More valve gear

[Deleted]

I have written to M.E. regarding No.4376 23rd April and the letter included a note about Don's valve gear article in that issue. Seeing that he has kindly responded to recent comments here about his Q1 article prompted me to post an extract on-line in addition to the letter.

"It has been mentioned that with indirect drive, through a rocker, 'the expansion links were driven from behind the link-slots', which I understand to mean if a rocker is used then launch type expansion links are used. Why is this so? I have prototype drawings for a production main line passenger locomotive where loco-type links are used with an indirect rocker drive to piston valves over the inside cylinders."

With best wishes to all
Dave

[donashton]

Dave,

Sorry for a late reply. It is not a question of 'rocker used therefore launch link'. It is a question of valve direction relative to connecting rod and eccentric rod direction, or rather angularities.

Taking your prototype - loco links through rocker to piston valves above the bores - it is assumed that these valves are INSIDE admission and all is good practice. (Q1 was OUTSIDE admission pv.) Your case has the main crank one side of axle centre and the eccentrics advanced opposite (roughly), so the two sources of angularity are opposing each other somewhat and the completion to clear up the last little bit is achieved by offsetting the trunnion a fraction ahead of the link centre.
A launch link has effectively a rather violent knuckle joint effect, and this can best be opposed by having both eccentric and connecting rod angularities summating (all on same side of the axle). Only a relatively small problem is then left to an offset trunnion behind the link slot centreline. GWR Hall/Manor/Grange gear is a very fine example.

Email me with pleasure if all is not clear (see members list)

Regards,

Don