|No. 2007: latest thinking|
The DeltaRail computer model of a P2 - groundwork for No. 2007 - DeltaRail
In much the same way that Tornado was constructed as the 50th A1 rather than a replica of the original members of the class, if given the go-ahead No. 2007 will be the 7th member of the P2 class, likewise allowing for improvements and variations in design. The decision to closely follow the pattern set by Cock o’ the North means that the locomotive will have the original semi-streamlining and rotary cam valve gear and will look, to all intents and purposes, like No. 2001. However, the Trust acknowledges that the original P2s had certain weaknesses and No. 2007 will have these eliminated at the design stage. Some fundamental criteria have already been decided:
Frames: The “foundation” of any engine will more than likely incorporate a modified leading pony truck to avoid the issues that afflicted the original P2s in this area. The L.N.E.R. solved the problem when the V2s showed a similar tendency and computer modelling can be used to re-design a more stable arrangement. DeltaRail can use the “Vampire” programme to construct a “virtual” P2 and analyse the performance of a modified front truck.
Wheel sets and running gear: Casting a full set of wheels and the forging of axles are processes the Trust is already familiar with. Unlike with the originals,it is proposed that No. 2007 will have roller-bearings throughout – our experience with those fitted to No. 60163 has vindicated the choice for that loco already. Many of the patterns will be common to both Tornado and No. 2007 and thus save a lot of expense, indeed a spare cannon box already exists!
Cylinders and valve gear: The idea to model No. 2007 on Cock o’ the North meant that rotary valve gear was preferable. Some changes may be made to the proportions of the cylinders to compensate for the increased boiler pressure available from the 250lb/sq in diagram 118A fitted to Tornado. However, the Lenz gear used on the original offered only limited cut-off settings and may have contributed, in part, to No. 2001’s high coal consumption. The Trust is therefore actively considering British Caprotti valve gear using a derived drive for the centre cylinder. This valve gear was perfected in its application to No. 71000 Duke of Gloucester and offers infinitely variable cut-offs, although the 71000 SLT is still developing and refining the gear.
Boiler: This major component is one of the most powerful reasons that the Trust is considering a P2. Although the original class had 220lb/sq in boilers the overall size is identical to the diagram 118A 250lb/sq in boiler fitted to No. 60163, thus giving the option to inter-change this component at overhauls. Although the internal design is slightly different to No. 2001’s boiler, the precedent was set by No. 2006 Wolf of Badenoch which had a boiler with a firebox combustion chamber. It is likely that the loco will have Davies-Metcalfe pattern injectors rather than a feed water heater!
Tender: It is proposed that the tender will be identical to that used by Tornado, featuring the modifications that allow more water and slightly less coal to be carried. It will run on roller bearings and will have spoked wheels (the pattern for these being held by the Trust).
Ancillaries: Braking will follow the design perfected during the construction of No. 60163, primary air brakes for locomotive and train with secondary vacuum brakes for working preserved stock. The electrical system will copy the excellent system fitted to Tornado although the original P2s did not have a Stones generator or electric lighting.
The P2 project already has a head start in the vast number of common components the locomotive shares with Tornado, many patterns for which are held by the Trust. However, no metal will be cut until the results of the computer modelling are known and we are sure that the design will be acceptable to Network Rail.
A superb study of Cock o' the North's unique front end - Rail Archive Stephenson
2012 - an update by Graham Nicholas
About ten years ago, I attended a training course entitled ‘an introduction to railway vehicle dynamics’, given by AEA Technology Rail (now DeltaRail, a UK railway engineering consultancy whose genealogy goes back to BR’s Research & Development department). In amongst the copious course note hand outs, a somewhat surprising picture leapt out at me – a computer generated diagram of a Class P2 2-8-2 steam locomotive, which sought to demonstrate the locomotive’s poor curving performance. At the time, I found this mildly amusing. The P2s had this supposed reputation for spreading the track and here was someone within the modern railway, using a state of the art technique to somehow prove this. More recently, as talk of a P2 as the Trust’s Lot 2 has gathered pace, it has occurred to me that many of today’s current railway engineers are unwittingly being taught that a steam locomotive called a P2 had a reputation for spreading the track!
