Drawing by Dr. Robert T. Rhode
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1. The boiler of a macadam, or three-wheel, steamroller consists of the firebox (shown at the left in the accompanying illustration), the barrel, and the smokebox (depicted at the right). The fireman or the engineer scoops egg-sized lumps of bituminous coal from the bunker and deposits them through a firedoor into the firebox, made of "firebox steel." For maximum efficiency, the layer of embers is kept to about four inches depth. At their hottest points, the flames can exceed two-thousand degrees Fahrenheit. Grates beneath the fire permit ash and clinkers to fall through to the ashpan below. A door at the back of the ashpan allows air to enter, pass upward through the grates, and aid combustion. (The hooked lever seen near the upper left-hand corner of the firebox opens and closes a door at the front of the ashpan so that the ashes can be periodically removed.) Apertures in the firedoor introduce air above the grates to ignite and consume the gas from the coal. The top of the firebox, known as the crownsheet, would melt, were it not for the layer of water above it. Water also fills the double wall surrounding the firebox. The heat boils the water, producing steam. Staybolts restrain the metal walls from ballooning outward from the pressure of the steam. 2. The wind, known as the draft, carries the hot gases, soot, and smoke from the fire into horizontal tubes extending through the front wall of the furnace and through the tubesheet at the smokebox end of the boiler. Water surrounds the tubes. Depending on the design of the steamroller, approximately forty tubes run the length of the boiler. The height of the chimney, called the smokestack, helps to create the draft along the tubes. A spark arrester at the top of the smokestack catches sparks, holding them until they go out. Steam in the exhaust from the engine comes through a pipe elbow near the base of the smokestack and is directed upward to increase the draft. The water absorbs heat from the tubes. By the time the gases from the firebox enter the smokebox, their temperature has dropped to just over four-hundred degrees. Each pound of coal evaporates about six pounds of water. About four and a half pounds of coal per brake horsepower are expended per hour. If the steamroller averages fifty brake horsepower (measured by the power of the flywheel) in an hour, 225 pounds of coal have been burned. On average, a steamroller uses between 1,500 and 2,000 pounds of coal in an eight-hour day. 3. In the open air at sea level, a pint of water can evaporate into nearly twenty-seven cubic feet of steam. In other words, the steam formed by a pound of water at the pressure of the atmosphere (14.7 pounds per square inch at sea level) can expand 1,600 times its volume in the liquid state. The expansive nature of steam is the principle on which the steam engine works. Imprisoned in the air-tight boiler shell, the steam exerts pressure outward. The temperature of the steam in a working steamroller is approximately 328 degrees Fahrenheit; at that temperature, the steam exerts a pressure of about 85 pounds against every square inch of the inner surface of the boiler. The steam, rich with potential energy, enters a dome on top of the boiler. An open valve admits the steam to a pipe leading to the throttle. The engineer pulls back the short lever depicted above the center of the firebox. A rod connected to the lever opens the throttle, shown just above and to the right of the dome. The channel of steam passes from the throttle, through another pipe, to the governor, the mechanism having three balls that rotate rapidly at the top. The faster the balls rotate, the farther they fly outward. When the engine suddenly begins to slow down and needs more power, the rotating balls of the governor also slow down, and their restraining springs pull them back toward the center. This action widens a valve opening that admits additional steam to the steamchest. A quick governor responds instantaneously to changes in the power demands on the engine and keeps the engine running at a fairly uniform speed. The governor receives its motion by means of a belt looped around a pulley on the crankshaft. 4. The steam from the governor passes into the steamchest. Inside the steamchest, a valve (in cross-section, shaped like the letter D) slides back and forth a very short distance. (In the drawing, the back of the steamchest has been removed, and you can just make out the light green highlights of the sliding D valve.) Two channels, called ports, lead from near the edges of the steamchest into the cylinder. (In the illustration, you can clearly see the front of the cylinder, which lies outside the steamchest.) Within the cylinder is a piston, a sort of plunger. (The drawing offers a ghostly view of the piston inside the cylinder.) When steam from the steamchest passes through the left-hand port into the cylinder, it pushes the piston forward. Alternatively, when steam enters the cylinder through the right-hand port, it shoves the piston backward. The steam pushes the piston because the steam is under pressure in a confined space, and it wants to expand outward. The sliding D valve mechanism opens first one port then the other. In turn, the piston travels back and forth in the cylinder. (The gray box in the illustration is an oiler. It dispenses special cylinder oil to keep the piston well lubricated within the cylinder.) 