Power Reverser questions

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Re: Power Reverser questions

Post by RET » Sun Feb 10, 2019 11:03 pm


I checked my copy of the Locomotive Cyclopedia (1941 version) and it says that the Alco units were air powered. These units were a simple servo mechanism. When the unit is at rest with the supply pressure on, if you look at the linkage, you can see that motion of the control lever will cause the control valve to be moved to one side or the other of its center position. The valve then admits and exhausts air from the appropriate sides of the double acting piston to move the crosshead and linkage to recenter the valve. In this way, small motions of the control lever are amplified to easily move the valve gear to the desired position, whether it is to change from forward to reverse or to adjust the valve cutoff. The control lever usually has a quadrant with notches so the valve unit can't "creep" although with a model, it likely would be sufficient to have some kind of a friction lock that would serve the same purpose.

There is no locking mechanism, just the air pressure on the double acting cylinder and perhaps the friction on the cylinder rod of the valve packing. Perhaps at first it was thought that some kind of locking or damping mechanism was necessary and this apparently was actually used in Britain, but in North America it seems that it was thought sufficient to use a large cylinder for the purpose.

Sometimes I guess its necessary to check my sources. Sorry about that. Nobody's perfect, not even me.

If you want to check the Big Boy thread, you'll see I'm building two of these things in 3 1/2" gauge. One of these days I'll get them finished and operating, then I can play with them myself and see.

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Re: Power Reverser questions

Post by NP317 » Mon Feb 11, 2019 1:08 am

The 2-6-6-2 I Engineered had an air-operated power reverser. With 4 sets of valve gear it was needed.
I remember steam-up procedures included opening the cylinder drains of the reverser to blow our water as part of verification of proper operation.
Of course the air pumps and reservoirs had to be in operation first.
The locomotive could not be moved under steam until air was available for the power reverse.
Snoqualmie # 11 mid 1980s shortly after rebuild..jpg

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Re: Power Reverser questions

Post by Dick_Morris » Mon Feb 11, 2019 1:09 am

As others have said, the steam reverse is just a type of servo. When a movement of the Johnson bar is detected, a valve opens to allow air to a piston until the position of the reversed reach rod is again the same as the reverser piston. For their power reverse gear, Baldwin (which was based on Rayonet) was operated with air. They advised that a three-way cock be fitted so steam could be used in an emergency. They gave piston cup packing and rod packing life as six months and one year. use of steam would materially shorten the packing life, rust cylinder parts, and wash away the lubricant. If steam was used, the gear should be inspected at the next terminal. Of course, a live steam model could be designed with materials appropriate to water.

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Re: Power Reverser questions

Post by BillF » Mon Feb 11, 2019 7:01 am

Just a thought: If you are trying to pack a servomechanism into the small space of scale power-reverser it might be better to make it electrically operated. The direct electrical analog would be opposing solenoids run by an analog proportional control, but that would drain a lot of current. Something with a more intelligent control mechanism that only powers one solenoid at a time to actively maintain the position the reverse lever might work. The electronics to control something like this can be hidden in a number of places; you don't have to put all of the hardware into the scale reverser.

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Re: Power Reverser questions

Post by kbarnett » Mon Feb 11, 2019 8:29 am

I've had a Mercer power reverse on my Hudson for 34 years, it uses hot water piped from the about 1/2 way up the side sheet. Other than having to replace the O-rings on the piston a couple of times it has worked great.

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Re: Power Reverser questions

Post by RET » Mon Feb 11, 2019 10:48 am

Hi Bill,

If you check the thread "Union Pacific Big Boy in 3/4" scale," you will see that the control valve body is already made. I still have to make the valve cover and valve plate. The valve body except for the mounting flanges is roughly a 3/8" cube which is held onto the top of the reverser cylinder with four #0-80 cap screws. Much smaller than any electronic assembly and it will work just like the original.

In a model, water should be used because its simpler (always use the KISS principle when possible) and because you have a ready source of water under pressure in the boiler. That way, you don't need to have a source for compressed air (although some time down the road I'm tempted to try building a couple of working cross compound air compressors in 1/16" scale). Who knows, they might even work if I happen to get it right. If you use steam from the boiler, it will condense into water anyway.

In the Locomotive Cyclopedia it says that the full size reverse units throttled the exhaust with bushings to control the speed of the unit, a smaller bore bushing was used if it moved too fast and the reverse if more speed was needed. Remember, the railroads only made things powered when it was too hard for a man to do things manually, like power reverse, stokers etc. so yard engines were likely too small to need something like this.

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Re: Power Reverser questions

Post by Moron » Mon Feb 11, 2019 12:06 pm

The power reverse on the full size locomotives used air with steam as emergency. The main type of power reverse gears were the ALCO gears. Starting with Type E, which had the air delivery pipes physically piped in. This was later cast into the cylinder itself creating the Type G, which for a while was most commonly. The huge locomotives latter on used Type H which was 2 inches bigger in bore. Type K I believe was the same as Type H except it did not have a crosshead. Franklin made a similar type which is found on many tourist railroad locomotives today. The gear operated with a disc/rotary valve. When the valve is centered, both admission ports were blocked with a tiny amount of lead on both sides. This allowed both sides of the cylinder to charge with air, with slightly more lead in the front and about 10 pounds less in the back. This was done to balance the weight of the piston trunk. Have air on both sides allowed
1. Less air consumption, more economical
2. Kept the valve from creeping
3. Allowed the gear to be controlled more or less on exhaust, so the gear would be sensitive and responsive to the finest little movement of the reverser handle. This created the ability to control the most precise cut-off next to using a screw reverse.

The exhaust “pipe” for lack of a better word would be choked down at times to create a slower movement.

The emergency steam use was not recommended unless there was a true emergency. This was made law by the ICC at some point. There was a case where the engineer on a cab forward lost all his air and had no brakes, nor air for the power reverse. He switched to steam and revered the engine, preventing a train wreck.

Hope this info has been helpful!

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Re: Power Reverser questions

Post by Asteamhead » Mon Feb 11, 2019 4:38 pm

Hello Moron,
Thank you for your precise information!
For power reversers were not common in Europe, any information would help for better understanding that system. I used Steam Cyclopedia as basis for my model construction. And yes, the precise, small and low loss valve is the heart of the reverser.
Thus a tiny rotary valve made of stainless steel with samall amouts of lead (as you descibed!) seems to be a good solution. Driven by water it`s tight, precise and stable. Steam as emergency backup will work, too.


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