Author Topic: Question on Stephenson value linkage  (Read 738 times)

Offline kvom

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Question on Stephenson value linkage
« on: January 03, 2016, 03:27:40 PM »
In observing the motion of the eccentric strap on my Westbury paddle engine, I see that the center pin takes a sort of oblong path over a full cycle.  This is without the reversing mechanism in place.  When I attach the lifting link to this pin, that motion causes a slight vertical movement of the lifting link and a resulting slight rotation of the weigh shaft.

Since a reverser will in principle lock the weigh shaft, the lifting link will be constrained to rotate about the end of the weigh shaft arm.  What effect should/will this have on the eccentric strap and valve motion?  Obviously with two engines 90 degrees out of phase the weigh shaft can't move in two directions simultaneously. 

Offline GailinNM

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Re: Question on Stephenson value linkage
« Reply #1 on: January 03, 2016, 06:01:39 PM »
It's been 20 years since I designed an engine with Stephenson's valve gear so I'm not really qualified to answer your question. But, I can recommend that you download one of the valve gear simulation programs that are available now. I used Dockstader's it's original DOS simulation program to check my calculations and to optimize the valve gear. Everything interacts so running things through a simulator makes life much easier to see what's going on. When Westbury design the paddle engine it was far before the age of computer simulation programs so everything was done with pencil, paper and lots of experience. This is achieved to design that was "close enuf" and was very well done for that time.

All valve gear design is a compromise. You pick what is important you and optimize that aspect and then see how bad some of the other characteristics are.

Here is a link to a short one page summary on valve gear software. At the bottom of the page are links to four different simulation programs. The only one I have ever used was Dockstader's DOS program and it is the granddaddy of them all but certainly somewhat outdated by today's standards.

Gail in NM

I would like to be the kind of person my dog thinks I am.