Thursday, April 03, 2014

Rolling contact fatigue root causes

Written by  Harry Tournay, TTCI Senior Scientist
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Figure 1: Forces on a wheelset in flange contact and under an angle of attack. Figure 1: Forces on a wheelset in flange contact and under an angle of attack. TTCI
Use of self-steering trucks can result in a significant reduction of high impact wheel removals, compared to traditional, non-steering three-piece units.

High impact wheels (HIW) have been a major cause for wheel removal. One of the root causes for HIW is high normal and tangential stresses generated at the wheel/rail contact patch. Tangential stresses are directly related to the steering forces (“tractions”) generated by the truck in order to negotiate a curve. As part of the AAR Strategic Research Initiatives Program, TTCI has:

• Associated HIW to the generation of high steering tractions developed by nonsteering (traditional three-piece) trucks at the lead axle, low rail contact patch in curves that are generally sharper than six degrees. Lesser tractions are also generated on lead axle, high rail contact; these may also contribute to HIW generation.

• Shown that the use of self-steering trucks can result in a significant reduction of HIW removals compared to three-piece trucks operating under similar heavy-haul (coal) conditions.

• Demonstrated that these high steering tractions are exacerbated by curving with excess cant (curving at speeds under balance), where both vertical loads and steering tractions are increased. Consequently, damaging steering tractions may be generated in curves shallower than six degrees under these conditions.

• Established, theoretically and under test, that steering tractions can be reduced by allowing the lead axle of the truck to steer into a more radial position in a curve.

TTCI has proposed an improved freight car truck (IFCT) design concept to reduce lead axle low rail steering tractions. TTCI has also developed a test procedure using instrumented wheelsets to evaluate IFCT designs offered by the supply industry. This procedure measures the lead axle, low rail traction ratio, or T/N, where T is the steering traction and N the normal load across the lead axle, low rail contact patch.

TTCILaboratory tests (Figure 1, top) were conducted using a single wheelset aligned at differing angles of attack to a set of rollers simulating the track; steering tractions were measured. This simulated lead axle contact in curves. Tests indicated that surface damage occurs on a C-class wheel under low rail tractions greater than approximately 14,000 pounds and a vertical load of 35,750 pounds (a traction ratio of 0.39, or 14,000/35,750). As Figure 2 (opposite) shows, the typical dispatch adhesion of high adhesion locomotives is 0.32. This suggests that railcar wheels are being stressed at least as much in curves as locomotive wheels. In addition, surface damage can occur under low repeated cycles (of the order of 5,000 cycles), which suggests that the damage may not be fatigue-related, but the initiator of subsequent fatigue damage.

TTCI’s research to date suggests that the median value of T/N measured under test in curves up to 10 degrees should be as low as 0.37. However, extended tests are envisaged through 2015 to simulate the spectrum of steering tractions and vertical loads encountered in revenue service. To this end, TTCI is acquiring a rolling load machine that, among other applications, will be used to establish more accurate performance limits for C-class wheels, determine the benefits of improved materials and friction modifiers, and study the progression of observed damage to this generation of HIW, including the role of ice and lubrication in this process.

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