Valve Gear Improvements for 'Doll'
Withdrawn from service at the Leighton Buzzard Narrow Gauge Railway 'Doll' received a new all welded boiler from Bennett Boilers and a full bottom end overhaul at the Works of Alan Keef Ltd. The valve gear was fitted with new eccentrics and repined and bushed to remove the massive amount of lost motion which had developed. It then became clear that at some point in locomotives' industrial past the wear had been compensated for by sawing part of the lap from the slide valves and filling in notches in the reversing quadrant near mid-gear. As the locomotive had a reputation for being a poor steamer and the lead was now excessive the key questions we were asked was what should the dimensions of the valves be and where should additional notches be provided in the reversing quadrant?
The Stephenson valve gear was computer modelled and it was determined that the valves needed to be increased in length by 6 mm to correct the valve events and that this would give full gear cut-offs of 77% in forward gear and 74% in reverse gear. The current notches in the reversing quadrant permitted a shortest cut-off of only 48% which would have been poor in terms of steam economy and additional notch positions were determined for 25% cut-off.
When the locomotive was delivered to Leighton Buzzard we attended with Alan Keef Ltd to finally set the valves and the locomotive was then tested. Everyone was delighted with the results and the hither to reluctant steamer has now got steam to spare and will happily pull heavy trains up the steep 1 in 25 gradients at 25% cut-off.