Determining how friction is measured by a tribometer requires some understanding of stick/slip mechanics in the contact patch at the wheel/rail interface. The contact patch can be composed of 'no slip' (stick) and 'slip' regions which vary as a function of creep. At 0 % creep, two bodies are considered to experience 'free rolling' (100 % stick) due to the absence of any torque. With increasing torque, the stick region of the contact patch shrinks as the slip region expands. Consequently, the boundary between stick and slip regions moves toward the inlet of the contact patch (the slip region is located at the outlet of the contact area). The rolling bodies in this regime experience a combination of rolling and sliding. As slip increases, the frictional force also increases linearly. When the stick region disappears altogether, creepage is deemed to be 'saturated' as the friction force reaches its maximum and remains constant (theoretically). The entire contact area is in the state of pure sliding, even though the bodies may appear to roll. To better understand similarities and differences among various measurement devices, a test matrix was completed. The following key parameters were explored: 1. Hertzian stress range, 2. aging effects on surfaces and third body materials, 3. range of expected friction values from various materials, 4. speed effects, 5. effects of slope in saturated creep regime. The runs to show the effect of changing the Hertzian contact stresses were performed on a mature surface of HPF material. lncreased stresses tend to reduce the saturated creep limit, which is consistent with Kalousek's third body studies where the increased pressure closes up asperities, etc. Aging is the process of running a particular application of third body material from its initial application until its influence on friction has diminished significantly. At the end of the 1 h run, the noise level indicated the film of HPF was depleted, although the friction level was still below dry steel. Runs of the TriboRailer wheel were performed across the available range of 6 - 60 mph (10 - 100km/h). No discernable change in friction coefficient My magnitude was observed; however, an expected increase in measurement noise was observed due to a magnification of dynamic effects. Although the various laboratory and field test devices reveal similar results, those results are often in different regions of the 'performance space' of the materials under test. A better understanding of the several factors important to friction behavior of third body materials between wheel and rail will help determine the specific usefulness of any one device. In particular, the relative age of the material, the region of the slip curve, and the Hertzian stress levels are all important factors in measuring a particular My. Future tests in both the laboratory and the field should be qualified by documenting these important factors to better evaluate the materials applied to the wheel/rail interface.
Recent developments in coefficient of friction measurements at the rail/wheel interface
Neue Entwicklungen zur Messung von Reibungskoeffizienten beim Rad/Schiene-Kontakt
2002
10 Seiten, 14 Bilder, 3 Quellen
Aufsatz (Konferenz)
Englisch
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