Steve asked:-
> At the National Summer Steamup Jeff Williams was kind enough to let me > inspect the valve gear on his C-16. I had a question about why prototype > Stephenson valve gear had two eccentrics per cylinder that each drove > one end of the expansion link. The C-16 has only one eccentric per cylinder. > > After a little bit of study I believe that the answer to my question is > that the two separate eccentrics allow setting valve "advance" for both > forward and reverse. With only one eccentric per cylinder driving the > expansion link it's possible to set valve advance for one direction > only. The other direction would then have the opposite effect applied to > the valve timing. Am I correct? Or close? Absolutely correct. The reason they get away with one eccentric is that the valve cut-off is 100% (zero lap) and the eccentric does not need to be advanced. This is an inefficient use of steam as it is not used expansively but makes for a cheap and cheerful valve gear. Most manufacturers of toy steam engines do it this way. It also allows for reversing to be achieved by using piston valves and reversing steam and exhaust connections. For a proper Stephenson's valve gear, take a look at a Catatonk logging engine, a Graham steam motor or a DJB Mikado. Mike Chaney
