Experimental study of wear of model friction pairs lubricated with contaminated oil

The results of tribotests of block-on-ring friction pairs, which are analogs of piston–liner friction pairs of axial-piston pumps, are presented. It has been shown that, when the pairs are lubricated with oil that contains inorganic contaminants, the wear rate of the steel specimen is more than 200 times higher than that of the bronze specimen. It has been found that the concentration of contaminants in oil of 0.02–0.03% is permissible for block-on-ring friction pairs. The dependence of the wear rate of the specimens on the concentration of the solid contaminations is described by a logarithmic function, while the dependences of the coefficient of friction and the contact temperature on this concentration are described by hyperbolic functions. This has made it possible to assume that the wear of the specimens is determined by the hydroerosion component, which enhances the abrasive effect of the solid contaminations.     

Experimental extrapolation model for friction and wear of polymers on different testing scales

The friction and wear behaviour for polyoxymethylene homopolymers (POM-H) and polyethylene terephthalate with teflon additives (PET/PTFE) is compared on small-scale cylinder-on-plate tests (50-200 N normal loads) and large-scale flat-on-flat tests (190-3880 kN normal loads). A common parameter to characterise tribological data is the contact pressure×sliding velocity (pv-value), but its use seems restricted to a single testing scale. Four experimental models are presented to extrapolate tribological data from one to another testing scale, based on (i) one single mechanical parameter (normal load or contact pressure), (ii) two mechanical parameters (normal load and sliding velocity), (iii) the contact pressure-sliding velocity model (pv-temperature limit), (iv) macroscopic geometry model. The latter model is most extensive, considering the influences of thermal effects (frictional heat generation and dissipation), sample geometry (geometry factor G) and visco-elastic contact (critical contact pressure p₀). For unfilled polymers, the introduction of macroscopic scaling factors allows for the extrapolation of coefficients of friction obtained on different testing scales. Specific or volumetric wear rates cannot be extrapolated because they strongly depend on the sample geometry, while linear wear rates are in better agreement when considering the transitions between mild wear, softening and melting. For internally lubricated polymers, extrapolation is more difficult. The differences depending on the testing scale are attributed to contact stress concentrations near the sample borders and limited wear debris mobility within large contact areas, promoting a homogeneous film formation onto the polymer surface.     

X-ray study of friction and wear of resin-bonded asbestos

Friction and wear of resin-bonded asbestos as an organic friction material were investigated by means of X-ray diffraction. It was seen that (1) the line profile of the diffraction tended to be the Cauchy type with increasing deformation or cycles of fatigue, (2) the temperature of the structural decomposition of asbestos in many cases, was lowered by addition of some friction modifiers and (3) the coefficient of friction was related to the lattice distortion of asbestos crystals; it decreased with the integral breadth.     

Effect of testing conditions on friction and wear of a polyetheretherketone-based composite

Friction and wear characteristics of a type of polyetheretherketone (PEEK)- based composite were evaluated under two different loading pressures and sliding speeds (P = 1.0 MPa, V = 1.0 m/s and P=2.0 Mpa, V=3.3 m/s). The material was in contact with steel surfaces of two different roughnesses (Ra=0.15 μm and Ra=0.33 μm). Interface temperature, coefficient offriction, depth wear rate, and specific wear rate of the polymer composite changed considerably with the PV value and the counterface roughness. The interface temperature increased with increasing PV value, whereas the friction coefficient decreased. The depth wear rate at the higher PV value was much higher than that at lower PV. In addition, the rougher counterface resulted in a higher friction coefficient, depth wear rate, and specific wear rate, when the PV value was fixed. The effect of counterface roughness on the specific wear rate at the higher PV value was smaller than that at the lower PV. Further variations in friction and wear with testing conditions are discussed along with the corresponding microscopic observations of the worn polymer surfaces and the polymer transferred counterfaces.     

湿式换档离合器摩擦片摩擦磨损特性试验研究

介绍了综合传动湿式换档离合器工作原理,基于离合器摩擦片的结构与材料成分特征,分析了转速、裁荷和温度对摩擦片摩擦特性的影响;通过搭建的离合器摩擦片摩擦磨损试验台,验证了摩擦片摩擦因数随滑磨转速增大而减小,随结合油压升高而降低的规律;通过温度测试发现了结合过程中摩擦片中部温度较高,内外边缘处温度较低,多次结合试验后内齿摩擦片中部靠外部分磨损比较严重.     

