I do not know of a chart showing how the excellent British staggered
double hole tracker system works, maybe there is a patent paper
somewhere.  It is certainly a system I would recommend, having
experienced most of the other types.  With the spools fixed and
accurately aligned, i.e., side to side and parallel, and the tracker
bar accurately following even damaged rolls, it just keeps on working
with no adjustment.  Anyway, this is my attempt to explain it in
detail.

In the tracker valve block, the valves are outside type (mushroom)
which means that when they are off they supply suction to the
appropriate tracker pneumatic and when on they don't.  They are turned
on by the outer holes and are forced off by the inner holes on the
opposite side.

In the following logic chart, XX and OO symbolise the double staggered
tracking holes on the tracker bar.

X = hole covered
O = hole open

XX..XX  Suction on both sides, the pneumatic is balanced and centred.

OX..XX  Suction on right side of pneumatic is turned off by left outer
hole being open, so tracker bar is pushed to the right.

OX..XO  Both sides of the tracking pneumatic are turned off by both
outer holes being uncovered.

OO..XX  Tracker bar is pushed to the right as left pneumatic has suction
by right outer hole being covered and left inner hole being uncovered.

OO..XO  Tracker still pushed to right as left inner hole is uncovered
providing suction to pneumatic even though both outer holes are uncovered.

OO..OO  The pneumatics are balanced and under suction because both valves
are in the off position controlled by the inner holes.

Without looking in the valve block, I would imagine that the inner
tracking hole uses a membrane to block the tube from the outer tracking
hole on the other side causing that valve to close and so supply
suction to the pneumatic.

If you can get you head around the above then you obviously did not go
drinking the night before.  And if I have got it wrong then I am not
surprised.

I would suggest that Gary Crawford contacts a British restorer to see
if they have a spare valve block and tracker bar, it would save hours
of work.

James Black