can someone explain the difference/advantages of the different types of valve gear: baker, stephenson and walschaerts? were both sides of the baker and stephenson cross linked thru the frame?
can someone describe a compound cyclinder, and what its advantage is?
are there engines with 3 cyclinders? if so, where is the 3rd cylinder located and what is it connected to?
i’ve read a little bit about this stuff in the locomotive encyclopedia, but don’t understand the advantages and trade-offs.
Oh my, that is a lot of ground to cover! But I’ll do my best. Let’s start with valve gear first…
Stephenson valve gear is located between the frames of the locomotive. It was an evolution of the old “Gab” or “Hook” type motion of the earliest locomotives. On one of the drive axles of the locomotive, it would have four eccentrics, a forward and reverse eccentric for each cylinder. The forward and reverse eccentrics were attached to a link that could be raised or lowered so that the either of the two could move the valve, or somewhere in the middle where a combination of the forward and reverse links moved the valve… this is what happened when the engineer “hooked up” the valve gear. As Stephenson is hooked up, the lead increases, which means steam is admitted to the cylinder earlier in the piston’s stroke, and cutoff happens earlier in the stroke, which allows the piston to be driven by the expansion of a smaller volume of steam, making the engine run more efficiently.
At this point, I would like to mention that with all three types of valve gear, the reverse lever (or Johnson Bar) is connected via linkage to the valve gear for both cylinders, so that the timing of both cylinders is adjusted at the same time. Other than that, there isn’t a connection between the valve gear from side to side on the engine except for the drive axles, which are quartered so that the two cylinders are 90 degrees apart from each other. All three valve gears can be hooked up so the steam will be used more efficiently.
Walschaerts and Baker were located outside of the drive wheels. Each cylinder had an eccentric crank located on the main crankpin that provided timing 90 degrees from the cylinder. Walshaerts used a link - similar to the link on Stephenson - which was pivoted in the center so the ends moved 180 degrees apart from each other, providing forward and reverse. Baker used an overcenter type arrangement with a bell
OK, after my greatly simplified post about valve gear, let’s tackle compound locomotives.
There are really four types of compounds - Tandem, Vauclain, Cross compound and Mallet. But before we get into specifics, let’s cover the generic stuff. Steam likes to expand. In a normal locomotive, steam is admitted to the cylinder and it pushes the piston during which time (for part of the stroke) it is expanded to some degree. However, when it is exhausted into the atmosphere, it is still under some pressure - not as much as normal boiler pressure, but some part of it nonetheless. What a compound locomotive does is take the steam that would normally be exhausted and run it through another “low pressure” cylinder so it can be expanded further - more work is recovered from the same amount of steam. Some marine engines would do this more than once! But compound locos generally just reused the steam once through a second set of cylinders. Compound locos are more fuel efficient than a regular locomotive, but there are extra maintenance costs due to the extra machinery involved. Many railroads - especially those using tandem, Vauclain or Cross compound locomotives - discovered that the extra maintenance costs were more than the fuel savings, so they converted the locomotives to “simple” (non-compound) engines.
Tandem compounds had a small cylinder in front of a large cylinder on each side of the locomotive. The cylinders were in line with each other and the pistons were on a common piston rod. The Vauclain was used by Baldwin; it was designed by Baldwin employee Samual Vauclain. It had a small high pressure cylinder mounted underneath a large low pressure cylinder on each side of the loco. Both piston rods were attached to a common crosshead. The cross compound had a small, high pressure cylinder on one side of the engine and the large, low pressure cylinder on the other side of the engine. Finally,
While Stephenson gear is usually found between the locomotive frames, there are some locomotives with outside Stephenson valve gear. The ones I saw were in Denmark.
The big advantage of Baker valve gear is that all moving parts were rotary, and were fitted with roller bearings. Walschaerts and Young valve gear have sliding blocks running in tracks - which wear and require adjustment.
Late in the day, several variants on poppet valves were developed and tested.
Three (and four) cylinder rod engines were common in Europe. In the US, there were only two big classes of three cylinder locos (SP 4-10-2 and UP 4-12-2) and the four cylinder rod locos were all compound (Cole, Vauclain, tandem or Mallet) or semi-articulated simple.
A one-off exception was the D&H triple-expansion four cylinder compound L. F. Loree, which had a high pressure cylinder under the engineer’s seat, a medium pressure cylinder under the fireman’s seat and two low pressure cylinders in the usual location. It was a 4-8-0, had a very high pressure boiler and used poppet valves. It wasn’t very successful. The next D&H locomotives were beautiful but conventional - 4-8-4 and 4-6-6-4, with normal cylinder layouts and Walschaerts valve gear.
Chuck (Modeling Central Japan in September, 1964 - with Walschaerts valve gear 2-cylinder steam)
I’ve seen at least one domestic example of outside Stephenson valve gear, but I would have to look through all my books to find it again… [sigh].
One other thing about three cylinder locomotives is the need for valve gear for the third cylinder. This could be done by adding the extra valve gear outside of the drivers on one side of the engine, or you could use a conjugated valve gear (such as Gresley) that combined the motion from both outside valve gears to drive the inside cylinder’s valve. The term “conjugated” makes me think of a bunch of extra linkage just begging to get out of whack!
There are a lot of different valve gears out there, many just variants of the established types. It is interesting to note that the economy of working steam expansively was known very early in steam loco development. There are a lot of early valve gears that were very complex - almost Rube Goldberg contraptions - just trying to accomplish a perfect variable cutoff. Many used an extra valve that rode above the normal slide valve (pre-piston valve days) called a riding cutoff. Of course, low maintenance & dependability won out over a complex movement with a minor technical advantage. That seems to be the theme with a lot of locomotive development!
One reason that compounding largely disappeared was the development of the superheater. A superheater is a system of steam pipes routed through the boiler flues, which allows the steam to be heated a second time before being sent to the cylinders. Superheating actually delivered the increased efficiency promised by compounding without the complications and design problems.
Older, and saturated (non-superheated) steam locomotives have a box-like valve on the cylinder. this is called a slide valve, and on a saturated steam locomotive, condensation from the steam will provide a bit of lubrication. On a superheated locomotive, the slide valve is replaced by a piston valve, and lubricants with a higher flash point need to be used. The piston valve is usually a smaller cylinder above the main cylinder, but on some older locomotives that were retrofitted, a box with a piston valve simply replaced the old slide valves.
Some three-cylinder designs were compounds with high-pressure cylinders on the outside, with both exhausting into a low pressure cylinder between the frames.
Towards the end of the steam era, some locomotive builders (I think American Locomotive Company - ALCo - was the main promoter of this) turned to three-cylinder locomotives in order to alleviate problems with dynamic augment as locomotives got bigger and heavier. The main drive wheel’s axle had a crank on a three-cylinder locomotive.