Role of MoS2 morphology on wear and friction under spectrum loading conditions

One of the ways by which grease is evaluated is by using a four‐ball wear test using ASTM D2266. However, actual applications may require bearings to be subjected to spectrum loading conditions. This study focuses on using ball milling to mitigate the wear from sharp edges in the MoS₂ particles. Two different blends of greases were formulated using MoS₂ in the as‐received state (unmilled) and milled MoS₂; they were tested under spectrum loading conditions where the load and frequency of the tests were treated as variables. It was found that ball milling of the MoS₂ significantly reduces the wear under spectrum loading condition both for ramp‐up and ramp‐down conditions. It was also shown that shortening the time step for both the ramp‐up and ramp‐down cycles resulted in larger wear for unmilled MoS₂ particles in comparison with milled MoS₂ particles in grease. The milling process did not play a significant role when frequency of the test was either ramped up or down. Copyright © 2015 John Wiley & Sons, Ltd.     

The friction and wear of different polymers under high‐load conditions

Engineering polymers are increasingly being used as self‐lubricating materials in demanding high‐load applications. In this study, a comparison has been made of the tribological behaviour under high‐load conditions of two grades of polyamide and polyethylene terephthalate as well as polyacetal. Large‐scale testing is used to reduce edge effects. It has been found that the two grades of polyamide and pure polyethylene terephthalate are prone to stick—slip, while internally lubricated polyethylene terephthalate and polyacetal show low friction and wear and no stick—slip.     

A hip simulator study of the influence of patient activity level on the wear of crosslinked polyethylene under smooth and roughened femoral conditions

Over the last three decades, tribological studies of polyethylene total hip replacements have been undertaken using a simplified model of normal walking. As hip prostheses are being implanted in younger and more active patients, coupled with the increased wear resistance of crosslinked polyethylene, such in vitro approximations in activity are very limiting. Using a hip joint simulator, the influence of a significant increase in patient activity was studied by applying a series of simulated walking, stumbling and jogging sequences, at varying cycle speeds, using crosslinked UHMWPE/CoCrMo components, with both smooth and roughened femoral heads. All tests were performed using 25% bovine calf serum, and all components were positioned physiologically. The effects on wear, frictional torque, counterface roughness, lubricant temperature, material deformation and particle morphology were measured and analysed. It was found that with smooth heads and non-constraining socket fixtures, the occurrence of excessive stumbling at 1 Hz (5 kN max) had a negligible effect on the wear rate of polyethylene, whilst simulated jogging at 1.75 Hz (4.5 kN max) only showed a median increase in wear volume of 40% compared to normal walking. Fast walking showed the largest wear rate, and was consistently greater than for simulated jogging, thus, suggesting that short periods of increased load and speed have a relatively small effect on polyethylene wear. However, increasing the femoral roughness R_a to 0.38 μm under simulated jogging, 1.75 Hz, led to a massive increase in wear and frictional torque, generating wear rates >3000 mm³/10⁶ cycles for crosslinked polyethylene. Surface topography, sliding speed and the type of socket fixturing were shown to be the most influential factors when simulating increased patient activity.     

An apparatus for the study of wear under dynamic loading conditions

A multipurpose wear testing apparatus has been designed, constructed and calibrated. The apparatus is primarily an impact wear testing device, but it may also be used for vibratory and oscillatory wear experimentation. The system utilizes a versatile displacement- and force-controlled device, which allows accurate control and measurement of the load cycles and their frequencies and the relative normal and transverse velocities between the wear surfaces as well as their time of contact. Features of this design permit testing at elevated frequencies and investigation of the effect of individual parameters on the wear process. These features include a facility to manipulate the system stiffness, the ability to control the impact and rotational velocities independently, feedback to maintain a constant nominal stress parameter, the ability to use spherical or cylindrical wear specimens and a method of applying the load and relative transverse motion in a constant, random or prescribed manner. The design facilitates modifications to include lubrication and environmental control, measurement of friction forces and fretting wear capabilities. Some initial results are included.     

A simulator study of friction in total replacement hip joints.

Frictional behaviour of 22 different femoral head-acetabular cup combinations was studied on a new servo-hydraulic microcomputer-controlled hip joint simulator using various flexion-extension angle and superior-inferior load set value waveforms and using distilled water at 37 +/- 1 degrees C as lubricant. Six different head materials were included in the study, whereas all cups were ultra-high molecular weight polyethylene (UHMWPE). Most head-cup combinations studied are commercially available. No distinctly superior joint design can be pointed out, but the frictional behaviour of alumina ceramic against UHMWPE proved overall most favourable (mu min was 0.02), whereas that of non-ion-implanted titanium alloy Ti-6Al-4V against UHMWPE proved strikingly poor (mu max was 0.15). The lowest frictional torque was in 22 mm joints, but frictional torque did not always increase straightforwardly with increasing diameter of the femoral head. The measurements form an extensive comparison between a wide variety of head-cup combinations. The simulator is apparently a useful instrument in the study of frictional behaviour of new designs, materials, surface treatments and coatings that are frequently introduced.     

