Interface - The Journal of Wheel/Rail Interaction

SUSPENSION IMBALANCE

Effects of Secondary Suspension Imbalance on Wheel-Climb Potential (continued)

Wheel and Rail Profiles
When a derailment occurs, wheels from the derailed truck should be measured with a profilometer, such as MiniProf®. Rail profiles of high and low rails should also be measured, prior to and after the point of derailment.

Analysis of worn wheel and rail profiles can determine the type of contact between the wheel flange root and rail gauge corner, and the wheel flange and rail gauge face. Two-point contact between the wheel flange and gauge face creates an additional moment acting as a lifting force on the climbing wheel. Gauge-face wear leads to “conformal” contact. The conformity between wheel and rail should be monitored and kept under prescribed limits. (An example of wheel/rail interface geometry for new and worn rail conditions is shown in Figure 3.)

Contact between the wheel flange and gauge face of the rail, i.e., “flanging,” typically occurs when there is either insufficient self-steering due to limited conicity of wheels or when steering capability has been exceeded. The effective conicity and COF of the interface play a key role in determining the steering (as well as stability) characteristics of the vehicle. This is especially important in curved track, where self-steering impacts wheel and rail wear. In the absence of lubrication, the COF between the wheel flange and rail gauge face is generally high (0.4 to 0.6). Wheel and rail wear, ride quality, generation of noise, and formation of corrugations are all directly influenced.

Modern rail vehicles are often equipped with a Vehicle Health Monitoring System (VHMS) capable of recording data, such as sudden changes and transitions from propulsion to coasting or brake modes. Post-derailment data required by the Federal Railroad Administration (FRA) may provide additional information regarding the vehicle speed and accelerations. Most wheel-climb derailments occur at low speeds. Sudden changes from propulsion to coasting or braking may affect the lateral friction between the wheel and top of the rail due to the accompanying change in longitudinal slip. Figure 4 presents an example of data captured by VHMS in close proximity to an actual point of derailment. The transition from accelerating to coasting then to braking is clearly visible.

Vehicle Leveling Systems
The secondary suspension of a modern rail vehicle consists of two air springs per truck and an independent damping system consisting of one lateral and two vertical hydraulic shock absorbers. The air spring provides the constant height of the carbody above the truck frame, regardless of passenger load. Secondary suspension height control is provided by leveling valves. For a four-point leveling system, this consists of two leveling valves per truck.

The car leveling system should not be confused with the load-weigh system. The load-weigh system latches the current load-weigh measurement when the car is at rest. The latched load-weigh value is based on the average air bag pressure on a per truck basis, and is used to compensate brake and traction effort for various passenger loads. Since the leveling system on one truck is independent of the system on the other truck, it is possible for the air springs on one truck to be set higher than those on the other truck. No adverse effect occurs, however, other than a threshold height difference and a small error in the signal given to the load-weigh system. This does not affect the wheel loads at each truck.

On a four-point leveling system, however, it is possible to have an imbalance in the air spring pressure between air springs of the same track. Such a pressure imbalance will cause a twist to the carbody, which will be resisted by an equal (but opposite) twist from the truck at the other end. This can result in a car that, while resting horizontal and appearing correctly leveled, has unequal vertical wheel loads.

Air spring imbalance also can be caused by end-to-end and side-to-side imbalances in car weight distribution. The higher the weight distribution imbalance, the greater the resulting air spring load imbalances. Weight distribution imbalance can be corrected by adding ballast at the appropriate locations. To minimize the operational costs associated with added weight, the imbalance requirements should meet those in IEC Standard 1133 (Rail Vehicle Traction).

IEC Standard 1133 specifies 60,000 in.-lbs. maximum for side-to-side imbalance. The end-to-end imbalance (measured at the trucks) should not exceed 2% of the vehicle’s ready-to-run (AW0) weight. Also, the effect of end-to-end imbalance should be verified with regard to the load-weigh system. Braking effort is based on the maximum of the front and rear truck load-weigh values, while tractive effort is based on the averaged load-weigh values. If adhesion demands exceed the available coefficient of friction, wheel spin or slide may occur. Analysis should be performed to confirm that the brake and the propulsion control systems can handle such imbalances in all circumstances (on wet and dry rail) without negative consequences.

Radovan Sarunac is Lead Mechanical Engineer in the Washington, D.C., office of Booz Allen Hamilton; Peter Klauser is an Illinois-based Vehicle Dynamics - Engineering Consultant.

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