0
Select Articles

No Hunting PUBLIC ACCESS

What Could Be Simpler than a Wheel Rolling Along a Rail? Plenty.

[+] Author Notes

Associate Editor

Mechanical Engineering 123(05), 54-57 (May 01, 2001) (4 pages) doi:10.1115/1.2001-MAY-2

This article reviews that wheels riding along parallel tracks have to balance diameters by “hunting.” Hunting wears down both wheels and rails, because a wheel must move sideways to find balance. The wheels, much like a pup chasing the phantom tip of a cropped tail, seek an accord they can never reach. Auto manufacturers might say all that energy used to go toward metal forming of the unwanted variety. American freight lines carry heavier loads than their counterparts in Europe. In Europe, passenger rail exerts more influence over railroad infrastructure than it does in the United States. Abc-Naco designed a variant of its domestic swing motion truck for European rail. Dubbed Axle Motion, the truck incorporates the same design philosophy of decoupling lateral forces from the car body that inspired the swing frame.

In profile, a railroad wheel tapers. Through the side-to-side motion of the taper along a track, a railcar negotiates curves. According to Abc-Naco’s Jay Schloemer, as pairs of wheels come around a bend, they move away from a curves center. Just how much they move depends on the radius of the curve, but inside wheels ride on shorter perimeters than outside wheels do. In that way, each wheel rolls without slipping along a correspondingly shortened or lengthened track sector. It’s the equivalent of a constant velocity joint or differential out of an automobile reduced to its simplest expression.

The same taper that makes turns possible throws a curve at straight running. Wheels riding along parallel tracks have to balance diameters by “hunting,” Schloemer said. Hunting wears down both wheels and rails, he said, because a wheel must move sideways to find balance. The wheels, much like a pup chasing the phantom tip of a cropped tail, seek an accord they can never reach.

As if the increase in rail and track wear from hunting wasn’t bad enough, the sinusoidal motion of the wheels rocks the railcar body, exciting a resonance that can damage goods on board. Schloemer, who heads up communications for the Chicago-based wheel and rail truck manufacturer, said the phenomenon had been especially troubling for automakers. In the past, many dollars worth of damage were directly attributable to hunting, he said.

It was in the early 1990s that U.S. automobile manufacturers and the railroad industry teamed up to find ways of smoothing the ride. Both groups expected that shipping by truck—while always an option—would increase delivery costs.

Eventually, a high-performance standard emerged that halved the allowable g forces for automobile carriers, or autoracks. Rail truck makers responded by manufacturing softer suspensions for freight service. Today, virtually every car carrier in this country wears, or is scheduled to be fitted for, new suspensions. That's about 40,000 autoracks in all, Schloemer said.

As a result, Abc-Naco sells a four-piece truck in the United States that it calls Swing Motion. In Europe, it offers a similar product named Axle Motion. Other manufacturers have developed softer freight suspensions as well. Buckeye Steel Castings of Columbus, Ohio, manufactures a freight truck that applies many of the techniques used in softening passenger transit. Meanwhile, Powell Duffryn PLC, in Wales, builds a truck that combines elastomer and hydraulic elements to manage forces. And the German firm Krupp makes freight trucks that rely on large leaf springs to smooth their ride over rails.

The Swing Motion truck decouples lateral inputs from the railcar body, the maker said, smoothing an automobile’s ride by train.

Grahic Jump LocationThe Swing Motion truck decouples lateral inputs from the railcar body, the maker said, smoothing an automobile’s ride by train.

Compared with a coal carrier, an autorack is lighter, both unladen and burdened with freight. However, the cargo is far more sensitive to jolts.

Grahic Jump LocationCompared with a coal carrier, an autorack is lighter, both unladen and burdened with freight. However, the cargo is far more sensitive to jolts.

Swing Motion suspensions slide smoothly under railcars in place of three- piece trucks. They cost more, but wear better, according to the maker.

Grahic Jump LocationSwing Motion suspensions slide smoothly under railcars in place of three- piece trucks. They cost more, but wear better, according to the maker.

Sit at any railroad crossing, Schloemer said, and watch a freight train squeak by. See how boxcars pitch, roll, and sway as they pass. Why so much noise? Why so much commotion?

The screeching nervousness of many boxcars stems directly from the interaction between wheels and tracks, explained Scott Duncan, Abc-Naco’s manager of product development. “An automobile riding as though it were on rails,” he said, refuting the advertisement of a few years ago, “is a bad thing.” Autos, unlike railcars, are not constrained laterally, he explained, so their environment is actually more conducive to a smooth ride. There are no abrupt lateral impacts affecting a car tire, he added, “unless it hits a curb.”

