Interface - The Journal of Wheel/Rail Interaction

ROCK AND ROLL

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

Car and Vehicle Factors
Freight cars are not created equal when it comes to harmonic rocking. Certain characteristics make some cars more susceptible to rocking. The principle factors in determining the probability of severe rocking are:
• Vehicle truck center distance and weight of car.
• Vehicle center-of-gravity height above rail.
• Spring constant or stiffness.
• Type and amount of damping in the suspension system.
• Amount of side bearing clearance.

Freight car geometry is generally fixed, depending on the car design and purpose. As stated, when the truck center distance is close to the rail joint spacing (39 feet, nominal), rocking becomes more severe. Thus, longer cars, and cars with articulated joints are not as susceptible to harmonic rocking. When the center of gravity exceeds 90 inches, the severity of rocking is increased. Hopper cars tend to have higher CG heights due to the voids formed by the slope sheets.

Suspension stiffness and damping are critical factors in determining susceptibility to rocking. While spring stiffness is nominally fixed, depending on the car weight and dimensions, damping levels can vary considerably, depending on the wear in the suspension system. Previous Interface Journal articles have addressed the importance of suspension damping and wedge rise. (See "Truck Warp: The Causes and Cures – Part 1" and "Truck Warp: The Causes and Cures – Part 2") As wear occurs on the damping elements, the ability of the suspension to dampen out the unwanted rocking motion is severely compromised. Thus, wedge rise should be maintained to manufacturer’s specification in order to control or prevent excessive rocking. Figure 2 shows a truck with excessive wedge rise.

Some cars require supplemental hydraulic damping to pass the AAR car stability acceptance criteria (M965). In these cases, it's incumbent upon maintenance personnel to ensure the effectiveness of the hydraulic damping system by checking the level of oil in the sight glass or by looking for signs of leakage.

Side bearing clearance for standard roller side bearings should be maintained between 3/16 and 5/16 inches. Excessive side bearing clearance is a prime contributor to the severity of car rocking. Side bearing clearance in the range of 1/2 inch will allow the car to lean or roll to excessive limits, which increases the energy input to the springs and car body.

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