Southern valve gear
Southern Valve Gear
The Miniature Locomotive, November-December 1954
The question about the Southern valve gear by G. B. Thomas in The Miniature Locomotive for September-October 1954 brings up a matter that is hard to explain. Almost everyone has his own pet reference book on the various valve gears and the methods of how to lay them out. I do not profess to be an authority of any kind on valve gears, but I agree 100% with one of the best locomotive designers of all time. He was once asked about what was a good reliable book to get. His reply was to the effect that they all had one outstanding feature; they were all notorious for What They Did Not Tell!
Now, we all know that the backset of the eccentric rod pin hole in the tail of a Walschaert's link is put there to correct for the angularities in the rods and that the backset is necessary to obtain an equal swing of the link. This is one of the most critical points in the design of the whole gear and yet almost all books skip lightly over this and one is still left out in the dark after studying them. The best method that I ever recall seeing was given by our always dependable instructor, LBSC, in The Model Engineer of July 4, 1935.
The same situation exists with the Southern valve gear, only in this case it is the angle of the two arms of the bellcrank. The included angle of the two arms is somewhere between 70 and 75 degrees. none of the books or articles that I have seen gives the angle nor do they tell how to figure it. If one puts a protractor on the various drawings, the angle comes out as stated above. My idea is that the angle here is essential for correcting angularity. The best dope on the Southern gear that I know of is in the book, The Locomotive Up to Date by McShane, the revised edition of 1920. With this book and LBSC's axiom that the eccentric crank pin circle should be two-thirds of the piston stroke, the necessary features are well covered.