I know this is a silly question for someone who knows anything about these things, but how does a steam engine go in reverse? Do the pistons merely move in the opposite direction?
Gabe
gabe wrote: I know this is a silly question for someone who knows anything about these things, but how does a steam engine go in reverse? Do the pistons merely move in the opposite direction? Gabe
Yes
Jay
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It's all in the timing.
Wikipedia has a decent explanation, along with some animations. Here you go.
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OK That's the short answer.
Put yourself on the engineer's (left) side of the locomotive so the engine going forward is moving from your left to right. Think of the driving wheel as a clock. When the pinion is at 16 and after steam starts to push the piston from front to back causing a clockwise turn of the wheel and that motion continues until the pinion is at the 45 second position. At 46 and after steam is injected in the chamber at the back of the piston moving the piston forward until the 15 second position.
In reverse the action is in the opposite direction. At 14 and after steam is injected in the front chamber moving the piston backward causing the wheel to turn counter clockwise, at 44 and after steam is injected into the back chamber moving the piston forward continuing the counter clockwise turn of the wheel.
You may know that the wheels are offset by 90 degrees. If that were not the case a steam engine could not move in any direction if the wheels were quartered, i.e., the pinions were at the 15 or 45 second position.
I read a story of an N&W engine that had lost the rods on one side being moved on its own power back to shops for repairs. With only one side working any stop the engineer made had to be on a down grade so the engine would not get stuck in a quartered position.
Yes. Once the locomotive is stopped, or very nearly stopped, the engineer pulls the reversing lever towards him...all the way. What that does, via a linkage, is to move the valve in its cylinder past an inlet port that will let steam expand on the face of the piston that will cause the main rod to be pushed backwards, in reverse, when the engineer next opens the throttle. Once this happens, the valve linkage will be caused to move in such a way that the process is sustained, just as it would have been in forward motion.
In automobiles, a gear on the other side of the driving gear is engaged which causes the vehicle to reverse since the newly engage gear will want to rotate in the opposite direction on the other side of the driving gear. In a steamer, the valve has to be physically shoved down its sleeve far enough that the next throttle action causes incoming steam to enter the drive cylinder in such a way that the piston is forced to move in the opposite direction it had been going up until the engineer stopped to reverse.
At least, that is what my brain tells me in its mental imagery. The piston still runs back and forth in its main cylinder, and the wheels turn because of it, but they start off in the other direction because the valve moved to the other end of its travel, forced by the engineer.
I hope someone straightens me out if I have some or all of it wrong. I'd appreciate it.
gabe wrote:I know this is a silly question for someone who knows anything about these things, but how does a steam engine go in reverse? Do the pistons merely move in the opposite direction? Gabe
Never too old to have a happy childhood!
selector wrote: gabe wrote: I know this is a silly question for someone who knows anything about these things, but how does a steam engine go in reverse? Do the pistons merely move in the opposite direction? Gabe Yes. Once the locomotive is stopped, or very nearly stopped, the engineer pulls the reversing lever towards him...all the way. What that does, via a linkage, is to move the valve in its cylinder past an inlet port that will let steam expand on the face of the piston that will cause the main rod to be pushed backwards, in reverse, when the engineer next opens the throttle. Once this happens, the valve linkage will be caused to move in such a way that the process is sustained, just as it would have been in forward motion. In automobiles, a gear on the other side of the driving gear is engaged which causes the vehicle to reverse since the newly engage gear will want to rotate in the opposite direction on the other side of the driving gear. In a steamer, the valve has to be physically shoved down its sleeve far enough that the next throttle action causes incoming steam to enter the drive cylinder in such a way that the piston is forced to move in the opposite direction it had been going up until the engineer stopped to reverse. At least, that is what my brain tells me in its mental imagery. The piston still runs back and forth in its main cylinder, and the wheels turn because of it, but they start off in the other direction because the valve moved to the other end of its travel, forced by the engineer. I hope someone straightens me out if I have some or all of it wrong. I'd appreciate it.
The "reversing lever" is called the Johnson Bar. Push it all the way forward to go forward, pull it all the way back to reverse. Middle position is neutral.
Really talented engineers -- after getting the train up to speed and having adequate steam pressure -- would ease the bar slowly backwards from full forward, a notch at a time. This altered the "timing" of the steam into the cylinders and made the locomotive run more efficiently at high speed and also cut fuel use. I believe the physics were it let some steam into the almost-compressed cylinder to aid in "pushing" it the opposite direction, sort of like bouncing the piston head off a "springlike" cushion of steam. A lot of railroads referred to this as the "company notch." It's where they wanted the locomotive run to conserve fuel.
