How A Steam Locomotive Works (Union Pacific Big Boy)

 With 25 in service between 1941 and 1944, the Union Pacific “Big Boy” was among the biggest and most potent steam locomotives ever built. About 604 tonnes is the combined weight of the engine and tender, which is enough to pull a 5-mile train by itself. On a level track, the maximum speed is about seventy miles per hour. When water is brought to a boil using coal, wood, or oil as a heat source, the steam it produces expands and produces workable pressure. Steam releases pressure and heat as it rotates the wheels by pushing pistons back and forth.

Coal is burned in the firebox to produce heat. Special movable metal grates support a bed of coals at the base of the firebox. Ash and spent coals can be released by shaking the grates, or you can fully tilt them to empty the coals completely. To completely empty the firebox, doors that open between the wheel axles allow waste to fall into ash pans below. A predetermined rate of airflow up into the coal bed is made possible by carefully sized gaps between the metal grates, which allow air for combustion to enter through apertures at the top of the ashpans.

The firebox is encircled by the boiler, which is primarily filled with water. This water is boiled by the firebox’s intense heat, which then causes it to expand into steam, which presses against the boiler walls with up to 300 pounds of pressure per square inch. Staybolts arranged in multiple rows between the boiler’s exterior and firebox help ensure that components maintain their correct shape even under extreme pressure. When the train is moving, the firebox sheets must always be submerged in water. The crown sheet is particularly susceptible to exposure since it is closest to the top of the constantly fluctuating boiler water level.

There is a lot more surface area available for heat transmission to boiler water through the tubes that hot firebox exhaust passes through. Steam gathers and exits the boiler through a steam dome at the top. Big Boy has a separator-style blowdown at the upper rear of the boiler that employs centrifugal forces to separate steam from sludge as it exits. Blowdown valves aid in cleaning the boiler’s interior.

An electrically controlled signal foam metre and a manually operated blowdown valve control the steam engine in a train. Particles and contaminants in water can create foaming, which increases the amount of water in the ensuing steam. When the blowdown is activated, steam or water at high pressure shoots outdoors, carrying pollutants with it. Water cannot be compressed at all, but pure steam, which is a gas devoid of water, can.

Boiler pressure must be overcome by pumping or injecting water into the boiler in order to bypass safety valves located close to the steam dome. Redundant feed water components, such as the main water connection from the tender and the mechanical pump under the fireman’s side of the cab, are present for efficiency and safety.

An exhaust steam injector located at the front of the engine runs on hot exhaust steam. Cylinder exhaust steam enters the injector at low velocity but high pressure. It then passes via a narrowing tube, where the pressure is changed to high velocity. This steam jet acts as a vacuum to draw water from a water supply nozzle into the stream by lowering the pressure in the chamber that follows. A swift-moving stream of water is produced when the hot steam condenses and combines with the water. The process is further enhanced by a second stage of steam nozzles, which gradually widens the cone-shaped route as it gets closer to the outlet on the opposite side.

When necessary, a backup intake can be used to admit live boiler steam. Numerous supporting appliances required to operate the locomotive are connected to a turret situated at the upper rear of the boiler.

A steam dryer, which rotates a wheel with fins to shoot water droplets out of the stream for the first stage of water separation, receives the steam as it exits the boiler. After that, steam moves via the dry pipe to the superheater unit, which is situated in the smokebox region outside the boiler. In order to properly superheat the steam, which turns it into a pure, dry gas, the superheater must be kept entirely apart from the water’s mass.

The superheater, housed in the smokebox, a sizable forward chamber, is responsible for a number of crucial functions that keep the train operating efficiently.

Steam engines emit smoke, cinders, and flaming embers into the compartment through flues and tubes called smokestacks. In order to delay these particles and lessen wear on the forward sections, baffle plates form a winding route. Hot embers are screened out by a metal screen, and any burning material that escapes the smokestacks increases the risk of pollution and trackside fires. Smokestacks descended farther into the smokebox to allow passage through tunnels as steam engines grew in size. Above blast nozzles, which convert spent exhaust steam into a draft appliance, are two smokestacks positioned in the middle.

Strong suction force created by the jets’ high output draws more air into the firebox, making the fire more lively. For a larger surface area in contact with the rapidly moving jet streams, blast nozzles split the exhaust. The bottom of the smokestacks are flared, and there are additional apertures to provide more room for smoke to enter the exhaust steam jets. The stacks, which are intended to raise the exhaust high enough above the train to avoid vision problems for the crew, are composed of a mixture of steam and smoke.

Steam travels to cylinders moulded into the locomotive frame by pipes running the length of the boiler’s exterior. Big Boy is equipped with four cylinders: two up front and two midway along the locomotive’s length. Compressed steam enters, expands, and leaves the cylinder chamber of these devices, driving pistons inside the cylinders back and forth. The crosshead, which travels in its own supporting channel, is fixed to the piston rod. The adjacent piston valve controls the flow of steam.

The valve gear, which has the ability to completely reverse valve timing and put the piston, wheels, and train in reverse operation, controls the interaction between the piston and valve. This specific arrangement, known as a Walshearts valve gear, is one of many that were developed during the steam era. Via a connecting rod and a separate rod that goes to the valve, a crank at the wheel provides a constant back and forth motion to the expansion link.

