Supplement to last post. This is the unique set of OUTSIDE Stephenson link valve gear as fitted to ex-LMS Black Five no 44767. There is one set per cylinder. Note it is in neutral or mid gear in this photo as the loco was parked up not in steam.
How point A is arranged on Pet is the one significant item I didn't get in the photos. It is the fixed point from where forwards the fore and aft motion of the valve rod spindle has to be on the centre line of the bore in the cylinder block and from there backwards the movement is in both the vertical and the fore and aft plane as the axle mounted eccentrics revolve.
The steam moves the piston, the piston moves the axle, the eccentrics are revolved and they drive the valve gear, that closes and opens the in/out steam ports and the piston is powered the other way and the engine moves (Cycle repeats).
The orange line shows how the power element of the system is off set from the centre line of the axle. The one on the other side of the engine has the corresonding orange crank line at 90 degrees.
On 44767 the valve motion is worked by cranks but to simplify the idea I have added the red blob to show how on an inside set of this gear the eccentrics surround the actual axle and move far less.
To make 44767 move forward the die block in the curved link is raised and the top rod becomes active. The turning of the eccentric is transmitted to a fore and aft motion in the valve rodding and the valve in the cylinder moves fore and aft opening and closing the relevant ports. If the block is dropped the converse action occurs, the lower rod is the active one and the ports open and close in the opposing sequence and it goes backwards.
You may have heard of cut off in relation to driving a steam engine. In the above example the die block fully dropped or fully raised gives a full movement of the valve rod in the cylinder and a full port opening per stroke - maximum input of steam and maximum power. Anything between all the way and the centre in the expansion link reduces movement in the valve rod thus reducing the port opening size/time and cuts off the steam flow early increasing efficiency of the engine.
Note - photo taken whilst working with the engine preparation team and therefore with full permissions to be wrong side of the fence!
NB Picture reloaded and broken link repaired 20 June 2013