The drawing (GA) reflects straightforward conventional design. Points noticed are the absence of backset on the expansion link tailpin and the shortness of both radius rod and eccentric rod. This latter turns out to be impression only – the expected compaction of the gear proves not to be so, as the dimensions apply to a mechanism requiring to deliver a mere 4″ valve travel from 23″ stroke. The expansion link swings for inside admission are considerably larger than the equivalent outside admission gear, yet the C19 manages perfectly well on less than 40o.
Appraisal of the motion drawing:
The cylinder inclination is taken from the railhead to bore centre dimension given and not the given inclination, which is slightly at variance. Some small amount of juggling is required to equate the vertical 3″ dimension with the 3.187″ of the combination lever upper pins, caused by two factors:
Firstly, the position 180o later is not a mirror image, and secondly one expects that the given lap and lead are nominal. As the backset is hardly likely to work out mathematically to zero this must have been deliberately chosen and one could therefore expect to see the union link used as a modifier, since perhaps the most primary one has been denied.
In the congested area of the return crank it is difficult to determine the dimensions, therefore a method of calculating the likely dimensions from the other data present was adopted. This quickly produces some initial figures to feed the simulator in order to fine tune from there and achieve the 4″ travel.
The weighshaft is slightly above the trunnion line to give greater cut offs in back gear for a similar angle of lifting arm, yet the table gives a 6″ depth in gear for both fore and back gears. We are dealing with nominals and variables here and the consequences are of no import. The results from the simulator are constructed from a different angular datum for back gear on the basis of maximum cut offs provided in the Ipswich works tables.
These tables infer that the cut offs are equal at 50% and deliberately contrived so to give the working range the best figures. The graph lines for front and rear ports frequently cross each other where the coincidence is unique, that is, the point at which front port distribution bias changes to one of rear port bias. It can be tuned in an otherwise symmetrical gear by minor valve setting slightly beyond equal leads and this has been carried out for the C19 simulations accordingly. The alternatives are not available.
Third axle drive option:
There is no question of an Ipswich third axle drive option: this is merely a paper exercise carried out to compare the given layout with a more relaxed envelope. The simulation quickly reveals an excellent gear for C19, rendering the third axle drive option somewhat superfluous. Tables and event diagrams are largely self-explanatory.
The results incorporate the revised lap and lead and show, in as much as Walschaerts’ is capable, a fine gear without compacted layout problems.