The invention relates to a method for detection of rail defects, in particular rail top defects, in a railway-track by measuring an axle box acceleration signal of a rail vehicle.

Rail defects, in particular rail top defects, as referred to in this document are local short vertical geometrical deviations that may cause impact between the rails of the railway- track and the rolling wheels of a rail vehicle. Aspects like indentations, differential wear and differential plastic deformation, inhomogeneous rail material and a defective manufacturing process of the rails may contribute to this problem. Unless repaired a light rail top defect or squat will grow into a moderate defect, and subsequently into a severe defect. Rail fracture and damages to its fastening, the rail pads, sleepers and ballast may also ultimately occur if no remedial action is taken. From the point of view of railway operation, safety and availability, rail defects, in particular rail top defects, should be detected and removed at the earliest possible occasion in order to prevent their further development into more serious rail defects .

Most commonly rail defects, and squats in particular are detected by human inspection or by an ultrasonic technique. For the human inspection inspectors walk along the rail to find the rail defects, or alternatively inspect photo's or a video record of the rails. In any case the naked human eye is needed to carry out the inspection. The ultrasonic inspection technique is only applicable when the cracks are deeper than approximately 7 mm in order to allow that the ultrasonic technique can be used for reliable detection of such cracks.

It has also been proposed to use eddy-current technology for detection of rail top defects, and even the use of acoustic detection has been proposed, however this latter technique is only applicable for detection of severe rail top defects, which emit detectable impact noise.

In the article ^λ A measurement system for quick rail inspec- tion and effective track maintenance strategy' published in Mechanical Systems and Signal Processing 21(2007), pages 1242- 1254, by M. Boccilione et al, instrumentation for measuring lat- eral and vertical axle box acceleration of a rail vehicle is proposed which is usable for detection of defects in a railway- track.

The measured vertical axle box acceleration of a rail vehi- cle as is known from said article is usable for the detection of a severe rail top defect. The measured axle box accelerations at a rail top defect are basically vibrations stemming from three sources, being

-1. Vertical vibrations of the track, including those of the rail, rail pads, fastening, sleepers, ballast etc.

-2. Vertical deformation and relative motion of the wheel and rail at the defect, and

-3. Vibration of the wheelset, including also those of the bearing and of the axle box.

The above-mentioned vibration source number 2, being the vertical deformation and relative motion of the wheel and rail at the defect is the signal that is of interest. For severe rail defects, in particular rail top defects, the vibration sources 1 and 2 are relatively strong. These sources can however be dis- tinguished because of their different frequency characteristics. For less severe rail defects, the vibration signals become less strong, and vibration source number 3 may become relatively more dominant than the other sources of vibration. Both aspects contribute to deterioration of the signal-to-noise ratio making it hard to detect light or moderate rail defects, in particular rail top defects.

EP-A-I 593 572 discloses a method for identifying locations along a track at which the wheel of a railway vehicle subjects the rail along which the vehicle is travelling to longitudinal forces, comprising the measuring of an acceleration signal of a wheel of the rail vehicle, wherein a longitudinal acceleration signal is used in combination and simultaneously with a vertical acceleration signal.

It is an object of the invention to provide a method for detection of rail defects, in particular rail top defects, in a railway-track, by which an accurate and reliable localization of such rail defects can be realized.

In order to meet the objective of the invention and to realize further advantages as will become apparent hereinafter, the method for detection of rail defects, in particular rail top defects, in accordance with the invention is characterized by one or more of the appended claims.

The method for detection of rail (top) defects in a railway-track in accordance with the invention is characterized in that the longitudinal axle box acceleration signal is used to remove from said vertical axle box acceleration signal a signal- part that relates to vibrations of the rail vehicle's wheelset, including the bearing and axle box.

As compared to the vertical axle box acceleration signal, the longitudinal axle box acceleration signal is of a relatively high strength, and moreover this longitudinal signal is a relatively undisturbed signal with a favourable signal-to-noise ratio. The longitudinal axle box acceleration signal is used in combination and simultaneously with the measured vertical axle box acceleration signal, in order to subtract from the latter signal the signal-part that relates to the vibration of the wheelset, including also those of the bearing and of the axle box. Due to the earlier mentioned different frequency characteristics, the vibration signal-of-interest relating to the defor- mation and relative motion of the wheel and rail at the defect can be separated from the vertical vibrations of the track. According to the invention it is therefore proposed that the longitudinal axle box acceleration signal is used to remove from said vertical axle box acceleration signal the signal-part that relates to vibrations of the rail vehicle's wheelset, including the bearing and axle box.

Further from the above it will be clear that according to the invention it is preferred that the axle box acceleration signals are filtered for removing signal-parts contributed by vibrations of the track, including the rail, rail pads and fastening, sleepers, and ballast.

It will further be clear that in order to be able to execute the method of the invention, instrumentation is required for measuring the axle box acceleration of a rail vehicle, com- prising at least one accelerometer that is known per se and is provided on said rail vehicle. This accelerometer is to be mounted for at least detecting the axle box acceleration in the longitudinal direction, that is in the direction of the railway- track. It will be clear that the actual measurement direction of the accelerometer may deviate some degrees from the exact longi- tudinal direction. A suitable type of accelerometer to be used for this purpose is the Endevco model 7259B lightweight piezo- accelerometer of the firm Meggitt.

Some measurement results with the application of the in- strumentation in accordance with the invention are shown in the drawing of figures 1 and 2 respectively.

In the drawing:

-Figure 1 shows the vertical axle box acceleration signal in accordance with the prior art;

-Figure 2 shows the longitudinal axle box acceleration signal in accordance with the invention; and

-figure 3 provides a schematic representation of an instrumentation system for measuring axle box acceleration of a rail vehicle .

In both figures axle box acceleration signals are shown to represent measured rail irregularities on a revenue track. In both figures the abscissa is the kilometre-position along the track, and the ordinate is the measured acceleration signal.

In comparison figures 1 and 2 show that the longitudinal axle box acceleration signal is more sensitive than the vertical axle box acceleration signal. There are for instance two clear peaks in the longitudinal axle box acceleration signal ( figure 2), the smaller peak of which is however hard to be distinguished in the signal representing the vertical axle box accel- eration (figure 1) .

Turning now to figure 3 a schematic representation is shown of a rail 1 of which the rail defects, in particular rail top defects, are to be measured and localized. One such defect is schematically represented by reference numeral 13. The measure- ment of this defect 13 is carried out by employing a rail vehicle having at least one axle box 3 that provides a bearing for a rail wheel 2. The axle box 3 is provided with both a vertical accelerometer 4 and a longitudinal accelerometer 5.

The vertical accelerometer 4 provides a vertical accelera- tion signal as represented by graph 6, which is comparable to what figure 1 shows .

The longitudinal accelerometer 5 provides a longitudinal acceleration signal as represented by graph 7, which is comparable to what figure 2 shows .

The acceleration signals 6, 7 are acquired in a data acqui- sition process by data logger 8. Data logger 8 concurrently monitors the speed of the rail vehicle by the application of a tachometer 9, whereas the data logger 8 also logs position data acquired by GPS system 10.

With a sender 11 which is optional the data may be transferred to a computer system 12 in which data processing and diagnosis can be carried out, in order to analyze the nature of the rail defects and their localisation along the track of the rail 1.