INTRO: Trials at the AAR Transportation Technology Centre have found that flange-bearing crossings which reduce wheel impact loadings have the potential to cut track maintenance costs and operating delays, with no significant problems in terms of wheel wear
BYLINE: David Davis and Semih Kalay*
BYLINE: * David Davis is Principal Engineer and Semih Kalay is Chief Technical Officer at the Association of American Railroads’ Pueblo-based research subsidiary Transportation Technology Centre, Inc
HIGH ANGLE crossing diamonds have long posed a headache for railway managements. Among the most short-lived, high-cost track components, they also create operational bottlenecks that limit the capacity of one or both routes. Recent moves toward higher axleload wagons and higher operating speeds do not bode well for the future performance of existing diamonds, as Heavy Axle Load test data suggests the service life of crossing diamonds is highly sensitive to wheel loads.
The Association of American Railroads has been exploring various concepts for reducing the impact of crossing diamonds. Vehicle dynamics modelling quickly showed that diamonds with unsupported flangeway gaps would not meet modern performance requirements. Most concepts for supporting the wheel over the flangeway gap involve moving parts with many complications, but the concept of a Flange Bearing Frog (FBF) is appealing because of its simplicity. It also has a proven performance record in light loading applications.1
In an FBF diamond, the wheel is supported on its flange while traversing the flangeway gaps. Typically the wheel load is transferred from tread to flange by raising the flangeway floor, enabling the wheel to be supported on its flange through the crossing. Fig 1 shows a wheel in the flange bearing section of the crossing with some of the critical dimensions and tolerances.
The FBF concept can easily be applied to heavy axleload operations using existing rolling stock. But since it involves loading wheels in a new manner, considerable effort has been needed to determine the effect of flange bearing on wheel performance. Evaluations performed by the AAR subsidiary Transportation Technology Centre Inc show that wheels are capable of flange bearing; with the only noticeable effect being a shiny, flattened flange tip.
Working with railroads’ track experts and specialist trackwork suppliers, the AAR has developed two prototype FBF crossing diamonds. Both exceed the performance requirements in simulations using TTCI’s Nucars computer modelling package. The first prototype was installed and tested last year in the Federal Railroad Administration’s FAST (Facility for Accelerated Service Testing) loop at Pueblo, Colorado. The second has been built and was due to start tests at FAST last month. Initial results have been very encouraging, to the point where improved designs will be ready for revenue service testing this year, subject to FRA approval.
Scale of the problem
Approximately 4700 crossing diamonds are in use on North American railroads, requiring an estimated $240m a year for maintenance and replacement. The initial cost of a heavy duty diamond (including materials and installation) can exceed $100000, and average maintenance costs are $700 per million gross tonnes.
High angle crossing diamonds have very short lives relative to conventional track or even main line turnout frogs, typically 100 to 200 MGT. The life expectancy of conventional diamonds is dramatically shorter in heavy axleload traffic compared to mixed freight operation with 117 tonne gross vehicle weights. A typical heavy haul diamond has an average service life of just 140 MGT.
In addition, frequent maintenance requires permanent or temporary slow orders, causing disruptions to train operations. This frequently creates traffic bottlenecks on high tonnage lines, generating delay costs estimated at $421m a year, which easily exceeds the actual maintenance costs. Permanent Slow orders are often imposed in an attempt to reduce to manageable levels the impact loading and related damage caused by the unsupported flangeway gaps. All this suggests that the limits of existing technology may have been reached for unsupported gap diamonds using conventional materials.
Three design types of crossing diamonds are widely used. They all function in the same way, but vary in the use and amount of austenitic manganese steel castings: