Steve, I cannot say for sure about the C-16, but the reason that the Aster C7S Mogul has only one eccentric, is because it isn't Stephenson's Link motion! It is a modified form of Walchaerts gear, but without and anchor and union link to impart the lap and lead functions to the valve, sepeartely from the valve motion imparted by the return crank or eccentric. An "true" Stephenson's Link Motion ALWAYS has two eccentrics, or two return cranks. When the eccentric that provides the forward motion is lined up with the end of the expansion link, the locomotive moves forward with full valve travel. As you move the reverser towards the center of the expansion link, the stroke of the valve is shortened, until at the mid point, the valve doesn't uncover the ports at all. Then, as you pass center, you begin to align the other end of the expansion link with the reverse running eccentric, ever increasiong the valve travel until you are lined up with the eccentric and the extreme opposite end of the expansion link. So, to make it short, if there is only one eccentric, it isn't Stephenson's valve gear! Ther are many types of valve gear, that all look similar, and it would probably help if you could find a copy of the book, Model Locomotive Valve Gears by Martin Evans, or the one by Henry Greenly, where there are good diagrams to show how each gear works. I have a Bassett-Lowke No. 3 gauge steam locomotive (2-1/2" gauge) that has a gear that also looks similar to Stephenson's Link Motion, but in this case, the link doesn't vibrate with each revolution of the drivers, and the link block slides up and down in the link with each revolution. This a the Greenly's "corrected" motion, based on Joy valve gear. So, unless you make an exhaustive study of valve gears, some can be a bit difficult to identify. Sincerely Keith Taylor Jefferson Maine USA ----- Original Message ----- From: "Steve Shyvers" <[EMAIL PROTECTED]> To: "Multiple recipients of sslivesteam" <[EMAIL PROTECTED]> Sent: Tuesday, August 31, 2004 10:05 AM Subject: Stephenson Valve Gear
> 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? > > Steve Shyvers > > > >