Unfortunate? Well, perhaps, but make no mistake about it – we are starting from a rather different place when it comes to considering the engineering design and ultimate acceptance for a P2. Despite the above, there appears to have been no recorded instances of any of the original P2s derailing of its own accord on the main line whilst working a train. However, as may be well known, the P2s were fitted with the same ‘swing link’ leading pony truck design as was originally fitted to the V2s, and there were no less than four recorded mainline derailments of the latter:
It was the latter two accidents that led to the front pony truck design being changed (the first two accidents had originally been put down to other causes but were subsequently determined to be due to the pony truck). Meanwhile, the P2s did suffer mechanical failures on the main line with no less than four instances of crank axle failure during their comparatively short working lives. Such things were more common in those days, with a general live and learn level of tolerance towards them. Nowadays, an axle failure on the main line is unthinkable and all rolling stock engineers live in fear of such a thing happening on their watch. As if to emphasise the seriousness of such occurrences, a particularly unlucky combination of circumstances (oil tank wagons passing through a station) led to multiple fatalities when an axle failed in Italy two years ago. Changes to railway legislation at European level have been rushed through as a result.
A drawing of the Doncaster designed swing link arrangement for the pony truck
Thus it is that the P2 feasibility study has a very clear purpose: to evaluate whether such issues can be addressed before deciding to embark on building a new P2. As was announced at last year’s convention, the study is now underway. This work is being undertaken by none other than DeltaRail themselves, based around the internationally recognised vehicle dynamics computer modelling package VAMPIRE®. Computer modelling using VAMPIRE is a recognised technique in the UK and is usually used to optimise the design of new or modified rail vehicles and to demonstrate that they will be safe (i.e. not fall off the track!) before they are actually tested out for real. This poses a slight problem for us as all modelling assumptions in VAMPIRE are based on testing undertaken with modern vehicles (e.g. passenger vehicles fitted with air suspension); there has been no fully validated modelling of a vehicle type characterised by a rigid frame chassis, different size wheels and coupled wheelsets – until now!
The first phase of the project has therefore been to develop a fully validated model of a steam locomotive within VAMPIRE. This has to be based on an existing locomotive – and Tornado is the obvious choice! Without doing this first it simply would not be possible to undertake an accurate study on the P2. A fully validated model is one whereby the predicted and actual ride performance of the vehicle have been compared and agreed to be a sufficiently close match. Fortunately we were already aware of all this when the time came to undertake the testing of Tornado. We were asked by Network Rail to undertake some ride testing anyway, but we arranged for a few extra measuring devices (accelerometers) to be fitted as the incremental cost was minimal. Thus as she roared through the night from York to Newcastle and back as part of her own testing on that memorable evening of 18th November 2008, Tornado was also helping to pave the way for her future stablemate.
So the current work (phase one of three) involves building a detailed computer model of Tornado within VAMPIRE. The work behind this is considerable; a particular challenge has been to build a working representation of the Cartazzi rear axle, with its inclined slides. Currently the process of validation is being undertaken – and this involves simulating the York-Newcastle test run. The additional data required is the measured track data for the piece of railway she ran over (specifically the ‘up fast’ road from Tyne Yard to Tollerton). But here another wonder of the modern railway lends a hand. Network Rail’s New Measurement Train (NMT) – the yellow HST – regularly roams the length and breadth of the country’s rails, measuring the track quality. The data so recorded can be fed into VAMPIRE to allow a vehicle model to be run over any stretch of track. The memory bank has been searched and the set of data nearest to 18th November 2008 over the Newcastle-York stretch found to ensure that the model simulation is as accurate as possible. By comparing the ride performance predicted by VAMPIRE with the actual ride performance measured on the night, the model can be tweaked until a good match is achieved. The modelling assumptions can then be confirmed as validated.
