Interface - The Journal of Wheel/Rail Interaction

SUSPENSION IMBALANCE

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

Flange Angle and Friction
In simple terms, a high coefficient of friction and low flange angle are a recipe for wheel-climb derailments. Increasing the flange angle alone can significantly increase the L/V threshold at which derailments are likely to occur. Assuming a constant COF of 0.5 (dry rail), the L/V ratio at which a derailment will occur increases with flange angle. For a 63-degree flange angle, the derailment limit L/V is 0.73; for a 65-degree flange angle, the derailment limit L/V is 0.79; for a 70-degree flange angle, the derailment limit L/V is 0.95; for a 72-degree flange angle, the derailment limit L/V is 1.01; for a 75-degree flange angle, the derailment limit L/V is 1.12.

A wheel profile with a flange angle of 72-75 degrees substantially increases the L/V wheel-climb derailment limit and improves safety even under extreme conditions. The standard freight car wheel profile required by the Association of American Railroads (AAR) has a nominal flange angle of 75 degrees. The draft APTA wheel standard proposes a flange angle of no less than 72 degrees.

The coefficient of friction between the top running surface of the rail and the wheel tread varies between 0.1 (with leaves, slurry of rust and water) and 0.6 (bone dry, newly ground surface on sunny days). Fortunately, the extreme conditions do not prevail; most of the time friction is in the range of 0.25 to 0.35, or at worst 0.5. Freshly trued wheels are more prone to derailment since the COF between the wheel flange and rail increases due to the rough wheel (tread and flange) surface. As an illustration, Nadal’s criterion for derailment (i.e., single wheel L/V ratio) for a 63-degree flange angle indicates that under "typical" friction conditions with a COF of 0.3, vehicles approach the derailment limit with an L/V ratio of 1.04. Under "dry" friction conditions with a COF of 0.5, vehicles approach the derailment limit with an L/V ratio of 0.73. APTA recommendations (RP-M-009-98) for new truck design suggest that a COF of 0.5 be used for L/V ratio calculations (Nadal’s criterion).

Lubrication and Friction Modifiers
Friction plays a pivotal role in wear and contact fatigue, steering, hunting, wheel squeal, skid-flats, wheel climb and low rail rollover derailments. Managing friction at the wheel/rail contact patch is the single most powerful method for providing safe and economical operation. While traditional wayside and vehicle-borne oil-based lubricants effectively reduce friction, field experience has shown that no matter how they are applied to the wheel flange or gauge face of the rail, they typically migrate to, and contaminate, the top running surface of the rail. Although top-of-rail lubrication has been shown to reduce lateral forces, rail wear, and energy consumption, uncontrolled contamination of the rail running surface with conventional lubricants adversely affects train traction and braking.

Friction at the wheel tread/top-of-rail interface is a function of pressure distribution and the creep condition (relative slip or slide). The most influential factor is the interface layer between the wheel and rail which typically consists of residual lubricants, wheel, rail and brake pad wear debris, environmental contaminants, and moisture. The only way to control this interface is to dispense a substance with a known and desired friction value, i.e., friction modifiers.

There are three primary types of friction modifiers. Low Coefficient of Friction (LCF) modifiers, with COF of 0.2 or less, are used to reduce friction at the wheel flange/rail gauge interface. High Positive Friction (HPF) modifiers, with COF of 0.2 - 0.35, are used at the wheel tread/top-of-rail to combat the growth of short pitch corrugation, reduce hunting or eliminate squeal (see Figures 1 and 2). Very High Positive Friction (VHPF) modifiers have the same friction range as HPF, but a "flatter" friction characteristic, with higher friction attained at lower creepage. Unlike grease and sand, the new-generation friction modifiers (which are generally applied in the stick or liquid form) do not migrate and tend to stay where they are applied and maintain the desired friction levels. Since lowering the COF between the wheel flange and the rail has the most significant positive effect on wheel-climb derailment protection, LCF modifiers should be used at critical locations.

< PREVIOUS PAGE | PAGE 2 OF 3 | NEXT PAGE >