The position and timing of the valve in the system are altered when the engineer in the cab adjusts the reverser lever. With this technology, the engineer can operate more economically by delivering less steam. When pulling hard to start the train moving, the lever can be closer to the fully forward position. The engineer can ease off the lever a little bit as the train moves along smoothly since it could require less steam to continue moving forward.

Since lubrication requires the use of mechanical lubricators on both the front and rear of the locomotive, it is an essential task in its construction. Connecting rods transfer the main rod’s power to the driving wheels, adding weight to ensure a balanced rotation. Big Boy is an articulated locomotive that possesses two independent main frame sections connected by a massive strengthened pin at their middle. While other frame parts hinge to fit, the rear frame guides the train into curves.

With four leading wheels, two sets of eight driving wheels, and four trailing wheels, the locomotive is categorised as a 4-8-8-4 locomotive. Leaf springs and coil springs provide suspension support, while equalisation bars connect each wheel pair. Axles are equipped with lateral movement mechanisms to provide cushioning when they shift a few millimetres on either side. To enable more lateral movement from a stiff point, such the third driver wheel set, frame parts are made to rotate around it.

Air pressure, produced by two steam-powered air compressors positioned on either side of the front pilot, powers the braking mechanism. Underneath the forward platform are aftercoolers, one for each compressor, which dissipate heat as hot compressor air passes through tubes to storage reservoirs on either side of the boiler. Every main frame segment has two brake cylinders with adjustable locations all the way around. Each driving wheel set has brake shoes at the back that apply pressure to the metal tyres of the wheels to slow the train. To activate the brake shoes at each wheel, the trailing truck has its own brake cylinders and linkages.

With controls in the cab to apply brakes for the locomotive and tender only, all of the cars simultaneously, or a combination of both, the air compressor provides the braking air for the entire train. There is a sand nozzle at each driving wheel that is used to blow sand at the wheel and track when more grip is needed between smooth metal surfaces. Above the boiler shell are large containers known as “sand domes,” which have plumbing extending below them. Valves let pressurised air into the system and regulate the flow of sand.

A steam whistle is tucked beneath the smokestacks, a bell is hanging at the front rather than on top for clearance, and there is a headlight with two sets of number plates at the front.

One of the biggest cabs of its day, the Big Boy locomotive had four seats for the engineer, fireman, head brakeman, and spare. The engineer is in charge of the locomotive, the safe running of the train, and visibility on the correct side of the train. The fireman is in charge of producing steam and keeps a tight eye on the boiler and all of its related systems despite constantly fluctuating power requirements. In order to prevent bearings in automobile axles from heating up and catching fire, the brakeman pairs and uncouples cars and keeps an eye out for journal box or hotbox fires. Track switches at the front of the train are also managed by the brakeman.

The front wall is formed by the backhead of the boiler, which extends straight into the cab. Crucial instruments, gauges, knobs, and more are all covered with it. Blower valves for clearing scale from the boiler, reverser lever, sander control valve, throttle lever, headlights, pull chain for the whistle, air brake stand, steam injector, cylinder cocks, blowdown valves, try cocks, dynamo valve, foam metre, blower, automatic firedoor, coal bunker, and stoker pressure gauges are among the controls on the engineer’s side of the cab.

A valve that regulates the train’s exhaust steam injector pressure, stoker pressure gauge, and steam-powered heating is located on the fireman’s side of the cab and backhead. Beneath the stoker pressure gauge are the exhaust steam injector starting lever, overflow indicator, and water regulator valve. Water sprinklers for cleaning ash pans are situated at the foot of the automatic stoker, which features separate steam jets to blast coal to different areas of the firebox. The automated firedoor features a foot pedal to operate the doors while shovelling coal into the firebox manually, as well as an air cylinder to assist in opening and shutting the heavy doors.

The tender transports the coal and water required for the locomotive to run outside the cab. Surrounding the coal bunker is a water tank with a capacity of over 24,000 gallons (90,849.89 L). When fully laden, a fully loaded tender weights 427,500 lbs (193,910.7 kg), and the coal bunker can contain 32 tonnes of coal. The coal bunker’s angled sidewalls are designed to funnel coal into the automatic stoker’s conveyor screw at the bottom.

The stoker has a front-mounted, two-piston steam-driven engine that drives the auger screw. A gear set at the back of the tender is connected to a series of revolving shafts. Big chunks of coal fall into the centre channel, where they are crushed to the appropriate particle size and moved forward in the direction of the firebox. To allow the tender and engine to move independently of one another while the stoker operates, the screw is separated into revolving parts, and the enclosing pipe contains ball joints.

The crushed coal fragments are forced up to a stoker table, which is a pan beneath steam jets directed at strategic firebox locations. For precise control over the coal bed and ensuing fire, the fireman can blow coal to particular locations as needed thanks to valves in the cab that regulate the pressure to these jets. Steam locomotives are amazing in a lot of ways; they combine strong and sometimes hazardous reactions to create a breathing, beautiful machine that is simple to enjoy on its own.