Thursday, July 29, 2010

Moving beyond M-976

Written by  Harry Tournay, Senior Scientist, TTCI, for Railway Age

TTCI’s “integrated freight car truck concept” is the next step in improved performance and reliability.

The Association of American Railroads’ M-976 truck design standard for 286,000-pound freight cars has improved wheel and rail wear and car dynamic performance beyond that of the standard three-piece truck. The advent of vehicle performance monitoring using wayside detectors, however, has enabled the root causes for poor car, truck, and component performance to be more closely defined, and this has led to a better understanding of the systemic nature of poor performance. The interaction between suspension and brake performance as well as that of the truck/carbody interface is better understood. Thus, now is the time for a more integrated approach to truck design.

TTCI, under the AAR’s Strategic Research Initiatives (SRI) Program, is developing integral freight car truck (IFCT) concepts and performance recommendations to further improve car and component performance and reduce track stresses. HIWs from unreleased hand brakes as well as thermal mechanical shelling (TMS), a condition related to overload of the rail/wheel contact patch, are the major contributor to maintenance costs.

TMS is caused by a combination of high tractions across the contact patch due to non-radial alignment of the wheelset in the curve, high wheel temperatures due to stuck brakes, and high wheel/rail friction conditions. Remedies include an improved-steering truck combined with improved truck brake rigging to control wheel temperatures, improved wheel steels, and controlled wheel/rail friction conditions.

The relationship between high tractions and wheel tread crack formation has been thoroughly studied by TTCI. Both are related to high angles of attack between the wheelset and the track developed in curves and under dynamic wheelset motions. These are related to poor wheelset steering associated with truck suspension characteristics. Crack formation is exacerbated by high wheel temperatures and wheel/rail friction conditions and can be inhibited with higher strength wheel steels. Improved brake performance, managed wheel/rail friction, and improved strength wheel steels are currently SRI initiatives. Improved wheelset steering required to eliminate TMS and HIWs will further improve car rolling resistance and fuel economy.

Asymmetric wheel flange wear also causes premature wheel replacement. One wheel is rejected for thin flanges when the opposite wheel’s flange on the same wheelset may be unworn—but the entire wheelset requires replacement. This wear pattern is associated with a combination of asymmetric brake forces and frictional stick/slip action of the truck in rotation to the carbody. Both of these are being addressed by IFCT. Loaded car hunting imposes high lateral forces on the track structure and leads to degradation of truck components (e.g., adapter pads). This is associated with wheel tread wear patterns leading to high wheel/rail contact conicities and resulting wheelset motions that excite carbody resonance. This reduces the ability of the friction wedges in the truck suspension to control truck warp and wheelset stability. One possible solution: provide truck warp resistance independent to the wedge system, also allowing a “softer” wheelset yaw constraint to improve motion and prevent HIWs.

Another problem is inconsistent (unpredictable) performance, between trucks in seemingly the same condition (whether new or partially worn) as well as for the same truck during its service life. This inconsistent or unpredictable performance is associated with the frictional nature of both the truck/carbody interfaces as well as truck warp. Again, a solution is to provide truck warp resistance independent to the wedge system. This approach is being considered in the IFCT program. Wayside detection and recent experience suggests that truck performance can be further improved by analyzing the root causes for this performance and introducing design alternatives to eliminate these root causes.

An integrated freight car truck concept design is being developed together with associated estimates of costs and benefits to provide improved car performance. This truck design will allow longitudinal motion of the adapter relative to the side frame by introducing a lower stiffness at the adapter pad than currently provided; sufficient longitudinal clearance will be introduced between the bearing and side frame at the pedestals to allow radial axle alignment and reduced angles of attack in curves. In order to provide adequate light- and loaded-car hunting stability, a separate shear structure will need to be introduced between the two side frames of the truck. Typically, this structure would be in the form of a transom, spring plank, or cross-braces that will reduce the stresses and functions of the secondary suspension and optimize vertical stiffness and damping to improve vertical and lateral ride quality. A means to introduce improved tolerance to the relative alignment of the wheelsets both laterally and longitudinally is being developed. The carbody/truck interface will be designed depending on the level of light- and loaded-car hunting performance achieved and the consequent need for truck rotational frictional damping.