Hot Axle Box Detection System (HABD)

Hot Axle Box Detection System (HABD)

Hot Axle Box Detection System (HABD)
The monitoring of axle box bearing temperature by track side equipment improves train safety by detecting wheelset bearings about to fail.

Failed bearings on rolling stocks present important safety risks, potentially leading to catastrophic events such as derailment or fire.

A rise in the heat generated by a bearing is found to be a good predictive diagnostic of a bearing about to fail. Thus, monitoring the hot axle box bearing temperature enables detection of bearings presenting a risk of failure. Acceptable temperatures and temperature variations are set: any value deviating from the norms will trigger alarms and, potentially, actions to guarantee safety.


The European standard EN 15437 deals with interface and design requirements for Axle box condition monitoring. The standard is divided in two, Part 1 describing bearing temperature measurement by trackside Hot Axle box Detectors (HABDs). Part 2 describes the monitoring of hot axle box bearing temperature by on-board systems.

Icon Standard   EN 15437-1: Railway applications - Axle box condition monitoring – Interface and design requirements - Part 1: Track side equipment and rolling stock axle box

Icon Standard   EN 15437-2: Railway applications - Axle box condition monitoring – Interface and design requirements - Part 2: Performance and design requirements of on-board systems for temperature monitoring

The illustration below depicts two methods to monitor the bearing temperature, one by onboard equipment (A), the other by by trackside equipment (B).

(A) On-board monitoring of axle box bearing temperature

EN 15437 refers to temperature monitoring of the axle box but also specify that the rolling bearing itself can be monitored directly. The most common setup up is to monitor the hot axle box temperature. The temperature sensors monitor the hot axle box temperature (more precisely, the grease temperature) at all times, and the train computer issues warnings if the measurements vary from the normal.

On-board monitoring overcomes number of limitations of the HABD trackside systems. Firstly, the temperature monitoring is continuous whereas track equipment only measure at one point in time, when the train passes by. Also, the track equipment is used for all trains with no possibility to develop specific solution depending on the train type, bogie design, bearing property, and so on while the on-board system can be tailored to match the train characteristics.

(B) On-track Hot Axle Box Detection (HABD) of bearing temperature

The Hot Axle Box Detection (HABD) systems are placed on the tracks. They measure the temperature of the axle box bearing boxes and often also wheels and brake discs (Hot Axle Box and Hot Wheel detection units, HABD/HWD) when the train passes over them. The system uses infrared beams to measure temperature then send the measured values wirelessly to the depot, fleet management team or even the train.

If the onboard method was developed to improve the bearing temperature monitoring, the 2 methods (A) and (B) can well be used together for optimal and redundant control.


On-board monitoring of axle box bearing temperature is a safety critical function commonly developed as SIL-2, providing probabilistic risk reduction factor from 10-6 to 10-7. SIL stands for Safety Integrity Level and is a concept defined in the IEC 61508 standard. For the rail industry, the CENELEC has developed the EN 50126, EN 50128 and EN 50129 standards. For more information on SIL, please visit our dedicated page here.


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