The British Aeolian 4-port tracking system with moving tracker-bar is
damned clever.  The two outer ports work as usual, and each inner port
turns off the outer port on the other side by means of a pouch valve.
The two tracking valves are "mushroom" valves, so when the roll is
centred and on track both linked tracking pneumatics are under suction.

It's quite a complicated thought experiment to determine what happens
with one or both edges of the roll curled or torn.  The system can go
quiescent (no suction on both pneumatics) until it picks up on a good
edge again, tracking on the two ports on one side, and since the inner
and outer ports are overlapped, the maximum tracking error is half a
roll perforation.  Properly adjusted, the system works extremely well.

The clearance between the dollies and the pouches must not be too
tight, and in each of the two pipes to the tracking pneumatics there
should be a constriction consisting of a brass tube about an inch long
partly filled with solder and drilled through about 1/32".

These factors have the effect of slowing the correction time which
stops the system hunting and making farting noises.  The bleed size is
about the same as for the note pneumatics, so the valve lift and drop
cycle is symmetrical.

In some instruments the pipes from the outer and/or inner ports have
dust screens which should be installed the right way round so they can
be cleaned effectively.  The system will hunt if any screen becomes
clogged.

Patrick Handscombe
Wivenhoe, Essex, UK