Annular Contact Research Articles

The effects of load conditions and structural parameters on fretting wear of annular flat in heavy-duty gas turbine

Heavy-duty gas turbine is the core equipment in energy and electricity systems. In the compressor of a heavy-duty gas turbine, the forces and torques are usually transmitted by the friction force of the annular flat contact interfaces between two adjacent discs. Aimed at the annular flat connection structure of the combined rotor in heavy-duty gas turbine, the distribution of fretting characteristics on the annular flat is analyzed. The wear depth was calculated on the contact interface based on the modified Archard's wear theory and finite element method. On the annular flat, the time-varying distributions of fretting characteristics with the progress of wear are investigated. The increase of fretting cycles increases the wear depth of the annular flat. Moreover, the effects of load conditions and disc geometrical parameters on the fretting characteristics are studied. The increase of tie rod preload force and transmitted torque will intensify the fretting wear on the annular flat. In addition, the decrease of the distance between the tie rod and the annular flat and the decrease of the width of the annular flat will exacerbate the fretting wear on the annular flat. A fretting wear experiment is conducted to verify the finite element model. The experimental results show the fitting curve of wear depth decreases from the inner radius to the outer radius, which aligns with the results obtained from the finite element simulation. Ultimately, the research of this paper will provide a reference for the design and engineering application of the annular flat contact interfaces.

Sustained release of proteins from contact lenses with porous annulus.

Ophthalmic drugs are administered to the front of the eye by eyedrops. The bioavailability of drugs delivered via eye drops is low due to tear turnover. Contact lenses can address some deficiencies of eye drops by sustaining the delivery of drugs, but commercial contact lenses have small pore sizes that cannot load biologics, which are becoming more common for treating ophthalmic diseases. This study aims to investigate novel poly(hydroxyethyl methacrylate) (pHEMA) lenses with transparent center and porous annulus for sustained release of model proteins. A novel hydrogel polymerization process was used to fabricate concentric, porous layer pHEMA hydrogel rods. The hydrogels were lathe cut into contact lenses which were explored for the delivery of proteins and gold nanoparticles. Lenses were characterized by partition coefficient and diffusivity, which was estimated by fitting experimental data to an analytical model. Transmittance measurements were made to compare transparency of porous lens centers to commercial contact lenses. Porous pHEMA lenses consisting of a concentric, porous layer made from 55% water content in precursor were successfully lathe cut into lenses with transparent center and opaque porous annulus. The porous lenses could load large model proteins of bovine serum albumin and human γ-globulin and provide sustained release. The core annular pHEMA contact lenses consisting of an outer annulus of opaque, porous pHEMA and an inner, center layer of clear, nonporous pHEMA can provide sustained delivery of biologics.

Study on the dynamic evolution of friction and wear of polyurethane material for pipeline inspection gauge

The dynamic evolution in friction and wear can lead to interface failure and induce vibrations in pipeline inspection gauge. The friction behavior between polyurethane and X70 steel was evaluated with annular face contact under 0.042 to 0.07 MPa. The temperature increase was recorded using infrared cameras. Dynamic simulations of frictional heat and stress-strain were conducted using Abaqus software. The results indicate that in the initial stages, abrasive wear, stress (1.5 MPa), and frictional temperature rise (65 °C) of PU were observed in the inner-ring region at 0.056 MPa. The surface roughness (3.394 μm) of the PU became comparable to that of the steel ring (3.768 μm) due to abrasive wear. In the later stages, a higher frictional temperature (80 °C) and increased stress (3.8 MPa) were observed in the outer ring. Concurrently, Schallamach waves and ridge-like stripes indicative of stick-slip behavior emerged in the outer ring. The viscoelastic hysteresis effect is the fundamental cause of the dynamic evolution of the friction coefficient and wear. Frictional heat exacerbates the evolution of stick-slip behavior and increases fluctuations in the friction coefficient. The findings of this research can be utilized to analyze the causes of interface failure and vibration in pipeline inspection gauge.

