"Preload" enhances cushioning - train loading technique

Railway Age, May, 1995 by S.K. Punwani, F.D. Irani

Slack is reduced because the preload - up to 100,000 pounds - prevents the cushioning unit from moving until a force greater than the preload is present.

Recent advances in cushioning unit technology have resulted in end-of-car (EOC) cushioning devices with a preload feature that controls train action forces and accelerations. These new devices have the potential to reduce damage to lading, reduce peak coupler forces, and reduce wear and tear on draft system components.

Slack, the unrestrained free movement between the cars in a train, has been a necessary part of train operations from the beginning of railroading, but can result in undesirable in-train forces. In addition, yard impacts occur as part of classifying cars for different destinations. To lessen the effect of these impacts, cushioning units are installed in cars carrying sensitive commodities to protect these commodities from damage.

However, EOC cushioning devices contribute to additional slack in the train, resulting in dynamic forces during train operation. As a result of AAR research and supplier response to the challenge of two recent AAR performance specifications, new cushioning unit technology has emerged that controls train action slack effectively without sacrificing protection from yard impacts. The two new specifications, M-921-B and M-921-D, were developed in 1989 and late 1993 respectively.

Under these specifications, train action performance requirements were made a part of the cushioning unit specifications for use on all car types. This represents a significant step forward in controlling undesired slack action in today's trains.

Train action performance requirements under these new specifications have resulted in the development of cushion units that have a "preload." This "preload" prevents the cushioning unit from stroking (moving) until a force greater than the preload is present. In the absence of a force, the unit returns to its fully extended position at a slow rate. Force levels of preload have been established for closure speeds of one, two, three and five mph. Selecting the performance requirements was a difficult task, particularly the selection of a preload level.

* A history of cushion devices. A short history lesson will explain why a preload is needed. The evolution of heavier freight cars and longer trains has been accompanied by increases in free slack, yard impacts, and train action. Commodities of a sensitive nature require added protection against yard impact shock and in-train shock resulting from slack action. Paper rolls, auto parts, finished automobiles, and other manufactured goods are all examples of commodities requiring additional protection from damage during transport.

The traditional railcar draft gear compresses completely at impact speeds of four to six mph. To protect against the possibility of higher speed impacts, mechanical cushioned underframes with a sliding sill were developed in the 1940s. These were used until the Hydracushion sliding sill arrangement was developed in the mid 1950s. This mechanical/hydraulic arrangement provided protection from impact speeds up to 14 mph. In train operations, however, free slack caused by couplers still contributed to in-train forces. The Hydracushion mechanism consisted of mechanical friction plates used along with hydraulic damping, including a metering pin.

Following the initial success of the Hydracushion unit. other sliding sill devices were designed and placed in service. High Cube box cars built in the 1960s were all equipped with sliding sill cushioned underframe devices. These devices have been remarkably reliable and maintenance free. and are still in operation today.

EOC cushioning devices were developed in the late 1960s after it was realized that the additional weight of the sliding sill devices was not cost effective (sliding sills add 5,000 pounds to the tare weight of the car). The purpose of these devices was to protect the lading against high speed yard impacts.

The use of EOC cushioning devices increases the effective car length in a train, and can cause high in-train forces. The true effect of changing train lengths was not realized until blocks of cars with EOC devices were operated together in a train. Test programs were conducted by the manufacturer with the assistance of the Canadian National Railways, the Seaboard Coast Line (now part of CSXT), and Tropicana. These tests showed the need for train action control and for specification requirements to control train action, and resulted in a general set of orifice changes to improve train run-out control. Train run-in was virtually unchanged since the impact requirements were the same.

Additional control valving was developed for train action control by one manufacturer, but had limited commercial success. This issue remained dormant for a decade until the AAR Track-Train Dynamics Program was initiated. In the late 1970s and early 1980s, guidelines were developed to reduce in-train action. These guidelines were the culmination of an extensive test program to obtain the force-displacement characteristics of draft gears and EOC cushioning units at low speeds encountered during train action. These characteristics are incorporated in AAR's Train Operations and Energy Simulation program (TOES).