The mystery of vertical split rims

A rare wheel defect is posing a mystery for researchers. The problem is called a vertical split rim, but why it happens and how to prevent it pose questions for those who try to keep good wheels from going bad.

The rims of freight-car wheels can split open when a crack spreads into the tapered tread, expanding, and eventually causing part of the rim or flange to fall off.

Railroads first recognized vertical split rims in the 1960s. While the defect has received more attention as other wheel defects have been resolved, it is still uncommon. The Association of American Railroads’ Pueblo, Colo.-based Transportation Technology Center Inc. says that while 13 million wheelsets are in North American freight service, no more than 22 vertical split rim-related accidents per year have been reported. Getting at the root of the problem has been difficult.

Researchers thought rail wheels changed little after leaving the factory and going into revenue service, says Cameron Lonsdale, vice-president of research and development for Chicago-based Amsted Rail. Instead, stress patterns show that wheels compress and stretch in service.

To learn more about the problem, researchers pounded wheels with drop hammers used to test draft gear, drilled core samples from wheels, examined new wheel slices with X-ray diffraction equipment, and questioned heat-treating techniques. They still came up short.

“At the end of the day, it is probably related to residual stresses,” Lonsdale says. “We knew the axial residual stress level from manufacturing was there, but nothing that was going to cause problems.”

The recipe for vertical split rim wheel failure seems to be a crack in the wheel tread that spreads, plus time. The way to prevent the defect from a well-worn set of wheels seems to be machining the wheel to eliminate the crack and other defects, thus removing the triggers.

“We did a machining test recently and by truing the wheel, you get to knock off the peak of the axial tensile stress in the wheel,” Lonsdale says. “I think the very best preventative solution is to make sure that your treads don’t get into really bad condition.”

Maintenance is one solution. Railroads that keep track of the miles put on wheelsets can remove them at specified intervals for truing. Lonsdale says this type of preventative maintenance is often seen on Australian mining railroads with heavier axle loads than those normally found in North America – 286,000 pounds.

He’s also observed it on a Canadian mining railroad. “The difference is it’s really well maintained. We listened to an empty ore train with car after car ‘whoosh’ by,” he says. At the sound of a single noisy car, the Canadian train crews marked the car for maintenance and set it out for work.

Another solution is to redesign wheels. Amsted offers a micro-alloy that resists cracking and pitting.

Railroads take wheel defects seriously. As part of an overall approach to detecting them, Union Pacific has ultrasonic detectors in a special wheel house in North Platte, Neb., a major stopping point for the frequent empty unit coal trains headed back to Wyoming’s Powder River Basin.

Trains creep through the detectors at 0.5 mph, slow enough for diagnostic testing devices to coat every wheel with a coupling fluid and check every wheel, says Michael Iden, UP’s general director of car and locomotive engineering.

Iden says his railroad singled out coal trains as the most susceptible to wheel failures, including vertical split rims, because of the continuous pounding they receive. “The predominance of cars that are running are empty or at 286,000 pounds. That’s where the risk was perceived as greatest,” Iden says.

Still, such defects are rare. “We have always referred to this as the needle in the haystack,” Iden says. On the triple-track main line out of North Platte alone are trains comprising 400,000 wheelsets. The percentage that will ultimately develop a defect and shatter is 0.0025 percent – a handful of wheels. “How do you find them without spending inordinate amounts of time?” he asks.
 
Then again, if railroad workers don’t find problem wheels, the wheels will find them. David McConnell remembers one of the first vertical split rims he saw 15 years ago while working in the research laboratories at UP. “All of the flange came off in one piece. They actually found the flange 30 miles from where they found the railcar,” he says. “Some track worker saw the wheel as it was going by and called in and stopped the train.” Other than the wheel, nothing happened and the train stayed on the track. “They were just lucky,” McConnell says.