QUOTE: Originally posted by trainjunky29 Just to nitpick, the 85% cutoff figure makes sense, but cutoff can go down to 0% when the reverser is in the center. Not sure what steam theory you're working with, but based on my experience in the backshops at Steamtown, the mechanical practicality of the real world of steam locomotives, once the reversing quadrant reaches a certain point near the center, the valve no longer moves far enough to allow steam into the cylinder, hence the 15% figure. 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. Again, not sure of what theory you're quoting, but that "infinite amount of steam" needs to be contained and controlled through valve(s) and piston(s) to be converted into mechanical energy. The steam passages through the valve for steam admission or exhaust are about the same size, since the pistons are double ended, so the volume of the steam does become a factor. In the real world, the reverser being at either limit will severely limit your maximum speed, not allow maximum speed. 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. The main problem was lubrication, as stated above. A slide valve was basically a plate with passages cast or machined into it, maiking it more difficult to lubricate than a piston valve QUOTE: Originally posted by marknewton 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. Um...oil can't burn when surrounded by steam, only in the presence of oxygen molecules, so the steam temperature doesn't really matter as far as the oil burning. 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. They both use Nathan hydrostatic lubricators, both using steam oil, also known as valve oil (as opposed to journal oil). Sincerely and respectfully, Daniel Parks
QUOTE: Originally posted by marknewton 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.
QUOTE: Originally posted by marknewton QUOTE: Originally posted by M636C 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. 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? QUOTE: 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. 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.
QUOTE: Originally posted by M636C 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.
QUOTE: 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.
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QUOTE: Originally posted by trainjunky29 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.
QUOTE: Um...oil can't burn when surrounded by steam, only in the presence of oxygen molecules, so the steam temperature doesn't really matter as far as the oil burning.
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. They both use Nathan hydrostatic lubricators, both using steam oil...
QUOTE: Originally posted by daveklepper 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?
QUOTE: As I remember, nearly all PRR locomotives used Walschearts and nearly all NYC Baker. Why?
QUOTE: Originally posted by M636C While Cossart Valve Gear was suitable for operating poppet valves, Cossart valves were vertical piston valves operated by camshafts.
QUOTE: 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.
QUOTE: Originally posted by 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.
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.
QUOTE: Originally posted by TomDiehl 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. 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. 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.
QUOTE: Originally posted by TomDiehl Another general rule, slide valves won't be used with a superheated engine.
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