Strain Gauges and Rail Forces
Using Strain Sensors to Measure Force in Continuously Welded Rail
Since rail forces occur in three dimensions, the key to using strain gauges is to install the gauges in specific configurations to measure vertical, longitudinal, and lateral forces independently.
Measuring Vertical Rail Forces
To measure vertical rail forces, a strain gauge is installed in the vertical position on the web of the rail. This point on the rail is closest to the vertical axis of the rail, where the rail force is centered. Two separate gauges are installed, on opposite sides of the rail, then wired together to complete the Wheatstone Bridge circuit. This gauge configuration eliminates the effects of any lateral bending that could reduce or exaggerate a strain reading taken on only one side of the rail.
Measuring Longitudinal Rail Forces
To measure longitudinal rail forces, a strain gauge is installed in the horizontal position on the web of the rail, along the neutral axis of the rail. The neutral axis is an invisible line through the cross-section of the rail, where half of the cross-sectional area is above the line, and half is below the line. The neutral axis location is known for all standard rail sections. Installing the gauge on the neutral axis eliminates the effects of vertical bending from the measurement.
Gauges to measure longitudinal force are generally installed at the midpoint between two ties, to minimize any local restraining effects that the tie plates may have on the rail. Installing gauges at this location also facilitates the installation of protective covers to prevent the gauge from being inadvertently struck by passing vehicles or debris. Two separate gauges are installed, on opposite sides of the rail, then wired together to complete the Wheatstone Bridge circuit. This gauge configuration eliminates the effects of any lateral bending that could reduce or exaggerate a strain reading taken on only one side of the rail.
Measuring Lateral Rail Forces
Measuring lateral forces is more complicated than measuring vertical and longitudinal forces. Lateral forces are applied by the wheels to the head of the rail, far above the neutral axis. There is no location on the rail head where a strain gauge can be installed that can provide accurate data on the lateral force without being susceptible to damage. The generally accepted practice is to install gauges on the foot of the rail. This location is safer in that it is far away from the wheel contact area on the head of the rail, and can be easily protected via an external cover. However, because the gauge location is not as close to the force application location, this configuration is more dependent on an accurate calibration than the other scenarios.
Two separate gauges are installed, on opposite sides of the rail, then wired together to complete the Wheatstone Bridge circuit. This gauge configuration eliminates the effects of any lateral bending that could reduce or exaggerate a strain reading taken on only one side of the rail.
Strain gauges provide a reliable method to monitor rail forces and to allow maintenance personnel to predict and prevent any potentially dangerous rail situations.
Sources
Kristoff, Susan. Rail Commuter Vehicle Curving Performance, April 2002. DOT/FRA/ORD-02/01
Kristoff, Susan and Schanoes, David,“Curving Performance of Newly Trued Wheels of Commuter Rail Passenger Cars”, ASME IMECE, Washington, DC, 2003.
