Steam Locomotive Walschaert Valve Gear Diagram

Walschaert Valve Gear --
Invented in 1844 by Egide Walschaerts, foreman of the Belgian State Railway
(Diagram based on 4-6-2 Pacific, Soo #2714)
Steam Locomotive Valve Gear - Component Names

 1. Eccentric Crank
 2. Eccentric Rod
 3. Reach Rod
4. Lifting Link
5. Lifting Arm
6. Reverse Arm & Shaft
7. Link (Expansion Link)
  8. Radius Bar
  9. Crosshead Arm
10. Valve Stem Guide
11. Union Link
12. Combination Lever
13. Valve Stem
14. Valve Spindle
For steam source and locomotive internal workings illustration, see Locomotive Insides (77K GIF)

The copy below is taken from page 88, Walschaert Valve Gear, of Locomotive Boilers and Engines, A Practical Treatise on Locomotive Boiler and Engine Design, Construction, and Operation, by Llewellyn V. Ludy, M.E., Professor of Experimental Engineering, Purdue University, American Society Of Mechanical Engineers, American Technical Society, Chicago, 1920.

The following steps for adjusting the Walschaert valve gear are given by the American Locomotive Company:
  1. The motion must be adjusted with the crank on the dead centers by lengthening or shortening the eccentric rod until the link takes such a position as to impart no motion to the valve when the link block is moved from its extreme forward to its extreme backward position. Before these changes in the eccentric rod are resorted to, the length of the valve stem should be examined, as it may be of advantage to plane off or line under the foot of the link support which might correct the length of both rods, or at least only one of these would need to be changed.

  2. The difference between the two positions of the valve on the forward and back centers is the lead and lap doubled and it cannot be changed except by changing the leverage relations of the combination lever.

  3. A given lead determines the lap or a given lap determines the lead, and it must be divided for both ends as desired by lengthening or shortening the valve spindle.

  4. Within certain limits, this adjustment may be made by shortening or lengthening the radius bar but it is desirable to keep the length of this bar equal to the radius of the link in order to meet the requirements of the first condition.

  5. The lead may be increased by reducing the lap, and the cutoff point will then be slightly advanced. Increasing the lap introduces the opposite effect on the cutoff. With good judgment, these qualities may be varied to offset other irregularities inherent in transforming rotary into lineal motion.

  6. Slight variations may be made in the cutoff points as covered by the preceding paragraph but an independent adjustment cannot be made except by shifting the location of the suspension point which is preferably determined by a model.
(& Links)
4-6-2: #2714
Steam Engine
PV Diagram

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