Effects of dimensional variability on failure of AAR coupler knuckle

Association of American railroads (AAR) standard automatic couplers are designed for much higher capacity than the normal operating loads. However, failure of knuckles and coupler bodies is still a common occurrence. Recent studies have shown that fatigue is the main reason behind such failures below the expected load. Moreover, knuckle failures occur more frequently than coupler body failures, which cause operational disruptions and also influence overall coupler life because of nonconforming contact between a new knuckle and an old coupler. In addition to new and old counterparts, undesired contact conditions are often the case with the new assembly due to casting-based manufacturing inaccuracies.

A study is thus carried out in this paper to understand the variation of load transfer paths and its consequences caused by dimensional variability. A finite element model of an E-type coupler’s knuckle is developed and different possible contact conditions of the knuckle with the coupler head are simulated. Knuckles generally fail in pulling mode, during which the possible contacting elements of knuckle are pulling lugs, pin protector regions and pinholes. Due to dimensional variability, contact conditions may exist where an individual or a combination of these elements are in contact.

Simulation results indicate that under regular operational conditions, having only the pulling lugs in contact reduces the risk of knuckle failure and maintains assembly integrity even if the knuckle fails. However, under extreme loading conditions, the safest scenario is when both pulling lugs and pin protector regions are in contact.

These findings are believed to assist in defining the dimensional variability limits to ensure the desired contacts between the mating surfaces of the knuckle and coupler body of railway couplers of AAR type. This work contributes to understanding implications of dimensional variability in the railway couplers. The insight presented are useful in design, manufacturing and maintenance of railway coupler’s knuckle.