Controlling Friction on Rail Transit Systems
By Bob Tuzik • January, 2007
Friction control plays a major role in reducing wear, noise and vibration, and managing wheel/rail interaction on rail transit systems. Controlling friction has become more achievable with the advent of engineered friction modifiers and improvements to wayside and onboard application systems for both traditional lubricants and friction modifiers.
Much of the benefit associated with improved lubrication/friction management materials and application systems has been gleaned from work done by the major freight railways to reduce lateral loads and gauge-face wear in curves. While the goals of transit systems (which are more sensitive to wheel/rail noise) may be different, the benefits and the net effects are the same.
Recognizing the need to make information on research and current operating practices such as these available to rail transit agencies, the Transportation Research Board (TRB), Federal Transit Administration (FTA) and Transit Development Corporation (TDC) — an educational and research organization established by the American Public Transportation Association (APTA) — established the Transit Cooperative Research Program (TCRP). A primary goal of the TCRP is to disseminate information to those who need it most — transit agencies, service providers and suppliers.
Track-Related Research Volume 4: Friction Control Methods Used by the Transit Industry is an example of the type of information collected and distributed through the TCRP. The report, which includes field reviews of vehicle-borne and wayside top-of-rail (TOR) application systems, and surveys of nine rail transit systems, shows that various types of friction management programs have yielded positive results. The report chronicles how the industry has moved from what were routinely called “lubrication” systems to “friction management” systems to reduce noise and vibration, and control wheel and rail wear.
While rail/wheel lubrication has been used extensively in the freight railroad environment to reduce wear and control curving forces, transit operators have been reluctant to apply lubricants or friction modifiers to the rail (or wheel) because of concerns about their effect on traction and braking conditions. Because of their inner city locations and use by the public, transit systems also have been more sensitive to contamination, grease buildup and other unsightly conditions typically associated with lubrication. Rail transit systems are also sensitive to wheel/rail-generated noise and wheel/rail wear that friction management can alleviate.
“The goal of friction control is to produce a specific friction level at specific locations on the wheel or rail, rather than simply to reduce friction to a low level at the gauge face,” Christopher Jenks, TCRP Manager – Transportation Research Board, points out in the introduction to TCRP Report 71 Track-Related Research Volume 4: Friction Control Methods Used by the Transit Industry. “This goal requires a higher degree of system control, applicator reliability, and lubricant (i.e. friction modifier material) development.”
Friction Modifiers
Friction modifiers, which are applied to the wheel tread or top of rail to produce and maintain a specific level of friction, have emerged as effective tools for controlling friction. They do, however, require a greater degree of system control and applicator reliability.
Water-based friction modifiers, such as Kelsan Technologies’ KELTRACK®, produce a micron-scale film of engineered solids that leave a thin, dry film on the rail (and wheel) after the water evaporates. These engineered products, which are applied to the top of rail or tread of the wheel, are designed to establish a constant coefficient of friction, typically in the range of 0.3 µ to 0.35 µ. By doing so, friction modifiers can reduce stick-slip (and resulting squeal) while maintaining enough positive friction for normal braking and traction operations.
Tests have shown that wheel squeal is in part generated by lateral creep at the wheel/rail interface. Under certain conditions, lateral creep can lead to lateral stick-slip oscillations, which, in turn, can excite high-frequency vibrations in the wheel plate and rail. During tests in which gauge-face and TOR friction have been independently controlled, the greatest noise reduction was achieved by controlling friction at the wheel tread/top of rail.
Compared to dry rail, lubrication can reduce wheel/rail-generated noise levels by 10 to 20 decibels. This noise reduction can be achieved after only 5 – 10 vehicles pass the wayside application point. Once achieved, the noise reductions associated with lubrication can continue for several days after a lubricator is shut down.
The TRB’s Chris Jenks points out that friction modifiers’ ability to reduce or increase friction to a specific range, has changed the way in which the industry thinks about friction. “We’ve moved from ‘lubrication’ to ‘friction control.'” This is not to say that traditional greases and wayside application systems (both of which have been modified and improved within recent years) are not part of the landscape of modern transit systems. Wayside lubrication systems continue to effectively reduce friction, noise and gauge-face wear on rail in curves.
