Measuring Rail Forces
How to Characterize Internal Forces in Continuously Welded Rail
CWR experiences stresses though environmental effects such as thermal cycling, track stability, and passing traffic. Understanding these forces and knowing what the normal range of acceptable forces is allows maintenance personnel to control the rail forces in their track and to prevent derailments and other incidents.
Types of Rail Forces
When rail forces are above acceptable levels, the integrity of the track is at risk, and can lead to failure of the rail in varying modes:
- Longitudinal Forces – Excessive tensile loads along the longitudinal axis of the rail can result in the rail breaking. Excessive compressive loads along the longitudinal axis of the rail can result in track buckling. Both rail breaks and track buckling can result in vehicle derailments.
- Vertical Forces – Ties that are spaced too far apart can result in the rail between the ties bending due to the load of a passing vehicle. Overloaded vehicles can result in degradation of the track foundation and premature rail wear.
- Lateral Forces – Lateral forces can be induced in rails as vehicles negotiate curves, or in tangent track if a vehicle is prone to hunting. Excessive lateral forces can result in track shift, gauge widening, or in extreme cases, rollover of the rail head.
Measuring Rail Forces
While there are several methods to measure the forces within CWR, strain gauges provide the most straightforward and reliable method. A strain gauge is a resistive sensor that measures the strain, or longitudinal deflection of an object along the axis of the gauge. Strain gauges can be attached to the rail surface in specific configurations to measure forces in each of the three axes of the rail. Sealed welded strain gauges are generally used to measure rail forces because the gauges can be installed in any climate and are more durable in the long term than bonded strain gauges.
Once the strain gauges have been installed, they must be calibrated so that the output strain can be converted to units of force. Lateral and vertical forces can be calibrated using a hydraulic test fixture that applies a varying load to the rail at the point of the strain gauge. Vertical forces can also be roughly calibrated by moving a vehicle of known weight along the rail so a wheel is located directly over the strain gauge.
Longitudinal forces are more difficult to calibrate. Unless the rail is cut, or a new rail is being installed, either of which results in a zero load state, it is not known what the absolute load is in a given rail section. A strain gauge can measure relative change in strain, which can be converted to load, an absolute value cannot be achieved unless data can be collected while the rail is in a zero force state. Hydraulic tensors can be used to induce a varying longitudinal force on the ends of a free rail section for the purpose of calibration.
Understanding the behavior of continuously welded rail and monitoring its performance allows maintenance personnel to maintain track integrity and prevent accidents from occurring.
