Interface - The Journal of Wheel/Rail Interaction

ROCK AND ROLL

Rock 'til You Drop: Starting and Stopping Harmonic Rock and Roll (continued)

Track Factors
Rock and roll occurs primarily on track with staggered joints. The FRA defines staggered joints as joints that alternate left and right and are spaced more than 10 feet apart. Many railroads’ past practice was to stagger the joints exactly one-half rail length (19.5 feet) apart. While this practice had merit in the early days of railroading, the advent of 100-ton hopper cars with nominally 39- to 40-foot truck center spacing caused severe harmonic rocking when operating over half staggered joints. Many railroads changed their practices to break this pattern and began to stagger joints at one-quarter to one-third rail length apart. This corrective action had good success in eliminating rock and roll. Another corrective action used by some railroads was to install longer 78-foot, jointed rail on one side of curves. This tended to break up the harmonic action by eliminating every other joint.

Simply operating a car over staggered joints does not automatically ensure that it will rock and roll violently. There must be sufficient vertical deflection at the joint to introduce sufficient energy into the car body to excite the rocking motion. In general, a dynamic joint deflection of more than 1 inch is required to begin significant rocking motion. Joints that deflect due to poor tie, ballast or pumping conditions are candidates to excite harmonic rocking. There also must be a number of consecutive joints, generally five or six in a row, to sustain the rocking motion. Maintaining track to FRA standards should prevent most rock and roll derailments. In addition to the well-known track geometry standards, the FRA made an important addition in 1999. FRA CFR49 213.63 states: "…to control harmonics on Class 2 through 5 jointed track with staggered joint, the crosslevel differences shall not exceed 1-1/4" in all of six consecutive pairs of joints, as created by 7 low joints…"

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