World Record
On 3 July 1938 4468 Mallard; the first of the class to enter service with the Kylchap exhaust, set a world speed record of 126 mph (202.8 km/h), pulling six coaches and a dynamometer car. Although the dynamometer car indicated a top speed of 126 mph (202.8 km/h), Gresley never accepted this as the record-breaking maximum. He claimed this speed could only have been attained over a few yards, though he was comfortable that the German speed record of 124.5 mph (200.4 km/h) had been surpassed. Close analysis of the dynamometer roll (currently at the NRM) of the record run confirms that Mallard's speed did in fact exceed that of the German BR 05 002(Speed on the East Coast Mainline p64, P Semmens). The Mallard record reached its maximum speed on a downhill run and actually failed technically in due course, whereas 05 002's journey was on level grade and the engine did not yet seem to be at its limit, though on the other hand the German train was only four coaches long (197 tons), whilst Mallard`s train was seven coaches (240 tons). One fact that is often ignored when considering rival claims is that Gresley and the LNER had just one serious attempt at the record, which was far from a perfect run with a 15mph permanent way check just North of Grantham. Despite this a record was set. Gresley planned to have another attempt in September 1939, but this was prevented by the outbreak of World War II. Prior to the record run on 3 July 1938, it was calculated that 130 mph (210 km/h) was possible, and in fact Driver Duddington and LNER Inspector Sid Jenkins both said they may well have achieved this figure had they not had to slow for the Essendine junctions.
At the end of Mallard's record run, the middle big end (part of the motion for the inside cylinder) was found to have run hot (indicated by the bursting of a heat-sensitive "stink bomb" placed in the bearing for warning purposes), the bearing metal having melted, which meant that the locomotive had to stop at Peterborough rather than continue on to London. Deficiencies in the alignment of the Gresley-Holcroft derived motion meant that the inside cylinder of the A4 did more work at high speed than the two outside cylinders - indeed on at least one occasion this led to the middle big end wearing to such an extent that the increased piston travel knocked the ends off the middle cylinder - and this overloading was mostly responsible for the failure.
Read more about this topic: LNER Class A4
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