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Effects of Rail Cant on Wheel/Rail
Forces and Derailment Potential
(continued)
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Undesirable rail cant is created when the
outside of the plate cuts significantly more than the inside.
There are three stages of differential rail cant:
• Stage 1: approximately 1/4-inch of differential cutting.
• Stage 2: 1/2- to 5/8-inch of differential cutting.
• Stage 3: 1 inch or more of differential cutting (which represents about
a 3-degree
cant angle).
The causes of negative rail cant include:
• Excessive superelevation, which causes high vertical forces on the low
rail of curves. This occurs when the elevation of a curve is not properly matched
to actual speeds. With more coal and other long trains operating on the system,
train speeds on some lines are considerably less than what they were 10 years
ago. In other instances, a curve may be properly balanced but trains may be running
under the balance speed because of slow orders or approach signals on the line.
Operating under balance speed can also cause spalling, flattening and pummeling
of the low rail.
• High-center-of-gravity cars such as grain or double-stack cars. The higher
the center of gravity, the higher the roll moment that they put on the rail,
which
can aggravate the condition.
• Operating at overbalance speeds, which can generate excessive force against
the high rail. While trains operated at less than design speed may vertically
load the low rail, operating above the design speed can throw more weight against
the high rail and cause it to roll to the outside. Running over balance leads
to heavy field-side plate cutting and gauge corner shelling on the rail.
Negative cant conditions increase the potential for rail rollover, wheel climb
and reverse wheelset steering. Rail rollover potential can be calculated by the
ratio of the base to the height of the rail. The lateral loads act on a moment
arm that is roughly 7 inches above the top of rail. That force tends to roll
the rail outward, in clockwise direction. Vertical loads are applied near the
gauge corner creating a moment at a dimension approximately 4-1/2 inches across
from the pivot point at the base (see Figure 1). This represents the point at
which the vertical loads are attempting to roll the rail back down in a counterclockwise
direction. When balanced, these opposing forces hold the rail in place. An L/V
of about 0.64 is required to initiate rolling in a typical new rail section.
Curve-worn rail, however, affects the positioning of these forces. The lateral
load might act on the gauge face at a height of 6.85 inches and the vertical
load might be transferred through two-point contact more closely to the center
of the rail, yielding a base dimension of 3 inches. So for severely curve-worn
rail, the L/V required to roll that rail section is about 0.4. As a result, rail
wear represents a significant factor in whether a given curve will have a tendency
to roll.
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