Effects
of Wide Gauge on Derailment Potential (continued)
In addition to elongation of the spike holes,
the roll moment on the rail produced by the combination of lateral
and vertical wheel forces causes the wood fibers under the outer
edge of the tie plate to become permanently compressed, resulting
in the rail canting outward (see "Effects
of Rail Cant on Wheel/Rail Forces and Derailment Potential"). As the rail
cants outward, the gauge is opened up at the gauge corner of
the rail head.
The lateral tie plate displacement due to elongation of the spike holes and the
outward roll of the rail due to canting combine to produce a dynamic widening
of the gauge that can result in wheels dropping down in the gauge, resulting
in derailment. (Figure 2. shows an example of dynamic gauge widening in a curve.
Note the evidence of lateral plate movement outward in the presence of gauge-widening
wheel forces.)
Two additional sources of gauge widening are wear on the gauge face of the rail — particularly
the high side of curves — and wear in the shoulders of the tie plate. Many
railroads consider changing rail when gauge-face wear approaches 1/2 inch. Tie
plate shoulder wear is typically in the range of 1/8 inch to 1/4 inch total,
and
is not a major contributor to gauge widening. In order to determine the total
gauge widening in the track, four factors must
be added together:
• Lateral plate movement due to elongation of spike fasteners.
• Lateral displacement of the rail head due to negative rail cant.
• Gauge-face wear on the head of the rail at the gauging line.
• Wear in the shoulders of the tie plate allowing rail movement.
If the total gauge exceeds roughly 59 to 60 inches, derailment becomes almost
certain. This factor is somewhat variable due to FRA allowances for wheel flange
wear. As wheels wear to a thin flange condition, the safety margin is further
reduced by approximately 1/2 inch.
Wide gauge is undesirable for two primary reasons: 1) The safety margin for wheel
drop is reduced; 2) The steering capability of the wheelset is compromised.
SEPTEMBER 2004 "Truck Warp:
Causes and Cures (Part 1 of 2)" READ
ARTICLEOCTOBER 2004 "Truck Warp: Causes
and Cures (Part 2 of 2)" READ
ARTICLEAUGUST 2004 "Effects of Rail Cant
on Wheel/Rail Forces and Derailment Potential" READ
ARTICLE
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The lateral tie plate displacement due to elongation of the spike holes and the
outward roll of the rail due to canting combine to produce a dynamic widening
of the gauge that can result in wheels dropping down in the gauge, resulting
in derailment. (Figure 2. shows an example of dynamic gauge widening in a curve.
Note the evidence of lateral plate movement outward in the presence of gauge-widening
wheel forces.)
