Could someone tell what a Johnson Bar is and what it does on Steam Engines? Watching some old videos and they kept referring to a Johnson Bar but I never got what it does. Thanks!
Larry
It is also called the reverser - it controls the timing of steam being admitted into the cylinders by adjusting an eccentric on the valve gear.
dd
Its a huge lever connected to the drive rod eccentrics.
Straight up was neutral, forward and reverse had notches that cutoff steam at specific intervals, the further out the bar was, the more steam got admitted to the cylinders.
Starting a heavy train meant having the bar full out and when moving it could be moved back without touching the throttle.
A few points:
1.) The reverser is not connected to the eccentrics, but rather to the “link” or “reversing link”
2.) It’s not always big. Later on, power reverse was used (required on locomotives with more than 400, 000 lbs. adhesive weight), and in this case the Johnson Bar is just a smaller lever in the cab
3.) Sometimes a screw-wheel would be used–it took a while, though, to change positions in large amounts, so an air-motor would sometimes be used for going from, say forward to reverse; wheels were popular in continental Europe and somewhat in Britain
Sincerely,
Daniel Parks
CP’s Royal Hudsons were also equipped with a screw reverse.
Maybe some steam expert can help me out with this: Many years ago, I was aboard a stern-wheel towboat which was a museum exhibit in Keokuk, IA. It was a tandem compound and had separate controls for reverse and cutoff. Is there any reason for the separate controls as opposed to a single control as on a locomotive?
I don’t have the answer to that, but note that the Johnson Bar or whatever should not be confused with the throttle, since the Johnson Bar controls timeing of opening and closing steam admittance to the cylinders while the throttle controls an opening to the steam pipe for the quantity of steam per millisecond while the valves to the cylinders are open.
Thanks for the inputs.
Larry
Without knowing what valve gear was used, I can’t say for certain, but my guess is that unlike a locomotive valve gear, the boat’s valves were not directly and solidly connected to the crankpin. Some valve gears operate whichever way the wheel is turning, but cannot inherently reverse it, and I hypothesize that this was one which could reverse, but in which there was no “reversing link.” The reversing link makes the cutoff tied to the position of the reverser. If you could tell me more, I might be able to come up with some more mindless gobbledimuck [:)].
Sincerely,
Daniel Parks