The boiler has a safety valve to stop pressure rising above the permitted limit.
Heat is usually produced by burning coal in the firebox at the rear of the boiler, though oil or wood may also be used. Fuel may be added to the fire manually or by using a mechanical stoker.
Special valves called injectors use steam pressure to force water from the supply tank into the boiler.
Steam is collected at the top of the boiler, usually in a projection called the dome, though some engines do not have this. The main inlet valve to admit steam to the cylinders (called the regulator in UK and the throttle in the US and Canada) is situated here.
Steam enters the cylinders and pushes the pistons inside them back and forth, the oscillating motion being converted to circular motion by cranks on the main drive axle which may be inside or outside the driving wheels. The pistons are connected to the main drive wheel by the connecting rod and the power is transmitted to the other driving wheels by coupling rods. The entry and exit of steam to and from the cylinders is regulated by valves worked by a system of return cranks and links from the drive axle (the valve gear). The timing of these can be adjusted to increase/decrease the point at which steam entering the cylinder is cut off (hence the name - 'cut-off') and also to enable the engine to run forward or in reverse. Depending on its size and power an engine may have 2, 3 or 4 cylinders. Some engines are compound types where the steam is used twice, in a high-pressure and then a low-pressure cylinder, before being exhausted.
Exhaust steam is expelled from the cylinders through a hollow chamber at the front of the boiler (the smokebox) and up through the chimney causing air to be sucked out of the smokebox and drawn through the firebox and fire tubes running the length of the boiler, thus maintaining the fire and helping to distribute the heat throughout the boiler. When regulator/throttle is closed a steam jet called the blower helps maintain the draught. Many steam engines also have a superheater, which consists of tubes looping back through the firebox which reheat the steam to dry out any water droplets, making the engine more efficient.
A steam locomotive usually has a minimum of 4 and a maximum of 10 driving wheels (2-5 axles) but some early machines had only 1 axle and a few were built with 6 or 7. Smaller wheels at the front and/or rear of the engine help to distribute the weight evenly and guide the engine round curves.
Steam engines come in 2 basic types: tender engines, in which the fuel and water are carried in a separate tender behind the engine, and tank engines, which are self-contained and carry water in tank(s) on, either side or beneath the boiler and the fuel in a bunker, usually at the rear of the cab. There are also various articulated types in which 2 sets of cylinders and driving wheels share a single boiler, and geared types in which the cylinders operate a drive shaft which in turn is geared to the wheels. Also locomotives designed for rack railways which have gears that engage in a rack between the rails and can climb much steeper inclines than conventional trains.
A steam locomotive often has three brakes: a train brake, which may be compressed-air or vacuum operated, a steam brake and a handbrake. Some have a train brake but no steam brake, or vice versa.
They have other fittings e.g. sanding gear, to aid traction on the track, lubricators, generators (when fitted with electric lamps), air or vacuum pumps for the train brake, steam heating for passenger trains, whistles, bells (mandatory in the US and Canada), and electro-mechanical devices to warn the crew of adverse signals.
The two that were most commonly used in later models of steam locomotives built in the US was the "Stevens" valve gear and the "Walschearts" valve gear, also referred to as "Monkey gear."
Stevens' mechanisms were a relatively simple mechanism and simple in appearance. These were used on engines designed for use as switch engines in yard service, called "goats." It was also found on the smaller "tank" engines that served in sea ports, called "docksides." Connecting rods and side rods were really all that was exposed, with the rest of the valve gear contained in the "steam chest", which is located right above the cylinders with the two being housed together.
The Walschearts valve gear was very complicated. Most of the gear was outside of and directly connected with the steam chest and cylinder housing. In this case, with the side rods and connecting rods driving the wheels, there were cranks and eccentrics that, as they were moved back and forth with the side rods, operated valves inside the steam chest to open and admit steam into the cylinder and on the other stroke open the valves to exhaust the expanded steam. All of this was outboard of the steam chest and at times moving in opposite direction of travel with reference to the side rods and connecting rods, which is why it got the moniker of "Monkey gear".
These were not the only two types of mechanisms used, but they were the most prevalent.
If you wish, click on HOGHEAD and navigate to my 360 page. The most recent blog post of "Since You Asked" has more information you may wish to know as it deals with steam locomotive classification, and further descriptions of drive mechanisms, so your timing with your question is good, as well as being a good question. Well done.
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