purpose of lead/trailing wheels on locomotive?
- Thread starter gregc
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What is meant by "larger" firebox is wider. the locomotives that have used that larger firebox, usually 2-8-2's and larger locomotive. because the firebox hung down so low and was so wide another driver could not be used under the firebox.
Booster engines were used to provide the extra tractive effort at start up when the large main drivers couldn't generate enough torque. With steam locomotives, large drivers meant higher top speeds but less initial pulling capability. Small drivers meant lower top speeds but more initial pulling force. Booster engines were added to create the extra initial starting torque/HP then cut out as the locomotive got up to speed where the main drivers provided everything needed.
The leading and trailing trucks provided better control at speed to help turn the locomotive instead of relying on the drivers to do it all. The drivers were never completely balanced anyway so leading and trailing wheels kept the locomotive on the track. Later on, the large fire boxes required more trailing wheels to support their weight as previous stated.
Near the turn of the century, steamers had relatively simple systems. They were smaller engines without feedwater heaters, without syphons, without superheaters, and they had small fireboxes that fit comortably between the drivers.
The guage of the rails, and therefore the distance between the wheels, could not change. Fireboxes were found between the drivers, or between the rear drivers and the trailing truck. But they needed heavier and more powerful engines with bigger boilers. Bigger boilers required more heat to turn water into steam. They had to enlarge fireboxes two and four times. They did this by making them rise above the rear trucks, but then widening them by a couple of feet, and then they began to lengthen them. Depending on the coal available, fireboxes in some cases were trully enormous, something near half the size of a city transit bus! As the boilers got longer, or as cabs and fireboxes got longer, the designers had choices to consider. One was to keep a given frame and wheel configuration, but add one more axle to a new truck, or add another pair of drivers. Or both. Or start from scratch with a new frame and wheel configuration. The Berkshire engine was a stretched superheated Mikado 2-8-2, but it was the famous 2-8-4.
The heaviest N. American locomotive is the C&O's H-8 Allegheny. It's Whyte notation is 2-6-6-6. The last 6 should tell you something about the weight distribution near the cab and the huge firebox.
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Selector
Well-Known Member
All true. In fact, once they had substantial trailing trucks under large fireboxes, they figured out that they could put a small booster steam engine on them. Some Hudsons had them, some Texas type engines, some Northerns...every little bit helps on start-up. The boosters would almost always be cut out by the time the engine had reached 10-15 mph.
What is meant by "larger" firebox is wider. the locomotives that have used that larger firebox, usually 2-8-2's and larger locomotive. because the firebox hung down so low and was so wide another driver could not be used under the firebox.
Tuna
Member
Booster engines were used to provide the extra tractive effort at start up when the large main drivers couldn't generate enough torque. With steam locomotives, large drivers meant higher top speeds but less initial pulling capability. Small drivers meant lower top speeds but more initial pulling force. Booster engines were added to create the extra initial starting torque/HP then cut out as the locomotive got up to speed where the main drivers provided everything needed.
The leading and trailing trucks provided better control at speed to help turn the locomotive instead of relying on the drivers to do it all. The drivers were never completely balanced anyway so leading and trailing wheels kept the locomotive on the track. Later on, the large fire boxes required more trailing wheels to support their weight as previous stated.
stationmaster
Crusty Old Geezer
I've been inside of the firebox of the NKP 765. It was cavernous!!!
Another note as to the fireboxes. They became so large that an automatic coal feeders(stokers) were installed on some models. A fireman could not shovel fast enough to keep the fire hot enough. Now that's a BIG firebox. Can you imagine hand shoveling 25 tons, or more, of coal in a day's work?
Also, adding drivers under the firebox would make for a "stiff" ride. The trucks had "suspensions" of a sort. Riding in the cab of some engines without trailing trucks, was like riding on the tailgate of a pickup truck....a little bit of a "bouncy" ride. Not really bouncy, more like a jolting ride. The crew would be beaten to a pulp. The addition of the trucks smoothed out the "jolts".
One of the gentlemen at the nursing home we are building the layout for was an engineer on steam engines in the 40's thru the diesel transition era and beyond, retiring in 1974(he's 97). He has told me many tales of the "old days" and being a crew member of some of the old steam engines and early diesels. Some of the early diesels were quite dangerous for the crew. And manning a steam engine was "real man work"(his words).
Bob
Another note as to the fireboxes. They became so large that an automatic coal feeders(stokers) were installed on some models. A fireman could not shovel fast enough to keep the fire hot enough. Now that's a BIG firebox. Can you imagine hand shoveling 25 tons, or more, of coal in a day's work?
Also, adding drivers under the firebox would make for a "stiff" ride. The trucks had "suspensions" of a sort. Riding in the cab of some engines without trailing trucks, was like riding on the tailgate of a pickup truck....a little bit of a "bouncy" ride. Not really bouncy, more like a jolting ride. The crew would be beaten to a pulp. The addition of the trucks smoothed out the "jolts".
One of the gentlemen at the nursing home we are building the layout for was an engineer on steam engines in the 40's thru the diesel transition era and beyond, retiring in 1974(he's 97). He has told me many tales of the "old days" and being a crew member of some of the old steam engines and early diesels. Some of the early diesels were quite dangerous for the crew. And manning a steam engine was "real man work"(his words).
Bob
Selector
Well-Known Member
Near the turn of the century, steamers had relatively simple systems. They were smaller engines without feedwater heaters, without syphons, without superheaters, and they had small fireboxes that fit comortably between the drivers.
The guage of the rails, and therefore the distance between the wheels, could not change. Fireboxes were found between the drivers, or between the rear drivers and the trailing truck. But they needed heavier and more powerful engines with bigger boilers. Bigger boilers required more heat to turn water into steam. They had to enlarge fireboxes two and four times. They did this by making them rise above the rear trucks, but then widening them by a couple of feet, and then they began to lengthen them. Depending on the coal available, fireboxes in some cases were trully enormous, something near half the size of a city transit bus! As the boilers got longer, or as cabs and fireboxes got longer, the designers had choices to consider. One was to keep a given frame and wheel configuration, but add one more axle to a new truck, or add another pair of drivers. Or both. Or start from scratch with a new frame and wheel configuration. The Berkshire engine was a stretched superheated Mikado 2-8-2, but it was the famous 2-8-4.
The heaviest N. American locomotive is the C&O's H-8 Allegheny. It's Whyte notation is 2-6-6-6. The last 6 should tell you something about the weight distribution near the cab and the huge firebox.
hamltnblue
Active Member
Here's a link that covers most of the history of steam and someof the reasons for the different configurations
http://railroad.union.rpi.edu/article.php?article=2593
http://railroad.union.rpi.edu/article.php?article=2593
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