I hope I have described it properly. If not, I'm sure someone will let us know.
jeaton wrote: I read a story of an N&W engine that had lost the rods on one side being moved on its own power back to shops for repairs. With only one side working any stop the engineer made had to be on a down grade so the engine would not get stuck in a quartered position. Jay
I don't believe it was necessary to stop on a downgrade. The engineer could observe the position of the main rod and as long as the wheel pins were not within a few degrees of parallel to the rail, he could still get it moving. In other words, if he stopped where the wheel pins were anywhere away from being quartered (9 o'clock or 3 o'clock), he could still get it rolling. He'd have to go very slow, as the driving wheels on the damaged side are are out of slightly out of counterbalance with the side and/or main rod missing.
Have fun with your trains
Poppa_Zit wrote: jeaton wrote: I read a story of an N&W engine that had lost the rods on one side being moved on its own power back to shops for repairs. With only one side working any stop the engineer made had to be on a down grade so the engine would not get stuck in a quartered position. Jay I don't believe it was necessary to stop on a downgrade. The engineer could observe the position of the main rod and as long as the wheel pins were not within a few degrees of parallel to the rail, he could still get it moving. In other words, if he stopped where the wheel pins were anywhere away from being quartered (9 o'clock or 3 o'clock), he could still get it rolling. He'd have to go very slow, as the driving wheels on the damaged side are are out of slightly out of counterbalance with the side and/or main rod missing.
PZ Yes, I believe you are correct on that point. As I recall, stopping on a grade was mentioned-perhaps it was considered as an additional measure to avoid quartering. I think it was an N&W J-Class. I don't remember what had to be taken off the non-working side, but the author remarked that even with only one side working, the engine was able to move at a good speed.
"The Company Notch" description is very good.
Poppa_Zit wrote: selector wrote: gabe wrote: I know this is a silly question for someone who knows anything about these things, but how does a steam engine go in reverse? Do the pistons merely move in the opposite direction? Gabe Yes. Once the locomotive is stopped, or very nearly stopped, the engineer pulls the reversing lever towards him...all the way. What that does, via a linkage, is to move the valve in its cylinder past an inlet port that will let steam expand on the face of the piston that will cause the main rod to be pushed backwards, in reverse, when the engineer next opens the throttle. Once this happens, the valve linkage will be caused to move in such a way that the process is sustained, just as it would have been in forward motion. In automobiles, a gear on the other side of the driving gear is engaged which causes the vehicle to reverse since the newly engage gear will want to rotate in the opposite direction on the other side of the driving gear. In a steamer, the valve has to be physically shoved down its sleeve far enough that the next throttle action causes incoming steam to enter the drive cylinder in such a way that the piston is forced to move in the opposite direction it had been going up until the engineer stopped to reverse. At least, that is what my brain tells me in its mental imagery. The piston still runs back and forth in its main cylinder, and the wheels turn because of it, but they start off in the other direction because the valve moved to the other end of its travel, forced by the engineer. I hope someone straightens me out if I have some or all of it wrong. I'd appreciate it. The "reversing lever" is called the Johnson Bar. Push it all the way forward to go forward, pull it all the way back to reverse. Middle position is neutral. Really talented engineers -- after getting the train up to speed and having adequate steam pressure -- would ease the bar slowly backwards from full forward, a notch at a time. This altered the "timing" of the steam into the cylinders and made the locomotive run more efficiently at high speed and also cut fuel use. I believe the physics were it let some steam into the almost-compressed cylinder to aid in "pushing" it the opposite direction, sort of like bouncing the piston head off a "springlike" cushion of steam. A lot of railroads referred to this as the "company notch." It's where they wanted the locomotive run to conserve fuel. I hope I have described it properly. If not, I'm sure someone will let us know.
All engineers must be this talented. Adjusting the cutoff as it's called, is necessary to attain the higher speeds. Once the train gets moving, the engineer must notch back the johnson bar (toward center) to allow boiler pressure steam into the cylinder for a shorter percentage of the stroke. The reason speed is limited with longer percentage of steam admission is backpressure: all the steam that goes into the cylinder must be pushed out the exhaust at the end of the stroke. With a shorter steam admission, the expansion of the steam, along with a bit of momentum, will complete the "push" on the piston.
Gabe, one thing not mentioned and may not be apparent if you're not familiar with steam engines, is that the cylinders are double sided. Steam can be sent in the front of the piston to push it back, and in the back of the piston to push it forward.
