Stephenson valve gear tutorial

For Austin, Nick, Adam, and everyone else who isn’t Alex Karnes. XD Stephenson valve gears can seem to be complicated devices since they’re hard to see, but drawing one really is easier than it looks. However, knowing how the parts fit together is crucial if you want it to look convincing... and while I hate to give up my trade secrets (“I have a competition in me... I want no one else to succeed”... XD) this diagram shows the basic geometry by getting the wheels and the frames out of the way and looking at the valve gear one “assembly” at a time.

The important thing to remember is that the “rocker arm” visible outside the frames is only one half of a longer lever that is symmetrical above and below the arm’s fulcrum (the arm’s bottom half, being behind the frames, is harder to see than its top half). The bottom half of the rocker arm ends in a pin (or “block,” to be technical about it) that is always inserted into the slot of the link (also behind the frames). (As you can see as well, the link (one on each side of the engine) is connected to the drive axle via two rods that end in circular “eccentrics.” Luckily, the frames nearly always block the eccentrics from view from the outside, so you hardly need to worry about them.) Remember this and you should be good to go.



(Note: If you’re Alex, don’t fuss at me because I didn’t draw the eccentrics set properly... it would have taken me forever to mentally figure out how they should be quartered. XD For everyone who isn’t Alex but read the previous sentence anyway, rest assured that as far as drawing (inside) Stephenson valve gears goes, it was irrelevant. XD)