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Effects of Rail Cant on
Wheel/Rail Forces and Derailment Potential
(continued)
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As differential or reverse rail cant occurs,
the downward vector of the vertical load tends to shift toward
the field side, effectively reducing the dimension of the base
of the rail (see Figure 2) and pushing the L/V ratio into the
0.3 to 0.4 range. This combination of rail wear and reverse rail
cant can lower the threshold by which the rail might roll. Hollow-worn
wheels, which contact the field side of the low rail, move the
point of vertical load toward the field side of the rail, producing
a very small effective base dimension, significantly increasing
the potential for the rail to roll. This is why the low rail
typically is the first to roll in rail rollover or gauge-spreading
derailments.
Research has shown that about 20% of L/V ratios in a given train exceed 0.4,
but very few exceed 0.6. If an L/V of about 0.65 (which is required to initiate
rail roll) is maintained, the risk of rail rollover derailments is minimal. But
if the required L/V drops into the 0.4 - 0.5 range, approximately 20% of the
wheels going through a typical curve generate L/V ratios that can initiate rail
rollover. And while it typically takes two or three consecutive bad actor trucks
to roll the rail, if the rail rollover L/V ratio falls into the 0.4 range, a
significant percentage of wheels and cars are exposed.
The lowering of the required L/V ratio to roll rail is exacerbated by hollow-worn
wheels, curve-worn high rail and flattened low rails with excessive flow on the
field side of the low rail. Rail grinding and profile management, along with
effective top-of-rail lubrication, are also factors to consider when assessing
derailment potential. If the built-up flow on the field side of the low rail
is not ground away, even good wheels can contact the field side of the rail,
increasing the propensity for the rail to roll.
Wheel Climb
The propensity for a wheel to climb depends on two factors. The coefficient of
friction and the angle at which the wheel meets the rail—typically 70 -
75 degrees for worn rail, or about 85 degrees for new rail.
M. J. Nadal was the first to formulate that the tendency of a wheel to climb
and is predicated on the angle that the wheel makes (angle Φv) and the coefficient
of friction (µ) (see Figure 3). On a heavily worn rail (Φ = 60) with a
nominal surface (µ = 0.40), the wheel will climb at about a 0.79 L/V. On
a normal worn high rail with an effective gauge-face angle of about 72 degrees,
a wheel will climb at an L/V of 1.2. Not many wheels develop an L/V ratio of
1.2, but if the rail is canted outward, the angle of incidence will be reduced.
At about 66 degrees, the L/V ratio required for wheel climb is reduced to 0.98.
At this range many more wheels are susceptible to climb.
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