Experimental study of the vacuum arc motion-diffuse transition in cup-shaped transverse magnetic field contacts

The arc in transverse magnetic field (TMF) contacts transitions to diffusion mode before current zero, affecting arc morphology and motion. TMF-generated force competes with local overheating's stagnation effect, impacting contact surface ablation. Cup-shaped TMF contacts exhibit slow, fast, and unstable transitions. Arc voltage declines gently, rapidly, or in repeated steps. Peak current, contact diameter, contact piece structure, and iron core influence transition characteristics. Transition energy increases linearly with peak current. Larger contact diameter and iron cores make transition constant earlier and transition current higher. Compared to circular contacts, annular contacts delay transition constant and widen transition current range. Scanning electron microscope (SEM) images showed that the ablation cathode surfaces exhibited five morphologies: melting pit, periphery, lamellar, particle, and pore. Energy dispersive spectrum (EDS) analyses show that the periphery has a higher Cu content than the interior, suggesting less Cu loss during fracture and less droplet splashing. The peripheral Cu content is comparable between the two contacts, 62.94 % vs. 62.37 %. However, the interior Cu content of contact NO.4 is higher than NO.6 at 60.54 % vs. 56.26 %. This indicates that the difference in the effect of fast and slow transitions on the cathode surface activity is mainly in the interior region.

Impact of Intraprocedural Pressure Changes on Hemodynamic Outcome During Self-Expanding TAVR.

During the transcatheter aortic valve replacement (TAVR) procedure, hemodynamic measurements can be used to evaluate transcatheter heart valve (THV) performance. We hypothesized that the occurrence of a significant decrease in invasive aortic pressure immediately after annular contact by a self-expanding THV indicates effective annular sealing. This phenomenon could thus be used as a marker for the occurrence of paravalvular leak (PVL). Thirty-eight patients undergoing TAVR procedure with a self-expandable Evolut R or Evolut Pro (Medtronic) valve prosthesis were included in the study. Drop in aortic pressure during valve expansion was defined as a decrease in systolic pressure of 30mmHg immediately after annular contact. The primary endpoint was the occurrence of more than mild PVL immediately after valve implantation. A pressure drop was seen in 60.5% (23/38) of patients. More than mild PVL requiring balloon post-dilatation (BPD) was significantly more frequent in patients who did not have a systolic pressure decrease > 30 mmHg during valve implantation (46.7% [7/15] vs. 13.0% [3/23], respectively; p = 0.03). Patients without a systolic pressure decrease > 30 mmHg also had a lower mean cover index on computed tomography analysis (16.2% vs. 13.3%; p = 0.016). The 30-day outcomes were similar between the two groups, and echocardiography at 30days demonstrated more than none/trace PVL in 21.1% (8/38) of patients, with no difference between the two groups. A decrease in aortic pressure after annular contact is associated with an increased probability of good hemodynamic outcome after self-expanding TAVR implantation. In addition to other methods, this parameter could be used as an additional marker for optimal valve positioning and hemodynamic outcome during the implantation procedure.

Investigating the Friction Behavior of Turn-Milled High Friction Surface Microstructures under Different Tribological Influence Factors

The coefficient of friction (COF) is an important parameter for mechanical engineers to consider when designing frictional connections. Previous work has shown that a surface microstructuring of the harder friction partner leads to a significant increase in the COF. However, the impact of the changes in the tribological system on the COF are not known in detail. In this study, the tribological influence factors such as the nominal surface pressure, the material pairing, lubrication, and the surface properties of the counterbody are investigated. Microstructuring is applied by turn-milling of an annular contact surface of cylindrical specimens. A torsional test bench is used to measure the torque depending on the displacement of the two specimens, thus enabling the determination of the COF. All tests with the microstructured specimens result in higher COF than the reference test with unstructured samples. The manufacturing process of the counterbody surface, the nominal surface pressure, and the materials in contact have a significant influence on the COF. While lubrication reduces friction in the case of unstructured specimens, the COF does not change significantly for microstructured samples. This proves that the deformative friction component dominates over the adhesive. Microstructuring the harder friction partner increases the transmittable torque in frictional connections and reduces the sensitivity towards possible contamination with lubricants.

Use of synchrotron X-rays for direct observation of wear damage in optically-opaque contacts by means of CT imaging and X-ray diffraction

In a simulated in-situ experiment annular contacts of 6082-T6 aluminium alloy were first exposed to various degrees of fretting wear, before synchrotron X-ray CT and diffraction were applied to examine the extent and development of wear damage. The wear volumes measured and development of the wear rate were consistent with the mathematical wear model of Fillot, Iordanoff & Berthier. Thickness and porosity of the worn zone were measured and the contact area between wearing elements estimated. Developments of residual strain and tribologically transformed structure were investigated through X-ray diffraction. This study demonstrates, for the first time, the value and viability of observing and quantifying wear through combination of X-ray CT and diffraction opening a path towards in-situ observations of wear.

