Design work has been concentrated in two areas; drawing and modification of the cab to comply with the reduction in overall height from 13′ 1″ to 13′ as was done with Tornado to ensure the widest possible route availability and redesign of frame major frame stays as full penetration welded fabrications with modifications to carry air pumps and air brake cylinders (see 3D models).

Frame assembly 2007 19-11-14

Frame assembly 2007 19-11-14 showing fabricated frame stay

The last of the initial batch of 24 frame stay and bracket castings – the drag box – has been completed by William Cook Cast Products including stress relieving and is due to be delivered to MultiTech at Ferrybridge along with other larger frame castings for machining. MultiTech will also be proof machining the small loco carrying wheels and tender wheels ready for wheelset assembly next year.  Smaller frame brackets will be machined by companies in the Darlington area.  Tyres, axles, crank pins and crank sweep forgings have been ordered and quotations are being sought for assembly of the wheelsets.

Castings ready for dispatch 10-11-14

Castings ready for despatch by WCCP – David Elliott

Drag box 10-11-14

The drag box casting – David Elliott

One of the problems experienced with the original P2 locomotives was fracture of crank axles. We are aware of at least four and possibly five occasions when a crank axle broke immediately behind the wheel. Whilst such occurrences were not uncommon in the days before routine non-destructive testing of axles by ultrasonic and more recently magnetic particle inspection, for so many events on a small class of six locomotives over an eight year period is exceptional and will not be tolerated on today’s railway.

The design of the P2 crank axle is essentially the same as that for the contemporary A3 Pacifics which were not prone to axle failure. The P2’s larger cylinders would have resulted in higher maximum piston forces and hence torque in the axle, and whilst a Pacific at high axle torque will slip dissipating the torque, the additional pair of coupled wheels on the P2s made them one of the most sure footed locomotives ever built.  Hence higher sustained torques were possible.  The fact that all these failures took place at low speed when the locomotives were accelerating hard had the fortunate consequence that none of the failures resulted in derailment.

When Tornado had its first four yearly in-service ultrasonic axle tests carried out by Graham Werrett of Serco Railtest during the 2012 winter maintenance, Graham showed David Elliott a fascinating photograph album that he rescued from Doncaster works several years ago. This was a record of axle, tyre and crank pin failures recorded by the metallurgists at Doncaster from the around 1919 up to the early 1940s. One page in particular caught David’s eye being the failure of the crank axle on No. 2005 Thane of Fife at Stonehaven in July 1939. This includes a picture of the broken axle in its axlebox and clearly indicated that a crack started from the sharp corner in the end of the keyway for the key that locates the wheel on the axle. The crack had grown slowly until it was about two thirds of the way through the axle when it failed completely.

2005 broken axle rotated

Thane of Fife’s broken crank axle – courtesy of Graham Werrett

The crank axle on Tornado has several significant design improvements developed by the Timken company including a better keyway design and a stress relief groove the surface of which was rolled to compress the material. These features improved the fatigue resistance by at least 60%.  We intend to further improve the design by incorporating the BR BASS 504 wheel/axle design techniques.

In order to assess that the improved design will fully address the problem, we are commissioning an independent design study by one of the leading railway engineering consultancies to examine and compare the original and improved designs to give us (and the certification authorities) confidence that the axle failure problem will be overcome.