Some aspects of the friction and wear of electrical sliding contacts under conditions of boundary lubrication

The friction characteristics of lubricated electrical sliding contacts are considered. Data are presented concerning the effects of the current, the lubricant properties, the velocity and the load on the friction and wear behaviour of the contacts. In light-current electrical contacts the effectiveness of the lubricant does not depend on its conductivity but on its ability to prevent the formation of a non-conductive film by lubricating action. Under semifluid lubricating conditions the heavy current acts by discharge through the lubricant film, and the lubricant conductivity generally determines the friction and wear characteristics of the contact. Colloidal metal particles produce additional conductivity in the clearance between the contacting surfaces, prevent electrical erosion and, in some cases, form plastic films which decrease the coefficient of friction and the wear rate of the contact.     

Mechanism-based modeling of friction and wear

Equations for “predicting” friction and wear that product design engineers can readily apply require new methods to develop. These include the following. 1. The planned cooperation of several technical disciplines.

2. The selection of a well studied system.

3. Parametric equations (models) for the observed friction or wear behavior, including all variables known by an interdisciplinary group.

4. Data that cover a wide range (e.g. several decades) of most relevant variables.

5. Methods for adjusting parameters in the models to closely match the data. The results of three research efforts are discussed in terms of the prospects for developing equations either by curve fitting the data or by describing underlying phenomena involved in the sliding process. Finally, possible contributions of several technical disciplines toward modeling the conditions for scuff prevention are suggested.     

Modelling of friction and wear for designing cryogenic valves

From a tribological point of view, the selection of materials for seals for valves of cryogenic rocket engines is a critical issue for the designer. Due to the lack of comprehensive information on this topic, data have been obtained in the framework of research programmes.In the first step, two polymers potentially usable for sealing cryogenic fluids were identified. They were submitted to general test conditions to gain some fundamental understanding of their tribological behaviour when immersed in a cryogenic fluid. This fundamental understanding proved to be helpful for an efficient technical approach.This approach with the test conditions as close as possible to those met in real valves has been used in the second step. In order to obtain the best information with the minimum number of tests, the statistical method of Doehlert has been adopted. The use of such a method led to the construction of surface response for modelling friction and wear. These surfaces and their associated equation are directly and easily usable by the designer. This way of treating technical tribological problem is illustrated by an example.     

弹性啮合与摩擦耦合离合器的实验研究

介绍弹性啮合与摩擦耦合离合器结构、测量方法以及实验数据的分析.试验表明该离合器有较好的启动性能,并介绍了一种新的弹性啮合与摩擦耦合结构离合器.     

Advances in friction and wear mechanisms

This review considers what inferences regarding the mechanism of wear can be derived from recent investigations into the functioning of machine elements operating under conditions representative of engineering practice. Among the subjects discussed are particle analysis, surface texture effects, stress system, thermal effects, chemical aspects of wear, and ehd effects     

A study on wear and wear mechanism of exhaust valve and seat insert depending on different speeds using a simulator

The wear of engine valve and seat insert is one of the most important factors which affect engine performance. Because of higher demands on performance and the increasing use of alternative fuel, engine valve and seat insert are challenged with greater wear problems than in the past. In order to solve the above problems, a simulator was developed to be able to generate and control high temperatures and various speeds during motion. The wear simulator is considered to be a valid simulation of the engine valve and seat insert wear process with various speeds during engine activity. This work focuses on the different degrees of wear at three different singular test speeds (10 Hz, 25 Hz & multi-Hz). For this study, the temperature of the outer surface of the seat insert was controlled at 350°C, and the test load was 1960 N. The test cycle number was 6.0×10⁶. The mean (±standard error) wear depth of the valve at 10 Hz and 25 Hz was 45.1 (±3.7)μm and 81.7 (±2.5)μm, respectively. The mean wear depth of the seat insert at 10 Hz and 25 Hz was 52.7 (±3.9) μm and 91.2 (±2.7) μm, respectively. In the case of multi-Hz it was 70.7 (±2.4)/on and 77.4 (±3.8) μm, respectively. It was found that higher speed (25 Hz) cause a greater degree of wear than lower speed (10 Hz) under identical test condition (temperature, valve displacement, cycle number and test load). In the wear mechanisms of valves, adhesive wear, shear strain and abrasive wear could be observed. Also, in the wear mechanisms of seat inserts, adhesive wear, surface fatigue wear and abrasive wear could be observed.