Examining wheel/rail interaction on rail transit systems
By Bob Tuzik • November 4, 2004
If the first unwritten rule in optimizing the wheel/rail interface on rail transit is: Know your system; the second rule should be: Recognize that the w/r interface is a system.
“No single department can attack the issues in isolation and expect to get very far,” Joseph Oriolo, Senior Project Manager – Maintenance-of-Way, Massachusetts Bay Transportation Authority, told attendees at Interface Journaland Advanced Rail Management’s Rail Transit ’04 Wheel/Rail Interaction Seminar in Baltimore, last month. “You need input and cooperation from Mechanical, Track and Operating people to address all of the issues affecting the system.”
And before modifying either the wheel-flange or the gauge-face angle through cutting or grinding, transit engineers should ask themselves what effect a change will have on the other half of the interface, and how the profile shapes will be maintained. What the Mechanical Department does to its wheels affects rail, just as what the Track Department does to its rail affects wheels.
Speakers at this—the first seminar devoted to wheel/rail interaction on rail transit systems — addressed these and other issues, including the “Principles of Wheel/Rail Interaction” in the areas of Vehicle/Track Interaction, Wheel/Rail Profile Design and Maintenance, Friction Management, and Noise and Vibration.
“Noise is the lightning rod of discontent on rail transit systems,” said Carl Hanson, Senior Vice President of Harris Miller Miller & Hanson Inc. Whether characterized as rolling, the most ubiquitous type of wheel/rail-generated noise, impact or squeal, the type which generates the most complaints, noise is a byproduct of rail operations. Excessive wheel/rail-generated noise can be controlled, however, by addressing the frequencies that are generated by wheel and rail roughness wavelengths and train speed.
Rolling noise is best addressed by keeping wheel and rail surfaces smooth, including preventing the occurrence of rail or wheel corrugations — or at least treating corrugations before they grow — Hanson said. Impact noise can be addressed by using cwr, aligning joint and frog surfaces to minimize impacts, and adjusting frog surfaces to make for smooth load transfer between different load-bearing surfaces. Squeal noise is best addressed through effective lubrication/friction management and optimized wheel and rail profiles.
Some systems have adopted the use of ring- or fin-dampened wheels that are tuned to control resonant frequencies and reduce noise, said Jim Nelson, Vice President – Principle at Wilson, Ihrig & Associates, Inc. Wheel vibration absorbers were shown to reduce under-car noise by an average 7 decibels (dBA) at the leading truck negotiating a large radius curve at 40 mph on one transit system. Wheel-flange/gauge-face lubrication and top-of-rail friction control have also been shown to provide effective noise control — especially at high frequencies where hearing is most sensitive.
Wheel squeal was particularly troublesome on The Port Authority of Allegheny County’s Gateway Loop, an approximately 119-meter loop with two 25-meter radius curves in the downtown subway portion of the system. These tight-radius curves require the outside wheel to travel more than 16 feet farther than the inside wheel through the curves, Jim Dwyer, the agency’s Director – Technical Support, told the seminar delegates. This slippage resulted in high-frequency wheel squeal of 100 dB or more. After trying a number of mitigation measures, the agency settled upon the use of Kelsan Technologies’ KELTRACK top-of-rail friction modifier, which reduced noise levels to 85 dB.
At Dorchester Station outside the subway, use of the top-of-rail friction modifier at a transition from a 91-meter radius to a 183-meter radius curve on an 8% grade reduced noise levels from 100-plus dB to 80 dB—”without negatively affecting traction on the grade,” Dwyer said.
Richard Reiff, Principle Engineer at the Transportation Technology Center, Inc., presented data on tests of a “drilled hole” approach to gauge-face or top-of-rail lubrication designed for in-street paved track applications. This approach features a 3/16-inch diameter hole drilled at 70 degrees into the railhead through which grease- or soy-based lubricants are applied to the rail in curves. Tests conducted through the Transit Cooperative Research Program (TCRP), a program that was funded to leverage research results from the freight sector and accelerate implementation to the transit industry, indicated that use of the system could reduce the coefficient of friction by approximately 0.2µ from dry conditions to an average 0.33 µ. Observed noise levels were reduced by 2 dB to 16 dB (between 2kHz and 20 kHz).
New York City Transit has adopted a number of measures—both car and track treatments—to mitigate noise and vibration along its more than 800 miles of mainline and yard tracks. The installation of ring-dampened wheels reduced screech noise by 15 – 20 dBA. New, quieter traction motors reduced noise levels another 5 – 7 dBA, and attention to wheel flats further reduced noise. “Trued wheels are 10 – 15 dBA quieter than ‘flat’ wheels,” said Antonio Cabrera, NYCT’s Director of Track Engineering.
Joint removal and the installation of cwr, where possible, further reduced noise by 5 – 7 dBA compared to jointed rail. The use of resilient fasteners in subways and on elevated structures reduced noise levels by another 3 – 5 dBA, Cabrera said. Gauge-face/wheel-flange lubrication and top-of-rail friction modifiers at stations, particularly in the underground portions of the system, provided an average noise reduction of 7.3 dBA—an average 5.2 dBA at 31.5 – 200 Hz frequencies, and 13.3 dBA at 1,000 – 20,000 Hz.
