Controlling Friction on Rail Transit Systems (continued)
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 (see Drilling Down to Top-of-Rail Friction Control).
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.
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