Steam & Excursion > Power reverser operation

Another question for the experts. How does the valve gear power reverser work? I'll just ask a ton of questions:
Is it steam or air powered?:
How does the control system maintain the position of the reverser? Suspect this is tricky valving.
What is the position feedback mechanism? Is that contained in the reverser valve assembly?
Is it ok to move the (power) reverser while the engine is running?
Is there any fail-safe scheme or a manual over ride?
Is it possible to use the reverser as a brake to slow an engine, that is, apply reverse while moving forward?

Always wondered  how this piece of magic works,

Thanks



Edited 1 time(s). Last edit at 11/07/19 20:33 by Illbay.

Is it possible to use the reverser as a brake to slow an engine, that is, apply reverse while moving forward?

Have you ever tried placing your vehicle in reverse while moving forward?

Danny McLean
Center, TX

You can use the reverse lever to slow a train so long as the throttle is closed.  It was not recommened by a lot of railroads including the Santa Fe. but was used on Cajon pass decending grades with no cars by some engineers. Expecially on lite  4-4-2's.    In some instances it could damage the gear.

All are wonderful,questions...

I've made notes and let’s take them in order.

Virtually all common power reversers are air operated.  HOWEVER, there is a possible exception: the FRA rule requiring ‘emergency’ operating pressure supplies: either steam or air... Steam was the older, simplest back-up for pressure.
The use of steam pretty much condemns the rubber ‘packing cups’ ( the ‘cups’ are the cylinder wall seals on the piston to prevent leakage).  So the more practical method was to use an “emergency storage tank” of compressed air —- enough to operate the reverser for a couple of strokes.

The emergency air tanks are in the airline from the loco’s Main Reservoir....But, protected from a possible broken MR air piping event by a simple  ‘check valve’ — a device with a flap that prevents air from flowing backwards...and out of the air tank.

The control valve operating mechanism is a from of a simple feedback lever, self-centering arrangement.
On lever-centered systems, the engineer’s control lever fastens at the midway-pivot of the lever’s three pivot points.
The bottom pivot connects to the outer piston rod-guiding block called the ‘cross-head’.  The main cylinder’s piston & crosshead moves  back and forth by compressed air, thus raising or lowering the main valve gear’s ( frame mounted) rock-shaft’ that moves the main steam engine’s valves that control steam to the pistons and wheels...

As described earlier, the middle pivot of the reverser’s control lever is connected to the engineer’s ‘reverse quadrant’ in the cab. As he moves the controlling reverse lever forward , the bottom pivot is ( temporarily) ‘fixed’, and that forces the top pivot also forward.
The top pivot is connected to arm of the pivoting block that controls the compressed air in the cylinder. 

There is compressed air on both sides of the main air-piston, —— as the block pivots forward, it allows air to exhaust out the front of the cylinder, the comrssed air behind the piston, moves tha main piston forward...

NOW, the bottom pivot, connected to the crosshead, also moves forward, ...That action forces the long lever to also move...The middle pivot is ‘fixed’ by the engineer’s desired setting..... so that NOW, the top pivot moves ( opposite), to close-off the open exhaust port as the piston slowly moves to the new ( desired) position.  

We we now have the valve gear moved a short distance, air has stopped flowing,  the control valve has re-centereditsekf,
and all air stops flowing and the air pressure on both sides of the reverser’s piston are equal.
The secret is the re-centering action of the feedback lever—- simple, but effective..

Yes, it’s very common to adjust while moving—- that’s its main function on the engine.

We’ve sidcussed The Emergency, back-up operating pressure schemes.

Yes, it’s possbke to reverse the engine while running.... the problem comes when you try to add steam to the cylinders... 
BAD THINGS HAPPEN....

But, in case all else fails, you can ...by very carefully controlling the steam to the pistons, try to retard the engine’s rolling momentum.  If you apply too much steam, you CAN actually reverse the drivers’ rotation....Which overheats the driver tires at the rail, sparking and sliding ... Sliding occurs because the steel of the divers actually melts at the rail surface...
The engine is sliding on liquid steel...the liquid becomes the lubricant!  That’s what all the spsrking is ABOUT..

Thus generating bad flat-spots on the driver tires....
However, at very low speeds, it is actually possible to slow and stop the engine with the reverser.
Dont try it,  if you don’t what you’re doing!