A generalized solution to the combo-crack problem—I. Pressure load on crack surface

The axisymmetric elasticity problem of crack combo containing an externally circular crack (ECC) and a coplanar concentric penny-shaped crack (PSC) is mathematically equivalent to the annular contact problem. This problem has been attempted by using Love's strain potential approach, which eventually comes down to solving a pair of simultaneous Fredholm integral equations. Finding the closed-form solutions to the integral equations is difficult, if not impossible. Approximate solutions have been proposed in power series representations, which suffer from two major deficiencies. First, the solutions apply only to a special loading case in which uniform pressure is applied to the whole surface of the interior PSC. Secondly, the accuracy of the solution becomes unsatisfactory when the interior PSC tip is close to the ECC tip. To address these issues, in this paper we revisit this problem by considering a more general loading case in which uniform pressure is applied to a circular region of any size at the center of the PSC's surface. To overcome the lower accuracy caused by power series with limited terms, we numerically solve the pair of simultaneous Fredholm integral equations based on the Gauss-Lobatto quadrature. The high accuracy of our solution in the whole size spectra of the PSC and ECC is verified by finite element simulations. Our paper provides a generalized and more accurate solution to the annular contact problem or the combo crack problem, which deserves to be included in the updated library of the solutions to basic crack problems.

WORKING LOAD SIMULATION ON THE CNC MACHINE

The CNC machining is performed according to a pre-prepared control program (CP). The CP records all the necessary actions of the machine to obtain the part by metalworking in accordance with the requirements of the drawing according to the developed technology. Precise formation of surface dimensions during machining is performed by a cutting tool, the trajectory of which is recorded in the CP.
 In real conditions, during CNC machining, errors may occur, the size and direction of which depend on many factors, which are difficult to anticipate. For example, the occurrence of dimensional variations is affected by the real technical condition and setting of the technological system of the CNC machine, elastic deformations of system elements from cutting forces, thermal deformations, etc. The value and direction of elastic deformations of the machine are influenced by the cutting forces, size and direction of which depend on the processing modes, the shape and size of the workpiece, the material of the tool cutting part and the material of detail.
 There are known methods of accounting for the expected processing errors in the calculation of trajectories of tools in the presence of data on the flexibility of the technological system of CNC machine. Such data can be obtained as a result of studies of the flexibility of machine tool elements when simulating workloads.
 To perform tests to determine the elastic deformations of a technological system in production conditions, a device has been developed to simulate working loads in a wide range and various places in the working area of ​​CNC machine. The device contains a loaded shaft with replaceable bushings on which annular contact grooves are made. The configuration of the device is assembled taking into account the design features of the CNC machine and the assigned research tasks.
 According to the control program or manual control, simulation forces are created by a load device with poppet springs installed in the cutter holder of the machine. Strain value is measured by linear displacement meter. The obtained data allow you to determine the stiffness of the machine elements and give a forecast for the amount of expected dimensional errors during processing.

Mechanics of adhesives under annular confinement: internal pressure, force, and interfacial area.

Closed annular adhesive interfaces are commonly found in nature as well as in many existing and developing technologies. Such contacts provide enhanced control of interfacial history by prescribing whether interfacial separation occurs at the outer or inner edge, and whether internal pressure affects the required force for separation. To facilitate the development of technologies involving annular contacts, we have experimentally measured the relationship between applied displacement, resulting force and internal pressure, and annular interface dimensions for the contact between a rigid annular probe and an adhesive layer with finite thickness. Experiments were validated by finite element analysis models, which were used to develop semi-empirical analytical relationships for the changes in contact compliance as a function of material properties and geometric constraints. Additionally, the change in internal pressure was modeled as a function of annular contact dimensions and adhesive layer material properties. This model predicts the critical volume where internal pressure changes alters critical force for separating an annular contact interface. The results discussed here provide a foundation for new experimental protocols for characterizing soft materials, including pressure-sensitive adhesives, as well as guidelines for designing annular interfacial materials with controlled separation histories.

