Building a new class P2
A CAD rendering of No. 2007 Prince of Wales – David Elliott
In much the same way that No. 60163 Tornado was constructed as the 50th Peppercorn class A1 rather than a replica of the original members of the class, No. 2007 Prince of Wales will be the 7th member of the Gresley class P2, likewise allowing for improvements and variations in design. The decision to closely follow the pattern set by No. 2001 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 class P2s in this area. The LNER solved the problem when the Gresley class V2s showed a similar tendency and computer modelling has been used to re-design a more stable arrangement. DeltaRail used the Vampire® programme to construct a virtual class P2 and analyse the performance of a modified front truck.
Wheel sets and running gear: Unlike with the originals, No. 2007 will have roller-bearings throughout – our experience with those fitted to No. 60163 has vindicated the choice for that locomotive already. Many of the patterns will be common to both Tornado and Prince of Wales 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 means that rotary valve gear is preferable. Changes will be made to the cylinder diameter, reducing it to 19.75 inches to compensate for the increased boiler pressure available from the 250lb/sq in diagram 118A fitted to Tornado. However, the Lentz gear used on the original offered only limited cut-off settings and may have contributed, in part, to No. 2001’s high coal consumption. Gresley class D49 No. 365 The Morpeth had infinitely variable Lentz Gear fitted in the 1930s and when rebuilt by Thompson this was fitted to No. 62764 The Garth and ran until 1958 in service. The Trust is therefore incorporating lessons from the Franklin derivative of the Lentz valve gear into its final solution.
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. A consideration of improving the design of air production will be looked at during the project.
A superb study of Cock o’ the North’s unique front end – Rail Archive Stephenson
Design study 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:
- 4844 – Newark 13th March 1944
- 4878 – Thirsk 24th February 1946
- 3645 – Hatfield 15th July 1946
- 905 (4876) – Marshmoor 10th November 1946
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
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.
At the A1 Convention in September 2013 the P2 Project was officially launched. This gave David Elliott the opportunity to bring the assembled company up to speed on the design work going into No. 2007. David surprised his audience with a CAD diagram of the frames showing that a great deal of development work had been quietly going on behind the scenes. The frames (illustrated here) will benefit from the adoption of a great many features carried by Tornado including roller bearings; the descision to fit these has led to a requirement for larger hornblocks and a consequent re-working of some of the stretchers. David also revealed that the rear (Cartazzi) frames would be better formed like Tornado’s to avoid the complicated shaping of the P2 frames, which, coupled with the A1 style dragbox, should create much more rigid design.
The 3D CAD model (drawn in Solidworks) shows some of the changes that will be incorporated in the frame design.
The grey parts are almost identical to the original design being principally the main frame plates. The only significant change to them is the plate thickness, eg 30mm today instead of 1 1/8″ in the originals.
The green parts are where A1 Tornado design will be incorporated and includes, roller bearing cannon/axle boxes and hornblocks with manganese steel liners. The whole of the rear frame area under the cab and firebox where the original design has a dished section in the inner frames to clear the bosses of the Cartazzi wheelsets (in common with Gresley A1/A3 classes). For the later Thompson and Peppercorn Pacifics, the dragbox was made narrower and the set in the frames moved forwards to avoid the need for the dished frames. The profile of the rear inner and outer frames are modified to make extra space to take the A1 style hopper ashpan.
The yellow components are original P2 design which requires minor modification, for example spring hangers which all alteration to clear the longer roller bearing hornblocks.
The red components are original P2 design which requires significant modification, for example large frame stays which were designed to support vacuum brake cylinders which will need redesign for air brakes, and the front boiler support which will need altering to accommodate the shorter A1 boiler.
The next stage is to draw the modified pony truck arrangement that was fitted to the V2 class after WW2 which was based closely on the design of the Stanier 8F locomotives a number of which were built at Doncaster during WW2.
This is the original arrangement for No. 2001 showing the inner frames dished to clear the axle.
This is the proposed re-design for No. 2007 employing Tornado architecture in the structure of the inner frames.
A further elaboration of the frames CAD shows the fitting of a fabricated cylinder monobloc (blue) and the arrangement of a V2 pony truck (purple).
The V2/P2 pony truck with side control springs rather than links
The design of the wheels will incorporate separate balance weights filled with lead
Further 3D design work concentrated on supporting the Delta Rail assignment – including wheels, connecting & coupling rods, crossheads and slide bars – to establish the maximum allowable side clearances in axle boxes in order to improve low speed negotiation of curves. Investigation of the original Lentz gear and its development by The Franklin Railway Supply Company (a subsidiary of Lima Locomotive Works) of the USA continued.
The frames with the wheels fitted
Detail of the fabricated monobloc cylinders
Detail of the frames and modifications for roller-bearings
The 3D CAD work focused on creating a 3D model of Tornado’s diagram 118a boiler to check that mounting points on the frames are correct. Footplate brackets and running plate angles have also been added to the model in order to position the holes in the frames as this is required to confirm the drilling pattern for frames.
Teesside University has taken on the Finite Element Analysis (FEA) of the crank axle wheel/axle interface to investigate the problem of axle failure that occurred on the original P2 locomotives. Initially this is being done by creating and comparing FEA models of the original design, the Timken design applied to the roller bearing A1 locomotives in 1949 (which is incorporated in Tornado) and a further development to the Timken design to incorporate the principles laid down in the modern BR BASS 504 standard. The work is being done as a post graduate Mechanical Engineering project with a view to ascertaining whether the later and better designs will increase the fatigue resistance of the axle to an acceptable level. According to contemporary Timken literature, their design increased fatigue resistance by 60%.
The wheelsets have been fitted with coupling rods and the motion brackets attached
A detail of the motion bracket and front frame stay to support the boiler
The boiler, coupling and connecting rods fitted
The extended smokebox fitted (the Diagram 118a boiler is shorter than the original’s)
Another view showing the boiler fitted to the frames
The CAD has been further enhanced with cladding and Franklin valve cylinders