Steam locomotives inner workings.
The boiler is typically placed horizontally and for steam locomotives designed to work on steep slopes, it may be placed vertically or mounted at an angle instead.
The steam generated in the boiler fills the steam space above the water in the partially-filled boiler. Its maximum working pressure is limited by spring-loaded safety valves.
It is then collected either in a perforated tube fitted above the water level or from a dome that often houses the regulator valve, or throttle, the purpose of which is to control the amount of steam leaving the boiler.
The steam then travels directly along and down a steam pipe to the engine unit. Then the steam goes inside each steam chest which is a sliding valve that distributes the steam via ports that connect the steam chest to the ends of the cylinder space.
The role of the valves is twofold, admission of each fresh dose of steam and exhaust of the used steam once it has done its work.
The cylinders are double acting, with steam admitted to each side of the piston in turn.
In a two-cylinder steam locomotive, one cylinder is located on each side of the locomotive and the cranks are set 90° out of phase. During a full rotation of the driving wheel, steam provides four power strokes; each cylinder receives two injections of steam per revolution.
The first stroke is to the front of the piston and the second stroke to the rear of the piston; hence two working strokes. Consequently two deliveries of steam onto each piston face in two cylinders generate a full revolution of the driving wheel.
Each piston is connected to the driving axle on each side by a connecting rod; the driving wheels are connected together by coupling rods to transmit power from the main driver to the other wheels.
Each piston transmits power directly through a connecting rod or main rod and a crankpin on the driving wheel. The movement of the valves in the steam chest is controlled through a set of rods and linkages called the valve gear, actuated from the driving axle or else from the crankpin; the valve gear includes devices that allow reversing the engine, adjusting valve travel and the timing of the admission and exhaust events.
The cut-off point determines the moment when the valve blocks a steam port, "cutting off" admission steam and thus determining the proportion of the stroke during which steam is admitted into the cylinder. Careful use of cut-off provides economical use of steam and, in turn, reduces fuel and water consumption.
The reversing lever or Johnson bar controls the cut-off which performs a similar function to a gearshift in an automobile, maximum cut-off provides maximum traction effort at the expense of efficiency, is used to pull away from a standing start, while a cut-off as low as 10% is used when cruising which provides reduced traction effort with lower fuel and water consumption.
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