- Wheel Climb
While a train wheel or wheelset rolls along the track on its tread, some guidance is provided by the steering ability of the coned or profiled tread as the track changes direction. However, it is the flange of the wheel ultimately contacting the gage face of the rail that really forces the wheel to follow the rail and change direction. The forces generated between the wheel and the rail can be resolved into two components — the lateral force (L) and the vertical force (V). These force components are illustrated in Figure 1.
In order for a wheel’s flange to climb up the gage face of a rail and over the rail head to the outside of the track, the wheel lateral and vertical forces must be such that the vertical force that acts to keep the wheel on the rail is overcome by the lateral force and the friction forces that exist between the wheel’s flange and the gage face of the rail. The ratio of lateral force to vertical force that must be exceeded in order for the wheel climb scenario to exist is specified by Nadal’s Limit, a criterion that is defined as follows:
where ì is the coefficient of friction between the wheel and the rail and a is wheel flange angle as illustrated in Figure 1.
- Gage Widening and Rail Rollover
The problems that can be encountered with excessive wheel/rail interaction forces are not limited to wheel climb scenarios. For instance, if the lateral force generated by the flange contact between the wheel and the rail is relatively high, this force can cause lateral rail displacement. This rail displacement produces what is known as gage widening and can lead to a wheel/rail separation as shown in Figure 2.
Rail rollover, as shown in the left side of Figure 3, is one of the most common sources of accidents especially when the vehicle travels over the spiral transition between tangent, or straight, track and the full body of a curve. The critical value of L/V for rail rollover is approximated by the ratio D/H with D and H defined as per the illustration on the right side Figure 3. In general, if the moment generated by the lateral force is higher than the moment generated by the vertical force, the rail can rotate about its corner.
- High Wheel Loads and Their Effect on Switch Components
As a wheel passes over the switch point and the tip of the stock rail, a high lateral pressure on the tip of the switch point will be generated. This can lead to battering of the switch point and the eventual fracture of the stock rail. This situation, in combination with thin, high flanges, can lead to the scenario where the wheels can get caught on the switch point and allow the wheel to ride up on the switch components.
- Vehicle Lateral Instability
The wheels of rail vehicles are tapered in order to allow for the wheelset to steer down the track. Small lateral displacements of the wheelset will result in the steering of the wheelset back and forth between the rails in an oscillatory or “swaying” motion. When that oscillatory motion of the wheelset and the vehicle continues to grow in amplitude as the vehicle moves down the track, the vehicle is said to be experiencing lateral instability.
For each vehicle there is a specific speed known as the critical speed, above which the vehicle will exhibit lateral instability. This speed is highly dependent on the characteristics of the vehicle suspension systems, mass and mass moment of inertia of the vehicle bodies, and the wheel profile. In general, track irregularities push the wheelset laterally. If the vehicle speed is above its critical speed, the wheelset lateral motion has the tendency to grow. The lateral motion of the wheelset will be limited only by the wheel flange coming in contact with the gage face of the rail. In the case of severe lateral instability, high forces can result from the impact of the wheels on the rail that can lead to damage of the track and create conditions that could eventually lead to derailments.
- The Effect of Hollow Worn Wheels
When a wheel exhibits significant wear on its tread to the point where a rut is formed around the circumference of the wheel around the center of the tread, it is said to be a hollow worn wheel. A typical measurement of hollow wheel tread is taken as the vertical depth of the lowest point on the tread of the wheel from a straight edge established as the plane of the original unworn design tread taper. This measurement procedure is illustrated in Figure 5.
Two conditions of concern can exist with hollow worn wheels. As the tread wears, the tip of the wheel flange “moves” further away from the top of the rail, creating a condition where the top of the flange is considered to be high and can strike switch components. Hollow worn wheels also adversely affect the steering of the truck and can increase the potential for derailment. A hollow tread can result in a shift of the wheel/rail contact location to the field side of the rail, thereby increasing the amount of vertical force that is located outside the gage of the rail. This situation can significantly increase the potential for rail roll and ultimately to a rail rollover derailment.
Strategies to Consider for the Prevention of Derailments
There is no easy way to provide complete protection from derailment. It requires commitment and diligence on the part of the operator to implement and adhere to sound practices aimed at minimizing or eliminating those factors that can contribute to a derailment scenario. Based on the discussion presented to this point, the following points are offered for consideration during the development of a derailment prevention strategy:
1. No industry can fully eliminate the occurrence of human error. In an effort to minimize these errors, operators need to be vigilant in enforcing their training requirements for all personnel. Just as critical, however, is the review and refinement of training and procedures. Operators must be able to objectively analyze and, when necessary, modify policies and training when issues arise to mitigate the chances of human error leading to a derailment.
2. A critical component to the prevention of derailments is reliable track and vehicle inspection practices. In the past, operators have responded to reports of excessive car body motions over locations that are exhibiting indications of failure by issuing slow orders and reporting them to the track department for attention before a derailment occurs. Not only is this approach reactionary in nature, but it is of little use in areas with low track speeds, in which locations of concern will result in a lack of significant car body motion. In cases like this, the operator is typically unaware of the hazard until the train derails.
Operators should consider being proactive in developing their inspection strategy. Detection of incipient component failures should be a top priority for operators. The measurement of the response of the vehicle to track can be a very effective means of identifying track locations of concern.
3. Railroads and transit operators must also aggressively perform maintenance as required. An operator with optimal inspection practices are only part way through an effective derailment prevention strategy. To that end, the following suggestions for key aspects of a maintenance strategy are offered:
A) Proper wheel flanges must be established and maintained. Depending on the design and maintenance of the truing machine, newly trued wheels can have “shallow” flange angles on the order of 60 degrees. Such wheels will most often wear to steeper flange angles (72 to 75 degrees) within 100 to 200 miles of operation. If they do not encounter any adverse track conditions in the meantime, they will be fine. However, if one of these wheels with a shallow flange angle encounters a low joint or a brand new switch point (with a “scaly” or “rusty” high-friction surface), a derailment can result. American Public Transportation Association (APTA) strongly recommended in its “Passenger Rail Equipment Safety Standards Task Force Technical Bulletin 1998-1” that organizations develop or adopt maintenance and inspection practices that ensure that a minimum sustained flange angle of 72 degrees to provide a margin of safety to protect against derailment. Assessment of the wheels following truing is a key aspect of this recommendation.
B) In addition to the level of attention that is required for wheel flanges, the operator should routinely inspect wheels for flaws that can lead to flats. The inspection process should also identify wheels exhibiting hollow worn treads for remediation to reduce the risk of gage widening and rail rollover. Restoration of the wheel profile can also lead to improved vehicle stability and hence reduce the potential for derailments due to lateral instability.
C) Those responsible for vehicle maintenance should maintain suspension clearances and springs to assure trucks (bogies) equalize properly. It is important to note that some truck designs are more sensitive to spring problems than others. Manufacturer’s tolerances should be followed closely. It should also be pointed out that some newer truck designs, such as those from Europe and Asia, are typically less tolerant to track warp or twist. They often include chevron, rubber springs, which are stiffer than “equalized” truck designs common in older designs.
D) Lubrication of center plate of the track will reduce the generated resisting moment to the steering moment during curve negotiation. The use of proper lubrication will reduce the required force for steering and hence reduce the tendency of derailment in curves.