Tuesday, June 19, 2012

Wheel failure is not an option

Written by  Scott Cummings, Principal Investigator, TTCI, for Railway Age

The industry spends close to $1 billion annually replacing bad wheelsets. What can be done to reduce this cost, and to improve safety?

To state the obvious, round wheels are a good thing. Unfortunately, there are many ways a railcar wheel can leave this ideal condition and begin to cause problems, such as high impact loads and potential accidents. Shelling, broken rims, and tread buildup are three major concerns of the railway industry because of maintenance costs and safety risks.

Wheelsets are the top rolling stock maintenance item in North America. High impact loads, due to tread damage from shelling and spalling, account for more wheelset removals than any other cause. Transportation Technology Center, Inc. (TTCI) estimates that 582,000 wheelsets are replaced annually at a cost of $828 million.

Each year, a very small but important minority of wheelsets fail in a way that causes a train accident. According to Federal Railroad Administration safety data, the top two causes of wheel related train accidents are broken rims and tread buildup. To address wheel problems, TTCI is testing improved wheel steels designed to resist wheel shelling, and conducting root cause analysis of broken rim and tread buildup failure modes.

Domestic and overseas wheel manufacturers produce high performance wheel steels that have the potential to improve wheel life by offering increased resistance to shelling. Eight types of these high performance wheels are being tested in a western railroad unit coal train to quantify the benefits compared to Association of American Railroads Class C wheels. This work is part of the AAR’s Strategic Research Initiatives (SRI) Program. Six wheel manufacturing companies from around the world each donated high performance wheels for this project. TTCI also developed and patented a high performance wheel steel for use in this test (the wheels were forged by MWL Brasil).

After more than two years of operation and approximately 200,000 service miles, the majority of the wheels remain in good condition. The high performance wheels are performing similarly to AAR Class C wheels in terms of rolling contact fatigue and shelling. Benefits of the high performance wheel steels should begin to become evident as the test progresses and more wheels are removed for cause. Three types of high performance wheels will be further evaluated in a Canadian unit coal train this year. Accelerated results from this train are expected from a combination of high traction forces, braking demands, and cold weather conditions.

The FRA has teamed with the AAR to co-fund research to better understand the conditions necessary to produce broken rims and tread buildup.

Broken wheel rims are increasingly manifesting themselves as vertical split rims (VSR), with an associated decline in the number of shattered rims. Tightening of the AAR rules for allowable defect size during ultrasonic testing and the addition of a microcleanliness test appear to have been effective in reducing the number of shattered rims. In the VSR failure mode, a vertical crack runs roughly perpendicular to the tread surface, often branching off a shallow subsurface horizontal crack. The vertical crack eventually grows large enough to produce a brittle fracture resulting in a broken rim.

TTCI is investigating VSR wheels in a number of ways. Microcleanliness testing was conducted on samples from 30 VSR wheels, and all but two of the wheels met current AAR microcleanliness standards. The two wheels that exceeded the current limits were manufactured prior to the establishment of the AAR microcleanliness criteria. Tensile axial residual stresses on the order of 30 ksi have been measured in service-worn wheels. These residual stresses are developed from rolling contact between the wheel and rail and likely act to propagate cracks in the vertical direction. TTCI is currently attempting to create a VSR failure by cycling a service-worn wheel with pre-existing subsurface cracks on a laboratory rolling load machine. The internal cracks are monitored with nondestructive methods to quantify crack growth and orientation as a function of load magnitude and number of load cycles.

Tread buildup can form when a wheel slides along the rail. The resulting material left on the tread surface can effectively reduce the flange height and flange angle leading to an increased risk of derailment at track switches and curves. While buildup is condemnable under AAR rules at 1/8-inch height, samples have been found that are multiple inches high. TTCI is exploring the conditions necessary to produce large amounts of tread buildup by pulling cars with locked brakes in a variety of test speeds, drag distances, car loads, and rail friction conditions.

By addressing common wheel removal causes like shelling, as well as more dramatic failure modes like broken rims and tread buildup, TTCI hopes to reduce maintenance costs while improving safety in the railroad environment.