The carbon dioxide (R-744) as a natural refrigerant has been an attractive alternative
refrigerant to replace HFC (hydrofluorocarbon) refrigerants currently used in air conditioning
systems, due to the environmental concerns. Because new compressors with CO2 are going to be
operated under the high pressure, the tribology of sliding surfaces in the compressor becomes very
important. To develop new compressor, especially scroll type, the friction and wear characteristics
of sliding surfaces between a fixed scroll and an orbiting scroll in the scroll compressor were
investigated in this paper. The sliding tests of pin-on-disk type were carried out between scroll
surfaces under various sliding speeds, normal loads, surface roughness, and refrigerant pressures.
Two types of lubricants were used, namely POE (polyol ester) oil and PAG (polyalkylene glycol)
oil. The friction forces, wear amounts and surface temperatures were monitored during the tests.
Test results showed that the wear amounts increased with increasing normal load and rotating
speed, and also decreased with decreasing surface roughness. Also, the results of the sliding tests
showed that using PAG oil has an advantage over POE oil in CO2 environment. As the pressure of
CO2 refrigerant became higher, the amount of wear and coefficient of friction became larger in
CO2/POE mixed environment, but wear and friction remained constant in CO2/PAG mixed
environment. The wear coefficients in CO2/PAG and CO2/POE mixed environment were 8.60×10-9
㎣/N·m and 2.35×10-8 ㎣/N·m, respectively.
The characteristics of the Rayleigh surface wave are investigated indirectly by measuring the angular dependency of the backward radiation of the incident ultrasonic wave in the scuffing specimens, and then compared to the residual stress distribution. The peak intensity of the backward radiation profile decreases and the right half width of the profile increases as the increase in the variation of residual stress distribution for the scuffing specimen. The peak intensity and angle depend on the amount of residual stress as well as the inhomogeneity in microstructure that causes scattering during the propagation of the surface waves. The perturbation theory about SAW dispersion was applied to understand the break-in procedure. The result observed in this study demonstrates high potential of the backward radiated ultrasound as a tool for nondestructive evaluation of the subsurface gradients of materials.
In this paper, the effects of the residual stresses on the life of structural engineering
components were investigated to predict the wear-life. Residual stresses are usually created by the surface treatment, such as shot peening or deep rolling. The objective of the experimental investigation was focused on the influence of friction and wear characteristics due to the residual stress under dry sliding condition and the measurement of the interleaf friction. Test specimens were made of SUP9, the leaf spring material. The residual stresses were created on their surface by shot peening treatment. Residual stress profiles were measured on the surface and subsurface by means of X-ray diffraction method. Also, the sliding tests were carried out under the different contact pressure and the same sliding velocity of 0.035m/s in order to investigate the friction and wear characteristics of leaf surfaces. Leaf spring assembly tests were performed by the ultimate tensile machine with sticking strain gauges on the leaf surfaces in order to evaluate interleaf friction characteristics. We compared friction coefficients, wear volumes and wear rates of the shot-peened specimens with those of the unpeened specimens, and evaluated the effects of residual stress on the tribological characteristics. We obtained the load-displacement curves, and measured interleaf friction force and interleaf friction coefficient of leaf spring assembly from these curves.
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