To identify the root causes of poor fitment of components in car coupling systems, TTCI, under the direction of the Association of American Railroads through the Strategic Research Initiatives Program, developed a digital gage to identify fitment issues. Though the digital gage allowed for more accurate measurements, its practical and technological limitations have driven the research to develop physical gages. These physical gages can be used to measure a larger population of coupling systems and support the development of clearly defined physical tolerances to ensure interchangeability of coupler components.
A clear understanding of the operational environment and how these tolerances affect performance reveals why gages are important. In draft, the coupler and knuckle are designed to transfer the longitudinal load through the pulling lugs of the knuckle to the pulling lugs of the couplers. From the coupler pulling lugs, the load is transferred through the coupler body and into the draft system, propelling the car down the tracks.
If the pulling lugs of the knuckle and coupler are not the first two surfaces to make contact in draft, then the area around the pin hole and pin bear that load. This can result in cracks around the pin hole and broken or bent pins. The bearing surfaces of the pulling lugs are subject to wear and plastic deformation. This causes the contact to change over the life of the components. When a new knuckle is installed in the used coupler body, because there is wear on the coupler pulling lugs, the gaps at the pulling lugs are likely larger than with a new coupler and new knuckle. This increased gap makes it less likely that the pulling lugs will be the first surfaces to come in contact. A gage indicating limiting wear conditions and avoiding possible alternative load paths would be useful.
Though the AAR Manual of Standards and Recommended Practices (MSRP) defines limiting dimensions in some areas, the dimension from the pin hole to the pulling lugs is not specified. To better understand this dimension on both the coupler and knuckle, TTCI began developing the Multiple Use Gage (MUG). After developing and prototyping a design, TTCI collaborated with a gage manufacturing company. Improvements were made to the gage to make it manufacturable, more accurate, and more rugged.
Using the MUG, a total of 268 E-type couplers have been measured by TTCI researchers. The couplers measured vary in style, age, and use. From the measurements, a few trends have developed. The bottom pulling lug on most couplers in the industry is closer to the pin hole than the top pulling lug. In the industry, the distance from the pin hole to the pulling lugs is between 5.5 inches and 5.9 inches. These measurements will be provided to the AAR Coupling Systems and Truck Casting Committee as a report on the health of the industry.
A simplified gage design has also been developed using the measurements from the 268 couplers. This go/no-go style gage is being used in a select few locations to estimate the number of couplers that could be affected if a dimensional specification were defined in the MSRP. This exercise is important to ensure that only couplers with severe wear or dimensional issues are removed from service.
The two-step process of gathering the data from industry samples and testing designs of simplified gages provides the industry a good starting point to improve coupler components.