The On-Line Magazine of Rideable Model Railroading
© November 26 2011
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"The Company Notch"
The Johnson Bar Explained
Photo of the cab of the Virginia & Truckee Railroad's #22 "Inyo" located at the Nevada State Railroad Museum, Carson City, NV. (photo by Todd Moore). On a typical steam locomotive, the reverser lever or "Johnson Bar" (above) controls the action of the valve gear that regulates direction of travel and valve timing. Railroads, in an effort to save money on fuel, instructed engineers to use a Johnson Bar position commonly called the "Company Notch". In that position, valve gear limited the power stroke to only what was needed to maintain train speed, saving steam pressure and, therefore, fuel.
Written by Ted Rita
Hesston Steam Museum
Expansion is what actually moves the piston. Some people think it's just the steam being emitted into the cylinder. IT'S NOT. If you would charge your boiler to 100 psi of air and ran on that, you would immediately notice how quickly your pressure drops way. Air works on volume, not expansion.
Steam expands aggressively when you drop its pressure. You can see it when you blow down a boiler. What's important to remember is that as it cools, the steam expands. This can be illustrated by thinking about a multi-stage turbine. With each stage, the steam cools until it's starting to condense. This need for the steam to expand is powerful. It wants to fill the container, and does so aggressively. When there is a "moveable wall", like a piston, it pushes it.
Using the company notch or "hooking up" will increase the efficiency of the locomotive. It does this by cutting off the valve or stopping steam from being emitted through the entire stroke of the piston. So now you are working with less volume of steam and using more of its expansion. This saves steam.
Here is an example of a main line situation:
A locomotive engineer starts off pulling, for example, a 70 car freight train. He puts the Johnson Bar in "full gear" or fully forward, opens the throttle and starts the train. As the train gains speed he is actually opening the throttle more and more. As this happens, he also is pulling the Johnson Bar back toward the center. Now, going down a reasonably straight, flat track he gets to a point where the throttle is wide open, and the Johnson Bar is in the company notch. At this point, full boiler pressure is on the valve chest and the expansion effort of the steam in the cylinder is being used at its full potential, or as close as we can come to that potential. Here's the curve ball: the track starts up a grade, which will take more effort to get up. As he attacks the grade, he needs to use more steam. So, he starts moving the Johnson Bar notch by notch back toward full gear. Reaching the top of the grade he might only be moving at around 10 mph and using all the steam he can to ascend the grade. The steam is still expanding, but you also have more volume of steam which will do more work. IF the locomotive is slowing, then the steam pressure in the valve chest, cylinder and the boiler might equalize, which results in a stalled locomotive. There is nowhere for the steam to move; think of it as one container as there is no differential in pressure.
What could happen if the engineer did not move the Johnson Bar forward on the grade (even if he had the speed and inertia) to get up the hill when he was attacking it? The train would slow and the small amount steam expanding in the cylinder would need to do more and more work. With no more steam molecules to increase the expansion force, the steam would cool more. The cylinder would also cool. By the end of the stroke you are working with wet steam (steam containing visible water drops or steam in the process of condensing back to water), which has very little energy in it. Most likely, the train would stall or, even worse, prime (when water is carried over into the steam causing the cylinders to lock up).
In the hobby:
In the hobby you need to avoid wet steam for a couple of reasons. First, it will cool off your cylinder casting, which already hurts what little steam is coming in, which it needs to heat things up. Cooling down robs the energy from the steam before it goes to work. You can actually prime the engine trying to run in the company notch because of the reason above. Years ago, one crew "primed" our Porter locomotive and bent a piston rod by using the "company notch". Wet steam hinders emulsion of cylinder oil with steam, leading to poor lubrication and cylinder damage.
So, I don't recommend hooking up a miniature locomotive, as we are talking about such a small bit of steam already. Some might say use the Company Notch, but if they were serious about saving fuel, it would be better to avoid popping off. That's what wastes fuel. They are losing so much more potential energy by lifting the safety valve than you could ever save by hooking up the engine. In my opinion hooking up the locomotive is just a way to show off, and when you need to do that, you have the wrong mindset.
Cab of the 2-8-2 at McCormick Railroad Park in Scottsdale AZ. Photo by Eric Mencis
Johnson bar in a hobby scale locomotive. It works the same way in all scales. Photo by Steve Muscato.
Written by Ted Rita
Hesston Steam Museum
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