5. A series of mechanisms is necessary to transform the linear motion of the piston into rotary motion. First, a short rod extends from the piston to a crosshead. (If you could see through the steamdome, you would be able to detect the crosshead, depicted in the illustration as the ghostly blue X-shaped object near the right of the dome.) The crosshead travels back and forth within guides that keep the linear motion of the crosshead uniform. A connecting rod reaches from the crosshead to the crankdisk. (In the drawing, the crankdisk’s position is shown by a ghostly blue circle. The crankdisk is at the far end of the axle called the crankshaft. All shafts or axles in the illustration are shown by red circles.) With one end connected to the crosshead and the other end attached to the crankdisk, the connecting rod is the part that translates the linear motion of the crosshead into the rotary motion of the crankdisk. In essence, the connecting rod pushes the crankdisk around. 6. The spinning of the crankdisk spins the crankshaft to which it is attached. At the opposite end of the crankshaft (the end closest to you as you examine the drawing) is the flywheel (depicted as a ghostly green wheel with spokes). The weight of the flywheel helps to keep the rotary motion of the crankshaft smooth. An eccentric, another disk, is located on the crankshaft. (It cannot be seen in the illustration because it is behind a geared pump.) The crankshaft pierces the disk off center. The eccentric, thus, appears to wobble while revolving. The wobbling motion is communicated to a series of rods extending to the sliding D valve, which gains its back-and-forth motion thereby. 7. The engineer moves the next-to-the-largest lever to adjust the length of travel of the sliding D valve and to determine if the engine will move forward or backward. The lever can be locked into position in any of a series of notches. The complicated linkage of parts from the lever to the valve is known as the reversing mechanism. By lessening the travel of the valve, the engineer can use less steam and therefore expend less water and coal. 8. A toothed gear (shown just to the right of the crankshaft) receives its motion from a pinion located on the crankshaft. The spinning gear powers a water pump (which can be seen above the back end of the boiler). The pump draws water from the tank, or reservoir behind the boiler. The reservoir is encased in steel, and the engineer stands on the tank. The water is pumped into the boiler to replenish the water that has been used up as steam to run the engine. (Another device to introduce water into the boiler is the injector. It is on the engineer’s left and cannot be seen in the illustration. A preheater, warmed by exhaust steam, heats the water before admitting it into the boiler. It is similarly out of view near the smokebox end of the boiler.) 9. The engineer works the largest lever to engage a clutch mechanism inside the flywheel. The clutch transfers the rotary motion of the flywheel to a series of gears (indicated by circles of red dashes in the drawing). These gears spin the main axle, the lowest axle of the steamroller. In turn, the big wheels, known as drivers or driver wheels, roll along the ground, thus moving the steamroller from place to place. (In the illustration, only the driver wheel farthest from the viewer is shown.) By moving a lever a short distance, the engineer steers the steamroller. The steering is steam powered. (You can see a part of the power-steering device to the right of the firebox. Chains from a worm gear pull the front roller to left or right.) 10. Scraping devices help to keep mud or asphalt from building up on the wheel and roller surfaces. A canopy protects the engine (and the engineer!) from rain. A smokebox door permits the engineer to clean the tubes at least twice a day. A scraper on a long rod extends the length of the tubes and dislodges built-up soot. When there is no pressure in the boiler, the boiler can be filled with water by removing a filler cap located near the smokebox. A set of steps allows the engineer to reach grease cups and oilers located throughout the engine. A code of blasts on the whistle attached to the dome communicates such information as when the steamroller will start or stop. (Not shown in the illustration are four safety devices: a steam gauge to measure the pressure in the boiler, a water glass to show the level of water carried in the boiler, trycock valves to test the level of water in the boiler, and, in the middle of the crownsheet, a fusible plug, which is a bolt with a soft metal center. If the water level drops too low and the crownsheet begins to grow too hot, the center of the bolt melts. The steam and water left in the boiler then rush through the hollow bolt into the firebox, thereby putting out the fire and averting more serious consequences. A fifth safety device is depicted: the safety release valve, also known as the pop valve or pop safety. It is attached to the side of the steamdome. When the steam pressure inside the boiler builds to a certain preset level—say, 175 or 150 pounds per square inch—the safety valve opens, delivering excess steam to the air above the canopy. Once the steam pressure within the boiler has reduced to the proper level, the safety valve closes.) |
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