A solidly modelled version of the 'Vampire' animation of the Cartazzi truck - Delta Rail
This then is the current activity. The next stage is to use the now validated modelling assumptions to build a model of the P2 as originally designed (with the swing link pony truck). We should then expect VAMPIRE to predict that the ride of the P2 will be worse (than Tornado). The final phase will be to use VAMPIRE to its full extent, i.e. to modify the P2 design (and fitment of the later V2 pony truck design is an obvious starting point) until an optimum configuration is achieved (but without noticeably altering its visual appearance!). Provided this redesign can be shown to be a noticeable improvement on the original P2 (and ideally as close to Tornado’s ride performance) then all should be well with the concept of embarking on the build of an Improved P2.
The biggest challenge has been to replicate the infamous swing link pony truck arrangement. Careful study of the original drawings shows how the arrangement worked, consisting of a top yoke (linked to the underframe) and bottom yoke (linked to the pony truck), with the two yokes able to rotate relative to each other (to allow for the turn of the pony truck). This has been replicated on the model by a single ‘lump’ (mass) but with a degree of flexibility.
The 'Vampire' plot of the original P2 arrangement - Delta Rail
The 'Vampire' model of the pony truck - Delta Rail
The mechanics of this arrangement have now been clearly understood, following its detail examination and modelling. Most significantly, the links were configured such that the vertical distance between the bogie and underframe increases as it ‘swings’ away from its centre line. Put another way, it is lifting the front end of the loco up as it negotiates the curve. This of course increases the weight on the pony truck wheelset, presumably in an attempt to help increase its steering capability. However, the unfortunate side effect of this is that it leads to the weight on the wheelset behind it (ie the leading coupled wheelset) to be correspondingly reduced (obviously, the overall weight of the locomotive has to stay the same!)
The work has already developed far enough to allow the movement of the suspension to be shown in animated form, which is an aid to understanding. Those that were able to be at the Convention would have seen this during Owen Evan’s presentation. As the virtual locomotive makes its way round a curve of increasing radius, so the lifting effect of the front pony truck starts to take effect. At first the suspension arrangement is able to compensate but it eventually gets to a stage where the inner wheel of the front coupled wheelset is lifted completely clear of the rail!
The Delta Rail animation used to illustrate the mis-behaviour of the leading truck - Delta Rail
2013 - The model is further refined
The results of the study were submitted to the Trust by Owen Evans of DeltaRail in report ES-2013-003 at the end of February 2013. The following is a summary of the report and its findings. The new models of No. 2001 (‘as built’ P2) and No. 2007 (‘improved P2’ with V2-style pony truck) include some graphical information to allow much better plots to be produced than for the Tornado model. Here is the complete ‘as built’ model, showing the suspension arrangement:
This is the detailed view of the pony truck:
The first simulations carried out with these models was to run them round an ‘assault course’ of smooth, sharply curved sections of track. This begins with a straight, then increasing curvature to a 6 chains radius curve, followed by a further increase to 4.5 chains (with a mirror arrangement back to straight). The models were run round this track at 5 mph, and results were produced for the forces between the wheels and rails, both laterally and vertically.
This simulation enables the study of the ‘track shifting’ forces; that is the total force from each wheelset that is trying to ‘straighten out’ the track. Because the coupled wheelsets are constrained to be parallel to each, they are not all aligned with the curve and have what is known as an ‘angle of attack’. In this case, not only do the wheelsets try to shift the track, they also try to spread the gauge, i.e. to force the rails apart. On gentle main line curves with good quality rail fastenings this is not too much of a problem, but in depots and sidings where curves can be much sharper there is a limit beyond which there is a real risk of derailment. There is also a link to the vertical loads on each wheel. If the lateral load (the track shifting force) builds up too much, the effect will be to push the wheel clean off the rail (ie reducing the vertical load to zero) – not a desirable result! Having explained all this, we can look at the results as Tornado, No. 2001 and No. 2007 were run round these curves.