“Proper friction management has been shown to effectively reduce lateral curve forces, rail and wheel wear, wheel squeal, derailment potential and energy consumption on rail transit and freight systems,” Gary Wolf, President of Rail Sciences Inc. said during his discussion of the Principles of Friction Management. “Other less tangible benefits include reductions in tie and fastener wear, environmental pollution and corrugations.”
Rail corrugation is the most-often cited reason for rail grinding on transit systems, Larry Daniels, Railroad Consulting Engineer, told the seminar delegates. “About 41% of transit track is prone to corrugation. Tangent and curved track are equally prone, although standard carbon rail is more likely to develop corrugation than higher-hardness premium rail,” he said. The use of premium rail and an effective lubrication/friction control program are the most effective means of mitigating corrugation. Once it occurs, however, grinding is the only way to get rid of it.
There are two basic types of rail grinding: corrective, which is performed to reduce or eliminate noise and vibration caused by corrugations; and preventive, which is performed to improve and maintain ride and wear characteristics before the onset of corrugation or other conditions requiring corrective action occur.
“Transit systems have a greater ability to optimize the wheel/rail interface than freight systems that have to deal with a wide range of wheel profiles on interchanged cars,” said Gordon Bachinsky, President of Advanced Rail Management Corp. Still, he said, every property is different and requires a unique set of profiles depending on the type of vehicles, track structure and operating parameters on the system. The first step in understanding what’s needed is to measure some percentage of the wheel and rail profiles on a given system.
Wheel measurements may be taken manually, with simple go/no-go templates or with precision contact or non-contact laser-measurement systems. Wayside measurements systems such as KLD Labs, Inc.’s WheelScan system, which uses a combination of lasers and video cameras to profile and measure wheels at operating speeds, are also available. A WheelScan system is in final commissioning stages on the MBTA in Boston.
Rail measurements also can be taken manually, using templates or taper gauges to measure vertical rail height and side wear, as well as the crown radius. Vehicle-mounted automated optical rail measurement systems that collect data as frequently as every 6 inches, if needed, along the rail are also in regular use on transit systems in North America.
“These systems are excellent tools for looking at a lot of data to determine how wheel and rail profiles are performing over time,” Bachinsky said. “This enables those responsible for rail and wheel maintenance to determine whether their grinding and truing are effective and if their planning is on target.”
There are two types of wheel cutting, or truing, methods: milling and lathe cutting, Oliver Cone, a consultant to Amtrak and Northeast Corridor on wheel truing and diagnostic systems, told the seminar delegates. Milling equipment, which profiles the wheel tread surface in one cut, typically leaves a rougher surface finish. Wheel lathes, which use a stylus to cut the proper profile across the tread, typically leave a smoother surface finish. Underfloor wheel lathes are suitable for turning the treads up to the top and rear face of the flange, machining the inner faces of the wheels, and unilateral re-profiling of one wheelset.
Wheel and rail wear rates are affected by rolling contact fatigue and, on a practical basis, by the ability to produce and maintain the desired profiles. “In order to design a matching wheel/rail profile pair it is necessary to consider a number of factors relating to its anticipated performance,” Roy Smith pointed out in his introduction to the “Principles of Wheel/Rail Profile Design and Maintenance.”
Each vehicle design has dynamic performance characteristics, which relate primarily to the effective conicity that will be created, and curving performance characteristics, which affect the lateral forces and L/V ratios that are generated. “In order to optimize these factors the designer must have a complete knowledge of the vehicle design parameters, the track characteristics and the performance expectations for the two working together,” Smith said.
Leading wheelsets tend to generate angles of attack toward the high rail, while trailing wheelsets generate angles of attack toward the low rail. These opposing lateral forces produce a net turning moment in the direction of contacting the high rail flange. “If the wheel/rail profile will not allow sufficient rolling radius difference to be produced before the flange is reached, the moment on the leading axle becomes negative and combines with that of the trailing axle to create a total moment that increases the flange force,” Smith said. Effective rolling radius difference on a given wheelset will minimize, or eliminate, wheel-flange/gauge-corner contact in all but the sharpest of curves. Restraining rails are typically used to prevent contact when sufficient rolling radius difference cannot be obtained to effectively steer through the curve.
North American transit systems have a variety of design flange angles, ranging from 60 degrees to 75 degrees. While increasing the maximum contact angle decreases the potential for derailment, the contact angle is function of both wheel and rail, said John Elkins, President, RVD Consulting, Inc. Consequently, transit systems must have consistent wheel and rail maintenance policies for maximum effectiveness.
Recent research by Transportation Technology Center, Inc., conducted on behalf of the Association of American Railroads, Federal Railroad Administration and the TCRP to develop flange climb distance and L/V criteria indicates that the lower the wheelset angle of attack, the higher L/V ratio required to derail, Elkins said. “The greater the wheel/rail flange contact angle, which is preferably greater than 70 degrees, the lower the derailment potential,” he said.
In order to improve overall wheel/rail interaction, rail transit systems should design vehicles with good steering and a soft primary longitudinal suspension with low sidebearing friction. They should also design high-conicity wheel/rail profiles and avoid large angles of attack, Elkins said.
These comments represent a glimpse of the insight into many of the issues that were covered at this groundbreaking Rail Transit Wheel/Rail Interaction Seminar. Interface will provide more details from the seminar in these pages over the next several months.