Re: Stephenson Valve Motion Dilemma...

Curtis: The link block is pivoted on the rocker arm. Forward or reverse motion of the engine is determined by which pair of eccentrics is moving the links.Standard practice is to have the backup
eccentric connected to the bottom of the link.
"BOB", "Backup on bottom" is the mechanics rule.
The links are suspended from the lifting arms by
a rod (similar to a side rod) which allows them
to swing freely. The most desirable location of the link lifting arm ends would be a point that
averages out the the difference between the points
where it is located when in forward or backup.
Direction of motion of the engine is determined by lowering the links for forward motion and raising it for backup.
Because of the weight involved the Johnson Bar usually was assisted by a spring arrangement that
counterbalanced the weight of the links.Even so, I understand that on some engines it was a real chore to move the lever when changing directions.
I read somewhere that the Stevenson Valve was the first gear to allow "hook-up" thus taking advantage of the expansive power of steam.
Jim