Walschaert valve gear

Radius Rod
by Charles A. Purinton

The Live Steamer, January-February 1951

With Walschaert valve gear the length of the radius rod (8) and the eccentric rod (2) is dependent on the fore and aft location of the expansion link (7). In most layous of Walschaert gear, the expansion link is located approximately halfway between the centers of the main driving axle and the valve stem guide (10), to which the combination lever (12) is attached. The radius of the expansion 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 (2) and position of the eccentric crank (1).

Setting
Rules for Timing Locomotives with Walschaert Gear

From Friends Steam Models Catalog 5A:


 * 1) Block up wheels in running position.
 * 2) Get front and back dead-center.
 * 3) Make dummy pin for link tail with shoulder sticking out.
 * 4) Then set the expansion link in mid position so that the radius rod (8) passes up and down without moving the link.  Have valve on center of seat.
 * 5) Use inside calipers. Set piston on front dead-center. Measure from dummy pin in link to eccentric crank pin and repeat for back dead-center.  Move eccentric crank until both distances are the same and clamp eccentric crank.  Then, make an adjustable eccentric crank rod.
 * 6) Then to check length of eccentric rod, again set piston on front dead-center and move gear up and down.  Valve should not move.  The same for back dead-center.
 * 7) Then set valve to get equal leads, front and back, when radius rod is in mid gear (or in the center of the link).

Video
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