with them. It is pointed out that, in general, tapered dividers make the stiffness of the lubricating film greater than do cylindrical dividers, as well as capillaries and constant-flow valves. An application to a grinder slide is also presented which confirms the good behaviour of tapered dividers.

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.

Stability of permanent magnet bearings under parametric excitations

Permanent magnet pairs offer negligible friction and no wear, but also static instability. Stability may be recovered if a suitable periodic axial motion is imposed on the levitated element of the pair. Review led by Al Palazzolo

The self-regulated hydrostatic screw and nut

Abstract The principle of self-regulating flow control, previously applied to hydrostatic bearings, has been applied to hydrostatic screws and nuts with trapezoidal threads. There is a continuous and automatic distribution of the total flow in the two recesses. The results of tests carried out on the self-regulated screw and nut confirm theoretical predictions

Hydrostatic Self-Regulating Multipad Journal and Integral Bearings

Self-regulation flow was applied to multipad radial hydrostatic bearings. Prior developments focused on opposing pads in hydrostatic bearings. This multipad radial bearing is designed to have a high load capacity and stiffness at arbitrary rotational speeds. In this article, formulas are derived to predict bearing performance. A combined or in-series hydrostatic bearing, integrating both thrust and radial components and supplied by a single source, is detailed and found to maintain a high load capacity and stiffness. In particular, these bearings are adequate for large plants, from low-speed tube mills to high-speed and large turbine shafts. A 1-m2 footprint integral bearing could bear a radial of about a hundred tons and an axial load four times larger.

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

The Self-Regulated Hydrostatic Opposed-Pad Bearing in a Constant Pressure System

It is shown that a self-regulated hydrostatic opposed-pad bearing which allows for the continuous and automatic distribution of the total flow into two half-flows in the two recesses is highly efficient in a constant pressure system. Its performance is comparable to that of a conventional hydrostatic opposed-pad bearing compensated with flow control valves, and is superior to that of a conventional bearing compensated with orifices or capillary tubes. Its performance may be further improved if the self-regulated bearing itself is compensated with an orifice or a capillary tube.