Abstract
American Steel Foundries introduced the Slackfree® Drawbar System to the North American Railroads in 1983. The system operated without significant problems until early 1993 when a series of yard derailments was reported on the CP Rail System. Derailments reportedly took place under empty or lightly loaded three platform container well cars. These cars were equipped with a shorter length drawbar system, developed in 1991, during facing point moves (in buff) through number 9 turnouts.
A series of yard tests conducted by the CP Rail in 1993 concluded that a sufficient lateral load applied to the car at the striker casting height would initiate near wheel lift on the truck ahead of the drawbar system under an empty platform as the car negotiated the number 9 turnouts. Yard tests also showed that the steel components inside the drawbar assembly could interact with the car geometry in a way that inhibits free lateral angling of the drawbar as the car advanced through certain curves.
ASF initiated a series of laboratory tests to determine the behavior of the system under conditions simulating the CP Rail Yard experiences. Tests were designed specifically to determine what might contribute to higher than intended lateral forces on the system including: (1) Component manufacture (tolerance, etc.), (2) Effect of non-lubrication, (3) Dynamic component behavior and (4) The mechanics of binding or jamming.
The results of the extensive laboratory investigation revealed that heavy draft loads on the system caused deflections that allowed the slack adjusting wedge to over-adjust, which, in turn, tended to inhibit free lateral angling of the drawbar. A solution was developed which allows the wedge to take up slack due to wear within the system without over-adjusting when the system is heavily stretched.