Avoiding the initial adhesive friction peak in fretting

An initial friction peak typically occurs in a dry self-mated quenched and tempered steel fretting contact in gross sliding conditions. The peak is related to adhesive friction and wear, which causes non-Coulomb friction. An early surface degradation including cracks may occur. To avoid such a peak, different media were studied using a flat-on-flat fretting test device with a large annular contact. All the media decreased the initial friction peak in comparison to the dry reference case, and in one series the peak was completely removed. The peak could often be delayed by lubrication. The steady-state coefficient of friction values mostly remained at similar levels to those of the dry contact, but decreased when oil was applied. Nevertheless, some surface damage occurred in every test, with varying amounts of wear.

In-situ X-ray tomography of wear – A feasibility study

Wear experiments using annular contacts were carried out to investigate the possibility of imaging wear scars and wear debris beds in-situ. Samples made of 6082T6 aluminium alloy were subjected to unidirectional wear under different conditions with the sample elements then locked in contact to preserve the end-of-wear condition. Post-wear 3D morphology was successfully imaged in-situ using X-ray tomography, with total depth of the wear scar, surface primary-profile roughness and real area of contact also obtained. This study is a proof-of-concept, illustrating the utility of tomography for studying wear, without the need to disassemble the contact or disturb the debris bed and serves as a starting point for a new approach to the investigation of wear phenomena.

DEVELOPMENT AND EXPERIMENTAL INVESTIGATION OF SELF-CENTERING DAMPERS WITH PARALLEL HIGH STRENGTH STEEL RING SPRING GROUPS

Based on the self-centering damper with high-strength steel ring spring, a self-centering damper with parallel high-strength steel ring spring is proposed. The seismic performance of the parallel ring spring self-centering damper under multiple seismic conditions was verified by a hysteretic test. The test results show that the parallel high-strength steel ring spring self-centering damper has good self-centering ability, and will not experience performance degradation after multi-time earthquakes. It does not need to be replaced after the earthquake. Compared with the self-centering damper with a single group of ring springs, the damper bearing capacity can be effectively improved under the same size, and the damper internal space can be fully utilized. The different frictional conditions of the annular spring contact surface have a certain influence on the performance of the self-centering damper. The theoretical prediction model proposed is in a good agreement with the experimental results.

Experimental investigation of debris entrapment in annular contacts

In wearing contacts, a part of the wear debris is trapped between the surfaces, creating a wear debris bed. Many researchers report such beds providing a degree of protection against further wear. It was hypothesized, that in an annular contact, the level of debris entrapment increases with the width of the annulus resulting in decrease in wear. To test this hypothesis, two series of experiments using EN1A steel and 6082T6 aluminium alloy annuli, measuring 1, 2 and 4 mm wide were carried out. When steel was used, there was no clear correlation between ring width and total mass loss or steady-state wear rate. When aluminium was used, wear decreased with the ring width according to both metrics. This behaviour appears to correlate with worn surface roughness, suggesting worn surface roughness to be a parameter controlling debris entrapment.

Torsional Fretting Wear of Annular Flat-Flat Contact Interface Under Different Road Clamping Force and Torque

For the rod fastening combined rotor in some heavy-duty gas turbine, the annular flat-flat contact interfaces are usually used between disks to transmit the torque. In this paper, a contact model of two disks assembled by 12 circumferential preload tie rods is established to study the fretting wear behavior of the annular contact interfaces under variable torque and different preload of tie rods. The changes of contact stress distribution, slide distance and wear depth of the annular contact interface with torque and rod preload are obtained. The results show that the wear depth is increasing with the growth of torque and rod preload.

Effects of Torque and Friction Coefficients on the Fretting Characteristics of Annular Face Gear Contact Interface

In present, the annular face gear connect structures are widely used in combined rotor of heavy-duty gas turbine or aeroengine to transmit the torque and force. The fretting wear will happen on the face gear contact interfaces after long-time operation. This paper investigates the evolution of contact characteristics including contact stress, slide distance, and wear depth of face gear contact interfaces under the conditions of different torque and friction coefficients by means of finite element analysis method. It can be concluded that the distribution of the contact stress and slip distance on the face gear contact interface are inhomogeneous, and the stress close to the tooth root is the largest, so the largest wear occurs near the tooth root in the inner surface. Increased torque can improve the contact stress and slip distance. The larger friction coefficient effectively reduces the wear of contact surfaces.

