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Peter Maloney08/07/2020 11:13:40
33 forum posts
12 photos

Hi All

I am 'playing about' with the valve gear on Pansy (my first model) to try to understand what 'is going on'. Am I correct in thinking that the valve gear is set up with 'crossed' eccentric rods and not 'open' rods because it uses a rocking shaft? Sorry if this question seems a bit basic.


Peter Maloney08/07/2020 12:44:49
33 forum posts
12 photos

Hi Again

Since my original post I am even more confused. Pictures I have of Pansy chassis seem to show 'crossed' eccentric rods which also seems to tie in with Martin Evans in his book 'Manual Of Model Steam Locomotive Construction'. When closely looking at the LBSC drawings, the eccentric rods seem to be 'open'. Obviously I am 'not seeing something'. Any help gratefully received.


John Baguley08/07/2020 14:47:01
477 forum posts
51 photos

Hi Peter,

It's a bit confusing as the rods of both types become open or crossed at some point in their motion!

The way to tell if the valve gear has 'open' or 'crossed' rods is how the rods look when the crankpin is on the opposite side of the axle to the eccentric.

The majority, if not all, of loco valve gears use open rods. I don't think it makes any make any difference if the gear uses a rocker or not.

With open rods the amount of lead increases as the cut off approaches mid gear. It is often necessary to set the gear to give zero or even negative lead in full gear to avoid the lead becoming excessive as the gear is notched up.

With crossed rods the amount of lead decreases as the gear is notched up. I believe crossed rods are normally used on traction engines rather than locomotives.

Hope that helps.


Peter Maloney09/07/2020 14:58:25
33 forum posts
12 photos

Hi John

Thanks for showing interest. Purely by chance I have come across what I call the 'John Baguley Pages'. Lots of very interesting and useful info. Thanks again.


Bruno Taylor11/07/2020 16:18:18
47 forum posts
14 photos

Hi John

Open and crossed rods are a bit of a misnoma as Stephensons gear the gears will always cross during rotation. Generally all drawings show open rods and the loco in mid gear for clarity.

The use of rocking shafts means the eccentrics are backwards but there is no effect on the rods themselves.

You say you are playing around with the valve gear.

In previous threads on Pansy errors in the valve gear are a keen topic. I suggest you read them if you haven't already.

I have recently built a Pansy valve gear with the assistance of Julian Atkins in particular. I modelled the gear using Don Ashtons theories and Docksanders simulation. I also built a 1:1 physical model.

The Chassis runs well on air.

Best of luck.

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