Steam locomotive valve gear

[marknewton]

QUOTE: Originally posted by tree68

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.

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.

[marknewton]

QUOTE: Originally posted by dldance

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.

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.

[TomDiehl]

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.

[Anonymous]

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 pressure, so elaborate systems of "balancing" them came, but the piston is in herently balanced anyway (theoretically). Remember that a steam engine has four cycles, just like a car engine or a jet enine. The difference comes on which cycle power is given in. On a steam locomotive, they are admission, cutoff, compression, and release, just remember that unlike the other types of engines, admission and cutoff on one side are happening at the same time as release on the other. On a car engine, the power comes in the ignition phase, and on a jet in the exhaust phase. In a steam engine, power comes in admission and cutoff.

One other correction. Not all valve gears are connected to the wheel, just most.

Interestingly, Stephenson and Walschearts valve gears move in slightly different patterns as far as relative positions of piston and valve. On Stephenson, the lead (the amount the valve is open at the beginning of the piston's stroke) increases as the reverser is brought farther from center. On Walschearts, the lead is constant, set by the dimensions of the valve gear.

Here some terms which might be useful to this discussion:

Inside Admission Valve--a valve which admits steam toward the "inside," or towards the center of the steam chest. Basically, synonymous with piston valves.
Outside Admission Valve--a valve which admits steam toward the outside, or away from the center of the steam chest. Basically synonymous with slide valves.
Steam chest--the place where the valve slides (or pops, or "does its thing").
Steam ports--the two ports in the steam chest, each one leading to its respective end of the cylinder, for the admission (or release) of steam.
Exhaust port--the port which carries away steam. (The ports are open, closed, and connected by the valve during its motion)
Reversing lever (or Reverser or Johnon Bar)--the control in the cab (could be a screw) which the engineer uses to control the valve gear.
Steam lap--the amount the valve over-covers the steam ports on the admission side (inside overlap for piston valves, outside overlap for slide valves), when it is in the middle of the steam chest.
Exhaust lap--the amount the valve over-covers the exhaust ports on the opposite of the admission side when the valve is in the middle of the steam chest. Do bear in mind that because piston valves exhaust rather differently, the "exhaust port" and "exhaust lap" don't exactly apply the same way.
Clearance--basically "negative steam lap"--the distance of "non-overlap" if the valve doesn't overlap the steam ports on the admission side when it is in the middle of the steam chest.
Lead--the amount the valve is open at hte beginning of the piston's stroke.
Admission--the portion of the stroke of the piston in which fresh steam is admitted from the boiler.
Cutoff--the portion of the stroke of the piston in which fresh steam is cut off and the steam already in the cylindr is left to expand.
Compression--the point at which the release of steam on the other side of the cylinder is stopped, so the expanding steam on one side of the piston comes against steam on the other side at the end of the piston's stroke, creating a nice "cushion," making a smooth ride and minimizing wear.
Release--the release of steam from the previously expanding side of the cylinder.

Most of this I have learned from reading books, so I'm sure that I've misunderstood something, so please correct me if I have any errors. I learned most of this from Charles McShane's Classic American Locomotives. It's from 1899, but was reprinted , and has a large section on valves (mostly balanced slide valves), as well as Stephenson valve gear, discusses these things about lap, lead, stuff like that.

Sincerely,
Daniel Parks

[tree68]

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.

[dldance]

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

[daveklepper]

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