About what time period did the transition occur from slide valves to piston valves? it seems like it was right after WW1 but it is so difficult to get a handle on it as most locomotives lasted so long and kept getting modified over time. - Nevin
My research indicates that locomotives began featuring piston valves at around 1910. Of course, it took a while after that before they were commonplace. My understanding is that they allowed operation at higher pressure.
Piston valves became the ‘standard’ before the turn of the century. What caused railroads to go to piston valves was ‘superheaters’ in the smokebox. This raised the steam pressure higher and the ‘slide’ valves could not be lubricated - slide valve failure. Piston valves have ‘rings’ like in your automobile for the sliding/wear surface. Lubrication of piston valves was quite a bit easier. Many times you read a story about ‘saturated steam’ engines. These are engines without superheaters. Most 2-8-2 & 4-6-2 engines built at the turn of the century had superheaters, and many 4-6-0 & 2-8-0 engines got upgraded with superheaters and piston valves. Fom a modeling standpoint, slide valve engines have a small ‘box’ on top of the cylinders. A piston valve engine has a smaller cylinder on top of the main cylinder.
Jim Bernier
In addition to their inability to tolerate superheated steam, slide valves, or “D” valves as they were sometimes called, were usually activated by Stephenson valve gear which operated inside the frame of the locomotive. The switch to piston valves was accompanied by the widespread application of Walchaerts valve gear which hung outside of the wheels where it could be easily inspected and serviced. By 1910 piston valves and Walchaerts gear denoted a modern locomotive and this pattern lasted pretty much until the end of steam.
Thanks to everyone for the answers. I appreciate it. The other question I had was how often a locomotive was converted from slide valves to piston valves? I know it was done, the D&RG K-27 started with one kind of valves and cylinders (compound) and the whole series was converted to different piston valves and cylinders. It does seem to be a involved procedure. - Nevin
Based on earlier replies I imagine any saturated engine that was superheated would have to get piston valves. Many engines were converted to superheating of course, for a minimal effort the RR could raise an engines power by a considerable degree (maybe 10% or more?).
The following is a quote from Model Railroader Cyclopedia Volume 1, Steam Locomotives by Linn Westcott:
“The first great step in efficiency was the acceptance of the superheater around the years 1910-1914. Its hot steam in turn required piston-type valves instead of the old D slide valves.”
I’ve read the book cover to cover and I don’t recall any mention of engines being converted from slide to piston valves. Not to say is didn’t happen, it’s just not in the book.
'T’ain’t necessarily so. I’m looking at a couple of photos of slide valve locos with Walschaerts valve gear, all built in the 1920-1930 decade. There were plenty of examples of piston valves driven by Stephenson gear (including some where the Stephenson gear was hung OUTSIDE the main rod!)
The main problem with slide valves was increasing boiler pressure, with superheating a secondary, but important, consideration. Due to their design, slide valves are held to the machined surface of the valve chamber by steam pressure. Higher pressure translated very quickly into greater wear, greater difficulty in getting proper lubrication and a decrease in time between failures. OTOH, piston valves really aren’t influenced by steam pressure, lubrication is much simpler and there is far less wear on the fixed and moving parts.
For a short period around the turn of the 20th century, slide-valve engines were upgraded by installing piston valves in valve chambers designed to replace those used by slide valves. It worked, to some extent - but the piston valve diameters were rather severely restricted by the low height available to the replacement machinery. Trainspotters would either note that the slide valve ‘box’ had a cylindrical appendage sticking out of it, or that there would be a mound-shaped ‘thing’ in place of the box, again with the valve cylinder p
the piston valve appeared fairly early in railroading, the piston valve was invented by John Marks master mechanic of the Boston & Worcester RR who made several test installations starting with the 4-4-0 “Meteor” in 1856. They reappeared at intervals until 1889 when Samuel Vauclain popularized them in his famous compound arrangement. The Schmidt superheater first appeared in North America in 1901 (on the Canadian Pacific) but it took about a decade before it became truly accepted. Many older locomotives were converted to superheated steam, a well known example would be the B&M’s Moguls which received Franklin’s “Economy Steam Chests,” a bolt on conversion valve.