The wheel and rail system, however, depends upon a curb’s equivalent—the flange-—to keep things on track. Three “inputs” affect wheel and track, Duncan said: vertical and lateral inputs, and a phenomenon he described as “angle of attack.” AH three must be absorbed and dissipated through the suspension in order to soften a railcar’s ride.

The source of vertical inputs is surprisingly counterintuitive. The likely place to find them is at the rail joints.

It isn’t the gap between rails that forces the biggest response, but the act of the wheel passing over the joint. As Duncan explained, an individual wheel, carrying upward of 15 tons, presses the rail section down as it approaches a joint. The subsequent rail, not loaded, does not deflect but rather presents an abrupt step to the oncoming wheel. Even continuous welded track uses joints at crossings and switches, he said.

Another vertical input comes at road crossings, Duncan said, where the rail bed leaves the normally compliant ground to cross a compacted, paved surface.

The track can also force lateral inputs, according to Duncan. Thermal expansion kinks the rails. Joints offset. Switches and crossings impart lateral energy to the system, he said.

Through hunting comes the primary lateral input, though. Watched from beneath a railcar moving along a straight stretch at 55 mph, the wheels bang back and forth between flanges, crashing into the rails with surprising frequency.

Auto manufacturers might say all that energy used to go toward metal forming of the unwanted variety. Duncan said he heard stories of car batteries shaking loose and damaging body components.

The third input relates to how a wheel behaves as it negotiates a curve. But to understand that, a look at standard three-piece freight truck construction might help.

Two sideframe castings and a bolster casting make up the namesake parts of a three-piece truck, Duncan said. The bolster connects the sideframes and bears the railcar body. Springs nest between sideframes and the bolster.

Bolster gibs, on the ends of the bolster, sandwich the sideframes and limit lateral movement to a half-inch, Duncan said. Clearance in the assembly makes the three- piece truck loose enough to “lozenge” or warp as it goes around a bend. This parallelogram effect, he explained, generates great friction as the outboard wheels rub or plow against the outside rail.

Deformation of the three-piece truck leads to more plowing as the outer wheels’ angle of attack increases. That brings on more deformation, Duncan said, and greater rubbing—a vicious circle. Run a train too quickly through a curve and the outer wheel can climb over the track and derail, he said. And every rail rub imparts a lateral input to the track.

A three-piece truck absorbs vertical inputs through multiple springs and a pair of friction wedges, explained Abc- Naco’s president and chief technology officer, Stephen Becker. The wedges sit on the springs, their hypotenuses facing upward against inclined faces on the bolster. As the bolster and siderails compress the springs, they force the wedges out against vertical faces of the siderails.

Coulomb friction damping is what Becker called this event.

This kind of damping is economical, he said, although it exhibits stick-slip properties, or stiction. Stiction describes the different friction coefficients that two surfaces can exhibit when they move past one another compared with when they are stationary, he said. Stick-slip causes a chatter that sounds like riveting, Duncan added.

The wedges are important, Becker said, because they are central to the Swing Motion design for damping lateral motion. That’s one reason Abc-Naco began facing them with a compositional material having the same static and dynamic coefficients of friction. Stiction needed taming.

In the old design, the friction wedges are limited to traveling vertically by the gibs, Becker explained. The new design works without gibs, eliminating a rigid lateral stop.

Abc-Naco engineers added a transom to the three-piece design, and created a pivot.

Actually, the new four-piece truck uses two pivots, Becker said. Separated by about 12 inches vertically, the two pivots make a swing hanger pendulum out of the entire side frame casting. What Swing Motion does, Becker said, is to increase the available lateral travel to 2Vi inches—a long way from the 1 inch of total travel of a standard three-piece truck.

The transom rests on a rocker seat that is itself supported by two bearings. Higher up, additional rocker seats support the side frame on the wheel bearings by way of a narrow adaptor.

The same friction wedges that dampen vertical motion dampen the large increase in lateral travel, Duncan said. The wedge plays an important role in decoupling the bolster from any hunting rail wheels.

The line at 0.26 g limits lateral acceleration for safety's sake. The limit at 0.13 g prevents lading damage.

Grahic Jump LocationThe line at 0.26 g limits lateral acceleration for safety's sake. The limit at 0.13 g prevents lading damage.

The line at 0.26 g limits lateral acceleration for safety's sake. The limit at 0.13 g prevents lading damage.

Grahic Jump LocationThe line at 0.26 g limits lateral acceleration for safety's sake. The limit at 0.13 g prevents lading damage.

In 27 years with The Baltimore Sun, newsprint manager Glenn Davis has seen a lot of paper come through the pressroom. Printing daily and Sunday editions, the presses run about 45,000 to 50,000 impressions an hour, he said. An impression is one copy of a newspaper.

