Interface - The Journal of Wheel/Rail Interaction

SIDE BEARINGS

Vehicle Side Bearings: Function, Performance and Maintenance (Part 1 of 2)
(continued)

Tight Side Bearings and Track Twist
Track twist can occur during spiral entry or exit, where twist rates of 1 inch in 50 feet are common. Track twist can also occur due to low spots or low joints in the track. The FRA sets limits on the amount of allowable track twist over a 62-foot chord or length. For instance, in class 4 track, the maximum allowable track twist is 2 inches over 62 feet. The result of track twist is vertical unloading, particularly on the diagonally opposite end of a car. If you hold a shoe box in your hand, and push down on one corner, you will note a tendency for the opposite diagonal corner to raise up. If this were a freight car, the raising action would result in an undesirable loss of wheel load. If side bearing clearance is nominal (1/4-inch), cars can accommodate the prescribed amount of allowable twist without exhibiting an excessive amount of wheel unloading. With tight side bearings, however, the effect of track twist on wheel unloading is worsened.

When negotiating twist, the car body leans and the pressure on the edge of the centerplate is increased. However, since the centerplate is only 16 inches in diameter, the resulting roll moment effect on the truck bolster is minimal. If side bearing clearance is tight, the vertical roll moment is instantly transmitted to the top of the roller and into the truck bolster, resulting in a tight compression of the bolster springs on one side of the truck. This creates a significantly larger roll moment on the bolster due to the effect of a 24-inch moment arm acting at the side bearing location. When this occurs, wheel unloading on the opposite side occurs. Cars with normal side bearing clearance can accommodate up to 2 inches of twist across truck centers with a loss of only about 20% of the vertical wheel load. However, cars with tight side bearing clearance can lose more than 50% of the vertical wheel load while negotiating track with only 1 inch of twist across truck centers. Thus, cars with tight side bearings are prone to wheel lift and wheel-climb derailments even on track with FRA-compliant track twist conditions.

Cars with excessive side bearing clearance tend to rock and roll excessively, especially when operating in the critical harmonic speed zone of 15 to 25 mph. The roll angle will increase proportional to the amount of side bearing clearance above 5/16 inches. Side bearing clearance should be checked at each corner of the car using a taper gauge, shims or other device. Always measure side bearing clearance on level, tangent track. Measuring side bearing clearance on twisted or curved track usually results in erroneous data. Figure 4 shows the use of a taper gauge to check the clearance between the roller and body wear plate. Consult AAR Rule 62 for further information on side bearing standards and maintenance procedures.

Part 2 of "Vehicle Side Bearings: Function, Performance and Maintenance" will examine design features and maintenance issues associated with constant contact or resilient side bearings.

Gary P. Wolf is President of Rail Sciences Inc.

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