Interface - The Journal of Wheel/Rail Interaction

WHEEL RAIL INTERFACE

Pre-engineering the Wheel/Rail Interface (continued)

Corrugation is a common problem in the wheel/rail interface. Corrugations are a result of vibrations exciting a wavelength-fixing mechanism that is characteristic of that particular vehicle/track system (see Figure 3). This gives rise to dynamic loads and possibly stick-slip, which act as the input to a damage mechanism (wear, contact fatigue, or plastic flow) acting on the rail. This causes periodic damage, which excites additional vibration at that same wavelength, and the process continues in a feedback loop.

fig 3

The wavelength-fixing mechanism could arise from the natural torsional frequency of the wheelsets, or a vertical bogie suspension frequency, or a track vibration frequency set by the stiffness of the rail pads. The damage mechanism for transit systems is usually wear.

Unfortunately, corrugation development is hard to predict. It is possible to design against damage mechanisms, but it may be impossible to remove wavelength-fixing mechanisms. Corrugations may still develop in a well-engineered system, but their rate of development can be significantly curtailed through proper design and effectively treated through regular preventive maintenance. Corrugations are self-reinforcing, however, and once they appear they can only be removed by rail grinding or rail replacement.

Wheel tread damage is a common symptom of wheel/rail interface problems. The wheelset shown in Figure 4a has slid along the rails, leading to a type of defect known as a spall. (Sliding can result from sticking brakes, or because of heavy braking in low friction conditions.) When a wheel slides, frictional energy flows into the wheel through the contact patch. As soon as the wheel stops sliding, the overheated steel in the contact patch is quenched by the large thermal mass of the wheel. The steel in the contact patch transforms into martensite, which is a very hard but brittle phase of steel. As a result of the skid the wheel has a flat spot at the martensitic area, causing an impact at each revolution. Cracks develop in and propagate through the martensite. When the cracks branch together below the surface, the martensite piece breaks out of the tread, leaving a spall on the wheel surface (see Figure 4b).

Friction Management
There are two friction zones on rail: the gauge face and the top of rail. Both have different friction requirements. The gauge face of the high rail in any curve where vehicle steering is accomplished through flanging should be lubricated. The gauge face of the low rail could also require lubrication on some tight-radius curves, if back-of-flange contact occurs.

The coefficient of friction (COF) on the gauge face of the high rail (or low rail, if required) should be 0.2 or less in order to minimize wear (see Figure 5a). The COF on the top of rail (TOR) of both rails in curves should be 0.30 - 0.35 in order to prevent loss of adhesion while reducing lateral forces on the curve rails (see Figures 5a and 5b). There are commercially available products that can maintain the TOR COF in this range. TOR friction modifiers can control friction to reduce wear and maintain traction. They have also proven effective in reducing or eliminating wheel squeal and slowing the formation of stick-slip corrugations.

Given the many competing demands in the wheel/rail interface, designing an optimized system can be daunting. Fortunately, with modern technology the effects of profile designs, bogie characteristics, rail hardness and lubrication can all be examined through quasi-static or dynamic modeling. The inputs needed for quasi-static modeling include the wheel/rail profiles, wheel loads, vehicle speed, suspension stiffness, curve radius, superelevation, track gauge/wheel back-to-back distance and COF values. Contact stresses, tangential forces, frictional work and surface damage are the parameters that must be controlled during the design process, and the effects of the inputs on these parameters can be investigated. Figure 6 is a screen capture of the quasi-static curving software developed at CSTT for modeling profile performance, but commercial modeling packages can also be used for this purpose. CSTT also applies fully dynamic simulations using Vampire, NUCARS and SIMPACK to design and optimize wheel/rail and vehicle-track performance, especially for systems where stability, curving and derailment are the dominating concerns.

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