Steam locomotive valve gear

Trains Magazine General Discussion
1

Other threads and questions have prompted this posting. I’m going to state what I think I know about basics, and then invite the experts to elaborate and or correct. I hope this can develop into a thorough and educational discussion that will cover all types of valve gear from the earliest Stephenson through to Franklin and Caprotti and whatever. Also, if someone can post a good figure for the difference betwen slide and piston valves, better explained in a drawing than by words, that would also be a big help.

Valve gear has two purposes, to reverse the locomotive, and to determine the duration of steam entrance to and exhaust steam exit from the cylinder during each stroke. This is done by its control of entrance and exhaust valves from motion of the rotating driving wheels.

Steam is used most efficiently if it is admitted to a cylinder when the volume between the piston that cylinder end is near or at minimum during the power stroke. However, the greates possible power is provided when the intake valve is open for a substantial part of the stroke, not just the start when the volume is minimum. So there is a trade-off between power and economy. That is why starting a heavy train is usually done with long cut-off, and running at speed with short cut-off. However, use of short cut-off can, as discussed on other threads, provoke highspeed slipping.

Exhaust valve control seems optimum when it provides minimum back-pressure and thus minimum wastage of power. This is when the exhaust valve is open for nearly the entire stroke on the exhaust side of the piston. This is impossible to arrange with slide valves, where exhaust and intake valve openings must be the same proportion of the stroke and is arranged only with great difficulty with piston valves. There is also the question of the effect of reduced back-pressure on smokebox draft.

I hope the real experts will continue this discussion. Thanks

2

I am certainly no expert on valve gear - but I noticed a long time ago that steam pressure helps the slide valve maintain a tighter seal - expecially as surfaces wear with time. Piston valves require sealing rings and lubrication.

dd

3

All I know about valve gear is what I read in the Trains article a few years ago. IIRC there is a third type of valves - poppets, roughtly analogous to the valves in an internal combustion engine. I should think that the difference between the timing of the intake and exhaust sides might be a little easier to adjust with them.

Feel free to point out the errors in my assumption… Always glad to learn more.

4

Dear everyone,
I am by no means an expert on valve gear, but I volunteer on a steam locomotive and know just enough to get myself into trouble [:)].

Firstly, maximum power is created when steam is admitted during the entire stroke (except at the very end, when the piston must reverse direction). The problem is just that the boiler can’t keep up with this. It is, however, more efficient to run with, say, 50% forward reverser and 100% throttle, than with 100% reverser and 50% throttle. This is because it’s better to use a small amount of fresh boiler-pressure steam when it is most needed (at the beginning of the stroke), and then let expansion do the work, than it is to use a large amount of steam, with less pressure, used throughout the stroke. For this, it is important to remember that, because of leverage, a force on the piston will put out much more force at mid-stroke than at the end, at the wheel. Because of this, you only need the fresh steam at the beginning, and expansion will do the rest, and keep up a decent amount of force. Now, of course, if you have a 2-10-2 with 114 cars behind you, and you stopped on Sherman’s Hill, you’re going to need fresh steam throughout the stroke to get you moving again, so you would put the reverser all the way forward. The rule is to take the reverser as far toward center as possible, to conserve on steam (and, what the railroads actually care about, coal).

Secondly, Dave, I have never heard of wheel slippage at high speeds because of too quick a cutoff. Remember that cutoff is proportional to power, so if any slippage would occur, it would be when there was a later cutoff (when the reverser was far forward or back). Also, slide valves and piston valves can in theory accompli***he same tasks, and almost always, the cylinder will exhaust steam for longer than it receives steam, with either slide or pison valves. To my understanding, piston valves became popular because traditional slide valves would be forced down on their seat by the steam pr

5

I don’t know what else you’re looking for, but I can add a few details.

Cutoof is controlled by the position of the Johnson Bar or reverser wheel (depending on which the loco has). Cutoff normally is adjustable from 15% to 85% of the stroke. There are probably some locos outside this range, but I’m talking the norm. Steam admitted during the longest part of the stroke will provde the most power, but the same quantity of steam must be exhausted causing back pressure. The back pressure will limit the top speed of the locomotive at the given cutoff setting. A quicker cutoff will admit less steam and give less to exhaust allowing for higher speed. This is much like shifting to a higher gear in your car (if you’re familiar with stick shift). The valve gear coordinates the valve motion with the position of the piston, much like the camshaft in an engine.

Another general rule, slide valves won’t be used with a superheated engine. The piston valve became more popular, even on saturated steam engines because they were easier to service, repair, and fabricate. All machine shops have at least one lathe.

6

Slide valves also require lubrication - older US-built locos with slide valves had individual lubricators for each valve chest. These were later replaced by feeds from the hydrostatic lubricator in the cab.

7

You’re quite correct - Franklin oscillating cam poppet valve gear allowed for the independent control of admission and exhaust. This valve gear was used on Pennsy T1 #5511, amongst others.

8

Generally, no. The main difficulty was in properly lubricating the valves. The temperature of superheated steam was well above the flashpoint of the mineral oils in use at the time.

9

Just to nitpick, the 85% cutoff figure makes sense, but cutoff can go down to 0% when the reverser is in the center.

Also, while it is true that back pressure slows down an engine, in any cutoff position, the force of the forcing steam will be much greater than that of the exhaust steam. Given an infinte amount of steam, maximum speed would be found with the reverser all the way forward or back. The only reason to bring the reverser back is to conserve on steam, since at high speed you would otherwise quickly lower your steam pressure.

About the slide valves and superheating, there is no real problem (as far as I know) with slide valves on a superheated engine. It seems to me that this is the case simply because piston valves were widely adopted before superheating was.

