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Effects of Rail Cant on Wheel/Rail
Forces and Derailment Potential
By Gary P. Wolf • August
28, 2004
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SPANISH VERSION CLICK HERE
There is a good deal of work being done to reduce the stress
state of the railway these days. With the number of 286,000-pound cars operating
over much of the North American rail network, efforts to reduce stresses and
derailment potential have never been more important. One way in which railways
can improve their margin of safety is by ensuring proper superelevation and
rail cant in curves.
Negative, or outward, rail cant has been shown to be a causative factor in
rail rollover and gauge-widening type derailments. Rail cant, and its effect
on lateral/vertical
(L/V) forces, can mean the difference between a safe operation and possible rail
rollover or wheel-climb derailments. Negative rail cant, which manifests itself
in the outward spreading of the rails, particularly in curves, is due to differential
plate cutting. This occurs as wheelset forces produce a moment that tends to
roll the rail outward, causing the tie plate to cut into the field side of the
tie more than the gauge side.
While the Federal Track Safety Standards address overall tie plate cutting, they
do not address the effects of asymmetrical differential tie plate cutting, which
typically generates a rail cant toward the outside of the rail. Most railroads,
however, do not approach the FRA Standard, which allows a plate to cut through
more than 40% of a tie's thickness before it must be changed out. Even if they
did, gross plate cutting isn't the issue. Differential cutting, where one side
of the plate cuts more than the other and causes the rail to cant by several
degrees outward, is what poses derailment potential.
What causes plate cutting forces?
On curves, flanging forces push the high rail toward the outside of the curve,
while lateral creep forces on the inside of the curve push the low rail out as
well. This lowers the L/V ratio required to produce rail rollover or gauge-widening
type derailments. It also lowers the L/V threshold for wheels to climb—especially
on lightweight, empty cars. This condition can further inhibit the positive steering
of a wheelset through a curve. VAMPIRE (a vehicle dynamics simulation model)
analysis of curving dynamics shows that as rail cants outward, it can defeat
the positive steering effect of wheelset conicity.
Rail Cant
In typical applications, the rail is canted inward toward the gauge side at an
angle between 1:20 and 1:40 cant. This is desirable and helps to keep the point
of the vertical contact centered on the railhead over the web of the rail. Normal
cant reduces the rail roll moment that tends to roll the rail outward. With zero
rail cant, the combination of lateral and vertical wheel forces will fall some
distance off the centerline of the track, producing a rolling moment and a tendency
to cut the plate. A positive, or inward, 1: 20 to 1:40 cant causes the L/V ratio
to fall closer to the centerline of the rail, creating a balanced force through
the base of the tie plate.
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