The Walschaerts Valve Gear for Steam Engines
This steam engine valve gear was invented by a Belgian, Egide Walschaerts, and patented in 1844, with an improved patent in 1848 being much closer to the way his invention became used in practice.
A German inventor, Edmund Heusinger, came up with the same idea shortly afterwards as a totally independent invention, and for this reason one will still often hear this gear referred to in Germany as the Heusinger valve gear (Heusinger-Steuerung, in German). A matter of national pride, one supposes.
In functionality, the Walschaerts gear is quite simple. The motion is taken primarily from the locomotive driving wheel. On the end of the crankpin, normally that one on the main driver, a return crank is fitted. The other end of this return crank is at a point just over 90 degrees ahead of the crankpin around the driving axle (in the normal case of an inside admission locomotive), and closer to the wheel's center. The return crank drives the return crank arm, which in turn is fastened to the lower end of the expansion link - this is a rectangular piece of metal with a slot in it, pivoted at the center so it can rotate. The return crank arm is fastened to the bottom end of it, and as the driving wheel rotates it therefore rocks the expansion link back and forth around its center.
I described the expansion link as rectangular, but that's not quite true; it's curved, as if it has been bent, the concave side facing forward. In the slot in the expansion link fits a slider, the die block. This is moved up and down in the expansion link by the reversing shaft, under the control of the locomotive's engineer (UK: driver). If the die block is in the center of the expansion link, of course, it does not move in a longitudinal sense and only rotates as the expansion link is rotated back and forth. The further it is moved from that central position the more it moves; if moved downward, it moves in the same sense as the return crank arm, while if moved upward it moves in the reverse sense.
The die block is attached to a rod named the radius rod, which provides the large part of the valve motion to the piston or slide valves. The expansion link and die block, then, are used to control the direction of movement of the locomotive, and the amount of valve travel; think of it as the equivalent of the gearbox in an automobile.
While the portions of the Walschaerts gear so far described make for a serviceable valve gear for a locomotive already, it was discovered that by adding a small proportion of the piston's movement to the valve motion produced better valve events. Thus, instead of the radius rod being connected directly to the valve spindle, it is connected to a more-or-less vertical rod called the combination lever. The radius rod is connected to the top of this lever; the valve spindle is connected to a position only slightly down the lever, since the radius rod's motion is the large proportion of the desired valve motion. The other end of the combination lever is connected to the crosshead at the end of the piston rod by a horizontal rod named the union link - the purpose of this was simply that the crosshead was not normally close to the locaton of the combination lever but rather substantially further back. The combination lever, thus, combines the movements of the radius rod and the crosshead in approximately a 10:1 proportion, adding only a small amount of the crosshead's motion to the valve motion.
While initially unpopular, the Walschaerts gear was probably applied to more of the twentieth century's steam locomotives than any other, replacing the former favorite gear, the Stephenson valve gear as locomotives got larger. Most applications of the Walschaerts gear were mounted externally, and it is the gear's suitability for external mounting that was one of its bigger selling points. If you've seen a steam locomotive with all kinds of rods and levers moving as it rolls, it's probably Walschaerts' invention that you witnessed working.
As the steam locomotive got larger and more powerful, things got more cramped to work on, as the boiler and driving wheels both grew in diameter, and all the locomotive's wheels got closer (so that the overall length and rigid wheelbase would remain as short as possible). It got increasingly hard to work on an internally mounted valve gear, like Stephenson's. The widespread adoption of the piston valve (due to the application of superheating) in the early twentieth century also pushed the adoption of an outside valve gear, since piston valves were optimally driven by an externally mounted gear.
Whether in the United States, Britain or continental Europe, most locomotives soon used Walschearts valve gear, and did until the end of steam. Since these nations also produced the locomotives for the vast majority of the rest of the world, the others followed suit.
Of course, the Walschearts gear wasn't the only external gear that could be used, nor was it perfect. It produced valve events about as good as any other regular locomotive valve gear, but its sliding die block needed regular lubrication and prevented it from being totally fitted out with roller or needle bearings for reduced maintenance. For that reason, many (although not a majority) of railroads in the United States favored the Baker valve gear, which contained no slides and furthermore was one self-contained system mounted in a cast steel frame. Other competing valve gears of the regular sort included the Southern valve gear and the Young valve gear, but these never gained the popularity of the Walschaerts or Baker gear.
Another class of competitors were the poppet valve gears, including the Caprotti valve gear, the Franklin Oscillating Cam valve gear, the Franklin Rotary Cam valve gear and others. Theoretically much more efficient, all of these suffered from the dual problems of not handling wear very well and the worse one of complexity, and were not widely adopted.
The Walschearts gear could be used with three or four cylinder locomotives, too. One set of valve gear could be installed per cylinder, of course. This either required the center gear(s) to be mounted inside, or for two sets of valve gear to be mounted on the same side of the locomotive (normally driven from two different driving wheels). Some four cylinder locomotives had the cylinders set up so that they operated in linked pairs, and thus needed only two sets of valve gear. It was also possible to use levers to derive the valve motion for a third cylinder from that of the other two, as was done in the Gresley-Holcroft Conjugated valve gear.
Thanks to many people and texts, but particularly to Charles Dockstader for his valve gear simulation program (available from his web site at http://www.tcsn.net/charlied/) which allows an understanding of these things in a way which way surpasses that possible in a text description. Charles' program allows one to see a valve gear in motion, to adjust it in all parameters and watch the effects of such modification, and even to plot graphs of the valve events, with sufficient accuracy to design a real valve gear for a working locomotive.