One of the challenges faced by track engineers, both in the UK and around the world, is keeping the rail profile in good condition. Writes Maurice Verheijen
Many studies have been undertaken to determine the interaction between rails and wheels, and all have determined that even small variations in rail profile can have an adverse effect on ride quality, noise and wear rates which will exacerbate the problem.
The traditional method of treating rails to maintain the correct profile is by rail grinding. A special work train is fitted with a series of grinding wheels which restore the profile as the train passes along the track. These trains do a good job, but they don’t remove much material in one pass, so they often have to make several runs along the same section of track, and the grinding process produces dust and sparks. The former is sucked up using in-built ‘vacuum cleaner’ heads, while the latter can cause problems in tunnels where there is dry refuse alongside the track.
Rail milling has been the topic of a few articles in The Rail Engineer. This alternative method uses special milling cutters fitted with interchangeable tungsten-carbide tips. It is a quicker process as more material can be removed in one pass and, although swarf is generated, that often causes fewer problems than the dust and sparks.
The fact that a deeper cut can be taken can also be useful in removing surface defects and in re-profiling badly worn rail. However, the milling process can leave ‘facets’ in the surface – small flats left by each individual cutting tip – and most rail millers are also fitted with grinding systems just to restore a smooth surface finish.
One of the pioneers of the use of rail milling is Schweerbau. In its fleet, it even has a rail milling train built to London Underground tube clearances that has been used throughout the London Underground network.
Now, however, Schweerbau is working on two new processes. It has developed a way to speed up the milling process even further and, even more interestingly, it has developed a method which combines planing and milling.
The first rail rotational planing train, the D-HOB 2500, went into service with the German network operator DB Netz last year and a second-generation machine, D-HOB 2500 II, is under construction and will be operational in 2015.
Rotational planing combines the characteristics of milling (accurate transverse profile) and planing (accurate longitudinal profile) into one single technique. Moreover, this new technology allows for a change of profile during the actual planing process. This way each rail can be machined independently with different transverse profiles, useful for anti-head-check profiles on high rails, and stock and switch rails in turnouts.
The rotational planing process uses a 1,400 mm diameter cutting wheel carrying 32 cartridges. The cartridges hold straight and curved planing blades and are arranged in such way that it follows the target contour of the rail head transverse profile, extending from 14mm below the gauge face of the rail all the way round to the field side of the rail.
The innovation is that each of the cutting tools performs a rotating movement combined with a short longitudinal movement (parallel to the running surface of the rail) at the moment when the cutting tools contact the rail. This longitudinal motion leaves a smooth finish, unlike the facets on a milled rail, removing the need for remedial grinding.
As the cutting depth can be varied from 0.2mm to 2.0mm, the new technology can be used both for regular maintenance of a good profile (preventative treatment) and for re-cutting the profile on damaged and/or worn track (corrective treatment). Furthermore, the ability to cut a different profile onto each rail allows the system to be used on almost every component of a set of switches or crossings. Set-up time from travel mode to working mode only takes a few minutes, and the accuracy to the set profile is within 0.1mm.
This low set-up time, together with the ability to remove so much material in one pass, really speeds up the time for the D-HOB 2500 to treat a section of track including S&C.
Deutsche Bahn has now approved the D-HOB 2500 for use on high speed lines where trains pass at up to 300kph.
The machine now under construction, the D-HOB 2500 II, uses the same rotational planing technology but has an added oscillating grinding system. This uses grinding stones that oscillate longitudinally, along the rail and as the train moves forward.
Due to the nature of the grinding system (it does not produce dust, smoke or fire) the new D-HOB 2500 II can be used for treatment of rails in tracks and S&C without the need for fire control and additional cleaning services. This offers major advantages, especially when working in tunnels.
Although the rotational planing leaves a good surface finish on the rail head, there are applications, such as in particularly-sensitive built-up areas, where the surface needs to be as smooth as possible to reduce noise. This the oscillating grinding technique does.
There are some areas of the German railway network where rail treatment is undertaken for acoustic reasons. In these areas, the oscillating grinding method is the only one approved for use since it can achieve the requirements of ISO 3095 and technical specifications for interoperability (TSIs) immediately after treatment.
The new train has a rotational planing section in one vehicle and the grinding in a second along with swarf and dust collection hoppers. The train will be propelled by power units (locomotives) suitable for the network it will be working on, and it can even be powered by battery locomotives. It meets the W6A and LU deep tunnel loading gauge requirements.
The need for speed
Although conventional rail milling offers many advantages over rail grinding, there are also some disadvantages. One is a minimum metal removal requirement while another one is the speeds at which milling can be undertaken. This is why, up to now, rail grinding has always been the preferred option for the preventative treatment of track and for maintaining the shape of rails. This so-called ‘little and often’ approach requires low metal removal combined with high output rates which is mainly achieved by an increase of the train speed during the grinding process.
For this reason, Scheerbau is building a second train – a high speed milling train (HSM). Most rail milling trains operate at speeds between 600 and 900 metres an hour and have limitations in running for longer periods due to the fact that the milling cutters need to be replaced regularly, a process which is typically undertaken during maintenance shifts.
The new Schweerbau HSM, which will also run for the first time at the end of this year, will be able to work at around 2,500 metres per hour. It uses two 1,400mm diameter cutting heads on each rail and these heads can be changed quickly, making it practical to exchange them during a working shift.
The HSM is a three-car unit and can run on-track at 100km/hr. It is built to both W6A and LU subsurface loading gauges so, sometime next year, there is no reason why they shouldn’t be seen out and about on the UK network.