Here is the comparison of Tornado and No. 2001 on the 6 chain curve:
Careful study will show the greater build-up of opposing lateral (green) forces for No. 2001 and that the red (vertical) arrows on No. 2001 are quite uneven from side to side – this is most noticeable on the front pony truck. Moving on to the 4.5 chain curve, here is No. 2001:
This is quite scary - the outer wheel of the third coupled wheelset has actually lifted off the rail, and you can see that the engine is doing its very best to straighten out the curve. No. 2007 actually fared little better. The work included a certain amount of adjustment of hornguide clearances to achieve the best performance for No. 2007. The conclusion was that Tornado can just manage a 4.5 chain curve whereas the minimum radius that could be achieved for No. 2007 with reasonable increases in the lateral hornguide clearances will be about 5.7 chains. It was established (from the original drawings) that No. 2001 already had thin flanges on the intermediate driving wheelsets and this feature will be retained with No. 2007. Given that such sharp curves only appear in yards and sidings, this was deemed not to be a ‘show-stopper’, so the simulations moved on to more general behaviour out on the main line. These were based around the high speed test run with Tornado between York and Newcastle on the evening of 18th November 2008. Much useful data was gathered during that test, mainly in the form of accelerations on Tornado and her tender, as well as a full speed profile for the southbound run. Courtesy of Network Rail, the measured track geometry data for the Up Fast line between Newcastle and York (as recorded by the New Measurement Train about two weeks after the test) was recovered and fed into VAMPIRE, and the Tornado model adjusted to improve its accuracy. All three models (Tornado, No. 2001 & No. 2007) could now be put through their paces over the same track data to provide a true comparison study.
A lot of information was generated, relating to ride quality, track forces and derailment risk. In some cases the P2s were better than Tornado, for example much of the ride (especially for No. 2007) and some of the track forces (remember this route is pretty straight). In some cases No. 2001 was better than No. 2007 and in others the opposite was true. Overall No. 2007 looked better than No. 2001, but would still benefit from further refinement.
One final check was ‘peak counting’. This is a somewhat complex calculation (done automatically by VAMPIRE). It looks at the lateral and vertical accelerations on the body of a rail vehicle and uses them to assess whether the vehicle is dynamically unstable. If you have ever pushed a supermarket trolley too quickly and seen the caster wheels start to vibrate rapidly from side to side, or seen a caravan snaking down the motorway, you will have witnessed dynamic instability. You can imagine that a railway vehicle behaving in this way would be quite frightening. Additionally, lack of vertical damping means that a vehicle can build up an increasing ‘bounce’ in response to features in the track to the point where it can momentarily jump clear of the rails!
Looking at our engines, the vertical behaviour is fine for all of them, even when the section between 75 and 80 mph is analysed. This is an excellent result, borne out by the actual test results from Tornado. Looking at the lateral results, some adverse behaviour is predicted, but some of this is caused by the piston thrusts causing the engine to ‘waggle’, and this is not the same thing as dynamic instability. Even so, only the fronts of the engines exceed the limits to any significant degree, and the P2s are generally better than Tornado (happily, this is an aspect where the longer rigid wheelbase is almost certainly an advantage!). Given that Tornado is now accepted as a mainline loco then this is an area where the comparison argument will be a likely way forward in terms of acceptance
Overall therefore, the verdict is that the P2 project should proceed on to the design development phase (involving more detailed dynamic analysis). Looking back, this has been quite ground-breaking work, as it is almost certainly the first time that VAMPIRE has been used in this way for steam locomotives. Delta Rail have completed an impressive report and we look forward to working with them further on the project.