The valve gear design for a locomotive follows an assessment of what is required within the cylinder. Port sizes are decided according to the required steam flow and the valve parameters derive from the lap and port opening in full gear. Lead is determined according to the required preadmission and compression characteristics and is a function of the set advance of the mechanism. Stephenson’s link motion with open eccentric rods generates increasing lead towards mid gear whilst that of Walschaerts’ gear remains virtually constant, though the preadmission increases towards mid gear in all cases.
As a rough guide early in the design stage, where the lap and port opening assigned are equal the cut off will be around 75%, quite adequate for good starting except perhaps for heavy loads on steep inclines. A more even torque may be had by using three cylinders set at 120o or four cylinders set at 135o.
Only if the gear design can support equal leads is it sensible to set the valves to equal leads, since this will directly affect the equality of the other events. Specific setting procedures were laid down by the design office, such was the importance placed on conformity. Generally, the engine’s stance was checked for level and all the dead centres determined accurately and marked so that they could be regained at any subsequent time.
Walschaerts’ gear was then tested on dead centres to verify that the eccentric rod allowed of no valve movement from the reversing mechanism and corrected if necessary by forging the rod, unless there was a specified non-conformity with this general rule. It is geometrically clear that this procedure assumes that both return crank and expansion link trunnion are correctly placed, but is a practical compromise in reality. Stephenson’s gear required that in by far the majority of cases the forward and backward eccentrics were advanced equally and in correct relationship to the main crank pin.
It is of no advantage to be able to see the valve when setting because the eye cannot substitute for accurate measurement. All that is required is a datum from which to work – the dimension from port edge to cover face, or on model slide valves the dimension from port edge to the inner steam chest face may be more convenient. In full size practice a device on which to mark valve positions was fixed to the front cover facing. Modellers will find that a digital depth gauge serves more easily. Means of adjusting the valve on its spindle, provided so that the valve can be moved independently of the gear and so preserve the gear’s known parameters, was not usual practice in full size. This is most regrettable, since altering eccentric rods or radius rods inevitably compromises the gear design, but this was generally tolerated in full size as a practical expedient and never reviewed. It really is incongruous to worry about 1/64″ setting accuracy on the one hand and to accept an alteration in the fine design of the gear on the other.
Today, with the advantage of computer simulation, valve setting can be achieved accurately and simply to preserve all the details of gear design. Also, if the eccentric rods incorporate the small addition in length to satisfy centralising of the leads, Stephenson’s gear can be set for equal leads at any reverser setting instead of achieving this at only one preferred notch. In theory, of course, the valve could be set by moving the whole cylinder, but this is purely academic.
SETTING TO SIMULATION
At any specific depth in gear (best near full gear) the total travel of the valve may be measured. The simulation’s reverser may then be set to record that same measurement of valve travel. The list of piston/valve relationships will then serve to find the point where the valve is exactly central over its ports (valve travel = zero). This occurs twice in a revolution – choose that as the piston approaches front dead centre. Read off the amount of piston travel before the dead centre and clock this through the front cover (removed for the purpose). With the engine in this position use the original datum to adjust the valve on its spindle to its central position over the ports. All positions of the simulation are then preserved – lock the valve setting.
If required, simple depth jigs can be made for a specific engine for future use. It was part of the valve setting procedure to check minimum dieblock clearances and record these on their cards. Rarely would the valve require a large movement to attain the required position and where the valve needed 3/16ths or more in adjustment the locomotive was the subject of detailed investigation to discover the cause.