Pistol boiler
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A pistol boiler is a design of
overtype steam wagons. It is noted for the unusual shape of the firebox
, a circular design intended to be self-supporting without the use of firebox stays.
Robey & Co.
'Express' steam tractor, with Robey's pistol boilerThe name "pistol boiler" derives from the smooth curve of the outer firebox flowing into the boiler barrel and a supposed resemblance to the stock of an early 19th-century pistol.
Need for a self-supporting firebox
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The
locomotive boiler had become well established since Stephenson's day[when?]; although the cost and complexity of its firebox remained a drawback, particularly for small boilers. If the top crown sheet of the inner firebox was made flat, so as to maintain a constant water depth above it, this required complex and expensive girder stays to support it. These stays were also a safety-critical part of the boiler, and many past boiler explosions
had been caused by their failure. This was especially so for boilers that were likely to be used at all carelessly or by crews who were less skilled or well-trained.
Clearly the market for small steam
London & Birmingham Railway's Bury locomotives, some small portable engines were already using cylindrical stayless fireboxes.[1] These combined a cylindrical vertical drum with a domed top, both shapes that could support themselves well under pressure. In extreme cases for larger railway locomotives, these became the massive brass-clad 'Haycock' fireboxes that were so distinctive on early Great Western Railway
locomotives.
Development of the pistol boiler
The firm of
Robey & Co., well-known builders of both large stationary engines and small steam tractors, developed its own version of this stayless domed firebox as the pistol boiler.[when?][2][3]
As the boiler was small, with a barrel diameter of only 2', it was practical to form the deeply curved plates for the inner and outer firebox with a
mud build-up in the corners.[4] The upper corner of the outer firebox was a separate plate, approximately a quarter of a sphere. Rather than the time-consuming and costly flanging of flat plates, these curved plates could be pressed and riveted together almost immediately. The number of boiler plates was also reduced from the usual eight to only five.[i] A relatively high working pressure of 250 psi could be used.[4]
The
firebox door
was also of novel design. As the backplate sloped so steeply, the door was top-hinged and opened inwards rather than outwards. This also had the effect of acting as a deflector plate, directing the cold draught down onto the firebed rather than directly across and into the tubes.
Robey used this design of boiler on their 6-ton steam wagons and 'Express' steam tractors,steam rollers. One of these rollers was the first artefact to be preserved by the Robey Trust.[5] When the boiler was re-barreled in 1988, this was the last boiler to be constructed by the Robey factory before closure. On their larger engines, Robey used a conventional boiler.
Related designs
A similar pistol boiler was also used by
Foden in their 'O-type' Speed Six and Speed Twelve steam wagons.[6][7] They were also used by Ransomes for their overtype steam wagons in the 1920s.[8]
The firm of
crown sheet was formed into a curved valley.[9] For a small firebox, this acted as a girder stay between the end sheets of the firebox and was sufficient to be self-supporting. The portable engines used this pattern alone.[10] For the slightly more powerful road tractor boilers, the inner firebox was still self-supporting but the outer wrapper now required crosswise sling stays to support it, where previously it would have been supported by the stays to the inner firebox.[11]
Notes
References
- ^ "Early French portable engine".
- ^ a b Gilbert, G.F.A. (2000). Robey boiler and engine unit. Traction Engine Design and Construction 1900-1930. p. 122.
- ^ a b Gilbert, G.F.A. (2000). Robey boiler for 'Express' tractor, longitudinal section. Traction Engine Design and Construction 1900-1930. p. 123.
- ^ a b c Wright, Philip (1959). Traction Engines. London: Adam & Charles Black. pp. 64–65.
- ^ "Tandem roller 42693 of 1925". The Robey Trust.
- ^ Illustrated Spare Parts List for the Foden Speed-Six Steam Wagon. Edwin Foden, Sons & Co. 1930s.
- ^ Owner's and Driver's Manual: Foden Speed 6 and Speed 12 models. Edwin Foden, Sons & Co. 1930s.
- ^ Thornton Rutter, H. (1922). Modern Motors. Vol. III. London: Virtue & Company. pp. 556–558.
- ^ Wright (1959), pp. 41–42.
- ^ "Garrett portable engine".
- ^ Dan Dyman (10 July 2008). "Guts of a Garrett boiler".
External links
- "Robey steam wagon, 42657". Steam Scenes. 30 May 2010.
- "Side view of Robey Tandem Road Roller, 41593". Steam Scenes. 24 Jul 2003.