Slip eccentric design considerations

[JMORG]

Hi all,
Currently drawing up the slip eccentric components on CAD. Just need abit of help for the eccentric and the stop collar.
I know that the offset for the eccentric is half the valve travel for the slide valve, but does the overall diameter matter for the eccentric?
As for the stop collar: Is there a recommended amount of travel for the drive pin? I am using the Wild Rose drawings as a reference and it has a 1.25mm offset from the center of the diameter.
I have included the drawing as a link so everyone knows what I am talking about!
http://www.davewatkins.pwp.blueyonder.c ... 281%29.pdf

[DaveWatkins]

Hi

The offset is related to valve lap.

If the valve just spans the ports (that is there is no lap), then there would be no offset. The slip eccentric is arranged to be exactly 90 degrees ahead of the piston. This will work in our size engines but does not make the best use of steam. No problem if you have a large boiler and adequate burner.

For a smaller boiler, to get a good length of run you would want to make more efficient use of steam. Because steam can continue to expand after entering the cylinder the steam supply can be cut off before the piston reaches the far end of the cylinder. This is achieved by making the slide valve wider (that is it has lap). The offset is added to the slip eccentric drive so the inlet port is uncovered by the valve as the piston reaches top dead centre. With lap and without the offset the port would not be uncovered until the piston was already past top dead centre.

I hope this helps.