Railway Track & Structures 12/07

Wheel climb Figure 5, above. The much longer wheelbase and stiff suspension of a typical commuter car results in higher angles of attack, higher L/V ratios and higher risk of wheel climb. Dry rail (Ì=0.45). Figure 6, above. Wheel-climb index for three different contact conditions, comparing a high flange angle wheel versus a lower flange angle variant. (see the example in Figure 3) which would suggest that lubricating rail invites wheel climb. But the Nadal relationship tells us that for the same wheel-flange angle, decreasing the friction coefficient from 0.5 to 0.25 leads to about a 60percent increase in the allowable L/V. Since the increase in L/V with lubrication is much more than offset by the allowable L/V value, there is a net reduction in wheel-climb potential. This permigs us to use a wheel-climb index to assess the risk of wheel climb. The wheel-climb index as proposed is simply the ratio of the current wheel/rail L/V ratio, divided by either the Nadal or Weinstock limits using the profiles and friction conditions considered: WCNADAL = (L) (V)HIGH Nadal Limit + (L) WCWEINSTOCK = (L) (V)HIGH (V)LOW Nadal Limit +µLOW An actual wheel-climb incident will always have some significant dynamic component associated with it, usually either a higher instantaneous lateral force from a lateral perturbation or vehicle resonance, or a momentary decrease in the vertical force due to wheel unloading. This vertical unloading may be very short in duration such as when running over a weld, or it may endure longer such as when the outer wheel unloads while negotiating a spiral. It is also possible for a wheel to climb on tangent track, either due to a truck that is severely parallelogrammed, or from very high lateral forces, e.g., from truck hunting, combined with large track cross-level errors. It has been noted6 that in most physical cases, high ratios of L/V on the flanging wheel are associated with a decrease in vertical load on the flanging wheel. This stage of the work considers only the possibility of wheel climb in curves, under quasi-static conditions. We purposefully and necessarily made the following base assumption: Any set of conditions that promotes high L/V ratios under quasi-static conditions is similarly more vulnerable to climb under dynamic conditions. Implementing wheel-climb index A useful range of parameters for consideration in wheelclimb studies includes gauge-face and top-of-rail friction coefficients, track curvature, track gauge, cant deficiency, suspension stiffness, wheelbase and wheel/rail profile conformality. We looked at a single curvature of 4.7 degrees (350m), slightly wide track gauge (56.7 inches = 1440mm), balanced 40 Railway Track & Structures December 2007 www.rtands.com http://www.rtands.com

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Railway Track & Structures - December 2007 Contents On Track Industry Today Supplier News AREMA News NRC News TTCI R&D Hand-Held Tools Provide the Power to Get Big Jobs Done AREMA C&S Update Striving to Solve the Riddle of Wheel Climb RTA Stages Warm Gathering in Sunny Florida in October Products and Literature People Calendar Website Directory Advertisers Index Sales Representatives Professional Directory Classified Advertising Chicago Perspective

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