Figure 1. Sensors must be designed, tested and manufactured to the most rigorous standards to endure the most laborious operating conditions on the underside of a railcar. Source: MNStudio/Adobe

In recent years, numerous train accidents have been attributed to a variety of factors, among which include the failure of bearings, axles and journals. These failures are often caused by excessive friction among these components, leading to high temperatures, thermal breakdown of lubricants, and the eventual seizing of these components. In some cases, these failures have caused train derailment, resulting in excessive down-time, high clean-up costs, late deliveries and even death.

The first step in understanding how bearing, axle and journal failures can be avoided is having a look into the causes of these failures. The next step in avoiding these failures is procuring the right parts for the job; sensors for speed, motion and temperature monitoring and control are crucial, and not all journal bearing temperature sensor products are equal.

Journal bearing failure

Numerous factors can lead to journal bearing failure, the most common of which include lubricant break-down. This can often lead to higher friction in journal bearings. In some cases, an improper lubrication may have been used that was not properly rated for the mechanical loads or thermal environments. In other cases, lubricants can become contaminated by foreign materials, such as dirt, water, metal chips or wear particles, and corrosion. These contaminants may increase friction or compromise the integrity of the lubricants, leading to excessive wear and eventual journal bearing failure. High mechanical loads may also damage or cause the journal bearing elements to become misaligned; this results in spalling, improper fit and out-of-roundness, which may contribute to excessive wear, vibration and high temperatures that cause lubricant break-down.

While it might be possible to inspect journal bearing elements for premature wear and damage, a common denominator that may indicate impending failure is excessive operating temperature. Although the bearing maximum operating temperature is dependent on the type of lubricant being used – grease or oil – excessive temperatures are generally due to inadequate lubrication, excessive loads or a combination of both. It is therefore incumbent upon operators and railcar owners to monitor the journal bearing temperature to ensure the safe operational temperature of the lubricants and bearing hardware.

Temperature sensing technologies

In the past, train axle bearings were housed in a so-called “hot box” that used oil-soaked packing materials to reduce friction within the bearings. When the oil leaked or dried out, the bearings overheated, emitting smoke, sparks or fire that was to be detected by train workers either visually or by smell. These workers would then sound an audible to alert other crew members that an emergency stop was needed to prevent further damage to the axle. This method was not very reliable since workers were not only required to be present, but the role of human error in seeing or smelling potential problems on moving trains was not fail-proof enough.

More recently, trains have been using “trackside” sensors to measure bearing temperatures. These consist of infrared thermal sensors that are installed on the track or rail bed to measure the heat emitted from the bearings, wheels, axles and brakes. These trackside sensors, however, are not an exact measurement of the absolute bearing temperature, but an indication of the temperature difference between the ambient air temperature and the bearing temperature.

There are about 6,000 trackside sensors installed on the North American freight-rail network with an average spacing of about 15 miles (around 25 km). The long spacing between trackside sensors is problematic, since moderate-to-high speed trains may require several miles to stop before imminent bearing failure.

Permanent on-board monitoring – where a temperature sensor is installed on every wheelset – would appear to be the most reliable alternative because the sensors can provide a continuous real-time indicator of bearing absolute temperature. While passenger rail vehicles already have such sensors, it is more difficult to implement these sensors on freight rail vehicles for a variety of reasons. Not all vehicles within a train are owned by a single company, with some of the cars being leased. Separately, on-board temperature sensing requires a way to connect the cars together, which poses a challenge when one is adding and subtracting cars at different stops. It is anticipated that federal law will soon require that temperature sensing equipment be installed on every wheel, axle or truck on a freight car.

Superior journal bearing temperature sensors

Smith Systems has developed and manufactured journal bearing temperature sensors for over 30 years. These sensors are available in several different configurations and suitable for integration on freight rail cars. Figure 2 shows a patented 90° right-angle temperature sensor that is currently being used on a passenger rail car.

Smith Systems' sensors are designed, tested and manufactured to the most rigorous standards to endure the most laborious operating conditions on the underside of a railcar. One key aspect of these sensors is that they utilize materials with an appropriate coefficient of thermal expansion (CTE) that is strategically matched to the environment and the parts with which they are designed to interface.

Smith Systems offers design capabilities to provide customers with improved products for long-termFigure 2. A 90° journal bearing sensor. Source: Smith Systems, Inc. reliability. The right-angle sensor exemplifies Smith Systems’ ability to design a solution with a specific customer need in mind. The 360° cable exit can be oriented anywhere around the center line of the sensor body after it is torqued in place, which eliminates large service loops in cabling, greatly reducing the incidence of fatigued, snagged or cut cables. This design allows for installation of the sensor and cabling close to the chassis of the vehicle when limited space is a concern. This feature is available in speed, motion or temperature sensors.

Because of the potentially high temperatures in the vicinity of the journal bearings, it is not feasible to integrate a wireless transmitter together with the temperature sensor. The journal bearing temperature sensors must therefore be connected through cable to a transmitter that can communicate wirelessly to a centralized monitoring system. Smith Systems partners with several different companies to provide high-reliability and robust wireless solutions for journal bearing temperature “smart” sensing that cannot be tampered with externally.

Smith Systems

Smith Systems, Inc. is a privately owned company based in North Carolina that has been manufacturing standard and customized sensor products in the industrial, transportation, military and aerospace applications since 1978. Specific products and applications include speed sensors, motion sensors and temperature sensors, as well as sensor controls, custom instrumentation, and cable harnessing associated with these devices.

Smith Systems engineers components and devices for high reliability in harsh shock, vibration, temperature and moisture environments for the industrial, military and aerospace applications. The company adheres to the highest quality standards, such as ISO 9001, ATEX (flammability and explosive environments), and IRIS (International Railway Industry Standard), so that customers can be confident the Smith Systems’ products offer the highest quality and safety at the best prices. Learn more about Smith Systems, their journal bearing temperature sensors and other products today.