Walschaert valve gear
Radius Rod
The Live Steamer, January-February 1951
With Walschaert valve gear the length of the radius rod and the eccentric rod is dependent on the fore and aft location of the swinging link. In most layous of Walschaert gear, the link is located approximately halfway between the centers of the main driving axle and the valve stem crosshead, to which the combination lever is attached. The radius of the link is the length of the radius rod measured from the center of its connection at the combination lever, to the center of the bearing on which the link swings.
If the radius of the link was not equal to this distance, it would be impossible to move the radius rod from top to bottom of the link without moving the valve and it must be possible to do this without moving the valve for accurate valve setting.
This condition is also a must for checking the length of eccentric rod and position of the eccentric crank.