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The Institution of Railway Signal Engineers
(Hong Kong Section)

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 IRSE(HK) Newsletter > Technical Articles
 Issue 05 @ March 2009 

Wild Impact Load Detector

Anthea NGAI
IRSE(HK) Executive Member
Hyder Consulting Pty Ltd.

Wheel impacts are serious problems for rolling stock and can lead to axle box failures. Early detection of wheel defects, such as skids (i.e. wheel flats), cracks and wheels that are out of shape, prevents major damage and reduces the cost of repairs. Wheel Impact Load Detector (WILD) is designed for the early detection of faulty wheels and to encourage rail operators to properly maintain their wagons. The maintenance crew will use the information from WILD to plan and perform wheel replacements before the wheels become so damaged that they cause service interruptions. This should reduce operational costs considerably.

Basic System Equipment

The basic component of a WILD system is the track mounted sensor array (Figure 1). There are two basic types of sensors, the original sensors are based on strain gauges and measure force and the new type is based on accelerometers and measures rail motion. The sensors are installed at strategic places along the track to monitor passing trains to investigate specific safety related symptoms.

Figure 1 Wheel sensors (Left circle) and the accelerometers installed on track.

The modern WILD system is usually equipped with on-site signal processor, a control PC, modems, Automatic Equipment Identification (AEI) and computers for local or remote diagnosis and monitoring. Together with other equipment, such as Weighbridge (Figure 2) and Hot Box Wheel & Bearing Detector, they form a comprehensive Wheel Condition Monitoring (WCM) System.

Figure 2 Weighbridge for axle balancing or to check the weight of wagons.
New type of weighbridge has weighbridge load cell in place the accelerometer.

Working Principles

Using rail mounted strain gauges as the wheel sensors, it can weigh each wheel of a train as it passes over, and detect skid flats in the wheels. A wheel with flat spots can create impact loads many times higher than the fully loaded weight of the car it carries and cause serious damage to the railroad.

New WILD systems use an array of accelerometers (Figure 3) to measure the change in motion. Air bags in cars are released when an accelerometer senses sudden extreme changes. When the wheel goes over them, they read positive and as the wheel rolls past, they read negative. Any irregularities in the wheel cause the signals to go both positive and negative as the wheel rolls over them.

Figure 3 Typical WILD site - Left circle: Accelerometers determine the impact of the wheel defect,
Right circle: Automatic Equipment Identifier (AEI) Readers are used to identify wagons.

Figure 4 Another typical site for WILD system

The array of sensors is mounted on track, together with an Automatic Equipment Identification (AEI) tag reader (Figure 5) which determines the Car ID when a train passes, identify and trace every wheel in the fleet for as long as that wheel is in service.

Figure 5 AEI tag reader to read the car identity

The data gathered for each axle is automatically recorded on a database by the signal processor and the control PC. It is then transmitted to the railway control centre or depot maintenance centre for remote monitoring and diagnosis.

When a wheel generates a force that causes too high an impact such as it reaches a predetermined impact level compared to the historical value (e.g. a freight car wheel exerts a peak impact load of 90 kips (Note 1) or above, is considered "out of round" and is on the path to failure.

Figure 6 A wheel experiences a 106 kip peak impact.

The train might be requested to slow down and come to a stop at the nearest sidings or yard to uncouple the car whose wheel is damaged. This minimizes the chance of damage to the tracks and bridges. However, reductions in the speed of the trains and the obligation to stop for repair introduce lots of delays in the delivery of goods and reduce the throughput of the railway network.


WILD allows early detection of many types of wheel defects. It can be used as a predictive and proactive maintenance system that detects and reports potential safety problems and poorly performing equipment before they result in accidents or undue rail damage. It enables the maintainer to track data on a single wheel as far back in time as data is retained. The defective wheels can be quarantined for immediate repair. They can pro-actively apply trim blocks to tread-braked rolling stock with mild to moderate wheel flats, too. The use of WILD system enhances safety, decreases disruptions of service, improves operating efficiency and reduces overall system costs. It reduces noise along with reduced incidence of audible wheel flats across the network, too.


The railway industry has undergone a remarkable technological upheaval during the past years. A wide range of Wayside Detection Systems such as the Hot Box Wheel & Bearing Detector (HBWD), Dragging Equipment Detector (DED) and the Wheel Impact Load Detector (WILD) have been developed. They are permanently placed at strategic locations along railroad tracks to monitor passing trains to investigate specific safety related symptoms. A comprehensive Wheel Conditioning Monitoring (WCM) system completed with the widespread implementation of AEI tag optimizes the productivity and minimizes the costs of operation with accurate and reliable condition monitoring information.

Note 1: kip - it is a unit of force equal to 1,000 pounds

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