Flat spots on rolls of newsprint can cause headaches for press operators, Davis said. Flat spots lead to vibration that can break the web. They can also undermine the splice between two rolls.

When a flat spot enters the line paster—a machine that applies glue to the ends of the running and incoming rolls—the brush that laps the two sheets together can miss that spot, Davis explained. Once the new roll is up to speed, a knife comes down and cuts away the old roll. The knife can hang up on the unglued section, he said, and tear the sheet.

“I’ve seen some rolls that are so out of round that when you put them in a reel and the press stops, you break the sheet out,” Davis said. “The rolls are so out of round that the brakes can’t even hold them.”

Davis remembered discussions that went on for years among the newspaper, the paper mill, and the railroad, all trying to decide the cause of these flat spots and potential solutions. As with the automakers, delivery of paper by truck, though more expensive and less efficient to unload, was looking more and more like the only solution.

“We started out by padding railcars,” Davis said. “That helped a bit.”

A technological fix was waiting in Swing Motion trucks. The paper mill, Bowater Inc. of Greenville, S.C., began shipping newsprint to The Baltimore Sun via CSX’s Big Blue railcars, Davis said. These special high-cube boxcars, designed especially to carry paper, rode on four-piece trucks from Abc-Naco.

Although Davis suspected that flat rolls were sometimes caused by humping, where cars roll into one another to couple up, he said that the route by which paper came from the mill seemed to correlate with damage levels.

Since the paper started arriving in Big Blue cars, however, “Our flat rolls have virtually disappeared,” he said.

Railcars carry lighter loads in Europe than they do in the United States. European trucks, or bogies, use primary suspension systems that provide independent movement of each axle.

Grahic Jump LocationRailcars carry lighter loads in Europe than they do in the United States. European trucks, or bogies, use primary suspension systems that provide independent movement of each axle.

Axle Motion bogies combine the best elements of the Swing Motion design with features specific to primary suspensions, the manufacturer said.

Grahic Jump LocationAxle Motion bogies combine the best elements of the Swing Motion design with features specific to primary suspensions, the manufacturer said.

American freight lines carry heavier loads than their counterparts in Europe, Schloemer explained. In Europe, passenger rail exerts more influence over railroad infrastructure than it does in the United States, he said. That translates to shorter trains there, along with requirements for lighter loads and shorter stopping distances.

In Europe, Schloemer continued, train tracks often pass very close to centuries-old buildings. The vibration from passing freight trains can damage these structures.

It is a concern that limits both the weight carried and speed attained by European freight wagons.

Yet, eyes in Europe increasingly look to U.S. freight operations in hopes of finding ways of relieving their congested roads of truck traffic. One obvious way is to increase the weight carried by each rail freight wagon, Schloemer said.

With that in mind, Abc-Naco designed a variant of its domestic swing motion truck for European rail. Dubbed Axle Motion, the truck incorporates the same design philosophy of decoupling lateral forces from the car body that inspired the swing frame.

In February, Abc-Naco released news of an order from Railtrack PLC, the United Kingdom’s privatized rail operator, to fit 490 new wagons with Axle Motion III trucks, or bogies, as they’re called there. Railtrack plans on using the ballast wagons for maintenance of track. The order followed approximately 18 months of testing.

For the test, freight wagons were fitted with a number of different suspension systems, Schloemer said.

Wheel forces acting on the track were of particular concern. Railtrack wanted to minimize damage to its assets—the track and overpasses—stemming from those forces. In addition, it sought a way to reduce the impact of freight movement past fragile historic buildings.

According to Schloemer, the very same freight wagons that had been restricted to 45 mph for fear of track damage were running at speeds of 60 mph fully laden and fitted with the new bogies from Abc-Naco. During the test, the trucks themselves were instrumented to gauge the force exerted on track. Resulting low-force measurements were then run in a computer simulation at Manchester University. Schloemer said that 100,000 simulated axle passes over a track corresponded to the low track forces measured in the field.

Railcars equipped with four-piece suspensions helped eliminate damage to newsprint. A big-city newspaper could unload five of these railcars in a single day.

Grahic Jump LocationRailcars equipped with four-piece suspensions helped eliminate damage to newsprint. A big-city newspaper could unload five of these railcars in a single day.

Copyright © 2001 by ASME
View article in PDF format.

References

Figures

Tables

Errata

Some tools below are only available to our subscribers or users with an online account.

Related Content

Customize your page view by dragging and repositioning the boxes below.

Related Journal Articles
Related eBook Content
Topic Collections

Sorry! You do not have access to this content. For assistance or to subscribe, please contact us:

  • TELEPHONE: 1-800-843-2763 (Toll-free in the USA)
  • EMAIL: asmedigitalcollection@asme.org
Sign In