• TECHNICAL: SUPER SENTINEL
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The Super Sentinel is fitted with a unique design of vertical boiler, fired from above. The boiler water tubes, of varying lengths, are arranged at steep angles (to stop fuel lodging on them) and tangentially to the central stoking hole. A double row of heater coils mounted in the upper combustion space super heats the steam to around 340°C.
With the external water gauge at its half-way point, the boiler holds some 227 litres of water, and when working hard it can evaporate up to 545kg of water per hour. The result is an engine output of up to 52kW at 240rpm, with a working pressure of 15 bar. The standard water tank contains 760 litres — Rivett uses an extra tank in the wagon body for extra range.
The Super Sentinel engine has two horizontal cylinders: it weighs a massive 1,100kg, and is suspended on three rubber mountings in the chassis. It is a double-acting engine, but not compound, and the cylinders are served by stainlesssteel thimble-type valves.
The two camshafts (for inlet and exhaust valves) can be slid sideways to give different cams for forward and reverse motion. They are moved laterally by a long lever in the cab: there are notches for 25% and 75% steam cut-off in the forward direction, and at 75% for reverse.
The Sentinel crankshaft, which has the final drive sprockets mounted on its outer ends, is also unusual in that it contains not only the differential gear, but also a primitive limited slip mechanism. The two crankpins (at 90° to each other) are connected by spur gears which allow the differential action, and the central spur gears contain spring-loaded clutches to give the limited-slip effect.
The body of Rivett's Sentinel is to the maker's own design, a three-way tipper. It is steam-operated, via a huge central underbody ram, and can elevate to 42°. Originally made of American oak and magnolia, it has been rebuilt in English oak.
ID by Bryan Jarvis and Richard Dames-Longworth.