eccentrics

[macmodels]

How do you determine the length of throw for the eccentris for stephenson valve gear I have a lot of other questions,but one at a time. thanks

[Harlock]

There is a good valve gear simulator software, written by Charlie Dockstadder, the guy who built the Mastadon engine I steamed a little while ago.

www.tcsn.net/charlied/

Download file 10A.

It's pretty neat, you can grab the sliders while it's running and change the reverser and the throttle. Somewhere in there you should be able to get the numbers you are looking for. I think you can modify the simulation to plug in your own dimensions for things.

--Mike

[gwrdriver]

The Dockstader programs are designed to test or proof a valve gear design rather than be a primary design tool but the base animation will certainly show you what the gear should look like in motion. You should be able to find several good Stephenson valve gear design texts on-line (or books) and from those develop your individual design on paper (or Cad). In a valve gear the location and/or angularity of everything affects everything else in some way. Stephenson appears simpler than most but isn't and for instance the eccentric throw, the location (in both X and Y) of the rocker box, length of the rocker arms, length and agularity of eccentric arms, the vertical location of drive axle centers, and the length of valve travel are all related and affect valve event timing, which is the sole purpose of the gear. While it can be done, using Dockstader to try to design all this by trial and error can become a rat's nest quickly, as I have discovered. When you go in Dockstader and set a new variable value (dimension) that suits your design, but which is substantially different from the base program design, dozens of other related values (and the animation geometry) goes a bit whacky and all of a sudden you are lost.

I would suggest establishing the basic dimensions of your design first before going further . . . the size and location of the cylinder bore, its relationship to the drive axles (co-linear, above, angled?), the height of the valve rod above piston center line, the rocker box pivot point, and the valve port dimensions. Of course this last is a science unto itself. I personally find Stephensons easier to grasp than most others but then I'm in the middle of trying to design an inclined Walschearts gear from scratch and I'm having all kinds of trouble.
Harry