Another well known example is the MA & PA 4-4-0
3T:
This is quite true, especially of the plain slide valve, which is completely surrounded by steam that presses it against the port face. When steam pressures were low, this was a Good Thing, but wear problems showed up as steam pressures increased during the 1800s. At some point, long before the piston valve went into common use, the balanced slide valve was developed to solve the wear problem.
Like a lot of successful ideas, it’s pretty simple. Various kinds of seal were installed in the slide top, to keep steam away from most of it. With less area exposed to steam pressure, there was less friction with the port face, and less wear.
http://www.sdrm.org/faqs/boilers/page95.html
Eventually the piston valve did take over. I don’t know why it didn’t come out earlier; maybe sealing rings weren’t developed enough,but you’d think the existence of at least two pistons with rings in every slide-valved loco would be evidence to the contrary. Maybe it was just a technological blind spot, like the prevalence of hook-type valve gear and patent cutoffs, before engineers realized they could just install a curved link and have Stephenson link motion.
the principle reason early installations of piston valves were unsucessful is the same as why early superheaters (James Millholland invented one that worked all too well in 1853) were not sucessful. The lubricants available at the time were made from natural sources, either animal fats or vegetables. It wasn’t until 1867 when Vaccum Oil Company (an early predecessor of today’s Exxonmobil) and Baum’s Castorine (a company still in business) developed high temperature petroleum based cylinder oils that higher boiler pressures, piston valves and eventually superheater became possible.
Very good point. Many of us forget that a simple thing like developing high temperature lubricants will allow ideas that have been around for a while to finally be implemented.
Thanks to everyone, this has been a very interesting discussion and I have learned plenty. We should have more of these kinds of threads rather than the endless repetative “John Allen is great, no he is not” type threads. - Nevin
TT:
Good point on the lubricants, but I’m not totally sold yet…because a piston valve is just that - made of pistons, and there was already a pair of pistons that needed to be lubricated and sealed, with steam pressure on them…and from all accounts, the slide valve was a pretty challenging exercise for lubricants…so I still have to wonder, what was the deal? Why no piston valves?
Perhaps they just cost more to make, needing more parts as well as piston rings? Maybe it was difficult to make a leakproof piston ring that wasn’t overly bulky - I recall reading an offhand remark in a 30s Model Craftsman that the piston rings of typical 1860s locos outweighed those of 1930s locos.
I’m going to have to dispute this one. I’ve studied literally thousands of pictures of steam engines and I’ve never seen Stephenson gear hung on the outside of the frames. Perhaps this was done on some very early locomotives but it was only a common practice on Shay geared engines. With the cylinders mounted vertically and in line like triple expansion marine engines, Stephenson links were the preferred gear for Shays. One other caviat: The Southern Pacific operated a large fleet of Harriman Standard 2-6-0s and 2-8-0s that used inside mounted Stephenson gear to drive piston valves. There’s practically no difference in mechanical efficiency between Stephenson gear and Walchaerts gear so the engines worked fine and lasted until the end of steam on the SP
Seen on an 0-6-0T IIRC, in Copenhagen, in the summer of 1956. Circular cams on CL of main driver axle (mounted on an eccentric crank that wasn’t eccentric) linked to the ends of a sector link that could be raised or lowered. The valve stem was fixed horizontally and vertically, driven by a block in the sector link. Maybe it has a different name if it isn’t inside the frames?
Edit. Just checked Wikimedia (category, Danish Steam Locomotives) and found a couple of examples of, ‘Looks like Stephenson,’ valve gear. One photo, DSB_F_694, might be a sister to the loco I remembered seeing.
Chuck (Modeling Central Japan in September, 1964)
i’ve seen photos showing outside stephenson gear, it may not have been common, but it was done