Front end progress
After half a century at Barry, exposed to the elements and sea air, parts of what is intended to become 4709’s pony truck, have predictably, corroded. Now, they need to be replaced.
The pony truck resembles the letter ‘A’, and was originally fabricated from many components that were common steel sections at that time, or heavy items which had been forged, bent and curved in three dimensions. Understandably, many of these sections are no longer available and so, renovation for mainline use involves a detailed and thorough assessment of each and every component.
Then comes the challenge of whether it would be more economic to replace with new, rather than repair. Interestingly, no modern day fabricators are able to bend the 5”x 5” x 5” x ½” T sections that the pony truck radius bars were fabricated with and so the originals have had to be sent away for specialist weld repairs to meet the approval of the VAB. Clearly Swindon had no problems in bending these sections in the day; providing a good indication of the capability and knowledge that has been lost.
It was always intended to fit the entire front end of donor loco no. 4115 to 4709, so we could be confident that it would work as a proven and self-contained unit. However, a general arrangement drawing of the proposed configuration quickly showed that this plan was unworkable; at 6 cwt, 4115’s leading frame stay was too light for a 47xx. The original 47XX stay weighed some 2 tons. Back to the drawing board - literally.
4709 centre stay pattern under construction
A new frame stay casting is needed as the function of this huge block of cast iron is to stabilise, stiffen and dampen the entire front end of the locomotive. In addition, the stay will contain a 10” diameter hole through which a heavy cylindrical counterweight will rise and fall as the wheels follow the track, moving compensating beams that connect to the front of the leading axle springs. This arrangement enables the leading axle to allow for the deflection encountered by the pony truck’s wheelset.
On 4709, this casting will be hidden under the running boards ahead of the smokebox. A bell shaped cowling will cover the 10” hole for the counterweight.
A 3D model was produced for the pattern. Caddick and Moss of Bristol – the company that previously produced patterns for 4709’s wheels and dragbox, has subsequently completed these patterns, which are due to be cast shortly. Patterns have also been commissioned for the compensating beams themselves which will be cast in steel in the next few weeks.
In the meantime, work on 4709 is progressing with the machining of fourteen new spring hanger brackets and the ongoing refurbishment of the (re)usable components from the loco’s pony truck assembly.