W.


 



Edited 3 time(s). Last edit at 11/07/19 18:59 by wcamp1472.

Santa Fe authorized  reversing the valve's direction in the train handling books that addressed water brakes.

Dreamer



Edited 1 time(s). Last edit at 11/07/19 19:08 by Dreamer.

Yeah.  To all that Wes said.

There were quite a few manufacturers of power reversers, but they all operated on pretty much the same principles. An Alco one is shown here.

The engineers reverse lever is a miniature version of the old "Johnson" bar, but has a lever only about 18" long in front of him instead of the 4 to 6 foot long lever required of old style manual Johnson bars.  Located ahead of him, controlled mostly by his left hand, though it's most convenient to use both hands, as there's quite a bit of resistance to movement - but not the body slamming feedback of the Johnson bars.  Sometimes, instead, a "steering wheel" was rotated to drive a screw mechanism instead of a Johnson bar type control.

The lever controls a double ported slide (or, rotary) valve thru a combination lever which senses movement of the crosshead, driven by the piston.  If, for example, the engineer moves the lever forward, that movement causes movement of the combination lever which moves the slide valve forward and admits air (or steam) to the back side of the piston, moving the crosshead forward.  This motion moves the combination lever to recenter the slide valve, and balances the pressures on both sides of the pistion to hold its new setting.

An engine may be set up to operate the reverser on air or steam, as Wes pointed out.  The supply pipe has an oil cup, as shown in the illustration, which is filled at the start of each day with either valve oil or with compressor oil.  Be sure to use the right oil for THIS engine.  A needle valve controls admission to the pipe.  It only takes one or two ounces of oil to supply the reverser all day.

The reverser should not be used as a brake.  As Wes points out, nasty things can happen, though it may accomplish your objective of slowing in an emergency.  Don't do it, otherwise.  Its purpose is to change the valve timing to give optimum use of steam for speed, power and efficiency.  Improper use of the reverser can be very hard on the wheels and tires, rods and their bearings, and driving box bearings and brasses.  Besides using two or three times the fuel and water as when properly used.

-Larry Doyle, aka John Stein.



Edited 1 time(s). Last edit at 11/07/19 19:31 by LarryDoyle.

---

I never knew Santa Fe had water brakes.  I know the Rio Grande did.

The Santa Fe's use of water brakes seem to end in the early 20th century.  The last instructional book to have a description is from 1906.  

Dreamer  



Edited 1 time(s). Last edit at 11/07/19 20:12 by Dreamer.

I can't tell you how pleased I am to learn how these reversers work. Thank you Larry for digging up that great photo of that reverser valve.  I can clearly see how the feedback works to allow air pressure to position the the valve gear.
And Mr. Wes, thanks for the descriptions and how the reverser is actually used.

If I may ask another question, what is the proper way to set the valve gear while running?  Suspect there are many vaiables, but there must be some basic rules to maximize efficiency.

Thanks,
...bill

Illbay Wrote:
-------------------------------------------------------
> I can't tell you how pleased I am to learn how
> these reversers work. Thank you Larry for digging
> up that great photo of that reverser valve.  I
> can clearly see how the feedback works to allow
> air pressure to position the the valve gear.
> And Mr. Wes, thanks for the descriptions and how
> the reverser is actually used.
>
> If I may ask another question, what is the proper
> way to set the valve gear while running?  Suspect
> there are many vaiables, but there must be some
> basic rules to maximize efficiency.
>
> Thanks,
> ...bill

There are a couple of ways to "hook and engine up".  First being listening and feeling, which comes with experience knowing when the engine is happy.  Then there would be a back pressure gauge which you could monitor to keep the cylinder back pressure low and proper. I guess the third would be a Valve Pilot installation, although someone else will have to explain this as I've never encountered that system.  It is fairly complex and was installed on many later engines.
BTW, the common term is "Power Reverse".  We used to tease one of our engineers who always called the power reverse the "reverser".   He ran diesels, so we did cut him some slack.

Illbay Wrote:
-------------------------------------------------------
> I can't tell you how pleased I am to learn how
> these reversers work. Thank you Larry for digging
> up that great photo of that reverser valve.  I
> can clearly see how the feedback works to allow
> air pressure to position the the valve gear.
> And Mr. Wes, thanks for the descriptions and how
> the reverser is actually used.
>
Thanks for your kind words.  It's nice to know someone actually reads and understands this stuff!