Um…oil can’t burn when surrounded by steam, only in the presence of oxygen molecul

10

Get a copy of THE ARROW, the magazine of the N&W Historical Society, from November/December 2002. The valve gear article therein is an updated and expanded edition of the story that appeared in TRAINS back in 1984. It does not cover poppet valves, but everything the casual railfan needs to know about slide and piston valves and the mechanisms that drive them.

Back issues of THE ARROW are available from the N&WHS website.

Old Timer

11

Note that my comment about slipping at high speed and its correction was verified by the Pennsy T-1 thread. From what I understand, both on the T-1 and on certain British 4-6-2’s, the management wanted short cut-off for fuel economy, but in certain conditions, engineers found these locomotives were prone to slipping at hight speed, and by reducing throttle and maintaining speed by longer cut-off at the same time, slipping was controlled better. If I am mistaken correct me.

Of course, it is really the reduced throttle setting the reduces slipping, but then the longer cut-off is required to maintain power and thus maintian speed.

Again, if I am not correct, please make the necessary correction.

Otherwise, I appreciate all the comments so far and hope there will be more!

12

Adding to this, Caprotti (Italy), Cossart and Renaud (France) poppet valve gear, all allowed independent control of admission, exhaust and cut-off.

13

While Cossart Valve Gear was suitable for operating poppet valves, Cossart valves were vertical piston valves operated by camshafts. This valve gear was extensively used by the French Nord (Northern) Railway and by the SNCF Northern Region. Two Pacific locomotives were rebuilt with this valve gear, but it was mainly used on a series of 2-8-2 tank locomotives for Paris suburban service.

The difference is of course, that piston valves admit steam by opening ports in the valve liner, while poppet valves actually close on a seat. Thus the piston valve can remain in continous motion while the poppet valve must stop with some force. The poppet valve can easily provide a larger port area, of course.

The valve gear was adjusted electrically, and the vaves could be set in in open posion for coasting with minimum resistance. Not the simplest solution, but the gear had the distinctive feature of the drive rod having a large mass at the valve gear end, and thus a taper in the reverse direction. This mass allowed the cross balancing of the reciprocating mass of the rods, to some degree.

Peter

14

Sorry, I should have made that distinction in my previous post. [:)]

I wasn’t aware of the two 4-6-2s. What were they? I was mainly aware of Cossart valves/gear from accounts of the Algerian passenger Garratts - now there’s an engine!

15

The two Pacifics were 3.1249 and 3.1250 which were built as experimental four cylinder compounds with poppet gear, one with Caprotti and one with Dabeg gear (if my memory is correct). The rebuild used the standard cylinders and valve gear from the 2-8-2s, thus making them two cylinder simples. While there were high hopes for these locomotives, they were not regarded as highly as the standard 3.1200 and no more were converted. I think one of these two was displayed at Paris Nord with Gresley’s 2001 “**** o’ the North” (another locomotive with experimental vave gear) during its period in France.

I think De Caso used the Cossart gear in conjunction with poppet valves on the three cylinder simple 232S class Baltics (or Hudsons, I can’t remember which name they had), but this may have been changed later.

Peter

16

Can anyone do a good job of explaining the basic differences between the two “conventional” valve grear systems used on North Americqn locomotives, the Walschearts and the Baker? As I remember, nearly all PRR locomotives used Walschearts and nearly all NYC Baker. Why?

17

Well, there you go! I wasn’t aware that “**** o’ the North” had been in France, either. That would have been something worth seeing. How long was it there?

Yes, they were the engines I had in mind when replying previously. As far as I 'm aware, they were referred to as “Baltics” - the preserved 232U1 at Mulhouse is labelled as such. Now there’s another magnificent loco - I’d happily trade my granny for a day on the footplate of that engine!

All the best,

Mark.

18

The significant difference is that Baker uses a bell crank rather than a link to alter cut-off and reverse the loco. It will permit valve travel of up to 9", and has good valve events with constant lead. The advantage over Walschaerts is that all wearing surfaces are pins and bushes, making maintenance cheaper and simpler.

I couldn’t say, other than to note that Baker valve gear was proprietary, being a product of the Pilliod Company. Perhaps the Pennsy were unwilling to pay royalties and/or a license fee for it’s use?

19

There is a very real problem. High temperature steam is a very effective cutting tool. With superheated steam the large flat surfaces of the slide valve quickly cut and score. Exacerbating this is the inability of the oils in use at the time to withstand high superheat temperatures. They broke down and lost their lubricant properties, further damaging the slide valves. But don’t take my word for it, read any of the contemporary trade journals and literature. Also, note the number of different piston valve adapter/conversions used on slide valve engines after they were superheated. If slide valves could be used successfully with superheat, these would have been superfluous.

Oh? The oil doesn’t need to burn to lose it’s effectiveness as a lubricant. The steam temperature matters vitally if it is above that at which thermal degradation occurs in the grade of oil you’re using. There was a large body of development work carried out over the years by the oil manufacturers to perfect an oil that could withstand the superheat temperatures of around 800 degrees typical of modern steam locomotives. If steam temperature didn’t matter, why did they bother?

[quote]
QUOTE: Consider these two locomotives: Grizzly Flats Railroad (ex-Nevada Central) no. 2 and Ventura County Railway no. 2. GF no. 2 was built in 1881, Baldwin 5575, has slide valves and is not superheated. Ventura County Railway no. 2 was built in 1922, Baldwin 55415, and has piston vavles and is superheated. Th

20

Which railroads used the Baker valve gear other than the NYCentral and its subsidiaries? The standard USRA WWI locomotives used Walschearts and locomotives were built to these patterns right up through WWII, Baker must have been in the minority. But I know others used it. Who?