The Britannia in 3.5 Gauge was designed by LBSC in close conjunction with the original drawings for the British Rail Standard 7. It was published in Model Engineer magazine Volumes 104-109 in the 1950s. A Pacific 4-6-2 with two cylinders of 1.25" bore and driving wheels of 4.625", it is a larger locomotive in this scale and a popular choice with more experienced model engineers. It is around 4.5 feet in length including tender.
Original Design - Prototype Edit
Robert Riddles, the designer of the full scale Class 7 Britannia, was enamoured with the work of LBSC and they had corresponded while the new locomotives were being built in secret. Riddles provided LBSC with a full set of drawings for the New Locomotive. On the day that Britannia was unveiled, the 1st of February 1951, the first instalment for the Britannia ‘words and music’ appeared. Model Engineer Magazine reputedly paid double their normal rate for the Scoop. The first in class was numbered 70000 and named "Britannia", this and 70013 "Oliver Cromwell" have survived into preservation. There were 55 produced, numbered 70000 to 70054. All were painted British Railways Dark Locomotive Green lined in Orange and Black, as befitting express passenger locomotives, however Britannia was briefly painted in unlined black when first unveiled. Towards the end of their useful lifetime with British Rail they were painted in a plain green livery which was cheaper to maintain and touch-up as required.
The LBSC Britannia has a screw reverser operating Walschaerts valve gear. The backhead usually features a single gauge glass. The valves are of the Piston variety, which does increase the difficulty of construction slightly compared to slide valves. There are a number of different regulator variants, including Poppet and disc type, two options were detailed in the original drawings. The regulator is operated by a lever that mimics the design of her full size sister. A Single water pump is fed from an eccentric fitted to the main driven axle, an injector is installed alongside the cab, and a hand pump provides backup in the tender. Unusually the entire locomotive was designed to be manufactured with Ball Bearing axle-boxes (although LBSC did provide drawings for conventional solid bearings too). Steam powered braking is also detailed. It follows typical LBSC design practise of large cylinders, with the pulling power that this accords.
Boiler design Edit
The tapered boiler uses a combustion chamber and six cross tube siphon pipes. The extended firebox crown is stayed by means of a number of vertical water tubes passing through the combustion chamber; LBSC/Martin Evans' 3.5 Gauge 9F Evening Star has the same combustion chamber arrangement. Several discussions have been raised regarding the efficiency of this setup, believing the increased boiler capacity gained from removing the combustion chamber and siphon tubes would result in a more efficient design (subject to boiler inspector approval). LBSC states no cladding is required, although it is usually added around the firebox area to hide the soldering of the stays.
There are some builder optional variants available for this design:
- The regulator can either be made as a poppet valve design in the smokebox, or as a disc valve design in the dome. Castings are available for the poppet valve system. LBSC details both in the original drawing set.
- LBSC details options for either roller bearing axles or solid bearing designs. The amount of effort required for each variant does not vary greatly. Although imperial sizes the bearings are still easily available. The coupled wheels require four bearings per axle (twelve in total) of dimensions 3/8 x 7/8 x 7/32. This bearing is commonly called an R6. The bogie uses two bearings per axle (four in total) of 1/4 x 3/4 x 7/32 designated R4a. Both R6 and R4a are available on eBay or from suppliers such as Bearing King.
- As LBSC produced the design at the same time as the first Britannia was unveiled the drawings represent the earliest evolution of the design. As a result the drawings show a BR1 Tender, whereas locomotives 70025–70029 were fitted with a BR1A tender with a larger water capacity of 5000 Gallons, which was visually very similar. However a large visual difference was shown by the introduction of the BR1D for locomotives 70045–70054. Model Engineers Laser offer the parts to create all these tender designs. Most existing 3.5 Gauge Britannias have been manufactured to the original specifications.
Drawing Errors Edit
- The original drawings show the water gauge glass bottom fitting sitting too low, allowing the top of the firebox to become uncovered whilst water still appears to be at safe levels. The lower fitting should be raised by approximately 1/4" to ensure that the crown is covered when there is no water visible in the gauge glass.
- The original boiler throatplate was designed to be Soldered using SifBronze, using alternative techniques can result in boiler joints which open up as they cool. Double flanging the throatplate is suggested.
- The original drawings show the leading coupled wheel's crankpin with an internal blind thread marked 3/32 X 40. In Model Engineering magazine Issue 2609 (24th May 1951) the same thread is shown as 5/32 X 40. Some drawings have had this corrected, some haven't. The correct thread for the leading coupled wheels crankpin is 5/32 X 40.
- Some concerns have been raised with regards to the assembly of the expansion links and trunnions.