> If I may ask another question, what is the proper
> way to set the valve gear while running?  Suspect
> there are many vaiables, but there must be some
> basic rules to maximize efficiency.
>
When starting out a train, or when just switching cars at low speed, the reverse lever is placed in the full forward or reverse position.  (For any engine - manual reverse or power reverse.)  The throttle is used to control the live steam supply to the cylinders, and steam is admitted for most of the stroke with very little use of the expansive nature of steam.  If too much steam is admitted the wheels will slip.  Then, immediately back off the throttle to stop slip, then turn on sand if necessary and slowly open the throttle again.  NEVER turn on the sand while drivers are slipping!!!!!  *   This is a real "seat of the pants" learning experience.  On most engines you can actually feel very slight sideways movement of the cab and its seatbox a moment before slip begins.  Quick response to back off the throttle will usually prevent the slip.  With a train, at a speed of about 5-8 mph speed will level off due to back pressure from unexpanded steam in the exhaust passages, but if you hook up the reverser a bit you will consume less steam on each stroke ane rely more upon steams expansive power.  It's an aquired skill.

* This can be very damaging to the engines rods and bearings, and can even actually twist the driving axles.  Listen carefully to some of the steam videos here on Trainorders and you can hear some engines with a sort of "limp".  Instead of an even "Choo-Choo-Choo-Choo"  you may hear "ChooChoo--ChooChoo".

-LD, aka JLS



Edited 1 time(s). Last edit at 11/10/19 12:59 by LarryDoyle.

Frisco1522 Wrote:
-------------------------------------------------------

> I guess the third would
> be a Valve Pilot installation, although someone
> else will have to explain this as I've never
> encountered that system.  It is fairly complex
> and was installed on many later engines.

I've not run into this either, and don't find anything in my didactic materials about it, but would lke to hear from someone who has.

Buehler?  Buehler?

-LD

I recall an article in one of the magazines, maybe Classic Trains, explaining the Valve Pilot.  A lot of later engines had it like the other Frisco 4-8-2s and Northerns, but not the 1500s.  They were just a plain ol steam engine.  No FWH etc, seat of the pants running.  1522 did have a back pressure gauge and I would get hooked up, etc, until I liked the way she sounded and felt and looked at it.
Our one engineer, who was a diesel engineer thought you were supposed to "match the needles" on the gauge.  Poor fireman was a busy boy until we got the engineer  straightened out.

I've read some about the Locomotive Valve Pilot, and it wasn't really complex at all, at least the version I read. It was basically just a speedometer with two scales, one reading speed and the other recommended back pressure. (There would be a separate standard actual back pressure gauge. Or possibly all could be provided on one gauge.) The VP would have to be calibrated by dynomometer car testing for each class of locomotives, so that the prescribed speed and optimum back pressure values could be found. Then for any given speed the maximum amount of back pressure that should be run would be readily read. Just as an example, at 30 mph the maximum back pressure might be 12 lbs. At 50 mph it might be 15. And in any case this was only to be a measure of how to hook up the engine with a wide open throttle for maximum power at a given speed. More back pressure than the reading and you're wasting steam and making no additional power. Less back pressure than the recommendation and you're going to develop less than the maximum amount of power. In fact that was the other recommendation: when looking for less than maximum power, the back pressure should be much lower than the valve-pilot recommendation.

The more common method, which Don describes, was usually called a Control Gauge, reading steam chest pressure and back pressure on one face, with two scales and needles. When starting the steam chest needle would be much higher than the back pressure needle. As speed comes up, the steam chest drops back (unless the throttle is wide open or close to it) and back pressure rises. At any given speed the idea was to keep the needles lined up for maximum power with the least steam used, or when low power is needed the back pressure needle could be somewhat behind the steam chest. Again, to really be accurate each class would have to be dyno-ed and find the correct values, and scale a gauge accordingly.

Scott Gordon

In reading stories from the early days of the last century when a train had to stop fast in the event of an impending collision, one can read examples in the own words of the engineeer like: "I threw the lever in reverse and took to the birds (jumped).

Victor A. Baird
http://www.erstwhilpublications.com