Interface - The Journal of Wheel/Rail Interaction

ROCK AND ROLL

Rock 'til You Drop:
Starting and Stopping Harmonic Rock and Roll
By Gary Wolf • February 1, 2005

It's said that "it takes two to tango," but it takes three critical elements to induce harmonic rock and roll:
• An operating speed between approximately 12 and 24 mph.
• A vehicle conducive to rocking due to its mass and spring characteristics.
• A series of vertical track perturbations that are severe enough to excite the vehicle.

Like many of the other legendary "rockers," freight car rock and roll came to the forefront in the ‘70s. The introduction of high-center-of-gravity, 100-ton hopper cars operating on jointed track caused a dramatic increase in the number of derailments attributed to the phenomenon of harmonic rock and roll. The problem was met head on with a number of industry research initiatives that clearly defined the problem and potential solutions. Even with those solutions, however, a number of derailments are attributed each year to harmonic rock off (FRA code M405). More than 260 reportable derailments were caused by harmonic rock off in the U.S. over the past three and a half years.

Any mechanical system comprised of springs will vibrate or resonate at a given natural frequency, know as its harmonic frequency, when excited. A good example of a simple harmonic system is a guitar string. Depending on the mass (weight/diameter) of the string, its tightness (spring constant), and location of striking, the string will resonate with a certain tone or frequency. A freight car is also a harmonic system comprised of mass and springs with a critical frequency at which it tends to resonate. For a typical freight car, the resonant frequency occurs at between 12 and 24 mph.

The energy required to excite car rocking comes from the track structure, principally the pattern of joint spacing. Joints tend to deflect vertically under the weight of the wheel, allowing the springs to compress in response to the joints. The energy stored in the compressed springs is then fed back into the car body between the joints, propelling it in the opposite direction. If the joints are staggered at approximately one-half rail length apart, the maximum energy is fed into the car body at just the precise frequency to violently rock the car back and forth, from side to side. In extreme conditions, wheel lift and derailment can occur. The forward speed of the car, in conjunction with the joint deflection and spacing, determines the exciting frequency. The spring characteristics, center of gravity height, and truck center distance determine the resonant frequency of the freight car. Therefore, operating at the resonant speed over jointed track has the potential to excite a car into its natural rocking mode. Figure 1 shows the harmonic response curve of a typical hopper car.

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