Lubricants and friction modifiers have been applied by wayside, onboard and hi-rail systems. They have also been applied by hand (using brushes and rollers) in tests or other localized applications. Because many noise-related issues are site-specific (often identified by complaints from the public), the majority of transit operators consider wayside systems to be the most practical and cost-effective solution.
Wayside application
Traditional wayside systems apply grease to the gauge face of the rail. TOR wayside application systems apply liquid friction modifiers to the top of rail through modified wiping bars that are mounted on the field side of the rail.
An alternate wayside system delivers a lubricant or friction modifier to the top of rail through a hole drilled through the ball of the rail, allowing friction control materials to be applied directly to the railhead.
In this type of approach a Teflon-based grease is pumped through a 3/16-inch-diameter hole drilled into the head of the rail. Unlike typical application schemes in which grease-based lubricants are applied to the wheel flange/gauge face of the rail, and friction modifiers are applied to the wheel tread/top of rail, the drilled-hole method can be used to control friction at both the gauge face and top of rail. This is done by angling the hole through the rail head toward the gauge corner for better gauge-face coverage, or toward the center of the rail for better top-of-rail coverage.
TriMet in Portland, Ore., uses a drilled-hole system to apply a lubricant to the rail in several locations on the system. Since it produces little spillage or waste, the drilled-hole approach is advantageous in embedded track and areas used by pedestrians. Once the rail is drilled, however, the hole placement is fixed; adjustment or relocation requires re-drilling. The drilled-hole method generally has been used to control site-specific noise at various locations.
Wayside TOR application systems are used by most transit operators to control wheel/rail-generated noise. Electric (rather than mechanical) pumps are typically used to control the amount of TOR material being dispensed. Wiper bars are mounted on the field side of the rail, higher than bars designed for gauge-face applications, to enable liquid material to migrate across the running surface of the rail where it is picked up and distributed by passing train wheels. Multiple wiper bars are sometimes used to apply the liquid friction modifier to the entire circumference of the wheel. This results in improved overall coverage and increased carry-down rates.
Transit operators have reported that rail/wheel profiles that produce a narrow contact band can inhibit the ability of wayside systems to properly transfer liquid friction modifiers to the top of rail. Some transit operators also reported that TOR friction modifiers can be washed off by rain. On systems where such problems were reported, however, the friction levels returned to a steady state within one to three days.
Onboard application
Onboard wheel flange and/or tread surface application systems have also been implemented on transit properties. The TCRP report emphasizes that successful implementation of onboard friction control programs requires the support of management. Maintenance and operating personnel — the first line of defense in detecting improperly operating application systems — must be trained to identify defective or inoperative applicators. Because of these cross-departmental requirements, the report points out that on-board systems are best suited to transit systems with small fleets (usually fewer than 50 vehicles) and a limited number of vehicle designs.
Onboard systems are often considered where systemwide friction control is intended. Instead of applying material at wayside locations to provide site-specific friction control, applicators are mounted on a large percentage of the car fleet. This allows friction control material to be applied during normal operations. Onboard systems can use liquid or solid forms of friction control materials. Experience has shown that regardless of whether liquid or solid material is applied, the mechanical alignment of the applicator is critical. Transit systems typically have not found off-the-shelf onboard applicators to be effective. They typically have had to re-engineer applicators to suit their vehicles’ truck design.
New Jersey Transit (NJT), which operates three-truck, articulated light rail vehicles, evaluated a modified onboard flange lubrication system to apply friction modifiers to combat noise and vibration. Tests showed that when the material was properly applied, friction levels remained in the 0.30 to 0.35 range. Under these conditions, the test car (operating on a 700-foot section of tangent track) repeatedly stopped within one foot of the stopping distance under dry rail conditions. No adverse braking or wheel slip conditions were created. However, when excess material was intentionally applied, friction fell below 0.25 µ and caused wheel slip during braking and acceleration. Under these over-application conditions, the stopping distance increased from 60 feet to about 80 feet at 15 mph. Results such as these emphasize that proper configuration and adjustment of TOR application systems are critical to obtaining and maintaining proper friction levels (on both rails). On systems where onboard friction management systems have been successfully implemented, vehicle operators are generally considered the first line of defense in detecting improperly operating application systems.
Regardless of the approach and type of equipment used, an effective friction management program requires buy-in and cooperation from Operating, Track and Mechanical Departments. And whether an onboard or wayside application is selected, inspection and maintenance of the application system are essential.