TomDiehl wrote: Poppa_Zit wrote: selector wrote: gabe wrote: I know this is a silly question for someone who knows anything about these things, but how does a steam engine go in reverse? Do the pistons merely move in the opposite direction? Gabe Yes. Once the locomotive is stopped, or very nearly stopped, the engineer pulls the reversing lever towards him...all the way. What that does, via a linkage, is to move the valve in its cylinder past an inlet port that will let steam expand on the face of the piston that will cause the main rod to be pushed backwards, in reverse, when the engineer next opens the throttle. Once this happens, the valve linkage will be caused to move in such a way that the process is sustained, just as it would have been in forward motion. In automobiles, a gear on the other side of the driving gear is engaged which causes the vehicle to reverse since the newly engage gear will want to rotate in the opposite direction on the other side of the driving gear. In a steamer, the valve has to be physically shoved down its sleeve far enough that the next throttle action causes incoming steam to enter the drive cylinder in such a way that the piston is forced to move in the opposite direction it had been going up until the engineer stopped to reverse. At least, that is what my brain tells me in its mental imagery. The piston still runs back and forth in its main cylinder, and the wheels turn because of it, but they start off in the other direction because the valve moved to the other end of its travel, forced by the engineer. I hope someone straightens me out if I have some or all of it wrong. I'd appreciate it. The "reversing lever" is called the Johnson Bar. Push it all the way forward to go forward, pull it all the way back to reverse. Middle position is neutral. Really talented engineers -- after getting the train up to speed and having adequate steam pressure -- would ease the bar slowly backwards from full forward, a notch at a time. This altered the "timing" of the steam into the cylinders and made the locomotive run more efficiently at high speed and also cut fuel use. I believe the physics were it let some steam into the almost-compressed cylinder to aid in "pushing" it the opposite direction, sort of like bouncing the piston head off a "springlike" cushion of steam. A lot of railroads referred to this as the "company notch." It's where they wanted the locomotive run to conserve fuel. I hope I have described it properly. If not, I'm sure someone will let us know. All engineers must be this talented. Adjusting the cutoff as it's called, is necessary to attain the higher speeds. Once the train gets moving, the engineer must notch back the johnson bar (toward center) to allow boiler pressure steam into the cylinder for a shorter percentage of the stroke. The reason speed is limited with longer percentage of steam admission is backpressure: all the steam that goes into the cylinder must be pushed out the exhaust at the end of the stroke. With a shorter steam admission, the expansion of the steam, along with a bit of momentum, will complete the "push" on the piston.
Almost no steam engines had sufficient grate and heating surface to maintain steam pressure at high speed without reducing the cut-off on the Johnson bar. The skill of an engineer is getting the most out of the steam that the boiler can generate and a good engineer makes the life of a fireman much easier.
dd
dldance wrote: Almost no steam engines had sufficient grate and heating surface to maintain steam pressure at high speed without reducing the cut-off on the Johnson bar. The skill of an engineer is getting the most out of the steam that the boiler can generate and a good engineer makes the life of a fireman much easier. dd
Absolutely. And if an engineer was pissed at his fireman, he didn't pull back to the "company notch" and made the poor guy's life miserable -- by making him shovel even harder.
Hugh Jampton wrote: gabe wrote: I know this is a silly question for someone who knows anything about these things, but how does a steam engine go in reverse? Do the pistons merely move in the opposite direction? Gabe If you go to this sitehttp://www.tcsn.net/charlied/and download the Windows Valve Gear Programs File 10A, 1.27 MB, Version 2.7and unzip it, there's some executables you can play with that shows what happens when you shift into reverse and vary the cutoff.
In so doing you will note, though, that the relationship between the movement of the pistons and the valves is the same either in forward or reverse. If it's an outside admission valve, it will move ahead of the piston by approximately 90 degrees, if inside admission it will follow the piston by approximately 90 degrees. The approximately comes in because of the lead, which in "outside" valve gears is provided by the combination lever.
Ol' Ed
SteelMonsters wrote:Can an engineer put the Johnson bar into reverse while traveling forward to slow down and reverse if needed? If so what are the parts to prevent a spike in pressure in the cylinders and how does that work?
IMHO, and IIRC, putting the engine in reverse is a bad thing - you break what traction you had. Earlier in the thread there was discussion on how to slow the engine down "using the cylinders." It wasn't a popular application or we'd have seen more of it.
tree68 wrote: SteelMonsters wrote:Can an engineer put the Johnson bar into reverse while traveling forward to slow down and reverse if needed? If so what are the parts to prevent a spike in pressure in the cylinders and how does that work? IMHO, and IIRC, putting the engine in reverse is a bad thing - you break what traction you had. Earlier in the thread there was discussion on how to slow the engine down "using the cylinders." It wasn't a popular application or we'd have seen more of it.
ndbprr wrote:Piston movement has nothing to do with direction..
Yes and no. When moving from a standing stop, the first movement of the piston (as determined by the engineer's positioning of the Johnson Bar, which determines which side of the steam chest gets steam first) determines whether the engine goes forward or reverse.
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