Sperry Rail's Sperry Walking Stick uses
Renishaw's non-contact magnetic encoders, developed by its associate company RLS, to locate microscopic cracks in rail track. Chosen for their overall performance characteristics, including resolution, power consumption, weight, size and I/O configurations, the encoders also help record auditable data. Network Rail is responsible for the UK's rail tracks and has embarked on a long-term programme of preventative maintenance. Sperry Rail International is helping Network Rail to find and monitor microscopic cracks in rail tracks before they cause track failure.
Sperry Rail uses Renishaw RE22 magnetic rotary encoders within the Sperry Walking Stick crack detector to help locate the exact position of faults. 'Rail cracks are generally born of two sources: small defects inherent in the rail at the time of manufacture and damage caused by the wheels of passing railway vehicles,' said George Dodd, field-service manager at Sperry. Boltholes and other fittings can also be the focus points for stress fractures but, says Dodd, there's no clear model of how cracks propagate. They depend on many factors, including frequency and weight of traffic, and extremes of weather.
'Cracks usually form at an angle of 20deg against the direction of normal travel, but a bi-directional line can grow cracks in both directions,' added Dodd. The most commonly used method of testing rail is an ultrasonic tool that can detect defects in the railhead, in the web and in the foot. The standard method of flaw detection on a pedestrian-operated device is called an A-Scan, where the ultrasound reflection is displayed on a screen. With a B-Scan flaw detector (BSFD) the signals received are displayed in a different format on the screen but can also be collected and stored in a file for later analysis and verification. A BSFD can store multiple data sets.
The Sperry Walking Stick is a pedestrian inspection tool, which the operator pushes along the line and can be operated in A-Scan or B-Scan modes. 'Our walking stick has a roller search unit and has nine transducers that project sound into the rail at a pre-determined angle to produce reflections from defects,' said Dodd Rail inspection 'walking sticks' in various forms have been around for at least 25 years. The Sperry device uses a polyurethane tyre containing liquid, incorporating fixed, non-contact probes. 'There's a zero probe, which is perpendicular to the rail foot, and there are probes that provide a refracted angle in the rail of 37deg, which look for defects in the web and the foot,' added Dodd.
The RSU tyre rolls along the rail on a thin film of water and antifreeze to dispel any air and provide a clean transition of sound into the rail and back out again. Graham Dale, group systems director, said: 'Sperry's walking stick covers the rail head, web and rail foot. 'It uses Renishaw RE22 magnetic rotary encoders to measure distance travelled. 'We knew we couldn't use the encoders fitted on our train-mounted inspection devices as they were far too large, heavy and power-hungry. 'The Renishaw RE22 magnetic encoder was the ideal weight, size, resolution, power consumption and suited the I/O configuration requirements.
'Of course, cost is always a factor, but the overall performance of the encoder was more critical for our application and was the main reason we chose the RE22,' he added. The Sperry Walking Stick uses many systems to log the location of the suspect signal. The magnetic encoder starts at a fixed point (zero), so that when the analyst goes through the data, they can see the specific encoder count, which is then translated into railway mileage or distance from the starting point of the inspection. Dale continued: 'The beauty of this model is that it can record data that is auditable. 'With the standard walking stick, the operator uses a defect form to record the location of a crack, but a form can be lost or destroyed.
'This particular model BSFD is used for collecting data that can be analysed later,' he said. GPS signals are also downloaded every second of the inspection and imprinted onto the data file. At the end of the test, suspect data is sent back to a portal, which Sperry sets up and maintains for the customer. The data is analysed and the suspect readings are reloaded on to the portal. The customer can then refer to the portal and, thanks to a combination of GPS data and linear positional data provided by the Renishaw rotary magnetic encoder, see exactly where faults exist.
Compared with other mechanised methods, the Sperry Walking Stick is relatively low-cost but can still be used to inspect up to 8km of track a day and is well suited to regular inspections of stretches that are particularly vulnerable or heavily utilised. Defects can be marked, monitored and dealt with before they become a serious threat. Dodd concluded: 'The incidence of rail breakages nationwide over the past year has dropped to zero, which, when one considers that some of the line in this country is 60 years old, goes to show that inspection techniques are effective and that Network Rail is on top of the problem,' he finished. The RE22 is part of Renishaw's range of magnetic encoders developed in close co-operation with its associate company RLS, based in Slovenia. It is a compact, high-speed rotary magnetic encoder designed for easy integration and for use in harsh environments.
A magnet is mounted to the shaft within the compact 22mm diameter encoder body and the rotation of this magnet is sensed by a custom encoder chip within the body. The encoder chip processes the signals received to provide resolutions to 13bit (8192 positions per revolution) with operational speeds to 20,000rpm and output signals provided in industry-standard absolute, incremental or analogue formats. With dirt immunity to IP68, the RE22 can be used in a range of applications, including marine, medical, print, converting, industrial automation, metalworking and instrumentation.