Non-intrusive measurement of thermal contact conductance at polymer-metal two dimensional annular interface

This work reports the development of a measurement technique for the estimation of thermal contact conductance at a polymer (Nylon)-metal(Copper) two dimensional interface having an annular contact under low external loading. The experimental setup consists of a vacuum chamber capable of achieving a pressure of 10−6 mbar. The chamber is fitted with electrical, thermocouple and fluid feed-throughs. Two dimensional temperature measurements are performed on the top surface of the polymer sample away from the boundary interface and are used to estimate the thermal contact conductance at the interface. The estimation process is accomplished by solving an inverse heat conduction problem using artificial neural networks coupled with Genetic algorithm. The actual pressure distribution at the annular Nylon-Copper interface is measured using a pressure sensitive film and the variation and the distribution of contact area between the mating surfaces is shown. The estimated values of thermal contact conductance obtained using the developed non-intrusive technique are found to be in good agreement with those reported in literature.

Observation and Analysis of Micro-Behavior Characteristics and Element Contents during Boundary Layer Evolution under Powder Particulate Lubrication

Typical characteristics of the boundary layer under powder lubrication are microscopic reflections of lubrication states and closely related to lubrication properties. Therefore, the current investigation is focused on the changes in typical features of boundary layer with an annular contact, flat-on-flat sliding. A 3D laser profile gauge, scanning electron microscopy, X-ray energy-dispersive spectrometer, and Raman laser confocal spectrometer are utilized to microscopically observe and analyze the boundary layers formed on the brass specimens during the process of graphite-lubricated 0.45 C steel (HRC56) sliding upon brass alloy (H62). The evolution of mixed powder lubrication can be divided into four typical stages: (A) running-in, (B) stable, (C) deterioration, and (D) damage. Accompanied by better lubrication performance, the boundary layers are flat and the surfaces are smooth at early stages. Meanwhile, the surface roughness keeps low level, and the carbon content is consistent. Then, the surface flatness worsens gradually and the surface roughness containing boundary layer increases. Simultaneously, the carbon content on the surface decreases, and the copper content becomes even more. Along with the deterioration of the lubrication, iron is observed on the lower brass specimen, indicating a significantly direct contact between the upper and lower specimens. Moreover, the graphite powder, which used to be an excellent solid lubrication material, transforms to the disordered state in the test. From the Raman spectrometer analysis, the stable phase is the most feasible graphitization stage coupled with optimal lubricity. Friction characteristics are closely associated with the boundary layer states in powder lubrication, and improving the corresponding conditions can delay the damage processes.

Johnson-Kendall-Roberts adhesive contact for a toroidal indenter.

The unilateral axisymmetric frictionless adhesive contact problem for a toroidal indenter and an elastic half-space is considered in the framework of the Johnson-Kendall-Roberts theory. In the case of a semi-fixed annular contact area, when one of the contact radii is fixed, while the other varies during indentation, we obtain the asymptotic solution of the adhesive contact problem based on the solution of the corresponding unilateral non-adhesive contact problem. In particular, the adhesive contact problem for Barber's concave indenter is considered in detail. In the case when both contact radii are variable, we construct the leading-order asymptotic solution for a narrow annular contact area. It is found that for a v-shaped generalized toroidal indenter, the pull-off force is independent of the elastic properties of the indented solid.

Late stage drainage of block copolymer stabilized emulsion drops.

Polymer covered emulsion droplets have a considerable number of applications ranging from active cosmetics to advance drug delivery systems. In many of these systems the emulsion droplets do not exist in isolation but interact with other drops, surfaces and particles. In a step towards understanding how these complex mechanical interactions take place, we examine the interaction between a block copolymer covered emulsion droplet (polystyrene-b-poly(ethylene oxide) (PS-PEO) covered toluene) and a flat mica interface. As buoyancy pushes the droplet upwards, it buckles in as it nears the mica and traps a droplet of the surrounding fluid. The trapped outer fluid (water/glycerine in our experiment) drains out through an annular region of PEO brush. This study focuses on the late stage drainage, unique to large molecule surfactants, and examines the effects of the polymer and droplet size on the drainage rate. We introduce a scaling model of the drainage which highlights the importance of three lengthscales in the problem - the brush height, the slip length along the emulsion drop interface and the width of the annular contact region.