Modern electronics should be able to easily solve a temperature-sensing challenge, but the harsh real-world complexities make it a difficult proposition.
Freight railroads are a very important part of the logistics infrastructure around the world for transporting raw materials as well as finished goods. They are a major constituent of intermodal transportation along with trucks, ships, and air freight. In the United States, there are seven major railroads (designated as Class 1) with 140,000 miles of track routes stretching in all directions.
What fraction of freight travels by rail in the US? According to the US Department of Transportation, about 28% of the total US freight movement by ton-miles was shipped by rail (calculated by multiplying shipment weight in tons by the number of miles that it is transported). In addition to the traditional tank, box, gondola, and other well-known freight cars, many of the shipments are “intermodal” where the freight is in standard shipping containers which can be loaded onto/off of a truck or barge using a crane, while the goods remain intact and untouched inside, safe from weather, pilferage, and other problems (Figure 1).
What about the all-important energy use? Here, analysis by various industry sources and technical research organizations shows that for a ton-mile of freight, rail shipping is about four times more fuel efficient and emits 75% less greenhouse gas emissions than trucking. Not only is this an environmental positive, but it lowers cost since fuel is a major operating-expense factor.
However, as with any heavy, moving object that is subject to weather exposure, vibration and impact, and heavy use, there are failure and maintenance issues with rail transport. This FAQ will look at one of the most challenging railroad failure modes due to overheating of bearings, wheels, axles, and brakes. The overall wheel and bearing assembly is colloquially known as the “hot box.”
Q: What characterizes the overall assembly?
A: The major differentiator between a railway and other means of transport is that it has a pair of wheels (often joined to a wheelset) rotating around a common axis and supported and guided through a pair of rails. Axle bearings are the connecting design elements from the wheelset to the non-rotating parts of the vehicle. They must transmit the weight of the vehicle to the wheelsets while providing a smooth rolling movement for the wheelsets.
The combination of significant relative movements and high forces creates the risk of wear. Recent rolling stock use with greased roller bearings; historically, these were plain bearings that required maintenance (mostly more lubrication) and monitoring (manually sensed).
Q: What is the potential danger and subsequent failure mode?
A: Although problems can occur with bearings, wheels, axles, and brakes, it’s the bearings which are the greatest concern (a dragging brake is the second-most-common problem) as they must allow smooth rolling of the heavily loaded wheel. Note that these bearings are now what mechanical engineers call tapered roller bearings, which replaced plain roller bearings and journal bearings, Figure 2. They are not ball bearings, which could be designed to support the load, but which could not withstand the repeated axial and radial impacts and shocks.
Q: Why does a bearing overheat or fail?
A: When the design assumptions on the operating conditions are fully met, the axle bearing will statistically reach its calculated life. But there can be a discrepancy between the real operating conditions and the design assumptions, leading to damage of the axle bearing before its calculated end of lifetime. At the same time, a bearing that is significantly overdesigned with a large “just in case” margin would be too large, heavy, and expensive.
Once the damage process begins, it tends to accelerate: the pre-damaged bearing is the source for local vibrations and heat, which both represent additional loads and subsequently a source for damage increase. The ISO 15243 standard classifies the bearing failure modes into six main groups and various sub-groups, with reference to features that are visible on the bearing’s functional surfaces. Just as with electronics, heat – here, both ambient and generated by operating friction and conditions – is both a cause and symptom of problems.
Q: What is done to detect bearing overheating?
A: Railroads use temperature detectors based on infrared thermal sensors (usually called hot-box or hot-bearing detectors) to measure the heat emitted from bearings, wheels, axles, and brakes (Figure 3). Note that some hot-box detector designs use several low-profile boxes laying alongside the tracks and even under them.
About 6,000 of these temperature hot-box detectors are in place on the North American freight-rail network, with an average spacing of about 15 miles (~25 km). Optical filters ensure that the detector is only looking at infrared radiation in the desired band, and the entire system is activated only when a train approaches, so it is not “looking at the sky” the rest of the time.
Q: What criteria are used to judge over-temperature conditions?
A: Although specifics vary from railroad to railroad, train crews are supposed to receive an alert to stop and inspect the equipment if sensor readings are between 170°F (~75°C) and 200°F (~°93°C) above the ambient temperature, or if the difference between bearings on the same axle is at or above 115°F (~46°C) (Figure 4). There’s also the issue of which exact point(s) should be sensed, and what are the thresholds for each. It’s a complicated real-world situation.
Q: What are the implications of the risk?
A: On one hand, there were just 119 train derailments due to overheated bearings from 2010 to 2016 in the US and Canada. On the other hand, it can be substantial. For example, on February 3, 2023, there was a widely publicized derailment in East Palestine, Ohio, involving 35 cars of a freight train, some of them carrying hazardous substances. The accident prompted major serious concerns for area residents and the surrounding environment. Although the formal analysis and report on the cause are not yet complete, it seems pretty certain that the cause was an overheated journal bearing on one of the wheels.
Q: Didn’t the hot-box detector work as intended?
A: Actually, it apparently did, (again, the final accident report is not done yet). The train passed temperature sensors that showed components of a hopper car getting warmer as it traveled. The detector that showed the bearing had reached a critical temperature was about 20 miles away from the prior detector. Crews worked to stop the train after receiving an alarm about the temperature reading but couldn’t stop the train before the bearing failed (unlike cars, a freight train traveling at moderate-to-high speed can take a few miles to stop, as there’s so much mass and momentum).
Q: What can be done?
A: Of course, pundits and politicians are insisting that the railroads “do something” or “do more” to prevent another such accident. However, the on-the-ground reality is that checking for overheated bearings and other impending problems is being done already. While more could be done, it’s a difficult problem technically.
The next part of this article looks in more detail and sensing options and issues.
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Electric locomotives and catenary power systems – Part 2: power needs
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Electric locomotives and catenary power systems – Part 4: maintenance and corona
Union Pacific, “How Much Freight Ships by Rail In the US?”
Total Connection Logistics Services, “Inland Freight Shipping: Rail vs. Truck”
The Wall Street Journal, “Norfolk Southern’s Ohio Train Derailment Puts Railroad Equipment Sensors in Spotlight”
Design News, “Rail Car Wheel Bearing Monitoring in the Spotlight”
University of Texas Rio Grande Valley, Joint Rail Conference, “An Analysis of the Efficacy of Wayside Hot-Box Detector Data” (very informative with excellent illustrations)
Apna Technologies & Solutions, “Hot Box Detector”
Global Railways Review, “Axle bearings and condition monitoring for railway vehicles”
IBT Industrial Solutions, “Ball Bearings vs. Roller Bearings: What are the Key Differences?”
Springer/Railway Engineering Science, “Defect detection in freight railcar tapered-roller bearings using vibration techniques”
Railway Engineering Science, “Defect detection in freight railcar tapered-roller bearings using vibration techniques”
Sage/Advances in Mechanical Engineering, “Wayside detection of faults in railway axle bearings using time spectral kurtosis analysis on high-frequency acoustic emission signals”