Currently the rail industry has available a number of methods to inspect solid axles. However, all these methods suffer from the problem that full access is required to the axle in order to perform a full inspection. This necessitates that for inspection of a solid axle, the associated bogey must be fully removed from the vehicle, and the wheelset fully disassembled from the bogey - including the removal of ancillary gear such as breaks and bearings. The resulting out of service disruption means that inspection cannot be carried out frequently and is only likely at major overhaul periods. MPI is the industry standard for inspection of axles during overhaul but it does not allow full axle inspection unless the wheels and bearings are also removed. Wheels are press fitted to the wheel-seat region of the axles; the process of press fitting can cause fretting which can lead to the formation of cracks in the wheel-seat region, which may not be detected using current techniques and procedures. Due to the time consuming nature of removing the wheels and bearings from an axle, the axles are usually tested with the wheels present. This makes it impossible to access the wheel-seat region thus resulting in only partial inspection.
If the axles could be inspected in-situ the actual disassembly required would only be removal of the end caps positioned on the axle end faces. To remove the axle end cap takes just 15 minutes. There is no existing solution/product that can currently meet the need of minimal disassembly. Although hollow axles present less issues in terms of accessibility, by virtue of being hollow and thereby allowing good probe accessibility, there are still challenges. Current inspection methods for hollow axles are slow and relatively inefficient. There are commercial solutions in existence that use conventional ultrasonic testing. However, ultrasonic inspection solutions are still not able to offer 100% coverage as there is a “dead zone”. The “dead zone” is the result of transducer ringing and reverberations from the interface between the transducer and the scanned object. This is normally a few millimetres into the surface of the sample just below the transducer.