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
It’s all in the timing.
Wikipedia has a decent explanation, along with some animations. Here you go.
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.
Jay
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.
Actually the pistions move in the same direction…it is just the timing of the application of steam to the pistons.
If you go to this site
http://www.tcsn.net/charlied/
and download the Windows Valve Gear Programs File 10A, 1.27 MB, Version 2.7
and unzip it, there’s some executables you can play with that shows what happens when you shift into reverse and vary the cutoff.
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
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.
If you’re really interested in steam, get the book “Guide to North American Steam Locomotives,” compiled by George Drury (Kalmbach). There’s a section on valves with diagrams of the various valve types. Looking at them you have to marvel at how much time inventors must have spent designing them. Not only did they have to be designed for backwards and forwards motion, but they also had to allow for varying timings, efficiency, ease of maintenance, etc.
Easiest answer is stop the loco, set the brakes, pull the Johnson bar all the way back, drop the brakes and then re-open the throttle, also helps if you turn around looking out the window[;)]
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.
[quote user=“Poppa_Zit”]
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 –
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.
[quote user=“TomDiehl”]
[quote user=“Poppa_Zit”]
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.
Reall
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.
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
Piston movement has nothing to do with direction. They function the same in forward or reverse. Steam is admitted to one side of the piston until the pressure pushs the piston toward the other end. At some point the steam flow is cut off and allowed to expand to push the piston completely to the end of the stroke. Just before the piston reachs the end of the stroke the exhaust valve on the other side closes trapping steam that was on the other side to cushion the piston as it reachs the end of its stroke. A steam inlet valve is opened on that side to push the piston to the first end where this repeated. Thus the piston is pushed back and forth to the same limits regardless if running forward or backward unlike an automobile engine that only gets pushed on one side. The valve gear on the engine is adjusted per previous posts to translate the back and forth motion of the piston connected to the crosshead to the driving wheels through the main rod. For example, if the engine was stopped with the side rods at 12:00 o’clock pushing against the back of the piston would cause a clockwise movement to the wheels BUT if the steam was admitted to the front of the piston it would cause a counterclockwise movement of the wheels. Since the rods on one side are usually 90 degrees out of phase with the other side it is impossible for an engine to be in a situation that would not permit it to move when given steam. The valve gear determines which valves are operated to move the engine in the proper direction usually by lifting or dropping a link.
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?