Why not use Variable Lead Valve Gear?

Readers have occasionally asked whether consideration was given to applying variable lead to the Walschaerts valve gear on the 5AT.

In fact, Wardale did give thought to this and he discusses it in FDC 5 (line 34) where he wrote:

Varying lead with cut-off is possible by

  1. shortening the eccentric crank length or
  2. slotting the combination lever top and suspending the front of the radius rod from a link held in one arm of a crank, and by raising or lowering it varying the effective length of the combination lever dimension between the radius rod and valve spindle joints.

The first method is simple but gives a lead which diminishes continually from FFG to FBG, i.e. it gives maximum lead in FFG, which hinders starting and is therefore potentially a significant drawback.

In the second method, by having the crank cam-operated and linking the cam rotation to cut-off, any reasonable desired variation of lead with cut-off (such as zero lead in full gear, then high lead in the long to mid cut-off range, decreasing for shorter cut-offs) could be arranged. This system is potentially advantageous, but it would be more so on a freight locomotive required to make difficult starts and habitually working in long cut-off, as did, for example, the D. & R.G.W. Railway 4-6-6-4’s to which it was applied, and whilst mentioned here it is not proposed to use it on the 5AT due to the not inconsiderable extra design complexity involved.